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Shock
Wave Basics
This page will cover the basic types of shock
waves that a rocket will experience in flight. Subsequent web pages will outline methods
of calculating the atmospheric changes that occur as air passes through
these shock waves (see page 2). This topic is much more complex than the
space on this page will allow, so the information presented here will be
brief. If you have questions on
these topics, please contact an ASA aerodynamicist.
The two main types of shock waves produced by a
supersonic body are Normal Shocks and Oblique Shocks. These two types are produced by the same
phenomena, but the shape of the supersonic object dictates which type of
shock wave is experienced.
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- Normal Shock
Waves -
A Normal
Shock is created by a blunt body in supersonic flow. The same body in a subsonic flow produces
waves of sound that propagate ahead of the body, basically
“warning” the approaching air stream of the approaching
body. These sound waves cause the molecules
in the air stream to begin to diverge around the body well in advance of
the actual body. When the object is
traveling supersonically, however, these sound waves cannot outrun the
object, and they pile up a short distance in front of the object. This stacking of sound waves is a Normal shock wave, and it
serves to instantaneously force the air to change direction around the
body. This effect is also referred
to as a Bow Shock, and is shown in the figure below, depicting a supersonic
bullet.
As a unit of air passes through the Normal shock wave, its
temperature, pressure, and density dramatically rise as its velocity
falls. In the case of the Normal
Shock, the air flow downstream of the shock (and therefore seen by the
bullet) is always subsonic.
A Normal Shock, though, is generally a special
case of a common Oblique Shock that
typically occurs on supersonic airplanes and rockets, as discussed to the
right.
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-
Oblique Shock Waves -
An Oblique
Shock is a sharp edged shock wave that is formed when supersonic flow
is turned on itself. These shocks
are weaker than Normal Shocks, and although the temperature, pressure,
density, and air stream velocity are reduced across the shock similar to
the Normal Shock, the air stream behind the shock is not necessarily
subsonic. The Mach number behind the
Oblique shock is calculated from the upstream Mach number, defined by the
angle at which the flow is tuned.
The figure below shows a typical oblique shock
formed by a sharp angle.
The next figure shows the companion of oblique
shocks, the Expansion Fan. The expansion fan is essentially an
infinite number of Mach Waves,
and has the opposite effects of an oblique shock. When the airflow is turned around a
corner, the temperature, pressure, and density fall as the Mach number
rises.
These two typical shock waves formations are
experienced in series on supersonic airplanes and rockets, and they dictate
the air properties down the length of the vehicle. The calculations used determine these air
properties may be found on Page
2.
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Now that the basic shock formations have been
discussed, the equations used to calculate air properties can be
covered. These equations and their
use can be found on Page 2.
This page summarized
the basic design issues that ASA has been working through for the design of
our space vehicle. The follow-up
presentation to this one will outline the CD vs. Mach relationship that
defines the aerodynamic efficiency and performance of a supersonic vehicle.
Future topics for this page include:
·
Aerodynamics
of the TLV-7
·
Shock
Wave Analysis
·
The CD vs. Mach Relationship
·
Reentry Issues for Our Space Flights
·
Ultra-High Altitude Rocket Recovery
·
The Aerodynamics vs. Structure Tradeoff
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