scramjet

A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion (hence ''ram''jet), but whereas a ramjet decelerates the air to subsonic velocities before combustion using shock cones, a scramjet has no shock cone and slows the airflow using shockwaves produced by its ignition source in place of a shock cone. This allows the scramjet to operate efficiently at extremely high speeds.

History

Before 2000

The Bell X-1 attained supersonic flight in 1947 and, by the early 1960s, rapid progress toward faster aircraft suggested that operational aircraft would be flying at "hypersonic" speeds within a few years. Except for specialized rocket research vehicles like the North American X-15 and other rocket-powered spacecraft, aircraft top speeds have remained level, generally in the range of Mach1 to Mach3.

During the US aerospaceplane program, between the 1950s and 1960s, Alexander Kartveli and Antonio Ferri were proponents of the scramjet approach.

In the 1950s and 1960s, a variety of experimental scramjet engines were built and ground tested in the US and the UK. Antonio Ferri successfully demonstrated a scramjet producing net thrust in November 1964, eventually producing 517 pounds-force (2.30 kN), about 80% of his goal. In 1958, an analytical paper discussed the merits and disadvantages of supersonic combustion ramjets. In 1964, Frederick S. Billig and Gordon L. Dugger submitted a patent application for a supersonic combustion ramjet based on Billig's PhD thesis. This patent was issued in 1981 following the removal of an order of secrecy.

In 1981, tests were made in Australia under the guidance of Professor Ray Stalker in the T3 ground test facility at ANU.

The first successful flight test of a scramjet was performed as a joint effort with NASA, over the Soviet Union in 1991. It was an axisymmetric hydrogen-fueled dual-mode scramjet developed by Central Institute of Aviation Motors (CIAM), Moscow in the late 1970s, but modernized with a FeCrAl alloy on a converted SM-6 missile to achieve initial flight parameters of Mach 6.8, before the scramjet flew at Mach 5.5. The scramjet flight was flown captive-carry atop the SA-5 surface-to-air missile that included an experimental flight support unit known as the "Hypersonic Flying Laboratory" (HFL), "Kholod".

Then, from 1992 to 1998, an additional six flight tests of the axisymmetric high-speed scramjet-demonstrator were conducted by CIAM together with France and then with NASA. Maximum flight velocity greater than Mach6.4 was achieved and scramjet operation during 77 seconds was demonstrated. These flight test series also provided insight into autonomous hypersonic flight controls.

Progress in the 2000s

In the 2000s, significant progress was made in the development of hypersonic technology, particularly in the field of scramjet engines.

The HyShot project demonstrated scramjet combustion on 30 July 2002. The scramjet engine worked effectively and demonstrated supersonic combustion in action. However, the engine was not designed to provide thrust to propel a craft. It was designed more or less as a technology demonstrator.

A joint British and Australian team from UK defense company Qinetiq and the University of Queensland were the first group to demonstrate a scramjet working in an atmospheric test.

Hyper-X claimed the first flight of a thrust-producing scramjet-powered vehicle with full aerodynamic maneuvering surfaces in 2004 with the X-43A. The last of the three X-43A scramjet tests achieved Mach9.6 for a brief time.

On 15 June 2007, the US Defense Advanced Research Project Agency (DARPA), in cooperation with the Australian Defence Science and Technology Organisation (DSTO), announced a successful scramjet flight at Mach10 using rocket engines to boost the test vehicle to hypersonic speeds.

A series of scramjet ground tests was completed at NASA Langley Arc-Heated Scramjet Test Facility (AHSTF) at simulated Mach8 flight conditions. These experiments were used to support HIFiRE flight 2.

On 22 May 2009, Woomera hosted the first successful test flight of a hypersonic aircraft in HIFiRE (Hypersonic International Flight Research Experimentation). The launch was one of ten planned test flights. The series of flights is part of a joint research program between the Defence Science and Technology Organisation and the US Air Force, designated as the HIFiRE. HIFiRE is investigating hypersonics technology and its application to advanced scramjet-powered space launch vehicles; the objective is to support the new Boeing X-51 scramjet demonstrator while also building a strong base of flight test data for quick-reaction space launch development and hypersonic "quick-strike" weapons.

Progress in the 2010s

On 22 and 23 March 2010, Australian and American defense scientists successfully tested a (HIFiRE) hypersonic rocket. It reached an atmospheric velocity of "more than 5,000 kilometres per hour" (Mach4) after taking off from the Woomera Test Range in outback South Australia.

On 27 May 2010, NASA and the United States Air Force successfully flew the X-51A Waverider for approximately 200 seconds at Mach5, setting a new world record for flight duration at hypersonic airspeed. The Waverider flew autonomously before losing acceleration for an unknown reason and destroying itself as planned. The test was declared a success. The X-51A was carried aboard a B-52, accelerated to Mach4.5 via a solid rocket booster, and then ignited the Pratt & Whitney Rocketdyne scramjet engine to reach Mach5 at . However, a second flight on 13 June 2011 was ended prematurely when the engine lit briefly on ethylene but failed to transition to its primary JP-7 fuel, failing to reach full power.

On 16 November 2010, Australian scientists from the University of New South Wales at the Australian Defence Force Academy successfully demonstrated that the high-speed flow in a naturally non-burning scramjet engine can be ignited using a pulsed laser source.

A further X-51A Waverider test failed on 15 August 2012. The attempt to fly the scramjet for a prolonged period at Mach6 was cut short when, only 15 seconds into the flight, the X-51A craft lost control and broke apart, falling into the Pacific Ocean north-west of Los Angeles. The cause of the failure was blamed on a faulty control fin.

In May 2013, an uncrewed X-51A Waverider reached 4828 km/h (Mach3.9) during a three-minute flight under scramjet power. The WaveRider was dropped at from a B-52 bomber, and then accelerated to Mach4.8 by a solid rocket booster which then separated before the WaveRider's scramjet engine came into effect.

On 28 August 2016, the Indian space agency ISRO conducted a successful test of a scramjet engine on a two-stage, solid-fueled rocket. Twin scramjet engines were mounted on the back of the second stage of a two-stage, solid-fueled sounding rocket called Advanced Technology Vehicle (ATV), which is ISRO's advanced sounding rocket. The twin scramjet engines were ignited during the second stage of the rocket when the ATV achieved a speed of 7350 km/h (Mach6) at an altitude of 20 km. The scramjet engines were fired for a duration of about 5 seconds.

On 12 June 2019, India successfully conducted the maiden flight test of its indigenously developed uncrewed scramjet demonstration aircraft for hypersonic speed flight from a base from Abdul Kalam Island in the Bay of Bengal at about 11:25 am. The aircraft is called the Hypersonic Technology Demonstrator Vehicle. The trial was carried out by the Defence Research and Development Organisation. The aircraft forms an important component of the country's programme for development of a hypersonic cruise missile system.

Progress in the 2020s

On 27 September 2021, DARPA announced successful flight of its Hypersonic Air-breathing Weapon Concept scramjet cruise missile. Another successful test was carried out in mid-March 2022 amid the Russian invasion of Ukraine. Details were kept secret to avoid escalating tension with Russia, only to be revealed by an unnamed Pentagon official in early April.

Design principles

Scramjet engines are a type of jet engine, and rely on the combustion of fuel and an oxidizer to produce thrust. Similar to conventional jet engines