Space - Crush Pressure

[munin]

deleted. deleted.

[Agemegos]

Originally Posted by cosmicfish

All of which is valid but almost all of which is beyond the resolution of most GURPS.

Maybe, but it can inform decisions about things that are beyond the scope of most GURPS. To wit, GURPS Spaceships doesn't say what crush resistance spaceships have, and there is no reason to suppose that most of them have any.

[Anthony]

Originally Posted by David Johnston2

Assuming that they are armoured to take missile hits they probably have decent compression resistance.

Unless you're dealing with an atmosphere capable ship, it's unlikely that armor is even sealed, you're generally concerned about objects on ballistic paths that won't wrap around a barrier, and many components in a ship prefer vacuum anyway. Explosions in space basically don't produce overpressure, they're nothing like explosions in atmosphere.

[cosmicfish]

Originally Posted by Brett

To wit, GURPS Spaceships doesn't say what crush resistance spaceships have, and there is no reason to suppose that most of them have any.

I would say that it certainly implies that most ships have at least some, simply by the observation that many of them are capable of atmospheric operations - that alone implies an ability to handle the range of pressures common to that type of craft, and for "reentry" vehicles that rely on friction braking to burn off orbital speeds those pressures get pretty high!

I would assume that "vacuum-only" ships that never land will have at least a modest external pressure resistance (at least a couple tenths of an atmosphere).

I would assume that most ships that enter atmosphere "gently" can handle at least a couple of times the "standard" atmosphere of the designing/using race. If your ship cannot handle 2-3 atmospheres, it is in danger landing on Earth! I would also apply this standard to internal sections of compartmentalized ships, even those built for space only - you might not be able to handle 1 atmosphere on a viewing port but compartmentalization means those internal barruers can take it.

I would assume that most ships that enter atmosphere violently can handle 10+ times the "standard" atmosphere mentioned above.

I would assume that any ship meant for atmospheric combat can handle the pressure indicated by its DR.

[Kimbo]

Originally Posted by vicky_molokh

Crush depth (in yards) = dDR × 150 / L.

Thanks! In my case dDR = 7 and L = 1.5 So crush pressure (in atmospheres) = 21 ?

And thanks to everyone else, for the informative discussion!

Originally Posted by cosmicfish

This is pretty complicated, and will vary between gas giants. If it is important to you, there are charts for some planets (Jupiter at least) that show experimental data. Personally, I would handwave it.

I did find some vertical pressure gradients for Jupiter and Saturn but no generic formulas that work for any gas giant. Glad to hear these are complicated and I wasn't failing my googlesearch-fu!

Originally Posted by cosmicfish

You could determine all this with some not trivial math, but is there a reason not to just handwave it? It is a lot of work for a very specific set of circumstances.

I will probably handwave it. I just wanted to have a reasonable, believably-close answer.

So, say the haze/opaque part of the gas giant's clouds is altitude zero. Is it reasonable to say that an atmospheric pressure of 21 will be reached within a negative altitude of, say, 130 km (80 miles) ?

Originally Posted by malloyd

Fall *from where*? A spacecraft in orbit won't fall, ever.

Fall from the cloud tops, just inside the atmosphere. Not an orbital trajectory, necessarily. Though I suppose the ship might have enough lateral velocity to be in a partial (degrading) orbit. But set that aside - assume the ship is hovering motionless when the contragrav partially malfunctions.

How do you incorporate drag from wind resistance when falling, to figure terminal velocity? Is it reasonable to just use the air-speed formula on page 35 of Spaceships, using an acceleration of 1.4G ? That yields a falling speed of 2958 mph. So the Mechanic has between 1-2 minutes before falling 80 miles and getting crushed, right? Tense.

Originally Posted by malloyd

But why the hell would you be hanging around in the atmosphere of a gas giant on contragravity lift anyway?

To flee from powerful enemies who cannot pursue you there. A sort of "out of the frying pan, into the fire . . . of hell" scenario.

Originally Posted by cosmicfish

I would say that it certainly implies that most ships have at least some, simply by the observation that many of them are capable of atmospheric operations - that alone implies an ability to handle the range of pressures common to that type of craft, and for "reentry" vehicles that rely on friction braking to burn off orbital speeds those pressures get pretty high!

I agree. This particular craft is designed to take off and land on terrestrial planets (hence the Contragrav Lifter), has armor for space and/or atmospheric combat, and hence is probably at a minimum designed for the highest atmospheric pressure one would likely find on a human-habitable, breathable, terrestrial world.


Thanks again everyone!

[malloyd]

Originally Posted by Kimbo

How do you incorporate drag from wind resistance when falling, to figure terminal velocity? Is it reasonable to just use the air-speed formula on page 35 of Spaceships, using an acceleration of 1.4G ? That yields a falling speed of 2958 mph. So the Mechanic has between 1-2 minutes before falling 80 miles and getting crushed, right? Tense.

Yes, but that air speed limit will vary with the density of the air. In this case you are going to need to correct for both the pressure and the average molecular weight of the atmosphere, both of which may vary over the course of the fall. But yeah, I'd expect an answer in the few handfuls of minutes range for pretty much any combination of variables here. Though I suppose at some point if your ship density is low enough and crush pressure is high enough you reach a level you float like a balloon rather than falling further.....

[Anthony]

Originally Posted by Kimbo

To flee from powerful enemies who cannot pursue you there. A sort of "out of the frying pan, into the fire . . . of hell" scenario.

An unfortunate issue here is that missiles are likely to be more pressure resistant than you are, and atmosphere generally enhances explosive effects.

[johndallman]

Originally Posted by Kimbo

Fall from the cloud tops, just inside the atmosphere. Not an orbital trajectory, necessarily. Though I suppose the ship might have enough lateral velocity to be in a partial (degrading) orbit. But set that aside - assume the ship is hovering motionless when the contragrav partially malfunctions.

Best to assume you have little lateral velocity. Being inside an atmosphere at a decent fraction of orbital velocity is called "re-entry" and is rather conspicuous, not to mention hard on hulls that aren't designed for it.

[roguebfl]

Originally Posted by Anthony

An unfortunate issue here is that missiles are likely to be more pressure resistant than you are, and atmosphere generally enhances explosive effects.

Um, actually unless your enemy is set up for orbital bombardment, missiles designed for space battles are probably going to be extremely degraded in effectiveness passing though an atmosphere, assuming reentry itself doesn't trigger them they going to be little better than smart bombs.

[sir_pudding]

Originally Posted by cosmicfish

(or part of the never-gonna-happen Vehicles 4E!)

Are you talking about Vehicle Design? Because it is still in progress.