Landing by manned space vehicles is constrained by the same considerations that apply to landings by aircraft, parachutes, etc. Thus this discussion is largely devoted to landings by unmanned vehicles. Landing a payload on the Earth, Moon, or distant planetary body or asteroid is influenced mainly by 1
The velocity of approach is the velocity of the payload just prior to contact with the "target" material. The kinetic energy possessed by the payload by virtue of this velocity must be completely dissipated in landing. The kinetic energy is a fundamental basis for comparison of impacts-a heavy body impacting at low velocity is about equivalent to a light body impacting at high speed, if their kinetic energies are equal.
The physics of impact suggests that the dissipation of energy can take place in many ways-and, in fact, the precise redistribution of energy cannot be predicted with current theory for even the simplest impact problem. Heat, friction, deformation of both target and payload, wave energy imparted to the surrounding atmosphere (if any) are some of the physical phenomena that take place in most impacts.
Landings on the Earth may take place in water or land. Landings may occur on a variety of surfaces, such as dry sand, clay, or rocks of varying degree of consolidation. Landing problems are markedly different in crushed rock, shale or pebbly material, with each piece relatively hard but free to move, as contrasted with an outcropping of solid rock.
Terrains are classified as to their compressive strength, determined by firing projectiles into material samples, and by compression tests in the laboratory. These are rather crude approximations to the very wide spectrum of terrain conditions on Earth and, most probably, on other planetary bodies. However, a considerable uncertainty in compressive strength is tolerable for properly designed landing systems.
1 Lang, H. A., Lunar Instrument Carrier-Landing Factors, The RAND Corp., Research Memorandum RM-1725, June 4,1956.
The nature of the payload determines the degree of sophistication required in the landing operation.
Experiments indicate that amplifiers, transistors, and other electronic gear can be designed and mounted to withstand very heavy deceleration loads-of the order of 3,000 g to 20,000 g or more. These are well above loads encountered in the majority of controlled landings. Consequently, excluding exceptionally delicate instruments, specifications on landing loads are not likely to be limited by electronic equipment.
One extreme limit on landing speed is provided by the need to keep communication antennas above ground-the payload should not hit so hard that it buries itself completely.
The landing loads tolerable by human beings are among the lowest likely to be specified in the design of landing systems.
There is a considerable difference in the total weight of landing arrangements required to land on planets having atmospheres, as compared with a body such as the Moon that has no appreciable atmosphere. Presence of an atmosphere permits use of lifting surfaces and other aerodynamic devices like parachutes for gradual descent to a light landing. Without an atmosphere, braking rockets, which are extravagant in their use of available weight capacity, are required to reduce the approach velocity to a permissible level.
Typical designs for landing on soils are depicted in figures 1 to 3. They all aim to reduce the landing load by taking up the shock in extended penetration of the target surface by a body of relatively small cross-section.
For terrain that is not level, a direct impact perpendicular to the ground surface cannot be guaranteed.
Spherical payloads with several spikes attached to a vehicle (fig. 2) have been suggested to increase the chance that in any orientation one spike will imbed. Figure 1 also illustrates the notion of using one or more devices for several purposes-if practicable. Thus, for a lunar landing, the retrorocket case can deform upon impact and partially dissipate kinetic energy. The soft landing vehicle of figure 3 is more complex and bulky but has the advantage that the spider legs, which may incorporate energy dissipative devices, would help position a landing vehicle appropriately for special purposes, such as takeoff for a return trip (fig. 4).2
Landings are classified, somewhat arbitrarily, as soft or hard, de- pending upon approach velocities less than or greater than 500 feet per second, respectively.
2 Figs. 1 to 4 are adapted from G. A. Olson, Lunar Vehicles, Proceedings of Lunar and Planetary Exploration Colloquium, July 15, 1958, pp. 10-11.
No planetary body other than earth is known to be covered to any appreciable extent by fluids. Consequently, landing in water is of importance only in recoveries on the Earth.
Water impacts may occur either because it is difficult to control the landing operation to impact within a designated land area or because weight limitations preclude the use of control devices.
Where a controlled landing is possible, it may still be desirable to impact in water because there is a high probability of recovering the payload from such a landing. Impacts on land involve the hazards of burial of payload, difficulty of access and search in some areas because of climate, terrain, or political factors.
Some other advantages of water recovery are: