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. So it's probably better than a turbofan engine, and much
better than turbojets, which are highly sensitive to FOD.
But I nevertheless agree with the general sentiment of things not
necessarily being identical to the laboratory.
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.
So you could avoid hitting the temperature line that hobbles the
thrust/weight ratio of virtually all other airbreathing jet engines.
Potentially you end up with really tiny, powerful engines. And IRC the
tests on those engines worked pretty well. The big problem is the humungous
liquid hydrogen-filled airframe you need to attach to it though. ;)
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.
On 15 February 2018 at 19:09, Norman Yarvin <yarvin@xxxxxxxxxxxx> wrote:
On Wed, Feb 14, 2018 at 01:39:29PM -0800, Keith Henson wrote:
Norman Yarvin <yarvin@xxxxxxxxxxxx> wrote:
I don't think either of those examples fully captures the glory of an
aircraft whose essential centerpiece would be a super-high-performance,
super-delicate heat exchanger filled with hydrogen and exposed to
incoming airflow.
It's helium in the fine tubes. Otherwise, you would have hydrogen
embrittlement. But you are close to the initial reason I rejected the
concept. Thousands of brazed joints and it's not going to leak? But
they built some furnace brazed test articles and they did not leak.
That's when I started to take them seriously.
There's a long way between "we made one and it works in the lab" and
"it survives the chicken cannon".
By the way, the engine for the Suntan project used a somewhat similar
heat exchanger, with no intermediate helium stage and "four and one
half miles of 3/16-inch diameter tubing... Hasteloy R because of its
compatibility with hot hydrogen." (quote from Dick Mulready, in
_Advanced Engine Development at Pratt & Whitney_.) The engine was
built and tested, albeit only on the ground.
(Its concept was that rather than using the normal turbine stage to
extract energy from the exhaust gases, they used the exhaust gases to
gasify and heat up the hydrogen fuel in the heat exchanger and then
ran the turbine off the hydrogen. I'm not sure what the attractions
of that arrangement were: did they think it was necessary to gasify
the hydrogen before injecting it? Or is hydrogen that much better of
a working fluid for a turbine?)