Evolution of the Jet Aircraft
The 20th century became the era of aviation and space flights. Rockets have reached space velocities (higher than 7.9 km/s), whereas the fastest planes still fly with moderate velocities almost an order of magnitude lower. Bridging this gap in velocity by hypersonic flying vehicles powered by air-breathing engines will open a new era — the era of aerospace planes. Then the flight from New York to Paris will take no more than 1 or 2 hours, which is commensurable with an everyday suburb travel. The distances between continents will become minor obstacles, and our home planet will become ‘smaller.’ A trip around the world in 8 hours will be tomorrow’s reality!
Today’s inhabitants of the sky are predominantly airplanes with turbojet propulsion operating on a commonly used hydrocarbon fuel: kerosene. Present-day engines can ensure airliner velocities slightly higher than the sonic one. Passenger airliners fly with subsonic velocities (except for the Concorde and Tu-144 that fly no more), and the maximum velocity of the fastest jet fighters is only three times the velocity of sound.
What hinders reaching higher velocities of aircrafts and does not allow them to enter the near-Earth space and to become a new type of transportation — aerospace planes, which can take off from conventional airdromes, fly through the atmosphere, enter the near-Earth space, and return back to the Earth? Obviously, the higher the aircraft velocity, the more powerful its engine should be. Maybe the future belongs to aircrafts powered by rocket engines?
Actually, the highest velocity for manned aircrafts was reached in 1967 by the American experimental rocket plane X-15. Yet, a rocket engine has an essential drawback: it operates on the fuel and oxidizer stored onboard the rocket. These components are used in such great amounts that the use of such engines for long-time atmospheric flights is out of the question.
Jet propulsion is based on the law of conservation of momentum. In a simple case it is described by the equality MV = mv with the product of the mass and velocity of a flying vehicle in the left-hand side and the product of the mass and velocity of the combustion products exhausted from the engine in the opposite direction in the right-hand side. It is owing to this law that jet planes can fly and rockets can leave the Earth and travel in the outer space; the same law is the reason for recoil during shooting
There is only one way out: reaching rocket velocities with flying vehicles powered by air-breathing jet engines. Such engines use oxygen contained in the air as an oxidizer, and the vehicle has to carry onboard only the fuel, which allows a substantial decrease in the vehicle weight and a sharp increase in the efficiency of the atmospheric flight. The fact is, the current aviation engines cannot operate at hypersonic velocities (М>3—4) because of an extremely intense heating of the air being slowed down in the engine inlet. To conquer hypersonic velocities, one has to develop principally novel air-breathing propulsion.
Why should airplanes fly like rockets?
High-velocity flying vehicles and aerospace planes powered by air-breathing jet engines are always of the greatest interest for military purposes. The US Department of Defense Instruction (1999) considered space operations as a sphere of vital importance for the state and as a cornerstone of the American military strategy in the 21st century. This is understandable: aerospace vehicles and hypersonic aircrafts can reach an arbitrary point on the Earth’s surface from any airdrome within dozens of minutes, attack strategic ground-based objects, and intercept high-altitude targets of various types far from the objects that are defended.
Such hypersonic vehicles, however, will be beneficial not only for military purposes. Multi-stage aerospace systems consisting of a launcher and a re-entry orbiter will not only provide multi-usable launch transportation, but also deliver a greater payload to the orbit. As the Earth-orbit-Earth transportation becomes more intense, the cargo delivery will become much less expensive, not to mention a possible development of space tourism.
The benefit for ordinary people will be a substantial speedup and intensification of passenger transportation on long-range intercontinental routes. During a time not too tiresome for passengers (half a workday only), an aircraft with a velocity of 10M will be able to cover the distance between the US or Europe and Australia, i. e., 16—17 thousand kilometers!
As a whole, the prospects of hypersonic aviation seem to be fairly promising. A question arises, however: is this realistic in terms of engineering, and are the most advanced countries in the field of aviation and aerospace technologies ready for this?
Industrial technologies are available at the moment only for the aircraft equipped with turbojet engines operating on kerosene and designed for velocities not exceeding 3M. For an aircraft capable of reaching velocities up to 5—6М, structures made of titanium and alloys that can withstand temperatures up to 500—600° С are adoptable. Turbojet or turbo-ramjet engines of such aircrafts should operate on a more heat-resistant hydrocarbon fuel. A certain progress has been achieved in this direction, though some improvement of already existing technologies is needed.
A hypersonic aircraft with М>5—6 and aerospace planes, however, require basically new technologies, which would differ from both the advanced aircraft and the rocket-space technologies. Engines in such vehicles should not be merely efficient. The propulsion has to operate in an unprecedented wide range of velocities — from subsonic to hypersonic values.
Solution of the problem — a hypersonic ramjet engine?
The first idea of creating an air-breathing engine for hypersonic flights was put forward and validated in the late 1950s — early 1960s by Shchetinkov, a Russian scientist, and by several researchers abroad. It is a ramjet with fuel combustion in a supersonic flow in the engine combustion chamber — the so-called ГПВРД (the Russian abbreviation belongs to Shchetinkov) or a scramjet (as it was called abroad).
