Would Or Could A Nuclear Saltwater Rocket Be Converted Into A Plasma Rocket With A Magnetic Nozzle?

[Spacescifi]

Yes or no?

Would this extend it's travel range greatly?

[AckSed]

Someone's been watching For All Mankind...

Okay, "plasma" is characterised by the presence of a significant portion of charged particles in any combination of ions or electrons.

A NSWR rides a continuous nuclear explosion held at bay by the flow rate, but the peak neutron flux should happen outside the craft and in the nozzle. Should.

A nuclear explosion contains some charged particles due to how hot it is, and maybe some alpha and beta particles. The magnetic nozzle will be able to corral them. So the advantage is that the ISP will rise. Slightly. But now you are stuck designing a superconducting nozzle that will withstand a constant barrage of neutrons, gamma rays and heat. I wouldn't want that challenge.

Edited May 18, 2024 by AckSed

[Spacescifi]

6 hours ago, AckSed said:

Someone's been watching For All Mankind...

Okay, "plasma" is characterised by the presence of a significant portion of charged particles in any combination of ions or electrons.

A NSWR rides a continuous nuclear explosion held at bay by the flow rate, but the peak neutron flux should happen outside the craft and in the nozzle. Should.

A nuclear explosion contains some charged particles due to how hot it is, and maybe some alpha and beta particles. The magnetic nozzle will be able to corral them. So the advantage is that the ISP will rise. Slightly. But now you are stuck designing a superconducting nozzle that will withstand a constant barrage of neutrons, gamma rays and heat. I wouldn't want that challenge.

I would say pulse fire it, but something tells me doing that with a nuclear reaction only limited by flow rate would be a challenge, albeit not an impossible one.

So the plasma drive version of a nuclear saltwater rocket would be designed to pulse fire rather than for continous exhaust and with a magnetic nozzle.

[AckSed]

Pulsing... wouldn't be a good idea either. The inventor, Dr. Robert Zubrin, the one who came up with Project Orion in the first place, said that this concept depends upon a steady stream of constantly-fissioning propellant, and water to shield the reaction chamber/nozzle.

For the why, read on.

From what I have read, a lot of the engineering and startup/shutdown processes in any rocket engine are attempts to mitigate/eliminate transient events.

Transient events, or transients for short, are the bane of any system in spacecraft, most often fluid-carrying systems. They happen when you turn something on and things are in the process of starting, or the opposite, when things are in the process of stopping. Sometimes it's when you have almost reached full power, but have to literally wait for the pumps to catch up.

We find this on Earth with normal plumbing. Closing or opening a tap/valve suddenly will cause a bang as the speed of a mass of an incompressible liquid (water) is reduced to zero, and the energy is dissipated at shockwaves ringing through your pipes. This is hydraulic shock AKA "water hammer".  If the system isn't engineered to mitigate it, such shockwaves can cause pipes to crack from the strain and bubbles of vacuum or vapour to form.

That's like a couple kilograms per minute in a good water system.

In a rocket engine pumping hundreds of kilograms of propellant per second, suddenly closing a valve that's feeding the propellant from the tanks is Bad. And explodey.

Citation: "Treatment of Transient Pressure Events in Space Flight Pressurized Systems"

It gets worse, though.

"Hard starts" are generally caused by fuel or oxidiser left in the engine or pipes meeting up with new oxidiser or fuel being fed in when you restart the engine. Certain mixture ratios or allotropes or frozen/semi-frozen mixtures of fuels explode. You must run the engine lop-sidedly by feeding in one part of the propellant to wash away any trace of the other, and  in the case of cryogenic fuel/oxidiser, do not boil when entering the pumps, causing vapour bubbles that the pumps will ingest, overspeed and then tear themselves to shreds. (This is what "engine chilldown" is prior to a Falcon 9 second stage engine igniting.) See here: https://space.stackexchange.com/questions/41473/how-does-lox-lead-startup-prevent-hard-starts

A NSWR, when the propellant is dissolved nuclear salts of a certain concentration that can boil off the water and become more concentrated, will need to be really, really, absolutely certain it is not leaving a crust of uranium tetrabromide on the reaction chamber walls that will not detonate when more nuclear fuel is fed in. Because water (which we are using to cool the chamber walls) is a good way to slow down highly energetic neutrons so that they can split fissile uranium i.e. it is a moderator. So if a restart doesn't feed multiple swimming pools of water into it beforehand, it's going to suddenly produce much more radiation and then blow up in a very dirty explosion.

