Friday, March 10, 2006

Space Travel for Dummies: Hands-On Tutorial


For those of you who care to download and configure some software, we can do a bit of a walk-through of a trip into orbit.

[NOTE: There are newer versions of Orbiter and its add-ons available. The new version of Orbiter is available here, and a selection of add-ons are available here.]

First, you want to download Orbiter from here. Orbiter, for those of you who don't know, is a freeware spaceflight simulator. It's fairly realistic, and if you're into that sort of masochism, it can be as realistic as you want it to be.

There are some add-ons we want, from Dan's Orbiter Page. Scroll down a bit, and click on the icons for Orbiter Sound and Delta Glider III. Basic Orbiter has no sound, and sound is a neat thing to have. As for DGIII, well, it has a couple of features the basic Delta Glider lacks. First, you can use the config tool to change up the paint job if you want. Second, and more importantly, it has an auto-pilot function. This makes life so much easier. I can hand-fly an ascent to orbit, but not to an accurate orbit. I can get into space, but if you're relying on me for an accurate rendezvous, you're SOL. With the autopilot, we have a fighting chance.

Now, let's fly.

From the start-up screen, you'll want to select the DeltaGliderIII folder, and Earth Scenery, and from that, the ascent from KSC to ISS. We aren't actually going to attempt a rendezvous, but we'll use that profile as a starting point.

When you get to the cockpit, use the left arrow key to go over to the orbit display. You'll see a grey circle, a green ellipse, and a bunch of numbers down the left-hand side. The grey circle is the Earth's surface, the green ellipse is your trajectory, and the numbers describe various things about your orbit. You won't need any of the numbers for this tutorial, so you don't have to worry about them unless you want to. The pictures on this display tell the story. If you're really interested, I'll have some definitions at the end of the post. One thing you'll want to do, though: punch the key on the upper right side of the panel. It'll change the reference of the display to ship-centered.

Now, hold down the right arrow key to get to the other display. Hit "SEL" to get the list of displays available. Take a look at what's available, but where you eventually want to end up is the Map display. It'll show your ground track, and the ground track of a selected target, if available.

Now that we've got that settled, let's program the autopilot. On your keyboard, type P903S43, then Enter. This loads the ascent autopilot, and then puts it into standby mode. You can activate it by hitting the "E" key. You don't want to do that just yet.

Now, if you hit F8, you get the HUD view. This is the view I like to fly from.

Hitting F1 toggles the external view. From here, you can hold down the right mouse button, and find an external view that you like the looks of. You can also use the scroll button, if you have one, to zoom in and out.

We're done setting up. It's time to light off our engines and get underway.

From the cockpit view, hold down the key and the <+> key on the keypad. Release both keys after full thrust is achieved. Now comes the trickiest part of the whole flight. If you've gotten the sound files in, this will be pretty easy. You'll hear a voice say, "80 knots." Don't do anything yet. Then, he'll say "V1." You still don't want to do anything. But stay on your toes. Shortly after V1, you'll be going fast enough to take off. The cue for that is when he says, "Rotate." The number keypad serves as a joystick. <2> pulls back, <8> pushes forward, <4> and <6> roll left and right. When you get the rotation signal, use <2> to pull back, but not to more than a 20 degree climb. Once you've left ground, tap "G" to raise your landing gear. Then, you can tap "E" to engage the autopilot. Once that's done, you can sit back and enjoy the ride.

This takes us to about T +30 seconds, referring from the time we started our engines.

The autopilot isn't particularly aggressive. It takes about forty seconds to turn to a 43-degree heading, and level the wings. Once it does that, it starts pulling up into a steep climb, a pitch attitude of 65 degrees. It reaches that attitude about two minutes into the flight. Remember, to get into orbit you have to do two things: gain altitude, and gain speed. Generally speaking, you gain altitude first, to get out of the thick, dense air near the surface.

