Apologies if this is the incorrect forum...

Basically, the key characteristic of my main character’s ship is that, whilst being the smallest of its class, it is the largest ship ever built that can make a rapid, emergency decent into atmosphere to facilitate mass evacuation.

My question is about the limitations on scale. How large can my craft be before the destruction it causes on approach completely undermines the goal of saving everyone?

To clarify, this is Not a question about how it’s propulsion systems effect the environment around it but a question of Size and Speed.

For the sake of the argument we can just say the planet in question is earth, that way we’re using familiar variables.


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    $\begingroup$ "How large" or "how fast"? $\endgroup$
    – Alexander
    Jan 19 at 19:47
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    $\begingroup$ Given that the ship has to soft land (or everyone onboard dies) then the propulsion system is pretty much the most important part of the question. If you have magically safe reactionless drives, then all that's at risk is whatever is directly underneath the ship. If it is going fast enough, it'll explode in an airburst and won't be much risk to stuff on the surface. We're going to need more details here! $\endgroup$ Jan 19 at 20:07
  • $\begingroup$ Basically I’m thinking of a ship that is tens of kilometres long entering the planets atmosphere and getting down to sea level as fast as possible (without touching the ground). I assume that the base of the craft shoving its equivalent volume of air out of the way at speed with cause chaos. Also I don’t know the speed limits to avoid things like plasma on the ship’s surface still being there as it arrives on the ground. Is the rule simply that, if the object is HUGE it must travel Slowly to avoid chaos? And if so, what happens when you just go full speed? $\endgroup$ Jan 19 at 21:10
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    $\begingroup$ Agree more detail is needed. How large and how fast are two separate questions. And the destruction potential is related to both. Size is irrelevant with slow enough speed, and vice versa. One or the other will need to be specified before the other can be worked out. Personally, I really hope this question gets an update, as it's one I personally find very interesting. As the question was originally posed, it would seem that speed is the one that should be specified, and allow answerers to determine the size from there. $\endgroup$
    – Harthag
    Jan 19 at 23:08
  • $\begingroup$ Note that it's not just size and speed - shape is going to matter a lot too. If we are magiking drives out of the way, then displacing air is going to be the main problem. Ideally the ship would be designed such that almost all of the air would be able to go through it rather than around it. $\endgroup$
    – Gene
    Jan 19 at 23:51

1 Answer 1


Air displacement is the bigger problem, not speed. That displaced air must go someplace, and we call that air movement "wind". Humans regularly see large amounts of high wind in a hurricane, so you're likely to see severe-hurricane-like devastation in the area for tens of kilometers around the ship's landing area. And lesser damage farther away.

Higher speed in the lower atmosphere will make the zone of devastation wider.

Ionization (plasma) during high-speed descent through the upper atmosphere will look spectacular, but won't have any other significant effect on folks on the ground.

If the ship approaches horizontally (aerobraking through the atmosphere instead of vertical like a returning rocket), then the zone of devastation will include several hundred kilometers of the approach path.


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