Julian Moxon/PARIS
Scramjet revival has re-awakened the dream of the SSTO concept for the millenniumThe dream of a single-stage-to-orbit (SSTO) vehicle first came into the public eye with US President Ronald Reagan's vaunted National Aerospaceplane (NASP) programme.
That dream faded as funding dried up and it became clear that there were to be no early breakthroughs in the propulsion and aerodynamics technology needed to field such a vehicle. As the millennium arrives, however, a new Franco-Russian programme has taken the lead on a form of propulsion that may bring the SSTO concept closer to reality.
The two countries are working together to produce a version of the type of engine that would have been used to power the NASP. It will be tested at the beginning of the new millennium, providing an appropriate milestone in what could turn out to be a new era in propulsion technology.
The engine is a supersonic combustion dual-mode ramjet - or scramjet - in which combustion takes place subsonically inside the chamber at flight speeds of up to around Mach 6, when combustion becomes supersonic. This is a step beyond the ramjet - the technology of which has been understood for many years - in which fuel, usually kerosene, is mixed with air and burned subsonically in a cylindrical chamber, expanding through the nozzle to accelerate the engine - and the vehicle on which it is mounted - to supersonic speeds.
Both types of engine have no moving parts, and therefore do not work from zero velocity, having to be accelerated by other means to speeds at which the compression caused by forward movement is sufficient for combustion and subsequent expansion to take place.
In the scramjet, the compression is such that the gas mixture self-ignites before expanding through a huge exit area to produce thrust, and the potential of hypersonic flight, between M6 and M12 (the supersonic Concorde cruises at M2.2). One of the main problems with such an engine is how to succeed, before combustion, in completely mixing the low-density hydrogen fuel with air. This is achieved with specially shaped hydrogen injectors - their design being one of the main specialities required for scramjet builders.
Aerospatiale has been involved in ramjet design since 1950, somehow keeping its expertise alive throughout major changes that have taken place in the management and orientation of the company. It is the European leader in ramjet design for weapons such as the ASMP-A air-launched nuclear missile.
In October 1996, Aerospatiale was awarded a Fr750 million ($120 million) contract for pre-development of the Vesta liquid-fuelled, variable-thrust ramjet. This will form the basis of a family of ramjet-powered missiles, including the planned Exocet anti-ship missile follow-on, the Supersonic Anti-Navire Nouvelle Generation. The company continues to develop other ramjet technologies, including a new small-diameter low-cost engine for light air-to-air missiles.
In 1992, the French Government launched a new programme for research into hypersonic propulsion. The Prepha (Programme de Recherche et Technologie pour la Propulsion Hypersonique Avancée) effort brought together Aerospatiale, Dassault, Onera (the French research agency), SEP and Snecma. Of the Fr526 million of initial funding (which was later reduced to Fr380 million), 76% came from the French Government via its defence ministry and the French space agency. The rest was provided by industry.
The six-year programme resulted in the development of the Chamois scramjet engine, which was tested repeatedly at equivalent speeds of up to M6 at Aerospatiale's Bourges facility near Paris. The work also included design of the highly specialised equipment for measuring the high temperatures and speeds of the gas within the combustion chamber, for comparison of different chamber shapes and of several types of fuel injector, and to look at ways of predicting flow in a full-sized engine.
The Chamois engine is built of stainless steel, and has an inlet area measuring about 0.05m2. It consists essentially of an injection box, with up to three struts located horizontally across it, through which hydrogen is injected. The position, number and angle of attack - as well as the internal shape of the struts - can be varied. The strut leading edges are water-cooled. The main combustion takes place in a fixed chamber, the dimensions of which can be varied between tests. Combustion lasts between 3s and 10s, and a stainless-steel heat sink carries away the intense heat, typically 1,650Kelvin (1,380¼C), generated within the combustor.
To supply compressed air, a "blowdown" system is used, in which air, pressurised to 80 times atmospheric pressure, is released into the engine. As it does so, the air cools, so warmed hydrogen is added, along with extra oxygen, to create a gas similar to air, heated to around 1,700K for a M6 entry condition.
