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Florida

FOR the past few weeks, NASA engineers have been anxiously scanning the
freezing skies above the dusty plains of eastern Kazakhstan, watching for a lone
missile. What interests them is not the missile itself, but its load—a
metre-long cylinder perched on the missile’s pointed nose.

For this cylinder is a revolutionary engine that combines the fuel efficiency
of a jet engine with the raw, crackling power of a rocket. And as the missile
rips through the atmosphere at speeds of over Mach 6, Russian engineers will
switch the engine on for a few seconds. If the test is successful, it could
clear the way for an ambitious American project: a small unmanned space plane
called Hyper-X.

Today’s rockets are little more than giant fireworks. And like fireworks,
they are pretty inefficient—much of their weight is made up of the fuel
and oxygen needed to get them off the ground. The space shuttle, for instance,
carries 2 million litres of liquid oxygen and hydrogen, while its payload makes
up only a measly 2 per cent of its total weight. The new engine could change all
that, believe the engineers who are developing the Hyper-X planes at NASA’s
Langley Research Center in Virginia and the Dryden Flight Research Center in
California.

Rather than carrying its own supply of oxygen, Hyper-X’s motor will suck in
oxygen directly from the atmosphere, powering the space plane through the skies
ten times faster than the speed of sound. If the trials are successful,
Hyper-X’s successors may carry heavier loads into space than existing rockets
can.

Boost to orbit

Such “air-breathing” engines may not be able to get spacecraft all the way
into orbit—a rocket booster might still have to provide the final thrust
necessary to reach escape velocity. Even so, NASA engineers calculate that up to
25 per cent of the weight of an air-breathing space plane could be payload,
replacing stored oxygen with thousands of kilograms of extra equipment,
satellites or people.

NASA’s air-breathing engine is based on a design that has been around for
over half a century—the jet engine. All jet engines work in the same way.
Air is compressed as it enters the engine, fuel is added and the high-pressure
mixture is ignited. Spewing these rapidly expanding hot gases from the exhaust
nozzle generates thrust.

What distinguishes NASA’s new engine from all the others is the way that it
handles incoming air. Turbojets and turbofans—which power most
airliners—use spinning fan-like blades to compress the air. By contrast,
supersonic jets such as the Lockheed SR-71 spy plane are powered by ramjets.
These engines do without moving parts such as compressor blades. Instead, they
use their own forward momentum to compress air by ramming it against a
spike-like surface on the engine’s inlet. As air moves into the engine inlet, it
is squeezed between the spike and the engine casing. This also slows the
supersonic airstream to subsonic speeds, which is vital—if the air moves
through the combustion chamber too fast, ignition will be inefficient and the
engine won’t produce any thrust.

Ramjets are efficient and extremely powerful at up to three times the speed
of sound. But if you want to go beyond this into hypersonic flight—Mach 5
or above—they just don’t cut the mustard. By the time a ramjet reaches
five times the speed of sound, the incoming air is travelling so fast that
supersonic shock waves—compression waves—form at the ramjet’s
inlet.

Those shock waves raise the temperature and pressure inside the engine until
oxygen and fuel molecules entering the combustion chamber split into fragments,
ignition stops and the engine loses all power. The shock waves can also produce
spots of intense heat and high pressure that may destroy the engine. To get
around these problems, and to turn the ramjet into a supersonic combustion
ramjet—a scramjet—requires some clever engineering.

Scramjets have been successfully tested in laboratories since the 1960s, but
getting them off the ground is a different matter. One of the main challenges in
designing a scramjet is to avoid generating supersonic shockwaves inside the
engine. And since a tiny shock wave is created as the fuel is added, something
as simple as injecting a stream of fuel becomes very complex, says NASA
aerospace engineer John Hicks of the Dryden Flight Research Center in
California.

But the biggest question is whether a scramjet can produce reliable levels of
thrust. Since it impedes the airflow much less than a ramjet, the problem is
that hot gases whoosh through the engine at many times the speed of sound. So
the scramjet’s combustion chamber must be long enough to allow the fuel and
supersonic air to mix and burn properly. Even so, the burning gases will remain
in a scramjet engine for a few milliseconds at most.

