NPPG-MPD Hybrid Rockets
Nuclear Pulsed Power Generation and Magnetoplasmadynamic thrusters theoretically offer strong combined performance characteristics.
NPPG was discussed in an earlier
post under the then title of stationary nuclear piston engines, and a review of that literature is a key informant about the potential of the system. In brief, the concept takes the Nuclear Pulsed Propulsion system, and re-evaluates it as an power generation system.
Magnetoplasmadynamic thrusters are a fairly well understood ion thruster type, although no real-world MPD has yet been tested at MW range energy inputs, making the concept somewhat akin to saying "besides, it works in Kerbal Space Program". They use electromagnetic forces to accelerate ionised plasma, and in CoaDE (which is at a technical level canon for the "Of Unity" series, and hopefully within acceptable margin of error, real life also) they consitute the primary choice of low thrust and high exhaust velocity rockets. Because of this, they are used in the RCS of Life2.0's NPP using ships. This premise therefore, explores combining the two.
As with all electric propulsion types, ion drives face fundamental limits because of the restrictions on power; even highly optimised (
read 'only just not about to meltdown') CoaDE reactors with 1.01 factor of safety radiators struggle to be 100MW/ton systems, and systems that 21st century Earth would be willing to sign off are a fraction of that performance. Which is why they are under-developed in the real world. At the 1GW region, Nuclear-electric MPD is simply inferior on every metric to NPP except for the fact it can be gimballed. (
NPP, for reasons relating to radiation exposure from off-centre nuclear detonations, cannot be gimballed)
However:
NPPG posits a reasonable possibility of 40-60% efficiency of turning nuclear explosions into electricity, turning the 212GW of a 12m NPP system into 84-127GW of electrical power.
Of note is that this must be split between two or more equally distributed MPD thrusters for forwards propulsion, as a single centre thruster cannot be mounted aftwards on a NPP or low-mass route NPPG using ship.
With two 40GW thrusters, or four 30GW thrusters, what are the performance characteristics? Well, using CoaDE designs:
2x40GW:
| Absolute Exhaust Velocity | 177 km/s |
| Absolute Total thrust | 2x448kN |
| 12m NPP Exhaust Velocity Factor | ~2x Life2.0 65kg PU; 4.9x Project Orion |
| 12m NPP Thrust Factor | 0.16x Life2.0 65kg PU; 0.2x Project Orion |
4x30GW:
| Absolute Exhaust Velocity | 163km/s |
| Absolute Total thrust | 4x365kN |
| 12m NPP Exhaust Velocity Factor | ~2x high end estimate of optimised system; 4.5x Project Orion |
| 12m NPP Thrust Factor | 0.27x Life2.0 65kg PU; 0.33x Project Orion |
In other words, a NPPG-MPD ship of broadly equal specification to a Life2.0 Vanguard corvette is far more delta-v efficient, at the cost of being limited to 0.2g accelerations, which may be further reduced by increases in mass in the system to accomodate the changes necessary for swapping the NPP to NPPG or to a dual-use NPP/NPPG system.
Now, quantifying the mass increase...
The initial estimate, posited 10kW/kg as a rough high-end number for whole system mass by extrapolating from today's best generators. Which in a planetary powerplant is acceptable.
On a spaceship, it definitely is not; 10MW/kg is far more preferable. How close to that can we get?
Well, MW/kg batteries are expected to be in service in the 2030s, as they are in the labs today for both Li-ion and solid state batteries. 212GW means 212 tons of these batteries. That is... A performance hit. But, we can work with that in civilian rockets. If we can get to 10MW/kg, then that drops to 21 tons, which is potentially acceptable for military warships. (
remember it is 65 tons per 1000 pulse units, and we've established we've doubled exhaust velocity already)
The limit is generators. Commercially available generators are barely 1kW/kg, while top end performance - for example, MGU-K systems in F1 and prototype endurance racing - is between 6-8kW/kg. And they restrict development on them to cut costs. To meet our MW/kg target to make this a competitive design, we need to radically rethink the generator.
Next generation superconducting generators are expected to be 50kW/kg capable, which isn't enough. Not by a long way...
We can't use the momentum transfer; the mass of conventional generators has ruled this out.
Likewise, we can't use thermoelectric or turboelectric, because the waste heat is best part of a hundred gigawatts, and even at a 3000K radiator temperature it's going to have radiators so big they will be clearly visible to the naked eye on other planets...
So, lets rethink.
We have a nuclear bomb, in the low kilotons, being kicked out the back and being blown up at point blank range. The pulse unit exists to turn that energy into a vastly more efficient directed plasma that exploits the kinetic energy.
But, thinking of it another way, that bomb will produce an incredibly lethal malestrom of X-rays.
So, we can break out another piece of the Cold War toolbox and invoke Project Excalibur?
Well...
Maybe.
Excitation of nuclei.
Obviously, Excalibur failed because while you can make a bomb-pumped X-ray laser, severe strategic issues and proliferation concerns meant it was canned. (
namely, despite Teller promising it could save the US in the event of Russian ICBM attack, it actually couldn't intercept them all due to a very short window while enemy ICBMs were still accelerating outside the atmosphere, and would instead see massively more ICBMs deployed to overwhelm the defences)
It didn't fail for actually making a working X-ray laser.
Now, we know there are materials that respond to excitation by X-rays - Hafnium-178 can be excited to 300keV, which equates to TJ/kg storage if it is all excited. And I was happy to take a millionth of that for the batteries...
If that's possible, we'd then need to discharge it in a controlled way - which we have no real idea of how to do with it's very long half-life - and top up the ablative layer.
Plus, there would need to be an extremely good radiation shield... We're literally talking about firing an XL-slot X-ray laser at the back of our own ship.
On the plus side, if these highly speculative elements can work, then this offers potentially 100MW/kg+ whole system power generation and 160+ km/s at 1MN thrust.
Alternatively, there's Thorium-229; it excites at UV wavelengths, so our nuclear flash has to be converted downwards, and it has a thousandth of the energy density of Hf-178. But, it does release practically instantly, which solves the biggest unknown of Hf-178. The biggest downside is it melts at just 2083K, which is not helpful for radiators to cool it. But then maybe a "shield" of some high-temperature UV transparent material could be used, which could allow liquid Thorium to be used as the coolant being used in the radiators, which could then be rejected at a much higher temperature. If one exists. If it doesn't, the radiator mass will be in the hundreds of tonnes.
Overall, I think it could be possible, but probably not superior without revolutionary leaps. Definitely deserves more research.
Edit:
The problem is now that, while we have now got kilotonnes of mass that goes for whatever generator selection that turns the released relatively low energy gamma rays from the thorium-229m into electricity...
Even the most simple approach of a solid tungsten plate with a solid osmium plate forming an extremely high temperature thermocouple and rejecting the heat at 2500K plus would be 2kt at best.
It is a workable idea as a propulsion system if you can tolerate 0.01-0.1g acceleration. But I don't think it offers anything over loading the equivalent mass in pulse units on a traditional NPP on near-future tech.
