Large-scale Space Engineering

The absence of a corrosive environment in space may permit the structural use of ultralight reactive metals whose terrestrial application is limited. Alloys based on magnesium-lithium in particular could enable a substantial reduction in the payload mass required for large space structures. Magnesiunllithium alloys are about half as dense as aluminum alloys. The direct in-situ preparation of shaped structural units by· metal foaming techniques appears to offer particular advantages in increasing structural stiffness. The tendency for elements to oxidize more readily as they decrease in density is illustrated for twenty-five elements in Figure 1. Aluminum alloys are currently the materials of choice for space satellites. The corrosion-free nature of the space environment, however, allows consideration of ultralight alloys. For large-scale space applications which do not involve crew quarters or other oxygen or water vapor containing environments, such an increased tendency to oxidize might not be as important as the concomitant reduction in density. There are five metallic elements-Li, K, Ca, Na, and Rb-which are less dense even than magnesium. In pure form, these elements are all quite soft and, indeed, little or no metallurgical effort has been aimed at improving their strengths. Such an effort WOUld, in any case, be made difficult by their high reactivity and concomitant low solubility for other elements. As it happens, however, lithium and magnesium do show substantial solubility. Beyond approximately 10 wt% lithium, magnesium-lithium alloys show a body-centered cubic structure which enormously improves the fabrication characteristics of these alloys. Above approximately 30 wt% lithium, however, the liquidus temperature decreases to below 250C. Because of this solubility, commercial alloys were at one time developed, as, for example, LA 141 A, which consisted of magnesium with 14 weight percent of lithium and 1 of magnesium with 14 weight percent of lithium and 1 weight percent of aluminum. 1 This alloy has a density of only 1.35 g/cm3 and, after heattreatment to produce age-hardening via the preCipitation of MgLi2AI, showed an ultimate tensile strength of 310 MPa (45Ksi). Importantly, the modulus of elasticity of this alloy (42. 7 GPa, 6.2 x 10 psi) is closer to that of magnesium (45 GPa, 6.5. x 10 psi) than to that of lithium (4.9 GPa, 0.7 x 10 psi). Regarding corrosion properties, if the rate of chemical reaction were assumed to be proportional to gas pressure, then the 10-11 decrease in pressure that occurs in going from sea level to 300 km would effectively reduce any reaction rate to a very low level. Materials that were rapidly and extensively attacked at sea level would be only minimally affected at 300 km. Because the elastic modulus of ultralight reactive metals, even magnesium-lithium, is relatively low, it could be beneficial to increase structure stiffness by increasing the crosssectional area of structural components by the use of metal foams. ConSider, for example, a beam of square cross-sectional area. If the cross-sectional area is increased by a factor of ten through foaming and the material modulus were decreased by this same factor, the resulting beam stiffness, as measured by its deflection under load would still be raised by a factor of ten. This increase in stiffness occurs because, for such beams, the second moment of the beam cross-section increases as the square of the cross-sectional area. A comparison of properties on this basis shows that LA 141 A foamed to a density of 0.135 g/cm could produce beams which are, on a stiffness to mass per unit length basis, several times better than those of solid aluminum. Unlike plastic foams, metal foams would not be expected to be affected . by ultraviolet radiation. It is also worth noting that, in the event of orbital failure, the possibility of any material reaching the surface of the earth would be considerably reduced through the use of reactive metals as the material of construction. Finally, on a mass-required basis, early work has shown that the micrometeriod impact resistance of LA 141 A is more than twice that of aluminum and 1.3 times that of magnesium. 1 To test these concepts, a'n experimental payload has been designed and built to be flown as part of the AcnVE ll'~' ~~,-----.---,-----.----,------.-----,----r-'~--'-'---'-'~ +30 "-:.co .80 t +20 No .Mg .s,