Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
depends on the difference of velocity of the air and of the injected gas.” Shock-free
internal flow appeared feasible: “The fuel is injected parallel to the stream to eliminate
formation of shocks [and] the combustion process can take place without the
formation of shocks.” He added,
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“The preliminary analysis of supersonic combustion ramjets…indicates
that combustion can occur in a fixed-geometry burner-nozzle combination
through a large range of Mach numbers of the air entering the combustion
region. Because the Mach number entering the burner is permitted to vary
with flight Mach number, the inlet and therefore the complete engine does
not require variable geometry. Such an engine can operate over a large
range of flight Mach numbers and, therefore, can be very attractive as an
accelerating engine.” 32
There was more. As noted, the inlet was to produce a bow shock of specified
character, to slow and compress the incoming air. But if the inflow was too great,
the inlet would disgorge its shock. This shock, now outside the inlet, would disrupt
the flow within the inlet and hence in the engine, with the drag increasing and the
thrust falling off sharply. This was known as an unstart.
Supersonic turbojets, such as the Pratt & Whitney J58 that powered the SR-71
to speeds beyond Mach 3, typically were fitted with an inlet that featured a conical
spike at the front, a centerbody that was supposed to translate back and forth to
adjust the shock to suit the flight Mach number. Early in the program, it often did
not work. 33 The test pilot James Eastham was one of the first to fly this spy plane,
and he recalls what happened when one of his inlets unstarted.
“An unstart has your full and undivided attention, right then. The airplane
gives a very pronounced yaw; then you are very preoccupied with getting
the inlet started again. The speed falls off; you begin to lose altitude. You
follow a procedure, putting the spikes forward and opening the bypass
doors. Then you would go back to the automatic positioning of the spike—
which many times would unstart it again. And when you unstarted on one
side, sometimes the other side would also unstart. Then you really had to
give it a good massage.” 34
The SR-71 initially used a spike-positioning system from Hamilton Standard.
It proved unreliable, and Eastham recalls that at one point, “unstarts were literally
stopping the whole program.” 35 This problem was eventually overcome through
development of a more capable spike-positioning system, built by Honeywell. 36
Still, throughout the development and subsequent flight career of the SR-71, the
First Thoughts of Hypersonic Propulsion
positioning of inlet spikes was always done mechanically. In turn, the movable spike
represented a prime example of variable geometry.
Scramjets faced similar issues, particularly near Mach 4. Ferri’s thermal-compression
principle applied here as well—and raised the prospect of an inlet that might
fight against unstarts by using thermal rather than mechanical arrangements. An
inlet with thermal compression then might use fixed geometry all the way to orbit,
while avoiding unstarts in the bargain.
Ferri presented his thoughts publicly as early as 1960. He went on to give a far
more detailed discussion in May 1964, at the Royal Aeronautical Society in London.
This was the first extensive presentation on hypersonic propulsion for many in the
audience, and attendees responded effusively.
One man declared that “this lecture opened up enormous possibilities. Where
they had, for lack of information, been thinking of how high in flight speed they
could stretch conventional subsonic burning engines, it was now clear that they
should be thinking of how far down they could stretch supersonic burning engines.”
A. D. Baxter, a Fellow of the Society, added that Ferri “had given them an insight
into the prospects and possibilities of extending the speed range of the airbreathing
engine far beyond what most of them had dreamed of; in fact, assailing the field
which until recently was regarded as the undisputed regime of the rocket.” 37
Not everyone embraced thermal compression. “The analytical basis was rather
weak,” Marquardt’s Arthur Thomas commented. “It was something that he had in
his head, mostly. There were those who thought it was a lot of baloney.” Nor did
Ferri help his cause in 1968, when he published a Mach 6 inlet that offered “much
better performance” at lower Mach “because it can handle much higher flow.” His
paper contained not a single equation. 38
But Fred Billig was one who accepted the merits of thermal compression and
gave his own analyses. He proposed that at Mach 5, thermal compression could
increase an engine’s specific impulse, an important measure of its performance, by
61 percent. Years later he recalled Ferri’s “great capability for visualizing, a strong
physical feel. He presented a full plate of ideas, not all of which have been realized.”
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Combined-Cycle Propulsion Systems
The scramjet used a single set of hardware and operated in two modes, sustaining
supersonic combustion as well as subsonic combustion. The transition involved
a process called “swallowing the shock.” In the subsonic mode, the engine held a
train of oblique shocks located downstream of the inlet and forward of the combustor.
When the engine went over to the supersonic-combustion mode, these shocks
passed through the duct and were lost. This happened automatically, when the flight
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