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.
Scott Manley has a good explanation but I can't find the video right now where he mentions it.
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. 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.
 This is fortunate as it makes down coats warm rather than cold.
“The steam is cooled by the cold engine nozzle, condensing and eventually freezing at the nozzle exit to form icicles.” https://www.nasa.gov/mission_pages/constellation/multimedia/...
In other words both sides of the base of the nozzle is cold.
PS: Though presumably closer it combustion it may still get hot on the inside.
Nuclear scramjets achieved 3 times this thrust "record" under similar test conditions more than half a century ago.
>> The SLAM, as proposed, would carry a payload of many nuclear weapons to be dropped on multiple targets, making the cruise missile into an unmanned bomber. After delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing tremendous ground damage with its shock wave and fallout. When it finally lost enough power to fly, and crash-landed, the engine would have a good chance of spewing deadly radiation for months to come.
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.
 http://hypersonics.mechmining.uq.edu.au/facilities (specifically X2 and X3)
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?
* Craft using an inertial mass reduction device 
* High frequency gravitational wave generator 
* Piezoelectricity-induced Room Temperature Superconductor 
(Doing this from memory so I may get something wrong here), the SR-71 usually cruised at mach 3.2, it could accelerate to mach 3.3 when fired upon, and could go to mach 3.4 if the pilot felt it was critical to preserve the crew/airframe however the plane needed to return to base after getting out of harms way and needed to be inspected before returning to the flight line. A lockheed test pilot apparently got the SR-71 to mach 3.5 (or maybe Mach 3.6?) but said it started to get scary. If this is accurate, it's probably the case that no one knows what fails first on the SR-71.
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.
Funnily enough, having read the Sled Driver, I don't remember any clear explainer whether the engine or the airframe was thermally limited. Perhaps it was there and I forgot by now?
Wikipedia to the rescue, "Maximum flight speed was limited by the temperature of the air entering the engine compressor, which was not certified for temperatures above 800 °F (430 °C)."  The inlet air was necessarily heated as side effect of being slowed down to subsonic speeds, necessitated by the nature of the turbine engine.
 by circulating the JP-7 fuel through critical sections, before sending it off to the engines
And all that was done without a computer.