3Nuclear Rocket Engines Nuclear Thermal Rockets : Propellant gets heated by conduction/convection from fuel.Nuclear Electric Propulsion: Electric power generated byheat engine or thermo-electric effects is used to drive electricPropulsion system.Specific Impulse more than twice that of chemical - ANDLarge-thrust possible because of high thrust/weight ratio.Shorter mission times:Proponents point out that for long-duration space missions, this leads to LOWER total radiation exposure per mission. (Less time exposed to the Big Nuclear Furnaces in the Sky)
4Nuclear Thermal Rockets (NTR) Originally researched by the U.S. Air Force and NASA from the late 1940s to the 1960s, the use of nuclear energy to power a rocket engine has many advantages:High thrust (as much as chemical rockets – temperature and pressure limits are similar)High Specific ImpulseMultiple re-startsDisadvantages:Heavy (reactor and shielding )Radiation concernsTesting / development / political issues“Thermal” reactor: neutrons slowed down below 1eV, moderator oflight elements used.“Fast”: broad spectrum of neutron energy up to 15eV. No moderator.
5NTR ConceptNeutron Reflector: typically Beryllium shieldPropellant: H2 (high Ue), CH4 (better storage density) etc.
6Types of ReactorsNERVA (Nuclear Engine for Rocket Vehicle Applications )Graphite fuel rods with uranium-carbide fuel particles – coated to protectfrom hydrogen.Coolant passes through channels in the rods.Most fully developed, but low T/W
8Particle Bed Reactor (PBR) 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.inisjp.tokai.jaeri.go.jp/ ACT95E/11/1104.htm
9lifesci3.arc.nasa.gov/.../thomas/ Adv.prop/advprop.html “The technical risks of the PBR include:+ 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;”lifesci3.arc.nasa.gov/.../thomas/ Adv.prop/advprop.html
10CERMET-core Source: www.ms.ornl.gov/.../ SCG/programs/OTT_HVPSM.html “Cermets: composite materials made by combining a ceramic and a metallic alloy.”“Cermets are made by 1) milling together powders of the desired materials, and 2) forming a shape, and then 3) sintering (heating) those materials together to form dense parts. The sintering process causes the component materials to bond together to form a new material with structural characteristics that are better suited for the intended use than either of the starting materials.”CERMET fuel: Hexagonal fuel elements. Uranium oxide particles imbedded in tungsten / tungsten-rhenium matrix; uranium oxide in molybdenum.Advantage: very long life ( > 40 hours).Uses fast fissioning reactor that does not depend as much on a “moderator” material to slow down the neutrons in the fission reaction.A newer system that still requires developmentHas potential for multiple restarts (very robust fuel elements)
11“ 6 - Cermet Reactor ( http://lifesci3. arc. nasa. gov/. /thomas/ Adv “ 6 - Cermet Reactor ( 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 This should produce less stringent containment and confinement restrictions on Ground Test Facilities.+ .. cermet fuel may offer improved swelling behavior, but this remains uncertain.
12Several issues remain open: - .. 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. “
14Candidate Working Fluids Working Fluids: coolant, exhaust gasesAs 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 =Methane:Carbon dioxideWater:Note: the above offer the possibility of replenishing propellant from extra-terrestrial sites.
15For a given temperature limit, we can determine the specific heats from equations or tables and find the other quantitiessuch as :
16NTR Performance Example 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.At 3000K,
19Not counting any turbine/pump losses (I.e., closed cycle)
20Reactor PowerThe reactor provides the power necessary to heat the propellant to the required temperature.Heat of vaporizationSpecific power required. Depends on propellant initial / final temperatures60H2is 48 MW/(kg/s)For H2 at 3000K,CH4CO22000T3500
21Reactor MassFrom Fig in Humble, (Reactor Mass vs. Reactor Power for differentNTR technologies ) the mass of the reactor is estimated as 500 kg.
22Reactor ShieldingWe also typically provide a “shadow shield” between the reactor and thepayload for in-space applications. A typical shield consists of layers ofLithium hydride (LiH2) which is a neutron absorber, Tungsten to shieldagainst gamma rays, Beryllium as a neutron reflector. A typical shield maybe 25cm thick and have a mass-density of 3500 kg/m2 of surface areaTotal mass = Reactor + shield + nozzle + turbopumps + core containment vesselTypical thrust-to-weight ratio of an NTR is from 3 to 10.
23Preliminary Design Decisions NERVA type core limited to ~ 2360 K temperatureCERMET: 2500KPBR: 3200KDetermine gas propertiesDetermine Nozzle expansion ratio and Isp.Sizing the SystemInert-Mass Fraction (when using hydrogen propellant): 0.5 to 0.7Lack of database of nuclear engines prevents good estimate of inert massfraction: iterate.From Isp and required thrust, find propellant mass flow rate.Determine required reactor power to heat propellant to required temperatureat the required flow rate.
24Determine system pressure levels: empirical correlations NERVA-1Enhanced NERVAParticle BedCERMETChamber Pressure3.1 MPa6.9MPa6.2MPa4.1MPaCore pressure drop: % of chamber pressure38%10.6%5%53.7%% of reactor head pressure27.5%9.6%4.8%34.9%
25Prometheus Nuclear Engine: JIMO mission Jupiter Icy Moons Orbiter: Nuclear electric primary propulsionTwo basic types of technology under consideration:radioisotope-based systems(2) nuclear fission-based systems.
26RadioIsotope Thermoelectric Generators Heat from plutonium dioxide and solid-state thermocouples toconvert directly to electricity. Cold outer space is the cold junction.General Purpose Heat Generator: 250 wattsMultimission Radioisotope Thermoelectric Generator: 100watts, 14+ years
27Stirling Radioisotope Generator http://spacescience. nasa Closed cycle Stirling cycle free-piston machine. Heat from GPHG with 600gPlutonium dioxide at 650C. Heat rejected from other end at 80C. Closed-cycleEngine converts heat to reciprocating motion – linear alternator produces62-65 watts AC – converts to 55w DC.