
Air Force Reveals Tests of Supposed Record-Setting Scramjet from N. Grumman - nwrk
https://www.thedrive.com/the-war-zone/29307/air-force-reveals-tests-of-supposed-record-setting-scramjet-engine-from-northrop-grumman
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fdavison
Johnny Armstrong, the Hypersonics Combined Test Force Chief Engineer for Air
Force, until his retirement in 2012, passed away just last week. He worked in
hypersonics from the X-15 to the X-51 Waverider.

[https://www.edwards.af.mil/News/Article/1922993/team-
edwards...](https://www.edwards.af.mil/News/Article/1922993/team-edwards-
loses-one-of-its-own-as-hypersonics-pioneer-johnny-g-armstrong-dies/)

[https://vimeo.com/37453969](https://vimeo.com/37453969)

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ChuckMcM
This is a pretty neat technical achievement. One of the things that has made
it possible has been the ability to much better CFD simulations of the whole
system. Rather than test/tail iterate cycle, they can get to a working system
that can be refined in a shorter period of time.

That said, its hard to beat the SABER engine's use of liquid hydrogen to cool
the air coming into the engine and just achieving better thrust. I still find
that concept mind blowing.

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positr0n
You probably know that using liquid hydrogen to cool engine bells is common in
rocketry as well. I found this video fascinating. On one side, the engine is
so cold icicles are forming. Centimeters away, the other side is so hot that
it would melt pretty much any metal except the most exotic of alloys. The
engine is so efficient that all that energy is pointed straight down. None
escapes to melt the icicles.

[https://youtu.be/4eM1mNNdguA?t=89](https://youtu.be/4eM1mNNdguA?t=89)

Scott Manley has a good explanation but I can't find the video right now where
he mentions it.

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ChuckMcM
Yes, it keeps the bell of the nozzle from melting off the back of the rocket.
And that is a really cool trick, extremely tough to pull off in the original
F1 engines of the Saturn V (lots of welds) but easier to do now that you can
3D print the engine.

However, the SABRE has a different challenge. Rather than cool metal (Inconel)
which is a very good conductor of heat, the SABRE heat exchanger is cooling
the air which has gained temperature for becoming compressed by the moving
aircraft. Air, for all of its wonderful properties, is not a particularly good
heat conductor[1]. Further, since heat transfer by conduction is a function of
time in which the two dissimilar temperature materials are in contact, and
hyper sonic craft are going very fast, it has to achieve this heat transfer in
a literal "blink of an eye." It has been compared to shooting an acetylene
torch through it and having a cool breeze come out the other side. Really
amazing stuff.

[1] This is fortunate as it makes down coats warm rather than cold.

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inamberclad
Not only that, but it has to not ice up in the process.

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beefman
> The Air Force recently broke the record for the highest thrust producing
> air-breathing hypersonic engine the service has ever tested in its history,
> at least that we know about.

Nuclear scramjets achieved 3 times this thrust "record" under similar test
conditions more than half a century ago.

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dtparr
Do nuclear scramjets count as air breathing? They don't use atmospheric oxygen
for combustion, iirc, which is what I think of when I hear air breathing.

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DuskStar
They're air breathing in the same sense that electric planes are, I suppose?
They use air for propulsion, but not for chemical reactions.

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killjoywashere
Here's a picture of the test apparatus. This barely resembles a typical wind
tunnel. Anyone who know these things care to provide a illustrated guide to
thing?

[https://media.defense.gov/2019/Aug/05/2002166487/-1/-1/0/190...](https://media.defense.gov/2019/Aug/05/2002166487/-1/-1/0/190220-F-IL571-080.JPG)

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namirez
Not sure about the details of this picture, but hypersonic wind tunnels look
very different than subsonic wind tunnels. The cross section of a hypersonic
tunnel is usually small and there are not many moving parts. The high Mach
number is achieved by discharging compressed air into the tunnel. You can find
more info about this specific tunnel on wikipedia [1].

[1]
[https://en.wikipedia.org/wiki/Aerodynamic_and_Propulsion_Tes...](https://en.wikipedia.org/wiki/Aerodynamic_and_Propulsion_Test_Unit)

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NamTaf
I can't add much to this, besides the point that in my experience with
expansion tunnels [1][2] from my time at uni, they are very short-term test
facilities, lasting for fractions of a second. They work by compressing a big
vessel of air with a piston, rupturing a diaphragm, and then blowing that
highly compressed air through a expansion nozzle (which, unlike subsonic flow,
_accelerates_ the flow velocity through the nozzle) before passing it over the
test piece.

I'm not familiar with the APTU, but it would not surprise me if this is a
similar concept, wherein it delivers a short-term high-velocity flow across a
test object. I think the main difference is it heats with a combustion air
heater, rather than pressurising with a big slug. In any case, because it's
quick and requires a small cross-sectional area, much of the surrounds of the
machine are all sensors to measure everything, since you don't want to have to
do it over and over again. I think that is much of what can be seen in the
photo of the APTU.

[1]
[https://en.wikipedia.org/wiki/Expansion_tunnel](https://en.wikipedia.org/wiki/Expansion_tunnel)
[2]
[http://hypersonics.mechmining.uq.edu.au/facilities](http://hypersonics.mechmining.uq.edu.au/facilities)
(specifically X2 and X3)

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throwaway3627
I remember scramjets in Popular Mechanics 30 years ago, but nothing seemed to
materialize.

Off-topic to scramjets but potentially military-related: Making a _big_
stretch by assuming the "white Tic-Tac" isn't an elaborate hoax to get
adversaries to overspend on propulsion technologies and/or troll the public,
what type of propulsion could allow it to accelerate so quickly? Casimir
effect thruster?

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kevin_thibedeau
Gamma ray to the visual cortex.

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sgt101
or bits into the video camera and hud.

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mikece
Is the SR-71 constrained by its engine or airframe? I know I’ve read that if
the plane went any faster _something_ would start melting and the plane would
be lost but I don’t recall if it was the airframe or the engine.

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starpilot
Presumably both, to a large degree, since it's probably a pretty optimized
aircraft. The engine & inlet are designed to compress the air using the shock
patterns that develop at certain Mach numbers. Beyond that Mach range, it
won't work. Likewise they probably didn't bother designing the airframe to
work at speeds in which the engine couldn't.

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cbanek
The SR-71 is also high altitude, so if there is less air (pressure), that can
change things like: how much lift you need or can generate, the outside
temperature, how much air can be used by the engine, heating due to air
friction, etc.

