Tuesday, January 10, 2006

Space Travel for Dummies, Part 1


For years now, I've threatened to write a book on space travel, but have never made the time to do the deed. There's a niche to be had, for a book somewhere between kids' picture-books and professional-level textbooks. I have a couple dozen pages of notes, somewhere. Provided that they haven't been thrown away in a cleaning binge.

Anyhow, it probably ain't gonna happen. So, I'm going to do the next best thing. I'll collect those thoughts here, in a series of posts. I'm not sure how many posts it'll be.

First off, let's talk about falling. Not just any kind of falling. This is falling with panache, daring, and style. Any fool can fall and hit the ground, but it takes great skill to fall ... and miss.

Imagine, if you will, Superman standing on the summit of Mount Everest with a pile of baseballs. Let's say that he picks one up and throws it. It flies eastward a bit, before smacking into the peak next door. He decides that was a pitiful effort, picks up another, and heaves it a bit harder. It flies over the next mountain, and lands somewhere in eastern Nepal. We can't see it from where we stand. It's gone a bit around the curve of the Earth. Still not satisfied, he picks up another ball and throws it with more force. This time, it plinks into the Pacific, somewhere north of the Phillippines. He keeps repeating the process, dunking the next ball off the cost of Ecuador, and then skipping it across the ground somewhere in Brazil. Then, at last, he pulls one back and lets it go ... and it never quite touches the ground. It screams through the sky, twenty-five times faster than sound, always falling but never hitting anything, until, about 90 minutes later, Supes turns around, and THWACK! He neatly traps it in his catcher's mitt.

That, ladies and gentlemen, is called an orbit.

It doesn't matter what's doing the orbiting, or what it's shaped like. It could be a baseball, a '57 Buick, an Orange Julius stand, or the Eiffel Tower. If you can drag it a couple hundred miles straight up, then set it moving sideways at 18,000 miles per hour, it will circle the Earth indefinitely.

The other thing to notice is that the Earth's gravity is still in full force. There's no such thing as Zero-G in Earth orbit. We sometimes use the word, but it's sloppy. Better to say "free-fall", because that's what's really happening. Something that's in orbit around the Earth is falling all the time. But it's going so fast, that the ground curves away before it can hit. Yes, it is possible to fall and miss the ground. All you need to pull that trick off is a bit of speed.

The next question is, how do you do it?

Well, remember, there are two things you have to do. First, you have to lift it up outside of Earth's atmosphere. Second, you have to accelerate it to orbital speed. It's pretty tough to do both at the same time. It's incredibly tough to try to do it with only one set of engines.

It ought to be fairly obvious that the "lift" job needs a really powerful, high-thrust engine. It's not as obvious what the "accelerate" job needs. For that, we'll have to look at two equations:

[1] V2 - V1 = Ve * EXP (M1/M2)

V2 and V1 are the final and initial speeds, respectively. Ve is the exhaust speed of the rocket, and M1 and M2 are the initial and final weights of the rocket. The other equation is:

[2] P = Ve * T

P is the power produced by the rocket engine, and T is the thrust.

Now, here's the sad and sorry fact: for chemical rocket engines, P is pretty much fixed. Energetic chemical reactions all produce about the same power level. There's not a whole lot of difference there. Where they are different is in how fast the exhaust moves. An engine with fast-moving exhaust gives us a better top speed. We see that from [1] above. But, looking at [2], we see that when Ve is big, and P is fixed, then T has to be smaller. Going the other way, if we want lots of T, we have to live with less Ve, and not as much speed.

But, because we have two different jobs to do, we need both kinds. Look at the Shuttle, for example. The solid rocket boosters aren't too terribly efficient, but they're absolutely swimming in thrust. When they light off, the Shuttle leaps off the pad with force and authority; as long as those SRBs are burning, you're going somewhere. When they're done, the Shuttle's main engines take over. They're not super-powerful, but they're about as efficient as chemical rockets get. Now that the heavy lifting's done, they can bend over sideways and start building up some speed.

Just about every modern rocket system uses the same scheme. The Ariane 5 uses parallel staging like the Shuttle, with solid boosters flanking a hydrogen/oxygen core stage. Others, like Russia's Soyuz rockets, use clusters of kerosene rockets to do the lifting, before handing over to a more efficient upper stage for orbital insertion. There was a proposal in the '50s to use balloons to take a rocket up out of the dense layers of the atmosphere, and shoot it into space from there. It was never tried, so far as I know. It wasn't a bad idea, and might have worked. Other proposals use a large 747-type plane as a mother ship, carrying a rocket plane piggy-back to 40,000 feet or so, before the rocket plane separates and flies into space. Also not a bad idea, but there are some kinks to work out.

That will do for an introduction. Next time, we'll figure out how to get around in space once we actually get there.