dietz@SLB-DOLL.CSNET (Paul Dietz) (12/08/85)
The January 86 issue of Discover has a very interesting article in
hypersonic aircraft ("The New Orient Express", pages 72-81).
The article starts by discussing conventional SST's, such as the
Concorde, that use turbojets. These planes can be made more efficient,
but turbojets cannot operate at a high enough altitude to muffle the
sonic booms, restricting them to ocean-crossing flights.
To get around sonic booms, one must travel at hypersonic speeds at
higher altitudes. The "Orient Express" is a hypothetical hypersonic
trans-pacific airliner. It would travel at Mach 5+. This is too fast
for turbojets: the heat is too much for the turbine that drives the
compressor. The proposal is to use a hybrid engine called a
turboramjet. In this design, the turbine is driven not by the exhaust
gases but rather by a "gas generator", a kind of combination rocket/
fuel injection system. The temperature of the gas generator can be
controlled to limit heating of the turbine to acceptable levels.
A compressor is driven by this turbine; at low speeds it would
make the engine act like a turbojet; at high speeds the ram pressure of
the incoming air would make the engine act like a ramjet, with a
small boost from the turbine.
For transatmospheric vehicles (TAV's), there are several possible
engines, all using liquid hydrogen. The first is the scramjet. This
uses a weak shock wave from a highly pointed intake to slow the air
slightly through the engine, plus variable-geometry ducting to keep the
flame from blowing out. These have been tested to Mach 8 (at which
point the wind tunnels overheat). There are two ways to get a scramjet
to hypersonic speeds. First, it could be carried on a first stage
propelled by turboramjets (which could also act as a endoatmospheric
transport, much like the 747 for the shuttle, and could be spun-off into
a commerical Orient Express). Second, the scramjet could use a
conventional rocket as a fuel injector. The rocket would burn a
highly fuel-rich mixture (even the space shutle main engines burn
a 4-1 molar ratio of hydrogen to oxygen), which would react
further with air before leaving the engine. The rocket would provide
thrust even at zero forward speed, eliminating the need for the
first-stage vehicle. The rocket exhaust may also drag air through the
engine at zero forward speed. The proposed military TAV will use
this scramrocket.
An alternate to the scramjet is the cryojet. This is a kind of
turboramrocket. Cryogenic liquid hydrogen is used to cool the incoming
air before combustion, allowing the engine to breath air at higher
speeds. The final boost to orbit uses the built-in rocket. A further
refinement, the LACE (liquid air cycle engine) uses the liquid hydrogen
to liquify a small quantity of air for use in the fuel-injector rocket.
The British HOTOL will use LACE. They hope to use it as a 60-passenger
transport that can fly from London to Sydney in 67 minutes, take-off to
touch-down.
This is exciting stuff. It's apparent that jet and rocket technology
is converging, and for the first time we can imagine in detail what
a commercial spaceliner might look like.
The article is highly recommended.