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.
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.
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.
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.
Sure doesn't seem that way to me. Blue rocket flame is coming out, and the operators talk about "ignition" and percentage thrust and shut down (when all the icicles fly off).
> 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.
Project Pluto belongs [0] in the "what could possibly go wrong" bin:
>> 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.
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.
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?
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].
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.
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?
They reported seeing the Tic Tac descend from 20,000 feet in just an instant, stopping at the surface of the ocean. This and other reports suggest a propulsion system that alters spacetime and mass, so that there's no inertia. We Earthmen have no such technology.
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.
In SR-71 Revealed, a book written by an SR-71 pilot and squadron commander, the author addressed this and to the best of his knowledge, the USAF and Lockheed both don't know the true top speed of the SR-71.
(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.
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.
The aircraft was actively cooled[1], so I suppose they could always force a bit more cooling if needed. My guess is on engines - and not necessarily thermal limitation, probably rather inlet speed.
Funnily enough, having read the Sled Driver[2], 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?
[edit]
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)." [3] The inlet air was necessarily heated as side effect of being slowed down to subsonic speeds, necessitated by the nature of the turbine engine.
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[1] by circulating the JP-7 fuel through critical sections, before sending it off to the engines
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.
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.
I've heard rumors that the mach number dial had a 4 on it; But the engine is rated to 3.6, anything past that would bottom out the inlet spikes and then you'd get an unstart.
https://www.edwards.af.mil/News/Article/1922993/team-edwards...
https://vimeo.com/37453969