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 ΔV

5. 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

6. Establish Parameters • NEP ΔV’s and flight times based on AIAA 2002-4729 – low thrust gravity assist trajectories • NTR data derived from NEP data

7. 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

8. Ariane 5 Payload Specifications Establish Parameters

9. Establish Parameters

10. Establish Parameters Design points established from Dumbkopff charts

11. NTR Design Size system so that it meets 3 specifications • Under max payload mass • Fits in payload fairing • Reaches required ΔV

12. NTR Design Inputs from Dumbkopff: finert, ΔV 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)

13. NTR Design • Equations for basic parameters

14. 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

15. NTR Design Turbo Pump Feed System Nuclear Reactor Radiation Shield standard SPAD design – 18 cm Be, 5 cm W, 5 cm LiH2

16. NTR Design Nozzle Columbium, designed to be ideally expanded in space (ε=100) Miscellaneous Avionics Reactor containment vessel Attitude thrusters Structural mass

17. Payload Propellant Tank Pump Shield Reactor Nozzle NTR Design Achievable ΔV verified with Rocket Equation Vehicle height determined by stacking parts according to Figure

18. NTR Design Final Results of NTR Design

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

20. 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?

21. 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

22. 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.)

25. NEP Design NEP Design Results

26. 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

27. 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

28. 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

29. 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

30. 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

31. Discussion and Conclusion Questions?