HOW TO STEER A ROCKET

As long as the rocket is in the atmosphere, steering it isn't very different from steering an airplane. Just as an airplane has adjustable surfaces -- rudders and so forth -- on its wings and tail, a rocket can be fitted with tailfins that have the same adjustable control surfaces (Fig. 1a). There are two obvious problems with this method. First the rocket must be going fast enough to provide enough airflow. At liftoff the rocket doesn't move fast enough, and that's when you need the most control. Second, rockets for space travel need to be able to operate in a vacuum. Nevertheless most rocket boosters and nearly all model rockets employ some form of passive aerodynamic stability. The fins on a rocket, even if they don't move, help keep the rocket pointed along its direction of travel.

Fig. 1 -Different methods for steering a rocket. (a) air rudders; (b) exhaust vanes; (c) V2/A4 exhaust vanes in the exhaust plume; (d) gimballed engines; (e) reaction control thrusters. (Photo (c) courtesy V2Rocket.com)

When screw propellers were first employed on ships, shipbuilders realized they could amplify the effect of the rudder by placing it behind the screw. The rudder would then deflect some of screw's thrust. Similarly vanes could be placed not in the rocket's aerodynamic slipstream, but in the exhaust itself so that by rotating the vanes some of the exhaust gas would be deflected at an angle and alter the direction of thrust (Fig. 1b). This method was used successfully along with aerodynamic control surfaces on the German V2 rocket (Fig. 1c).

But the best way of directing rocket exhaust is to direct all of it. That means swiveling the engine itself by mounting it in a gimbal (Fig. 1d). Since the entire thrust is directed, it doesn't need to swivel all that much -- only a few degrees. Most modern rockets, including the space shuttle main engines, use gimballed engines.

A rocket that must control its attitude without firing its main engine uses small RCS thrusters to effect a rotation. These can be used while the rocket is firing to correct the attitude errors caused by off-axis thrust (Fig 1e). The disadvantage to this method is that the rotation rate can generally only be changed by a fixed amount since RCS thrusters have a fixed thrust.

HOW TO GUIDE A ROCKET

Being able to control the orientation of a rocket is only half a solution. A steering wheel is no good without a driver and a road to follow.

We can use a gyroscope mounted in a gimbal to provide a fixed reference for orientation. The gyroscope will always maintain the same orientation in space regardless of how the spacecraft turns. This same principle operates the artificial horizon instrument in a conventional airplane.

Fig. 2 - Cutaway drawing of the Inertial Measurement Unit (IMU) used for the Apollo command and lunar modules. It was about the size of a basketball and contained gyroscopes and accelerometers.

In between the reference gyroscope and the exhaust vanes or engine gimbal sits the guidance system. This can be as simple as an electrical circuit which reads the gyroscope error angles with a variable resistor and applies voltage to electrical actuators. But usually the task of interpreting the position of the gyroscope and applying corrections falls to a digital computer. Modern guidance systems can sense the gyro position and adjust the thrust vector many times every second.

Another advantage to using digital computers is that by combining gyro information with altitude, precise timing, and speed, the rocket can be made to fly exactly along a preplanned trajectory. It need not go straight up, or even in a straight line. Some of this could even be accomplished prior to digital computing. The V2 rocket used timers and more simple electronics to achieve a ballistic trajectory for use as a weapon.