Material Selection

An important factor that must be considered when designing a structure for space is the type of materials the structure will utilize. Choosing the proper materials will provide the optimal operational environment for the structure. The materials selected must meet three main criteria. These criteria are the number of launches required, the structural support provided, and the ability to survive the space environment. The material selection criteria relates to the reliability of the solar power satellite. The reliability is an important measure because of the lack of accessibility for maintaining the structure. Consistent operation during the 50 year duration of the mission is essential for the satellite to be a feasible power source.

Number of Launches

Among the main criteria considered in material selection, the number of launches required to transport an SPS is one of the most important factors. The number of launches is directly related to cost. At a present cost of $230 million dollars per launch vehicle (Titan IV), it is important to minimize the number of launches. Mass and volume of the structure and solar cell array are contributing factor to the number of launches. A goal volume of 300 cubic meters and mass of 4500 kg of payload per launch is assumed when evaluating how many launches are required for transport of a structure. These figures are loosely based on present vehicle capacities for future vehicle designs. The Titan IV has an ability to deliver 4540 kg in a payload volume 270 m3 into geostationary earth orbit (GEO). The space shuttle has a payload capacity of 304 m3 though it can only travel to lower Earth orbit (LEO).3

Survivability of Space Environment

The survivability of materials in space also influences material selection. The interaction between the materials and the space environment must be intensely considered to design an effective spacecraft. The spacecraft must be able to survive many elements of the space environment. These elements of the harsh space environment include radiation, magnetic influences, plasma fields, large thermal differentials, near zero atmospheric pressure, and traveling debris.3

Radiation

A selected material must be able to withstand the effects of radiation in space. Radiation in space can come from radiation belts, solar emissions, and cosmic radiation. This radiation can remove structural material. The removal of material due to radiation is minimal, therefore, there are no serious effects on most structure designs. However, due to the long duration of this mission, 50 years, the degradation of structural material should be a design consideration. The deployment structure will be hidden from the sun by the solar cells. This shielding provided by the solar array will minimize the radiation effects on the structure itself.

Magnetic Influences

The effects of magnetic influences in space must be considered when choosing a material. A non-magnetic material should be used for most of the structure due to the magnetic field produced by the Earth. A ferromagnetic material would be susceptible to orientation changes due to the magnetic dipole moment induced by motion through the magnetic field. Attitude control boosters must be added to combat this effect.

Plasma Fields

The interaction between the Earth's magnetic field and the solar wind results in an energized plasma environment. These plasma fields can affect materials in different ways. The hot plasma charges the surfaces of GEO spacecraft. This surface charging can exceed breakdown voltages, produce static discharges, and disrupt electronic components. An electrically conductive material allows the structure to act as a ground plane, thus mineralizing the charging effects.

Thermal Differentials

The thermal differentials in space must be considered when choosing materials. Thermal differentials in the structure are caused by uneven heating of the spacecraft. The space environment has a temperature of -168 oC (-270 oF). Heating of the structure is due to the sun's radiation and occurs only on the side facing the sun. Therefore, one side of the structure will be hot and the other will be cold. A material with a high thermal expansion coefficientmay have uneven expansion causing large thermal stresses. It would be beneficial to use a single material throughout the structure or materials with similar thermal expansion coefficients. The obvious choice would be to use a material with little or no expansion coefficient. A net coefficient of zero for a structure is theoretically possible within the temperature limits. Composites such as Kevlar, are currently being utilized in this manner.

Near Zero Atmospheric Pressure

Materials react differently in various pressure situations. Outgassing and sublimation of materials are direct results of the near zero (1.3*10- 12 kPa) atmospheric pressure in the space environment. Sublimation is state change of a material from solid to a vapor without becoming a liquid. This can cause whiskers and deposits to form as the material reverts back to the solid state. Sublimation rate is directly proportional to the vapor pressure of materials. Therefore, materials with higher vapor pressures like many composites should be avoided. Outgassing is the release of foreign materials previously absorbed in the structural material. Both effects may result in short circuits and disruption of sensors.

