[AR] Re: SSTO
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 15 Feb 2018 23:50:57 -0500
On Thu, Feb 15, 2018 at 09:36:02PM +0000, Ian Woollard wrote:
FWIW Skylon's SABRE engine was designed by ex-Roll Royce engineers and
certainly 'ought' to be able to ace the chicken test- neither the heat
exchanger nor any of the turbomachinery is inline with the engine inlet, as
the airflow into the compressor turns a corner towards the core after the
inlet, and ballistic coefficient and momentum considerations mean the
chicken should go straight through the external burners, which are more or
less just ramjets.
Unfortunately "should" is the right word there, whereas the word they
need is "will". Although the heat exchanger isn't inline with the
engine inlet, it isn't far away from being inline, at least in the
diagrams I saw. And chickens (and other foreign objects) can do
things like bouncing off part of the inlet structure. Aerodynamics is
complicated, as is chicken-dynamics.
Of course this is still just a paper engine, so it could easily change
before getting built. But if they add enough stuff to really shield
the heat exchanger, they might lose the light weight that they need to
make orbit.
So it's probably better than a turbofan engine, and much
better than turbojets, which are highly sensitive to FOD.
Often brute strength (which jet engine blades have) beats clever
design which incorporates something delicate on the grounds that bad
things should never happen to it.
I think with Suntan the hydrogen expansion cycle they planned was mainly
intended to avoid having a gas turbine in the hot exhaust gases, as these
have strict temperature limits, although they're much higher these days
with all monocrystalline blade malarkey. Not having them there means you
can run stochiometric combustion, which is more energetically effiicient,
even at high speed, without worrying about burning out the turbine blades.
No, that explanation doesn't work; it even cuts the other way. As
regards efficiency, it doesn't matter what temperature the combustion
gases reach: what matters is what temperature the hydrogen reaches,
since it's the working fluid. And that temperature can't exceed the
melting point of the heat exchanger tubing. With the usual turbine
arrangement, in contrast, the gases that surround the turbine blades
_can_ exceed the melting point of the blades, since the blades are
cooled by blowing bleed air through internal passages.
What I think is meant by "hydrogen is a good working fluid" is not
thermodynamic efficiency (ideally (T_high-T_low)/T_high) but rather
the size and weight of the turbine. The numbers that Mulready cites
are that the whole engine consumed only 3.5 pounds per second of
hydrogen, which in the heat exchanger received 72 million BTU per hour
of heat (21 megawatts), and drove a turbine producing 12000 horsepower
(8.9 megawatts), the turbine being only a foot in diameter.
The thing is, yes (as he says) hydrogen has a specific heat that is
about 15 times greater than that of air... but that's heat capacity
per unit mass; the heat capacity per molecule (or per unit volume) is
about the same as air's. Which leaves the question of what makes
hydrogen better. I think it's that hydrogen, being lighter, moves
through the turbomachinery faster: to get the same force on the
turbine blades, you need hydrogen moving sqrt(15) times faster. (The
speed of sound in hydrogen is also higher by the same factor, so
that's not an obstacle to it moving faster.) So in terms of
throughput multiplied by heat capacity per unit volume, you get
sqrt(15) times more of that out of hydrogen, for the same size
rotating machinery (but moving sqrt(15) times faster and thus
producing sqrt(15) times more power).
Whether the savings in weight from making that turbine a foot in
diameter compensated for the weight of the six-foot-diameter heat
exchanger is another question. (Despite the way that sounds, it's
possible.) As for whether they compensated for the troubles of the
heat exchanger -- well, Mulready blames one designer's gray hairs on
those troubles, and another designer's heart attack.
Suntan might well have been more susceptible to FOD than SABRE, because
it's more like a turbojet, stuff would go through the compressor and then
go carreering through the delicate heat exchanger. Stones could have been
particularly problematic, but with care that may have been soluble too.
Foreign objects weren't going to get through the compressor on a
ballistic trajectory, though, and the heat exchanger was in a region
of relatively slow speed airflow. And 3/16 inch tubing can take a few
knocks. Still, yes, there'd be a chance of stuff bouncing through in
just the wrong way. And even if it arrived too slowly to break the
heat exchanger, it would have a good chance of accumulating and
clogging it up.
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