I listen to a lot of space podcasts and when discussing interstellar travel, the subject of nuclear pulse propulsion is always offered as a solution, as if it’s a given. But I’ve not heard anyone discuss the practical way it would work.
Reading the Wiki page, the projected top size of a manned rocket is a the “super” Orion design; at 8 million tonnes, it could easily be a city. An interstellar ark.
A more reasonable design, at the smallest end, an Orion vehicle that was extensively studied, would have had a payload of around 100 tonnes in an 8 crew round trip to Mars.
I don’t understand a lot of the technical stuff in that article, but what I wonder about, first, is how the crew would absorb the G forces? I see they mentioned some sort of device to make them more tolerable, but bringing the forces of a nuclear explosion down to 2 Gs seems to be asking a lot.
Also, the article mentions to reach Alpha Centauri, would take about 36 days at constant acceleration. That seems like a lot of nukes and a lot of wear and tear on the ship.
I assume a 44 year trip to Alpha Centauri would require the ark version. That’s even more nukes and a lot of folks that would need to be cradled against the G forces.
Again, I hear the distance to Alpha Centauri hand-waved away because of the viablity of nuclear pulse propulsion vehicles, but is this really a viable idea?
2 gs isn’t a force; it’s an acceleration. F = ma, so what acceleration you get from a nuke depends on your mass.
And yeah, an Orion ship would need to be pretty durable. Mostly just at the pressure plate and shock absorbers, though: Everything else, it’s just a matter of building things to survive an acceleration of a g or two, and we have a lot of experience with that.
I would think a nuclear explosion would create quite a bit more acceleration than 2 Gs, it would have to, right, to propel the ship to (eventually) .12 c?. What would a mechanism look like that could diminish that much force down to something manageable?
Size matters. You (as in you personally) can push a bicycle much faster than you can push a car given the same amount of force. A big-ass ship is going to be accelerated slowly by a small-ass nuke.
Yes, you are right to be skeptical about the interstellar version. Even though the Project Orion ship was fantastically powerful, it would be just a bit too feeble to accelerate to interstellar speeds,
Here’s a fairly comprehensive look at various designs: none of them could get to Alpha Centauri in a reasonable timescale. http://www.projectrho.com/public_html/rocket/realdesigns2.php#id--Project_Orion
I suppose you could build a really big one, and be resigned to a long, slow trip; but hopefully there are better options than that.
I still think overall that NTP (Nuclear Thermal Propulsion) makes more sense than Orion platforms. For more see this related thread, and especially @Stranger_On_A_Train’s informed follow up on my comment:
Fun fact: 1 g is just about exactly c divided by 1 year. You still can’t get to c by accelerating at 1 g for 1 year, because of relativity, but it works for smallish fractions. Like, to get to 0.1 c, you’d need to accelerate at 1 g for 0.1 years (that’s where that figure of 36ish days for a 44 year trip comes from).
I’m currently in a structure designed to withstand 1 g. So are you, and everyone else reading this thread. Structures designed to withstand 2 g are perhaps a bit less common, but no engineer would blink if you asked them to design a structure with twice the usual strength (actually, probably most structures could withstand 2 g, too, because engineers usually build in safety margins).
Oh, and yes, there probably are better options than Orion. Orion is mostly a sort of proof of concept, to show that it’s definitely possible to make a nuclear-powered spacecraft, because we’re certain that we know how to make bombs. But detonating bombs behind something is a horrendously inefficient way to make a rocket. I mean, you have enough energy available that you can afford that inefficiency, because nuclear, but you’d still be better off with something more efficient, like NERVA.
I guess I’m not understanding. I do understand that if you are in a rocket that has a constant acceleration of 1G and that if if continues at a constant rate for 36 days you would reach your percentage of C. But a nuclear pulse propelled ship wouldn’t experience a smooth 1G transition like a chemical propulsion system would be capable of, would it? From eburacum45’s link: “It is like a naughty little boy lighting a firecracker under a tin can. Except the tin can is a spaceship and the firecracker is a nuclear warhead.” At some point that initial jolt (after every detonation) will be considerably more than 1G.
When I asked for an example, I was asking for examples of technologies that take the force of a nuclear explosion down to 1G. To say ‘just put shock absorbers on it’ seems to be sweeping a pretty issue under the rug. Compared to nuclear explosions, humans are pretty frail, I would think.
The acceleration depends on the mass of the object getting accelerated. A firecracker accelerates a tin can multiple gees, but would accelerate a manhole cover much less. If there’s suspension system between the manhole cover and the frail humans, the vibrations can be reduced to tolerable or barely noticable (a modest speed bump could cause several gees of acceleration - if it weren’t for the springs). The tricky part is making the manhole cover thick enough that your atomic fire cracker doesn’t destroy it
There was a point made many years ago, that designing an Orion spaceship has some options that are entirely counter to designing traditional rockets. The big one was that, making it heavier actually helps, because it smooths out the ride. The limit for Orion after all isn’t the energy available, or the forces generated, but, as discussed above, the much higher accelerations possible, that complicate things.
So, combine a heavy vehicle, with big shock absorbers, and a variety of bomb yields, and you can tailor the ride pretty well.
Footfall is one of the novels I think of every time Orion comes up. It seems to be in fiction (and at a guess, IRL if we ever saw a ELE incoming) the go-to plan for something we could hypothetically build fast and lift a large mass to orbit without major handwavium involved and money and consequences became a non-issue for the short term.
