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Above: The S-200 rocket with a Kholod scramjet vehicle. (Click red arrow to rotate)

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A 1 to 5 scale model of the Kholod scramjet engine.

One of the early Kholod vehicles launched in 1991 or 1992.

The 58L scramjet developed at KB Khimavtomatiki, KBKhA, in Voronezh was tested at speed of more than Mach 6 in 1998.

A Kholod laboratory lifts off from Sary Shagan.

Hot breath of Kholod

During the 1990s, when the Russian space program was largely left for dead, the nation's engineers flight-tested a pioneering propulsion system that one day might revolutionize space travel. Known as scramjet, for supersonic combustion ramjet, the engine still remains a cutting-edge technology, while its early history in Russia has ended up largely forgotten.

Previous chapter: Supersonic vehicles

Origin of the program

The project of an experimental scramjet engine that came to fruition in the waning days of the Soviet Union, started at least two decades earlier at the Moscow-based Baranov Central Institute of Aviation Motors, TsIAM. Its pioneering vehicle became known in the West as Hypersonic Flying Laboratory, HFL. (686) In the USSR, the project was code-named Kholod, the Russian word for "cold." Indeed, the engine at the heart of the program used cryogenically cooled liquid hydrogen as a fuel, while compressed air "sucked" from the stratosphere served as an oxidizer.

Due to the absense of an air compressor like those inside traditional aircraft jet engines, the scramjet promised lower mass, higher efficiency and, most importantly, much higher speed. However for the same reason, it would need initial acceleration with an alternative source of thrust, before atmospheric air could start flowing into its inlet and enable combustion. Soviet engineers resolved the problem by fitting the scramjet vehicle on top of the anti-aircraft missile from the S-200 system (known in the West as SA-5), which was able to accelerate the miniature vehicle three times faster than the speed of sound, or Mach 3, at an altitude of more than 20 kilometers. After firing for slightly more than a minute, the Kholod vehicle was designed to accelerate to Mach 6.5, climbing as high as 35 kilometers above the Earth surface. (331)

Test flights of Kholod scramjets would be all preprogrammed without any attempt to steer the rocket from the ground. However, an onboard telemetry system would transmit engineering data to mission control. The hydrogen scramjet would be fired after the separation of the solid-propellant boosters of the first stage. The scramjet would remain attached to the second stage of the S-200 rocket and fire simultaneously with its own liquid propellant engine. (Critics charged that such a mixed propulsion mode had complicated the analysis of the scramjet's performance.)

After consuming around 17 kilograms of liquid hydrogen, the vehicle would enter a free fall and slam into the desert. Still, engineers planned to search for the mangled debris for additional analysis.

Propulsion system design

The Kholod experimental vehicle was designed to be small enough to replace the explosive warhead of an anti-aircraft missile. In order to minimize changes to the shape of the converted missile, engineers placed the scramjet engine into the nose of the vehicle with its hydrogen propellant tank located right behind it. As a result, the scramjet would pull rather than push its rocket. With such an unusual configuration, the engine would suck the air with its doughnut-shaped inlet, then compress it and feed it into a circular combustion chamber. The exhaust would then escape through a ring-shaped nozzle, essentially flowing around the body of the rocket. (686)

The hydrogen fuel liquefied at a temperature of around 30 Kelvin would first flow through a cooling loop enveloping the combustion chamber. After fulfilling its job as a coolant, the hydrogen would quickly heat up, turning from liquid to gas. It would then be sprayed into the combustion chamber through three rows of nozzles, where it would mix with air and be ignited by electric sparks.

The combustion chamber had three section of combustion: the first zone located right behind the air inlet enabled subsonic movement of air, while the two following sections provided super-sonic combustion.

This purely experimental propulsion system became known as axisymmetric, because the engine's cylindrical body was perfectly symmetrical all around a single axis. However such a configuration was expected to be followed by larger non-symmetrical designs that were envisioned for practical use. In turn, next-generation scramjet engines could pave the way for more advanced propulsion systems, which could use atmospheric air during flight in the atmosphere and then switch to onboard supplies of oxidizer upon entering airless space. As a result, one-stage space planes could reach orbit following horizontal takeoff from a conventional runway. The technology also had potential military applications, such as propelling cruise missiles and hypersonic planes.

First flight test

Following years of theoretical development initiated in mid-1970s, the Turaev branch of the TsIAM institute successfully tested the scramjet engine at its Ts-16KV test stand, apparently the largest facility of its kind. These trials cleared the technology for actual flight tests. The first Kholod rocket lifted off from the Sary Shagan test range in Kazakhstan on Nov. 27, 1991, just a few weeks before the dissolution of the USSR. The vehicle reached an altitude of 35 kilometers and developed a speed of 1,653 kilometers per second or Mach 3.6. The hydrogen-powered scramjet engine logged a total of 27.5 seconds of firing during the flight. (688)

Second launch

Following the collapse of the USSR in 1991, both French and American engineers raced to team up with TsIAM, in order to learn from the Kholod project. However initially, the US industry bogged down in bureaucratic hurdles, while the French aerospace research establishment ONERA quickly initiated a $95-million program in the field. In April 1992, ONERA started negotiations on cooperation with TsIAM and in September the two sides signed an agreement, which would give French industry access to a wealth of data along 50 different parameters to be measured during an upcoming test flight. According to US estimates, ONERA invested a modest sum of around $200,000 into the second flight of the Kholod vehicle in 1992. (687)

On November 17, 1992, the Kholod lifted off from Sary Shagan again, while eight representatives of the French aerospace industry were looking on. The rocket was programmed to fly a slightly lower trajectory than in the first test to give the scramjet engine better flying conditions. Combined with improved fuel controls, the new trajectory promised a longer firing of the engine in the so-called supersonic combustion mode, which produces maximum performance.

