T I T A N E L E C 9 7 6 2 : P R O J E C T

Meiyappan Muthu z5026869

Hassan Wahab z5025960

Cheng Wei z5036213

Jason Chan z3256518

Contents

1. Executive Summary ............................................................................................................................... 1

2. Background ........................................................................................................................................... 2

2.1 Historical observations and explorations ............................................................................................. 2

2.2 Unanswered Questions about Titan .................................................................................................... 3

3. Mission Statement ................................................................................................................................. 5

3.1. Mission Statement .............................................................................................................................. 5

3.2. Mission Objectives ............................................................................................................................. 5

4. Stakeholders ......................................................................................................................................... 6

5. Timeline ................................................................................................................................................. 7

6. System Concept .................................................................................................................................... 8

7. Concept of Operations ........................................................................................................................... 9

8. Requirements ...................................................................................................................................... 11

9. System Architecture ............................................................................................................................ 16

9.1. Launch Vehicle ................................................................................................................................ 16

9.2. Orbits and Trajectories ..................................................................................................................... 16

9.3. Payloads .......................................................................................................................................... 17

9.4. Subsystems ..................................................................................................................................... 21

9.4.1. Command and Data Handling Subsystem (CDHS) .................................................................... 21

9.4.2. Structural Subsystem ................................................................................................................. 22

9.4.3. Attitude Determination and Control Subsystem (ADCS) ............................................................ 23

9.4.4. Power Subsystem ...................................................................................................................... 23

9.4.5. Thermal Subsystem ................................................................................................................... 25

9.4.6. Communications Subsystem ..................................................................................................... 26

9.4.7. Propulsion Subsystem ............................................................................................................... 27

9.5. Ground Architecture ......................................................................................................................... 27

10. Propulsion Subsystem Analysis ....................................................................................................... 30

10.1 Statement of Works ......................................................................................................................... 30

10.2 Fault Tree Analysis ......................................................................................................................... 32

10.3. Preliminary Failure Mode, Effects and Criticality Analysis (FMECA) ............................................... 34

10.4. Risk Matrix ..................................................................................................................................... 37

11. Conclusion ............................................................................................................................................ 38

Appendix .................................................................................................................................................... 39

References ................................................................................................................................................. 40

List of Figures

Figure 1: Titan as observed by Pioneer 11 ................................................................................................... 2

Figure 2: Titan as observed by Voyager 1 .................................................................................................... 2

Figure 3: Titan as observed by Cassini ......................................................................................................... 3

Figure 4: The first images from the Huygens probe during descent .............................................................. 4

Figure 5: Mission timeline diagram ............................................................................................................... 7

Figure 6: Titan boat concept ......................................................................................................................... 8

Figure 7: Titan Boat profile views .................................................................................................................. 8

Figure 8: Landing operations for the Huygens probe which the Titan Boat will largely mirror ........................ 9

Figure 9: Map of Ligeia Mare on which science operations will be conducted ............................................ 10

Figure 10: Function tree for the Titan Boat Project ...................................................................................... 11

Figure 11: Function block diagram for landing on Titan............................................................................... 14

Figure 12: Function block diagram for mission operations .......................................................................... 14

Figure 13: Functional matrix: Requirements vs. subsystem as used for subsystem design ........................ 15

Figure 14: Trajectory to Saturn (Labreton et al. 2002. Edited to remove dates) .......................................... 16

Figure 15: Orbit around Titan (Labreton et al. 2002. Edited to remove dates) ............................................. 17

Figure 16: Example of a twin camera system on the Mars Curiosity Rover ................................................. 18

Figure 17: Sample Analysis payload for the Mars Curiosity mission ........................................................... 18

Figure 18: Gas collector payload used on the Huygens probe .................................................................... 19

Figure 19: How environmental sensors can be mounted on the mast ......................................................... 19

Figure 20: Mounting locations for payload systems on the Titan Boat (based on Huygens configuration) ... 20

Figure 21: Subsystem block diagram .......................................................................................................... 21

Figure 22: Titan Boat Subsystem interface N2 diagram .............................................................................. 21

Figure 23: The Huygens probe outer structure from which the Titan boat will draw heavilty upon .............. 23

Figure 24: MMRTG unit used on Mars Curiosity Rover ............................................................................... 24

Figure 25: Space mission power sources from AERO9500 lecture slides, week 3 ...................................... 24

Figure 26: The Huygens front shield design which the Titan Boat will reuse ............................................... 26

Figure 27: Example of electric boat propulsion unit from Volt master .......................................................... 27

Figure 28: Ground architecture for the Dawn Mission on which the Titan Boat Mission is based ................ 28

Figure 29: Requirements for Propulsion subsystem .................................................................................... 31

Figure 30: Fault Tree Analysis: operational but inaccurate direction ........................................................... 33

Figure 31: Fault Tree Analysis: Operational but less thrust than expected .................................................. 33

Figure 32: Fault Tree Analysis: Non-operational ......................................................................................... 34

List of Tables

Table 1: Science objectives and the payloads used to investigate ................................................................ 5

Table 2: Other mission stakeholders ............................................................................................................. 6

Table 3: Mission timeline description ............................................................................................................ 7

Table 4: Operational Requirements and Limits/Constraints ........................................................................ 12

Table 5: Function Matrix for the Titan Boat Project mapping level 2 functions ............................................ 15

Table 6: Summary of payloads used on the Titan Boat ............................................................................... 17

Table 7: Power budget estimate ................................................................................................................. 24

Table 8: Plutonium mass estimate .............................................................................................................. 25

Table 9: Failure Mode, Effects and Criticality Analysis ................................................................................ 35

Table 10: Risk Matrix for the Titan Boat Propulsion System ....................................................................... 37

1

1. Executive Summary This report describes the conceptual design of a probe that will operate on the surface of Saturn’s moon, Titan. The probe is called the Titan Boat and will cruise on the surface of Ligeia Mare, the second largest lake on Titan, selected because it has been fully imaged by Cassini. While the Titan Boat probe will reach Titan via an orbiter/probe configuration like Cassini-Huygens, the scope and focus of this report is principally on the design of the surface probe itself. The Titan Boat mission is a follow on mission to the successful Cassini-Huygens mission and aims to further uncover the mysteries of Titan to investigate two broad scientific objectives: how conducive are conditions on Titan to support life and what is the topology of the polar lake environment? The Titan Boat mission will launch in late 2020 and will arrive at Titan late 2027. The Titan Boat probe will be launched via a SpaceX Falcon 9 vehicle and will utilise a similar orbit trajectory to the Cassini-Huygens mission, sling shotting by Venus, Earth and Jupiter. The Titan Boat’s science mission will last 100 days. This report considers the payloads and sub-systems of the Titan Boat. The Titan Boat will utilise as much of the heritage from the Cassini-Huygens mission as possible. The payload system, however, comprises newer technology appropriated from the Mars Curiosity rover including cameras, spectrometers, a gas collector and dissociator, and a host of environmental sensors. The Titan probe structure is constructed from aluminium and will have a double storey mounting structure for the payload and subsystems. The attitude and determination control subsystem consists of a passively designed hull structure that stabilises the Titan boat on the liquid surface of Ligeia Mare. The Command and Data Handling Subsystem and the Thermal Subsystem will be extremely similar to that used on the Huygens probe as these were worked very successfully. A distinct feature of the Titan Boat is its propulsion system. Unlike the Huygens probe, the Titan Boat will have the functionality to navigate and propel itself on the surface of Ligeia Mare using two 50W electric motors, which although small in size is sufficient for the purposes of surveying new locations. Eight multi-mission radioisotope thermoelectric generators will power the Titan Boat. This report also develops a statement of works to approach prime contractors to build and test the propulsion system. A preliminary fault analysis of the propulsion system is also conducted which was utilised to revise the design of the propulsion system into its current form. Team member contributions to each section of the report may be found in appendix A.1.

2

2. Background

2.1 Historical observations and explorations

Titan is Saturn’s largest moon. At Saturn's orbit, more than nine times farther from the sun than Earth, the

solar illumination is weak, and beneath Titan's smoggy skies it is even weaker. An observer on Titan's

surface would experience daytime as dim as deep twilight on Earth (NASA JPL n.d).

First glance of Titan

On 1 September 1979, the first artificial probe entered the Saturnian

system, Pioneer 11 visited Saturn and conducted scientific research

about its largest satellite, Titan. Pioneer 11 is a 259 kilogram deep

space scientific satellite. Its mission was to study the outer solar

system, including the asteroid belt, Jupiter and Saturn surrounding

environments, solar wind, cosmic rays and finally the boundary of the

solar system. Through temperature measurement of Titan, scientists

concluded that Titan is not likely a place for life, because its

temperature is too cold (The Pioneer Missions 2007). Pioneer 11 also

took a picture about Titan together with Saturn, but the most

significant is that it opened the door of Titan research age.