While you're doing this, you'll probably notice that the green ellipse on the left display isn't changing much. Gaining altitude doesn't change that situation very much. Your trajectory is still what we call a degenerate ellipse. If the ground weren't in the way, your path would go straight down to the center of the Earth. This will change in a few minutes, though.

At about T +200 seconds, three minutes and change, the autopilot begins to rein in the climb. You've hit about 100 kilometers in altitude, so you can begin to bend over sideways to build up speed. But you don't do it all at once. It's more efficient to execute a "gravity turn": you turn a slight bit off of your climb, and let gravity pull your path towards the horizontal. It's a neat trick, but it only works if you have little to no air resistance.

But look over at your left-hand display. Now, the orbit begins to fill out. It changes from a degenerate ellipse to a normal ellipse. As you gain speed, you gain energy, and this is expressed as a bigger orbit.

Now, look to your right. The map display shows your ground track mostly in pink. That's the "underground" portion of your trajectory. If you ever try to shoot a Moon landing, that'll come in handy. Where the green trace ends, that's where your path intersects the surface. (If you spend a bit of time learning how to use these displays, there's little to no actual cipherin' involved in flying this thing. Which is a good thing, becuase most rocket jockeys aren't mathematicians.)

At about six minutes into the flight, the autopilot has brought the pitch attitude to zero. It's all about the speed, now. The magic number is about 7,700 meters per second. You're not near that yet, but you're getting closer all the time. The ellipse on the left-hand display is growing visibly.

Now is a good time to do a bit of sight-seeing. Again, F1 toggles between inside and outside views. You can look back at Florida, and south to the Bahamas. The scenery is pretty good.

At about nine minutes, the autopilot will do something that might surprise you at first. You're getting close to your target altitude, so the autopilot pitches down to soak up excess vertical speed. Don't worry about it if that happens. It means you're almost done.

And then, at T +580 seconds, the engines shut down, the autopilot terminates, and you're in a stable circular orbit.

As I've said earlier, this is the easy part. Landing is much, much trickier ...


Here are some definitions of some of the terms on the Orbit display. I haven't defined all of them, just some of the more important ones.

SMa: Semi-major axis. This is one-half of the total distance, end-to-end, of the long side of the orbital ellipse.

SMi: Semi-minor axis. This is one-half of the total distance of the short side of the orbital ellipse. This is almost never used except to compute the eccentricity.

PeD: Periapsis Distance. The Periapsis is the point of closest approach to the Earth's center. If it's less than the Earth's radius (6,378.137 km) then your trajectory intersects the Earth's surface at some point. If unprepared for said intersection, you're going to have a bad day.

ApD: Apoapsis Distance. The Apoapsis is the highest point on an orbit.

Rad: Radius. This is your current distance from the center of the Earth.

Ecc: Eccentricity. This is a number, 0 to 1, describing how flat your ellipse is. Zero describes a circle, one describes either a degenerate ellipse or a parabola.

T: Total orbital period in seconds. This is how long it will take to go around and come back to your starting point. For a low circular Earth orbit, it's about an hour and a half, give or take.

PeT: Time to periapsis. This is how long you have to go to reach periapsis. It's important for timing orbital maneuvers: if you want to raise your apoapsis, you need to do your burn at periapsis.

ApT: Time to apoapsis. This is how long you have to go to reach apoapsis. It's the same deal: if you want to change your periapsis, you have to do the burn at apoapsis.

Vel: Current velocity. 'Nuff said.

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.

Tuesday, March 07, 2006

Space Travel for Dummies, Part 3


If you've been following along, by now you know the basics of how to get into orbit in the first place, and how to move around in Earth orbit once you get there. Which will do as far as things like going to the space station or putting a new satellite up go, but sometimes you'd like to step a bit further out. It's not all that much different than what we've already been talking about. You know the old saying, "What goes up, must come down?" Well, it's not always true.

If you throw something hard enough, it won't be coming back.

Let's revisit our good friend Superman up on Mount Everest. He still has a few baseballs left, and intends to put them to good use.

