# Could a “nuclear turbo-jet” be built?

Being familiar with jet engines myself, I wonder whether a similar device could be built to provide high $$I_{\text{sp}}$$ and high thrust in vacuum and space. Using the tried-and-true turbojet formula as an analogy --

• Inlet. This would be the storage of propellant, say hydrogen, always at local $$M=0$$ for a rocket engine. Because we aim for a high thrust and a high $$I_\text{sp}$$, the mass flow rate is very low, so an electric drive will be sufficient to drive a propellant pump. The very low-density gaseous propellant is subsequently by an RF antenna ionized to a low-temperature plasma.
• Multiple-stage compressor. The jet engine needs this to establish a stable direction of flow, plus it'a necessary part of the Brayton cycle. So I imagine for plasma this is more or less the same. For a nuclear turbo-jet, this would be cascading stages of magnetohydrodynamic drives with converging cross-section and diffusers which is simply a section with a diverging cross-section, I assume this rather than the other way because the plasma is hot and the absolute number of local speed of sound must be very high so compression happens subsonic. Or this could be done in a simple stage for plasma.
• Fuel injection and burn. Here is the part that distinguishes nuclear turbo-jet from conventional ones. For fission to happen the fuel has to be squeezed as well. I don't know if you can do this simply by jetting ionized Plutonium or Uranium and squeezes it or you need an ionized tamper around it to contain it for a more complete burn, and that if the burn time for fission can be made less than the dwell time in the combustion section. As with jet engine, pressure peaks before this stage, and only goes downhill from here, so exhaust comes out from the backend. I also don't know if there's a magnetic equivalence of a flame holder to induce vortices inside the plasma to help the combustion.
• Turbine. This would be a magnetohydrodynamic generator that feeds the combuster magnetohydrodynamic drive.
• Nozzle. A variable de Laval magneto nozzle.

A jet engine is streamlined, has very low vibration compared to its pulsed peer the piston engine, and has the amazing ability to burn nearly every hydrocarbon on the market provided that you can jet it out of the fuel line. I imagine plasma is not that different than gas and a nuclear turbo-jet could offer similar advantages while providing very high exhaust speed and thus strong thrust at a low mass flow rate.

Is this engine feasible, if we handwave the superconducting coil generating the magnetic field part, and the exact pattern of the magnetic field part?

• – uhoh Mar 26 '19 at 13:52
• This question is probably more aviation related than space. en.wikipedia.org/wiki/Nuclear-powered_aircraft – JCRM Mar 26 '19 at 14:02
• You're confusing the requirements for a fission chain reaction and those for a magnetically confined fusion reaction – Steve Linton Mar 26 '19 at 14:48
• The system uses stored hydrogen as a reaction mass and a fuel that doesn't need an oxidizer, and so it would work in space (if it worked). It's on-topic here. – uhoh Mar 26 '19 at 14:50
• @SteveLinton No, the continuous fuel jet must stay subcritical in the fuel line and go supercritical when it's supposed to burn (or the engine will just end up nuking itself), so squeezing is the proper way, as in a nuke you squeeze using dynamite to make it go boom and it's safe when stored. – Meatball Princess Mar 26 '19 at 15:03

No, it won't work.

• Fuel injection and burn. Here is the part that distinguishes nuclear turbo-jet from conventional ones. For fission to happen the fuel has to be squeezed as well. I don't know if you can do this simply by jetting ionized Plutonium or Uranium and squeezes it or you need an ionized tamper around it to contain it for a more complete burn, and that if the burn time for fission can be made less than the dwell time in the combustion section...

Squeezing a little bit of fissile material through the engine at a time will not produce much fission. The neutrons come out fast and need to be moderated by a lot of intervening material to slow them down before they are captured by the next nucleus.

To make the chain reaction, you need a nuclear pile with a lots of fuel and lots of moderator mass all in one place. This is why reactors are big and heavy and don't really fit into aircraft or spacecraft very well.

Neutron damage would make your fan blades brittle quickly if there was a lot of fission, which there won't be.

Compact sources of nuclear power like RTGs use spontaneous nuclear decay rather than chain reactions. But that means you can't induce "ignition" inside the engine.

A really interesting idea was proposed based on a nuclear isomer and is discussed at length in this excellent answer to the question How did Northrop Grumman propose to make the Global Hawk nuclear powered?.

From the New Scientist's article Nuclear-powered drone aircraft on drawing board

Tightly controlled reaction

The reaction works because a proportion of the hafnium nuclei are “isomers” in which some neutrons and protons sit in higher energy levels than normal. X-ray bombardment makes them release this energy and drop down to a more stable energy level.

Here the idea was that an X-ray generator (which can be compact) is used to stimulate isomeric states to decay and release energy promptly. It's a little bit like the X-ray laser concept except the excited state lasts days or years rather than microseconds, and there's no optical gain. It's not a LASER, it's only a SER (stimulated emission of radiation) without the LA.

• There have been some small reactors within spacecrafts, for instance the TOPAZ reactors. Of course not as "turbo jets". – Uwe Mar 26 '19 at 14:27
• @Uwe but that wasn't used for propulsion. 300 kg for 5kW would make a ridiculously small thrust to weight ratio. – uhoh Mar 26 '19 at 14:31
• @uhoh What if we use weapon-grade HEU and add a dense vapor or plasma tamper around it? – Meatball Princess Mar 26 '19 at 14:53
• Wait, the Hafnium X-ray simulation actually works? It has been demonstrated experimentally? I thought it had been discredited. – Eth Mar 26 '19 at 15:31
• @Eth no, don't wait, go ahead and re-read what I've written to see that there's no suggestion of that. – uhoh Mar 26 '19 at 15:40