A Comparison of Nuclear Thermal to Nuclear Electric Propulsion for

Interplanetary Missions

Mike Osenar

Mentor: LtCol Lawrence

Overview

Introduction Objective Establish parameters NTR Design NEP Design Discussion and Conclusion

Introduction

NASA is developing Nuclear Electric Propulsion (NEP) systems for Project Prometheus, a series of interplanetary missions

What happened to Nuclear Thermal Rocket (NTR) systems? Should NASA only invest in NEP systems?

Objectives

Prove the feasibility of different nuclear propulsion systems for interplanetary missions which fit in a single launch vehicle

Compare NTR and NEP system designs for given missions

Method: take a set of inputs, use a series of calculations and SPAD process along with reasonable design assumptions to design a spacecraft to reach a given ΔV

Establish Parameters

Establish ΔV’s and flight times for both NEP and NTR systems to Jupiter and Pluto

Determine launch vehicle payload restrictions Obtain design points – inert mass fractions

based on thruster specific impulses

Establish Parameters

NTR ΔV (km/sec)

NEP ΔV (km/sec)

NTR TOF (years)

NEP TOF (years)

Jupiter 3.83 7.66 4.13 4.13

Pluto 6.70 13.40 19.00 19.00

•NEP ΔV’s and flight times based on AIAA 2002-4729 – low thrust gravity assist trajectories

•NTR data derived from NEP data

Establish Parameters

Relationship between NEP ΔV/TOF and NTR ΔV/TOF

Table shows that NTR has same TOF for 50% of the ΔV

NTR numbers based on AIAA 1992-3778

Mission ΔV (km/s) TOF (yrs)

Pluto NEP 13.4 19

Pluto NTR 6.52 16

Pluto NTR 12.9 10

Establish Parameters

Ariane 5 Payload Specifications

Mass to orbit (kg) 18000

Height (m) 12.5

Diameter (m) 4.5

Establish Parameters

Dumbkopff Chart - Jupiter NTR 1000 kg

0

18000

0 500 1000 1500Isp (sec)

Init

ial M

as

s (

kg

)

0.1 0.2

0.3 0.4

0.5 0.6

0.7 0.8

0.9

Dumbkopff Chart - Jupiter NEP 1000 kg

0

18000

0 1000 2000 3000 4000 5000Isp (sec)

Init

ial M

as

s (

kg

)

0.1 0.2

0.3 0.4

0.5 0.6

0.7 0.8

0.9

Dumbkopff Chart - Pluto NTR 500 kg

0

18000

0 500 1000 1500Isp (sec)

Init

ial M

as

s (

kg

)

0.1 0.2

0.3 0.4

0.5 0.6

0.7 0.8

0.9

Dumbkopff Chart - Pluto NEP 500 kg

0

18000

0 1000 2000 3000 4000 5000Isp (sec)

Init

ial M

as

s (

kg

)0.1 0.2

0.3 0.4

0.5 0.6

0.7 0.8

0.9

Establish Parameters

Design points established from Dumbkopff charts

Design Isp (sec) ΔV (km/sec) f-inert

Jupiter NTR 1000 3.83 0.65

Jupiter NEP (Ion) 3500 7.66 0.80

Jupiter NEP (Hall) 1500 7.66 0.60

Pluto NTR 1000 6.70 0.50

Pluto NEP (Ion) 3500 13.40 0.65

Pluto NEP (Hall) 1500 13.40 0.32

NTR Design

Size system so that it meets 3 specifications

1. Under max payload mass

2. Fits in payload fairing

3. Reaches required ΔV

NTR Design

Inputs from Dumbkopff: finert, ΔVAssumptions

Po = 7 MPa

Isp = 1000 s – hydrogen

Tc = 3200 KT/W = .3 – experimented, balance between

high thrust short burn time and low reactor mass (low power)

NTR Design

Equations for basic parameters

0

0

1

11

gI

V

inert

inert

gI

V

pay

prop

sp

sp

ef

fem

mimW

FF

0gI

Fm

sp

715417.5018061.0 TmP

NTR Design

Subsystem Sizing (note: volume constraint height)

