TESTS last week on a new type of engine have taken NASA a step closer to the
dream of a reusable spacecraft that takes off like an aeroplane. To be light
enough to do this, such a craft needs a powerful air-breathing engine to propel
it to the edge of the atmosphere. Using oxygen from the air instead of liquid
oxygen saves on weight.
The power unit NASA is pinning its hopes on is called a pulse-detonation
engine. It repeatedly ignites a fuel and air mixture in a simple cylinder, using
a spark plug. This produces plenty of thrust, but until now getting the engine
itself to supply the electricity for the spark has been a problem for rocket
engineers.
Now NASA and its partner company ERC have successfully tested a scaled-down
pulse-detonation engine at the company鈥檚 laboratory in Tullahoma, Tennessee. In
the test engine, a mixture of acetylene and air is injected into a cylindrical
detonation tube, where the mixture is ignited a hundred times each second. Such
frequent detonations should help to avoid a bumpy ride. 鈥淎t that rate the thrust
is almost continuous,鈥� says John Lineberry, vice-president of ERC.
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The key to the engine鈥檚 success is a magnetohydrodynamic (MHD) generator that
extracts energy from the engine鈥檚 ionised exhaust gases and converts it into
electricity. 鈥淲hat we鈥檝e done is to use the energy that鈥檚 produced by the
detonations to generate an electromotive force鈥攗sing principles laid down
long ago by Michael Faraday鈥攖o power the spark,鈥� Lineberry says.
The team, led by John Cole, head of NASA鈥檚 Advanced Space Transportation
Program, placed a magnet around the outside of the open end of the detonation
tube. In this position, hot ionised exhaust gases from the engine flow through
its magnetic field. The field steers positively and negatively charged ions
towards opposite edges of the exhaust tube, close to an electrical conductor or
鈥渓oad鈥�. Because the flow of ions varies continuously as the engine pulsates, it
induces a current in the conductor. 鈥淭he current generated in the load is then
conditioned to drive the spark plug,鈥� says Lineberry. 鈥淪o we are controlling the
engine by tapping the forces it generates.鈥�
Lineberry says ERC envisages multiple pulse-detonation engines being ganged
together, with each one firing in turn, to produce a low-cost, lightweight power
source to accelerate spacecraft to many times the speed of sound. 鈥淵ou could
place several engines together in a kind of Gatling gun arrangement and get them
to fire sequentially,鈥� he says.
Next, NASA and ERC plan to experiment with an MHD unit fitted in reverse. 鈥淏y
putting an MHD device in backwards, we could make the hot ionised gas move
faster,鈥� says Lineberry. They will also test an engine that uses both forward
and reverse MHDs鈥攖hough there is no clue yet as to how they plan to power
the reverse MHD.