Min/Micro Shuttles with hypergolic noncryogenic liquid rocket bipropellant

Introduction M INI and Micro Shuttles are members of the hypothetical Space Shuttle family, whose largest member is the Shuttle II (a second-generation Space Shuttle). One Shuttle II concept, among several considered by NASA in its Access-to-Space study of 1993 is for a reconstituted, full-size Space Shuttle with essentially the same external tank (ET) and solid rocket boosters (SRBs) but having an orbiter upgraded with the latest technology and an improved thermal protection system. Moreover, the Shuttle II would also incorporate a streamlining of systems to simplify ground operations so as to shorten the launch preparation time and lower costs. The concept of a Space Shuttle family had its origin in the idea of downsizing the Shuttle Orbiter while replacing liquid hydrogen as fuel with kerosene (RP-1). This partial downsizing results in a slightly smaller version of the Shuttle (called a Mini Shuttle) with a |-size orbiter and ET but the same size SRBs. For about the same liftoff weight as the full-size Shuttle, the Mini Shuttle has a payload capability about 75% as large in both size and weight. The Mini orbiter is still large enough for crewed missions with space for a crew of up to five and a payload bay measuring 12 x 48 ft. The three Space Shuttle main engines (SSMEs), producing 1.125 x 10 Ib of thrust at sea level, could be replaced by a single, kerosene-fueled Russian RD-180 engine, which can be throttled from 37 to 100% of its rated thrust of 9.33 x 10 Ib while remaining fixed in specific impulse. Although the Mini Shuttle could prove to be less costly to operate than either the Space Shuttle or the Shuttle II, with a lower cost per pound of payload delivered to orbit, it is still a huge vehicle and larger than necessary for small-to-medium-size payloads (1,00015,000 Ib). To provide even lower payload-delivery costs, the size of the vehicle must be scaled to the size and weight of the payload. Thus, the Mini-Shuttle concept has been extended to obtain four successively smaller versions of the Mini Shuttle that are similar in function and shape but differ greatly in size and weight. These so-called Micro Shuttles have payload capability commensurate with their total liftoff weight. Together with the Shuttle II and Mini Shuttle, they make up a proposed six-member Space Shuttle family. The orbiters in this family vary from full size down to | size with the same planform loading so as to ensure having similar aerodynamic characteristics. In the initial conception of Micro Shuttles as scaled-down derivatives of the kerosene-fueled Mini Shuttle, it was assumed that the bipropellant for the liquid rocket engines would be kerosene and liquid oxygen. However, because some potential applications of the Micro Shuttles require the ability to launch on short notice, it is highly desirable that both the fuel and the oxidizer be noncryogenic and storable. It is fortunate that there is such a bipropellant that is hypergolic (autoigniting) and able to yield a sufficiently high specific impulse to be considered a candidate for rocket use. It is, in fact, currently used by the Air Force in its Titan II, III, and IV space launch vehicles (SLVs). Both the first and second stages of the Titan SLVs use Aerozine-50 as fuel and N2O4 (nitrogen tetroxide) as oxidizer. Aerozine-50 is a blend of 50% (by weight) unsym-