The Voyager missions

Just following Pioneer 11 footprint, Voyager 1 and Voyager 2 visited Titan in 1980 and 1981 sequentially

(Voyager the interstellar mission 2014). Voyager 1 and Voyager 2 are two deep space scientific satellite

with the same configurations, both satellites are 722 kilogram. They were launched by NASA on 5

September 1977 and 20 August 1977 to study the outer solar system and Interstellar space.

On 12 November 1980, Voyager 1 came within 6490 km distance

from Titan. During its mission, using remote sensing instruments,

Voyager 1 studied the atmospheres of Titan. Due to limited

technology, Voyager could not see through Titan’s atmospheric haze

and neither could Hubble or ground-based observations such that the

nature of the surface remained largely unknown (Lebreton et al.

2002). However, through the initial investigation of Titan, Voyager

took many valuable images and predicted that 90 percent of Titan’s

atmosphere was composed of nitrogen. It also found that the

atmosphere pressure and temperature near the Titan’s surface was

about 1.6 atmospheres and -180 degrees Celsius (Missions to Jupiter

2014).

The Cassini/Huygens Mission

In the late seventies and early eighties, NASA studied several scenarios for missions to Saturn as the next

natural step to flow the Galileo orbiter/probe mission to Jupiter. The Cassini/Huygens mission was

proposed in 1982 as a collaboration between ESA and NASA. Cassini/Huygens weighed 5650kg and was

launched on 15 October 1997 and arrived at Saturn 1 July 2004. The mission was designed to explore the

Saturnian system and all its elements: planets, moons, rings, magnetosphere and their interrelationships.

(Lebreton et al. 2002)

Cassini/Huygens scientific objectives were to determine atmospheric constituents, measure winds and

global temperatures, investigate cloud physics, determine the topography of the surface, infer internal

Figure 1: Titan as observed by Pioneer 11

Figure 2: Titan as observed by Voyager 1

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structure, investigate upper atmosphere. Huygens’s objectives are to make detailed in situ measurements

of the atmosphere structure composition and dynamics (Lebreton et al. 2002).

The Cassini-Huygens mission revealed a lot about Titan. It uncovered that Titan is rich in mixtures of

organic chemicals. The chemicals on titan are mostly nitrogen but it is mixed with methane. Titan has

mountains, dunes, rivers and lakes, but all of them are filled ethane and methane. Titan has a weather

system and seasons like earth (although the seasons are 7 earth years long).

Figure 3: Titan as observed by Cassini

2.2 Unanswered Questions about Titan

Titan has two factors that may suggest it can host the building blocks for life – a chemically active weather

cycle and liquid, both surface and potentially sub-surface.

Liquid

It is thought that Titan may have a warm and watery interior. Rogez and Lunin (2010) concluded that Titan

may have a warm hydrous silicate core overlain by a shell of high pressure ice 500-600km deep. The icy

shell might also contain a liquid layer some tens of kilometres below the surface (Beghin et al. 2010).

Norman and Fortes (2011) identify four possible areas for possible astrobiological potential: the silicate

core, subsurface ocean and crust, and surface ocean. For the purposes of this project, only two areas,

subsurface ocean and crust, and surface liquids will be considered.

1. Subsurface ocean and crust: it is thought that life could be supported if this region has liquid

ammonia given appropriate conditions (e.g. temperature, pressure, access to nutrients etc).

Methane/sulphate oceans are similar to conditions on Earth’s cold seep ocean floors. “At cold seeps,

sulphate reduction and anaerobic oxidation of methane are syntrophically linked. The metabolic

products are hydrogen sulphide and dissolved carbon ionates in liquids erupted from Titan‟s surface are

strong indications of microbial activity in the subsurface ocean.”

2. Surface liquids: Titan’s liquid surface of methane could play the same role for life that water does on

Earth. Methane based organisms, while only theoretical, has regained attention in a 2015 Cornell paper

published in Science Advances on a proof-of-concept blueprint for methane based life (Ju, 2015). It is

also thought that there may be temporary surface water, possibly unfrozen by geothermally heating

liquid methane (Fortes and Grindrod 2006) or geysers (Lorenz 2002). Artemeva (2003) suggest water

4

may be present from comet/asteroid impacts. Sarker et al. (2003) pointed out that “aqueous

cryovolcanic flows may remain partially molten for very long periods if they contain significant ammonia.

These flows may induce hydrolysis of tholins to produce amino acids, the building blocks of RNA and

DNA (Neish et al. 2007, 2008, 2010)

Atmospheric

According to the literature review by Norman and Fortes (2011), methane based life would “produce

anomalous depletions of hydrogen, acetylene and ethane, as they consumed these substances”. There

are many open questions relating to this proposal. Based on Cassini data, Lorenz et al. (2008) points to an

unexpected lack of ethane on the surface. Strobel (2010) found that the Cassini data suggests a lack of

hydrogen. According to an article by Cowen (2010), “Darrell Strobel of Johns Hopkins University in

Baltimore found that 10,000 trillion trillion hydrogen molecules fall out of the atmosphere per second. But no

corresponding build-up was seen at the surface.” Clark et al. (2010) describe an unexpected depletion of

acetylene at the surface given expected rates of atmospheric production and subsequent deposition on the

surface. Furthermore the Huygens probe did not detect acetylene on the surface. Huygens did not have

equipment to test for bio-signatures.

Figure 4: The first images from the Huygens probe during descent

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3. Mission Statement

3.1. Mission Statement

Based on the existing information known about Titan and the tantalising questions that remained

unanswered, the mission statement of the Titan Boat Project is:

Interplanetary study of Titan‟s polar sea, Ligeia Mare by autonomous boat for 100 days in

order to enhance humanity‟s understanding of life in the solar system.

3.2. Mission Objectives

The Titan Boat’s science objectives seek to answer two broad questions: can Titan support life? And what

is the lake topography? Science objectives were designed to help answer these questions as shown in

Table 1. The payload systems are explained in more detail in section 9.3. Each payload package contains

a multitude of sensors. The underneath package describes the sensor kit that is mounted on the base of

the Titan Boat for below surface remote sensing.

Table 1: Science objectives and the payloads used to investigate

Objective A: Can Titan support life? Payload systems used to investigate objectives

A.1. Lake environment

What are the chemicals in the lake? Spectrometer package

What is the temperature? Underneath package

What is the density? Spectrometer package

A.2. Atmospheric environment

What is the chemical composition? Spectrometer package, gas collector and dissociator

What is the radiation? Environmental Package

What is the weather system like? Environmental Package

Objective B: What is the lake topography?

How deep is Ligeia Mare? Underneath package

What is the composition of the lake bed? Underneath package

What does the lake look like? Camera system

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4. Stakeholders

Customers

This report defines customers as the intuitions paying for the Titan Boat Mission. The Titan Boat mission

will be a worldwide collaborative program including National Aeronautics and Space Administration

(NASA), the European Space Agency (ESA), as well as several separate American and European

academics and contractors. The international composition of Titan Boat team guarantees that the mission

responsibility and expenditure would not be borne by any single organisation. Through sharing investment

and participation, the risk and cost will be largely alleviated.

End customers

The international science team investigating Titan will comprise some 250 experts from 15 countries and

regions. Their research will be made publically available such that the end customer is ultimately the

international general public.

Operations

In America, this mission will be supervised under NASA and led by the Jet Propulsion Laboratory (JPL).

JPL will provide project management, systems engineering, mission assurance, payload, SEP, navigation,

mission operations and data management. At JPL, Robert T. Mitchell will be proposed to be the Titan Boat

program senior supervisor. Dr Linda J. Spilker will be the Titan Boat mission scientist and Dr Amanda R

Hendrix will be the mission deputy scientist. At NASA, Bill Knopf will be Titan Boat program management

director and Curt Niebur will be Titan Boat program chief scientist.

Prime Contractor

Lockheed Martin is the prime contractor in America who will construct the Titan Boat and Titan Boat

propulsion units and the electricity generators. In Europe, the Titan Boat will be supervised by the

European Space Technology and Research Centre. Alcatel will be responsible for assembling the Titan

Boat with equipment furnished by many European contractors. At ESA, Dr. Jean-Pierre Lebreton will be the

Titan Boat chief manager and chief project scientist.

Other mission stakeholders Table 2: Other mission stakeholders

Stakeholders Function

UCLA (University of California, Los Angeles) Science lead, science operations, data products,

archiving, and analysis

KSC (Kennedy Space Centre) Responsible for launch operations

DSN (Deep Space Network) Responsible for data return from spacecraft

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5. Timeline

The mission timeline is divided into the major mission components (Osborne 2015). Phase A and B take

approximately one year while Phase C/D will take around three years (the minimum time for space

mission1) for the completion of detailed design and manufacturing. The Phase E operations schedule is

based on the Cassini-Huygens Mission (Munsell, n.d.) and comprises almost seven years of interplanetary

travel to Saturn, followed by three months of orbit preparation around Titan. Once the Titan Boat probe

detaches from the orbiter it will spend roughly twenty days from release to travelling toward Titan before

entering its atmosphere. Landing will take up to three hours after which mission operations will be

conducted for 100 days. After the 100 days the Titan boat is parked at a suitable area on the lake and the

mission will cease.

