Space - Crush Pressure

[Kimbo]

Two related questions:

(1) What is the crush pressure of a space ship?
and
(2) At what depth inside a gas giant, would that atmospheric pressure be reached?

Taken together, these two questions become: How far down inside a gas giant can I fly my spaceship before it implodes like a sinking-submarine in a WWII movie?

I realize this question might be highly hypothetical and thus arbitrary. "A spaceship can withstand as much pressure as it is designed to withstand. If its specs say 50 atmospheres, then it is good to 50." Okay, but what is a believable, reasonable, number for that upper pressure (crush limit)?

Probably, the answer to this question will be complex - an equation of multiple variables. For instance, Tech Level probably plays a role. (The Millenium Falcon probably can withstand higher pressures than our 1970's Apollo spaceships). What else is part of the equation? Size? Armor? Shape (Streamlined vs. Not)?

The second part of the question also most certainly depends on variables - the size, diameter, mass, and density of the particular Gas Giant. A generic formula works fine, but in this case, the specific gas giant I am considering is roughly 5 times the size of Jupiter, or a mass 1650xEarth, with Diameter 14xEarth, and Surface Gravity of 8.4G.

Lastly, a third related question: (3) Supposing an acceleration of 1.4G on a streamlined hull, how long (assume terminal velocity) would it take to fall to this hypothetical crush depth?

That is, suppose a 10G Contragravity Lifter malfunctioned, or was only working at 70% efficiency (only canceling 7G above a world with 8.4G, thus net downward force of 1.4G), how long would the ship's Mechanic have (while the ship is falling) to fix the Contragravity Lifter before the atmospheric pressure crushes them? Seconds? Minutes?

Thanks in advance!

[vicky_molokh]

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

dDR is precisely that, while L is the length of the hull in hundreds of feet.

That's for water under Terran conditions.

Divide crush depth by 33 to get pressure in atmospheres.

[munin]

deleted. deleted.

[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.

[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.

[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.

[cosmicfish]

Originally Posted by Kimbo

(1) What is the crush pressure of a space ship?

To paraphrase Futurama, "Somewhere between zero and one!" Seriously, vicky has the formula from Vehicles (I think!) and that will tell you for more rugged and futuristic ships.

Originally Posted by Kimbo

(2) At what depth inside a gas giant, would that atmospheric pressure be reached?

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.

Originally Posted by Kimbo

(3) Supposing an acceleration of 1.4G on a streamlined hull, how long (assume terminal velocity) would it take to fall to this hypothetical crush depth?

This is also depends on the density (not just strength) of the vessel in question - the ship has to be dense enough to sink in that high-pressure atmosphere!

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.

[malloyd]

Originally Posted by Kimbo

I realize this question might be highly hypothetical and thus arbitrary. "A spaceship can withstand as much pressure as it is designed to withstand. If its specs say 50 atmospheres, then it is good to 50." Okay, but what is a believable, reasonable, number for that upper pressure (crush limit)?

Honestly, not much. Maybe not even 1. In most settings spaceships aren't *supposed* to be diving into oceans or gas giants, they're for flying around in space. There's no reason to design them for more than highest pressure atmosphere they will encounter minus the interior pressure.

(The Millenium Falcon probably can withstand higher pressures than our 1970's Apollo spaceships).

Actually, I bet Apollo will withstand more. It's much more symmetric, and designed to take pretty substantial reentry stresses the Falcon would never need to worry about.

Lastly, a third related question: (3) Supposing an acceleration of 1.4G on a streamlined hull, how long (assume terminal velocity) would it take to fall to this hypothetical crush depth?

Fall *from where*? A spacecraft in orbit won't fall, ever. A vessel on contragravity lift just above its crush depth when the generator goes out is doomed in seconds. But why the hell would you be hanging around in the atmosphere of a gas giant on contragravity lift anyway?

That is, suppose a 10G Contragravity Lifter malfunctioned, or was only working at 70% efficiency (only canceling 7G above a world with 8.4G, thus net downward force of 1.4G), how long would the ship's Mechanic have (while the ship is falling) to fix the Contragravity Lifter before the atmospheric pressure crushes them?

If you are far enough down in the atmosphere of a gas giant to be feeling 8.4 G, you're way past the crush depth of anything not designed as a dedicated gas giant probe *already*, and were crushed well before the contragravity problem came up.