Principles of Space Flight
To accomplish the launching of even the simplest satellite into orbit around the earth, scientists must deal with several complex factors. These include the earth's gravitational attraction, the control of the satellite after launch, and the nature of the space environment in which the satellite must function.
Getting Into Space
Rockets provide the power needed to launch a spacecraft and carry out its mission. The reactive force of a rocket is called its thrust. If the thrust is twice the weight of the launch vehicle at lift-off, the spacecraft assembly will rise at an initial acceleration of one gravity (g), or 32.2 feet (9.8 meters) per second per second. In order to enter orbit, the spacecraft must attain a final velocity of about 18,000 miles (28,800 km) per hour.
Rocket pioneers grasped a way to increase the final velocity achieved by a spacecraft by separating its rocket propulsion system into separate stages. They found that by arranging stages on top of one another and discarding the empty propellant tanks of each stage, the loss of deadweight resulted in a great increase in the velocity attainable. Staging is extremely important in space flight because the weight of a spacecraft, or payload, is usually only a small fraction of 1% of the total weight of the multistage launch vehicle at takeoff.
As a multistage rocket vehicle moves upward, propellants in the first stage are consumed, decreasing the vehicle weight. Acceleration increases until staging occurs, at which time the first stage (consisting of the propellant tanks, rocket motor, and supporting structure) separates and falls away. The second stage motor then ignites, and the spacecraft, traveling at several hundred or several thousand miles per hour, begins to accelerate again. The procedure continues until the last stage has achieved the desired flight velocity and is discarded.
Speed of Ascent
Within certain limits, the longer it takes to leave the earth and its atmosphere on a space mission, the less economical the procedure becomes. At low accelerations the spacecraft wastes great amounts of rocket propellant, because each second it loses in effect a velocity of 32 feet (9.8 meters) per second as a result of gravity. Thus the quicker the craft attains orbital or escape velocity, the less propellant it need waste in counteracting gravity.
The resisting effect of earth's gravity on the upward motion of a spacecraft subsides slowly as the distance between the earth and the spacecraft increases. At an altitude of 100 miles (160 km) the gravitational attraction of the earth and the spacecraft is 1% less than at the earth's surface, at 1,700 miles (2,700 km) it is % that at the earth's surface, and at 60,000 miles (96,000 km) it is %o- The gravitational attraction of the earth, for practical purposes, would be negligible for spacecraft at distances of a few million miles out in space.
There usually are limits to which a spacecraft can be accelerated without risk of structural damage. Also, in manned space flight, a properly positioned and secured space traveler cannot comfortably experience more than 5g or 6g during takeoff. High-g launchings also encounter aerodynamic drag loss due to high-speed flight in the dense lower atmosphere. Thus in determining initial launching speeds and rate of achieving high velocity, certain lower and upper limits of acceleration are considered.
The sounding rockets used for upper atmospheric research that appeared shortly after World War II were fired vertically to altitudes of over 100 miles (160 km). These single or two-staged rockets reached maximum speeds of the order of 3,000 to 5,000 miles (4,800-8,000 km) per hour at the moment of completion of burning of the propellants (burnout). Burnout occurred at altitudes from about 15 to 20 miles (24-32 km), and from this point on the rockets coasted upward, gravity slowly reducing their speed to zero at peak altitude. The rockets then descended, picking up speed until they finally crashed into a desert or ocean. The maximum altitudes and speeds attained by these rockets were not great enough to achieve an orbital path, or a closed path around the earth. Since the type of path they followed had a definite beginning and end and was not repetitive, it is often called a trajectory.
Earth Orbital Flight
A rocket (final stage and spacecraft) that achieves a burnout velocity of at least 18,000 miles (28,800 km) per hour at an altitude of over 125 miles (200 km), and that is directed on a path essentially parallel to the earth's surface, will establish an orbital flight path around the earth. At this altitude, molecules of air are so widely dispersed that aerodynamic drag is almost negligible. Thus the orbiting spacecraft, or artificial earth satellite, would remain aloft for years, circling the earth in the same manner as the moon, the earth's natural satellite. At this velocity, the satellite develops a centrifugal force that exactly balances the pull of earth's gravity. Less orbital velocity is required in orbits that are a greater distance from the earth, because the force of earth's gravity decreases with increasing distance.
