Electric Propulsion Continued. Nuclear Rocket Engines. Nuclear Rocket Engines. Nuclear Thermal Rockets : Propellant gets heated by conduction/ convection from fuel. Nuclear Electric Propulsion: Electric power generated by heat engine or thermo-electric effects is used to drive electric
“Thermal” reactor: neutrons slowed down below 1eV, moderator of
light elements used.
“Fast”: broad spectrum of neutron energy up to 15eV. No moderator.
Neutron Reflector: typically Beryllium shield
Propellant: H2 (high Ue), CH4 (better storage density) etc.
Graphite fuel rods with uranium-carbide fuel particles – coated to protect
Coolant passes through channels in the rods.
Most fully developed, but low T/W
Particles of uranium-carbide fuel (coated) are packed between two porous cylinders. Hydrogen (or helium) is directly used to cool them.
PBRs have a higher fuel density and thus higher T/W. The configuration also allows a higher temperature in the working fluid before the fuel melts – Thus higher specific impulse.
+ Challenges in fabricating the high temperature fuel particles that are the key to this technology -- efforts to date have failed to conclusively demonstrate that fuel particles can withstand the rigors of the reactor operating environment;
+ The low thermal capacity of the reactor core increases the risk of thermal damage to the core in off-normal conditions, or during reactor cool-down;”
“ 6 - Cermet Reactor ( http://lifesci3.arc.nasa.gov/.../thomas/ Adv.prop/advprop.html )
General Electric (GE) is the leading proponent for the Cermet Reactor, which has been evaluated for SDI(16) and SEI applications.(17) .. advantages :
+ .. improved thermal conductivity compared to metal oxide fuel elements.
+ .. extensive fuel test engineering heritage exists from the ANP and 710 programs.
+ … high retention of fission products in the fuel matrix. .. experimentally demonstrated in the 710 Program, in which most test fuel elements demonstrated fission gas fraction release of less than 10-9, while some fuel elements released fission fragment fractions in the range of 10-5 to 10-4. This should produce less stringent containment and confinement restrictions on Ground Test Facilities.
+ .. cermet fuel may offer improved swelling behavior, but this remains uncertain.
- .. lower fuel density relative to metal fuel elements, a potential disadvantage of cermet fuels is large core size, and thus greater core and shielding mass.
- In order to improve weldability, Rhenium is a potential Cermet cladding material. “
Working Fluids: coolant, exhaust gases
As the propellant, we want something that can be easily heated and results in a high Isp.
Low molecular weight is preferred, but propellants with higher molecular weight may be used depending on storage volume constraints.
Hydrogen: MW = 2.016
Carbon dioxide 44.01
Note: the above offer the possibility of replenishing propellant from extra-terrestrial sites.
For a given temperature limit, we can determine the specific heats from equations or tables and find the other quantities
such as :
Assume a PBR NTR. Hydrogen is heated to 3000K at a total pressure of 60 atm The engine operates in vacuum with e = 200, At = 100 cm2, hCF = 0.97, hC* = 1.0. Find vacuum specific impulse, thrust and mass flow rate of hydrogen.
This gives heats from equations or tables and find the other quantities
At heats from equations or tables and find the other quantities = 200, =1.289,
Not counting any turbine/pump losses (I.e., closed cycle) heats from equations or tables and find the other quantities
The reactor provides the power necessary to heat the propellant to the required temperature.
Heat of vaporization
Specific power required. Depends on propellant initial / final temperatures
is 48 MW/(kg/s)
For H2 at 3000K,
From Fig. 8.21 in Humble, (Reactor Mass vs. Reactor Power for different
NTR technologies ) the mass of the reactor is estimated as 500 kg.
We also typically provide a “shadow shield” between the reactor and the
payload for in-space applications. A typical shield consists of layers of
Lithium hydride (LiH2) which is a neutron absorber, Tungsten to shield
against gamma rays, Beryllium as a neutron reflector. A typical shield may
be 25cm thick and have a mass-density of 3500 kg/m2 of surface area
Total mass = Reactor + shield + nozzle + turbopumps + core containment vessel
Typical thrust-to-weight ratio of an NTR is from 3 to 10.
NERVA type core limited to ~ 2360 K temperature
Determine gas properties
Determine Nozzle expansion ratio and Isp.
Sizing the System
Inert-Mass Fraction (when using hydrogen propellant): 0.5 to 0.7
Lack of database of nuclear engines prevents good estimate of inert mass
From Isp and required thrust, find propellant mass flow rate.
Determine required reactor power to heat propellant to required temperature
at the required flow rate.
Jupiter Icy Moons Orbiter: Nuclear electric primary propulsion
Heat from plutonium dioxide and solid-state thermocouples to
convert directly to electricity. Cold outer space is the cold junction.
General Purpose Heat Generator: 250 watts
Multimission Radioisotope Thermoelectric Generator: 100watts, 14+ years
Closed cycle Stirling cycle free-piston machine. Heat from GPHG with 600g
Plutonium dioxide at 650C. Heat rejected from other end at 80C. Closed-cycle
Engine converts heat to reciprocating motion – linear alternator produces
62-65 watts AC – converts to 55w DC.