Figure 5: Mission timeline diagram

Table 3: Mission timeline description

1 According to The Lunar and Planetary Institute (2012), 2016 is the most efficient launch window with last chance for a low cost mission in 2023. Due to this

programmatic constraint Phase C/D was minimised to 3 years since phases A,B are as short as they can be and phase E is beyond control.

Phase C/D

Phase E

2027 2028

Phase A

Phase B

2021 2022 2023 2024 2025 20262015 2016 2017 2018 2019 2020

Phase F

Start date Phase Function

2015 April A Feasibility phase: define system concept and assess various functional concepts

2015 October B Preliminary definition: define system and sub-system designs in detail to progress to

phase C

2018 April C/D Complete designs and analysis, prepare drawings and procedures, complete

development and qualification testing, manufacture of hardware and acceptance testing

2020 October E Launch

2021 April E First Venus flyby

2021 June E Second Venus flyby

2021 August E First Earth flyby

2024 December E Jupiter fly by

2027 June E Arrive Saturn

2027 October E First Titan orbit

2027 December E Titan Boat probe release

2028 January E Titan Boat probe enters Titan atmosphere

2028 April F End of mission

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6. System Concept

The Titan Boat mission will land a boat probe on Titan’s lake, Ligeia Mare. It will float on the lake’s surface

and propel itself to navigate and conduct its science mission for 100 days. Longer mission duration allows

for the return of valuable scientific data and for the observation of Titan’s dynamic weather system. More

detailed discussion of the features on the Titan Boat can be found in the following section

Figure 6: Titan boat concept

Figure 7: Titan Boat profile views

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7. Concept of Operations

Landing Operations

When the Titan Boat and its Orbiter arrive at Titan it puts itself into the correct trajectory for Ligeia Mare.

The Titan Boat Probe is ejected from the orbiter using explosive bolts on its trajectory to Ligeia Mare before

the Orbiter boosts itself out of the same trajectory into orbit around Titan.

The Titan Boat will enter Titan's atmosphere at a velocity of 6.1 km per second. The entry phase will last

about 3 minutes, during which the probe's velocity will decrease to about 400 meters per second as it is

converts its kinetic energy into heat as it soars through the Titan atmosphere (Clausen et al. 1999).

Three parachutes will be used during the probe's descent. When the on-board accelerometers detect a

speed of Mach 1.5 near the end of the deceleration phase, a 2­meter­diameter pilot parachute will deploy,

pulling off the aft cover. This will follow immediately by deployment of the 8.3­meter main parachutes. The

parachutes are made of Kevlar and nylon fabric (Clausen et al. 1999). About 30 seconds after deployment

of the main chute, the Titan Boat will slow from Mach 1.5 to 0.6.

Following this, the front heat shield will then be released and the Titan boat will descend slowly for 15

minutes. The main parachute will then separate. Another smaller 3­ meter drogue parachute is deployed

until it hits the surface of Ligeia Mare with an impact velocity of about 7 meters per second (ESA 2015 and

Clausen et al. 1999).

Figure 8: Landing operations for the Huygens probe which the Titan Boat will largely mirror

Mission operations

After landing on Ligeia Mare, the Titan Boat will start its 100 days science mission. This mission has two

goals, exploring for evidence of whether life could exist on Titan and the topography of Titan Sea as

described in section 3.2. In order to fulfil the first goal, the Titan Boat will need to survey various areas

around Ligeia Mare. There are three objectives during the lake survey part. Firstly, Titan Boat needs to

cruise around Ligeia Mare to determine the chemical composition, through deploying spectrometers.

Secondly, Titan Boat should using thermal scanner to determine the temperature change and distribution of

10

Ligeia Mare and the Titan atmosphere. After that, Titan Boat will deploy fluid density instruments to

research the density of Ligeia Mare.

For the atmospheric science objectives, Titan Boat will use its environmental instruments pack to determine

the weather system and near surface radiation of Ligeia Mare. The Titan Boat will deploy its gas collector

and dissociator to dissolve and separate gas samples for the spectrometer instruments pack to analyse.

To investigate the lake topology, the Titan Boat will use its underneath instruments package to measure its

depth and composition. The acoustic scanner will uncover the depth and topography of Ligeia Mare. Its

camera system will create the panoramic images of Ligeia Mare’s surface during its science operations.

Figure 9: Map of Ligeia Mare on which science operations will be conducted

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8. Requirements

The system functional requirements are captured in Figure 10. These function actions must be performed

in order to achieve mission success.

Figure 10: Function tree for the Titan Boat Project

Table 5 expresses the information in the above function tree into table form. It is used to complement the

function tree and represent the relationship between higher and lower functions.

Operational Requirements, Limits and Constraints

In addition to the functional requirements, every space mission is also subject to operational requirements

and limits and constraints. Operational constraints describe how the system is used including interaction

12

with system operators and users while constraints and limits are those imposed beyond the control of the

mission designers such as budgets, schedule and implementation techniques, operating environments etc.

Table 4: Operational Requirements and Limits/Constraints

O1.Mission duration: 100 days on the surface of Ligeia Mare

O2. Reliability

a. Design robustness: High priority given to robustness to the

detriment of mass, power and data returned. Critical functions

shall have triple redundancy including probe-wake up function and

measurement of deceleration profile

b. Failure tolerance

c. Redundancy (duplicate / backup / temporal)

i. Electrical sub-system shall have two duplicate parallel units

ii. Command and Data Handling System shall have two

duplicate parallel units, fully physically separated. Shall

have a flexible system in which its software can be updated

remotely. Mission Timer Unit to be triple duplicate

redundant. Central Acceleration Sensor Unit to be

supported by backup system

iii. Communications sub-system shall have two duplicate

redundant system

iv. Pyrotechnic devices shall have a duplicate parallel units

O3. Data distribution: From JPL operations team to broader NASA/ESA

O4. Science mission: Payloads need to be mounted to maximise achievability

of science mission

L1. Programmatic constraints: 2016 launch window more most efficient

trajectory (7.5 year cruise) with a last chance for low cost mission in 2023-

24 (Lunar and Planetary Institute 2012). October 2020 is selected for the

mission launch.

L2. Environmental

a. Launch

I. Mechanical: up to 3g acceleration during launch

b. Space

I. Radiation: Ensure electronics hardened to withstand

particle radiation from the sun and from outside the solar

system (NASA 2015)

II. Thermal: Survive temperatures as cold as 2.7 kelvin (BBC

2013)

III. Communications with Earth from Titan will have a one hour

Operational

Requirements

Limits and

Constraints

13

lag (Lebreton et al. n.d.)

c. Titan

I. Mechanical: Up to 12g of deceleration during re-entry.

Parachutes must operate in supersonic conditions of Mach

1.5. Up to 5m/s (18km/h) splash down speed (Lebreton et

al. nd).

II. Thermal conditions during entry and descent: Survive both

atmospheric entry heat and descent convective cooling

conditions. Actual temperatures unknown since Huygens

heat shields did not have sensors but it was designed to

survive max heat flux of 1400kW/m2 for the front shield and

30-120kW/m2 for the rear (Bouilly 2005)

III. Surface conditions: Pressure 1.5bar, methane and

Nitrogen environment. Surface temperature of minus 180

degrees Celsius. Thick atmosphere rules out solar power

systems (Lebraton et al. nd). Methane is explosive.

IV. Ligeia Mare: located at 78°N, 250°W, expect hydrocarbon

precipitation near poles (Stofan et al. 2010), possibly has

methane ice on the lake surface, waves about 2cm high,

and wind speed of 0.75m/s (BBC 2014)

L3. Interfaces: subsystems and payload mass shall be evenly distributed

around the centre axis of the Titan probe. Furthermore, the heaviest

components should be mounted as low as possible to improve stability on

Ligeia Mare

L4. Launch vehicle payload limits: 4850 kg (orbiter plus Titan Boat) due to

SpaceX Falcon 9 launch vehicle limits to Transfer Orbit.

L5. Cost: $3bn (replicate Cassini/Huygens cost)

Functional Block Diagrams

A functional block diagram emphasise the interfaces and interrelationships between functional entities

(inflows and outflows). It is based on the system requirements already defined in Figure 10. The following

figures describe the key functional interrelationships of the Titan boat mission: landing on Ligeia Mare

(Figure 11) and conducting the science mission operations (Figure 12). Landing operations have three

concurrent functions identifying and tracking Ligeia Mare’s location, maintaining the correct trajectory and

surviving the conditions during atmospheric entry. These are all important functions while entering Titan’s

atmosphere. Science operations are much more sequential. Monitoring the health of the Titan Boat via

telemetry, however, is a parallel function on top of the science operations.

