RoverNERVA Program Overview

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Rover/NERVA Program Overview

• Brandon Cunningham

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Nuclear Thermal Rocket Roots

• Nuclear Thermal Rockets provide twice the
  specific impulse as chemical propulsion
• Initiated in 1955 at Los Alamos National Lab
• 1.5 billion spent or about 10 billion in
  todays dollars
• Goal of Rover program
• Achieve highest possible propellant temperature
  and therefore specific impulse for the duration
  of the mission
• Operate for 10 hr with 60 restarts with
  reliability of 0.995
• Test at the Nuclear Rockets Development Stations
  (NRDS) at Jackass Flats in Nevada

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Rover/NERVA Program Categories

• Fuel development
• High temperature and resistant to erosion and or
  corrosion
• Find a fuel inert to hydrogen propellant
• Reactor development series
• KIWI, NRX, PHOEBUS, PEWEE
• Engine testing capabilities

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Ideal NTR Design
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Propellant Choice

• Hydrogen chosen because of low molecular weight
  and high specific heat
• Hot hydrogen passes through fuel element coolant
  channels then a converging- diverging nozzle and
  expansion of gas gives thrust
• High performance requires high gas temperatures

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Fuel Neutronics and Properties

• Epithermal neutron spectrum
• Graphite moderated
• Graphite is not a strong neutron absorber
  yielding a reactor with a smaller critical mass
• Poor fission-product retention
• Graphite reacts with hydrogen
• Graphite melting point at 3650 and has high
  temperature strength
• To control reactivity half the control drum is
  composed of boron carbide and half composed of
  beryllium
• Core surrounded by beryllium reflector and which
  also contributes (n,2n) reaction

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Fuel Performance

• Graphite fuel elements ejected or damaged during
  Kiwi B1B test
• Theories why
• Flow vibrations
• Thermal shock at startup
• Flow and pressure oscillations

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Fuel Performance

• Only graphite-based fuels were considered in
  Rover/NERVA program
• UO2, UC2, UC, (U,Zr)C
• Graphite fuels are dependent on coating or else
  corrosion will occur rapidly
• ZrC or NbC
• For 1hr
• Standard graphite-matrix fuel had coolant exit
  temperature between 2400-2600K
• Projected for 2 hr
• Advanced composites and pure carbides will have
  coolant exit temperature about 2450K

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Reactor Developments

• Kiwi series demonstrated feasibility of high
  temperature gas-cooled reactor space propulsion.
  
• NRX line had a reactor to reached a propellant
  exit temperature of 2280K or an 730 s specific
  impulse.
• Phoebus series demonstrated higher power levels,
  power densities, reactor duration, and coolant
  exit temperature
• Pewee reactor demonstrated 2550K coolant exit
  temperature corresponding to 845 s specific
  impulse and 2340W/m3 power density is feasible

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Engine Testing

• Interest
• Startup, shutdown, restart characteristics, for
  different initial conditions
• Evaluated various control concepts
• Testing the performance of non-nuclear components
  in a nuclear environment
• Cold flow test
• Maximizing specific impulse
• Engine longevity

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Alternate NTR Design

• Metal Dumbo Rocket Reactor
• Refractory metal fuel element
• Thermal spectrum due to polystyrene moderator
• Laminar flow heat exchanger
• Predicted coolant exit temperature 2500K

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What Rover/NERVA Demonstrated

• Using a NTR will give approximately twice the
  specific impulse as chemical rockets
• High propellant temperature is possible
• Graphite-based fuels are brittle
• Could cause fuel failure
• Fission-product release
• Graphite-based fuels are dependent of a coating
  if using hydrogen propellant