Scramjet
A scramjet (supersonic combustion ramjet) is a type of air breathing engine, of which the jet engine, ramjet, pulse detonation engine and pulse jet engine are other examples. Scramjet engines are intended to propel aircraft at hypersonic speeds. They consist of a constricted tube through which air is compressed, fuel is combusted, and the exhaust is vented at higher speed than the intake air. However, like a ramjet, there are either few or no moving parts, the compression being provided by the fact that the engine is moving through the air, and air is being rammed into the air intake (and thus compressed). In a scramjet the combustion takes place at supersonic air speed inside the engine.
Like a ramjet, the scramjet must already be moving extremely fast before it will start working but, theoretically, speeds in excess of Mach 20 are possible. The upper limit of operation of a scramjet engine without additional oxidiser input is a subject of debate within the scientific community, with values between Mach 12 and Mach 24 being proposed.
History
During and after World War II, tremendous amounts of time and effort were put into researching high-speed jet- and rocket-powered aircraft. Fast planes had proven their usefulness in that conflict, and development continued afterward. Supersonic flight was attained in 1947 by the Bell X-1, and by the early 1960s, the rapid progress towards faster aircraft suggested that operational aircraft would be flying at "hypersonic" speeds within a few years. Except for specialized rocket research vehicles like the American X-15 and other rocket-powered spacecraft, the speeds of operational aircraft have remained level since that time, generally in the range of Mach 1 to Mach 2.
In the realm of civilian air transport, the primary goal has been to move large numbers of passengers point-to-point cheaply rather than quickly. Because supersonic flight requires significant amounts of fuel, airlines have favored subsonic jumbo jets rather than supersonic transports. The Concorde, which carried about 100 people at Mach 2, had always cost more to operate than the airlines charged fliers. In the military arena, the goal was to create aircraft that would be maneuverable with low radar or infrared signatures, which weighed against hypersonic aircraft as they are less maneuverable and have a high infrared signature.
Hypersonic flight concepts haven't gone away, however, and low-level investigations have continued over the past few decades. At the present time, the US military and the National Aeronautics & Space Administration (NASA) have formulated a "National Hypersonics Strategy" to investigate a range of options for hypersonic flight. Other organizations around the world from places such as Australia, France, and Russia have also gone forward with significant research.
In the US, the different organizations have different agendas, but they have all realized they need to coordinate their activities to make progress. The Army, for example, wants to develop hypersonic missiles that can attack mobile missile launchers before they leave their launch site and disappear. NASA wants to develop new, economical, reusable launch vehicles. The Air Force is interested in a wide range of hypersonic systems, from hypersonic air-launched cruise missiles to orbital spaceplanes, that the service believes could transform it into a true "aerospace force".
Theory
Much of scramjet experimentation remains classified. Several groups including the US Navy with the SCRAM engine between 1968-1974, and the Hyper-X program with the X-43A have claimed successful demonstrations of scramjet technology. Unfortunately, since these results have not been published openly, they remain unverified and a final design method of scramjet engines still does not exist.
All scramjet engines have an inlet, which compresses the incoming air, fuel injectors, a combustion chamber and a thrust nozzle. Typically engines also include a region which acts as a flame holder, although the high stagnation temperatures mean that an area of focussed waves may be used, rather than a discrete engine part as seen in turbine engines. An isolator between the inlet and combustion chamber is often included to improve the homogeneity of the flow in the combustor and to extend the operating range of the engine.
A scramjet is reminiscent of a ramjet. In a ramjet operating supersonically, the supersonic inflow of the engine is decelerated to subsonic speeds and then reaccelerated to produce thrust. This deceleration, which is produced by a normal shock, creates a total enthalpy loss which limits the upper operating point of a ramjet engine. At high speeds, the kinetic energy of the freestream air entering the scramjet engine is large compared to the energy released by the reaction of the oxygen content of the air with a fuel (say hydrogen). Thus the heat released from combustion at Mach 25 may be around 10% of the total enthalpy of the working fluid. Depending on the fuel, the kinetic energy of the air and the potential combustion heat release will be equal at around Mach 8. Thus the design of a scramjet engine is as much about minimising drag as maximising thrust.
Changing from subsonic to supersonic combustion makes the control of the flow within the combustion chamber more difficult. Since the flow is supersonic, no upstream influence propagates within the freestream of the combustion chamber. Thus throttling of the entrance to the thrust nozzle is not a usable control technique. In effect, a block of gas entering the combustion chamber must mix with fuel and have sufficient time for initiation and reaction, all the while travelling supersonically through the combustion chamber, before the burned gas is expanded through the thrust nozzle. This places stringent requirements on the pressure and temperature of the flow, and requires that the fuel injection and mixing be extremely efficient.
