Scramjet technology - how a hypersonic engine was created

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

Combat missile "surface-to-air" looked somewhat unusual - its nose was lengthened by a metal cone. On November 28, 1991, it took off from a test site near the Baikonur cosmodrome and self-destructed high above the ground. Although the missile did not shoot down any aerial object, the launch target was achieved. For the first time in the world, a hypersonic ramjet engine (scramjet engine) was tested in flight.

The scramjet engine, or, as they say, "hypersonic direct-flow" will allow to fly from Moscow to New York in 2 - 3 hours, leave the winged machine from the atmosphere into space. An aerospace plane will not need a booster plane, as for Zenger (see TM, No. 1, 1991), or a launch vehicle, as for shuttles and Buran (see TM No. 4, 1989), - delivery of cargo to orbit will cost almost ten times cheaper. In the West, such tests will take place no earlier than in three years …

The scramjet engine is capable of accelerating the aircraft to 15 - 25M (M is the Mach number, in this case, the speed of sound in the air), while the most powerful turbojet engines, which are equipped with modern civilian and military winged aircraft, are only up to 3.5M. It does not work faster - the air temperature, when the flow in the air intake is decelerated, rises so much that the turbocompressor unit is not able to compress it and supply it to the combustion chamber (CC). It is possible, of course, to strengthen the cooling system and the compressor, but then their dimensions and weight will increase so much that hypersonic speeds will be out of the question - to get off the ground.

A ramjet engine works without a compressor - the air in front of the compressor station is compressed due to its high-speed pressure (Fig. 1). The rest, in principle, is the same as for a turbojet - combustion products, escaping through the nozzle, accelerate the apparatus.

The idea of a ramjet engine, then not yet hypersonic, was put forward in 1907 by the French engineer Rene Laurent. But they built a real "forward flow" much later. Here Soviet specialists were in the lead.

First, in 1929, one of N. E. Zhukovsky's students, B. S. Stechkin (later an academician), created the theory of an air-jet engine. And then, four years later, under the leadership of designer Yu. A. Pobedonostsev in the GIRD (Group for the Study of Jet Propulsion), after experiments at the stand, the ramjet was first sent into flight.

The engine was housed in the shell of a 76-mm cannon and fired from the barrel at a supersonic speed of 588 m / s. The tests went on for two years. Projectiles with a ramjet engine developed more than 2M - not a single device in the world flew faster at that time. At the same time, the Girdovites proposed, built and tested a model of a pulsating ramjet engine - its air intake periodically opened and closed, as a result of which the combustion in the combustion chamber pulsed. Similar engines were later used in Germany on FAU-1 rockets.

The first large ramjet engines were created again by Soviet designers I. A. Merkulov in 1939 (subsonic ramjet engine) and M. M. Bondaryuk in 1944 (supersonic). Since the 40s, work on "direct flow" began at the Central Institute of Aviation Motors (CIAM).

Some types of aircraft, including missiles, were equipped with supersonic ramjet engines. However, back in the 50s it became clear that with M numbers exceeding 6 - 7, the ramjet is ineffective. Again, as in the case of the turbojet engine, the air that was braked in front of the compressor station got into it too hot. It did not make sense to compensate for this by increasing the mass and dimensions of the ramjet engine. In addition, at high temperatures, molecules of combustion products begin to dissociate, absorbing energy intended to create thrust.

It was then in 1957 that E. S. Shchetinkov, a famous scientist, a participant in the first flight tests of a ramjet engine, invented a hypersonic engine. A year later, publications about similar developments appeared in the West. The scramjet combustion chamber begins almost immediately behind the air intake, then it smoothly passes into an expanding nozzle (Fig. 2). Although the air is slowed down at the entrance to it, unlike previous engines, it moves to the compressor station, or rather, rushes at supersonic speed. Therefore, its pressure on the chamber walls and the temperature are much lower than in a ramjet engine.

A little later, a scramjet engine with external combustion was proposed (Fig. 3) In an aircraft with such an engine, the fuel will burn directly under the fuselage, which will serve as part of the open compressor station. Naturally, the pressure in the combustion zone will be less than in a conventional combustion chamber - the engine thrust will decrease slightly. But the gain in weight will turn out - the engine will get rid of the massive outer wall of the compressor station and part of the cooling system. True, a reliable "open direct flow" has not yet been created - its finest hour will probably come in the middle of the XXI century.

