The Fission Fragment Nuclear Rocket

Robert Sheldon and Rod ClarkNational Space Science & Technology Center

Grassmere Dynamics, LLCNSSTC, Huntsville, Alabama

May 13, 2005

Abstract● NASA's Human Exploration Initiative has refocussed on high-efficiency, high-

thrust rocket propulsion, which has returned attention to the potential of nuclear rockets to provide a unique, high-efficiency, high-thrust propulsion technology. There have been many nuclear rocket designs suggested over the past 50 years, some that were developed here at MSFC, but one that has not received much attention is the extreme high-efficiency "fission fragment" rocket, first proposed to our knowledge by George Chapline.

● Possessing a specific impulse ISP > 100,000 sec makes fission fragment propulsion second only to pure light (or anti-matter) for raw efficiency. Previous designs suffered (as do most nuclear rocket designs today) from concerns about keeping the nuclear core cool. A recently studied material called "dusty plasma" (such as Saturn's rings) held the secret to a clever solution to the heating problem, since it provides a density intermediate between gasses and liquids. That is, basic research into space physics has provided new materials that can solve old technological problems resulting in improved space capabilities. Think of it as a debt repaid. We will discuss the principles of operation, a schematic design with a weight/size breakdown of the components, and potential mission profiles for this breakthrough technology, with particular attention to radiation hazards.

– Vexhaust= Isp * g [d/dt(MV) = 0]–dV = Vexhaust* log( final mass / initial mass) Material Isp Limitation Solid fuel 200-250 fuel-starvedLH2/LOX 350-450 fuel-starvedNuclear Thermal 825-925 efficiency-

starvedGas Core Nuclear ~2,000 efficiency-starvedMHD < 5,000 energy-starvedIon < 10,000 energy-starved

Fission Fragment ~1,500,000 fuel-starvedMatter-Antimatter ~10,000,000 fuel-starvedPhotons 30,000,000-both-starved

The Rocket Equation

Mission to the Gravitational Lens at 550AU

● Assume we accelerate half-way, decelerate the other half. (Not the most intelligent approach, but good for comparing technologies) so T_trip = 10 years.

● Acceleration = 550AU / (5yr)2 = V / 5yr=.0027 m/s2

● So V = 425,000 m/s● Isp (m/s/10) Mrocket / Mpayload1,500,000 1.0291,000,000 1.04 500,000 1.09 MORAL of Story: 100,000 1.5 V ~ V_exhaust 10,000 70.6 450 1.2e41

Mf/Mi Comparison Missions

\ Mission Technology

Grav. Lens 550au in 10y

Oort Cloud 0.5Ly in 30y

AlphaCentauri 4 Ly in 50y

LH2/LOX 450 s ISP 1.2e41 exp(2222) exp(10666)

Xe Ion 10,000s ISP 72 2.69e43 2.9e208

Fission Frag 1,500,000s 1.029 1.95 24

Fusion Frag 2,000,000s 1.021 1.65 11

Ideal Rockets

Semi-Ideal Carnot Efficiency● So whether we have gas or plasma confinement, hotter is

better for thermal rocket propulsion, but worse for engineering the confinement. Carnot Effic. = (Tf - Ti)/Tf In order to achieve better than thermal efficiency, we must have a non-thermally accelerated rocket.

● This can be done with plasma:– Hot gas is ionized = a plasma, with much higher temperatures

possible because of magnetic confinement– Plasma responds to additional forces, electric & magnetic, so it

can be accelerated (or heated in 1-dimension), with better than Carnot efficiency.

● Non-thermal acceleration = high specific-impulse rocket

How to maximize thrust withnon-ideal rockets

● Rocket engines convert thermal energy into kinetic energy by means of a Laval nozzle. Therefore maximizing thermal velocity => high temperature + hydrogen atoms– Chemical: Heat=Propellant (LH2/LOX 350s) =H2O @14kK– Nuclear thermal: Heat+propellant (Nerva 800s) =H2 @2kK– Gas Core Nuclear: High heat+H2 (2000s) = H @50kK

