# Questions about Project Orion and nuclear propulsion **URL:** \ **Category:** Factual Questions\ **Created:** [June 7, 2023, 6:29pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161 "2023-06-07T18:29:07Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 7, 2023, 6:29pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/1 "2023-06-07T18:29:07Z")
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](https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)), 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? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 7, 2023, 6:39pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/2 "2023-06-07T18:39:52Z")
> [@Lucas\_Jackson](#): > > 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. 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. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 7, 2023, 8:44pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/3 "2023-06-07T20:44:46Z")
> [@Chronos](#): > > it’s just a matter of building things to survive an acceleration of a g or two 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? > [@Chronos](#): > > … and we have a lot of experience with that. What would be an example of that? ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [June 7, 2023, 8:52pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/4 "2023-06-07T20:52:06Z")
> [@Lucas\_Jackson](#): > > 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. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 7, 2023, 9:13pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/5 "2023-06-07T21:13:30Z")
Ah, that’s a fair point. > [@Darren\_Garrison](#): > > small-ass nuke Is that a technical term? ;~) ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [June 7, 2023, 9:57pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/6 "2023-06-07T21:57:34Z")
Sorry, typo. > **[Hyphen](https://xkcd.com/37/)** > > I do this constantly ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [June 7, 2023, 10:08pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/7 "2023-06-07T22:08:31Z")
> [@Lucas\_Jackson](#): > > 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. 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](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. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 7, 2023, 10:50pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/8 "2023-06-07T22:50:55Z")
Lots of interesting info in that article, thanks. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 7, 2023, 11:03pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/9 "2023-06-07T23:03:44Z")
> [@Darren\_Garrison](#): > > Sorry, typo. And right on cue, [this add popped up](https://s3.amazonaws.com/gs-geo-images/3a8e591e-43ae-49ae-9e8c-be4cd0d186a6.png). I don’t know what that is, but it looks for all the world like a small ass-nuke. ---
**Author:** ![ParallelLines](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/parallellines/32/3575_2.png) [@ParallelLines](https://boards.straightdope.com/u/ParallelLines)\ **Post date:** [June 7, 2023, 11:30pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/10 "2023-06-07T23:30:36Z")
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: > [@Alternate applications of nuclear energy?](https://boards.straightdope.com/t/alternate-applications-of-nuclear-energy/951975/12): > > Since Orion has already been mentioned, I’ll toss out the NERVA style rockets, for a less-crazy option for nuclear assisted heavy lift capacity. Which isn’t a completely dead technology (well, more modern derivatives of nuclear thrust rockets are still being researched at least) - from the link above. An engine for [interplanetary travel](https://en.wikipedia.org/wiki/Interplanetary_spaceflight) from Earth orbit to Mars orbit, and back, was studied in 2013 at the MSFC with a focus on nuclear thermal rocket (NTR) engines.[[116]](https://en.wikipedia.org/wiki/NERVA#cite_note-117) Since NTRs are at least… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 8, 2023, 2:26am UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/11 "2023-06-08T02:26:06Z")
> [@Lucas\_Jackson](#): > > 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?. 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). > [@Lucas\_Jackson](#): > > What would be an example of that? 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). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 8, 2023, 2:31am UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/12 "2023-06-08T02:31:33Z")
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. ---
**Author:** ![peccavi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peccavi/32/388_2.png) [@peccavi](https://boards.straightdope.com/u/peccavi)\ **Post date:** [June 8, 2023, 3:15am UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/13 "2023-06-08T03:15:41Z")
> [@Lucas\_Jackson](#): > > What would be an example of that? Human beings > **[G-Force](https://www.formula1-dictionary.net/g_force.html)** > > A physical force equivalent to one unit of gravity. Drivers experience severe g-forces as they corner, accelerate and brake. > Formula One drivers usually experience 5 _g_ while braking, 2 _g_ while accelerating, and 4 to 6 _g_ while cornering. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 8, 2023, 10:12pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/14 "2023-06-08T22:12:57Z")
> [@Chronos](#): > > 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). > [@Chronos](#): > > I’m currently in a structure designed to withstand 1 g. So are you, and everyone else reading this thread. 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. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [June 8, 2023, 10:39pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/15 "2023-06-08T22:39:29Z")
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 ---
**Author:** ![Ponderoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponderoid/32/19221_2.png) [@Ponderoid](https://boards.straightdope.com/u/Ponderoid)\ **Post date:** [June 9, 2023, 4:20pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/16 "2023-06-09T16:20:51Z")
Here’s a common visualization. BIG shock absorbers compared to the size of the vessel: > **[KSP Orion/Fireworks/Kraken drive 2](https://www.youtube.com/watch?v=Z-aRyBA1KkQ%2F)** > > This video shows a prototype of my stock orion drive (unlimited fireworks power glitch). The finished craft can be downloaded on KerbalX here:https://kerbalx... That one leaves out any FX from the bombs. They’ll explode at the point in the cycle when the giant shock absorbers are fully extended. ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [June 9, 2023, 4:44pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/17 "2023-06-09T16:44:36Z")
> [@Andy\_L](#): > > 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. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [June 9, 2023, 8:04pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/18 "2023-06-09T20:04:44Z")
Yeah. I should have mentioned that - “bigger the better” is a counterintuitive thing for space travel. P.S. In Pournelle and Niven’s Footfall, the experience of being in an Orion spaceship is described as “God was knocking, and he wanted in bad.” ---
**Author:** ![ParallelLines](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/parallellines/32/3575_2.png) [@ParallelLines](https://boards.straightdope.com/u/ParallelLines)\ **Post date:** [June 9, 2023, 9:29pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/19 "2023-06-09T21:29:08Z")
> [@Andy\_L](#): > > P.S. In Pournelle and Niven’s Footfall, the experience of being in an Orion spaceship is described as “God was knocking, and he wanted in bad.” 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. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [June 9, 2023, 9:34pm UTC](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161/20 "2023-06-09T21:34:06Z")
Outside of an alien invasion type emergency, an Orion craft will never lift from the surface of Earth under the Orion drive. Most likely, if we ever make such a thing, it will be built in space. [Next page](https://boards.straightdope.com/t/questions-about-project-orion-and-nuclear-propulsion/985161.md?page=2)