Having achieved LEO the next question is the energy requirements to change orbital altitude. The orbital altitude of the International Space Station (ISS) is higher than the nominal LEO by some 500 km, so additional propellant is required to reach
ISS altitude. The ISS is also at a different inclination than the normal United States orbits (51.5 degrees instead of 28.5 degrees) and the inevitable increment in propel-lant requirement will be discussed in Chapter 5 when describing maneuvering in orbital space. As orbital altitude is increased, the orbital velocity required decreases, with the result that the orbital period is increased. However, because the spacecraft must first do a propellant burn to accelerate to the elliptical transfer orbit speed, and then it must do a burn to match the orbital speed required at the higher altitude, it takes a significant energy expenditure to increase orbital altitude. Figure 3.5 shows the circular orbital speed required for different orbital altitudes up to the 24-hour period GSO at 19,359 nautical miles and 10,080 ft/s (35,852 km and 3,072m/s). Figure 3.6 shows the circular orbital period as a function of orbital altitude, and at GSO the period is indeed 24 hours. Translating this velocity increment requirement into a mass ratio requirement calls for specifying a propellant combination. The two propellant combinations most widely used in space are the hypergolic nitrogen tetroxide/unsymmetrical dimethyl-hydrazine and hydrogen/oxygen (see Table 1.4 in Chapter 1). The hyper-golic propellants are room-temperature liquids and are considered storable in space without any special provisions. Hydrogen and oxygen are both cryogenic and require well-insulated tanks from which there is always a small discharge of vaporized propellants. Both the United States and Russia have experimented with magnetic refrigerators to condense the vaporized propellants back to liquids and return them to the storage tanks. Had Buran continued development, the author saw a magnetic refrigerator to be used for the all hydrogen/oxygen propellant maneuvering and station-keeping systems used for Buran. The resulting mass ratios for the two propellants are shown in Figure 3.7. The propellant for this orbital altitude
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