Wednesday, March 08, 2006

Space Travel for Dummies, Part 4


Now, we come to the really hard part. It's worth mentioning that while nobody has actually ever died IN space, a total of nineteen have cashed in their chips going and coming. Breaking that down, it's seven on the way out, twelve on the way back. It's a very, very tricky thing to do, and it's easy to find yourself well and truly up the river without a paddle.

There's an odd little factoid about the first Space Shuttle mission that I find amusing. Both John Young and Bob Crippen were wired with medical sensors, recording heart rate and other parameters. Crippen, the rookie, recorded his highest heart rate during the launch. He wasn't too excited about re-entry and landing. Young, the veteran, took launch in stride. His pucker factor soared to its heights during re-entry. Obviously, he knew something Crippen didn't.

In short, it's the launch problem in reverse. You need to lose 18,000 MPH of speed and a couple of hundred miles of altitude without getting anything important broken, bent, or burned.

Easy to say, harder to do.

The first part is the easy part: you need to adjust your orbit so that the low part dips back down into the atmosphere. It doesn't take all that much fuel. You tap your maneuvering thrusters for about a minute or so, firing the jets out in front, soaking up a little of your orbital speed. Then, you start to descend. If there were no atmosphere present, your lowest approach would be about fifty miles up. But, there are a few billion air molecules standing in your way...

The basic idea is that you use friction and pressure drag to slow your vehicle down to a more manageable velocity, so that you don't have to use fuel to do the job. The problem is that your vehicle can only stand so much deceleration stress and thermal heating. There's a fairly narrow corridor, expressed as an altitude-versus-speed plot, that you can safely fly in. Too high, and you don't get enough deceleration, and skip back out into space. Too low, and it's a race between thermal stress and G-loading. One of them's gonna kill you, it's just a question of which one gets there first.

It's vitally important to keep control all the way through re-entry. Early ballistic capsules maintained control by putting the ship into a slow roll during re-entry. Kind of like a quarterback's spiral, only slower. More modern versions add active, computer-controlled guidance for extra control. Winged vehicles like the Shuttle can use their aerodynamic control surfaces once the dynamic pressure gets high enough. But if you lose control before you get down below Mach 1, you're in for a whole world of hurt. Vladimir Komarov found this out the hard way on Soyuz 1, when his capsule began spinning too fast. The parachute lines snarled up when they were deployed, and the Soviets were saved the trouble of cremation. They didn't find many pieces bigger than a soda can after the capsule struck the ground.

And then there's Columbia, which gamely tried to fly along missing its left wing for a little bit. A piece of icy foam tore a hole in its heat shield during launch, which let super-hot gases inside to romp and play with the aluminum wing structure during re-entry. It sagged, melted, and eventually let go. Parts were found in six or seven states.

Not that those are the only problems to be had. Soyuz 11 ran into a bit of trouble on its way back from Salyut 1. Russia very nearly scored another first on us. And it must be said, they did have the first space station. We, on the other hand, had the first space station crew who were able to brag about it afterwards. You see, a cabin purge valve had become stuck open during re-entry. It let all of the air out of the capsule, and the crew weren't wearing space suits. The predictable thing happened.

Coming down onto an airless world is just as exciting, but in a different way. Because there's no air, you have to burn enough fuel to do the deed. It's a more predictable process, since you can cipher out beforehand how much gas you need to bring along. They nearly got caught short the first time. Neil Armstrong landed on fumes. But they got better, and more precise, with each successive landing. It's interesting to note that while the computer was in control during most of powered descent, every final approach was flown by hand. No one trusted the guidance well enough that far down. Having had the chance to work with the algorithm, I can't blame them. It has an evil tendency to bend over sideways and burn for the horizon when the time-to-go gets short.

We seem to have lost a bit of the knack, though. The Mars Polar Lander had a senior moment during landing, and cut the engines about a hundred feet off the deck. Back to the drawing board...

Everyone says space is a dangerous place. That's rubbish. Getting there? Dangerous. Coming back? Really dangerous. Being there? Mostly harmless. Well, except for the time the oxygen converter caught fire, or when they rammed themselves with a cargo tug, or when the toilet stopped working... But those are stories for another day.

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