The high cost of flight testing and the unavailability of ground facilities have hindered scramjet development. A large amount of the experimental work on scramjets has been undertaken in cryogenic facilities, direct-connect tests, or burners, each of which simulates one aspect of the engine operation. Further, vitiated facilities, storage heated facilities, arc facilities and the various types of shock tunnels each have limitations which have prevented perfect simulation of scramjet operation. The HyShot flight test showed the relevance of the 1:1 simulation of conditions in the T4 and HEG shock tunnels, despite having cold models and a short test time. The NASA-CIAM tests provided similar verification for CIAM's C-16 V/K facility and the Hyper-X project is expected to provide similar verification for the Langley AHSTF, CHSTF and 8 Ft HTT.
Computational fluid dynamics is also only recently coming to grips with the problems inherent in scramjet operation. Boundary layer modeling, turbulent mixing, two-phase flow, flow separation, and real-gas aerothermodynamics continue to be problems on the cutting edge of CFD. Additionally, the modeling of kinetic-limited combustion with very fast-reacting species such as hydrogen continues to make severe demands on computing resources, as the full reaction schemes are numerically stiff, having typical times as low as 10E-19 seconds, requiring reduced reaction schemes.
The final application of a scramjet engine is likely to be in conjunction with engines which can operate outside the scramjet's operating range. Dual-mode scramjets combine subsonic combustion with supersonic combustion for operation at lower speeds, and rocket-based combined cycle (RBCC) engines supplement a traditional rocket's oxidiser in that atmospheric oxygen undergoes supersonic combustion with the exhaust of a rocket run fuel-rich.
Applications
Scramjet technology has significant potential, so governments and organizations around the world are researching it for use in hypersonic vehicles. The most likely near-term usage of Scramjets is in missiles since this is technically the easiest task, requiring only cruise operation instead of net thrust production- much of the money for the current research comes from governmental defence research contracts.
Another use of such an engine is on a launch vehicle. Whether this vehicle would be reusable or not is still a subject of debate. A scramjet stage of a launch vehicle could provide a higher specific impulse whilst in the atmosphere, potentially permitting much cheaper access to space.
An aircraft using this type of jet engine could dramatically reduce the time it takes to travel from one place to another, potentially putting any place on Earth within a 90-minute flight. However, there are questions about whether such a vehicle could carry enough fuel to make useful length trips, and there are obvious issues with sonic booms and acceptable g-loads on passengers.
Recent progress
In recent years, significant progress has been made in the development of hypersonic technology, particularly in the field of supersonic-combustion ramjet, or scramjet, engines. While American efforts are probably the best funded, the first to demonstrate a scramjet working in an atmospheric test was a shoestring project by an Australian team at the University of Queensland. The University's HyShot project demonstrated scramjet combustion in 2002. This demonstration was somewhat limited, however; while the scramjet engine "worked", the engine was not designed to provide thrust to propel a craft.
The US Air Force and Pratt and Whitney have cooperated on the Hypersonic Technology (HyTECH) scramjet engine, which has now been demonstrated in a wind-tunnel environment. NASA's Marshall Space Propulsion Center has introduced an Integrated Systems Test Of An Air-Breathing Rocket (ISTAR) program, prompting Pratt & Whitney, Aerojet, and Rocketdyne to join forces for development.
To coordinate hypersonic technology development, the various factions interested in hypersonic research have formed two integrated product teams (IPTs): one to consolidate Army, Air Force, and Navy hypersonic weapons research, the other to consolidate Air Force and NASA space transportation and hypersonic aircraft work. Current funding levels are relatively low, no more than US$85 million per year in total, but are expected to rise.
The most advanced US hypersonics program is the US$250 million NASA Langley Hyper-X X-43A effort, which flew small test vehicles to demonstrate hydrogen-fueled scramjet engines. NASA is worked with contractors Boeing, Microcraft, and the General Applied Science Laboratory (GASL) on the project.
The NASA Langley, Marshall, and Glenn Centers are now all heavily engaged in hypersonic propulsion studies. The Glenn Center is taking leadership on a Mach 4 turbine engine of interest to the USAF. As for the X-43A Hyper-X, three follow-on projects are now under consideration:
- X-43B: A scaled-up version of the X-43A, to be powered by the ISTAR engine. ISTAR will use a hydrocarbon-based liquid-rocket mode for initial boost, a ramjet mode for speeds above Mach 2.5, and a scramjet mode for speeds above Mach 5 to take it to maximum speeds of at least Mach 7. A version intended for space launch could then return to rocket mode for final boost into space. ISTAR is based on a proprietary Aerojet design called a "strutjet", which is currently undergoing wind-tunnel testing.
