In article ,
Joe D. wrote:
Wasn't HOTOL meant to use the same engine for both jet and rocket
propulsion? IIRC, to switch to rocket propulsion the idea was that
they'd start injecting oxidiser as well as fuel into the engine?

Yes, it envisioned a Liquid Air Cycle Engine (LACE).

HOTOL's engine wasn't exactly a LACE system, because it didn't liquefy the
air, just cooled it drastically and compressed it before injecting it into
the rocket chamber. (This was a significant innovation, reducing the
amount of extra LH2 that had to be carried for cooling the air.)

In theory that would
reduce the number of discrete propulsion systems to two...

To one, actually, for the HOTOL scheme. HOTOL's engine *wasn't* a
scramjet; it switched to pure rocket at around Mach 5. And since it used
a turbocompressor, it could run at zero airspeed. The key idea was a
different approach, as Alan Bond commented:

"Any hybrid engine must end up being a very efficient rocket for most
of the flight. I began with a good rocket engine and made it a bad
air-breather. Everybody previously had done the reverse."

He also had some unkind things to say about scamjets, er excuse me I
meant scramjets:

"Scramjets do not have an intrinsic performance benefit and even if they
did the cost of engineering the hardware required would be much greater
than the other options. The demise of the X-30 NASP may be a belated
realisation of this fact."

(Both quotes are from an article/interview in the May 1993 Spaceflight.)
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