Launching into orbit could become a little easier and
cheaper, thanks to a futuristic space plane that looks like it might have flown
straight out of a Star Wars film.
The European Space Agency and British government have
awarded $1 million euros ($1.28 million dollars) to Reaction Engines Limited
(REL), a British aerospace company, as part of a multi-million dollar
development program for an air-breathing rocket engine that could power the Skylon
spaceplane. The unpiloted, reusable vehicle is designed to take off from an
airstrip, deliver cargo into orbit and return to the same runway.
"This is an example of a British
company developing world beating technology with exciting consequences for the
future of space," said Lord Paul Drayson, U.K. Minister for Science and
Innovation.
A breakthrough could provide relief for space-faring
nations, which currently rely on expensive rocket launches that can cost
thousands of dollars per pound. But the project faces a host of competitors as
well as its own technical challenges.
Breaking down the competition
The Skylon spaceplane would boast a unique SABRE hybrid
engine which can act as both an air-breathing jet engine and a rocket engine in
space. This gives it a potential advantage over other space plane designs,
according to Mark Hempsell, director for Future Programs at Reaction Engines.
Unlike Virgin
Galactic's SpaceShipTwo or other vehicles designed for suborbital flight, Skylon
aims to replace the aging space shuttle, as well as rockets such as Ariane or
Ares.
Hempsell noted that a two-stage design such as Rocketplane
Kistler's faltering
K-1 project is possible, but has higher development and operational costs.
Scramjets that would channel supersonic airflows appear to be decades off.
As for a pure
rocket single stage, such as the Lockheed's long-defunct Venturestar design,
those are "right on the borderline of feasibility," Hempsell told SPACE.com.
"I think in reality they are not practical even if possible."
"We have the full advantages of being single stage but
with low technical risk (but admittedly not as low as a two stage),"
Hempsell added.
Keep it cool
Unlike most rocket engines, the Skylon's SABRE engine would
use hydrogen as its fuel rather than as a coolant, and instead would use liquid
oxygen for cooling.
That means the air-breathing mode would rely on a revolutionary
heat exchanger pre-cooler, which cools the air that gets compressed and fed to
the rocket engine along with hydrogen fuel. Once the spaceplane enters rocket
mode, the hydrogen would be burned along with liquid oxygen.
"We have a working demonstration of the frost control
system at flight engine scales, and we have construction of a flight standard
heat exchanger module," Hempsell said. That system is slated for testing
on a jet-engine-powered rig.
Pressure is on
Skylon would also try to squeeze more power from its engine
by using a redesigned exhaust nozzle that makes the most of the space plane's
vital fuel.
Ideally, hot exhaust gases should press fully against the
walls of a rocket engine's nozzles to push a rocket upward and onward, assuming
that the exit pressure matches the atmospheric pressure.
However, current nozzles cannot adjust to different
atmospheric pressures, and only work efficiently at certain altitudes. Exit
pressure lower than atmospheric pressure (i.e. low altitudes) means that the
escaping gases may not expand fully against the nozzle wall. That means less
thrust for the rocket and more wasted fuel.
By contrast, exit pressure greater than atmospheric pressure
(i.e. high altitudes) means the exhaust gases still expand after escaping the
rocket nozzle — representing wasted fuel spent outside the rocket and loss of potential
thrust.
"In practical terms this places a maximum on the area
ratio that may be used on a system that flies from sea level to vacuum,"
said Neil Taylor, an aerospace engineer at the University of Bristol in the
U.K. He has helped investigate the new nozzle concept for Skylon.
The new nozzle contains a central "plug" that
forces exhaust gas to flow between the plug and outer walls. The escaping gases
eventually separate from the plug and leave a void in between, which should
provide a buffer that allows for atmospheric pressure flexibility as the spaceplane
flies to higher altitudes.
But will it fly?
Skylon could realistically see a 10 percent increase in
carrying payload, if the new nozzle design pans out. The technology is based a
prototype engine called STERN, which managed a stable flow of exhaust gas under
many different atmospheric conditions.
It has even worked with max efficiency at just 116 pounds
per square inch (PSI) inside a test chamber, as opposed to the space shuttle
main engine which needs a test chamber pressure of least 2,900 psi.
But there are still hurdles to surmount, researchers said.
One such major obstacle is to make sure Skylon's engines remain cool enough to
avoid melting during flight. The STERN prototype fired for just one second to keep
it from overheating, Taylor noted, but added that the problem should not be more
challenging than for a regular engine.
As for Hempsell, he envisions a time when customers
can buy a launch on just weeks or days notice. "Currently you have to
give years notice," he said.
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