- X-43C: NASA is in discussions with the Air Force on development of a variant of the X-43A that would use the HyTECH hydrocarbon-fueled scramjet engine.
While most scramjet designs to date have used hydrogen fuel, HyTech runs on conventional kerosene-type hydrocarbon fuels, which are much more practical for support of operational vehicles. A full-scale engine is now being built, which will use its own fuel for cooling. Using fuel for engine cooling is nothing new, but the cooling system will also act as a chemical reactor, breaking long-chain hydrocarbons down into short-chain hydrocarbons that burn more rapidly.
- X-43D: A version of the X-43A with a hydrogen-powered scramjet engine with a maximum speed of Mach 15.
Hypersonic development efforts are also in progress in other nations. The French are now considering their own scramjet test vehicle and are in discussions with the Russians for boosters that would carry it to launch speeds. The approach is very similar to that used with the current NASA X-43A demonstrator.
Several scramjet designs are now under investigation with Russian assistance. One of these options or a combination of them will be selected by ONERA, the French aerospace research agency, with the EADS conglomerate providing technical backup. The notional immediate goal of the study is to produce a hypersonic air-to-surface missile named "Promethee", which would be about 6 meters (20 feet) long and weigh 1,700 kilograms (3,750 pounds).
Scramjet programmes
On July 30, 2002, the University of Queensland's HyShot team successfully conducted the first ever test flight of a scramjet.
The team took a unique approach to the problem of accelerating the engine to the necessary speed by using an Orion-Terrier rocket to take the aircraft up on a parabolic trajectory to an altitude of 314 km. As the craft re-entered the atmosphere, it dropped to a speed of Mach 7.6. The scramjet engine then started, and it flew at about Mach 7.6 for 6 seconds. [1]. This was achieved on a lean budget of just A$1.5 million (US $1.1 million), a tiny fraction of NASA's $US 250 million to develop the X-43A.
NASA has partially explained the tremendous difference in cost between the two projects by pointing out that the American vehicle has an engine fully incorporated into an airframe with a full complement of flight control surfaces available.
No net thrust was achieved.
NASA's Hyper-X program is the successor to the National Aerospace Plane (NASP) program which was cancelled in November 1994. This program involves flight testing through the construction of the X-43 vehicles. NASA first successfully flew its X-43A scramjet test vehicle on March 27, 2004 (an earlier test, on June 2, 2001 went out of control and had to be destroyed). Unlike the University of Queensland's vehicle, it took a horizontal trajectory. After it separated from its mother craft and booster, it briefly achieved a speed of 5,000 miles per hour (8,000 km/h), the equivalent of Mach 7, easily breaking the previous speed record for level flight of an air-breathing vehicle. Its engines ran for eleven seconds, and in that time it covered a distance of 15 miles (24 km). The Guinness Book of Records certified the X-43A's flight as the current Aircraft Speed Record holder on 30th August 2004. The third flight of a Boeing X-43A set a new speed record of 6,600 mph (10,621 km/h), nearly Mach 10 on November 16, 2004. It was boosted by a modified Pegasus rocket which was launched from a Boeing B-52 at 13,157 meters (40,000 feet). After a free flight where the scramjet operated for about ten seconds the craft made a planned crash into the Pacific ocean off the coast of southern California. It was the third X-43A. The X-43A crafts were designed to crash into the ocean without recovery. Duct geometry and performance are classified.
Russia and France (and NASA)
On November 17, 1992, Russian scientists with some additional French support successfully launched a scramjet engine in Kazakhstan. From 1994 to 1998 NASA worked with the Russian central institute of aviation motors (CIAM) to test a dual-mode scramjet engine. Four tests took place, reaching Mach numbers of 5.5, 5.35, 5.8, and 6.5. The final test took place aboard a modified SA-5 surface to air missile launched from the Sary Shagan test range in the Republic of Kazakhstan on 12 February 1998. Data regarding whether the internal combustion took place in supersonic air streams was inconclusive, according to NASA. No net thrust was achieved. The tests also included French partners.
GASL projectile
At a test facility at Arnold Air Force Base in the U.S. state of Tennessee, GASL fired a projectile equipped with a hydrocarbon-powered scramjet engine from a large gun. On July 26, 2001, the four-inch (10-centimeter) wide projectile covered a distance of 260 feet in 30 milliseconds (roughly 5,900 mph or 9,500 km/h). The projectile is supposedly a model for a missile design. Many do not consider this to be a scramjet "flight," as the test took place near ground level. However, the test environment was described as being very realistic.