Let's return, however, to the scramjet engine, which was tested on the eve of last winter. It was fueled by liquid hydrogen stored in a tank at a temperature of about 20 K (- 253 ° C). Supersonic combustion was perhaps the most difficult problem. Will hydrogen be evenly distributed over the section of the chamber? Will it have time to completely burn out? How to organize automatic combustion control? - you can't install sensors in a chamber, they will melt.

Neither mathematical modeling on super-powerful computers, nor bench tests provided comprehensive answers to many questions. By the way, to simulate an air flow, for example at 8M, the stand requires a pressure of hundreds of atmospheres and a temperature of about 2500 K - liquid metal in a hot open-hearth furnace is much "cooler". At even higher speeds, engine and aircraft performance can only be verified in flight.

It has been thought for a long time both in our country and abroad. Back in the 60s, the United States was preparing tests of a scramjet engine on a high-speed X-15 rocket aircraft, however, apparently, they never took place.

The domestic experimental scramjet engine was made dual-mode - at a flight speed exceeding 3M, it worked as an ordinary "direct flow", and after 5 - 6M - as a hypersonic one. For this, the places of fuel supply to the compressor station were changed. The anti-aircraft missile, which was being removed from service, became the engine accelerator and the carrier of the hypersonic flying laboratory (HLL). The GLL, which includes control systems, measurements and communication with the ground, a hydrogen tank and fuel units, were docked to the compartments of the second stage, where, after the removal of the warhead, the main engine (LRE) with its fuel tanks remained. The first stage - powder boosters, - having dispersed the rocket from the start, separated after a few seconds.

Bench tests and preparation for the flight were carried out at the PI Baranov Central Institute of Aviation Motors, together with the Air Force, the Fakel machine-building design bureau, which turned its rocket into a flying laboratory, the Soyuz design bureau in Tuyev and the Temp design bureau in Moscow, which manufactured the engine. and the fuel regulator, and other organizations. The well-known aviation specialists R. I. Kurziner, D. A. Ogorodnikov and V. A. Sosunov supervised the program.

To support the flight, CIAM created a mobile liquid hydrogen refueling complex and an onboard liquid hydrogen supply system. Now, when liquid hydrogen is regarded as one of the most promising fuels, the experience of handling it accumulated at CIAM can be useful to many.

… The rocket launched late in the evening, it was already almost dark. A few moments later, the "cone" carrier disappeared into low clouds. There was a silence that was unexpected compared to the initial rumble. The testers who watched the start even thought: did everything really go wrong? No, the apparatus continued on its intended path. At the 38th second, when the speed reached 3.5M, the engine started, hydrogen began to flow into the CC.

But on the 62nd, the unexpected really happened: the automatic shutdown of the fuel supply was triggered - the scramjet engine shut down. Then, at about the 195th second, it automatically started up again and worked until the 200th … It was previously determined as the last second of the flight. At this moment, the rocket, while still over the territory of the test site, self-destructed.

The maximum speed was 6200 km / h (slightly more than 5.2M). The engine and its systems were monitored by 250 onboard sensors. Measurements were transmitted by radio telemetry to the ground.

Not all the information has been processed yet, and a more detailed story about the flight is premature. But it is already clear now that in a few decades the pilots and cosmonauts will ride the "hypersonic forward flow".

From the editor. Flight tests of scramjet engines on the X-30 aircraft in the USA and on the Hytex in Germany are planned for 1995 or the next few years. Our specialists could, in the near future, test the "direct flow" at a speed of more than 10M on powerful missiles that are now being withdrawn from service. True, they are dominated by an unresolved problem. Not scientific or technical. CIAM has no money. They are not even available for the half-beggarly salaries of employees.

What's next? Now there are only four countries in the world that have a full cycle of aircraft engine building - from basic research to production of serial products. These are the USA, England, France and, for now, Russia. So there would be no more of them in the future - three.

The Americans are now investing hundreds of millions of dollars in the scramjet program …