Air Force Reveals Tests of Supposed Record-Setting Scramjet from N. Grumman

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.

That’s not what’s going on in the video. This is a cryogenic test, no part of the engine is hot.

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

Thank god we have MSNBC to keep the rooskie from sapping and impurifying our precious bodily fluids.

https://thediplomat.com/2019/02/russia-conducts-test-of-nucl...

More importantly, that was an ordinary ramjet, where the fluid flow inside the engine is subsonic.

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 [2] http://hypersonics.mechmining.uq.edu.au/facilities (specifically X2 and X3)

or bits into the video camera and hud.

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.

> Presumably both, to a large degree, since it's probably a pretty optimized aircraft.

And all that was done without a computer.

The SR-71 had a moving engine inlet spike, so that would be an operating limit based on temperature and speed.

Fun fact: the SR-71 flew its entire mission in radio silence, including multiple aerial refuelings, to avoid pesky Russian trawlers, etc. tracking it.

Generally aluminum is good to Mach 2.2, then you go to steel or titanium for either leading edges or the entire fuselage.

Which makes the F-5 all that more impressive, as its fuselage was designed for Mach 2.0, near the limit of aluminum, even theough the engines limited it to Mach 1.3 to Mach 1.6. (The F-5 was so small that it was optically stealthy - adversaries couldn't see it outside 5 miles.)

Titanium was used as a last resort in the F14 to reduce weight in the central fuselage box, which meant building the world's largest titanium casting plant at the time. Your tax dollars at work.