On Space Travel — Part 1: Another Anniversary

Another Anniversary

(... something that's been brewing over the past month; this is going to be Part 1 of n...)

Among other things, I've been reading up on the Interplanetary Transportation Network,

but I was also reminded that last month was the 45th anniversary of the Apollo 7 launch, which in itself wasn't much — fly a Saturn 1B and Command/Service Module into low earth orbit and splash down again just to make sure the basic system works. It was mainly notable for being the first manned flight after the Apollo 1 disaster killed Grissom, White, and Chaffee, which resulted in everything at NASA being grounded for a year while they figured out what they did wrong...

Which I knew nothing about at the time because I was seven years old, as old as my youngest kid is now. For me, Apollo 7 was an introduction to whole idea that we even had a space program. It started as a stupid little "Hey look, we launched something!" article in the Weekly Reader that was maybe a paragraph long and had pictures of astronauts. Sometime later that year they showed the Disney "Man in Space" series in class, by which point I was vacuuming up all kinds of colorful books on space exploration.

By then we were maybe a year or two away from an actual moon landing. It was a big deal for everyone.

What amazes me now is how much we didn't know.

Take orbital mechanics, the study of how things move in space (or everywhere, really). As it's covered in the physics curriculum, you'd think it was a dead subject. Isaac Newton, Joseph-Louis Lagrange, and William Rowan Hamilton had everything we needed to know worked out by 150 years ago, and then Physics moved on to more exciting topics (electricity! magnetism! relativity! quantum mechanics! quantum electrodynamics! quantum chromodynamics! string theoryloop quantum gravity? hell if I know what's next …)?

Classical Mechanics? Been there, done that. Boring?

Guess again.

Granted, I'd already had some inkling of this, what with one of my friends in grad school being a Control Theory student who burst out laughing at the thought that there wasn't anything more to learn about mechanics. And to be fair, they didn't have computers 150 years ago, nor did they know what we know now about numerical analysis or non-linear differential equations. Never mind the general Playing Around With Numbers that you just couldn't do even 40 years ago. And now that people are actually shooting crap into space and needing it to go to particular places, there's a bit more than just abstract interest, now.

So, to take an example,…

Remember how in the late 1970s there was this serendipitous lineup of the outer planets -- Jupiter, Saturn, Uranus, Neptune, all in a nice row so that if we launched something Just Right, even with very little fuel it could go bing-bing-bing-bing, each planet doing a gravitational assist to boost the probe to the next, and so we get this really cheap odyssey that visits them all? Obviously this was a golden opportunity since it would be hundreds of years before we'd get that kind of lineup again. And so we launched Pioneer 10+11 and then Voyagers 1+2 and got back all of those nice pictures.

Turns out,… that "golden opportunity"? Total lie.

Not only can we can do this any time we want, but if we'd known about this back in the 1970s, we could not only have saved a bit of fuel but also done it in such a way that, instead of being stuck with single flyby for each planet, the probe could instead have been arranged to loop around each one as many times as we wanted before going on to the next.

But before I get into how the Interplanetary Transportation Network works, let's start with something much simpler that I'm pretty sure we'd have done differently had we known what we know now:

The Apollo Debate

In the early 1960s, shortly after John F. Kennedy issued his famous challenge, there was a huge debate about the best/cheapest way to actually get somebody to the moon.

Calculating what it takes to launch from Earth something big enough to hold a few astronauts, land it on the moon, and bring it back—this being the "Direct Flight" scenario—you find out that you need this insane, huge-ass rocket, something like two or three times the size of the Saturn V that was eventually built, something that maybe we'd be able to do by 1975 or 1980 (cue maniacal laughter from The Future), but if the goal is to get to the moon by 1970, we'd have to come up with Some Other Plan.

So they focused on what they could do, which is build smaller rockets. You put the astronauts on one of them, and have the rest carry spare fuel tanks, have them meet in low earth orbit, bolt everything together, and then send that to the moon (and back). Obvious, really. This, the "Earth-Orbit Rendezvous" (EOR) scenario, became NASA's game plan. Wernher von Braun gave his blessing and we were off to the races.

But then there was this annoying group that had this other, completely bizarre idea: "Lunar Orbit Rendezvous" (LOR), they called it.

