Astronautical Engineering

Astronautical engineering:

The engineering aspects of flight and navigation in space, also known as astronautics. Astronautical engineering deals with vehicles, instruments, and other equipment used in space, but not with the sociological or economic aspects of space flight, except as they influence the equipment.

There is a lack of parallelism between astronautic and aeronautic vehicle terminology. An aircraft is a self-contained vehicle, having within its structure essentially all the equipment required to transport its payload from one place to another. A spacecraft, in the more restricted sense, is the container for the payload. Sometimes the word is used to denote the container and payload. Most spacecraft, to date, have had either very limited propulsion or none at all. Since enormous speeds are the hallmark of all astronautic missions, unpowered spacecraft require a “booster,” or “launch vehicle,” usually a rocket many times as large as the spacecraft. The weight of the spacecraft, in fact, seldom exceeds 5% of the total launch vehicle weight.

It is extremely expensive to put a pound of payload into Earth orbit. Thus designers have been justified in going to great lengths to convert a pound of structure into a pound of payload. Great improvement appears possible in this respect; only the cost of the propellant seems to be irreducible. In view of the high cost of space operations, it is especially important that space vehicles operate long enough to successfully fulfill their missions. A severe reliability requirement is thus imposed upon vehicles and equipment intended for missions, such as journeys to the planets, which may require up to a year or more to accomplish. For complex equipment in space vehicles, operating lifetimes of this order of magnitude are difficult to attain. The requirements for high reliability and low weight add tremendously to the cost of the payloads themselves, to the extent that their cost approaches that of the launch vehicle. All space missions through 1975 used expendable launch vehicles. The space shuttle, a reusable launch vehicle, is expected to reduce the costs of Earth-to-orbit transportation.

Gravity is a dominating influence in the design of space launch vehicles. Despite the fact that the pull of gravity extends to infinity, it is nonetheless possible to escape permanently from the Earth's gravity in the sense of never being drawn back to the ground. The key is speed. Circular velocity is the minimum at which a space vehicle can remain permanently above the Earth. At low altitudes, this velocity is about 25,000 ft/s (7.9 km/s). As the speed is increased above the circular velocity, the path of a vehicle becomes a larger circle or an elongated ellipse. When the speed reaches 37,000 ft/s, or about 7 mi/s (11.2 km/s), the path becomes a parabola and the vehicle will travel along one of the legs to infinity without further propulsion.

These velocities are tremendous by any previous standard. To reach them, a vehicle must carry the corresponding amount of energy in the form of propellant.

Even with the most energetic propellants and the lightest structures, it has not yet been possible to reach orbital velocity with a single rocket. To overcome this seemingly insurmountable obstacle, one rocket is carried as the payload of a larger one. When the larger burns out, the second is ignited and adds its velocity to that of the first. This is known as the step-rocket or staging technique. For lunar and planetary missions, lightweight vehicles, powerful propellants, and many stages are used. The lunar orbit rendezvous method required a total of six stages to take the Apollo astronauts to the Moon and back. See Rocket staging

Although propulsion is the key to space flight, other elements are essential and present numerous new problems. One such element is guidance and control. For the ascent phase of space vehicle flight, guidance systems similar to those used for ballistic missiles are employed. Another control requirement of many types of space vehicles is that of maintaining the desired vehicle attitude over long periods of time. Displacement gyroscopes, even excellent ones with very low drift rates, cannot provide an accurate reference for days or weeks. Such devices must be corrected frequently by an external reference.

At least two such references are available: sources of electromagnetic radiation, and the gravitational gradient. The first might be used by such devices as a Sun seeker, a star tracker, or a horizon scanner. In the vicinity of the Earth (or any large celestial body) the difference in the pull of gravity between points on the craft having different distances from the Earth can be usefully employed.

Reaction wheels or other devices capable of storing angular momentum may be used to provide the torque to effect or maintain a given orientation. Such devices are very efficient, both from a weight and an energy standpoint, where disturbing torques on the spacecraft are small, random, and long continued. At the opposite end of the torque spectrum, torques that are large and uncompensating, rocket engines are the most suitable.

Vehicle and payload equipment require electric power. For small amounts of energy, chemical sources, such as batteries or chemically fueled generators, may be used. A great deal more energy can be obtained from a nuclear reactor. Energy also comes continuously from the Sun but at a fairly low density at Earth's distance.

Communications equipment comprises an essential item of nearly all space vehicles. This equipment is designed for light weight, low power consumption, and, usually, long life.

Although a large percentage of the problems of space flight are associated with the vehicles, it would be a mistake to assume that these constitute even a major fraction of the total operating system. Indeed, the cost of overcoming the Earth's gravity is so great that any portion of the total operation which can be performed on the ground should be done there. The supporting ground equipment consists of the preparation and launching equipment, and the tracking, communications, and payload-oriented equipment for turning the received data into usable form. For missions which involve return of space vehicles or booster rockets, recovery equipment may also be required.

In their interaction with the terrestrial and atmospheric environment during reentry, space vehicles resemble ballistic missiles. However, although ballistic reentry techniques have been proved successful, the use of winged vehicles also has certain attractive aspects. There is a basic difference in these two methods in respect to the way atmospheric heat is handled. The ballistic approach absorbs the heat in the reentry body or rejects it back to the air by mass transfer. The winged vehicle dissipates the heat by radiation. Considerable research has been done on compromise reentry vehicles, such as the lifting body approach. The orbital stage of the shuttle is a winged craft designed to land like an airplane. It utilizes a combination of techniques to overcome the reentry heating problems: lift, temperature-resistant materials, and local ablative cooling.

Astronautical engineering must contend with the unique environment of space outside the Earth's atmosphere. Although gravity is present in space, whenever a vehicle is coasting unpropelled, the shell and everything in it are acted on equally by gravity and therefore appear weightless. Fluids do not flow naturally, but must be confined and extruded. Liquids exposed to the vacuum of space evaporate or freeze. External transfer of heat takes place only by radiation. Metals exposed to the ultraviolet rays of the Sun emit electrons. Small particles of cosmic dust strike external surfaces at fantastic velocities and gradually erode them. Cosmic radiation creates a spectrum of secondary radiation that may reach levels damaging to equipment or personnel.