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Friday, December 19, 2014

Reentry

Bill is... concerned, about this reentry trajectory.
I got to thinking about the Orion spacecraft the other day, in particular splashdown and recovery of the capsule. The US Navy had a ship standing by and they were able to successfully retrieve the capsule, which is good. But then I got to wondering about how accurate was the splashdown? Was it within a few yards of the predicted point of impact, or a few miles? Or should we be talking about hundreds of miles? I mean the thing is traveling 5 miles per second, it wouldn't take a very big mistake to have you end up on the wrong side of the world. Even if you miss your landing site by only 100 miles, it is still going to take the recovery ship hours to get to you, and that could lead to a sticky situation. One way to compensate would be to deploy several ships in the vicinity, but that would raise the cost. Sending even a small Navy ship out has got to be expensive. I wouldn't be surprised if a destroyer cost something like a million dollars a day to operate. Makes that million dollars a year to field a soldier in Afghanistan look like a real bargain.

MX re-entry vehicles over Kwajalein, following their launch aboard an MX missile some 30 minutes earlier from Vandenberg Air Force Base in California, October 1985.
     So I started poking around, looking for answers, and not finding much of anything. And then, snap, I realized this is probably all classified because of nuclear warheads and ICBM-s (Inter-Continental Ballistic Missiles). You don't need to be real accurate with ICBM-s because, well, nuclear bombs. Still, you would like to be within a mile or so of your target. You can bet the military has spent billions on this very problem. When I started looking into this from the ICBM angle I found all kinds of information, everything that is except how accurate they are.
    The big factors affecting reentry are the time when fire your retro-rockets, and how much you slow down. You want to be accurate about this because a little too much or too little, too soon or too late, can easily kill you. But after you've fired the retro-rockets there isn't much you can do except pray.
    Except I seem to recall something about Orion being able to adjust it's attitude, which could easily affect it's trajectory, except how can you tell? GPS will be useless as the fireball you generate as you plunge into the atmosphere pretty much ruins any chance to sending or receiving any radio communications. Well, how about inertial guidance? That used to require big heavy chunks of equipment that was marginally accurate and marginally reliable, but I think we've gotten better at it. So it's not inconceivable that Orion was able to steer itself quite accurately.
    Once you deploy the parachutes, you kind of lose your steering capability, but you also become much more visible, so the pickup crew should be able to find you. As if they haven't been tracking you on radar since you appeared over the horizon ten minutes ago.

Minuteman III attacks Kwajalein

This video has some odd bits, but it also has some good shots.

Other posts about Orion. Most of them are about the spaceship, only a couple are about the airplane.
A couple of other posts about reentry.
Some posts about inertial navigation.

Update January 14, 2015. Just received an automated reply from NASA about this issue. As you might expect it contained no useful information.

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