Around 17 seconds before reaching the summit of its trajectory, as the rocket was flying at Mach 3.5, the scramjet engine fired in subsonic combustion mode lasting from six to seven seconds. Upon acceleration to Mach 5, just 10 seconds from reaching the highest point of its trajectory, the supersonic combustion was initiated. This unprecedented flight mode lasted for 15 seconds accelerating the rocket to a speed of Mach 5.5, before it ran out of liquid hydrogen. Overheating of the engine was also apparently detected. The scramjet logged a total of 23 seconds of firing during the mission. The unpowered rocket then smashed into the ground 180 kilometers from the launch site, after a 100-second flight. (687)

At the time, plans called for the development beginning in 1995 of a six-meter scramjet rocket, which could be accelerated to Mach 8 by a traditional booster, then separate and fire its scramjet for around one minute. However during the third test flight of the Kholod vehicle in March 1995, the scramjet failed to fire, even though its launcher delivered it to an altitude of 30 kilometers. A similar fate befell on the next Kholod experiment launched in September 1997.

NASA involvement

In November 1994, NASA finally joined the Kholod program. During the same year, Department 101 at the KB Khimavtomatiki propulsion bureau, KBKhA, in the city of Voronezh took over the development of the scramjet engine, which was now designated 58L. (331) The engine was re-designed to withstand higher temperatures, which would result from sustained operation of the engine in a supersonic combustion mode.

A NASA-sponsored test mission, which featured an upgraded engine supplied by KBKhA lifted off on February 12, 1998. With the goal of reaching a speed of Mach 6.5, the Kholod vehicle accelerated from Mach 3 to around Mach 6.41-6.47, after successfully firing for record-breaking 77 seconds at a maximum altitude of 27.1 kilometers. (688, 331, 689) Ironically, despite its terrible economic woes in the 1990s, Russia became the first to fly a scramjet vehicle.

Back to US

NASA likely used the experience from the Kholod project to build an unmanned experimental aircraft with a scramjet engine designated X-43A. The program was conducted jointly by NASA's Langley and Dryden research centers. Not coincidently, Dryden had participated in the last launch of the Kholod vehicle on behalf of NASA. Record-breaking flights of the X-43A vehicle were first attempted just three years after the last launch of Kholod.

Research into scramjet technology was also conducted in France, Germany and Japan, however the results of this work still await their practical application into space launch systems or civilian aviation. (686)

Next chapter: ISS missions

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APPENDIX

Chronology of the Kholod and X-43A projects:

1991 Nov. 27: The Kholod vehicle lifts off for the first time, firing its scramjet for 27.5 seconds and reaching a speed of Mach 3.6 at a maximum altitude of 35 kilometers.

1992 Nov. 17: The Kholod vehicle develops a speed of Mach 5.35, while climbing to an altitude of 22.4 kilometers, with its scramjet engine firing for 41.5 seconds.

1995 March 1: The Kholod vehicle reaches an altitude of 30 kilometers and a speed of Mach 5.8, however its scramjet engine fails to fire.

1997 Sept. 1: The Kholod vehicle reaches an altitude of 33 kilometers and a speed of Mach 6.2, however its scramjet engine fails to fire.

1998 Feb. 12: The fifth Kholod vehicle reaches an altitude of 27.1 kilometers and a speed from 6.41 to 6.47 Mach, with its scramjet engine firing for 77 seconds. (688)

2001 June 2: NASA makes an attempt to test-fly its X-43 experimental vehicle powered by a scramjet engine. It fails due to a failure of the Pegasus rocket, which was designed to accelerate X-43 after a mid-air launch from a B-52 aircraft.

2004 March 27: NASA's second X-43A hypersonic research aircraft flies successfully after being dropped in mid-air by NASA's B-52 and accelerated by a Pegasus rocket. For the first time, an air-breathing scramjet accelerated a free-flying vehicle. The unmanned vehicle's supersonic combustion ramjet, or scramjet, ignited as planned and operated for the duration of its hydrogen fuel supply, which lasted about 10 seconds. The X-43A reached its test speed of Mach 7.

2004 Nov. 16: NASA's X-43A scramjet-powered research vehicle reaches speed of around Mach 9.8, or 7,000 miles per hour, as it flies at an altitude of about 33 kilometers, following a mid-air launch from a B-52B aircraft onboard the Pegasus rocket booster. The mission concluded the program, discontinuing the development of scramjet technology at NASA.

Participants in the Kholod project:

Known specifications of the 58L engine for the Kholod vehicle:

Page author: Anatoly Zak; last update: February 16, 2014

Page editor: Alain Chabot; last edit: February 16, 2014

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