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 Pluto NEP ΔV’s and flight times based on AIAA – 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 Mission Δ V (km/s) TOF (yrs) Pluto NEP Pluto NTR Pluto NTR12.910

Establish Parameters Ariane 5 Payload Specifications Mass to orbit (kg)18000 Height (m)12.5 Diameter (m)4.5

Establish Parameters

Design points established from Dumbkopff charts Design Isp (sec) Δ V (km/sec) f-inert Jupiter NTR Jupiter NEP (Ion) Jupiter NEP (Hall) Pluto NTR Pluto NEP (Ion) Pluto NEP (Hall)

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: f inert, ΔV Assumptions P o = 7 MPa I sp = 1000 s – hydrogen T c = 3200 K T/W =.3 – experimented, balance between high thrust short burn time and low reactor mass (low power)

NTR Design Equations for basic parameters

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

NTR Design Turbo Pump Feed System Nuclear Reactor Radiation Shield standard SPAD design – 18 cm Be, 5 cm W, 5 cm LiH 2

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

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

NTR Design Final Results of NTR Design Δ V (km/s)f-inert Initial Mass (kg) Height (m) Power (MWe) TOF (years) Jupiter NTR Pluto NTR

NEP Design Size system so that it meets 2 specifications 1. Under max payload mass 2. 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 m 2 solar panel in earth orbit, would need 27 m 2 panel at Jupiter and 1562 m 2 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:

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

NEP Design NEP Design Results Δ V (km/s)f-inert Initial Mass (kg) TOF (years) Power (kWe) # of thrusters Jupiter (Kaufman) Jupiter (MESC) Jupiter (RIT) Jupiter (Hall) Pluto (Kaufman) Pluto (MESC) Pluto (RIT) Pluto (Hall)

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?