“One of the fundamental physical questions is whether there will be a long
enough dwell time in the combustor for the fuel to properly mix and burn,” says
Hicks. If it turns out that most of the energy is released after the burning
gases have spewed out from the back of the vehicle, then the scramjet might not
generate enough thrust to stay in the air.

NASA engineers believe they have the answer. As the air speed inside the
engine increases, fuel will be injected progressively closer to the inlet. That
way, Hicks says, there should be enough time for the fuel to burn. “If the fuel
finishes burning 50 feet behind the engine then you’ve lost all your thrust,” he
says.

The thrust question was one of the great unknowns that helped kill NASA’s
previous scramjet project—the $11-billion National Aerospace Plane
project, which was cancelled in 1994. “Some nay-sayers actually predicted that
the engines would produce no thrust at all,” Hicks says. “You turn them on and
they act as a big drag device.”

Because of the limitations of even the best wind tunnels, NASA engineers
cannot say for sure whether or not that will happen without proper test flights.
In a wind tunnel, fuel combustion products corrupt the air and, more
importantly, the tunnel can’t create the high speeds at which Hyper-X will fly.
This is why the Americans have turned to Russia for help: the Russians already
have experience of getting scramjets into the air. In November 1991, Russian
engineers scooped the world by mounting an experimental scramjet on an
anti-aircraft missile.

The flight stunned—and piqued—NASA engineers. “To the people at
Langley, it was a relatively old design. But they went and did it. You’ve got to
give them credit,” says Vince Rausch, the Hyper-X programme manager.

NASA has paid Russia $1.5 million for a scramjet test flight in
eastern Kazakhstan. Although the Russians will not measure the engine’s thrust,
the test flight should tell NASA if supersonic combustion is possible. The data
will help to calibrate NASA’s fluid dynamics design programmes, used to predict
the airflow through Hyper-X’s engine. “We’re using the Russian engine design to
hone our design tools,” says one NASA engineer. The agency has even procured an
exact copy of the Russian engine and will test it inside a wind tunnel at the
Langley Research Center in Virginia to check their predictions.

Wing and a prayer

There’s more than just money at stake. American engineers say that the
project could boost the US to the fore of scramjet research. “The Russians were
clearly first, as far as I know, with what they’ve done with captive carries. We
should be first flying a scramjet on a free-flying vehicle,” says Rausch.

New aerospace technology does not come cheap. The Hyper-X
programme—with its series of three flight tests—will cost the US a
cool $160 million. At the start of each test, far out over the Pacific
Ocean, a high flying B-52 bomber will release a modified Pegasus rocket, a
delta-winged launcher designed to place small satellites in low Earth orbit.
Pegasus will carry Hyper-X on its nose, and the plan is to accelerate the rocket
to Mach 7 and reach an altitude of 30 000 metres before releasing the unmanned
space plane for its high-speed test run.

Even the early stages of the test, before the Hyper-X is released, are risky.
Much depends on the modified Pegasus rocket. The engineers at Orbital Sciences
in Virginia who are building the rocket booster will use ballast to make the
Pegasus fly a flatter trajectory than it would when launching a satellite. But
the biggest trick will be to separate the Hyper-X from the rocket without
destroying both of them. The force of the air at Mach 7 could make Hyper-X move
in unexpected ways, and even a slight movement might be enough to push it back
against the launcher.

Project engineers admit to a good deal of separation anxiety about this part
of the mission. Pegasus is designed to release cylindrical loads, which have
predictable aerodynamics. And these separations also occur much higher in the
atmosphere where the air is thin and there are fewer aerodynamic worries.

All that will be different with Hyper-X, Hicks says. Engineers are building a
special adapter to eject Hyper-X safely away from Pegasus. Engineers learnt
decades ago how to dispense conical warheads from fast-moving ICBMs, but it’s
much more difficult with an irregularly shaped space plane. “Hyper-X looks more
like a surfboard than a cylinder,” says Lowell Keel, programme manager for
MicroCraft, the Tennessee company chosen by NASA to build Hyper-X. “Hypersonic
separation with an asymmetric vehicle is not easy to do.”