If you remember, last time he threw the ball a little harder than he had to in order to achieve a circular orbit, and put it into an elliptical orbit. If he puts more muscle into the throw, he can get it to sail higher and higher into the sky. The orbits get bigger and bigger, and they take longer and longer to get back to their starting point. Starting from 90 minutes, the orbit times extend to two hours, three hours, six hours, and longer. Then, you start measuring the orbits in days, not hours. They blast up to ten, twenty, a hundred thousand miles high. Eventually, he starts going home after he throws, coming back a couple of days later for the catch.

He's got one ball left.

He winds this one waaaaaaaaay back, and lets it go. It leaves his hand at over 36,000 miles per hour. If he weren't Superman, the shock wave would knock him down. Then, he packs up his stuff and goes home. He's done.

This one, you see, isn't coming back to Earth. Ever.

Here's what's happening: Gravity loses strength at the square of the distance from Earth. But, gravity can only drain your speed away at a linear rate. Most of the time, that's enough. It saps all of your vertical speed and pulls you right back. But if you're going fast enough, gravity can't suck speed away quickly enough, and no matter how far away you get you've always got a little speed left over.

So, that last ball Superman threw will sail across the Solar System forever. It's in a permanent Solar orbit, and probably won't ever come back to Earth.

This is an important principle, because that's what lets us leave Earth and strut our funky stuff across the Solar System. While gravity extends everywhere, it's not infinitely powerful, and can be overcome with enough effort. Sometimes, you even get to cheat a little bit and make gravity work for you.

That's the essence of what they call the gravity assist or "slingshot" maneuver. It's a way that you can get a speed boost and direction change for free, if you line everything up just right.

Ordinarily, changing speed and direction takes energy. And the only practical tool we have right now for doing that is a chemical rocket. Which means you have to bring along fuel to do it. Since you have to bring along fuel, you have to bring along fuel tanks, and that drives up the ship's weight.

That's bad. As the current NASA administrator, Mike Griffin, once wrote: "Spacecraft, like turkeys, are bought by the pound."

But, sometimes, you get to pull one over on Mother Nature. Here's how it works:

Imagine a skater, carrying a grapnel hook on a bungie line. He wants to round the corner, but doesn't want to expend the effort to turn his skates. So, he throws out his line, hooks a pole, and swings around.

By timing your approach and direction just right, you can swing around a planet just like the skater swung around the light pole.

As you fall in, coming from behind and below, you speed up as the planet pulls you in. As you speed away, the planet's gravity isn't able to suck your speed away quickly enough to get it all back. Basically, you've stolen a tiny bit of the planet's orbital momentum, and used it to speed up and change the direction of your orbit.

We've gotten pretty good at this. Voyager, Galileo, Cassini, all of them went through numerous slingshot encounters to achieve their missions.

And that's about it for basic orbital maneuvers. Once you escape Earth orbit, getting to and from another planet in the Solar System works just the same as getting to and from something in Earth orbit. You burn in the direction of travel to gain energy and go farther out, and you burn against the direction of travel to lose energy and go in.

Which brings us to a trick question. Remember this: you can amaze your friends at parties, because almost no one ever gets the right answer the first time.

Here's the problem: you've got a package of dangerous waste that you want to be rid of forever. You've got two options. One, drop it into the Sun, and two, send it into interstellar space.

Which one is easier?

Sending it into interstellar space, of course.

This stunned me the first time I heard it, but the numbers absolutely work out.

Earth's orbital velocity is about 30 kilometers per second. As a rule of thumb, escape velocity is about one and a half times orbital velocity. So, once you've escaped Earth, you have to gain about another 15 km/sec to achieve Solar escape velocity.

But to drop something straight down, you have to shed ALL of your orbital velocity. So, to drop something into the Sun means you have to find a way to shed a full 30 km/sec.

It's a hard fact to swallow, but I assure you, it's true.

Next time: How the heck to we get back home?