● Plasmas with “magnetic walls” & nonthermal acceleration– MHD Engines use magnetic fields to produce a 1-D magneto-

fluid nozzle for a gain of about 3X. H~100V ISP~2000– Ion Engines achievie non-thermal velocities from kV electric

field acceleration. Xe at 10kV = 100V/nuc ISP ~ 10,000– Fission-Fragment achieves non-thermal Velocity from MV

nuclear forces. 2MeV/nucl ISP ~ 1,500,000

● 1.5GW Pu239 reactor cooled with GH2 run for >30 minutes, stopped and restarted without incident at Jackass Flats Nevada test site. One version made 4.08GW for 12 minutes. Held the record almost 30 years for the highest power nuclear reactor on Earth. By comparison, the largest hydropower dam is 12GW.– Mass (dry) = 34 ton– Diameter = 10.5 m– Thrust = 867 kN in vacuum– ISP ~ 820second at 1.2GW

● Could place men on Mars by 1980. Cancelled in 1972.

NERVA nuclear thermal circa 1968

JPL Nuclear-Electric Concept

Reactor

Cooling Fins

InstrumentsIon Thrusters

Shielding, Fuel

Shield shadow terminator

Power Lines, Coolant tubes

Fission Fragment Concept

● Nuclear-Electric converts nuclear energy to heat, heat to electricity, then electricity to propulsion. The overall efficiency isn’t very high. There’s abundant nuclear power, so low efficiency can be tolerated, but now we also have much heat to remove, which in space can only be done with radiators.

● If the fission fragments, which contain 90% of the nuclear energy, can be used directly for propulsion, not only is the nuclear power extracted more efficiently, but much less waste heat is generated.

Fuel Fibers● Fuel coated micron-thick fibers, emit >50% of fission

fragments away from fiber. Fragments can be directed out of the system as propellant. Since 90% of energy is in fission fragments, then <55% energy is wasted as heat. Still, fibers get hot.

Carbon fiber

Chapline’s Fission Fragment Rocket

Magnetic yoke

Moderator & magnet coils

U235 coated micron-thick spoke-fibers rotating fast

Fission-fragment exhaust

Enabled Missions

Heat: The hidden killer● So the problem with space nuclear propulsion is

NOT raw power, but how to eliminate waste heat. The more efficiently we can generate thrust, the less waste heat produced.

● Can we have our cake and eat it too? Can we have a non-thermal nuclear propulsion minimizing waste heat?

● Yes. ● Fission fragments can escape < 1 micron U235 dust

without heating the grains much. The dust radiates heat very effectively, permitting high power levels.

Schematic

● Chapline’s rocket with nuclear fissioning dust.

Cool Dust

How do we control, suspend, manipulate such a dust grain(s)?Electrostatically.

Dust Clouds

Since we need a total amount of U235 to achieve criticality, how do we collect enough dust grains without heating them? Organization.

What is a dusty plasma?Charged dust + plasma = a “plum pudding” Coulomb

crystal, or as Cooper-pairs in BCS theory. Note surface tension & crystalline interaction.

Auburn University University of Iowa

Fragment Confinement

More on confinement .● B=0.6 T over 1-meter bore is an awesome

energy density = pressure. If we could do that we’d be flying a fusion reactor! Instead, we use a multipole magnet toroid, such that the field strength drops as |R – R0|-N , with N>2, from the wall.– This has a magnetic gradient near the wall, producing a strong

mirror force, “insulating” the wall from fission fragments.– By Liouville’s theorem, n/B=constant, so fission fragment

density peaks at the wall, low in the dusty plasma center. E.g, one pass through dust.

● Because the escaping fragments are positive, net negative charge in the dust cloud. An ambipolar electric field (=some fraction of MeV) develops at edge as well, confining the fragments.– Proper treatment will require full kinetic simulations.

Toroidal MultipoleMagnetic Trap

Power & Thrust

● One mirror can be adjusted for either better reflection (more thrust) or better transmission (electric power).

● Field coils on the end control thrust & power

● U235 dust● Moderator is

lightweight LiH● Multipole

permanent magnets on sides contain fragments

Concept

Dust suspension FAQs

● Can the dust be suspended while the rocket is accelerating?

● Yes, 1g is typically no problem for labs.

● Will B-field change the dusty-plasma dynamics?

