“DESIGN OF A FOOD WARMER FOR A MARS TRANSIT MISSION”

PROJECT MID-TERM REPORT

For

Texas Space Grant Consortium

By

Brandon Henry

“Team Spacewalkers”

Team Members Allen Omardeen

Jack Ranken Tim Crispin

Brandon Henry

INDE/MECE 4334 Capstone Design IV

Spring, 08

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Table of Contents

Table of Contents............................................................................................................... iiiTable of Figures ................................................................................................................. iiiTeam Spacewalkers ............................................................................................................ 1Introduction and Background ............................................................................................. 2Statement of Goals.............................................................................................................. 4Progress Description ........................................................................................................... 5Cost Analysis .................................................................................................................... 14Scheduling ........................................................................................................................ 15Conclusion ........................................................................................................................ 16

Table of Figures Figure 1: Team Spacewalkers patch ................................................................................... 2Figure 2: Food pouches used by NASA (picture provided by NASA) .............................. 4Figure 3: Existing food warmer (picture provided by NASA) ........................................... 4Figure 4. Side view of Design 1 ......................................................................................... 6Figure 5. Top View of tray for Design 1 ............................................................................ 7Figure 6. Isometric and side view of Design 2 ................................................................... 8Figure 7. Wire racks for Design 2....................................................................................... 9Figure 8. Third design food warmer. All dimensions are in inches.................................. 11Figure 9. Concept Art of Design 3................................................................................... 13Figure 10: Updated Gantt chart of the project schedule ................................................... 15

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Abstract

The current mid-term report was prepared by team Spacewalkers (Team 4), which

consists of Allen Omardeen, Jack Ranken, Tim Crispin and Brandon Henry. The

University Houston (UH) faculty advisor is Jagannatha Rao and the engineer-in-charge at

NASA is Dr. Michele Perchonok. This project is conducted as part of the Texas Space

Grant Consortium (TSGC) Design Challenge. The lead coordinator for TSGC is Debbie

Mullins. Developing a food warmer is crucial to mission success. The journey to Mars

will take up to 6 months for a one way trip. The food warmer is required to heat up to 12

packages per meal prior to consumption. In addition, specific constraints for weight, size

and temperature exist. The weight and size requirements of 13 pounds and 1 cubic foot

respectively were achieved by last semester’s design team, but the main challenge is to

attain a temperature of 155-175°F for heating the food packages in less than 25 minutes

using minimal power of only 280 watts.

The team has already developed several ideas to modify the current design and

this may include, but is not limited to, modifying heating source, redirecting airflow, and

a possible modification to the outer casing. The team is currently using COMSOL

software to analyze the system. Initial analysis for the food packet was generated using

COMSOL, 2D convection and conduction transient heat analysis. COMSOL models

verified that the Fall 2007 design was not able to meet the time requirements of 25

minutes. The team is on schedule to meet the deadlines for validation and verification

testing of the new design. Currently, there are no obstacles that will prevent meeting the

deadline for the final report on April 14th, 2008.

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Team Spacewalkers

Team Spacewalkers carefully choose a patch (Figure 3) that best symbolizes all

the elements of this year’s Design Challenge.

Figure 1: Team Spacewalkers patch

The school logo dawns in the center of the patch to show from where team Spacewalkers

come. The three acronyms in the middle show the three groups responsible for bringing

together the 2008 Design Challenge. The words “CHALLENGING THE FUTURE” at

the bottom of the patch represents the spirit of engineering. Progress is made by going

beyond the norm and challenging the notion that certain things just can’t be done.

Introduction and Background The current project is being conducted by conducted by team Spacewalkers

(Team 4), which consists of four team members: Allen Omardeen (Industrial

Engineering), Jack Ranken (Mechanical Engineering), Tim Crispin (Mechanical

Engineering), Brandon Henry (Mechanical Engineering). The faculty advisor is

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Jagannatha Rao and the engineer-in-charge at NASA/Johnson Space Center is Michele

Perchonok. This project is being conducted as part of the Texas Space Grant Consortium

(TSGC) Design Challenge coordinated by Debbie Mullins and as a requirement for

graduation by the University of Houston (UH) Mechanical and Industrial Engineering

department’s Capstone Design course.

The Fall 2007 UH Capstone Design Team developed a 2nd generation food

warmer to be used during trips to and from Mars. The food warmer had to meet several

requirements set forth by NASA which included: it must be able to heat 12 food packets

to 155-175F within 25 minutes, it must have a maximum volume of 1 cubic foot, it

cannot weigh more than 13 pounds, and it can only use 280 Watts power. The Fall 2007

team was able to meet all of the requirements, except for the 25 minute time limit.

Spacewalkers objective is to take the Fall 2007's design and analyze it to determine its

deficiencies and recommend several modifications for improving the design. Material

cost and vibration analysis will be considered but will not be extensively covered.

The project is proposed by NASA’s Space Human Factors and Habitability

(SHFH) project at Johnson Space Center and is sponsored by the TSGC. During long

space flights, NASA uses a quad-laminate pouch made of polyolefin, aluminum foil,

polyamide, and polyester to store meals in as seen in Figure 1. The packets are stored at

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Figure 2: Food pouches used by NASA (picture provided by NASA)

room temperature and during meal times are warmed and must be consumed within 60

minutes of heating in order to adhere to safety standards. The current version of the food

warmer as seen in Figure 2, takes about 40 minutes to warm the food packets and is

bulky. Also, the vehicle for which the food warmer is designed is smaller than the

Figure 3: Existing food warmer (picture provided by NASA) current shuttle. Microwave technology cannot be used because the microwaves can

interfere with communications and sensitive on-board equipment.

Statement of Goals

The main goal of this project is to develop a working prototype of NASA Food

Warmer. To achieve this goal, the following milestones are used to track the progress

throughout the project lifecycle:

Milestone 1: Analysis of current prototype

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• Meet with person involved in last years’ NASA Food Warmer design project

• Analyze pros and cons of previous design

• Determine needed modification to current design

Milestone 2: Analysis of heating system

• Test and collect data in lab on current heating method

• Investigate alternative power consumption methodology

• Re-visit internal size and shape requirements

• Develop improvements for current design

Milestone 3: Validation and verification testing

• Implement modifications to current prototype

• Construct re-designed prototype

• Verify prototype up to specification

• Test and collect data in lab on revised heating method

Milestone 4: Final report and presentation

• Generate final technical report and presentation

• Submit final technical report and presentation

Progress Description

Three major modifications were made to the Fall 2007 design team’s prototype

and are described below. All three designs will be constructed of an aromatic (aramid)

polymide fiber and carbon fiber as the inner and outer shell and supported by a thin

aluminum frame. Carbon fibers are strong and light weight and aramid fibers can

withstand high temperatures. Both materials, however, are expensive to purchase. A full

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body carbon fiber frame is expensive and takes a long time for production and may be

considered at a later date if weight requirements cannot be met.

Design 1

The first design (Figure 3) will take advantage of conduction and forced

convection. The design consists of an outside chamber (case), heating element,

convective fan, insulation, an electrical circuit to control the fan, heating elements, and

thermostat. Concerning convection, the heat is fan-circulated from a dedicated

convection element in the rear and the side of the oven cavity to break the thin layer of

air, which breaks the “air insulation”. The fan motor will be placed in a separate section

to avoid overheating. The main heating element will be placed at the bottom and side of

the oven.

Figure 4. Side view of Design 1 By installing the heating element as part of the oven, conduction will play a significant

role in heating the food. This is one of the simple heating methods used in regular ovens.

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The combination of conduction and forced convection methods make this design more

efficient. The following is a detail of the trays:

• Dimension of the 3 trays (see Figure 4) • Width 7” • Length 16” • Thickness 0.125” • Holes are 1” in diameter and 1” in between them with 1” at each end. The holes

were designed to assist in the air flow.

Figure 5. Top View of tray for Design 1

The fan will be installed at the backside of the oven. The thickness of the fan is 1”, which

is accounted for in the dimensions listed above.

An aluminum fan, usually used in regular microwave ovens, will be used.

Aluminum was selected due to the light weight of the device and the temperature inside.

The fan motor selected will have four-pole motors. The team is currently searching for

the best motor to fit the device. Due to power limitation, 4 cartridge heaters will be used.

Two heaters will be placed on the sides of the oven, as well as one at the bottom and one

facing the fan in the back of the oven. The heating source that will be used is a 1/8"

diameter by 1-1/2" long SunRod Split Sheath cartridge heater (24V, 60W, 9.6 ohms).