14

Figure 11: Function block diagram for landing on Titan

Figure 12: Function block diagram for mission operations

Functional Matrices

Functional matrices are used to describe any interrelationships within functional requirements (Osborne

2015), hardware systems or any other area of concern during system design. Table 5 maps lower level

15

functional requirements with higher level requirement while Figure 13 maps requirements to hardware

systems for the individual Titan Boat teams to design their respective subsystems.

Table 5: Function Matrix for the Titan Boat Project mapping level 2 functions

Figure 13: Functional matrix: Requirements vs. subsystem as used for subsystem design

F1.1. F2.1. F.2.2 F2.3 F3.1. F3.2. F3.3. F3.4. F3.5 F3.6. F3.7. F.4.1. F4.2. F4.3. F4.4. F4.5. F4.6. F4.7. F5.1. F6.1.

F1.0 x

F2.0 x x x

F3.0 x x x x x x x

F4.0 x x x x x x x

F5.0 x

F6.0 x

Level 2 FunctionsLevel 1 Functions

ADCS

Pow

er

CDHS

Com

ms

Pro

pulsi

on

Therm

al

Boat

struc

ture

Pay

load

Therm

al

Struct

ure

Par

achute

Gro

und A

rchite

ctur

e

Jason Felix Jason Hassan Meiyappen Meiyappen Jason Felix Meiyappen Meiyappen Meiyappen

F1.  Leave Earth

a.        Launch satellite into space

F2.  Get to Titan

2.1.        Enter into the correct trajectory for Titan

2.2.        Thrust to Titan

2.3.        Orbit Titan

F3.  Land on Ligeia Mare

3.1.        Detach from the Orbiter X

3.2.     Identify Ligeia Mare's location X

3.3.     Maintain and enter into the correct trajectory to land on Ligeia Mare X X X

3.4.        Survive atmospheric entry X X X X X X

3.5.        Splash down on Ligeia Mare X

3.6.          Float on Ligeia Mare X

3.7.        Establish and maintain communications with Earth X

F4.  Perform Mission Operations

4.1.     Identify current location on Leiga Mare X X X

4.2        Transmit location to Earth X

4.3. Receive new target location and science plan X X

4.4. Navigate to new location on Leiga Mare X X X

4.5. Activate and operate payload systems according to science plan X

4.6. Store science mission data X

4.7. Monitor health of Titan Boat X

F5.  Transmit Results X

F6.  End of Mission X X X

O1. Mission duration: 100 days X X X X X X X X

O2. Reliability

a.        Design robustness: High priority given to robustness to the detriment of mass,

power and data returned

b.        Fault tolerance X X X

c.        Redundancy (duplicate / backup / temporal)

                                               i.     Electrical sub-system shall have two duplicate parallel units X

                                              ii.     Command and Data Handling System shall have two duplicate parallel

units, fully physically separated. Shall have a flexible system in which its software can be

updated remotely

X

                                             iii.     Communications sub-system shall have two duplicate redundant

systemsX

                                            iv.     Pyrotechnic devices shall have a duplicate parallel units X X

O3. Data distribution: From JPL operations team to broader NASA/ESA X X

O4. Science mission: Payloads need to be mounted to maximise achievability of science

objectivesX

L1.  Programmatic constraints: 2016 launch window more most efficient trajectory (7.5 year

cruise) with a last chance for low cost mission in 2023-24 (Lunar and Planetary Institute 2012)

L2.  Environmental X X X X X X X X

a.        Launch

                                          I.         Mechanical: up to 3g acceleration during launch X X X X X X X X X X X

b.        Space

                                          I.         Radiation: Ensure electronics hardened to withstand particle radiation

from the sun and from outside the solar system (NASA 2015)X X X X X X X X X X X

                                         II.         Thermal: Survive temperatures as cold as 2.7 kelvin (BBC 2013) X X

                                       III.         Communications with Earth will have a 1 hour lag X X

c.        Titan

                                          I.         Mechanical: Up to 12g of deceleration during re-entry. Parachutes must

operate in supersonic conditions of Mach 1.5. Up to 5m/s (18km/h) splash down speed

(Lebraton et al. nd).

X X X X X X X X X X X

                                         II.         Thermal conditions during entry and descent: Survive both atmospheric

entry heat and descent convective cooling conditionsX X

                                       III.         Surface conditions: Pressure 1.5bar, methane and Nitrogen

environment. Surface temperature of minus 180 degrees Celsius. Thick atmosphere rules

out solar power systems (Lebraton et al. nd). Methane is explosive.

X X X X

                                       IV.         Ligeia Mare: located at 78°N, 250°W, expect hydrocarbon precipitation

near poles (Stofan et al. 2010), possibly has methane ice on the lake surface, waves

about 2cm high, and wind speed of 0.75m/s (BBC 2014)

X X X X

L3.  Interfaces X X X X X X X X X X X

L4.  Launch vehicle payload limits

L5.  Cost: $3bn (replicate Cassini/Huygens cost)

Functional Requirements

Operational Requirements

Limits and Constraints

Titan Boat

Atmospheric Entry System

Ground Architecture

16

9. System Architecture

9.1. Launch Vehicle

The launch vehicle will be SpaceX’s Falcon 9. This vehicle was selected because it is currently delivering

commercial spacecraft services and represents another opportunity to grow the private space sector. It can

launch a 4.8 tonne geosynchronous transfer orbit and costs USD61 million degrees (SpaceX 2015). A

geosynchronous transfer will be useful for maximising apogee for the orbiter craft (on which the Titan boat

will be piggy-backing) to break out of earth orbit.

9.2. Orbits and Trajectories

The Titan Boat mission will use the same orbit and trajectory design as the Cassini/Huygens mission which

successful delivered the spacecraft onto Titan’s surface. The flight time to Saturn is around seven years

and requires gravity assists from Venus, Earth and Jupiter (Labreton et al. 2002) as described in Figure 14.

Unlike the Cassini/Huygens mission, however, the Titan Boat mission will not include a targeted flyby of the

moon Phoebe.

Figure 14: Trajectory to Saturn (Labreton et al. 2002. Edited to remove dates)

Upon reaching Saturn orbiter/probe will enter a highly eccentric capture orbit as described in Figure 15. The

first two orbits are used to prepare for the required geometry that needs to be achieved for the Huygens

mission on the third orbit where the orbiter and the Titan Boat probe are targeted for a collision path to

Titan. 22 days before a collision, the Titan Boat probe separates. The orbiter will then manoeuvre to avoid

Titan and place itself in orbit around Titan. Like Huygens, the Titan Boat Probe will coast for 22 days with

no possibility of changing the attitude parameters acquired at separation (Clausen, KC et al. 1999). Given

17

this operational constraint it is critical that the orbiter/Titan Boat probe is placed in the correct trajectory to

Ligeia Mare.

Figure 15: Orbit around Titan (Labreton et al. 2002. Edited to remove dates)

9.3. Payloads

The payload systems are the critical for performing the science mission and completing the objectives

described in section 3. The payload drives the design of the subsystems since they support the operation of

the payloads. The payload used on the Titan Boat largely comes from the Mars Curiosity Rover due to it

being the relatively new technology with solid space heritage. A summary of the payload systems used in

on the Titan Boat is shown in Table 6. More detailed descriptions of the payload systems is discussed in

the following sections.

Table 6: Summary of payloads used on the Titan Boat

Main Cameras (MAC)

The Main Camera, or MAC for short, can take colour pictures of Titan’s surface. These pictures can be

used to produce a set of panoramas Titan by stitching them together. Similar to the cameras on the Mars

Exploration Rovers Curiosity, the MAC is comprises two coaxial camera systems mounted on the top of the

Titan Boat. The MAC is designed to study the Titan landscape, observe frost and weather conditions and

also to support the cruise and sampling actions of the Titan Boat (Mast Cameras n.d.).

Payload Function Power (W)

Camera systems Photography and navigation 13 based on Mars curiosity

Spectrometers Examine elemental composition 42 based on Mars curiosity

Gas collector and dissociator Breaks gases down for the

spectrometers 28 based on Huygens probe

Environmental sensors Measure weather conditions 10 based on Mars Curiosity

Underneath package Measure lake bottom topography 10 based on Huygens probe

Total Payload Power 100

18

Figure 16: Example of a twin camera system on the Mars Curiosity Rover

Spectrometers

Spectrometers can examine the composition elements of compounds. These elements that this mission

mainly focus on, concluding carbon, hydrogen, and oxygen, methane, since they are associated with life

that existing on earth. Because compounds are the key factors to form life on existing knowledge, their

kinds and amounts will contribute to a vital section of evidence for assessing life form of Titan and the

evolution procedure of life on it. There are three instruments in spectrometers package, including a mass

spectrometer, gas chromatograph, and rotatable laser spectrometer. Sample analysis on Titan will also

search and measure the existence of other assumption light elements, such as phosphorus, sulphur and

silicon.

