Orbital Base Jumping

[Tai]

The backstory to why I'm wondering this is a long story, which I'll share if people ask, but in short: I'm wondering the rough levels of DR/other advantages that'd be necessary to jump from orbit and survive re-entry with little to no ill effects. The drop afterwards and accurately landing are handled; I'm just unsure whether the re-entry itself would be plausible.

The two scenarios I'm looking at are

a) A pair of psychics in light spacesuits harnessed together, one providing the shielding and propulsion for re-entry, the other handling the landing after. How much DR would the shielder need to be able to provide, for roughly how long?

b) A TL9 battlesuit upgraded to the gills as the signature gear of a one-man-army type character - what DR would be necessary, and is it reasonable for it to be added onto such a suit? Also, would it be possible to 'add' ablative DR, or would it have to be built with it to begin with, thus meaning it'd only get one drop?


If I'm looking at it the entire wrong way with DR and there's another, better way to model being able to survive reentry, then please tell me that as well. Thanks in advance!

[Braun]

I think that GURPS: Traveller - Armed Forces has the "re-entry capsule" for an orbital insertion. Sorry I don't have my book right now.

[Agemegos]

For that matter Ultra-Tech has "life pods", "drop capsules", and "stealth capsules" (p.232). These have DR 100 ablative armour on the underside (which is noted as "all of which is usually gone after the re-entry") plus DR 20 for the composite body.

Well, strictly speaking it says that only of the "drop capsule", but the others had the same "100/20" listing for their DR.

I am a little puzzled. The description of the drop capsule says that it breaks up at an altitude of one mile. Surely the DR 20 from the composite body is just as gone after that happens.

With regard to the OP, I note that all of these gadgets use parachutes to enhance aerobraking. They do not kill orbital velocity with skin friction and ram pressure alone. I am far from assured that 100 DR ablative armour backed up by 20 DR is sufficient to survive re-entry.

[vicky_molokh]

Yeah, the idea of WH40k-style fast drop capsules is another reason to see the stats. Surprisingly, Spaceships is very vague regarding the damage levels of a 'rough' (i.e. non-engine-assisted, wingless etc.) reentry.

[Agemegos]

Originally Posted by vicky_molokh

Yeah, the idea of WH40k-style fast drop capsules is another reason to see the stats. Surprisingly, Spaceships is very vague regarding the damage levels of a 'rough' (i.e. non-engine-assisted, wingless etc.) reentry.

Well, that's likely because of two things. First, the physics of re-entry depend acutely on mass, cross-sectional area, streamlining, orbital speed, atmospheric density, local gravity, and angle of descent, in complicated ways hard to reduce to a gameable formula. Second, the results are poorly modelled by the GURPS damage system, which is based on almost-instantaneous penetrating injury, not sustained surface heating or ram pressure. The kind of thing that resists bullets well doesn't make a good re-entry shield, and vice versa. For that matter, the kind of thing that makes a good ablative heat shield for a steep re-entry might be no good for a re-entry only a few degrees shallower.

As a very rough approximation, a rough re-entry on Earth involves dissipating a minimum of 31 MJ per kilogram, which for a 70-kg man is 2.1 GJ. Modelled as an impact, that comes to about 5,275d + 1 Cr.

[Pomphis]

VE2 (for 3e) p.23 says "reentry vehicles should have DR 100+", the sidebar on page 164 gives a more complicated formula, depending on the velocity you have at start and the velocity you want to aerobrake to (a capsule typcally to 0, but something like a shuttle to it´s top air speed).

[Tai]

Some interesting replies already, thanks all! A little more info

a) WRT aerobraking - one of the psychics is a potent short-range teke, the other is a very talented water/fire elementalist. I had initially considered the teke to be doing all the shielding and the elementalist to pull condensation out of the air to form 'wings' to glide once within atmo. (Forcefield DR for the shielding, and Create Water followed by Control Water with the Wild enhancement to use Flight (Gliding, Winged).)

However, I did consider the elementalist might be able to provide some heat shielding - would Control Fire be any good, d'you think, and if so, what would, say, level 4 do to help the teke with shielding? (I figure it's a bit of a gray area - technically one could use Control Fire to -prevent- fire by keeping it down, and I'm presuming it might have some influence on temperature, I'm just not entirely sure how much)

b) About the velocity, assuming a psychically controlled (and thus reacting at the speed of thought) glider, what would be top safe speed to decelerate to before making any attempt at steering? Would the flier's advantages and skills have any noticeable bearing? (3D Spatial Sense would be the big one I'm pondering)

c) D'you think I ought to put a shark under them? (Orbital base jumping the shark, ha ha I'm so funny. */sarcasm*)


Also, no word on the big question about the battlesuit - would it be reasonable to put enough DR on that to survive re-entry, and if so would it be plausible to 'add' Ablative DR to an existing suit or would it need to be built into it?

[Braun]

The orbital re-entry capsule would be the ablative armor required. It is used up and needs to be replaced after each re-entry. But the Traveller marine would not be shooting out of this capsule, he is enclosed in it.

Guess I have to say "Depends on what you want. Starship Troopers (the book) or an Anime version battlesuit?"

[Tai]

Ah, hadn't realized that. My bad. Somehow I got it in my head that was for troop drops and didn't think 'well a battlesuit pilot is just a soldier in really big armor derp'.

I'd like him to be able to put some combat use out before he lands, but he wouldn't have to be firing from the moment the jump started - if the capsule breaks up a mile above the ground, that's plenty high enough for some light air support.

(This is initially for an NPC in a sci-fi game - he's the company's heavy hitter, deployed for the really big fights, but knowing my players if they see an NPC do something they'll want to do it too. :P)

[Pomphis]

Originally Posted by Tai

Also, no word on the big question about the battlesuit - would it be reasonable to put enough DR on that to survive re-entry, and if so would it be plausible to 'add' Ablative DR to an existing suit or would it need to be built into it?

You only need DR for aerobraking on the underside if the reentry vehicle is designed properly (like an apollo capsule). Given what a battlesuit looks like and that´t won´t be very stable in the air you probably have to protect all sides. So IMO a battlesuit should use some sort of drop capsule, which doesn´t have to be fully enclosed. More like an armored shield to stand or kneel on plus a cage structure with fins. Armoring the suit would probably add significantly more weight.