This is for a base that has been built up into a big, complex place. For the radiation safety of long-term residents, the walls are very thick. There are windows, that are quite large. They have quite strong glass, reinforced and double glazed with about half a meter between the panes, so the outer layer acts as Whipple shielding against meteoroids. The rockets are nuclear but their fuel is hydrogen, so there isn't radiation exposure from them. Still, I put the nearest landing pad half a kilometer away in case of explosions. But I wonder if I'm being too conservative.

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The ships do use liquid oxygen in an afterburner design, it's injected into the nozzle just after the throat so it's never exposed to the nuclear reactor. So the ships are carrying LOX and liquid hydrogen, but they are in separate tanks, in a vacuum. (The LOX is in the smaller tank sticking out near the bottom, the LH2 is in the larger tank to the left of it.) Does the low explosion potential of that arrangement allow the launch pad to possibly be even closer? Or does prudence say just never do that? These ships fly themselves - to hit other structures they'd have to be quite off their final approach and their emergency systems would have to all fail. (I don't know that 'self-destruct' is the way to go in this case. Maybe...)

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    $\begingroup$ You assume that the explosion happens on a plane on which you've build the base as well. However geography is a huge factor. If the settlement is in a lava tube or in a crater wall, the rocket could be placed over a ridge and wouldn't do much damage in any case. Distance won't do much against an air burst, especially as all the debris are on pure ballistic trajectories. Point Defense cannons are probably a good idea in any case. $\endgroup$ Commented Sep 17, 2021 at 17:45
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    $\begingroup$ Related on our sister site Space Exploration. $\endgroup$ Commented Sep 17, 2021 at 17:46
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    $\begingroup$ @TheDyingOfLight I just considered it from the perspective of the closest point at which something could go drastically wrong. This is for a game, this decision is for convenience of good visuals and game navigation. I just wonder how reasonable it is. $\endgroup$
    – kim holder
    Commented Sep 17, 2021 at 18:04
  • $\begingroup$ @TheDyingOfLight I like the idea of point defense cannons... although I also think about the relative risk of having cannons around, versus the risk of a launch or landing failure leading to large impacts. $\endgroup$
    – kim holder
    Commented Sep 17, 2021 at 18:29
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    $\begingroup$ @ChristopherJamesHuff I would need a reference for this. I've been told by people I feel know what they are doing that this system will work fine. The LOX is an afterburner system called (LANTR)[large.stanford.edu/courses/2017/ph241/mangram2/docs/…. It raises thrust and allows the use of a smaller reactor. The extra shielding mass is supposed to be feasible. The base is capable of handling the kinds of failures you mention. I'd have to dig into my references to say more. $\endgroup$
    – kim holder
    Commented Sep 18, 2021 at 2:14

2 Answers 2


The velocity of a detonation wave in a suitable hydrogen-oxygen mix is probably not going to be higher than about 2km/s for a well mixed cloud with ideal fuel-oxideser proportions, and what you'll be dealing with is not going to be a well mixed cloud. Lets take that as a worst case figure.

Bits of debris are not likely to come firing out of the blast any faster than that. The blast is going to be approximately spherical, which means that most of the shrapnel will not be heading towards your viewpoint. It'll arrive as a cloud of irregularly shaped chunks of lightweight metal and composites.

This compares favourably with the speed that you might be expecting meteor strikes to have, which could come in a lot faster than that... up to 72km/s, in the extreme.

The minimum safe separation distance, then, is a function of how much energy your windows can absorb per unit area, and some model of the most dangerous bits of debris coming out of the explosion. That seems entirely too hard to work out, as far as I'm concerned, but with suitably over-engineered windows you could handwave almost any distance away.

does prudence say just never do that?

There are good reasons to be cautious here. You're flying a nuclear reactor strapped to a bomb in front of a viewing deck.

Personally, I'd consider being about 1km away from a debris cloud propagating at 2km/s to be an OK distance. Consider that you have half a second to activate emergency protective systems... a set of safety radars might be able to trigger an explosively-closed blast-shutter fairly promptly, throwing up a curtain of rock and metal that could be as over-engineered as you saw fit.

These ships fly themselves - to hit other structures they'd have to be quite off their final approach and their emergency systems would have to all fail. (I don't know that 'self-destruct' is the way to go in this case. Maybe...)

One possible scenario here is an uncontrolled descent onto the spaceport at de-orbit velocities, which is also going to be at about 2km/s, only there's going to be just one big lump of debris instead of an expanding cloud most of which will miss you.

The roof of the any of the buildings on the surface will probably be quite thick! You might consider that point defense systems (that after all, might be powerful fast tracking laser weapons that could present a serious threat to anything hundreds or even thousands of kilometres away) or range safety systems that would allow someone to remotely destroy a ship, problematic for other reasons (especially for the owners and any passengers!) but then you're in the territory of viewing areas being too risky to allow.

There's also an additional problem you may want to consider.

A bang which throws up debris at 1-2km/s in a vacuum is going to throw that stuff a long, long way. A lot of it is going up and out, and it could pose a serious threat to anything in orbit. That's a reason why you have landing pads by the way, instead of landing on unimproved regolith, because otherwise the debris kicked up by your landing burn could trash the person behind you in your original orbit...

  • $\begingroup$ This is a really good answer. It now puts me in a dilemma, as I never considered the 'spaceship crashes into roof' scenario. I can now move all the launchpads several kilometers away and hope for the best, or redesign the habitats. $\endgroup$
    – kim holder
    Commented Sep 17, 2021 at 20:13
  • $\begingroup$ I've moved the pads. But I'm thinking of putting a 'crumple zone' on the roof of the hab, which is the vulnerable portion. If that can absorb enough of the initial impact, the rest of the hab can remain unchanged. $\endgroup$
    – kim holder
    Commented Sep 17, 2021 at 20:57
  • $\begingroup$ If you want them nearby, it will be a balance between safety, and logistics; while putting them on the opposite side of the moon would be safest, you need to get people, supplies, fuel, air etc. to and from the launch pad. Maybe put habitation somewhere safer and have a viewing area as some sort of command deck? $\endgroup$
    – JeffUK
    Commented Sep 17, 2021 at 22:56

"This is for a game, this decision is for convenience of good visuals and game navigation."

I found that in the comments. That is key! Leave the pad where it is. Then when something blows up the players can see. When it blows up real good it will mess with the base, knock stuff over, hole the windows etc. Best - the players will see the flash and then see and hear the rain of stuff coming across and down. No shockwave though being as it is the moon.

Stuff blowing up is one of the main reasons to play a game! If nothing blows up you might as well stick to cribbage.

I love the idea of the crumple zone on the roof too. Because a player might then survive having something big come down on the roof, then go out and survey the crumplage. That is a French word I am pretty sure.

  • $\begingroup$ Clearly I have very mixed feelings about that because high realism is the main point of the game. So, I end up only being willing to accept unrealistic design to a small degree. Right now, I've moved the pads, but the crumple stuff is way too complicated and overkill and I abandoned that. $\endgroup$
    – kim holder
    Commented Sep 18, 2021 at 1:09
  • $\begingroup$ Sounds like your cribbage game needs some house rules. $\endgroup$ Commented Sep 18, 2021 at 13:47

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