Defense Intelligence Reference Document Space Access Where We ve Been And Where We Could Go

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Appendix B: Aeropropulsion Integrated Vehicle
A launcher that uses air-breathing propulsion in a portion of its flight to exit the
atmosphere has the same entry issues as the rocket-boosted hypersonic glider.
However, the capture of atmospheric air to create thrust by chemical combustion is a
different issue, as it configures the underside (aerodynamic compression side) as a
propulsion system that produces more thrust than drag and also produces lift. For the
propulsion system to function efficiently, the dynamic pressure and air mass flow per
unit area must be higher than a rocket exit trajectory, as it is the airflow mass that
enables the propulsion system to produce thrust in excess of drag so the vehicle can
accelerate. So in this case we have a propulsion-configured vehicle. Neither the shape
of the vehicle nor the trajectory it flies is arbitrary. The air breather does not exit the
atmosphere as quickly as the rocket but stays in the atmosphere to the point where the
transition to rocket propulsion occurs-usually set when the air-breather propellant per
unit change in velocity is equal to or greater than the rocket propulsion, usually at
about mach 12 to 14. The air-breathing propulsion system mechanical, aerodynamic,
and thermal loads act longer and are of greater magnitude than the rocket-powered
vehicle. In fact, the dynamic pressure-that is, the pressure of the air impacting the
vehicle-is about 10 times greater than the entry dynamic pressure of the hypersonic
glider. In this case the principal thermal load is encountered during exit from the
atmosphere and the vehicle must be configured to generate sufficient thrust to provide
a strong acceleration. So an air-breather configuration is different from the hypersonic
glider, because the hypersonic glider has not been configured to fly extensively in the
atmosphere and produce thrust from captured airflow. Like the hypersonic glider, this
vehicle needs the same glide performance at entry. However, with the thermal
protection designed by the high exit loads, the entry design is one of detail in
maintaining stability and control and of achieving a comparable glide L/D ratio. The
carried oxidizer is heavy and requires more engine thrust to lift it into space. A
hydrogen/oxygen rocket, vertical-launch vehicle with a 7,000-kg payload has a gross
weight in the 450,000- to 500,000-kg range and a 50,000-kg operational weight empty
(that is, with the payload loaded). The engine thrust for a vertical takeoff is about
607,000 to 820,000 kg. A modest-performance combined-cycle air breather with a
7,000-kg payload and a 50,000-kg operational weight empty has a gross weight in the
200,000- to 225,000-kg range. The engine thrust for a vertical takeoff is about
270,000 to 304,000 kg. Most of the gross weight reduction is from the lesser amount
of oxidizer carried and the lighter propulsion system weight.
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