Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
design was trickier than it looked. They could not be too wide, or the V-2 would be
unable to pass through railroad tunnels. Nor could they extend too far below the
body of the missile, or the rocket exhaust, expanding at high altitude, would burn
them off.
The historian Michael Neufeld notes that during the 1930s, “no one knew how
to design fins for supersonic flight.” The A-3, a test missile that preceded the V-2,
had proven to be too stable; it tended merely to rise vertically, and its guidance
system lacked the authority to make it tilt. Its fins had been studied in the Aachen
supersonic tunnel, but this problem showed up only in flight test, and for a time
it was unclear how to go further. Hermann Kurzweg, Rudolf Hermann’s assistant,
investigated low-speed stability building a model and throwing it off the roof of his
home. When that proved unsatisfactory, he mounted it on a wire, attached it to his
car, and drove down an autobahn at 60 mph.
The V-2 was to fly at Mach 5, but for a time there was concern that it might
not top Mach 1. The sound barrier loomed as potentially a real barrier, difficult to
pierce, and at that time people did not know how to build a transonic wind tunnel
that would give reliable results. Investigators studied this problem by building heavy
iron models of this missile and dropping them from a Heinkel He-111 bomber.
Observers watched from the ground; in one experiment, Von Braun himself piloted
a plane and dove after the model to observe it from the air. The design indeed
proved to be marginally unstable in the transonic region, but the V-2 had the thrust
to power past Mach 1 with ease.
A second test missile, the A-5, also contributed to work on fin design. It supported
development of the guidance system, but it too needed fins, and it served as
a testbed for further flight studies. Additional flight tests used models with length
of five feet that were powered with rocket engines that flew with hydrogen peroxide
as the propellant.
These tests showed that an initial fin design given by Kurzweg had the best
subsonic stability characteristics. Subsequently, extensive wind-tunnel work both
at Peenemunde and at a Zeppelin facility in Stuttgart covered the V-2’s complete
Mach range and refined the design. In this fashion, the V-2’s fins were designed with
only minimal support from Peenemunde’s big supersonic wind tunnels. 7 But these
tunnels came into their own later in the war, when investigators began to consider
how to stretch this missile’s range by adding wings and thereby turning it into a
supersonic glider.
Once the Germans came up with a good configuration for the V-2, they stuck
with it. They proposed to use it anew in a two-stage missile that again sported fins
that look excessively large to the modern eye, and that was to cross the Atlantic to
strike New York. 8 But there was no avoiding the need for a new round of windtunnel
tests in studying the second stage of this intercontinental missile, the A-9,
which was to fly with swept wings. As early as 1935 Adolf Busemann, another
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colleague of Prandtl, had proposed the use
of such wings in supersonic flight. 9 Walter
Dornberger, director of V-2 development,
describes witnessing a wind-tunnel test of a
model’s stability.
The model had “two knifelike, very
thin, swept-back wings.” Mounted at its
center of gravity, it “rotated at the slightest
touch.” When the test began, a technician
opened a valve to start the airflow. In
Dornberger’s words,
First Steps in Hypersonic Research
“The model moved abruptly, turning
its nose into the oncoming airstream.
After a few quickly damping oscillations
of slight amplitude, it lay quiet and
stable in the air that hissed past it at 4.4
times the speed of sound. At the nose,
and at the edges of the wing supports
and guide mechanism, the shock waves
could be clearly seen as they traveled
diagonally backward at a sharp angle.
As the speed of the airflow fell off and the test ended, the model was
no longer lying in a stable position. It made a few turns around its center
of gravity, and then it came to a standstill with the nose pointing downward.
The experiment Dr. Hermann had wished to show me had succeeded
perfectly. This projectile, shaped like an airplane, had remained absolutely
stable at a supersonic speed range of almost 3,500 mph.” 10
A-4b missile ready for launch. (U.S. Army)
Work on the A-9 languished for much of the war, for the V-2 offered problems
aplenty and had far higher priority. But in 1944, as the Allies pushed the Germans
out of France and the Russians closed in from the east, Dornberger and Von Braun
faced insistent demands that they pull a rabbit from a hat and increase the V-2’s
range. The rabbit was the A-9, with its wings promising a range of 465 miles, some
three times that of the standard V-2. 11
Peenemunde’s Ludwig Roth proceeded to build two prototypes. The V-2 was
known to its builders as the A-4, and Roth’s A-9 now became the A-4b, a designation
that allowed it to share in the high priority of that mainstream program. The A-4b
took shape as a V-2 with swept wings and with a standard set of fins that included
slightly enlarged air vanes for better control. Certainly the A-4b needed all the help it
could get, for the addition of wings had made it highly sensitive to winds.
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