In the launching of an earth satellite, the launch vehicle takes off vertically and then slowly tilts toward the east, achieving a 90° turn at burnout with at least sufficient velocity to orbit at the desired altitude. Satellite launchings are invariably made toward the east, since the earth rotates in that direction at an average speed of about 1,000 feet (300 meters) per second. The actual rotational speed is 1,040 feet, or 317 meters, per second at the equator, and decreases with increasing latitudes toward the poles. It is quite possible to launch a satellite in a westerly direction, but about 2,000 feet (610 meters) per second additional launch velocity is required.
As the orbital altitude increases, the orbital period—the time required to circle the earth—also increases, and the orbital velocity decreases. At an altitude of 1,075 miles (1,718 km) the period is 2 hours, and at 26,000 miles (41,600 km) the period is 24 hours. Since the latter is the same ;ngth of time that it takes for the earth to rotate once, the satellite (if in equatorial orbit) is said to be geostationary. This means that the satellite will always be above the same point on the earth's surface. Satellite orbital velocity at this altitude is about 7,000 miles (11,200 km) per hour. The moon, which has an orbital altitude of 240,000 miles (384,000 km), has a period of about a month and an orbital velocity of 2,300 miles (3,100 km) per hour.
Earth satellites usually travel in elliptical paths. However, they can be put into circular orbits. Circular orbits are more difficult to achieve because they require more precise control of speed and direction during launching. If the velocity at burnout is greater than required for a circular orbit, the point of burnout will be at perigee, the point closest to the earth in the spacecraft's elliptical orbit. If the burnout velocity is less than required for a circular orbit, the point of burnout will be at apogee, the point farthest from the earth in the spacecraft's orbit.
To escape the earth's gravitational attraction, a spacecraft must achieve a minimum velocity of 25,000 miles (40,000 km) per hour. This is also the minimum velocity required to reach Mars or Venus, whereas a flight to distant Jupiter would require a velocity of at least 32,000 miles (51,000 km) per hour. A flight to Pluto or the nearest star would require a minimum velocity of 37,000 miles (59,200 km) per hour.
A flight to the moon or another planet requires precise timing as well as precise aiming and control of speed, because of the motions of members of the solar system. Ideally, a spacecraft is kept on a flight path, or trajectory, that requires a minimum expenditure of energy. Minimum energy trajectories are elliptical paths called transfer orbits. In the transfer orbit to Mars,
speed must be reduced in climbing out of the earth's and sun's gravitational attraction, because the orbital velocity of Mars is less than that of the earth. Venus has an orbital velocity greater than earth's, and therefore the transfer orbit to Venus requires an increase in velocity. Mid-course application of rocket power is usually necessary to effect the changes in velocity. If a fly-by rather than a landing is planned, an exact match of the planet's orbital velocity is not necessary. Launch timing is particularly important because of the changing relative orbital positions of the planets. For example, the relative orbital position of Mars and Venus gives favorable opportunities for minimum-energy transfer orbits only about every two years.
Space flight requires navigation in three dimensions rather than essentially two, as in the case of travel on the earth's surface or in its atmospheric envelope. Furthermore, the rotation of the earth, the orbital speeds of the planets and other bodies, the varying gravitational influences and the immense distances and varying relative positions of members of the solar system, require a precision in navigation stretching the limits of scientific and technical capability. Navigation in flight paths to the moon and the planets requires continuous tracking from the earth and various changes in the spacecraft's velocity and direction of flight.
The requirements of guidance and control of a spacecraft vary widely, depending upon the type of spacecraft and its mission. Basic to most guidance systems is a positional memory system known as inertia} guidance. By the use of spinning gyroscopes, precise measurements are made of any deviation or change in the velocity of a spacecraft from the one originally planned.
Any significant deviation from the flight plan must be corrected to achieve the desired flight path. During the launch phase, the corrections may be made at once by changing the angles of the vernier thrust rocket motors, jet vanes in the rocket motor exhaust, or rocket motor which is hung on a gimbal-ring mount. In the case of a lunar mission, the data on deviation and corrective thrust requirements may be stored in a computer memory system on the spacecraft, at the same time being transmitted to tracking stations. A mid-course and late mid-course correction maneuver can then be made by firing a rocket motor in a precisely determined direction for a precisely determined length of time. This application of thrust places the spacecraft on a corrected trajectory to accomplish its mission. During a power maneuver, acceleration measurements continue to be made. Any differences in the corrected flight path from the original course are taken into account in further power maneuvers, such as in terminating a flight.
