Selected Articles from the
April 2001 Odyssey
Editor: Terry Hancock
- Taking Part in the Search, Craig Ward
- The Rarity of Sol-Type Systems, Terry Hancock
- A Matter of Attitude, Part 3, Robert Gounley
- The Surf Report, Diane Rhodes
A Matter of Attitude, Part 3
Robert Gounley
At the start of our adventure we were left tumbling through space trying to get ourselves properly pointed to return home, or at least stabilized so that our spaceship would once more operate properly. Since then, we've learned how to tell which way we're pointed and the direction and speed we're turning. Now it's time to put everything together.
Before we stop our tumble, it would help to explain exactly what it means to be tumbling in space. (By now, your spaceship's gyrations may be giving you a touch of vertigo. Take heart. Well have things under control soon.)
An object in space that stays pointed in the same direction is, by definition, maintaining a stable attitude. Thats not always what we want. Sometimes, we may want to point a spacecraft down at the Earth for cameras or antennas to work properly. In that case we want it to make one full revolution in the time it takes to go around the Earth. Other spacecraft spin to gather data about the streams of particles emanating from the Sun (think of holding up a damp finger to sense the prevailing winds).
If your spacecraft is fairly stiff and disc-shaped, you're off to a good start. When these are set spinning like an old-fashioned phonograph record, they hold their orientation quite well. Except for small jitters caused by property called ``nutation'' (easily moderated in the spacecraft's design), ``spinners'' hover implacably like the flying saucers in 1950s science-fiction films. Not surprisingly early spacecraft exploited this properly and Earth orbit promptly littered with rotating discs, spheres, and cones.
The catch is that not all spacecraft are born to be spinners. Objects spin best when most of their mass is far away from the axis where it is rotating. A majorette's baton twirls well because it has heavy rubber tips at both ends that help keep it stable. Distribute the mass unevenly and the results are far less predictable - imagine twirling a lawn chair.
(Oh, motion sickness has you feeling a bit irritable? I'll try to hurry things along.)
Things get worse when spacecraft parts can flex and move. Launch costs depend on launch weight, so most spacecraft save mass by allowing parts that are springy to the touch. Solar panels flutter. Rocket fuel sloshes in its tanks. With masses moving erratically, the whole structure can be affected. Without measures to control then, flexible spacecraft can be sent gyrating into a chaotic dance.
America learned this lesson from its first satellite: Explorer 1 had no sensors or actuator to control attitude. Instead, this pencil-shaped spacecraft stabilized by spinning about its long axis like a top. In theory, a rigid body could remain like this indefinitely. The catch was that Explorer was built with thin wire antennas that stuck out its sides. The wires were still vibrating erratically when the Redstone rocket released the spacecraft into orbit. The vibrations unevenly tugged on the outer shell, causing the spinning spacecraft to rock about. In time, the physics of rotating bodies coaxed Explorer 1 into a much more stable form of rotation - somersaulting end over end like a baton.
Now that we have explained tumbles, we're ready for action. (Hope your color comes back soon.)
For fast, erratic motions, we'll want to use our gyroscopes for reference the way a pilot in his cockpit feels the way his plane is pitching ``by the seat of his pants.'' Instead off wing-flaps, we'll fire small thrusters in short bursts. This is most easily done with the aid of an on-board computer.
Since were going to need a lot of stabilizing, it's important that we don't fire any one thruster too long, or we'll change our orbit. For this reason, most spacecraft are built with several attitude control thrusters grouped in clusters. When some extra spin is required, the spacecraft simultaneously fires two thrusters, set on opposite sides and pointing in opposite directions like a rotating garden sprinkler. The thrusters apply torque, but impart no net velocity.
Now that we're no longer tossing about, we can use other attitude sensors to see how were pointed. (You're already looking better, by the way.) For a coarse, initial estimate we can get our bearings by viewing the Sun and the Earth. Later, measurements of star positions will orient us precisely.
Knowing where we are, it's time to turn about to where we want to be. We could use the attitude control thrusters again, but perhaps we'd rather save the fuel. If we're in no great hurry, we can take use Earth's magnetic field to good advantage: By activating selected electromagnets on our spaceship's sides, we can turn it into a giant compass needle. Applying power to different magnets can gently nudge us to our final orientation. That's the way the Hubble Space Telescope moves about with out relying on chemical thrusters that could contaminate mirrors and other sensitive surfaces.
In more of hurry? Reaction wheels give a more robust response. These are groups of large flywheels spinning in different directions. When we want to turn, electric motors speed up or slow down one or more of the wheels. Physics conserves angular momentum, so when a reaction wheel changes speed, the rest of the spacecraft is driven in the opposite direction. If we build the reaction wheels very heavy and keep them spinning very fast, they can turn even a large spacecraft very quickly. Thats how military planners foresee to control orbiting lasers as they track fast-moving targets.
Well, it looks like your spaceship is now under control. Hope you've enjoyed this lesson. (You're looking much calmer.)
File translated from TEX by TTH,
version 2.25.
On 15 May 2001, 18:18.
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