To solve the problem, NASA officials sought the advice of engineers at Sandia
National Laboratory in Albuquerque, New Mexico. “Everyone said—make it
happen as quickly as possible,” says Rausch. So quick it will be. Once Hyper-X
reaches the test altitude, explosive bolts will snap loose, gas-powered pistons
will thrust Hyper-X 20 centimetres forward and the adapter will swing free, all
in a tenth of a second.

The scramjet engine will fire for between five and seven seconds, but that
should be long enough to see whether the vehicle accelerates. Even at 30 000
metres, there will be enough oxygen for the scramjet to breathe. And if the
engineers are right, Hyper-X will accelerate. Years of calculations and
wind-tunnel tests say the scramjet will work—but if the engineers are
wrong, says Hicks, the vehicle will plummet into the Pacific like a rock.

One of the big lessons from early high-speed flights is that the engine has
to be properly integrated into the aerodynamic design of the vehicle. Ramjet
engines like those on the SR-71 are essentially pods mounted on wings, with
protruding spikes to compress air for combustion. The Russian scramjet engine is
designed along similar lines. But Rausch says that this approach will not work
at the incredible speeds needed to reach space. “Having engine pods in the flow
field generates a lot of drag,” he says.

Hyper-X’s designers have taken a different approach, embedding the scramjet
in the plane’s belly. The engine will not need a spike or the heavy turbine
blades that conventional jet engines rely on. Instead, its own knife-shaped,
flattened fuselage will produce shock waves that will compress the air as it
enters the scramjet’s square inlet. Since Hyper-X will be flying high where the
atmosphere is thin, its fuselage will be shaped to maximise the volume of air
entering the scramjet.

Maximum thrust

And the experimental plane does away with the exhaust nozzle of a
conventional jet. Instead, the airframe behind the scramjet will taper upwards
in a graceful curve. This allows the hot exhaust gases to expand against the
fuselage, maximising the engine’s thrust.

Since the scramjet will be integrated into the plane’s fuselage, when Hyper-X
is manoeuvred, engine performance will change significantly. Likewise, when the
engine’s thrust is increased or decreased, the space plane’s pitch and lift will
change. The complicated flight surfaces will be subjected to 48 kilonewtons per
square metre of pressure, Rausch says. Engineers must take this into account in
their designs if Hyper-X is to reach hypersonic speeds. “Everything about this
project is new,” says Hicks.

Although NASA has released sketches of the vehicle, the exact shape of the
space plane is a closely guarded secret. There’s a lot at stake, says Hicks. If
successful, a scramjet-powered space plane could capture a big chunk of the
lucrative market for space launches in the 21st century. And high-speed
airliners powered by airbreathing engines would make globe-trotting a
cinch—flying London to Tokyo in just a few hours. “I can’t say anything at
all about the geometry,” says Tony Castrogiovanni, propulsion manager at GASL of
Ronkonkoma, New York State, the company which is building the engine. “It is
technology being developed by the US to protect the national interest.”

From their wind tunnel tests, NASA engineers think they have all the problems
in hand. “It doesn’t look like we have as big a problem as the initial analysis
showed,” says Rausch. The scramjet is almost halfway to completion and will be
tested later this year. The 3.6-metre plane will be equipped with thermal
protection tiles similar to those on the space shuttle—for good reason.
Hyper-X must withstand temperatures of 1400 °C in hypersonic flight.

Despite the optimism, NASA managers have delayed the first flight until
January 2000. The revised schedule has left some engineers anxious that they
will be scooped once again by an international competitor. And while Hicks,
Rausch and other officials predict that they will be first to fly a scramjet on
a free-flying vehicle, the question of whether the US will ever muster the money
to build a full-size space plane capable of putting satellites or people in
orbit remains open. At least the latest news from Russia is good. The scramjet
finally flew on 12 February, and though NASA engineers are still evaluating the
data, Hicks believes the flight was a success. “It was pretty satisfying to see
it go,” he says.