Actually, the ramjet engine can operate effectively only at supersonic velocities (М>2); it is not suitable for take-off and landing regimes. In the latter cases, therefore, a hypersonic aircraft should additionally use a conventional turbojet or a rocket engine. Another possible variant is the use of various types of hybrid propulsion.
The efficiency of air-breathing engines can be increased by using the rocket fuels: liquid hydrogen or, e. g., liquid methane. Hydrogen is an ideal fuel for aviation. First, it has a high calorific value and releases a maximum energy per unit mass of the fuel during its combustion. Second, the product of its combustion is ordinary water; hence, hydrogen is an environmentally safe fuel, which is important.
It is commonly recognized now that it is most reasonable to use special experimental unmanned vehicles to test full-scale scramjets under in-flight conditions. These vehicles are launched to the test flight trajectory by a rocket or by a launcher airplane. Such experimental systems were called hypersonic flying laboratories or test beds; the Russian flying lab KHOLOD is one example.
The development of the new-generation hypersonic flying lab IGLA was initiated in the 1990s at the Central Institute of Aviation Motors (CIAM) within the framework of the governmental program OREL. The objective was to test a new modular scramjet with an inlet possessing vertical backward- and forward-swept compression wedges.
Similar projects were developed in the US within the NASP (National AeroSpace Plane) and Hyper-X programs. Concerning the latter, its challenge was the in-flight demonstration of achievements in the field of hypersonic scramjet development. A small unmanned experimental vehicle X-43 of length 3.7 m was developed for this purpose. In 2004, in its flight, X-43 reached a velocity exceeding 11,000 km/h, which is 10 times greater than the velocity of sound.
The investigations of a scramjet started more than 40 years ago, and are now continued in many countries with advanced aviation and aerospace industry: Russia, US, UK, France, etc. Despite a significant success, however, the problem of designing an engine that could be used in a real hypersonic flying vehicle project has not been solved yet.
Fighting against aerodynamic heating
One of critical problems in creating hypersonic and aerospace planes is intense heating of flying vehicles moving with hypersonic velocities. This problem refers both to the powerplant and the airframe (whose flight lifetime should be greater than 30—60 thousand hours) and to the aviation fuel, which was mentioned above.
The currently available technologies of heat-resistant structures were developed for re-entry orbital vehicles. For instance, the so-called “hot” (non-cooled) structures made of refractory alloys were developed in the late 1950s; cellular paneling is one example. Similar structures can also be used for hypersonic planes.
In addition to the “hot” structure, two more types were proposed. The first one is the so-called shielded structure with thermal insulation and a shield separating the thermoprotective layer from the force-loaded elements of the engine and airframe. In this case, the latter operate at moderate temperatures and, hence, can be made of conventional lighter materials.
The other approach involves active cooling of the outer shell of the vehicle. An attractive coolant is liquid hydrogen used as a fuel for the engine. The most efficient structure implies a combination of a cooling system with a system for liquid fuel convection and with thermal shields separated by an air gap from the cooled elements. In this case, the cooling capacity of the fuel is spent on cooling both the engine itself and the airframe.
The US research has shown that almost the entire cooling capacity of the fuel has to be spent to cool a hypersonic transport airplane reaching velocities of 6M and equipped with a scramjet of a traditional (e. g., axisymmetric) configuration. Therefore, it is important to develop inlet configurations that can reduce inevitable total heat loads onto the engine walls. Three-dimensional inlets possessing vertical swept compression wedges and providing smaller total heat fluxes were developed in NASA Langley (US) within the NASP program and in CIAM (Russia).
Since the early 1970s, ITAM has been engaged in studying the so-called convergent three-dimensional inlets. The air stream captured by such an inlet is compressed along directions converging in space. As a result, the cross-section of the inlet duct acquires a compact (close to a circular) shape, which ensures a comparatively small (wetted) area of the inlet and the combustor walls with the highest heat loads. Because of these factors, it is easier to protect the engine from heat loads than, e. g., the structure with an axisymmetric inlet with a slit-like engine duct. Convergent inlets also ensure a higher degree of compression with smaller slopes of the compression surfaces.
Hypersonic aviation yesterday and today
The most intense research into the field of hypersonic aircraft and aerospace planes was performed in the USSR/Russia, in the US, and in the NATO-member countries (UK, France, Germany, and Italy). The projects of aerospace systems developed in 1980—1990 in these countries had a large effect on similar research in the Asian region: India, Japan, and China.
The most extensive program on promising aerospace systems was carried out in 1985—1994 in the US. The NASP program mainly implied the development of aerospace systems for military applications. In addition, the NASP program involved the development of a hypersonic passenger airplane ORIENT EXPRESS designed for intercontinental flights.
Germany began the development of a two-stage aerospace system SANGER within the national program of hypersonic technologies in 1985—1986. One of the challenges was to provide an independent access to space for Europe.
