Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
for instance, if one had a jet velocity of 10,000 feet per second while flying in the
atmosphere at 400 feet per second. The ejector promised to avoid this by slowing
the overall flow.
The ejector ramjet thus offered the enticing concept of a unified engine that
could propel a single-stage vehicle from a runway to orbit. It would take off with
ejector-boosted thrust from its rocket, accelerate through the atmosphere by using
the combination as an ejector-boosted ramjet and scramjet, and then go over completely
to rocket propulsion for the final boost to orbit.
Yet even with help from an ejector, a rocket still had a disadvantage. A ramjet or
scramjet could use air as its oxidizer, but a rocket had to carry heavy liquid oxygen in
an onboard tank. Hence, there also was strong interest in airbreathing rockets. Still,
it was not possible to build such a rocket through a simple extension of principles
applicable to the turbojet, for there was a serious mismatch between pressures available
through turbocompression and those of a rocket’s thrust chamber.
In the SR-71, for instance, a combination of inlet compression and turbocompression
yielded an internal pressure of approximately 20 pounds per square inch
(psi) at Mach 3 and 80,000 feet. By contrast, internal pressures of rocket engines
started in the high hundreds of psi and rapidly ascended into the thousands for
high performance. Unless one could boost the pressure of ram air to that level, no
airbreathing rocket would ever fly. 44
The concept that overcame this difficulty was LACE. It dated to 1954, and Randolph
Rae of the Garrett Corporation was the inventor. LACE used liquid hydrogen
both as fuel and as a refrigerant, to liquefy air. The temperature of the liquid
hydrogen was only 21 K, far below that at which air liquefies. LACE thus called for
incoming ram air to pass through a heat exchanger that used liquid hydrogen as the
coolant. The air would liquefy, and then a pump could raise its pressure to whatever
value was desired. In this fashion, LACE bypassed the restrictions on turbocompression
of gaseous air. In turn, the warmed hydrogen flowed to the combustion
chamber to burn in the liquefied air. 45
At the outset, LACE brought a problem. The limited thermal capacity of liquid
hydrogen brought another mismatch, for the system needed eight times more liquid
hydrogen to liquefy a given mass of air than could burn in that mass. The resulting
hydrogen-rich exhaust still had a sufficiently high velocity to give LACE a prospective
advantage over a hydrogen-fueled rocket using tanked oxygen. Even so, there
was interest in “derichening” the fuel-air mix, by making use of some of this extra
hydrogen. An ejector promised to address this issue by drawing in more air to burn
the hydrogen. Such an engine was called a ramLACE or scramLACE. 46
A complementary strategy called for removal of nitrogen from the liquefied air,
yielding nearly pure liquid oxygen as the product. Nitrogen does not support combustion,
constitutes some three-fourths of air by weight, and lacks the redeeming
quality of low molecular weight that could increase the exhaust velocity. Moreover,
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First Thoughts of Hypersonic Propulsion
a hydrogen-fueled rocket could give much better performance when using oxygen
rather than air. With oxygen liquefying at 90 K while nitrogen becomes a liquid at
77 K, at atmospheric pressure the prospect existed of using this temperature difference
to leave the nitrogen unliquefied. Nor would it be useless; it could flow within
a precooler, an initial heat exchanger that could chill the inflowing air while reserving
the much colder liquid hydrogen for the main cooler.
It did not appear feasible in practice to operate a high-capacity LACE air liquefier
with the precision in temperature that could achieve this. However, a promising
approach called for use of fractional distillation of liquid air, as a variant of the
process used in oil refineries to obtain gasoline from petroleum. The distillation
process promised fine control, allowing the nitrogen to boil off while keeping the
oxygen liquid. To increase the throughput, the distillation was to take place within
a rotary apparatus that could impose high g-loads, greatly enhancing the buoyancy
of the gaseous nitrogen. A LACE with such an air separator was called ACES, Air
Collection and Enrichment System. 47
When liquid hydrogen chilled and liquefied the nitrogen in air, that hydrogen
went only partially to waste. In effect, it transferred its coldness to the nitrogen,
which used it to advantage in the precooler. Still, there was a clear prospect of greater
efficiency in the heat-transfer process if one could remove the nitrogen directly from
the ram air. A variant of ACES promised to do precisely this, using chemical separation
of oxygen. The process relied on the existence of metal oxides that could take
up additional oxygen when heated by the hot ram air and then release this oxygen
when placed under reduced pressure. Only the oxygen then was liquefied. This
brought the increased efficiency, for the amount of liquid hydrogen used as a coolant
was reduced. This enhanced efficiency also translated into conceptual designs
for chemical-separation ACES units that could be lighter in weight and smaller in
size than rotary-distillation counterparts. 48
Turboramjets, ramjets, scramjets, LACE, ramLACE and scramLACE, ACES:
with all these in prospect, designers of paper engines beheld a plenitude of possibilities.
They also carried a strong mutual synergism. A scramjet might use a type of
turboramjet for takeoff, again with the scramjet duct also functioning as an afterburner.
Alternately, it might install an internal rocket and become a scramLACE.
It could use ACES for better performance, while adopting the chemical-separation
process to derichen the use of hydrogen.
It did not take long before engineers rose to their new opportunities by conceiving
of new types of vehicles that were to use these engines, perhaps to fly to orbit as
a single stage. Everyone in aerospace was well aware that it had taken only 30 years
to progress from Lindbergh in Paris to satellites in space. The studies that explored
the new possibilities amounted to an assertion that this pace of technical advance
was likely to continue.
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