Comment by Animats

Comment by Animats 9 days ago

Pictures of Orion concepts have been widely distributed for decades, but not this one. Going into space with an air-breathing nuclear bomb powered jet engine. That is truly weird. How fast was it supposed to be going in atmosphere, I wonder. Did it carry reaction mass for vacuum so it could keep going out of the atmosphere?

The trouble with nuclear rockets is that although you have plenty of energy, you still need to carry reaction mass - air, or water, or something - and a lot of it. That becomes the limit on delta-V.

(The great frustration of rockets: not only do rockets need something to push against, they have to carry it with them.)

cryptonector 9 days ago

> not only do rockets need something to push against, they have to carry it with them

I think you meant "something to push with. But anyways, nuclear explosions make their own hot gasses out of the reactants, and the problem with that is getting that to be directed and also to not get the ship -you know- blown up in the process.

A better approach is to have a nuclear reactor to heat reaction gasses to much higher temps than chemical reactions allow for, thus increasing ISP a lot. The main problem with this is that nuclear reactors need a lot of cooling, even and especially after you shut them off, and the reaction gasses are going to be your only way to cool them. Alternatively you can carry huge radiators -- really huge, because in space you don't get to exchange heat with an atmosphere or a body of water, so you can only radiate or ablate away the excess heat, and either way would require huge amounts of extra mass. Another problem is that stop/start latency with nuclear reactors is huge, so they would only work for interstellar travel, I think. I'm sure if you carry enough reaction mass then nuclear reactors can work well for interstellar trips.

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M95D 8 days ago

Nuclear explosions don't have "reactants". There's just a few Kg of Uranium or other fuel and a little conventional explosive to trigger it, and that's all. That's not enough mass to propell anything of significant size outside of atmosphere. The speed at which the mass is ejected from the engine counts, of course, but a nuke won't give the same speeds as an ion engine for example.
Aldebaran spacecraft proposed to "intake" air. But it's mass means it would have to intake A LOT of air. And that engine, as described in the article, doesn't seem to compress the air before each bomb expands it. I don't think it would work.
IIRC, the Orion project proposed to wrap each bomb in one ton of polypropylene. It would provide mass and also reduce radiation fallout.

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glompers 8 days ago

By pushing against it they are pushing with it

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unwind 9 days ago

Uh, is it really clear to say that rockets "push against" something? That makes it sound as if they could absolutely not work in space, which they obviously do.

"They push against their own exhaust" seems to be the idea that motivates the pushing explanation, but to me that just invites the question "using what?" and makes things even more complicated. At least in my opinion.

I think Newton's 3rd ([1], forces and reaction forces) is a better explanation.

[1]: https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

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richrichardsson 9 days ago

> rockets need something to push against, they have to carry it with them.
(Emphasis added)
The rocket exhaust is "pushing against" the rocket itself, propelling it forward.
Rocket exhaust goes <- which creates the equal and opposite force -> which pushes against the mass of the rocket sending it on its way.

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