[Spaceships]/[UT] Launching life pods, drop capsules, and stealth capsules

[martinl]

Originally Posted by ericbsmith

I've thought on this and my take is that you aren't launching the drop capsules out actual "missile launchers," but rather that "drop capsule launchers" capable of launching a 1 ton capsules have the same stats as "missile launchers" capable of launching 1 ton missiles. The two types of launcher shouldn't be interchangeable.

You still have the acceleration limits to deal with.

Cybernetic re-enforcement might help.

Various smart armors/suits might be be able to selectively compress parts of the body.

The launcher might be pretty long, even the entire length of the ship + a centrifuge at the start, but that's still pretty limited.

There might be a disposable rocket on the non stealth systems.

Could the stealth pod use the planet's EM field to break more quietly?

[Darekun]

Originally Posted by Brett

Am I? Re-entry significantly faster than three hours or considerably sharper than 0.04 gee requires superscience? Snoman314 says that fifteen minutes and 8 gee was achieved by Mercury capsules.

That was in reply to "much more quickly and discreetly"; it looks like Mercury used thrust-braking, and was therefore horribly indiscreet. I guess you could use a "cold" reactionless engine to thrust-brake? It's still superscience to get both quick and discreet.

Originally Posted by Brett

Sure it does. I use engine braking by the ship to reduce the amount of aerobraking I have to do.

Ah, difference of perspective; I figured the launcher normally covered that. But with hangar launch, yes :J

Originally Posted by Brett

Not to me. Mercury, Gemini, Apollo, and Soyuz re-entries all involved decelerations way higher than 0.04 gee, and braking times far less than three hours.

Looks like Mercury used thrust-braking, can't tell for Soyuz, looks like Gemini used thrust-braking, can't tell for Apollo.

Originally Posted by Brett

Hang on. That entry is talking about using aerobraking to capture an interplanetary craft on a hyperbolic orbit, i.e significantly faster than escape velocity.

The significance is that it's talking about sufficiently-rapid aerobraking using real-world materials, which implies that a quick indiscreet reentry should be possible using only aerobraking. Looks like the Zonds were unmanned, though. It's possible that rate of aerobraking isn't considered human-safe, but at least in America there's accusations of space exploration being too conservative about human safety.

Originally Posted by martinl

You still have the acceleration limits to deal with.

Would crash tanks help, or are they flat-out too much mass? Would lend a neat "reverse nautical" feel to evacuation by life pods :J Might also be trouble to have them disintegrate around the troops…

[Agemegos]

Originally Posted by Darekun

That was in reply to "much more quickly and discreetly"; it looks like Mercury used thrust-braking, and was therefore horribly indiscreet.

Not so. Mercury used aerobraking. They had retro-rockets to take them out of LEO and put them on a re-entry trajectory, but nothing like the 150-ton Titan rockets they would have needed for thrust braking to land.

I guess you could use a "cold" reactionless engine to thrust-brake? It's still superscience to get both quick and discreet.

I am only asking to be discreet compared to circling the planet twice in a sheath of incandescent plasma. Not super-discreet. Just more discreet than that. The actual level of discretion and speed actually achieved by the actual Gemini re-entry capsule will do me fine.

Looks like Mercury used thrust-braking, can't tell for Soyuz, looks like Gemini used thrust-braking, can't tell for Apollo.

No. Mercury used retro-rockets to put it into a re-entry trajectory, but it used aerobraking with an ablative heat shield and parachutes to get rid of its orbital velocity. The retro-rockets each produced 1,000 lbf. for ten seconds, and there were three of them. That is enough for 330 feet per second delta-v given a Mercury spacecraft's 2,900-lb mass at the beginning of re-entry injection. You might get a fraction more thanks to Tsiolkovski, about 0.063 mi./sec., that the is still insignificant compared to the delta-v requirement of powered re-entry. Thrust braking to landing would have required 5 mi./sec. of delta-v, eighty times as much as the Mercury retro-rockets provided. Similar figures pertain to Gemini and Apollo re-entry capsules.