The design of the injectors is obviously critical, since, when hydrogen is injected, combustion must take place near the struts, at "shock interaction locations", where the hydrogen self-ignites (because of the very high local static temperature) and combustion stabilises in specific, well defined, areas. The flow is stratified, alternating between subsonic and supersonic flow areas in the injection box, becoming fully supersonic in the divergent section, just downstream of the struts.
At this point, the walls of the combustion chamber achieve their highest temperatures, which, in an uncooled, all-metal, chamber, have to be limited to around 1,000-1,200K. In future versions, with metal matrix combustor walls, the temperature will be of the order of 1,700¼K.
The physics of how the engine produces positive thrust are not easily understood, since the thrust derives from the fact that the "engine" is actually a vehicle, comprising inlet, combustion chamber and exhaust nozzle. Combustion of the hydrogen essentially provides a small increase in momentum to the incoming air, which, because of the speed of the vehicle (which would have already been accelerated by other means), is invested with high momentum.
The Prepha programme put Aerospatiale in a good position to join forces on a new concept with another scramjet authority, the Moscow Aviation Institute (MAI). This is a variable-geometry, dual-fuel, scramjet engine, which is proposed as one way of optimising the powerplant for the changing conditions as the vehicle ascends and speed increases to ever higher Mach numbers.
The MAIhas 30 years of experience in scramjet design, and is responsible for design manufacture and development of the variable-geometry engine. Aerospatiale will provide its considerable Prepha experience, including computational fluid dynamics knowhow, and will also provide the Bourges test facilities, where the engine should have its first test run in the beginning of 2000.
Supersonic combustion
Vadim Levine leads the Russian effort at MAI. He says: "The aim is to develop an engine the size of the Chamois, but capable of running from Mach 2.5 to Mach 12." From M2.5 to M5, combustion will be subsonic, the end of the combustor being restricted by a moveable "throat", which will be opened into a divergent form to enable supersonic combustion. "We want to be able to simulate the part of the vehicle's acceleration during which combustion goes from subsonic to supersonic, while optimising the flow for maximum thrust at each point of the process," he adds.
While, theoretically, this would dramatically improve the performance of a scramjet, the technological challenge associated with very hot moveable panels and the associated increase in weight, through the use of actuators, "-could counter the increase in performance", says the Aerospatiale/MAIteam.
A further novelty is the use of kerosene and hydrogen fuels, the former giving way to the latter as the vehicle's speed increases. The reason is that kerosene is more dense than hydrogen, and can be carried in a relatively small fuel tank. Hydrogen has a higher energy content, but its low density means that tankage occupies considerably more volume. Burning off the kerosene at an early stage means that only a low-weight empty fuel tank has to be carried into orbit under hydrogen power.
The technology for changing from one fuel to the other is being developed at MAI, which has found a way of injecting the kerosene into a "bubble" of hydrogen gas just before combustion. "We've tested the mixer," says Levine, "and demonstrated all of the performance points individually on a fixed-geometry engine. But there is a scale effect and, at Bourges, we will be able to investigate that."
Paper studies have already been done by the team on a full scale ramjet able to power a 500t vehicle into space. These give an idea of the scale of the propulsion system - the engine is 10m wide at the inlet, with a moveable intake leading to a combustion chamber 0.66m high. The full sized engine would have around 50 struts for injecting fuel, each 200mm wide.
In the last three years, MAI has carried out 150 scramjet trials on a fixed-geometry engine, building up an immense technology database through using four concepts and 45 different combustor structures. Computer analysis is also used, but, according to one of the Aerospatiale scramjet team, Francois Falempin, three-dimensional computer analysis "-still cannot cope with the extraordinarily complex flow around the combustor. We've done a pretty good job on the inlet and nozzle, but we're still entirely dependent on actual testing to find out if we're on the right track."
Under the tests, the variable-geometry scramjet will be run for up to a minute at a time - longer than for any previous engine of this type. If it operates as hoped, it will be a significant step towards the dream of a scramjet-powered SSTO, although Falempin has no illusions about the amount of work to be done before such a vehicle will be built. "I'm optimistic that we'll get there eventually. Maybe 10 years is a reasonable goal for an experimental vehicle, and maybe 30 before we see an operational version," he concludes.
Source: Flight International
