[AR] Re: Precooled engine
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 4 Jun 2016 15:33:03 -0400 (EDT)
On Mon, 30 May 2016, Keith Henson wrote:
The Reaction Engines precooled engine may be the most advanced in the
world. It gets good performance up to 26 km on air before it switches
to internal LOX for the last 6 km/s to orbit.
...on paper.
With the exception of the precooler, it's still a paper engine, and a
complex and novel one too. I'll grant that the precooler was the
trickiest (and least orthodox) bit, but there's still plenty of room for
development difficulties and underperformance to happen along the road to
flight. High-supersonic jet engines are not easy at the best of times,
and this would be the *very first* one to operate all the way from the
ground to Mach 6-ish.
Even the precooler might not be entirely a done deal. Its basic
feasibility has been demonstrated, but I'm not sure how thoroughly the
practical issues like durability (e.g., birdstrike tolerance!) and working
lifetime have been addressed. That tubing is awfully thin.
The really sticky part is the precooler, but Reaction Engines has
installed a huge capacity to make them and just might be willing to
supply some.
There are at least three other issues that are manageable for pros with
big budgets, but could be major obstacles for amateurs: liquid hydrogen,
high-performance turbomachinery, and a wide-range variable-geometry
supersonic intake. (You're unlikely to get to Mach 3, never mind Mach 6,
without variable geometry in the intake, and even the pros find such
intakes difficult to develop.)
If you imitate Skylon's layout, there is another possibly-serious problem
that's an unresolved issue for Skylon itself. There was a recent NASA
paper, in Journal of Propulsion and Power I think, which pointed out that
as altitude increases, the engine exhaust plumes naturally start to spread
out more and more. CFD simulation suggests that with Skylon's flight
profile, at around Mach 8, the plumes start to impinge on the tail, and by
Mach 15 or so, much of the rear fuselage is inside them. The heating
problems are potentially severe. (The HOTOL layout may have been better
after all, despite the CG problems it caused...)
The only thing more impressive than a SSTO would be an amateur SSTO.
Agreed. Given the other challenges it would involve, though, almost
certainly it would use pure rocket propulsion -- simpler, lighter, fewer
technical difficulties. Would-be amateur SSTO builders should focus hard
on simplicity and high mass ratio and incremental improvement based on
repeated flight testing. Trying to develop a heavy tricky complex engine
that promises reduced mass ratio -- once you get it working -- *might* be
a better choice for the pros, but is almost certainly a project killer for
amateurs.
Henry
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