The Future of Space Flight – Nuclear Propulsion

Nuclear propulsion is the simplest thing in the world. Obviously, the word NUCLEAR is scary. Apparently, the presence of this world has destroyed more projects than the Congressional Budget Office. But still, Nuclear Thermal Propulsion, to be specific, is easy peasy lemon squeezy.

nuclear thermal propulsion

Figure 1. The simple view of a Nuclear Thermal Rocket.

To start with, you get a tank of water. Then you boil it. Then you squirt it behind you. Yeah, that’s about it. You can use Helium, Hydrogen, Water, Liquid just about anything will work. The boiling process is run by a hot nuclear fission reactor.

Now, a couple of points:

  1. It doesn’t turn off and on like your stove. It takes hours to heat up and days to cool off.
  2. The waste is slightly radioactive.
  3. The thing could melt if run too hot without propellant. So, accidents could happen.
  4. You get a lot of thrust per pound of fuel.
  5. It is very efficient. ISP around 1000, twice as good as a chemical engine.
  6. It can last for tens of years, used carefully.

Well, looking at those points, what is the engineering argument? You probably shouldn’t light one off on the ground. They save literally tons of fuel, but with a high thrust, so you can use them in a gravity well. (Super high efficiency engines often have almost no thrust, gravity and atmospheric drag can defeat their efforts.)

The best locations for nuclear rockets are planetary orbits, possibly the occasional moon landing or Mars launch. Good bang for the buck.

So, when it comes time to move Man from low earth orbitĀ (LEO) to geosynchronous earth orbit (GEO), or even the Moon. Nuclear Rockets are the best engineering choice.

Again, we first need to develop a solid method of delivering payload and fuel to a LEO space station. Then, we need to develop a strong work horse to deliver these payloads where they need to go. A deep-space dock at a location like L2 (shown below) might be the best choice. But satellites have lots of locations to go to, a good delivery service is worth hundreds of millions each year.

1024px-Lagrange_points

Figure 2. Lagrange (L) points on a map. The closest to Earth are L1 and L2. The others could simply be called “co-orbit with Earth.”

So, when you are looking at the future of space flight. It starts with delivery to LEO, but that is too close for real space work. To get human projects out of the gravity well, we need a workhorse. I recommend a nuclear rocket for near-space travel.

3 thoughts on “The Future of Space Flight – Nuclear Propulsion”

  1. Hawk,
    Yeah, when I got to teaching my students about gas laws and why we do not have a cool future like that shown in “2001, A Space Odessy” I point out that we don’t have cities on the Moon, or a giant spinning space station with artificial gravity or even a homicidally insane artificial intelligence like HAL 9000 on the fact that chemical rockets throw backwards water vapor at 3000 degrees Celsius, which sucks and means just to get to LEO you have to have a ship that is over 90% fuel and only one or two percent actual payload. Thus, no cities on the Moon, just space junk from Apollo, no cool orbital space wheel as a base or construction site for manned missions, rather a barely functional ISS that sucks up all the money for manned travel. using the ISS as a measure for our ability for space travel is like saying that we are an ocean-going species because we have a lashed-together raft anchored offshore and we row dinghys at great expense back and forth to that raft. To get to another harbour on a different island you need an actual transport.

    Chemistry is just physics for atoms and molecules and the energy of a gas molecule is E=1/2 mu(squared) with u(rms) dependent on temperature in Kelvins. Rocket action (moving forwards) is dependent on propellant reaction (gas being thrown backwards. Things can go faster forwards if the propellant can be thrown faster backwards; i.e. the reactant gases are hotter. However one hits a wall in the fact that the rocket engine is made of solid stuff and things can only get so hot before they melt and vaporize, or are burned up by reacting chemically with the insanely hot propellant. NERVA, which used graphite to hold the uranium fuel and molecular hydrogen as the propellant, had the graphite erode away. C+2H2 =CH4. Steve Howe has proposed containing the uranium fuel for the reactor core in a tungsten/rhenium alloy container, which would actually not be bad in the sense that tungsten is the most refractory metal and rhenium is second or third (I think it is tungsten, rhenium, tantalum and osmium for metals with melting points above 3000 Celsius) Hafnium (10B) boride has a melting point at 3300 C and would work as a control rod as both hafnium and boron 10 are effective neutron absorbers. Zirconium (11B) boride might be used as a refractory liner for the reactor and the rocket motor itself, though I have no Idea about how well the ceramic would stand up to thermal expansion or radiation embrittlement, or just being assaulted by 3000 degree hot hydrogen. One might just as well line the walls with graphite or polycrystalline diamond and just let the surfaces ablate away and have to reline the rocket motor with fresh carbon every hundred hours run time or fifty million miles, (whichever comes first) This would have to be done by teleoperated, shielded robots. Once that reactor had been running for a while the workers could picowave their burritos by just having the robot crawlers carry them in to the reactor before lunch! (Mmmm! Picante sauce with just a hint of transuranics! Just like what my abuelita used to make! She worked at the cafeteria at Los Alamos! just kidding!)

    Actually, one of the things that rang true about 2001 was the positioning of the drive unit on the end of a very long boom from the living quarters. Presumably the great distance and the propellant tanks would protect the crew initially. Would extra shielding, like a beryllium disc between the crew compartment and the reactor to provide a radiation shadow be necessary as propellant is used up?

    Finally, on leaving Earth for solar system travel. Neil Stephenson in his book SevenEves explains the value of the Earth-Sun L1 point as a useful transit point from circumEarth orbit to reach a nearby comet for water. He uses a nuclear rocket, powered by a 4000 MW thermal capacity fission reactor to move a multimegaton chunk of water ice back to Earth orbit to provide reaction mass, shielding and drinking water for our protagonists. However the crew that went on the mission die a particularly gruesome death from ‘fuel fleas’ pieces of irradiated fissile material that escape from a cracked fuel rod and are brought into contact with the original crew. The ice is brought in by a second, salvage crew who find the original crew who have died from having their intestinal linings cooked by the “fleas” One more reason to keep the toasty badness of the core well contained.
    Yours in irradiated burrito science,
    Dr. DNA.

  2. Great comment as always. Yeah, 3000 degree Hydrogen stinks. That stuff will cut through most materials like… a plasma cutter. (roughly identically)

    Re-coating the surfaces with an ablative diamond/graphite is probably a good idea for lifetime. We can keep the reactor temperatures cooler and then use a nozzle to get the last bit of high temperature out, but since the magnetic nozzle is only 90%, we’re still losing a lot of surface.

    But, one of my points is that the Nuclear Rocket isn’t the inner solar systems dream transportation. It beats the lashed together raft (chemical rocket), but only near shore. Maybe its a good rowboat, or a decent fishing boat, nothing to travel to Jamaica on, but good for getting to our Ship. It really is only needed in gravity wells. For my Ship’s engine, I like temperatures in the million degree range, which pushes our efficiency up significantly.

    1. Hawk,
      So, for the Ship, serious interplanetary travel like to say Ceres or Saturn, are we talking about VASMIR? Hall style Ion drives are highly efficient,(exhaust velocities in tens of thousands kps) but very slow initially. I would like a working fusion drive, but working continuous fusion is for me one of those wish for things, like gainful employment, more hair on my head, a winning lottery ticket or world domination! (Obligatory Bwa! Ha! ha! inserted here) The only fusion drive (noncontinuous) that could be done with present day technology would be Orion, and politically, that sucker is not just dead, but as they might say here, DAID!!! (just channeling my inner redneck, now! Ahhh, much better!)
      Yours in you-know-what kiching science,
      Dr. DNA.

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