| Someone asked on another list for an explanation of the press release. This is my try. Hypersonic engines are up against hard physics. The ram air heats so
much in the inlet that it's hard for combustion to add much energy to
make it go faster out the back. The idea behind the SABRE engines is to cool the ram air before it is
compressed. The heat exchanger to do this is what the press release
is all about. With not much more than a ton of mass, it sucks 400 MW
of heat out of the incoming air, dropping the temperature from 1500 C
to -150 C in a few inches of heat exchanger that looks much like
fabric because the tubes are so tiny. The engine cycle also uses the temperature difference between the ram
air and the LH2 to run the compressor. It takes close to 2/5th of the
energy from burning hydrogen to liquefy it. The engines recover much
of this by running a helium turbine on the temperature difference
between the ram air and the liquid hydrogen flow to the engines. The
turbine powers the compressor stage that raises the pressure of the
-150 C air to rocket chamber pressure. The design is extremely clever thermodynamics which also avoids most
of the metallurgical problems of high temperature. Fabricating the
air to helium heat exchanger was a very hard task. They have miles of
tiny tubing, tens of thousands of brazed joints and they don't leak! Using these engines and breathing air, the vehicle reaches 26 km and
about a quarter of the velocity to orbit giving an equivalent exhaust
velocity (back calculate from hydrogen consumption) of 9 km/s.
That's twice as good as the space shuttle main engines. It is
expected to go into orbit with 15 tons of payload out of 300 or 5%
even though the rest of the acceleration is on internal oxygen that
only gives 4.5 km/s exhaust velocity. Leaving out the oxygen and using big propulsion lasers to heat
hydrogen reaction mass, such a vehicle would get 25% of takeoff mass
to LEO, reducing the already low cost by a factor of 5. That's enough
to change the economics of power satellites from being too expensive
to consider to a cost substantially less expensive than any fossil
fuel. But try explaining any of this in a press release. |