QUESTION:
What are the average values for Landing Stall Speed?
ANSWER from Paul Schwindt on December 8, 1997:
The question: "What is the Space Shuttle stall speed?" is a bit of a
trick question because, as a delta wing aircraft, the Orbiter does not
have a clearly defined stall point. Discussions of Space Shuttle flight
mechanics need to be treated separately for ascent powered flight and
descent unpowered flight.
During ascent, the vehicle follows a prescribed trajectory which has
been tailored on the day of launch to best accommodate the actual day of
launch winds aloft. Steering/guidance commands are defined in near real
time to minimize the effects on angle of attack, sideslip, and the
vehicle loads that might result. The criteria is similar to that used
for other rockets and does not involve the "stall" speed considerations
typical for conventional aircraft. Ascent studies typically address
angle of attack, sideslip, and dynamic pressure dispersions that result
from combinations of winds and system effects such as booster
performance variations. The typical ascent angle of attack limits
during first stage are between 0 and -7 degrees with a target of about
-3.5. The sideslip limits are typically +/- 4 degrees with most
experience less than 1 degree. The dynamic pressure reaches a maximum
of 819 psf at around Mach 1.25. Typically the trajectories target a
lower value to protect against load limit exceedances.
During the descent unpowered flight phase, the vehicle also flies a
prescribed trajectory. However, unlike the ascent phase, the Commander
actively controls the vehicle with various energy management maneuvers.
Because of the crew involvement, our analyses are similar to
conventional aircraft which includes criteria such as the Vn diagram.
We have determined the minimum flyable speed based on the longitudinal
stability characteristics. We start by evaluating the maximum angle of
attack, alpha, for which the Orbiter is aerodynamically stable in the
pitch axis. This is the angle of attack at which the pitching moment
partial derivative with respect to alpha changes from negative (stable)
to positive (unstable). This angle of attack is identified as the
maximum usable angle of attack and it varies by Mach number.
Conversely, the minimum usable angle of attack for the Orbiter is -10
degrees which represents a "pitch down" condition. The minimum maneuver
speed, VA, is defined in accordance with the Vn diagram criteria, as the
speed corresponding to the maximum usable angle of attack (or the point
at which the flight controls reach their limits) at the maximum required
load factor. This is the upper left corner of the Vn diagram. We
present the Orbiter Vn diagram as a "Qn" diagram with nz as a function
of dynamic pressure (qbar) for convenience since structural loads are
linearly proportional to dynamic pressure.
The VA point varies with Orbiter weight. It typically varies between
150 and 250 knots depending on the mass properties and Mach number. The
minimum speed, the point at which the pitch attitude is at the maximum
angle of attack and the vehicle is at 1 g ("trim"), is typically between
60 and 110 knots depending on the weight, c.g, and Mach number. Note
that the concept of "trim" is not quite the same for the Orbiter as it
is for conventional aircraft because it is an unpowered glider - trim
condition implies powered level flight.
The "Qn" diagram was determined as a composite of several envelopes that
included the full range of permitted control surface excursions. The
maximum abort weight load factor is reduced to keep the design wing
loading, represented by the product of vehicle weight and normal load
factor, at a constant values of
Max Wing Loading = 211000 lbs x 2.5 g's = 256000 lbs x 2.06 g's.
Orbiter flight operations typically vary between dynamic pressure limits
of 250 and 300 psf, with load factors varying between 0.8 and 1.8 g's.