The materials used in the construction of rocket boosters and space vehicles range from special high-density material for heat absorption to high-strength, lightweight materials to carry flight loads. For each application, the requirement for minimum weight is dominant. Any unnecessary pound of material used in the construction of the flight vehicles reduces the useful payload by at least 1 pound.
Some possible future improvements in structural materials are discussed in the following paragraphs.
At present, designers have achieved structural configurations which have more than two-thirds of the maximum possible strength per pound of material. Some further gain can yet be expected from novel designs, closer control of material properties and manufacturing tolerances, 1 and the use of very large single shapes. Today's most efficient structural materials for normal temperatures, such as aluminum and titanium, can be surpassed in the future by new materials such as beryllium 2 and composite materials using high-strength filaments.3
Extreme thermal environment 4
The best current high-temperature metals, e. g., nickel and ferrous alloys, may soon be replaced by molybdenum. A better future prospect for higher temperatures is tungsten; however, there is today rather little metallurgical work being done on tungsten, and no effort toward alleviating such problems as its affinity for oxygen. Another excellent prospect for high-temperature use is carbon, possessing a host of attractive properties. But structural use of carbon will be severely restricted by its brittle behavior and the need for protection against oxidation, hydrogenation, and nitrogenation, problems on which little research is being done at present.
Ceramics such as carbides have very high melting points and show much promise for high-temperature use. They do not exhibit any ductile behavior, except in a few rare cases under meticulously controlled surface conditions; but they remain an attractive field of investigation.
Since each available material possesses only one outstandingly good property (for example, tungsten's ductility, ceramics' high-temperature strength),
1 Hoffman, G. A, A Criterion for Choosing Sheet Tolerances in Aircraft Materials, The RAND Corp. Research Memorandum RM-2127, Mar. 7,1958.
2 Micks, W. R., and G. A. Hoffman, A Reevaluation of Beryllium as a Potential Structural Material for Use in Flight Vehicles, The RAND Corp., Research Memorandum RM-1642 May 7, 1956
3 Hoffman; G. A, Fibered Materials for Flight Structures, The RAND Corp., Research Memorandum RM-1868, Feb. 18, 1957.
4 Hoffman, G. A., Materials for Space Flight, The RAND Corp., Paper P-1420, July 1, 1958.
the ideal material would be a composite of two or more materials, each component being utilized only for its best property. In some applications it may be advantageous to protect conventional structures from severe thermal environment, rather than to make a structure of high-temperature materials. Relatively brief encounters with a hot environment can be survived by the protection method, as in the insulation of rocket nozzles and reentry nose cones.
In attempting to reduce weight, vehicle skin thickness must be as low as possible. But since very thin sections of material possess appreciable strength only under tension (stretching) loads and in no other direction, unique design and handling problems are created that will continue to require study and experimentation.5
There are several methods for using thin-walled structures. In some cases the structure can be internally pressurized to keep the walls from buckling. The net stretching force due to internal pressure is made greater than the compressional force due to flight loads, so that the tank walls experience no compression, and buckling is avoided.
Another method of stabilizing thin sheets is commonly used in the construction of conventional aircraft (fig. 1). In this case (sheet and stringer construction), stiffening members (stringers) are fastened to the skin in the direction of the compressive load.
5 Sandorff, P. E., Structures for Spacecraft, American Rocket Society Paper No. 733-58. Nov. 20. 1958.
The same results can be obtained in a single piece by chemical milling or machining of a solid sheet to remove all metal except ribs or "waffles" that act as stringers (fig. 2).
Thin sheets can also be stabilized against buckling by placing a lightweight supporting core between two sheets to form a "sandwich." The core might be in the form of honeycomb (fig. 3) or corrugation, or possibly a light plastic or metal foam. sandwich construction is becoming increasingly important in higher temperature applications. This type of construction should be useful in such vehicles as hypersonic gliders.6
6 Braun, M. T., and E. G. Czarnechi, Structural Aspects of Earth Glide Reentry Vehicles. Boeing Airplane Co., Aug. 18-19, 1958.