Design 2

The second design (Figure 5) is based on the idea of heating the food through

forced convection and conduction. The forced convection concept will be implemented

by using the fan motor. The conduction concept will be implemented by placing the

heating

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Figure 6. Isometric and side view of Design 2 element in contact with the oven racks. Convection ovens or fan ovens augment a

traditional oven by circulating heated air using a fan. The fan motor is in a separate

enclosure to prevent it from overheating. Food warms faster in a convection oven since

the moving air strips away the thin layer of air, which otherwise would surround and

insulate the food. By moving fast hot air past the food, convection ovens can operate at a

lower temperature than a standard conventional oven and yet can cook food more

quickly. The air circulation, or convection, tends to eliminate "hot spots" and thus food

may bake more evenly. The two fan motors and the heating elements are connected in

parallel with the power supply through connecting wires attached to the outer motor

casing and the inner side walls of the oven respectively.

Each heater will draw 60 Watts of power and the fan motors will draw 50 Watts

each, bringing the total power used to the 280 Watts limit for this conceptual design. The

power will be submitted to all elements of the circuit at the same time through a

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connection at the side of the food warmer. The two motor fans are installed in the center

of the back side of the oven. This is done to force the airflow circulation in a horizontal

plane within the oven cavity to minimize the potential for airflow paths to be blocked by

the objects placed in the oven.

There are three wire racks (Figure 4) in the oven to hold 12 food pouches. Each

rack consists of 2 major parts: the bottom part is used to carry the food packets; the top

part holds the food packets in place (like a lid) due to zero gravity. A wire rack is used to

minimize weight and to allow for more heat exposure to the food pouches.

Figure 7. Wire racks for Design 2

The heating elements are custom built high-resistance wires. Nichrome is used to

make the resistance wires because it has a high resistivity and resistance to oxidation at

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high temperatures. Two aluminum fan motors are used for airflow circulation.

Aluminum fans are used because they can withstand heat and are light in weight. There

are two return vents, which are located on the side walls of the food warmer. For internal

insulation, Fiberfrax will be utilized. Fiberfrax ceramic fiber products are manufactured

from alumina-silica materials and offer such characteristics as high-temperature stability,

low thermal conductivity, low-heat storage, excellent thermal shock resistance, light-

weight, and superior corrosion resistance. Fiberfrax ceramic fiber products exhibit

thermal stability at temperatures up to 1260°C (2300°F).

Design 3

The third design (Figure 5) entails the use of pure convection. This design uses

three rows of five thin aluminum fins with a heating coil embedded in the fins. Each fin

is slightly smaller than the approximate size of each food packet to reduce excess weight

and to assure that contact with the heating area is maximized. Durablanket insulation is

used to prevent heat escape. The exterior walls are made of carbon fiber, which is light

in weight and durable. The interior walls are lined with aluminum foil to take advantage

of (and contain) any radiation heat.

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Figure 8. Third design food warmer. All dimensions are in inches.

Aluminum was used for the rack because it is light in weight, conducts heat well,

and is cheap as compared to other metals of similar densities. Aluminum is also readily

available. Fin theory controls how efficient heat can be transferred to and from the fins.

The fins need to be thick enough to encase the heating element and thin enough to

minimize the weight of the rack. The spacing between the racks will snugly hold the

food packets in place during heating. Since the fins are slightly smaller in stature than

the each food pouch, the edges of the pouches can be grabbed after cooking without

touching the aluminum rack.

A 0.125” diameter by 223” long Watlow cable heater will be used as a heating

source. It has a maximum wattage of 240 and can sustain temperatures of 630˚F for

extended periods and is capable of a wide range of voltages. Of course the maximum

wattage will not be used due to power needed for other components and a 280 Watt limit.

With an aluminum frame (1” trim), the weight of this design comes in at about 15.2

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pounds. Using a completely carbon fiber body will reduce the weight by 3 to 4 pounds.

The volume for the outer case comes in at 1691 in3 which is just under one cubic foot

(1728 in3). All of the specifications given by NASA are met by this design. Also, initial

calculations predict the warm time of about 27 minutes. This is slightly over the required

time, therefore a further evaluation will need to be conducted. Ideally a lab test would

confirm the results, but time may not permit that.

