A space vehicle consists of two principal parts: the launch vehicle and the spacecraft proper. The launch vehicle, also called the carrier vehicle, is the rocket that lifts the entire space vehicle off the ground and thrusts it into orbit or into some other flight path, called a trajectory. The vehicle usually consists of several rockets, often sitting on top of one another in a stack. These rockets are called stages. The Saturn 5 launch vehicle shown on the previous page, for example, consists of three stages. The spacecraft proper is the part that contains the payload, or objects that are designed to fulfill the purpose of the mission. The spacecraft can be considered additional stages on top of the launch vehicle stages, though the word "module" is used more often than "stage" in reference to the spacecraft. The spacecraft usually includes a guidance and control system, communications equipment, and a power supply. A payload for an unmanned space flight consists of instruments for scientific observations, such as cameras and radiation counters. In a manned space flight the payload includes life-support equipment.
To date, the multistage launch vehicle has been thrown away after use- that is, it is dropped into the sea or it burns up on reentering the earth's atmosphere. From a complex and expensive manned lunar mission, only the little command module returns. Engineers are now developing a winged "space shuttle" that will land at a predetermined site in the manner of a heavy glider when it returns from orbit and will be usable at least 100 times. Rocket engines will be included as part of the spacecraft, but there will also be a separate launch vehicle to help boost it into orbit. The launch vehicle will land by parachute and may be recovered and repaired for further use.
Why space vehicles are made up of stages becomes apparent when one compares the weight of the propel-lant to the structural weight (weight of the vehicle itself, including tanks, engines, and controls). The amount of propellant in a vehicle varies, depending on the type of vehicle; it generally accounts for 70 to 92 percent of the total weight of the vehicle. The structural weight is kept as low as possible. As the propellant in a single-stage rocket is burned, the structural weight soon makes up the larger percentage of the vehicle's total weight. Under these circumstances the vehicle cannot achieve the desired trajectory velocities, because part of the energy of its engines is wasted on the propulsion of now-useless structures.
Staging overcomes this difficulty. Instead of a single-stage large rocket vehicle, several smaller vehicle stages are used. As each stage is operated, it burns and expends its propellant. When all the propellant in a given stage has been ejected, this stage can no longer push or accelerate the remaining stages on top. Therefore, the empty stage is separated from the vehicle and dropped off, so that energy need not be expended on its dead mass. Stage separation is usually accomplished by the thrust of separate small rocket engines that operate only briefly. By dropping off stages that are no longer useful, it is possible to attain the very high velocities necessary for space flight.
The first stage must carry, as dead mass, all the upper stages. For this reason it must develop more thrust, and therefore has larger rocket engines and carries more propellant than the other stages. Each successive stage is usually lighter and smaller than the one that precedes it.
Each stage of a multistage vehicle has its own mission, or flight purpose. For example, the first stage, or booster, lifts the vehicle off the ground and accelerates it to a velocity of about one mile per second. For complex missions, such as an Apollo manned lunar landing, the staging becomes quite complex.
A space vehicle contains a large number of rocket engines, each having a different purpose, typically including a primary propulsion system for increasing the vehicle's forward velocity and auxiliary propulsion systems for attitude control and emergency escape. (Rocket engines and their uses are discussed in the article rocket.)
Primary Propulsion Systems. For primary propulsion, a rocket engine cluster, consisting of several engines in parallel, has often been used. The number, size, and form chosen for the rocket engines depend on considerations of engineering readiness, reliability of the equipment, and cost. For example, in 1960 it would have been impossible to build a launch vehicle of only one stage that could reach orbit; it is possible to build such a vehicle now, but questions of cost, efficiency, and reliability still condition its development.
Attitude Control. For maneuvering space vehicles, small attitude-control rocket engines producing 2.2 to 4.5 kilograms (5-10 pounds) of thrust are used, often in pairs. Attitude-control engines are designed to be started, shut off, and restarted many times.
Emergency Escape. Emergency escape rockets are designed for the safety of the crew in a manned spacecraft in case a malfunction should develop in the launch vehicle. They are usually mounted at the tip of a small tower built on top of the spacecraft. Should the booster explode, or should any other launch-vehicle malfunction occur, the escape rockets ignite and propel the spacecraft far ahead of the launch vehicle. A parachute is then released automatically, which carries the spacecraft and its occupants safely to the ground. If the launch vehicle behaves normally, lifting the spacecraft into its correct orbit, the tower and escape rockets are jettisoned.
Retro-rockets. Retro-rockets produce thrust in a direction opposite to that of flight. They slow down vehicles just prior to landing, and slow down the speed of orbiting satellites. In addition, retro-rockets are used in stage separation.
