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Old 02-26-2017, 07:16 PM   #21
sjard
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Default Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry

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Old 02-27-2017, 07:24 AM   #22
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Default Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry

Looks like my question generated a little ... ahem ... friction and the atmosphere has become a bit heated. That wasn't my intention. Some of the math provided is very interesting (I'll save it for future reference) but a bit complex for gaming. I think I'll stick with minimums of 10dDR on the leading edge and 2dDR for other hull sections as a general rule-of-thumb. (Had to tweak my cargo lighter design by reducing the control room size to make room for more armor. |:-/ )

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Old 02-27-2017, 11:54 AM   #23
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Default Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry

No idea why the math would be too much for gaming. It's not like it's rocket sci...oh, um...

I'd probably just go with a Soft Landing Module for simplicity's, because wouldn't even straightforward atmospheric shielding need to be replaced each time anyway?

If you want fiddly bits for in setting crunch, demand that each such module be specialized for an atmospheric density and gravity range.
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Old 02-27-2017, 12:34 PM   #24
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Default Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry

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Originally Posted by Flyndaran View Post
I'd probably just go with a Soft Landing Module for simplicity's, because wouldn't even straightforward atmospheric shielding need to be replaced each time anyway?
I wouldn't think that's necessarily the case. If you've got minimally ablative reentry insulation and (like most fictional spaceships) are operating on generous enough margins to overspend mass here and there, I don't see anything stopping you from having a reentry shield that is good for multiple uses. Unlimited use without repair might be impossible.
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Originally Posted by Flyndaran View Post
If you want fiddly bits for in setting crunch, demand that each such module be specialized for an atmospheric density and gravity range.
You could probably stretch that considerably depending on how much control of the landing profile you have.

Or rather, the reentry shield part can be used in a very wide range of conditions with good trajectory selection. The part that softens your actual landing is a bit more specific.
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Old 02-27-2017, 01:05 PM   #25
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Default Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry

Quote:
Originally Posted by DaltonS View Post
Looks like my question generated a little ... ahem ... friction and the atmosphere has become a bit heated. That wasn't my intention. Some of the math provided is very interesting (I'll save it for future reference) but a bit complex for gaming. I think I'll stick with minimums of 10dDR on the leading edge and 2dDR for other hull sections as a general rule-of-thumb. (Had to tweak my cargo lighter design by reducing the control room size to make room for more armor. |:-/ )

Dalton “who'd still like an aerobraking equation Spence
TL:DR version: Aerobraking is complex, and probably too complex for the abstract system in Spaceships to tackle beyond saying "buy a softlanding system" and assuming that system includes the design features needed to survive aerobraking from orbit to surface.

In reality, DR doesn't represent a vessel's ability to survive aerobraking and (re)entry. The Space Shuttle's Thermal Protection System (TPS) was actually very delicate. The tiles were made of foamed silica, and you could scratch them with your fingernail. The nose cone and wing leading edges where carbon-carbon, and that part was damaged catastrophically by a piece of foam once. The top and sides of the Shuttle were covered with fabric.

It's more about moving the keeping the hot air away from your body to reduce heating, and being able to deal with the heating you do experience. How much heating takes place can vary A LOT depending on circumstance.

From my experience with KSP, aerobraking from a low energy transfer orbit (e just about 1) to a capture orbit (e just below 1) doesn't take a lot of delta-v, and even today's space probes can do that just be grazing the top of the atmosphere. Then they can keep grazing to circularize, doing a little each pass.

The big deal is slowing from orbit (or faster) to hit the surface. From LEO, that means getting rid of 8 km/s of velocity. Not a mean feat, but there's more to surviving that just lots of DR. (Apollo had to dump 11 km/s, and only got one pass. Galileo hit Jupiter's atmo at 47.8 km/s and lived (for almost an hour), but it went through 300 gees deceleration). And most of it is beyond the scope of an abstract system, beyond giving it a softlanding system, and saying that that system includes all the design considerations and equipment needed to survive the deorbit.

Now Lithobraking...that's another matter. ;-)
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Old 02-27-2017, 01:39 PM   #26
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Default Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry

As far as aerobraking being hard, though, what matters isn't really the delta-V involved, it's the total energy you're dissipating. A vehicle aerobraking is dissipating energy at a rate of force * velocity.

Let's say a spaceship's approach velocity towards Earth started at 3 km/s, which is a kinetic energy of 4.5MJ/kg. At the top of atmosphere, it has gained another 62.5 MJ/kg from potential energy, so its total energy is 67 MJ/kg for a velocity of 11.57 km/s. To be captured, its energy needs to drop below escape energy (62.5 MJ/kg), so we need to dissipate 4.5 MJ/kg and about 400 meters per second.

Now, the big trick here is that atmospheric density varies substantially with altitude, and our altitude during this pass will vary a lot. You can reasonably approximate this pass as if we were falling 'up', so at 10s away from apogee you have gained 500 meters; as the scale height of the atmosphere is about 8 km, we can figure our total time in atmosphere as if it were about 80 seconds (it's actually longer than that but a lot of it is in very thin atmosphere), during which time we need to shed 400 m/s and 4.5 MJ/kg. 400m/s in 80s isn't hard (it means a peak deceleration of about half a G), the challenge is the 4.5MJ/kg. If we assume we have a 10 ton probe that is a 4 meter diameter sphere (33 cubic meter volume), it has a surface area of 50 m^2, so we're looking at 45000 MJ/50 m^2 = 900 MJ/m^2 (it's actually uneven heating, max is about 4x that).

It takes about 7.5 MJ to ablate a kilogram of steel, so if we just ablated away it would be 120 kg/m^2 (about 15mm), or a total of 6 tons of steel gone. Since we're only a 10 ton craft and we still have another 625 GJ of orbital energy we want to get rid of, that's not really a viable option. Thus, we have to figure out how to resist it without ablation.

Our peak energy flow rate is 900/80 = 11.25MW/m^2. An idealized blackbody sheds heat at a rate of 5.67e-8 W*m^-2*K^-4, which we can solve for temperature, T(K) = (1.125e+7/5.67e-8)^0.25, or 3750K (and as noted, it's not going to be evenly spread, so it actually reaches 5300K on the leading edge). That's higher than any realistic material can withstand, but it's not dramatically higher; it we can find some trick that causes heat to be radiated outwards without ever striking the craft, we should be okay.

At that point, the surface of the craft isn't ablating -- and we no longer need armor at all, we just need a framework that can hold our shielding in place without buckling, plus insulation. This roughly describes the way the space shuttle's tiles work -- while they would likely stop attacks for a little while, they aren't really armor as such.
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