The mass spectrometer is designed to identify components by mass for analysis and measurement.

The gas chromatograph is designed to identify gases components by heating samples, these gases source

form vaporized samples in different temperatures. The laser spectrometer is designed to identify a variety

of isotopes of carbon, hydrogen and oxygen in near-surface atmosphere, including methane, water,

ammonium and other likely element (Mahaffy 2013).

Figure 17: Sample Analysis payload for the Mars Curiosity mission

Gas collector and dissociator (GCD)

The gas collector and dissociator is used to collect and heat sample from liquid and atmosphere and then

transfer them to the Gas Chromatograph (Lebreton & Matson 2002, p.59 ).

19

Figure 18: Gas collector payload used on the Huygens probe

Environmental Sensors (ES)

The environmental sensors pack is designed to observer the weather, near-surface and atmosphere

temperature and also pressure (Rover Environmental Monitoring Station n.d.). Through these

measurements, Titan Boat could collect detail data about everyday pressure and temperature, humidity,

space particle radiation at the Titan surface, atmosphere wind speed and direction (Armiens et.al 2012,

p.583).

Figure 19: How environmental sensors can be mounted on the mast

Underneath Package (UP)

The underneath pack consists of five separate working units, including lean sensor, thermal scanner,

acoustic scanner, permittivity detector and density detector. They are used to research the topography,

temperature distribution and range and also possible floating materials in the Titan sea (Lebreton & Matson

2002, p.59 ). The lean sensor consists of a gravity sensor, Doppler lidar and a set of gyroscopes. It could

measure the sea current and sea wave variation trend. The thermal scanner unit consist of infrared heat

radiation imageries which can detect sea temperature at different layer and areas. The acoustic scanner

unit consist of two ultrasonic sound detector and one laser distance detector. They are used to detect

seabed structure and possible suspended materials. Also, these detection data could be used to depict

topography on Titan. The permittivity detector unit contains two electrodes. Through detect material fluid

between two electrodes, liquid permittivity can be found. In addition, this unit could be used to detect any

possible polar molecules in Titan Ocean. The density detect unit consists a set of Archimedes buoyancy

sensors around Titan Boat. It also contains four gasbags which could to help instrument floating on the

surface of Titan Sea through filling gas in Titan surface.

Payload layout on the structure

Figure 19 displays the layout of the payloads on the Titan Boat structure. It is only conceptual for the

moment and would require more detailed evaluations for further optimise the layout. The Titan Boat needs

to be stable on the surface of Ligeia Mare. This means that the heaviest items need to be monuted lower

20

than lighter items to lower the overall centre of mass. Another balance requirement is that the payload and

subsystems need to be distributed evenly around the centreline of the Titan Boat’s circular structure.

Figure 20: Mounting locations for payload systems on the Titan Boat (based on Huygens configuration)

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9.4. Subsystems

Design Summary

Figure 21 demonstrates a very high level block diagram of the major subsystems on the Titan Boat Mission

and their interrelationships based on power and data distribution. It highlights the major components of the

individual subsystems.

Figure 21: Subsystem block diagram

Figure 22 documents the design interfaces between subsystems and the payloads via an N squared

diagram. During the conceptual design phase where the Titan Boat project team designed the subsystems

according to the functional matrix diagram in Figure 13 , it was found that changing one subsystem design

would have knock-on effects on other subsystems. The N squared diagram captures these inter-

relationships so that the team is aware of the impact of their design updates.

Figure 22: Titan Boat Subsystem interface N2 diagram

9.4.1. Command and Data Handling Subsystem (CDHS)

The CDHS for the Titan Boat will have the same functional requirements as Huygens, that is, autonomous

control of the probe after separation from the orbiter. The CDHS will execute ‘Pre-programmed sequence

triggers parachute deployment and the heat-shield ejection’ (Clausen et al. 1999). Given the success of the

Huygens architecture and design heritage, it will be implemented for this mission.

As described by Couzin (2005) and Clausen et al. (1999), the major components of the CDHS are:

Payload Data handling rate Power required by payloads Minimum operating temperature for payload Data transfer rate required Payload mounting locations

Sends commands to payload CDHSSend commands to power system

Power required by CDHS

Minimum operating temperature for CDHS

Sends commands to thermal systemSend commands to propulsion system

Send commands to communications

systemMounting location

Power Minimum operating temperatures for battery Mounting location

ADCS Mounting location

Power required by thermal system Thermal Mounting location

Power required by propulsion

Battery design Propulsion Mounting location

Receive software upgradesPower required by communications

system

Minimum operating temperature for communications

system Communication Mounting location

Available volume Available volume Available volume Available volume Available volume Available volume Available volume Structure

22

2 x Command and data handling unit (CDMU) system. This system controls the probe operations.

Each will operate its own software independently in parallel and are fully physically separated. It

was designed such that no failure of one chain would impact the other chain. This allowed major

simplification and increased robustness. It is a fully flexible system that had the capability to have

large portions of its software updated.

3 x Mission Timer Units (MTUs). This is used to activate probe after end of coast phase after

separating from the orbiter.

2 x mechanical g-switches (MTU backup). These ensure the Titan Boat will wake up in the event of

atmospheric entry without time-out signal from any timer boards. They are purely mechanical

devices that work when deceleration reaches 5.5-6.5 g (Clausen et al. 1999).

3 x Central acceleration sensor unit (CASU): These measure the axial deceleration to calculate the

time at which the parachute will deploy. Three devices mitigate failure and reduce uncertainties from

single measurements.

2 x Radio Altimeter Units (RAU). These provide the altitude data at heights lower than 25km

(Clausen et al. 1999) for each of the two CDMUs.

The CDHS will run software to meet three operational requirements including: mission management,

telemetry management, and telecommand management (Clausen et al. 1999). For mission management,

the software will take sensor readings from the accelerometers, altimeters, and mission timer units to

determine when to deploy the parachutes. It will also forward commands to the subsystems and payload

equipment according when it receives instructions from the operations team on Earth. It will also continually

assess the state and health of the Titan boat from the sensor readings. For telemetry management, the

software will collect and store data and transmit to the communications system to deliver back to Earth. For

Telecommand management, the software allows for receiving instructions when the Titan Boat is still in

transit to Titan for any software updates and to forward commands to the payload and subsystem if

required.

9.4.2. Structural Subsystem

The structure provides mounting support for the payload and subsystems and protects these systems from

the harsh environment of Titan’s lakes. The mounting platforms need to be strong yet lightweight. Using

Huygen’s materials heritage, the Titan Boat mounting structure will also be constructed from aluminium

honeycomb sandwich platforms to meet these requirements. (Clausen et al.1999). There will be two

mounting platforms. The top level is for the communications subsystems while the lower level is for the

payload and all other subsystems.

The overall casing will comprise and after cone and fore-dome made from aluminium shells. Aluminium can

withstand liquid methane and has a successful history for storing liquefied natural gas storage on earth:

„[Aluminium‟s excellent mechanical properties at cryogenic and ambient temperatures, combined with

superior corrosion resistance, make it attractive for applications such as LNG tankers or storage tanks‟

(Alcoa, 2015). The aluminium casing is also hardened to withstand any collisions with hydrocarbon ice on

Ligeia Mare whether that be during landing (base impact) or mission operations (side impacts). The casing

is fully sealed except for a vent hole on top to handle depressurisation and pressurisation when it goes

through environments (earth, space, Titan). The after cone and fore dome and mated by a central ring. The

exterior of the structure will have spin vanes to provide spin control during descent.

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Figure 23: The Huygens probe outer structure from which the Titan boat will draw heavilty upon

9.4.3. Attitude Determination and Control Subsystem (ADCS)

Attitude determination in the case of the Titan Boat requires measuring the orientation of the boat on the

surface of Ligeia Mare. This is critical for the science operations since the boat will need to rotate itself and

for health monitoring and telemetry. Attitude determination will come from three redundant gyroscopes and

three redundant accelerometers (3-axis) that serve as backup systems. When combined they will feed

multiple measurements equivalent of aircraft pitch, yaw and roll that will be used to navigate the Titan Boat.

Roll control is achieved through passive structural design. The driving reason for this design decision is that

it is expected that the liquid conditions on Ligeia Mare is calm (2cm waves, wind speed 0.75m/s, BBC

2014). Thus active control systems were not considered since they would add unnecessary complexity and

weight. The fore-dome structure will be designed to passively balance and return to an upright position

(positive stability) on the lake’s surface. This is achieved by ensuring the centre of gravity of the Titan Boat

is kept as low as possible by mounting the heaviest items around the toward the bottom of the Titan Boat

and evenly around it’s centre line (Maritime New Zealand 2011). The fore-dome base will have a wide

surface area and a flat hull design given calm liquid conditions on Ligeia Mare. Flat hull designs are good

for slow speeds and calm condition (Hull types 2015). Liquid methane ingress is prevented by sealing the

case.