A number of space-flight applications, such as collection of solar radiation for power generation, involve very large structures-covering areas of millions of square feet in extreme cases. The weight of these structures must be kept very low in order to make the associated systems feasible at all. Added to the problems of operating such structures in space are the problems (perhaps more serious) of packaging extensive amounts of fragile material compactly for carriage on launching rockets.
The bending and vibration of very light rocket structures interact with the flight-control system to such an extent that the structure and control system must be approached as an integrated design problem.
The equilibrium temperature of a space vehicle is determined principally by the nature of the structural surface.7 The radiation properties of this overall surface determine the relative rates of absorption of solar energy by the vehicle and the radiation of vehicle heat into space. This balance, along with the quantity of heat internally generated, determines vehicle temperature. Measures available to adjust this temperature balance include choice of surface color and smoothness of finish, as on Vanguard, Explorer, and Pioneer.8 The overall surface characteristics can also be controlled in flight by operating "flaps" that cover or expose more or less surface area finished black, as was done on Sputnik III.9 10
Data on entry of meteorites into the earth's atmosphere can be used to estimate crudely the number of encounters with meteorites that can be expected for a space vehicle by a simple comparison of the surface area of the earth with that of the vehicle. A vehicle close to the earth, such as a satellite, would be sheltered somewhat by the earth from meteoroid collision. However, this reduction (by a factor of about 2) is small compared with other uncertainties-estimates of numbers of meteorites vary by factors of 1,000 or more.
The depth of vehicle skin penetration due to meteoroid impact is a somewhat speculative calculation, since no facilities are yet available for experiment on effects at such high relative velocities.
The combination of great uncertainties in number and size of meteorites, in relative velocities, and in the phenomenology of high-speed impact, lead to widely uncertain estimates of the hazard to space vehicles due to meteoritic matter.
7 Sandorff, P E., and J. S. Prigge, Jr., Thermal Control In a Space Vehicle, Journal of Astronautics, spring 1956.
8 Medaris, Maj. Gen. J. B., The Explorer Satellites and How We Launched Them, Army Information Digest, vol. 13, No. 10, October 1958, p. 5.
9 Sputnik III-Laboratory in Space, U. S. S. R., No. 7 (22), p. 1.
10 The Third Soviet Artificial Earth Satellite Pravda, May 18, 1958, p. 3.
As a specific example, consider a spherical vehicle with a diameter of 1 yard. Calculations based on available information indicate that the average period between punctures would be somewhere between 3 months and 170 years, if the skin were 1 millimeter thick. If the skin were one-quarter inch thick, the mean interval between punctures is variously estimated at 300 to 150,000 years. In spite of the wide degree of uncertainty, the implication is clear; any skin thickness likely to be used in a substantial vehicle carrying men and machinery will be virtually free of hazard from this source. However, very thin structures like solar-radiation collectors and the like should be designed with a fair likelihood of puncture in mind.
The encounter with smaller particles (i. e., too small to penetrate) would be more frequent, of course. Thus, one might expect a "sandblasting" on the surface before penetration occurs. It has been estimated that surface erosion by small particles would be comparable to that produced by solar-particle radiation and by interaction with the gases of the solar corona. The erosion due to these 3 effects would, it is estimated, destroy the optical properties of a surface after about 1 year.
Because of the great premium on weight reduction in a space vehicle, designs that use a single item of structure for more than one purpose would be highly desirable. It has even been suggested that material for propellant tankage, say, be made of combustible material (like lithium, for example) that might itself be used as fuel.
Areas in which important uncertainties may exist include the effects on various materials of prolonged stay in total vacuum (particularly critical with respect to lubricants and paints and containers of gases and fluids) and prolonged exposure to radiations.