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Aerodynamics
of the TLV-7
The aerodynamic design of the TLV7 was a study
of the supersonic modeling capability of several analysis tools. There are three major aerodynamic
regimes: Subsonic, transonic, and
supersonic. The subsonic realm is
easily defined and studied, as these speeds are relatively easy to
reproduce in low-technology wind tunnels.
The transonic speed range (approximately Mach 0.8 to Mach 1.2) is an
extremely dynamic phase of flight for both airplanes and rockets. It was the “unbreakable wall”
to the pre-X1 aerodynamicists, and still is a troublesome area for aircraft
design. Once an airplane or rocket
is reaches the true supersonic regime, it is past the transonic danger zone
but shock waves and temperature issues still cause problems. At this point in ASA rocket design, we
are most concerned with the transonic and supersonic phases of flight, as
they are less understood and more difficult to model.
As a rocket nears the speed of sound, shock
waves will begin to form on the leading edge of every forward surface (nose
cone, fins, body farings, antennas, fuel loading ports, access door hinges,
etc.). These shock waves begin
weakly (some are experienced even before the rocket reaches sonic speeds)
but as the vehicle travels farther beyond the speed of sound, the shock
waves become more pronounced, stronger, and at a closer angle to the rocket
body. These shock waves result in
rapidly increasing temperatures and pressures along every surface inside of
the shock wave.
As a unit of air passes through the shock wave,
its temperature, pressure, and density dramatically rise as its relative
velocity falls. These shifts must be
taken into account when designing a rocket; otherwise the material
properties and structural integrity in those locations may not be adequate
to handle the extreme environment.
The ASA designs minimize these forces by using very small ramp angle
nose cones, minimum external components, and very smooth paint.
The following picture is a simple example of the
shock waves and expansion fans that the TLV7 experienced during
flight. Note that each successive
body structure on the rocket experiences dynamic forces in the wake of the
previous shock wave. This can be beneficial
(lower velocity airflow) and detrimental (compounding temperature rises).
In this picture, the red lines show the three
major shocks experienced by the TLV7 (on the nose cone, the camera mirror
faring, and one each on the four fins).
The blue lines represent expansion fans, which are basically
“negative” shocks.
Opposite of the shock wave dynamics described above, an expansion
fan results in lower temperatures, pressures, and density while Mach number
rises. As noted above, each
successive shock in this picture is weaker; since it is downstream of the
shocks before it (weaker shocks have larger angles than strong shocks).
Note: The previous picture depicts a
two-dimensional (2D) flow field over the rocket. In reality, the air flow has farther to
expand as it flows around the three-dimensional rocket, and therefore the
shock forces are lower than what is calculated for the two-dimensional
field.
The q-B-M equations may still be used at transition
angles, but CFD analysis must be utilized to model the flow between
shocks. A simplifying assumption may
be used, therefore, conservatively modeling the symmetric 3D flow as a 2D
flow.
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The picture below is a
scale drawing of the TLV7, and on this drawing you can see the calculated
aerodynamic center range (the shape that looks like an “I”
shows how much the aerodynamic center moves from Mach 1 to Mach 3).
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We have used several
analysis tools to calculate the rocket’s performance in flight. Although many of the tools agree (within
large tolerance bands), testing is the only way to truly understand how a
rocket (or any airframe for that matter) will perform in-flight.
Since the supersonic
realm we are interested in researching is very expensive to reproduce on
the ground, we must find other solutions for gathering data.
Therefore, we have
opted to design the test rockets similarly to the full-scale rocket, and
then fly these test rockets faster than the speed of sound carrying a full
complement of data gathering instruments.
Our intent is to use
the atmosphere itself as a test bed, rather than purchase time in a wind
tunnel. The data gathering equipment
on-board (along with known atmospheric conditions and motor performance)
enables us to reconstruct each flight, and determine the rocket’s
performance through the sonic barrier.
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Admittedly, this is
not the most effective or sterile test environment, but it does help us to
prove the capability of our modeling tools.
The last two flights of the ½ scale rocket have provided enough data
to show that the tool predictions are “in the ballpark” but
further refinements to our testing process will be needed to satisfy our
test requirements.
The tools that we have
been using range from basic hobby-rocketry performance analysis tools
(which use empirical data gathered from hobby rocket flights) to
Computational Fluid Dynamics programs available commercially. Note for those interested in analysis
tools: The results generated by the “AeroLab” analysis program
have been notably accurate through the transonic region. AeroLab is available on the internet.
One additional problem
with supersonic flow is that the “aerodynamic center” of the
rocket moves forward as the rocket reaches higher Mach numbers (this is
shown on the drawing above). This
effect is called the Munk shift, and if not accounted for can cause a
rocket to become unstable during flight.
A rocket is considered
stable if its center of gravity (CG) is in front of its aerodynamic center
(AC) – meaning the CG is closer to the nose of the rocket. As the Munk shift moves the AC forward,
it may cross the CG, rendering the rocket unstable. At this point, without a sophisticated
guidance system, the rocket will flip end over end and will be destroyed. Since the CG of a rocket is not
necessarily fixed either (it can move forwards or backwards as the rocket
uses up its fuel) the CG/AC interaction must be carefully analyzed during
the design process.
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:
·
Shock Wave Basics
·
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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