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NASA's Project Orion created the concept of a spacecraft that detonated nuclear bombs behind it, and rid the shockwaves as a form of propulsion, but the project was abandoned because nuclear bombs are dangerous and there is no need for something that powerful.

However, let's say that today, an apocalypse scenario was predicted in 5 years that would wide destroy all life on earth, maybe a meteor or a supernova or something.

With our current technology + 5 years of research and development, could we use Project Orion's concept to build city-sized spacecraft and evacuate the entirety of humanity from earth?

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  • $\begingroup$ ... you terrify me! $\endgroup$ – Cort Ammon Aug 20 '15 at 1:23
  • $\begingroup$ I think it would be easier to move the asteroid. $\endgroup$ – JDługosz Aug 20 '15 at 1:27
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    $\begingroup$ Best solution: build an ORION spaceship to move the asteroid, mine the asteroid and create a screen in space to deflect radiation from nearby supernovas... $\endgroup$ – Thucydides Aug 20 '15 at 3:35
  • $\begingroup$ First lets try to launch the Empire State Building(~370,000,000kg) with ideal specific impulse of 100,000s. Change of velocity(dV) needed for object to leave surface to achieve orbit is 8600m/s, so I calculated that the total mass of your "fuel" is ~366,700,000kg! are you still planning to launch your city? $\endgroup$ – user6760 Aug 20 '15 at 5:32
  • $\begingroup$ Oops I've made a mistake in my calculation the specific impulse used for Project Orion is 2000s but still a lot more powerful than chemical rocket(300s). My earlier approach uses Hydrogen-Boron fusion(99,898s) and consume 0.89% of Empire State Building's mass to reach orbit and I found out that using antimatter collision(10,193,680s) you only need 0.0086% of said building's mass it's super effective! wait I need to find 183,350 ton of antimatter WTF! $\endgroup$ – user6760 Aug 20 '15 at 7:51
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Given the very short timelines, I will say "no".

The largest Orion craft under serious study were 4000 ton behemoths for flights to Mars, Saturn or to create "Space Battleships" to dominate the Earth, and as we all know, no actual hardware was ever made for these. More "serious" studies were done for much smaller ships capable of being lofted into orbit on Saturn V boosters (i.e. much more detailed studies) but even than no metal was actually bent.

This means that the actual parameters of some of the most critical aspects of ORION, like the mechanisms of the shock absorbers are relatively unknown. The size of the shock absorbers for a 4000 ton ORION is the size of a building, and no one to my knowledge has ever built something like that, much less tested it under extreme conditions (nuclear shockwaves, the vacuum of space), so how they would perform is unknown. The secondary shock absorbers are also a bit of a mystery (I am a bit dubious of a stack of ring shaped airbags as the secondary unit), so much of the R&D will be to actually test these 50 year old concepts.

Now it is possible that there are new technologies that would work "better" (I am picturing something like a giant solenoid for the shock absorber unit), but now we are talking about adapting a different technology for very extreme conditions.

And of course no one ever built or tested the drive plate "for real". I think the only thing which was ever physically tested were concept models of the "physics package" which would power the ship, mostly to determine if such small devices would have other sorts of utility (nuclear artillery shells, the ignition units for very small thermonuclear weapons, nuclear shaped charges, etc.)

A five year crash program would probably be enough to build ORION ships of modest size, perhaps similar to the ones planned for liftoff to orbit via Saturn V booster (even if you launched them from the ground), since the size of the components would be reasonable to design, build and test with existing infrastructure. A 4000 ton ORION might be possible at the outside edge of possibility, but multi million ton vessels like the ones Freeman Dyson postulated as starships when calculating the "ultimate" possible ORIONs would be far beyond us even today.

