INTO ORBIT

Solving Problems

In the early days of rocketry many aircraft features were adapted to the new vehicles. Dr. Robert Goddard placed vanes, similar to the tail surfaces of an airplane, on the nozzle section of his early rockets.

Turning these one way or the other deflected a portion of the exhaust gases, pushing the rocket’s tail in the opposite direction.

Gimballed NozzleThe mere presence of the vanes, however, reduces the efficiency of the rocket. Whatever amount of downward thrust pushes against the vanes cancels out an equal amount of the thrust’s upward push inside the rocket.

The solution developed for large rockets and space boosters was to “gimbal” the nozzle. A gimbal is a pivot device that allows the entire nozzle, and the flow of exhaust gases, to be swiveled in any desired direction. This is how the rocket’s course in the opposite direction is controlled.

Rockets can be guided by radio signals from the ground or by equipment and prerecorded instructions placed on board.

Inertial guidance is a common form of on-board control. It is an automatic navigation system that senses, through gyroscopic devices and acceleration-measuring devices, the vehicle’s position and speed, compares them with the planned flight path, and gives orders for necessary adjustments.

A number of missile and space programs use inertial guidance. Inertial guidance signals cannot be detected outside the vehicle or be disrupted by radio commands.

Another problem is stability in flight. There are three distinct motions to be controlled and corrected quickly before the vehicle begins undesirable tumbling. These motions are pitch, yaw, and roll.

Extend your arm straight out in front of you. When you swing your arm up and down, you are describing pitch.

When you swing your arm from side to side horizontally, you are describing yaw.

Keeping your arm straight out in front of you, rotate your arm so that the palm of your hand faces upward then downward. This is roll.

Airplane fins are of no help for controlling roll, pitch, or yaw outside Earth’s atmosphere because there is no air to push against in space. Instead, spacecraft have small gimballed engines or extremely small jets, using the reaction principle, to provide control.

Those small rocket engines are used to produce short bursts of very small thrust in the direction opposing the undesirable motion, damping it out. They are often powered simply by a small supply of cold gas under pressure.

Rocket motionsAnother method for stabilizing certain unmanned payloads is to make them spin like a gyroscope. The payload spinning at a preplanned rate gives it stability in flight, just as the rifling in a gun barrel spins a bullet for stability.

Reentry into the atmosphere is a critical period in any space vehicle’s return to Earth. Shooting stars, or meteors in the night sky, are the most dramatic example of what can happen when an object plunges through Earth’s atmosphere at high speeds. As the air becomes thicker, its particles create such friction that the descending body becomes increasingly hotter until it first glows, then burns.

Covered by heat shields, spacecraft can survive their descent through the atmosphere. Three kinds of heat shields prevent fatal heat from reaching astronauts or instruments.

The first type is known as the heat sink. Good examples of heat sink are the early nose cones of the Atlas and Thor missiles, which were made of copper and could absorb and store a large amount of heat without melting. The second type, which is now used instead, is called the ablative type, where special materials coat the forward surface of the vehicle and dissipate the reentry heat by ablation—boiling and bubbling away into vapor, using up the heat energy before it reaches the inside. The third type is a protective surface capable of withstanding high temperatures, such as the ceramic tile used on the space shuttle. The shuttle is built to survive intense heat, while insulation prevents that heat from reaching the internal parts of the spacecraft.