Final Design

Out of the three designs mentioned above, a simple analysis was performed using

a Pugh chart (see Table 1). None of the designs were able to meet the weight

Table 1: Pugh Chart. 0=Does not meet requirements, 1=Meets requirements, 2=Exceeds requirement

CriteriaImportanceWeight

CurrentFood

Warmer Design1 Design2 Design3

HeatingTime 0.26 0 1 2Weight 0.18 0 0 0

Size 0.18 1 1 1

RequiredPower 0.18 1 1 1

EaseofUse 0.15 1 2 2

AvailabilityofMaterials 0.05

DATUM

2 1 2

TotalPoints 0.61 0.97 1.28

requirements, which means some work will need to be done to lighten the food warmer.

Design 3 was chosen based on the Pugh score but in the following weeks some tweaks

will need to be made. For example, the weight will need to be reduced. Below is a

concept design of Design 3.

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Figure 9. Concept Art of Design 3.

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Cost Analysis

Table 2 shows the breakdown of cost to potentially build the third design from

scratch. Up to this point no money has been spent. In the next few weeks the team is

planning to start purchasing the materials needed to build the agreed upon design. The

decision to select the appropriate design should be accomplished no later than March 16,

2008. The total cost listed in Table 2 is a rough estimate based on core parts alone. Man

hours and cost of producing the model itself is not included. The cost for the other two

designs is not listed because the cost was much more than Design 3. Most components

were priced through Grainger [3] and Watlow [4].

Table 2: Design 3 cost analysis

Items Description Cost

Construction 6+ Man Hours and Cost of Construction $250.00

Aluminum 2 x 6’Aluminum Flashing, 1” trim, 0.125” Thick $65.00

Carbon Fiber 1 Yard US Composites Carbon Fiber, 5.7oz x 50" Width

$46.00

Axial Fan Motor Square AC Axial Fan, Air Flow 105 CFM, Voltage Rating 115 Volts, Width 4 11/16 Inches, Speed 2900 RPM, Current Rating 0.18 Amp, Bearing Sleeve

$24.90

Heating Element 2 xWatlow Cable-Coil heater, Length 30”, Voltage 120, Watts 175

$160.00

Insulation Auralex 2" Mineral Fiber Insulation - 24" x 48" x 2" Sound Absorption and Insulation Panels - 6 Pieces

$119.00

Miscellaneous + Project Reserve

Bolts, nuts, accessories, etc. $100.00

Total $764.90

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Scheduling

The team remains on schedule for the overall project. The project is expected to

be completed by the deadline of April 14, 2008. The updated Gantt chart is provided in

Figure 10: Updated Gantt chart of the project schedule

As can be seen from the Gantt chart, the team has decided on a final design. This

is expected to be completed by March 16, 2008, as indicated on the chart. Concurrently,

other tasks are being completed, because of the team’s structure and flexibility. The

project is expected to be completed by the deadline of April 14, 2008.

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Conclusion

The team remains confident about the successful completion of the project by the

deadline of April 14, 2008. Progress is being made in multiple areas of the project. Over

the past four weeks, the team has met with NASA enginner-in-charge Dr. Michele

Perchonok, and contacted UH faculty advisor Dr. Jagannatha Rao to discuss the project

parameters and to gather all necessary information and recommendations needed to

proceed. The team has gathered background information about the project. Additionally,

the team has studied and analyzed the current design that was developed by last

semester’s Capstone Design team. Due to the limited time that the team has, three

different designs were obtained to allow the team to select one or a combination of any of

the three designs to save time. Concurrently, other tasks were completed through the

team’s structure and flexibility. As of now, the team is expected to complete the new

design by the deadline of April 14, 2008.

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Appendix A

Budget Report

No funds have been spent toward this project at this point. There are plans to

build a prototype, but only if time permits.

Team Trip Report

The team has not made any field trips due to the tight schedules of each member.

Also, two of the members work at the facility of the engineer-in-charge. The project did

not facilitate ant field trips at this point.

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Reference

[1] Design of Food Warmer for a Mars Transit Final Technical report; D. Perez, I. Castro,

T. Stoner, E. Lopez; 2007

[2] A.F. Mills, Heat Transfer, Prentice Hall, Inc., Second edition, 1999.

[3] www.grainger.com

[4] www.watlow.com