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
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
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
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
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
The force on the vehicle can be modeled using simple
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
of the hypersonic regime, at Mach numbers from 5 to 7.
These aircraft would be powered by air-breathing
propulsion systems. Because ramjets and scramjets can not generate
thrust statically, the vehicles would likely employ
turbine-based combined cycle (TBCC)
rocket-based combined cycle (RBCC)
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
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
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
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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