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  • $\begingroup$ Small scale, (non-nuclear, RDX) explosive plate drives were tested and proved the basic concept of an explosive pulse drive. youtube.com/watch?v=E3Lxx2VAYi8 $\endgroup$ – Russell Borogove Aug 20 '15 at 5:16
  • $\begingroup$ In Larry Niven's and Jerry Pournelles Footfall part of the story tells of how a large dome is propelled into space by nuclear explosions (based on Project Orion). They build a dome under which they set off a nuclear bomb which shoots the dome into space. On the top side of the dome are other smaller spacecraft that fly off, once in space. Its not a city, and I don't know what the dome was made off that it could resist being destroyed by the blast, but Larry Niven and Jerry Pournelle thought it was possible(?) $\endgroup$ – Len Feb 1 '18 at 20:00
  • $\begingroup$ My reading of the Battleship Micheal is a bit different, there is a normal ORION drive plate covering the bottom of the dome, and my impression was the magazine of pulse units was in the dome. Of course the writers didn't go into a lot of detail, so your interpretation may be correct. However, actual ORION studies suggested a hemispherical drive plate would absorb too much energy and melt, so flat drive plates are the feature of every serious "real life" design, for example: projectrho.com/public_html/rocket/realdesigns2.php#orionbomb $\endgroup$ – Thucydides Feb 2 '18 at 4:16
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Short answer: Yes, but I don't think you'd want to.

Long answer: Nukes would be cheaper so it's feasible, but nukes can be significantly more dangerous.

Wikipedia actually has a calculation for something along these lines: (Paraphrasing)

With the "Momentum Limited" Orion, a 50,000,000 kg payload could be brought from Earth to Alpha Centauri and would cost around \$367 Billion USD. That's about $7,340/kg.

Source: Wikipedia: Project Orion

The SpaceX Falcon 9 rocket can put 4,850 kg into Geosynchronous Transfer Orbit (it's a cakewalk after getting to GTO) for \$61.2 million. That's about $12,618/kg. Add an additional chunk to get us further into space and we're even more expensive (I can't give an exact figure).

The real problem is that nukes give off loads of radiation. And you can't really get much away from it. You could shield the effect with lead, but the more lead, the more it costs, and the less you could bring up with you. Traditional rockets don't have this issue.

Additionally, traditional rockets have to be made small just due to the cost of leaving Earth. The Orion craft mentioned above from Wikipedia, you'd be putting a lot of your eggs in a single basket. Specially 10,309 eggs for each traditional rocket into a single orion basket.

Most rockets have a 98%+ success rate. For the same weight in the orion ship, you'd have around 10,309 traditional ships. Around 206 would fail (around 1,000,000 kg). At an average of ~80.7kg/person, at most that's 12,391 lives lost (likely less due to luggage)

If the orion craft breaks, you'd lose all 50,000,000 kg. At most, that's 619,578 lives lost. That's quite a lot of lives.

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In addition to the size problem that Thucydides brings up there's also the fact that the bigger something is the longer it takes to construct. Even if they were handed known-good plans for the city-sized craft that doesn't mean they would go that route.

Besides, this is the sort of problem where you try every approach and hope something works. You're not going to build one big craft, you're going to build a bunch of small craft. They may dock together during the freefall part of their voyage but they'll boost separately.

I don't think we have anything like the bombs to lift Earth's entire population anyway. At best you'll get enough off to form a colony.

Given either of the threats you name I don't think the right answer is evacuation, anyway.

1) Supernova. Looking at the sky we don't find any prospects for a star that could actually fry the surface of the Earth. The threat is frying the ozone layer and ending up with really brutal UV for some years. Survival consists of sunshades and greenhouses. Even if there is something that can fry the surface every 3' of dirt provides 1000x attenuation. There are houses today that provide that kind of shielding--not preppers but rather surrounding the house with enough thermal mass that you have basically no need of heat or AC.

2) Meteor. Here things get nastier. The area for hundreds of miles around the impact would have to be evacuated. Beyond that you need protection from the fires and the splash--the same houses I mentioned in the supernova case with some filters and bottled air would do the job.

If the impactor is really big (large asteroid size) evacuation would be the only option, though.

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