Yaw and pitch control is achieved by the propulsion system described in section 9.4.7.

9.4.4. Power Subsystem

Power generation system

The Titan Boat will be powered by 8 multi-mission radioisotope thermoelectric generators (MMRTGs),

fuelled by 40kg of plutonium 238, to support a maximum total power requirement of roughly 1000W. Each

MMRTG weights 45kg. The MMRTG unit has space heritage on the Mars curiosity rover, which itself is

powered by three MMRTG units.

The two main components of a nuclear thermoelectric generator is a heat source containing the radioactive

material and a set of solid-thermo couples that convert heat into electrical energy (Jiang 2014). Plutonium

238 was chosen for its ability to emit high energy radiation. The power system uses non-weapons grade of

radio isotope. The thermo couples convert heat energy into electricity by relying on the „seebeck effect‟,

where differential temperatures creates electric voltage (JPL 2013). To prevent contamination of radioactive

material in the worst case of leaks the plutonium is stored in high strength blocks of graphite surrounded by

a layer of iridium metal.

Estimating total Power Required

To estimate the total power required for the Titan Boat, the AIAA estimation method was used based on the

known payload power requirements (100W). The total power requirements are estimated using the AIAA

„Total spacecraft power estimating relationship‟ formula based on a planetary mission. Then using the

24

margin estimate guide from AIAA recommended power contingencies, a figure of 80% was selected based

on a Class 1 (New Bid) category BP (500-150W) design.

Figure 24: MMRTG unit used on Mars Curiosity Rover

Table 7: Power budget estimate

Justifying the selection MMRTGs

Given the high amount of power required of 1000W and the mission duration of roughly 3 months, only a

nuclear energy source can provide the required amount of power per Figure 25 (Tsafnat 2015).

Furthermore, a nuclear power system is advantageous on Titan since Titan’s atmosphere would be too

thick for any reliable use of solar panels (Clausen et al. 1999). Approximately 40kg of plutonium is required

to power 8 MMRTGs (see Table 8).

Figure 25: Space mission power sources from AERO9500 lecture slides, week 3

Power estimation Method

Total power required (excl.

propulsion)

( ) From AIAA Total spacecraft power estimating relationship for planetary missions

Total power required incl.

margin (excl. propulsion)

From AIAA recommended power contingencies for Class 1, Category BP

Total power required incl.

power and propulsion

880W + 100W = 980W ~ 1000W From adding 2x 50W electric propulsion systems

25

Table 8: Plutonium mass estimate

Power storage

The Titan Boat mission will use the same battery technology from the Huygens probe. The Titan boat will

have 14kg of lithium sulphur dioxide batteries with enough capacity to supply 210W of power. This

technology was selected because the Huygens batteries were specifically designed to handle the extremely

cold temperatures of Titan. Commercial lithium ion batteries degrade in performance as temperatures

decrease below zero. According to Buchmann (2015), these batteries stop functioning at -20 degrees

Celsius. The Huygens batteries, however, had a minimum operating temperature of -40 degrees Celsius.

These batteries will still need a external heating from the Thermal subsystem.

The batteries uses lithium metal foil as the anode and sulphur dioxide as the cathode reactant or

depolarizer. The cathode itself is a Teflon-bonded porous carbon matrix pressed into a metallic screen.

These batteries are highly energy density and highly voltage, and are potentially long shelf life (Halpert and

Anderson 1982).

The batteries will have sufficient capacity to service 210W of power demand. 100W is dedicated to the

propulsion system while the remainder serves as a backup for critical subsystems like communications. A

1kg lithium sulphur dioxide battery supplies 15W. So to supply 210W, 14kg of batteries is required. In the

event of fire, a graphite-type compound or extinguisher such as Lith-X-type (class D) will extinguish burning

lithium (Halpert and Anderson 1982).

9.4.5. Thermal Subsystem

The purpose of a thermal subsystem is protect the payload and subsystems in the spacecraft by keeping

their temperatures within limits. This is critical because electronics can only operate within a temperature

range, beyond which they either stop operating or worse, sustain physical damage. There are two types of

Thermal Subsystem that will be used on the Titan Boat. Active thermal control, although expensive, allows

for direct control of temperature by use of powered electric heaters and coolers. Both will be employed on

the Titan Boat to maintain minimum temperatures during science operations and for emergency cool down

during landing operations. The other is passive thermal control, which is less complex and relies on heat

pipes, coatings and insulation for example. This is employed on the Titan Boat as well as the heat shields

during atmospheric entry into Titan. The thermal system will draw extensively from the heritage from the

Huygens space probe.

Front heat shield

The purpose of the front shield is to decelerate the probe in the upper atmosphere of titan to reduce entry

speed from 6km/s to 400m/s (Mach 1.5) at altitude of 150-180km as well as protect the Titan Boat from

extreme heat (Clausen et al. 1999). During this period the Titan Boat will create a plasmas shock wave of

around 12,000 degrees Celsius (Piazza, n.d.). To survive these extreme conditions, the shield is made

Carbon fibre reinforced plastic honeycomb shell and ablative AQ60 heat tiles on its exterior to resist a heat

flux of 1.4MW per metre squared (Clausen et al. 1999).

Power generated (W) Estimated mass of plutonium required

870 4.8kg (Capotini 2008) Based on the Mars Curiosity rover which uses 1 x MMRTG offering 120W

1160 40.0kg

Assuming the above relationship is linear and estimating number of MMRTGs needed to meet 1000W

requirement

26

Figure 26: The Huygens front shield design which the Titan Boat will reuse

Back shield cover

The back cover provides insulation protect the Titan Boat during cruise and coast phase of landing

operations on Titan. During launch a hole assure depressurization and repressurisation during entry. It is

made of aluminium shell, which is stiffened just like on the Huygens probe (Causen et al. 1999). It has

access door for integration and emergency cool down of the probe and a breakout patch for firing the first

parachute.

Titan Boat thermal system

To survive the cold conditions of -180 degree Celsius on Ligeia Mare, the Titan boat will combine active

and passive thermal technologies. It will have multi-layer insulation and heat pipes that conduct heat from

the thermoelectric power generator to distribute heat to the temperature-sensitive critical componentry. It

will also have electric coolers and heaters around vital subsystem components which activate when

temperatures are measured to be too cold or hot.

9.4.6. Communications Subsystem

The Titan Boat mission will use the orbiter relay communication system like the Cassini-Huygens mission. It

comprises two redundant parallel communications systems. It has two s-band channels for each of the

CMDU’s consisting of a dedicated transmitter unit, 10W RF solid state power amplifiers (Couzin et al.

2005). The communications system also had two low noise amplifiers (Clausen et al. 2002). The orbiter will

have two high gain antennas pointed in the direction of the Titan Boat probe to receive its signals and relay

to Earth (Couzin et al. 2005).

A huge constraint on the communications systems on Huygens was its limited power. According to Couzin

et al. (2005), Huygen’s uplink rate could be boosted up to 2.6 times if it reduced its mission life time from 3

hours to 30minutes. Since the Titan Boat has significantly more power available to it, the Titan Boat mission

will be able to send unprecedented amount of data back to Earth. A problem encountered during the

Huygens mission was Doppler shift during entry into Titan’s atmosphere. This will be corrected from the

body of knowledge about this problem for the Titan Boat mission.

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9.4.7. Propulsion Subsystem

The Titan Boat mission duration is 100 days. In order to maximise the science value during these 100 days

it was required for the Titan Boat to be able to navigate on the surface of the Ligeia mare. The propulsion

system on the Titan Boat comprises 2 x 50W electric motors.

Figure 27: Example of electric boat propulsion unit from Volt master

Electric propulsion motor will be suitable choice for this operation because of its efficiency and light

compact structure. They can provide powerful torque and full power instantly with no need of warming up. It

has a smooth operation on very low speed with high level of reliability (Whisper Power, n.d.). The general

configuration of electric propulsion motor is generator, electric motor and static converter. To be clear

usually we use a diesel generator in boats, which is going to be replaced by MMTRG here that is, going to

generate electrical energy and to store the energy we need a Li/S02 cell, which is 14 in number each, can

store 15W(Explained briefly in batteries). Combined propulsion is in which the motor is supplied by a

separate source power being added to the direct propulsion. Electric motor gives power to the propelling

shaft.

The simple calculation based on resource and assumption since this concept has not yet be tested before.

Generally, a 1kW power motor can make a boat to travel 10km/hr, which is 3m/s. (Eco boats, n.d.)

Liquid Methane is only 45% as dense as water. It means it is less viscous than water. (PHYS.ORG,

April 2014), which can be approximated as 50%. If we take the above condition and assume that

density is proportional to boat speed, then Titan Boat can travel 20km/hr in liquid methane.

