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Pictures of three hypersonic missions; re-entry, air-breathing
 accelerator, and air-breathing cruiser

The design of any aircraft or spacecraft begins with a definition of the mission of the aircraft. Aircraft mission dictates the size of the vehicle, the materials to be used, the environment in which the aircraft must operate, and the type of propulsion system. For hypersonic aircraft, three principle missions have been identified with each mission having its own unique vehicle requirements.

Re-entry from Orbit

The earliest studied and most often encountered hypersonic flows involve the re-entry of a spacecraft from orbit around the earth. NASA's Mercury, Gemini, and Apollo spacecraft experienced hypersonic flows as they safely returned their crews to the earth's surface during the 1960's. The current NASA Space Shuttle, Russian Soyuz, and Chinese Shenzhou must also pass through the hypersonic flow regime. Because the mission of a re-entering spacecraft is to slow from 17,500 mph to zero at the surface, the spacecraft is designed to have high drag. During re-entry, the craft is unpowered and strong shock waves generate tremendous heat on the windward side of the vehicle. All of the space "capsules" used ablative heat shields to protect the crew from the heat; the surface of the spacecraft was designed to slowly burn away. The Space Shuttle uses a different mechanism for thermal protection. The bottom of the shuttle is covered with silicon tiles that insulate the aluminum skin from the heat of re-entry.

Re-entry hypersonic flows are typically at Mach numbers from 25 to 10, with the vehicle constantly decelerating. The surface may be fully insulated, or it may undergo a physical change of state from solid to liquid to gas as it burns away. Because of the high temperatures, the gas is an electrically charged plasma. And because of the high altitudes where re-entry begins, the air is highly rarefied, having very low density. The force on the vehicle can be modeled using simple Newtonian flow.

Air-breathing Cruiser

There have been several design studies to build air-breathing, hypersonic cruising aircraft. The military has proposed hypersonic cruise missiles, high altitude, high speed reconnaissance vehicles, and piloted global reach vehicles that could deliver cargo or weapons to any location on earth in just a few hours. On the civilian side, the Orient Express was proposed to carry passengers from California to the Pacific rim in a few hours. All of these aircraft would cruise at the lower limits of the hypersonic regime, at Mach numbers from 5 to 7. These aircraft would be powered by air-breathing ramjet or scramjet propulsion systems. Because ramjets and scramjets can not generate thrust statically, the vehicles would likely employ turbine-based combined cycle (TBCC) or rocket-based combined cycle (RBCC) engines. Besides the unique propulsion systems, these vehicles would likely include special materials for thermal control, or actively cooled surfaces.

Cruising hypersonic aircraft would be flown at high dynamic pressure, or high-Q, conditions. Hypersonic flow experiences shock waves, thick boundary layers, and complex interactions between the shocks and boundary layers. Because of the high temperature generated at Mach 5 - 7, real gas effects must be evaluated during design. The flow can be properly modeled by the Navier-Stokes equations, if provision is made for the real gas effects and boundary layer transition models.

Air-breathing Accelerator

Besides hypersonic cruise vehicles, there have been several proposed hypersonic accelerator vehicles. The distinction from the cruise vehicle is that the accelerator must continually produce excess thrust, thrust greater than drag, in order to accelerate; the accelerator is never flown in a steady cruise condition. The accelerator can be used as a single stage to orbit vehicle, like the National Aerospace Plane (NASP) shown on the figure, or as the re-usable first stage of a two stage booster, like the German Sanger configuration. Depending the exact mission, the accelerator may employ either TBCC or RBCC engines. The NASP design used an ejector ramjet for low speed operation. Many of the flow problems associated with the cruise vehicle also apply to the accelerator. Although, depending on the exact mission, the accelerator may have to operate over a larger Mach range.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 07 2021

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