[RuBisCO]

Here are my spreadsheet calculations.

Consider an NSWR of 147 GW with a thrust of 5 MN and an Isp of 6000 s. It would have a total mass flow of 85 kg/s. If we assume an amazing fission efficiency of 90% while using a 2% uranium solution of 80:20 U235:U238, then only 0.224 kg/s of the uranium solution would be used, of which only 2 g/s of U235 is consumed. This means almost all of the 85 kg/s is water coolant, with some lithium salt and deuterium as well to provide fusion boosting, but that can be included in the uranium solution.

Now the big question is: can 85 kg/s of water cool the nozzle and plenum chamber? This engine would produce 147 GW, which would heat the uranium solution as high as 46.3 x 10^6 Kelvin (assuming a heat capacity of 14.18 J/g°C for a plasma of 2 parts H and 1 part O). Fission of Lithium 6 and Lithium 7 to tritium, as well as T+D fusion, should be possible and can thus assist in the burn. I will assume this is occurring and is covered in that amazing fission efficiency of 90%. Now the rest of the 85 kg/s of water coolant would have an average temperature of 122,000 Kelvin, still a plasma that will vaporize anything.

Obviously, the temperature is not the average of the water coolant at the nozzle and plenum chamber walls. Maybe, just maybe, we could get by with film cooling, having a thin layer of non-plasma water steam along the walls keeping the plasma from melting the walls. Otherwise, the only other solution is a magnetic nozzle.

For a magnetic nozzle, we would likely need to switch to liquid hydrogen as coolant to cool the "high temperature" superconducting magnets. We would also still need film cooling as well. The advantage of liquid hydrogen is that our Isp would increase considerably. The average molar mass of water plasma is 6, and the average molar mass of hydrogen plasma is 1, so the specific impulse would increase by sqrt{6/1} or 14,697 s from 6000 s. Also, our thrust would decrease to 2 MN from 5 MN (with a mass flow now of 14 kg/s). This is not a problem considering that shrinking down an NSWR to 147 GW to begin with is likely asking a lot; we can expect terawatt NSWRs easily!

The disadvantages of a magnetic nozzle include how to keep the high-temperature superconductors cool with all the heat from the continuous nuclear explosion, as well as neutron irradiation. Again, we would need to have film cooling with hydrogen gas as well as refractory materials that will not impede the magnetic field of the superconductors much.

Next, hydrogen has very poor volume and mass density compared to water. Even with the superior Isp, the tanks for hydrogen would still be 5.6 times larger in volume than water tanks. Assuming spherical tanks, the surface area needed for hydrogen tanks is 3.1 times more than water tanks. Even still, 27 tons of water would need a tank of 27 m³ and would weigh 0.2 tons at 5 kg/m². This would be equivalent to 10.5 tons of LH2 in a 150 m³ tank that would weigh 1.4 tons at 10 kg/m². Either would provide a ΔV of ~3 km/s for a 500-ton spacecraft, so clearly even at that low ΔV, Hydrogen would still save ~15 tons, and the weight savings only goes  ΔV goes up.

In conclusion, a superconducting nozzle may be necessary to keep the plasma from touching the nozzle and plenum chamber walls. It would have the added benefit of using hydrogen fuel with higher Isp and reducing the minimum thrust an NSWR can provide. The extra tankage for hydrogen is negligible.

Edited July 13, 2024 by RuBisCO