NERVA has about 250 kN of thrust at 841 seconds of specific impulse.
Merlin 1D has about 1000 kN of thrust at 311 seconds of specific impulse.
That suggests NTRs have around 4x less thrust, but around 2.7z more efficiency than chemical rockets. At a large enough scale craft, the weight of the engine could be considered irrelevant. But for smaller crafts, the Merlin has a thrust to weight ratio of 184:1, whereas NTRs only ever achieved 7:1 (they hoped to get up to around 35:1 eventually).
I imagine the thrust to weight could be substantially improved by making lightweight reactors, but I’m wondering if anybody knows anything about concepts for higher thrust NTRs?
https://en.wikipedia.org/wiki/NERVA
https://en.wikipedia.org/wiki/SpaceX_Merlin
The NTP applications I’ve seen rely on chemical rockets to get the vehicle into space, and then the NTP to maintain acceleration after the chemical is exhausted. The chemical engines are never going to store enough propellant to keep firing for very long, but the NTPs can theoretically maintain acceleration for, say, a whole journey to Mars, flipping around to decelerate halfway. Even with the smaller thrust, a vehicle doing this can get up to a nontrivial percentage of light speed.
but for a hypothetical combat scenario, you’d want something (maybe extra chemical boosters) for maneuvers
leaving the forces aside, have you thought about the mass and the resulting acceleration? that would be pretty important if your crafts were used by living people and not for transporting corpses.
for transporting
corpsesbiological slurry.
Ah yes! Nuclear thermal rocketry! Thankgod!
All the thrust to weight ratio tells you is what percentage of the craft is the engine.
The next component is the propellant mass you eject to get your thrust. To escape the Earth’s gravity you need 11200m/s of acceleration which with the best nuclear thermal rocket specific impulse of 900 means 72% of the ship is propellant. If the rest is engine with no payload at 28% of the mass and thrust to weight of 7:1 you get a max acceleration of 1.8Gs at liftoff which is… Okay… Again that’s with no payload.
Basically your options would be to invent a way to propel the exhaust at a higher velocity to increase your ISP (maybe you could ionize the propellant, use the heat to generate electricity and then use the electricity to blast the propellant to ridiculous velocities), or make your rocket engine much lighter to improve thrust to weight and make room for payload. But my favorite option is just make a monstrously huge rocket with a small command module at the top of it and the rest is all engine and propellant. These things could replenish by consuming massive amounts of mass from nebulas or small meteors or something. Just giant nuclear punk monstrosities that take a chunk out of the solar systems they operate in and create jets of high speed ionised exhaust that obliterate anything that happens to pass by.
One thing to note is the thrust to weight ratio only matters in relation to some gravity you’re fighting against. It isn’t a thrust to mass ratio. The only time thrust to mass matters in space really is if your burn time needs to be short. If the same amount of ∆V is used, it doesn’t matter if it’s done over seconds or months (or years for that matter). Turning as much of your propellant into as much ∆V as possible is the most important thing.
nuclear powered ion drives are hyper efficient, but the thrust is so low. im thinking about developing a space fleet doctrine. ion drives are definitely necessary for carriers and cargo, but i’m wondering about which ships use what engine types during combat, and how those tactics could play out.