Voskod re-entry capsules had retro-rockets fit to give only 215 m/s delta-v. Voskod and Soyuz capsules slow first with a heat shield, then with parachutes, and fire their soft-landing rockets only when within a metre of the ground.

Engine braking from orbit to surface requires nearly as much engine and propellant as takeoff to orbit, and re-entry capsules didn't and don't have that by any means. They would have needed rockets the size of Titan IIs, which would have taken stupendous efforts to launch at the beginning.

It's possible that rate of aerobraking isn't considered human-safe, but at least in America there's accusations of space exploration being too conservative about human safety

I'm satisfied with the rate of aerobraking actually used in the Voskod, Soyuz, Mercury, Gemini, and Apollo re-entry vehicles. If astronauts can survive them, so can space marines.

[Agemegos]

Originally Posted by martinl

You still have the acceleration limits to deal with.

Cybernetic re-enforcement might help.

Various smart armors/suits might be be able to selectively compress parts of the body.

The launcher might be pretty long, even the entire length of the ship + a centrifuge at the start, but that's still pretty limited.

Well, it looks as though the Gemini craft used about 100 m/s delta-v to take them from a circular orbit to an aerobraking trajectory. And it was an aerobraking trajectory that suits my requirements for speed and discreteness. If we assume that space marines in combat suits and crash couches can tolerate eight gee for a short period during capsule launch, launch will take 1.25 seconds and 62.5 metres. 70 yards is the indicated length of an SM+9 (3,000-ton) frigate-sized spaceship.

If necessary, drop capsule launchers could include a spar that projected from the hull, mounting a linear accelerator. That might even be collapsible.

There might be a disposable rocket on the non stealth systems.

A cheap solid-fuel retrorocket for a de-orbit burn is certainly possible. And it would be suitable to the so-called "stealth" capsule, too. You would fire it for a few tens of seconds near your ship, where it would give itself away only to someone with a telescope or other suitable instruments trained on the ship anyway. That wouldn't give as much warning or create as much alarm as flying twice around the planet in a sheath of incandescent plasma. The stealth capsule isn't actually stealthy in the way of being difficult to observe. It screams its head off with ECM emissions, scatters decoys and flares, and probably fills the sky with chaff, too, besides being sheathed in incandescence plasma. It's "stealth" difference from the simple and cheaper drop capsule lies in being difficult to target (-5 to hit with homing missiles).

Could the stealth pod use the planet's EM field to break more quietly?

Well, the capability has been demonstrated experimentally. In 1993 NASA used a 20-km electrodynamic tether to de-orbit a spend Delta upper stage, and it worked fine. The tether was deployed for one orbit, which is to say about 80 to 90 minutes.

The thing is that a piddling 100 m/s trajectory change to de-orbit is not very conspicuous compared to screaming down through the atmosphere in either a sheath of incandescent plasma or on top of a pillar of rocket exhaust. Which means that you don't get true stealth in the sense of an undetected landing unless you also adopt a slow careful glide-in re-entry, shedding speed at high altitude, slowly and with low peak temperatures in a winged vessel. That means that you're very unlikely to be shot at by ground defences, and can actually perform covert and clandestine operations on the ground if it all goes right. But it means six hours or more de-orbiting and re-entering, which imposes operational limitations.

It sounds like a very useful capability to have up your sleeve, for inserting intelligence agents and special forces operators. I'll certainly add it to my Imperial marines' repertoire. But I think it requires something other than the drop capsule or even the stealth capsule described on UT p.232. And it doesn't directly address the question of how I design a ship to launch those.

But I think we're approaching an answer to that question.

1. Delta-v of only 1/16 mi./sec. should suffice to de-orbit a capsule for fifteen-minute re-entry.
2. A capsule launcher ~60m long will suffice.
3. A capsule launcher suitable for a 1-ton capsule might have stats and costs similar to those of a 32-cm missile launcher, but would be a different bit of kit.
4. A ship would need only 0.125 mi./sec. of delta-v to put itself on a suitable re-entry trajectory, launch capsules from a hangar bay, and then recover its orbit. A ten-second burn at one gee, followed by launch and another ten-second burn at one gee, would do the job.