● Yes, but not much.

Terrella Lab ( NSSTC)

Levitated Dusty Plasma w/Magnets

The Dust Trap

• Arc discharge on 3μ SiO2 dust grains charges them

negative. Probable charge state on dust is –10,000 e/grain.

• They are trapped in a positive space-charge region adjacent to ring current. The RC is formed by -400V DC glow discharge on NIB magnet, streaming electrons ionize the air, maintain the RC. Phase-space mismatch of streaming electrons and trapped ions produces the space charge. Highly anisotropic B-field contributes as well.

Langmuir Probe mapping

Discharging Dust

● Won’t negatively charged dust discharge from thermionic emission? And won’t 100nm dust have huge corona discharge current?

● Yes, but not as much as one might think.

Discharge vs Dust Size

Photoelectrons vs. size

550 AUPower, Mass, Acceleration

● Acceleration = V / 5yr= 0.002 m/s● The following values are scaled from Chapline’s

Am242*-fueled rocket. We have not done a separate neutronic analysis to get the appropriate volumes for LiH moderator and U235 dust.– 10m x 0.5m radius, with 30cm moderator = 5.4 ton– Co-Sm magnets 2cm thick w/Al windings = 1 ton– Graphite superstructure, radiators, liquid Na = 1.6 ton– Assuming that the payload is 1 ton, then total=9 tons– For a trip to 550AU, the fuel is then .02*9=.18 tons

● 350 Megawatt reactor (Nerva was 4.08 GW) ~3mg/s● 0.5Ly Oort Cloud5.6 GW consuming 50mg/s

Nuclear Pollution?● Since radioactive fission fragments are emitted from the

rocket, how dangerous is this for the Earth? ● From the two missions analyzed, we calculated how long

each rocket is withing 10 Re of the earth, and how much fuel is burned during this time.– 550 AU mission = 720 g U235 = 3 moles– 0.5 Lightyr mission=3.7 kg U235 = 15 moles

● We modelled the transport through the radiation belts, ionosphere & stratosphere and decay lifetimes of 60 decay products. Short-halflife products decay before reaching the surface of earth. Long-halflife products produce almost no radioactivity. We list radioactive products that make it to Earth from 10 moles U235, both by number and curies.

Modelled Pollutionfrom 10moles U235/P239

● By moles (total radioactivity ~10% of U235)– Rb87 .1 = 1 uCu– Sr90 .2 =1800 Cu– Cs135 .3 = 4 mCu– Cs137 .3 =3600 Cu– Nd144 .05 = .01 nCu

● By Curies fast diff slow diffusion– Sr90 1800 1800– Ru108* 204 110 Cosmic Ray production– Cs137 3600 3600 C14 = 266 Cu/yr– Ce144 1900 770– Pm147* 2300 930

550AU Mission Concept

350MW Fission Fragment Rocket

Conclusions● An interstellar probe is still a challenge with a nuclear

fission-fragment rocket, but 550AU gravitational lens or 1 Lyr Oort Cloud missions are eminently feasible.

● We chose these missions to illustrate how close the fission fragment rocket comes to the stuff of science fiction but using the materials found already at hand.

● For example, 550 AU is very promising. At 350MW, the rocket is still 1/10 of Nerva power, and could accomplish an even shorter mission than 10yr (or bigger payload than 1 ton.) Nor is pollution a real problem.

● Therefore high V missions are enabled by a promising high-efficiency nuclear technology.

4Lightyear Alpha Centauri Found-ET-must-go-now-scenario

● Acceleration = V / 25yr= 0.06 m/s● The following values are scaled from Chapline’s

Am242*-fueled rocket. We have not done a separate neutronic analysis to get the appropriate volumes for LiH moderator and U235 dust.– 10m x 0.5m radius, with 30cm moderator = 5.4 ton– Co-Sm magnets 2cm thick w/Al windings = 1 ton– Graphite superstructure, radiators, liquid Na = 1.6 ton– Assuming that the payload is 1 ton, then total=9 tons– For a trip to Alpha Centauri, the fuel is then 24*9=240 tons

● 208 Gigawatt reactor (Nerva was 4.08 GW) ~1.8g/s