How is a scramjet engine cooled?

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The US military research agency DARPA is now putting out one project after the other in order to catch up with the USA's great deficit in relation to Russian missile technology. It's not about the rocket engine, but about the controls.

From Erich Moechel

The successful test of a new type of Russian missile system at the end of December created a hectic rush among the US military. A so-called "Proposers’ Day "for suppliers is scheduled for January 22nd to solve the outstanding problems of their own hypersonic project. The US currently has nothing comparable to oppose the Russian “Avangard” rocket, which has reached more than twenty times the speed of sound.

The project of the military research agency DARPA bears the acronym MACH, which is the unit of measurement for the speed of sound. However, the focus is not on hypersonic drives themselves, but on materials that keep their shape even when extremely heated. This is now being researched at full speed in order to catch up Russia's great lead. A new arms race is looming.


The deadline for submission is January 15th. The participating institutions and companies will meet a week later, and another date is scheduled for mid-February.

Cooling systems for tail units

In addition, a “Broad Agency Announcement” from DARPA is announced for mid-January, ie a comprehensive invitation to tender that is aimed at all government organizations beyond the field of rocket technology. The main reason why the US still does not have such a missile weapon after so many decades of research is the lack of very specific materials that remain stable even at temperatures in excess of thousands of degrees Celsius.

In all symbol drawings, the hypersonic vehicles known as “gliders” are therefore shown with red-hot edges or tail stubs. This is precisely the problem facing American missile designers, which is due to be tackled again on January 22nd. It is imperative that these exposed parts be cooled, otherwise they will deform and glow. This means that any control of the glider is lost and the almost wedge-shaped vehicle crashes.


The only requirement of the MACH program so far is to develop “sharp, dimensionally stable, cooled control systems for hypersonic aircraft”.

How Mach 20 can be achieved

As early as 2004, a NASA X-37 rocket completed a very short test flight at Mach 7 for the first time. The expected breakthrough did not materialize, however.

The details of the failed US tests of this rocket technology are kept secret, but in the previous large test series from 2010 to 2013 each attempt ended with an uncontrolled crash of the projectile at speeds beyond Mach 7. These edges determine the entire aerodynamics of this Gleiters, it's the tail units of this weapon. Unlike a conventional warhead, which flies to its target in a wide parabola without further acceleration or control, the “scramjet” drive only starts up after re-entry into the atmosphere.

The almost wedge-shaped flying object, which was carried with a conventional ballistic missile at an altitude of more than 1,000 kilometers, then descends in a dive. So far there is no data on this, but it can be assumed that a fairly steep ballistic curve is flown for this. As soon as Mach 5 is reached and the atmosphere is already tight enough again that there is such a strong build-up in front of the air inlets of the scramjet turbine that the air in the combustion chamber is compressed, the device can be accelerated to more than 20 times the speed of sound.


The top diagram shows a conventional aircraft turbine drawing, compressing and igniting air. In the middle is a hybrid system (“ramjet”), which already works according to principles similar to the “scramjet” turbine below. Wikimedia | CC BY-SA 3.0

How a scramjet engine works

Unlike conventional jet turbines, the scramjet engine has no moving parts, it just needs a huge backlog in front of the inlet, which presses the oxygen into the combustion chamber. The name is therefore also from the verb to scram ("Press in") derived. Furthermore, only a relatively small hydrogen tank is required - there is enough oxygen available - because the scramjet engine is only in operation for a short time before the impact. This also requires a fast injection pump for the hydrogen, an equally fast ignition element and comparatively simple electronics that control all of this and keep the glider realigning its target.

For the last few hundred kilometers, the vehicle can still fly maneuvers and vary the flight altitude at this insane speed. Any attempt at defense against such a flying object in this last phase of flight is completely futile, because the glider can also vary the flight speed, short engine thrusts are sufficient. None of the three known US air defense systems - neither Patriot, AEGIS (ship-based) nor THAAD, which operates at altitudes more than 100 kilometers - has any chance of defense.

The militarization of the ionosphere

The turn of the year 2019 marks the start of a new arms race that will increasingly shake the entire strategic balance. The greatly reduced reaction times to a hypersonic rocket attack contribute to this, but the consequences that this disruptive technology brings with it weigh much more heavily. Since all defense options have to be taken into account in the development of any offensive weapon, it can be assumed with a high degree of certainty that Russia is already working flat out on the development of a defensive weaponṫ.

The US is already on it. In the summer, DARPA started a program for an “interceptor” that is an interceptor weapon against hypersonic missiles; not entirely surprisingly, it was called “Glide Breaker”. The special thing about this weapon is that it has to be stationed high up in the ionosphere, at a height of hundreds of kilometers, because otherwise it cannot function at all. For the coming years there is nothing more and nothing less ahead of us than a militarization of the universe.

More on that comes in Part Three, due mid-week.