It is a design decision to significantly reduce the speed of the boat since speed is not a mission

critical. The maximum speed will be fixed to 1km/hr, which is 0.3m/s.

Assuming the the 1kW to 10km/hr relationship is linear, then for 1km/hr the power of the motor is

1kW/20 = 50W.

Two propulsion units are desired in order to have reliability and redundancy purpose. Since it

doesn’t put too much load on single propeller.

The two propulsion units will draw 100W from the batteries.

In case of worst scenario, a single propulsion unit is designed to be capable to propel and steer the

boat.

9.5. Ground Architecture

The ground system architecture describes the interface between the end users and the space segment. It

comprises overall operations, science operations and the communications uplink and downlink process.

The most iconic hardware system in a ground station is the radio antenna, which is capable of receiving

and transmitting electromagnetic waves to and from the Titan Boat and its orbiter. The ground architecture

will be extremely similar to that used on NASA’s Dawn mission per the below figure (Polanskey 2011).

28

Figure 28: Ground architecture for the Dawn Mission on which the Titan Boat Mission is based

The Deep Space Network (DSN)

The DSN includes three ground stations which are located in USA, Australia and Spain. The DSN is used

for interplanetary missions so it is ideal for the Titan Boat mission. Each ground station consists various

34m and one 70m diameter antenna platforms. Separate platform can be collect together to perform a

wider downlink area. Every platform has a control centre and a group of technical staff to link with deep

space network control centre and each space mission control centre to offer tracking assistances. In each

ground station, 34 m antenna platforms are mainly used for Dawn mission. And the 70m antenna platform

is only used transfer data which requires high reliability. Also, 70m antenna platform is an optimized choice

when receiving downlink data from spacecraft low gain antenna.

Dawn science operations are managed by the Science Operations Support Team at University of

California, Los Angeles (UCLA). They design the science operations processes and procedures, and

troubleshoots instrument issues.

During the uplink process, the science team defines science objectives which are passed to the

instruments team. The instruments team defines activities that meets these science objectives and passes

this plan to DSC and checks that the plan can be implemented within available spacecraft resources

(memory buffers, downlink capability etc). Mission planners review the plan to verify that sufficient margin

remains for spacecraft engineering activities such as orbit maintenance, optical navigation, and downlink.

During the downlink process, telemetry is captured by the Deep Space Network (DSN). The data is then

transferred to JPL. After a process of decompressing, decoding and formatting the data sets and is stored

in the Science Database (SDb). The science team can also create details Geographic Information System

products like maps and mosaics. The data is then delivered and archived for distribution to the broader

scientific community and the public.

Support from European Space Agency

29

In addition to NASA’s DSN, the European Space Agency (ESA) tracking station, including Kourou, South

America and Perth station, are also used since the Titan Boat mission is a cooperative mission between

these two organisations. The ESA tracking station supplements are followed:

X-band communication.

Navigation symbol recording.

Integration of management with deep space network.

Data line connection with Jet Propulsion Laboratory

Voice connection with Jet Propulsion Laboratory

30

10. Propulsion Subsystem Analysis

10.1 Statement of Works

The Statement of works is a formal document for the prime contractor to develop and manufacture the

propulsion units for the Titan Boat. It includes the requirements, terms and conditions of their contractual

obligations.

10.1.1 Purpose

The purpose of the propulsion system is to propel the Titan Boat on Ligeia Mare with high reliability based

on periodic instructions sent by the Science operations teams.

10.1.2. Scope of work

The scope of work involves the design and manufacture of two electric propulsion units according to the

requirements described in section 10.1.7. followed by user, operational and integration testing.

10.1.3. Main entities

The user of this system is NASA and the prime contractor is Lockheed Martin as identified in section 4.

10.1.4. Location of work

The location of work includes software coding by NASA at JPL laboratory, manufacturing by Lockheed

Martin and its sub-contractors around the word and integration and testing at JPL, ESA and the Kennedy

Space Centre.

10.1.5 Period of Performance

The total period of performance is from October 2020 to April 2028. The propulsion system must survive

launch and a minimum of seven years of interplanetary travel to Titan. It must then survive atmospheric

entry and splash down on Ligeia Mare. Following this it must reliably operate for at least 100 days, starting

from the Titan Boat landing on Ligeia Mare.

10.1.6. Deliverables Schedule:

Phase Time line Entities/ principals Procedure

A 2015 to 2016 NASA, ESA NASA to provide operational concept and

functional requirements

B 2016 to 2018 NASA, JPL, ESA NASA, JPL to supply Lockheed Martin with

preliminary detailed specifications

31

C 2018 April to July Lockheed Martin Design and analysis conducted by Lockheed

Martin

C 2018 August Lockheed Martin, NASA,

JPL, ESA Critical Design Review

D 2018 September to March Lockheed Martin Manufacture

D 2019 April to May Lockheed Martin Assembly

D 2019 June to 2020

January

Lockheed Martin, NASA,

JPL, ESA Test

10.1.7. Applicable Requirements

Figure 29: Requirements for Propulsion subsystem

10.1.8. Acceptance Criteria

In order for the final product to be accepted a number of tests must be passed. At a minimum, NASA and

its partners will examine if this system achieves all its functional requirements.

User acceptance testing

The science and mission operations team at JPL will conduct tests to validate that the propulsion system

behaves as they instruct and that any bugs or failures on the propulsion system to obey the commands of

the operations teams are rectified before the propulsion system is accepted.

Operational acceptance testing

32

Lockheed martin must supply operational test results to NASA to demonstrate that it meets the functional

requirements. To replicate the conditions experience by the propulsion system during its period of

performance the following tests must be conducted by Lockheed Martin

Launch simulation

Deep space travel simulation

Heat and cold tolerance testing

Splash down simulation

Titan environment simulation

Physical damage testing

Software testing

Fault testing and resolution

NASA and its partners will then conduct its own independent verification tests by. Should this test be

passed then the operational acceptance testing phase is complete.

10.1.12. Type of Contract/Payment Schedule

NASA will provide one out of three part of the total value of the contract to Lockheed Martin as start-up

capital. After successful validation and verification from NASA, NASA will transfer the remaining money to

Lockheed Martin. Any schedule slippages will result in penalties to Lockheed Martin. A performance

incentive bonus will be paid to Lockheed Martin if the Propulsion system passes all testing and is delivered

earlier than the contracted deadline. This amount will be paid as a fixed sum per week for the number of

weeks the propulsion system is delivered ahead of schedule.

10.2 Fault Tree Analysis

Fault tree analysis (FTA) allows for the understanding of the logic leading to a fault event and the

prioritisation of those causes. It is a proactive tool used to prevent fault events occurring as well as a design

evaluation tool (NASA 2002). It must be emphasised that it is not an exhaustive description of failures.

Instead an FTA should describe faults that are realistically expected. The first step in FTA is to describe the

objective for creating one. In this case the objective is to evaluate and further enhance the design of the

Titan Boat propulsion sub-system. The next step is to define the boundaries of the FTA. In this case only

the landing and science operations are considered. The manufacturing, assembly and launch operations

for the Titan Boat are excluded. The orbiter is completely excluded.

33

Figure 30: Fault Tree Analysis: operational but inaccurate direction

Figure 31: Fault Tree Analysis: Operational but less thrust than expected

CDHS failure

Operational but inaccurate direction

Propulsion subsystem Hardware damage

Sensor Hardware issue

Software issueCommunications

interruption

Titan weather interference

Titan-Earth access

Titan-orbiter access

Wrong command sent By operations team

Data corruption on CDHS memory

Heat shield failure

Camera hardware Issue

Expected location of Titan boat is incorrect

Onboard location beacon failure

Data correct but incorrect interpretation of the data

Miscalculation by Operations team

CDHS software glitch

Environmental contamination

No Power

Mechanical damage

During descent and landing operations

During science operations

Software glitch

Collision with the environment

Parachutefailure

Cyclic loading beyond design

Thermal stress fatigue beyond design

PROPULSION SYSTEM FAULT TREE

Non-operational Operational but less thrust than expected

Damage to propulsion blades

Insufficientpower

Software issue

Power gen. degradation

Battery storage issue

Electrical distribution issues

CDHS failure

Wrong command sent By operations team

Data corruption on CDHS memory

Software glitch

Operational but inaccurate direction

PROPULSION SYSTEM FAULT TREE

Non-operational Operational but less thrust than expected

34

Figure 32: Fault Tree Analysis: Non-operational

10.3. Preliminary Failure Mode, Effects and Criticality

Analysis (FMECA)

FMECA is used to help the contractor , Lockheed Martin to find the likely failure status, and relative causes

and following outcomes in system design stage. Through failure mode analysis, contractors can identify the

reliable and safety design and modify the risk ones. This is because, each failure mode should be attached

with causes, effects, severity level, probable level, critically level, failure detection methods, short time

solution, long time solution and some critical comments which could give contractor a thorough

understanding of each likely failure modes’ capacity. The FMECA criteria are based on MIL-STD-1629A

(Department of Defence 1980). After FMECA analysis, designers could modify their projects to alleviate

dangerous and increase components or functions reliability. Also, the entire design time will be largely

reduced, due to identify and correct relative problems.

