A Comparison of Nuclear Thermal to Nuclear Electric Propulsion for Interplanetary Missions

1
A Comparison of Nuclear Thermal to Nuclear
Electric Propulsion for Interplanetary Missions

• Mike Osenar
• Mentor LtCol Lawrence

2
Overview

• Introduction
• Objective
• Establish parameters
• NTR Design
• NEP Design
• Discussion and Conclusion

3
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?

4
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 ?V

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Establish Parameters

• Establish ?Vs 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

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Establish Parameters
NTR ?V (km/sec) NEP ?V (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 ?Vs and flight times based on AIAA 2002-4729
  low thrust gravity assist trajectories
• NTR data derived from NEP data

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Establish Parameters

• Relationship between NEP ?V/TOF and NTR ?V/TOF
• Table shows that NTR has same TOF for 50 of the
  ?V
• NTR numbers based on AIAA 1992-3778

Mission ?V (km/s) TOF (yrs)
Pluto NEP 13.4 19
Pluto NTR 6.52 16
Pluto NTR 12.9 10
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Establish Parameters

• Ariane 5 Payload Specifications

Mass to orbit (kg) 18000
Height (m) 12.5
Diameter (m) 4.5
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Establish Parameters
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Establish Parameters

• Design points established from Dumbkopff charts

Design Isp (sec) ?V (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
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NTR Design

• Size system so that it meets 3 specifications
• Under max payload mass
• Fits in payload fairing
• Reaches required ?V

12
NTR Design

• Inputs from Dumbkopff finert, ?V
• Assumptions
• Po 7 MPa
• Isp 1000 s hydrogen
• Tc 3200 K
• T/W .3 experimented, balance between high
  thrust short burn time and low reactor mass (low
  power)

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NTR Design

• Equations for basic parameters

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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 density

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NTR Design

• Turbo Pump Feed System
• Nuclear Reactor
• Radiation Shield
• standard SPAD design 18 cm Be, 5 cm W, 5 cm
  LiH2

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NTR Design

• Nozzle
• Columbium, designed to be ideally expanded in
  space (e100)
• Miscellaneous
• Avionics
• Reactor containment vessel
• Attitude thrusters
• Structural mass

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NTR Design

• Achievable ?V verified with Rocket Equation
• Vehicle height determined by stacking parts
  according to Figure

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NTR Design

• Final Results of NTR Design

?V (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
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NEP Design

• Size system so that it meets 2 specifications
• Under max payload mass
• Reaches required ?V
• No size requirement analysis showed that NEP
  systems would violate mass constraints before
  volume no low-density hydrogen propellant

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NEP Design

• Power Source
• Nuclear Reactors (Pgt6 kWe)
• Critical reactors designed as small as 6 kWe
• Radioisotope Thermoelectric Generators (RTG) (Plt6
  kWe)
• Solar?

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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

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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

23
NEP Design

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

24
NEP Design

• Subsystem Design
• Power system
• Propellant tank
• Thruster mass
• Power conditioning mass
• Other mass (structural, feed systems, avionics,
  etc.)

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NEP Design

• NEP Design Results

?V (km/s) f-inert Initial 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
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Discussion and Conclusion

• Overall, ?Vs were low real science mission
  would need higher ?V to capture orbit of planet,
  maneuver
• Accurate data on EP trajectories was desired over
  ?Vs for realistic missions

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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

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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

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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

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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

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Discussion and Conclusion

• Questions?