Үстінкү дыбыс жылдамдығында жану камералы реактивтік қозғалтқыш

Jet engine where combustion takes place in supersonic airflow

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 ramjet), but where as 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. Although scramjet engines have been used in a handful of operational military vehicles, scramjets have so far mostly been demonstrated in research test articles and experimental vehicles. An exception to this is the Russian attack of Ukraine with a scramjet powered missile in March 2024.

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 the mid 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.

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. 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 70000 feet (21336 m). 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 X 51A Waverider reached 4828 km/h (Mach3.9) during a three minute flight under scramjet power. The WaveRider was dropped at 50000 feet (15240 m) 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.

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.

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, scramjet powered aircraft carry the fuel on board, and obtain the oxidizer by the ingestion of atmospheric oxygen (as compared to rockets, which carry both fuel and an oxidizing agent). This requirement limits scramjets to suborbital atmospheric propulsion, where the oxygen content of the air is sufficient to maintain combustion. The scramjet is composed of three basic components: a converging inlet, where incoming air is compressed; a combustor, where gaseous fuel is burned with atmospheric oxygen to produce heat; and a diverging nozzle, where the heated air is accelerated to produce thrust. Unlike a typical jet engine, such as a turbojet or turbofan engine, a scramjet does not use rotating, fan like components to compress the air; rather, the achievable speed of the aircraft moving through the atmosphere causes the air to compress within the inlet. In May 2013, another flight achieved an increased speed of Mach5.1. While scramjets are conceptually simple, actual implementation is limited by extreme technical challenges. Hypersonic flight within the atmosphere generates immense drag, and temperatures found on the aircraft and within the engine can be much greater than that of the surrounding air. Maintaining combustion in the supersonic flow presents additional challenges, as the fuel must be injected, mixed, ignited, and burned within milliseconds. While scramjet technology has been under development since the 1950s, only very recently have scramjets successfully achieved powered flight. Scramjets are designed to operate in the hypersonic flight regime, beyond the reach of turbojet engines, and, along with ramjets, fill the gap between the high efficiency of turbojets and the high speed of rocket engines. Turbomachinery based engines, while highly efficient at subsonic speeds, become increasingly inefficient at transonic speeds, as the compressor rotors found in turbojet engines require subsonic speeds to operate. While the flow from transonic to low supersonic speeds can be decelerated to these conditions, doing so at supersonic speeds results in a tremendous increase in temperature and a loss in the total pressure of the flow. Around Mach3–4, turbomachinery is no longer useful, and ram style compression becomes the preferred method. Ramjets use high speed characteristics of air to literally 'ram' air through an inlet diffuser into the combustor. At transonic and supersonic flight speeds, the air upstream of the inlet is not able to move out of the way quickly enough, and is compressed within the diffuser before being diffused into the combustor. Combustion in a ramjet takes place at subsonic velocities, similar to turbojets but the combustion products are then accelerated through a convergent divergent nozzle to supersonic speeds. As they have no mechanical means of compression, ramjets cannot start from a standstill, and generally do not achieve sufficient compression until supersonic flight. The lack of intricate turbomachinery allows ramjets to deal with the temperature rise associated with decelerating a supersonic flow to subsonic speeds. However, as speed rises, the internal energy of the flow after diffusor grows rapidly, so the relative addition of energy due to fuel combustion becomes lower, leading to decrease in efficiency of the engine. This leads to decrease in thrust generated by ramjets at higher speeds. Thus, to generate thrust at very high velocities, rise of pressure and temperature of incoming air flow must be tightly controlled. In particular, this means that deceleration of the airflow to subsonic speed cannot be allowed. At such setup not only mixing of fuel with air presents an engineering challenge, but also speed of combustion in air fuel mixtures becomes a concern. In addition the relative increase of internal energy in combustion chamber must be maximized. Consequently, current scramjet technology requires the use of high energy fuels and active cooling schemes to maintain sustained operation, often using hydrogen and regenerative cooling techniques.