Classification scheme for the severity of effects of each failure mode

4. Catastrophic (Death or system loss)

3. Critical (Severe injury, occupational illness, or system damage)

2. Marginal (Minor injury, occupational illness, or system damage)

1. Negligible (Less than minor injury, occupational illness, or system damage)

Estimate probability of failure mode.

4. Probable (Likely to occur immediately or within a short period of time)

3. Reasonably Probable (Probably will occur in time)

2. Remote (Possible to occur in time)

1. Extremely Remote (Unlikely to occur)

Communications failure

No power Catastrophic failure

Explosion from naked spark in methane environment

Power generation failure

Battery storage failure

Electrical distribution short circuit/leak

Does not receive commands

Receives commands but cannot take action

CDHS failure

Antenna system failure

Connection between antenna and CDHS failure

Operational but inaccurate direction

PROPULSION SYSTEM FAULT TREE

Non-operational Operational but less thrust than expected

35

Table 9: Failure Mode, Effects and Criticality Analysis

Function Failure modes Causes of failure Failure effects Severity Probability Criticality

Failure

detection

methods

Immediate

intervention Long term intervention Comments

1

Communication

1.1

Communication

interruption

Titan weather interruption

Titan atmosphere interruption

Titan orbital interruption

No functional signal

receiving 2 4 8

Detect

through

frequently

signal

comparison

Stop instruction distribute and transfer Titan Boat to auto-drive mode

Deploy different antenna to work in various frequency communication mode

Periodically check communication packet loss probability

Probably

occur but not

deadly

1.2 No

command

received

Antenna system failure

Data transmission failure

No message transfer 2 4 8

Periodically

check the

system

hardware

working

condition

feed back

Check each node of

communication link to

find problem or try to

contact the station

Deploy at least

one

communication

subsystem as

backup

frequently to

occur , not

deadly

indeed

1.3 No valid

action under

command

instruction

Data transmission lose because of hardware fault

Propulsion structure damage

Communication will be

affected or have to

replace the hardware

4 2 8

Periodically

system

conduct

system

self-check

Start backup system

mode (including back

up circus, propulsion

facilities)

Deploy programmable logic controllers to modify system working mode

Deploy high reliable hardware

Fatal and

hard to

correct

2 Cruise

2.1 No thrust

Instruction transmission failure

propulsion blade damage(erosion, structural damage)

power leakage

Titan Boat cannot cruise

on Mare sea 4 1 4

Periodically

system

conduct

system

self-check

Start backup system

mode (including back

up circus, propulsion

Deploy robust data and power transmission channel

Increase hardware strength

Rare to occur

but deadly to

function

2.2 Collision

when shipping

Failure to detect barriers on channel

Data transmission error

Structure damage of Titan Boat

Inner instruments shock

3 1 3

Sensors

detect

unexpected

collision

and shock

Modify survey plan

and shipping lane

Improve reliability of auto drive system

Improve structure strength of Titan Boat

Less chance

to occur

36

2.3 Boat

capsize

Shipping over speed

Bad weather

Instrument unreasonable distribution

BT mission early

termination 4 1 4

Tilt sensor

send failure

mode

signal back

Using compensation

mechanism to keep

balance

Decrease

gravity centre

when design

Unimpressive

but could

bring huge

catastrophe

2.4 Ship with

un wanted

speed

Propulsion blade damage

Liquid Viscosity change

Cannot achieve

preinstall mission 1 1 1

Periodically

location

detect

Calculate the failure

trend and modify

following mission

plan

Increase blade structure strength

Increase navigation self-revise ability

Could be

revise easily

in operation

3 Charging

3.1 Not enough

power supply

Power generation failure

Battery storage failure

Electrical distribution short circuit/leak

Cannot achieve

preinstall mission 2 1 2

Periodically

status

check

Check

power line

Modify

power

allocation

quota

Apply backup

power supply

system

Rare to

happen but

easy to

modify

3.2 No power

supply

Power generation failure

Battery storage failure

Electrical distribution short circuit/leak

BT cannot cruise on

Mare sea 4 1 4

Periodically

status

check

Transfer to backup

power supply line

Deploy robust

power supply

system

Rare to

happen but

deadly to

system

function

4 Navigation

4.1 Ship to un

wanted

direction

Propulsion blade damage

Navigation computer failure

Titan Boat lost in

direction 2 1 2

Periodically

location

detect

Calculate the failure

trend and modify

following mission

plan

Increase blade structure strength

Increase navigation self-revise ability

Could be

revise easily

in operation

37

10.4. Risk Matrix

The risk matrix is used for the assessing the safety of risk. It is the popular method for safety and decisions.

It helps in building the consensus. It is a formal and structured method easy to understand by the

managements. Risk matrix shows the uncertain and consequences of the product or the design and

highlights the damages/consequences with different levels of the uncertainty. It shows the probability of the

product success or the failure and to achieve the acceptable risks through a systemic approach of analysis

design a risk matrix throughout its life cycle (Ho 2010).

Table 10: Risk Matrix for the Titan Boat Propulsion System

Consequence

Negligible (1) Marginal (2) Critical (3) Catastrophic (4)

Pro

bab

ilit

y

Probable (4) 1.1

1.2 2.1

Reasonably probable (3) 1.3

Remote (2)

Extremely remote (1) 2.4 3.1,4.1 2.2 2.3

3.2

Legend

Green: Acceptable risk

Yellow: Acceptable risk

Orange: Moderate risk

Red: Unacceptable risk

Classification scheme for the the severity of effects of each failure mode

4. Catastrophic (Death or system loss)

3. Critical (Severe injury, occupational illness, or system damage)

2. Marginal (Minor injury, occupational illness, or system damage)

1. Negligible (Less than minor injury, occupational illness, or system damage)

Estimate probability of failure mode.

4. Probable (Likely to occur immediately or within a short period of time)

3. Reasonably Probable (Probably will occur in time)

2. Remote (Possible to occur in time)

1. Extremely Remote (Unlikely to occur)

38

11. Conclusion

Since the Cassini-Huygens mission, there have been numerous tantalising mysteries about Titan that wait to be investigated. These can be summarised into two broad questions: ‘how conducive is the Titan environment for life’ and ‘what is the lake topography and environment system’? Previous science missions to Titan suggest that the compounds present in the atmosphere and surface could be supportive for prebiotic conditions. Answering both these questions will further improve humanity’s understanding of life in the solar system and the nature of life itself. There is no better time for another mission to Titan. Indeed, the window for a low cost mission will close beyond 2024 due to orbital inefficiencies. This report details a conceptual design by which such a mission could be conceived. It leverages the enormous technology and operational heritage from previous Titan missions to maximise mission success while simultaneously multiplying the investment return in science value by incorporating the latest space vetted payload technologies. This report demonstrates the conceptual design process for the Titan Boat’s 100 day mission on Ligeia Mare, the second largest lake on Titan. The Titan Boat mission will launch in late 2020 and will arrive at Titan late 2027. To maximise the scientific value during the 100 day mission the Titan Boat will have a novel propulsion system driven by two 50W electric motors to slowly cruise on the surface of Ligeia Mare. This report also develops a statement of works to approach the prime contractor, Lockheed Martin, to develop the propulsion system. A preliminary fault analysis of the propulsion system is also conducted. This methodology was used to perform several design revisions of the propulsion system. This study confirms that the mission is ready to proceed to the next stage.

39

Appendix

A.1. Team member contribution

Report Section Contributors

Executive Summary Jason

Background Hassan, Jason, Felix, Meiyappan

Mission statement Hassan, Jason, Felix, Meiyappan

Stakeholders Felix

Timeline Jason

System concept Hassan, Jason, Felix, Meiyappan

System concept CAD Jason

Concept of operations Hassan, Jason, Felix

Requirements: tree, block diagram and table Jason

System Architecture Meiyappan, Felix, Jason, Hassan

Launch vehicle Hassan, Jason

Orbits Jason

Payloads Felix

Subsystems Felix, Jason

Subsystem block diagram Felix

N2 Diagram Jason, Felix, Jason, Hassan

CDHS Jason

Structure Jason

ADCS Jason

Power Meiyappan, Felix, Jason, Hassan

Thermal Meiyappan, Jason

Communication Hassan, Jason

Propulsion Meiyappan

Ground Architecture Jason, Felix, Meiyappan

Statement of works Felix, Jason

Fault tree diagrams Jason

Risk Matrix Felix, Hassan

FMECA Felix, Hassan

Conclusion Jason

Report compiling, editing, formatting Jason

References Meiyappan, Felix, Jason, Hassan

40

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