[andrewla]

I only skimmed the article; but as I understood it the liquid hydrogen here is being used strictly as ejecta. It is not being combusted, the way that it is used in the space shuttle boosters.

If that is the case, liquid hydrogen seems like an awful substance -- hard to produce and transport, hard to store and the mass required to support keeping it is not insubstantialy, all for a mass density of ~70g/L.

Why not just use water? Water is 1000g/L and can be stored without any real effort at all. If all you're doing is shooting it out the rear end of the rocket with as much energy as possible, it's mysterious why you would even consider liquid H2.

[paulsutter]

The article covers this. Virtually all nuclear engine proposals have used H2 as propellant because of the specific impulse, but this article mentions methane as an alternative. Of course, Starship today uses methane and Elon plans to create methane on mars.

> Most significant for our purposes is methane as propellant. It is six times denser than liquid hydrogen, can be stored at 100K, which is compatible with liquid oxygen, and it can be produced using water and carbon dioxide. At high temperatures, it breaks down into hydrogen and carbon, turning it from a 16 g/mol molecule into a 3.25 g/mol plasma. That is how it achieves a specific impulse only mildly lower than what is achievable using liquid hydrogen. Zubrin lists its specific impulse as

>Previous calculations using hydrogen propellant revealed how volume-limited the Starship design was. There was no room for the bulky liquid hydrogen, and getting to orbit meant sacrificing the payload mass and volume advantages that the Starship is built around.

>These could be solved by using denser liquid methane as propellant for the nuclear propulsion system. The Isp will be lower, but the mass ratios become so much better that more deltaV is available overall.

[CarVac]

For specific impulse. When you have unlimited power, you want the lightest possible molecular mass exhaust for the best total delta V.

[klyrs]

So, since beta radiation is even lower mass... just put a block of nickel-63 into a magnetic lens?

[ben_w]

Shooting just electrons out the back of your ship is fine for the brief period before your ship gains a positive electric potential comparable to the net potential those electrons experience due to the inside of the particle accelerator you were using, after which your ship's own field is a major source of drag.

If your ship has a positive electric potential of over 1.044 MeV[0], you also start getting positron-electron pairs forming on your hull.

[0] less in practice, because any electrons you're going to encounter were already moving

[moffkalast]

Don't ionic thrusters typically solve this problem by also emitting oppositely charged particles from a thin rod behind the motor? Maybe that still causes drag, but overall the main problem is that while the ISP is high, the thrust is almost non-existent.

[ben_w]

They do indeed; the oppositely charged particles being electrons. But there's no charged particles lighter than an electron, and the only one with the same mass is the positron. If you try to balance it with protons, then what you've really got is a slightly over-complicated hydrogen-based ion drive.

[klyrs]

Twin beta+ and beta- engines?

[ben_w]

First, you have a way to store an absurd quantity of positrons. For a sense of scale, without shielding the electric fields, 1 picogram of positrons (or electrons) confined within a 10 cm radius is going to trigger positron-electron pairs formation, thanks to free electrons in the area responding to the surface potential.

Second, AFAICT if you can do that then you're either going to want to use them as an energy source to propel your cheap reaction mass even harder than a fission rocket would, or you're going to want to react them with electrons to make a photon rocket.

[klyrs]

I proposed nickel-63 for my electron source. Let's say, aluminum-26?

And, yes. The idea is patently absurd. But I'm waiting on a ridiculously long compile.

[perihelions]

- You'd need to account the mass of the copper-63 waste, in addition to the electron

- This math is specifically about thermal propulsion; your 17 keV electron wasn't in thermodynamic equilibrium with anything, it's a different sort of problem

[abecedarius]

https://en.wikipedia.org/wiki/Nuclear_electric_rocket

[klyrs]

That's not quite what I'm picturing (and let's be honest, the idea is silly). That page describes capturing the heat of a nuclear reactor, converting it to electricity, and then using that electricity to force ions out of a thruster. I'm saying, the radiation itself has momentum; don't bother with the contraption.

[abecedarius]

Yes, just thought you might not know of the related more practical version, and you might be pleased to encounter it.

[andrewla]

I need to play Kerbel Space Program. I have absolutely no intuition about this sort of thing. Some intuition around orbital mechanics from my physics days but rocketry is a bit of a mystery.

[idlewords]

For me it helped to remember that the term that matters in the rocket equation is exhaust velocity, not momentum. You want to be shooting the lightest things you can out the back of the rocket, at the highest speed.

Unfortunately with chemical rockets, the energy source and reaction mass are the same thing, so you're kind of stuck with whatever burning your fuel gives you. But when you can separate reaction mass from energy source (like in nuclear or electric rockets), hydrogen is always the best bet.

[Kim_Bruning]

Mostly true. However, there's tradeoffs qua thrust and specific impulse for different propellants. For pure vacuum work hydrogen is a good candidate. For operating in a gravity field (eg. the moon), you might be able to use a smaller nuclear engine with a denser propellant. Obviously you ARE trading in some range when doing this. But if the question is whether you can get off the ground in the first place, you may have less of a choice. It might also depend on what you can actually lay your hands on qua ISRU (if/when relevant)

[matheusmoreira]

Do it! You'll know you got it when you're maneuvering the rocket on pure intuition without even using maneuver nodes.

Just steer clear of the planet Eve. It's been years and I still haven't been able to leave it.

[Kim_Bruning]

Yes, absolutely: https://xkcd.com/1356/

[rsynnott]

Some explanation here: https://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Princip...

I think all real-life actually-tested nuclear thermal rockets have used hydrogen as propellant, though use of other substances has definitely been proposed.

[Kim_Bruning]

Water is annoying: when you heat it, some of the energy goes into dissociating it rather than accelerating it. In the end you apparently get pretty bad performance out of it.

In deep space you don't really care about the volume of your fuel tank, so then it does make sense to use something light like hydrogen.