

How SR-71 Blackbird Designers Overcame Canonical Jet Engine Limitations - signa11
http://www.reddit.com/r/aviation/comments/vouxj/this_sr71_holds_the_flight_airspeed_record_at/c56rwsn

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D_Alex
Unfortunately, while the reddit post seems authoritative, it contains a
multitude of errors and ends up somewhat misleading. The most significant
error is:

"What the front half of a jet engine (the intake/diffuser, and the compressor
blades, i.e. all the stuff that happens before fuel is burned) does is; it
heats up the air until it's hot enough for fuel to ignite" ...etc.

The heating up of air in the gas turbine cycle (Brayton cycle) is an
UNDESIRABLE, but difficult to avoid consequence of the ideal gas law. A higher
efficiency and power output would be possible if this heating did not occur,
hence measures such as water injection or intercooling on large stationary gas
turbines.

There are a number of other more subtle errors in the post.

~~~
screwt
This.

If you have any interest in manufacturing, IMO the turbine blades in a jet
engine are incredible.

Without going into loads of detail, 2 points that blow my mind:

1) Due to compression in the intake (and as per the ideal gas law D_Alex
mentions above), the temperature of the air when it reaches the turbine is
_higher than the melting point of the turbine blades_. The workaround for this
is the blades are made with hollow channels through which cold air is blown,
keeping them just cool enough.

2) You may be familiar with 'creep' - where materials under stress gradually
relax to relieve that stress, even when the stress is lower than the yield
stress of the material. Creep is made worse by (i) repeated stress cycles and
(ii) high temperatures. In a typical jet turbine engine, you have blades
spinning at 10000 rpm, at high tempratures, with a few microns clearance to
the edge of the turbine housing. If they were to elongate and rake the edge of
the housing at that speed ...

~~~
SageRaven
Point #1 is pretty cool.

I worked as an IT intern for Pratt and Whitney in the Middletown, CT plant in
the mid 90's. I got to see samples of the blades straight from the factory
floor, and indeed they had those cool-looking grooves and channels in the
them.

My occasional trips to the shop floor for some PC maintenance or another was
really the coolest part of that job.

~~~
tcpekin
Point #2 is also pretty cool. In order to prevent overcome problems of creep,
the turbine blades can be made in a single crystal [1] [2]. This prevents
much/all of the creep, as creep is associated with grain boundaries. It's
pretty phenomenal that these single crystals can be manufactured into jet
turbine blades.

[1] <http://www.appropedia.org/Single_Crystal_Turbine_Blades>

[2] <http://www.freepatentsonline.com/6673308.html>

*edit fixed links.

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bcrescimanno
While it is fascinating, this is a really long-winded and sort of rambling
approach to an explanation of how the J58 worked. Even the Wikipedia page does
a better job of describing the engine without all the filler of the author's
need to type through his "geek out"

<http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58>

~~~
ktizo
How is it long winded? I thought it was pretty concise. You could fill books
on just that one engine.

~~~
bcrescimanno
What's long-winded is the amount of text that's not actually related to the
design and operation of the engine. The information he provided was actually
fairly slim but there were a lot of proverbial "oohs and ahhs."

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jluxenberg
Neat anecdote about the SR-71's speed from a former pilot
<http://www.jumbojoke.com/the_king_of_speed.html>

(couldn't find a better link; this is an excerpt from _Sled Driver: Flying the
World's Fastest Jet_ by Brian Schul)

~~~
torrenegra
Even more fun when you listen to it: <http://soundcloud.com/torrenegra/the-
king-of-speed-sr-71>

(This is one of my favorite stories ever. As such, I couldn't resist to get a
professional voice to record it. I have tried to contact Brian Schul to
transfer the ownership of the recording to him, but I have not been able to
get a reply from him)

~~~
ZoFreX
Wow, thank you for this - one of my favourite stories too. This is worthy of
its own post in my opinion (Reddit would probably love it, too)

Edit: How much did it cost to get this done, and is the same voice actor still
available? He did a really good job of capturing the ATC tone! I'd love to
hear more sled stories read by him.

~~~
torrenegra
$70 or so in VoiceBunny.com. Disclaimer: I'm it's co-founder.

~~~
ZoFreX
Is it possible to request the same voice actor that did the one you linked?

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ballooney
Interestingly, right now in Oxford UK is being tested a precooler for the
sabre jet engine designed to power skylon spaceplanes to Mach 6 in air
breathing mode (before going to orbit once out of the atmosphere with stored
oxidiser).

This precooler is a crucial and fascinating bit of technology, cooling the
incoming air down by 1000Kelvin in 100ms without itself frosting up, so it can
be compressed and burnt. It is designed to solve exactly the problem outlined
in the reddit comment. There was a fascinating BBC article about this
recently:

<http://www.bbc.co.uk/news/science-environment-17864782>

Looks like a startup rather than 'dinospace' too, for what that's worth round
here.

~~~
gcb
Why would you cool down to later compress (and heat up again)?

