Monday, January 01, 2007

Combat resupply and rescue

I'm not a military guy, I don't know much about how they do things. But I have read Blackhawk Down, and I have some sense that a casualty is a much bigger problem than one guy getting shot. If there is no well-linked rear to which to send a casualty, a fire team has a huge liability. I think the usual rule is that one casualty effectively soaks up four people. It reduces the fire team's mobility and effectiveness, and can rapidly send a mission down a cascade of further problems. So, I got to thinking about how you could improve combat rescue.

Let's assume you control the airspace about the battlefield, or at least the portion of it above small-arms range. Helicopters work pretty well when you want to insert your fire teams, because folks near the target can often be taken by surprise and the choppers can dump their loads and be off before serious resistance is organized. But helicopters are not a good way to get people back out, because they move slowly near the landing zone and are thus pretty easy targets. What you need, getting out, is a lot of acceleration and altitude, right away. You want a rocket.

The wounded guy goes into a stretcher. I'm imagining something like a full-body inflatable splint: it squeezes the hell out of him, totally immobilizing him, and insulating him from cold and wind. You'd design the thing so that it could be popped in a couple of places and still work fine. The stretcher gets attached to a rope attached to a reel at the bottom of the rocket.

The rocket fires a very short exhaust pulse, which sends the thing up 50 feet or so. At this point the rope is entirely unreeled. When the rope goes taut, the main burn starts, accelerating the stretcher at, say, 5G straight up. The exhaust plume is directed out two symmetrical nozzles slightly away from straight down so that the poor guy at the bottom doesn't get burned. Acceleration drops to 1G for ten seconds or so once the guy is at a few hundred miles per hour, and then cuts out. The rocket coasts to a stop at 10,000 feet or so, at which point a parasail pops out.

At this point an autopilot yanking on the control lines can fly the guy ten miles away to get picked up on the ground, or a helicopter or C-130 can grab him out of midair. A midair grab sounds ridiculous but apparently they already use this technique for recovering deorbited film capsules and they haven't dropped any yet. A midair pickup at 2000 feet would have 8 minutes to snatch a guy falling from 10,000 feet at 16 feet/second, which seems plausible with good communication.

[Update: apparently they already use midair grabs for picking up people, too. They use a helium balloon and snag that. The trouble is that when they winch the guy in, he generally spins around in the vortex behind the airplane, and when he gets to the tail of the airplane he can get bashed against the fuselage a fair bit before they get him inside.]

A rocket sufficient to boost 300 lbs of payload to 3200 meters needs about 300 m/s delta-V. With a mass ratio of 80% and an Ve of 2600 m/s, the rocket will weigh 120 pounds. That's not something you want to be carrying around with you, but it is something that one guy can manhandle into an upright position. So you have to deliver this heavy, bulky thing to a fire team in the middle of a combat zone which is already distracted by tending to some casualties. Luckily, you can make the rocket pretty tough.

I suggest dropping the recovery package (ascent rocket, stretcher, medical kit, ammunition) on the fire team as you might drop a smart bomb. Instead of exploding a warhead, this munition pops a parachute or fires a retrorocket right before impact to minimize the damage to whatever it hits and cushion the blow to the medical kit. Someone on the fire team might use a laser designator to pick the landing spot, so that they have good control over the difficulty of recovering the thing. You'd want to be careful: bomb there, recovery kit here.

I posted about this three years ago in this thread: http://groups-beta.google.com/group/sci.space.tech/browse_thread/thread/efb906c8dd19915a/a355a9c6b2ed55f5?hl=en
Back then I thought you needed the robot paraglider to deliver the recovery package. Now I suspect something more like the smart bombs we already have would be okay.