Keep in mind that at this point, we hadn't rendezvoused anything yet in space, so nobody had any idea how hard EOR might be. Think about how you might go about catching up with a meteor. It's going thousands of meters per second and your job is to match it's velocity. And now we're supposed to be doing this in lunar orbit, instead? WTFF?

And they just would not shut up and get with the program.

It's weird to me now, remembering all of those colorful books with illustrations of all the possible ways to get to the moon. They actually mentioned this debate, even if they didn't do a very good job explaining well what the actual pros and cons were.

Or maybe they did, and it just whooshed over my head because I was seven years old and hadn't had any actual physics, yet, and anyway, hey, look, rockets!

But now that I have, it's bloody obvious. In fact, it's so obvious it's a bit appalling to me that NASA had to spend an entire year figuring this out:

Climbing the Wall

(to be continued in Part 2)

Space Travel, Part 2 — Climbing the Wall

(continued from Part 1, introducing the 1961-62 NASA debate on how to get to the moon, which you might want to read first)

Climbing the Wall

I don't know who first got the idea to picture the Earth's gravity well as this huge funnel, with the Earth at the bottom and the moon and various other satellites as ball bearings rolling or sliding around the top. I'd draw it but you've seen it already in every science museum on the planet. For all I know, Newton may well have had it in his Principia even if they hadn't quite figured out how to make ball bearings at that point.

What's annoying is just how deep the hole in the middle is, and this number I first heard from either Arthur C. Clarke or Gerard O'Neill:

Getting out of the earth's gravity well takes the same amount of energy as climbing a wall 6,000 kilometers high (i.e., if you had to climb the whole way against earth's surface gravity).

O'Neill's point was pretty simple: Why would you ever want to live at the bottom of a hole? Let's build space colonies!

Unfortunately, my point is a little more subtle, so we need some more numbers.

The energy you need just to get to low earth orbit is like climbing the first 3000km, half way out. Even though you've only gained a few hundred kilometers in real altitude, it's a huge accomplishment to go from standing still on the surface to going fast enough to stay in orbit. And it's useful enough to get to a place outside most of the atmosphere, where you have time to think about what you want to do next. But you still have at least another 2000km of wall to climb before you can get anywhere useful…

Like, say, geosynchronous orbit, where most of the communication satellites live — which is already much farther away than people give it credit for. At this point you're a bit more than a tenth of the way to the moon and about 500km from "the top" of the wall, though by that point the funnel has flared out pretty far so that you're actually going 50km outward for every one that you're going "up" (yay inverse square law...). Meaning instead of climbing El Capitan, we're now doing the leisurely stroll from the house to the supermarket — in my case this happens to be a fifty foot elevation gain over half a mile that I'll hopefully still be able to do when I'm 70.

Now as it happens, since our goal is just to get to the moon, we don't need to get all of the way out of the hole. The moon itself isn't all the way out; it's still in orbit around the Earth, see. But it's most of the way out.

Continuing outward from geosynchronous orbit towards the moon, the "wall" continues to flatten out until finally, when you get about 5/6 of the way there, it flattens out completely. You're now just 170km from "the top", in this saddle point where, to either side of you, the wall continues to rise, but in front of you it drops off and you're staring at another big hole with the Moon at the bottom.

Welcome to L1, the first of the Lagrange Points, the five magical places in the Earth-moon neighborhood where moon gravity, earth gravity, and centrifugal force all cancel each other. Put a ball bearing here in exactly the right place and, if it could be undisturbed by perturbations from the sun and the other planets, it would just stay there forever, i.e., orbiting the earth once a month exactly in synch with the moon.

And I really do mean magical, here. Forget Stonehenge, the Bermuda Triangle, or even Disneyland; they've got nothing on L1, as we'll see.

So now we're staring down into this second hole. To be sure, it's not anywhere near as deep, only 450km down to get to the lunar surface, child's play after having come this far. But it's still deep enough to inspire the Lunar Orbit Rendezvous (LOR) people to ask this stupid question:

"Why the hell are we taking everything with us down this 450km hole and back out again?"

Keep in mind that not only do you have to reach the bottom, but you want to be standing still when you get there (so as not to make a fresh crater), which means burning off all of the velocity you accumulate as you fall into the hole.

Which you have to do with rockets because there's no atmosphere.

And then, of course, you have to add all of this velocity back in order to get yourself out of there.

Which gets us to

The Stupid Thing About Rockets

(to be continued in Part 3)