Sounds like a shotgun solution to the passer by such as myself

Edit: it is a shotgunsolution... They just added an old turbine after the
cooler component to test it.

~~~
omegant
They had to develope a special fuel for the SR 71. One with very high flash
point. I imagine that the temperatures at mach 6 are too high for normal fuels
too.

~~~
z303
The JP7 was also used as a coolant. The fuel was pumped around the hottest
part of the plane to soak up the heat before it was burnt

~~~
matthavener
Yep. Skunk works describes how they pumped it around the cockpit to help cool
the aviation electronics and pilot. Just to add to the scariness of flying at
mach 3+, you're also surrounded by jet fuel.

~~~
excuse-me
Jet fuel is remarkably non-flammable as anyone who has ever tried to light a
Aga will discover

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omegant
If you like aerospatial history but also if you'll like to know how all this
birds were made, you should read "Skunk Works". I have just finished reading
it, and is a great book. It tells all the development of the U2, SR71, F117,
and more.

It's also greatly recomended for entrepreneurs, they worked as an small team,
a very talented and focused group of people building unbeliable planes. At the
end it has a great rant against all the bourocracy that has built up alog the
years at the government.

Just an amazing read!

~~~
SkyMarshal
Second _Skunk Works_. They really did some amazing things, and that is
probably the best book on it. It reads like _folklore.org_ , in that it's one
of the principal people involved retelling the stories.

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WalterBright
It always kind of irks me when people talk about the heat of reentry coming
from friction with the air. It doesn't come from friction, it comes from
compressing the air.

~~~
Retric
Nope, if it was just compression you would not leave a plasma trail that far
after the shuttle. Drag aka Friction is rapidly slowing the ~100,000 kg
shuttle down from Mach 22. You can get disk breaks to glow slowing down a
1,000kg car from 100mph to zero quickly this is dumping over 2.3 million times
that energy mostly into a lot of vary low density air.

PS: ((6.9 km/s / 100mph)^2 * 100) = 2,382,349.16

~~~
powertower
<http://www.newton.dep.anl.gov/askasci/eng99/eng99460.htm>

~~~
Retric
Sure, pressure does increase the temperature, but what's happening is your
increasing the pressure of significantly heated air. As I understand it the
shuttles bow shock stays under 3ATM during reentry which is hardly enough to
be all that interesting by it's self. But, dump enough energy to get air to
~1000c at STP and increase the pressure and you get some real heat.

Also the shuttle is descending (aka falling) from LEO which adds significant
kinetic energy, but the ride get's a lot more boring at low mach numbers so
when you look at the energy needs to dump in that 'glow/burn' phase it's about
the same as orbital velocity.

~~~
powertower
The point was that heat of re-entry of a blunt object results from compression
rather than friction (negligible part), once it reaches the atmosphere (at
beginning there is hardly any air and hence temp/heat/compression is small).

See more of the discussion here:
[http://www.rocketryforum.com/showthread.php?31212-Reentry-
he...](http://www.rocketryforum.com/showthread.php?31212-Reentry-heat-air-
friction)

~~~
Retric
There is a lot of miss information on that form.

First off, both energy and velocity is conserved even though Energy is a
function of velocity squared.

So when you mix air of two different velocity's you get direct heating along
with the change in velocity. You also get quite a bit of thermal radiation
which heat up the air before it feels the physical effects from the craft. So,
while the pressure does increase the actual pressure never get's all that high
despite the huge increase in temperature. For a sanity check if you assume the
energy of the air starts at the equivalent of ~30c at 1ATM and 1ATM =
~15lb/square inch then pressure required to get anywhere close to the observed
temperatures get's ridiculous as in 100's of g's of deceleration.

Also, the blunt surface in front of a hyper-sonic aircraft collects a pressure
wave off the surface. With the air next to the skin moving at the same
relative speed as the craft. So the friction heating precedes the craft, and
is negligible right next to it. The aircraft is also cooling the air next to
it which is what creates the cooler buffer zone. (This is why they use blunt
surfaces in the first place.)

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tocomment
Has anyone thought of strapping a rocket onto the SR-71 and getting it to
orbit?

(Obviously much more complicated than my whimsical comment but maybe do-able
...?)

~~~
starpilot
The SR-71 is probably too small to carry a space-faring rocket, but the
concept is reasonable:
[http://en.wikipedia.org/wiki/Air_launch_to_orbit#Supersonic_...](http://en.wikipedia.org/wiki/Air_launch_to_orbit#Supersonic_air_launch)

This splits combined-cycle propulsion into multiple vehicles: air-breathing
propulsion for the aircraft, chemical rocket for the spacecraft, thereby
improving the efficiency of the whole launch.

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elteto
It's funny how the Internet works sometimes: I clicked on a link on HN to a
random comment on a Reddit post, just to find out that I know the original
Reddit poster (saw his pic on the post)!!! Haha :)

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cjdentra
What is really cool is to actually see an SR-71. The Intrepid in NYC has one
on deck along with the retired shuttle Enterprise. There's a Concorde and some
other cool aircraft too.

