Ramjet engine tested by the Chinese

The problem with getting above Mach 5-6 is that once you enter that regime in the atmosphere you have even less time to scoop the air and even higher temperatures. Known reusable airframe materials have issues at, say, operating even at Mach 8. Titanium for example will lose its elasticity and stiffness and will deform at those temperatures. Let alone more than that. The whole idea about using a cryogenic fuel (it does not need to be liquid hydrogen, you can use liquid natural gas as well) is that you can use it to actively cool, well, everything from the engine to the surface of the aircraft. The SR-71 also cooled the airframe with the fuel for example. It is just that because the speeds were lower on the SR-71 the fuel didn't need to be at such low temperatures so Kerosene worked fine. Because you have to scoop more air at higher speeds, you start to see people switch to things like Waverider airframe designs, which are poorly tested designs. Plus the engine gets complex and heavy like heck, more than a combined turbojet+rocket like SABRE.

On the other side of things there are also designs like air-augmented rockets. The Soviets at one point had a ballistic missile design called Gnom and the modern Meteor air-to-air missile uses a Throttleable Ducted Rocket engine. Those engines have more Isp than a regular rocket engine because they scoop the oxidizer from the air. If the speeds are low enough and you only need to fly the whole vehicle once, like for a missile, you can basically make the whole thing of heat resistant materials that you know can survive the trip even if the whole skin or engine gets cooked and partially burned by the time you hit the target.

Folks working on rocket engines on aircraft which operate at Mach 20, like orbital space launch vehicles, also use ablative surfaces on disposable rocket engine nozzles. But those are typically more expensive, heavier, and less efficient than cooling the nozzle with the propellant. Checkout the history the SpaceX Merlin engine for example. The Soviets invented in the 1960s a process where you simply use a circular saw to machine channels into the rocket nozzle and then you just weld and braze an outer jacket on the engine. The propellant flows through the coolant channels before either going back into the combustion chamber to aid Isp, or is vented out, to simplify design. This is basically called the Sänger-Bredt engine design. It is quite cheap and reliable.

 

About the report of TBCC, I suspect that is a mistake. It is actually TRRE which is in the same category as TBCC but different. Here is the evidence.

The report stated that the test was conducted by CASIC 3rd institute. This institute is "中国航天科工集团北京动力机械研究所" which is CASIC (中国航天科工集团) Beijing engine mechanic institute. The institute published the article that I posted before, see the bold texts.

The article was published in 2017, meaning the research has been concluded well before that. There are more than 2 years to work on construction of the test piece, 2018 launch is good match.

The reason of the confusion is that TRRE is essentially an improved concept of TBCC by adding small rockets inside the main air duct of TBCC, therefor the "Turbo-aided Rocket-augmented Ram-jet Combined Cycle Engine".

 

The problem is to turbo based jet (Sabre, SR-71) and Ramjet (with the cone to lower the air speed to subsonic) because they all compress the air before burning the fuel mix. That compression increase air temperature a lot. The increase reduce the efficiency of the engine because there is less temperature differences between the in and out. At some point the engine will chock because the shockwave produced by the compressor (turbine or Ram cone) will push air out rather than suck in.

However, a scram-jet does NOT compress air, it mixes fuel and burns the mix in super-sonic. When TBCC/TRRE moves to the scramjet mode, their turbine section has been shut out from the air path and becomes part of the fuselage whose shape is designed NOT to compress air that much to suffer the compression induced temperature rise (to the dangerous point).

And there is no need for TBCC/TRRE to have the kind of cooling effect in SABRE as you suggested. SABRE has to have it, TBCC/TRRE do not need it because they are scramjet while SABRE is turbo-ram.

Also note, the purpose of SABRE's cooling is NOT due to the compression-induce high temperature to the engine, BUT to retain the efficiency of engine without which the increase inlet air will drastically reduce the turbine's compression efficiency (reduced temperature difference between inlet and exhaust). That makes SABRE goes to somewhere around Mach 4 or 5 with turbine mode, BUT no more.

About your last paragraph, I don't think we should brought in anything more than Mach 10, TBCC and TRRE is not designed to work above that, for the moment at least. Both NASA's TBCC "NASA Overview of the Turbine Based Combined Cycle Discipline" in 2011 and the CASIC 3rd's article presented their diagrams with Mach 6 as the max. Remember these combined circle engines are NOT to replace conventional rocket engines for orbital launch, they are for primarily for aircraft fly within atmosphere in scramjet mode.