NERVA operation

[peadar1987]

I'm not a regular user of the nuclear engine in KSP, but I was curious about how it compares to the actual NERVA in terms of operation.

I was thinking in particular about heat production.

So in KSP, when you throttle up the NERVA, it produces more heat. If you've constructed things badly, presumably it will explode. Would the real-life NERVA have had the same sort of heat production issues?

From what I understand, it works in theory by having a nuclear reactor running, passing propellant through the reactor, and using the thermal expansion of the propellant to drive the craft.

Most nuclear reactors don't particularly like rapid power excursions, so I would have assumed that any rapid throttling of the NERVA would have happened by changing the mass flow rate of the propellant through it. Slower power excursions would have been possible by adjusting control rod position in the reactor.

With this in mind, would throttling up a NERVA not, at least in the short term, actually cool it down, as you're effectively increasing the flow of coolant through the reactor? Or am I thinking about this all arseways?

[Silverchain]

Sounds reasonable to me after a quick look through "how the NERVA works" pages.

Oh, how I

one! Edited February 25, 2014 by Silverchain

[Red Iron Crown]

As I understand it, under thrust the propellant in a nuclear rocket cools it as it passes through. When not thrusting, it is put in a lower power state (control rod position) to generate electrical power in a closed coolant loop, with excess heat being removed with radiators.

[architeuthis]

The KSP LV-N is infinitely throttleable without any effect on isp. IRL reactors take a bit of time to 'warm up' and the NERVA2's isp would suffer initially because the hydrogen would still be cold. After you withdraw the control rods and turn on the LH2 turbopumps you burn as normal, then stick the rods back. Since the the reactor stays hot for a while after you put the rods back in, you don't want the fuel elements to melt and the LH2 propellant is effectively acting as reactor coolant you would have to keep running the turbopumps (effectively continuing the burn, but at very low isp) until it was safe to turn them off.

If the reactor is bimodal (unlike NERVA), then it is essentially always on and you don't have to go though all this rigmarole. This is what Red Iron Crown describes.

[MajorThomas]

As I understand it, under thrust the propellant in a nuclear rocket cools it as it passes through. When not thrusting, it is put in a lower power state (control rod position) to generate electrical power in a closed coolant loop, with excess heat being removed with radiators.

That would be efficient...if not producing thrust, have the Nuke engines give you electrical power like an RTG. Would remove the need for solar panels. Daddy want.

[Blackstar]

That would be efficient...if not producing thrust, have the Nuke engines give you electrical power like an RTG. Would remove the need for solar panels. Daddy want.

That's what the reactors in the Interstellar mod do. Attach an electric turbine to one end, and a thermal rocket nozzle to the other, and the heat generated by the reactor can be used by either. Throttling up lessens the rate of electric power generation, though the Megajoule -> Electric Charge conversion rate is so high that you effectively have unlimited power to any stock instruments even at full throttle.

[Red Iron Crown]

Nuclear thermal rockets really are a different breed than chemical rockets, in that what you use for propellant is orthogonal to your energy source, whereas in a chemical rocket the propellant is the energy source. This lets you choose a propellant based on things other than the energy it stores. If you want the highest Isp, LH2 is a good choice (and no heavy oxidizer is required). If you want higher fuel density to reduce tank volume, water is a good choice as it's cheap, dense, and has other uses (life support, can be electrolysized to produce H2 and O2 for life support and powering auxiliary vehicles).

If realism was the goal, the LV-N should be able to use liquid fuel by itself for propellant, or oxidizer or monopropellant for that matter. Would certainly make jet/rocket hybrids easier to design as no oxidizer would be required in any mode.

architeuthis, thanks for the correction, I had forgotten that NERVA wasn't bimodal.

[DerekL1963]

Most nuclear reactors don't particularly like rapid power excursions, so I would have assumed that any rapid throttling of the NERVA would have happened by changing the mass flow rate of the propellant through it.

Most reactors, but by no means all - it's all a question of the how the reactor is designed. I see no particular reason why a NERVA wouldn't have been designed to handle rapid throttle changes, especially as long throttle-up and tail-off periods play merry aitch-e-doublehockeysticks with your guidance and navigation.

[StrandedonEarth]

the LV-N should be able to use liquid fuel by itself for propellant, or oxidizer

If it used oxidizer at high temp it would oxidize (i.e. burn up) very quickly. No more engine.

[Pockrtplanesairways]

they should seriously add liquid "Kydrogen" (yes you heard me correctly) tanks and add the LV-Nb from the KSPX pack.