Payload

1000 kg to Jupiter, 500 to Pluto

based on densities of actual space mission

sized as 2 m tall cylinder

Tank

biggest part – hydrogen has low densitynkta

totbk g

Vpm

.0tan

NTR Design

Turbo Pump Feed SystemNuclear Reactor

Radiation Shieldstandard SPAD design – 18 cm Be, 5 cm W, 5 cm LiH2

3549512 )10(1703.1)10(946.8)10(655.2 corecorecorecore PPPR 34.3132955.2)10(427.7 23

corecore PP

9883.171427.0)10(027.4 25 corecorecore PPH

NTR Design

NozzleColumbium, designed to be ideally expanded in space (ε=100)

MiscellaneousAvionicsReactor containment vesselAttitude thrustersStructural mass

NTR Design

Achievable ΔV verified with Rocket Equation

Vehicle height determined by stacking parts according to Figure

f

isp m

mgIV ln0

Pump

Shield

Reactor

Nozzle

Propellant Tank

Payload

NTR Design

Final Results of NTR Design

ΔV (km/s) f-inert

Initial Mass (kg)

Height (m)

Power (MWe)

TOF (years)

Jupiter NTR 4.191 0.6094 9100.41 7.23 281.23 4.13

Pluto NTR 8.103 0.4182 14853.83 12.29 281.23 19.00

NEP Design

Size system so that it meets 2 specifications1. Under max payload mass2. Reaches required ΔV

No size requirement – analysis showed that NEP systems would violate mass constraints before volume – no low-density hydrogen propellant

NEP Design

Power Source Nuclear Reactors (P>6 kWe)

– Critical reactors designed as small as 6 kWe

Radioisotope Thermoelectric Generators (RTG) (P<6 kWe)

Solar?

NEP Design

Solar Power proportional to inverse square of distance from sun

to receive power equal to 1 m2 solar panel in earth orbit, would need 27 m2 panel at Jupiter and 1562 m2 panel at Pluto

does not factor in degradation – significant for long lifetimes

engineering, GNC concerns with huge solar array

mass too much

NEP Design

Thrusters based on actual designed thrusters from SPAD

Baselines used: T6, XIPS-25, RIT-XT Design allowed thrusters to be clustered in

groups of up to 3 – proven to work, increases force and power appropriately

NEP Design

Use NTR equations for propellant mass, thrust, mass flow and power

NEP equations:

0gIV spe

2

21

eVmP

NEP Design

Subsystem Design Power system Propellant tank Thruster mass Power conditioning mass Other mass (structural, feed systems,

avionics, etc.)

NEP Design

NEP Design ResultsΔV

(km/s) f-inertInitial Mass

(kg)TOF

(years)Power (kWe)

# of thrusters

Jupiter (Kaufman) 15.860 0.5266 4068.58 4.13 10.258 2

Jupiter (MESC) 14.051 0.5685 3673.06 4.13 8.425 2

Jupiter (RIT) 15.433 0.5622 3768.34 4.13 9.555 2

Jupiter (Hall) 12.242 0.3351 6645.87 4.18 6.180 3

Pluto (Kaufman) 42.725 0.2656 9495.62 18.79 10.258 2

Pluto (MESC) 41.420 0.2849 8079.27 19.40 8.425 2

Pluto (RIT) 44.626 0.2826 8352.61 19.19 9.555 2

Pluto (Hall) 13.771 0.3433 6719 19.02 1.471 1

Discussion and Conclusion

Overall, ΔV’s were low – real science mission would need higher ΔV to capture orbit of planet, maneuver

Accurate data on EP trajectories was desired over ΔV’s for realistic missions

Discussion and Conclusion

NTR Design Almost failed Pluto design – tank volume High thrust, impulsive burn more reliable –

operates for short time Much less efficient then NEP Other applications? launch vehicle, human

Mars exploration

Discussion and Conclusion

NEP Design Low thrust, long trip times Lifetime analysis – electric thrusters tested to

3.5 years – less than Jupiter TOF Space Nuclear reactors require extensive

testing

Discussion and Conclusion

Testing – extensive testing needed for either system – facilities, money needed to test for operational lifetime

Safety – perennial concern with nuclear systems, real hazards to be considered

Radiological hazard – higher with NEP (low power but long burn time), must be addressed for either system

Discussion and Conclusion

NASA probably right to go with NEP for interplanetary missions

Much stands between now and operational nuclear propulsion system

Much to be gained from nuclear propulsion technology

Discussion and Conclusion

Questions?