An advantage of a hypersonic airbreathing (typically scramjet) vehicle like the X 30 is avoiding or at least reducing the need for carrying oxidizer. For example, the Space Shuttle external tank held 616,432.2 kg of liquid oxygen (LOX) and 103,000 kg of liquid hydrogen (LH) while having an empty weight of 30,000 kg. The orbiter gross weight was 109,000 kg with a maximum payload of about 25,000 kg and to get the assembly off the launch pad the shuttle used two very powerful solid rocket boosters with a weight of 590,000 kg each. If the oxygen could be eliminated, the vehicle could be lighter at liftoff and possibly carry more payload. On the other hand, scramjets spend more time in the atmosphere and require more hydrogen fuel to deal with aerodynamic drag. Whereas liquid oxygen is quite a dense fluid (1141 kg/m3), liquid hydrogen has much lower density (70.85 kg/m3) and takes up more volume. This means that the vehicle using this fuel becomes much bigger and gives more drag. Other fuels have more comparable density, such as RP 1 (810 kg/m3) JP 7 (density at 15 °C 779–806 kg/m3) and unsymmetrical dimethylhydrazine (UDMH) (793.00 kg/m3). One issue is that scramjet engines are predicted to have exceptionally poor thrust to weight ratio of around 2, when installed in a launch vehicle. A rocket has the advantage that its engines have very high thrust weight ratios (~100:1), while the tank to hold the liquid oxygen approaches a volume ratio of ~100:1 also. Thus a rocket can achieve a very high mass fraction, which improves performance. By way of contrast the projected thrust/weight ratio of scramjet engines of about 2 mean a much larger percentage of the takeoff mass is engine (ignoring that this fraction increases anyway by a factor of about four due to the lack of onboard oxidiser). In addition the vehicle's lower thrust does not necessarily avoid the need for the expensive, bulky, and failure prone high performance turbopumps found in conventional liquid fuelled rocket engines, since most scramjet designs seem to be incapable of orbital speeds in airbreathing mode, and hence extra rocket engines are needed. Scramjets might be able to accelerate from approximately Mach5–7 to around somewhere between half of orbital speed and orbital speed (X 30 research suggested that Mach17 might be the limit compared to an orbital speed of Mach25, and other studies put the upper speed limit for a pure scramjet engine between Mach10 and 25, depending on the assumptions made). Generally, another propulsion system (very typically, a rocket is proposed) is expected to be needed for the final acceleration into orbit. Since the delta V is moderate and the payload fraction of scramjets high, lower performance rockets such as solids, hypergolics, or simple liquid fueled boosters might be acceptable. Theoretical projections place the top speed of a scramjet between and For comparison, the orbital speed at low Earth orbit is
The scramjet's heat resistant underside potentially doubles as its reentry system if a single stage to orbit vehicle using non ablative, non active cooling is visualised. If an ablative shielding is used on the engine it will probably not be usable after ascent to orbit. If active cooling is used with the fuel as coolant, the loss of all fuel during the burn to orbit will also mean the loss of all cooling for the thermal protection system. Reducing the amount of fuel and oxidizer does not necessarily improve costs as rocket propellants are comparatively very cheap. Indeed, the unit cost of the vehicle can be expected to end up far higher, since aerospace hardware cost is about two orders of magnitude higher than liquid oxygen, fuel and tankage, and scramjet hardware seems to be much heavier than rockets for any given payload. Still, if scramjets enable reusable vehicles, this could theoretically be a cost benefit. Whether equipment subject to the extreme conditions of a scramjet can be reused sufficiently many times is unclear; all flown scramjet tests only survive for short periods and have never been designed to survive a flight to date. The eventual cost of such a vehicle is the subject of intense debate since even the best estimates disagree whether a scramjet vehicle would be advantageous. It is likely that a scramjet vehicle would need to lift more load than a rocket of equal takeoff weight to be equally as cost efficient (if the scramjet is a non reusable vehicle). Space launch vehicles may or may not benefit from having a scramjet stage. A scramjet stage of a launch vehicle theoretically provides a specific impulse of 1000 to 4000s whereas a rocket provides less than 450s while in the atmosphere. A scramjet's specific impulse decreases rapidly with speed, however, and the vehicle would suffer from a relatively low lift to drag ratio. The installed thrust to weight ratio of scramjets compares very unfavorably with the 50–100 of a typical rocket engine. This is compensated for in scramjets partly because the weight of the vehicle would be carried by aerodynamic lift rather than pure rocket power (giving reduced 'gravity losses'), but scramjets would take much longer to get to orbit due to lower thrust which greatly offsets the advantage. The takeoff weight of a scramjet vehicle is significantly reduced over that of a rocket, due to the lack of onboard oxidiser, but increased by the structural requirements of the larger and heavier engines. Whether this vehicle could be reusable or not is still a subject of debate and research.