Internal Power Supply
A supply of electric power is needed in spacecraft and launch vehicles. It is needed for operating guidance, navigation, and communications systems; for starting, stopping, and controlling rocket engines; for operating crew-support systems, recording equipment, and scientific instruments; and for separating vehicle stages if retro-rockets are not used.
The electricity for such systems is provided by an internal power supply system. The type and size of the system depend on the mission's duration and on the size and complexity of the spacecraft. Different systems use different sources of energy. The sources may be solar radiation, atomic energy, or the chemical energy of devices such as fuel cells.
The internal power supply is not used while the vehicle is on the launch stand. Instead, electricity is supplied to the vehicle through a cable from a generator on the ground. This cable connection is broken just before the vehicle lifts off.
All spacecraft are designed to fulfill particular space missions, and their shape, size, weight, equipment, power supply, and other characteristics vary widely. Spacecraft range from simple, small, unmanned space-probe payloads containing a few measuring instruments to prospective large, multi-module space stations.
Some spacecraft escape from the earth's gravitational attraction, others remain in orbit, and still others reenter the atmosphere and return to the earth. The reentry vehicle must withstand the severe heat and deceleration caused by friction when it enters the atmosphere. When such a vehicle reenters on a path similar to that of a ballistic missile, its surface can be exposed for several minutes to temperatures above 5000°C. (9000°F.). Thus, special heat-dissipating devices, such as ablative shields, are necessary. An ablative material absorbs a great deal of heat as it slowly melts and evaporates (its chemical bonds are broken down). Commonly used ablative materials include special types of plastic, glass, ceramic, and carbon.
A vehicle with wings or lifting surfaces, such as the planned space shuttle, would enter the atmosphere at a shallow angle and would not be subject to such intense heating.
Spacecraft, of course, contain a complex array of instruments, machines, and equipment.
Orbits and Trajectories
The flight path of a vehicle in space is called its trajectory or orbit. Strictly speaking, the terms "orbit" and "trajectory" are almost synonymous. In practice, however, they are used with slightly different meanings. The term "orbit" is applied to a flight path that is closed or is constantly repeated. Thus, we speak of the orbit of the earth around the sun, or the orbit of the moon around the earth. The term "trajectory," on the other hand, is commonly applied to flight paths that have definite starting -and ending points. For example, it is usually applied to the flight path of a bullet, a discarded launch vehicle, or a spacecraft on its way to the moon. Some orbits and trajectories are shown below.
Satellite Orbits. Satellite orbits may be either elliptical or circular, although most artificial satellites have been placed in elliptical orbits. It is easier to attain an elliptical orbit, because placing a satellite in a circular orbit requires extremely precise control of its velocity and the angle at which it is projected. The slightest deviation from the correct values results in an elliptical orbit. Elliptical orbits are also generally more desirable because a satellite in an elliptical orbit passes through a range of altitudes, making it more useful for most purposes of scientific research. For communications satellites, however, synchronous orbits (circular equatorial orbits with a 24-hour period) are often more useful. (See also satellite, artificial).
Parking Orbits. A parking orbit is a satellite orbit in which a vehicle is kept for a certain time before its rocket engines are restarted to lift it into either a higher-altitude earth orbit or an earth-escape trajectory for a flight to the moon or one of the planets. Radio communication with a parked vehicle is relatively simple, making possible the coordination of more precise flight plans before the vehicle departs from the vicinity of the earth.
Interplanetary Orbits. In travel from the earth to the moon, or from any planet to another, the amount of propellant required and the size of the space vehicle depend on the relative positions of the launching and target planets. Since the planets are themselves in orbit, their relative positions vary continuously. For this reason the time and date of a launching are most important. Also, in the case of the rendezvous of a newly launched spacecraft with a spacecraft already in orbit the relative positions of the two vehicles are influenced by the rotation of the earth about its own axis and by the orbit of the circling spacecraft. In some cases a rendezvous can be achieved only if the launch time is precise to within one minute.
Determining the flight path necessary for a vehicle to reach the moon or a planet is extremely complicated. First of all, the earth and the target planet are moving at different rates. The space vehicle must be aimed, not at the target planet's position at the time of launching, but at its position at the time the vehicle is expected to reach it, usually several months or more in the future. Additional complicating factors include the elliptical orbits of the planets and the variation of their speeds with time. The gravitational forces of the sun and the other planets have a great influence on the flight of a spacecraft, and must be allowed for in the determination of a flight path. Even minor forces, such as atmospheric aerodynamic drag and the pressure of solar radiation, have an effect and must be considered. Yet another factor is the duration of powered flight. The duration must be determined accurately, because a spacecraft's mass decreases rapidly as it expends its propellants and as a result the spacecraft accelerates rapidly. Even one second more or less of powered flight at this point would mean a tremendous difference in the final velocity of the spacecraft.