Traveling Debris

Finally, another concern when selecting a material is the materials ability to survive collisions with high velocity space debris. Before the first man-made rocket was launched into space many objects have passed through near-Earth space. Now, since over 4,500 spacecraft have been launched into space, thousands of man-made objects also pose a hazard to orbiting spacecraft. The continuous launching of new spacecraft and collisions between objects already in orbit will increases the amount of debris in orbit around the Earth. Although much space debris is destroyed upon reentry of the Earth's atmosphere, much more debris is appearing.

Spacecraft are concentrated into three orbital regions including low Earth orbit (LEO), semisynchronous Earth orbit (SEO), and geostationary Earth orbit (GEO). LEO's altitude ranges up to 2000 km above the Earth. GEO s altitude is around 36,000 km. The SPS will be located in a GEO.

The amount of debris and the relative velocities of objects in GEO are small, therefore collisions will be minimal. Initial origins of orbital debris include explosions, solid rocket motor firings, hypervelocity collisions, and the degradation of orbiting spacecraft structures. Meteoroids and micrometeroids are other sources of space debris. Meteoroids originate from the breakup of asteroids and comets. These meteoroids can orbit around the Sun and eventually pass through Earth orbital space. Micrometeroids consists of small particles and dust that travel through space high velocities. These particles are in the range of microns and travel at velocities 20 to 30 times the speed of a bullet.

The debris environment is difficult to accurately characterize. The debris population changes continually with the creation of new debris and the loss of debris reentering the Earth's atmosphere. Only large debris objects can be repeatedly tracked by ground-based sensors. The detection and tracking of numerous smaller pieces of debris leads to many difficulties. Optical telescopes are the primary sensors used in GEO. These sensors have the sensitivity to track meter-sized objects in GEO. However, this capability does not mean that all meter sized objects in GEO are cataloged. Current space surveillance systems have difficulties cataloging some space objects in highly elliptical orbits and low-inclination orbits. Objects in low-inclination orbits are difficult to detect because of the relative lack of sensors at low latitudes.

Several techniques have been developed to characterize the orbital debris environment, however, there is still a high level of uncertainty in theunderstanding of the debris population. There is an extremely vast population of small (<1mm in diameter) debris particles in Earth orbit. Knowledge of the distribution of these particles comes primarily from examination of returned spacecraft material. Because spacecraft from GEOs are not returned to Earth, it is challenging to accurately determine the debris population in GEO.

The probability that debris will collide with a spacecraft is dependent on the spacecraft's size and debris flux through the orbital region. The relationship between the probability of collision and the orbital region is very complex and varies significantly with altitude and degree of inclination. The estimates of collision probabilities are less accurate in GEO than the LEO due to sparse information available in the GEO debris population. The chance of collision with cataloged objects decreases sharply when the distance from the geostationary orbit for GEO spacecraft changes. The cataloged space object flux in the GEO region varies as a function of altitude above and below GEO. Going 50 km above and below the exact geostationary orbit causes the flux to drop by almost a factor of ten. At an altitude of 35450 km, the probability of an object hitting the inflatable tube structure is around one in twenty. This probability is assuming a life of fifty years.5 See Appendix 9.2 for calculations.

Structural Support

A third criteria that must be considered when choosing materials for spacecraft is the amount of structural support the material will provide. Any material must supply the required amount of structural support to prevent the structure from folding or buckling. Specifically, the SPS structure must keep the array planar and pointed continuously at the sun. A material must resist fracture and fatigue, while possessing high strength and stiffness. The structure must also be able to withstand gravitational forces from the sun, Earth, and moon. These gravitational forces acting on the structure were calculated to be:6

Fsun = 0.00581 N (0.001306 lbf)
Fearth = 0.3172 N (0.0713 lbf)
Fmoon = 0.000009 N (0.0000064 lbf)

The material must be strong enough to endure the forces and torques acting upon the structure as it simultaneously rotates about two axes.