Tracking and Communications
In orbital flight and in lunar and planetary space missions, the rotation of the earth makes many tracking stations necessary in order to keep continuous radio and radar contact with the space vehicle. For this reason a worldwide network of large-antenna tracking stations has been established. Through telemetry—the production and transmission of radio signals that can be translated into such meaningful values as temperature, pressure, and acceleration—the flight path and various conditions are monitored at tracking stations. Instructions or queries also may be transmitted to the spacecraft. Thus a dialogue is maintained, although the spacecraft is thousands or millions of miles away. Proper performance of on-board transmitters and receivers requires precise orientation of antennas and solar-cell power systems. A deep-space probe will be programmed to "lock on" to the sun or to another star as a reference point in flight. In earth orbital space flights, both sun-seeking and earth-horizon scanners may be used for orientation of the spacecraft.
Since large amounts of data are desired at tracking stations, and the transmission power requirements of the spacecraft transmitters and receivers are limited, techniques of compressing data and achieving high-speed transmission are used. On-board memory-storage systems for data and photographs facilitate the transmission of information when it is asked for by coded signal from the earth.
Manned Flight Control
With man on board a spacecraft, the availability of his judgment and selection facilitates some of the guidance and communications activities on the craft. However, man's presence also introduces the need for additional monitoring equipment
Generally, instructions for maneuvers of manned spacecrafts are transmitted from earth to the vehicle, and the space traveler executes the orders at the determined time. Override control may be provided, in case the space traveler is unable to perform the maneuvers because of his physical condition or because of nonoperative on-board controls. In an earth-orbit rendezvous, the data for transfer of orbit is provided from the earth and is based on tracking data of both orbiting vehicles. Once the rendezvousing spacecraft establishes radar contact with the other spacecraft, the space traveler can close on the target and eventually use direct visual observation for the final docking, or contact, maneuver.
In the case of a manned spacecraft, provisions must be made for the space traveler's protection and needs. These include radiation shielding and an environmental control system to provide oxygen, remove carbon dioxide from the cabin atmosphere, and regulate temperature and pressure. Food and water must also be supplied, as well as a means for collecting physical wastes. The space traveler must be provided with instrument displays, controls, warning signals, and a means of visual observation and communication. Finally, an even higher reliability called man-rating is called for in the design and test of manned craft.
Recovery of earth satellites requires the use of retrorockets, or braking rockets, which apply thrust in the direction opposite to the flight path. The resultant loss in velocity of several thousand feet per second causes the spacecraft to drop toward the earth. As the vehicle enters the outer fringes of the atmosphere, aerodynamic drag begins to occur, and the spacecraft arcs increasingly sharply toward the earth. Heating due to friction produced by air passing over the spacecraft becomes so intense that the spacecraft reaches incandescence if not properly shielded. To withstand this reentry heating, a special heat shield is provided that vaporizes and absorbs heat while the insulated spacecraft slows down. At an altitude of about 15,000 feet (4,600 meters) a small, drogue parachute is released, followed by one or more main parachutes. In the United States manned spacecrafts are recovered at sea. In the USSR recoveries of spacecraft have been made on land. Soviet cosmonauts could alternatively eject from the spacecraft in final descent and land separately by parachute.
In lunar landings, the absence of atmosphere eliminates the possibility of aerodynamic drag recovery. Accordingly, retrorockets are used in the final descent phase. Radar sensors ignite the braking rockets at the precise moment, bringing the spacecraft to a hovering condition a few feet above the lunar surface. The rocket motors then shut down, and the spacecraft drops the final few feet to the surface. Since lunar gravity is about 1/6th that of earth's gravity, the problem of impact is lessened significantly.
Automated research packages have thus far been landed on the planets Venus and Mars by means of drag devices, since both planets sustain atmospheres. The atmosphere of Venus is thick and the hidden surface of the planet is very hot, so the design problem is to develop a package that can endure the high temperatures it must encounter. The atmosphere of Mars, on the other hand, is very thin in comparison to the earth's atmosphere, while surface temperatures are not a problem; the difficulty is to obtain sufficient aerodynamic drag.