A promising issue in the development of aerospace planes is the creation of an air-breathing propulsion with the so-called liquid air cycle engine — LACE. In such propulsion, the inlet-captured air stream is liquefied, and the oxidizer (oxygen) is extracted from it in the liquid state. Liquid oxygen is accumulated during rather a long-time atmospheric flight, after which the oxygen-hydrogen rocket engines are switched on, and the vehicle reaches the orbit. Such a research is carried out in Russia, the US, and other countries; one of the most famous systems (the reusable aerospace system HOTOL) was developed in the UK.
The first Soviet project of the single-stage scramjet-powered aerospace plane was developed in 1966 at the Research Institute No. 1 (now the Keldysh Research Center), which was the first governmental rocket institution in the country, where Prof. Shchetinkov worked at that time. The design of hypersonic flying vehicles for various purposes was started almost simultaneously in several research institutes and design bureaus supervised by the Ministry of Aviation Industry of the USSR. The first Soviet project of a re-usable two-stage aerospace system called SPIRAL was triggered in the Mikoyan Design Bureau only four years after Gagarin’s space flight.
In the 1970s — 1980s, the Tupolev Design Bureau (now the Tupolev Public-Stock Company) developed the project of a re-usable single-stage aerospace plane Tu-2000. A subsonic research airplane Tu-155 was designed to study issues of practical importance of the cryogenic fuel technology, and a special experimental vehicle close in size to the actual airplane was under development to study the combustion processes in the scramjet and aerodynamic phenomena that occur in flight with М>6—8. In the difficult period of “perestroika”, however, the activities on the project were terminated and then continued only after a long pause.
In the 1995s — 2000s, the Mikoyan Design Bureau developed the aerospace system MiGAKS including a hypersonic launcher airplane and an orbiter designed for delivering a payload (up to 17—18 tons) to a circular orbit at an altitude of 200 km. Various possible types of launcher propulsion and a potential fuel (kerosene, methane, and hydrogen) were considered to comply with the existing level of technologies and that predicted for the nearest two decades.
Other research institutions also developed hypersonic flying vehicles of different levels of complexity and with scramjets or hybrid propulsion: the Central Aerohydrodynamic Institute (TsAGI), CIAM, and ITAM in Novosibirsk. These institutes carried out experimental and theoretical studies devoted, in particular, to problems of fuel combustion in a supersonic flow, hypersonic aerodynamics, and aerodynamic heating important for flight efficiency of the vehicle as a whole.
Some words about AYAKS
In the early 1990s, the State Research Enterprise of Hypersonic Systems in St. Petersburg developed the national program AYAKS (AJAX). It was based on the concept of creating an unusual hypersonic vehicle with an active energy interaction of the airframe and the engine with the ambient medium — the atmosphere, where the vehicle flies.
A unique air-breathing engine of AYAKS was expected to ensure ionization and magnetogasdynamic (MGD) slowing down of the inlet-captured air stream; therewith, part of the kinetic energy of the stream was to be converted to the electric energy.
The “free-of-charge” heat from aerodynamic heating of the airframe and the engine duct had to be used to improve the characteristics of the fuel used. The essence of the idea was to subject the primary energy carrier (conventional kerosene) to vapor conversion, which is done by adding water and driving the resultant mixture through the system of active cooling of the vehicle and the propulsion unit. The cooling system includes reactors of chemical heat generation built into the shell of the airframe and engine duct.
As a result, the hydrocarbon fuel releases free hydrogen, which mixes with kerosene to form the fuel called “convertin.” This reaction is accompanied by intense absorption of heat, which ensures the cooling of necessary elements of the flying vehicle. Convertin enters the combustor and provides better combustion than the primary fuel.
The project implied the development of flying vehicles for various purposes: a cargo-passenger plane, a launcher airplane, an aerospace system, test modules, etc. The program was scheduled for 20 years, but the fact is, the government did not support this program. It should be noted, however, that unique technologies that could be developed within the AYAKS project could find numerous applications in the national economy.
What can be expected tomorrow?
Practical implementation of the projects of hypersonic missiles and some projects of scramjet-powered experimental flying vehicles can be expected in the next 10—15 years.
The creation of hypersonic military airplanes and launchers does not cause principal engineering difficulties. The main obstacles are a high cost and the uncertainty of demand in the current international situation.
There may be numerous rational arguments for the necessity of developing ‘hypersonic’ technologies, but anyone, somewhere deep in his heart, seems to have an understandable childish desire to have a look at the home planet from the flight altitude of an aerospace plane…
The commissioning of hypersonic cargo and passenger airplanes will be determined by two factors: the growth of long-distance traffic flow and a possibility of increasing the efficiency of vehicles, which requires a significant technological progress.
The future of hypersonic aviation is quite obvious. But even if hypersonic and aerospace planes would never take off from the Earth, advanced high technologies developed for them would never be redundant for the mankind. Heat-resistant materials and structures, results of the combustion research in subsonic and supersonic air flows, heat-resistant and cryogenic fuels, various subsystems operating under extremely severe conditions, and other developments will find application in versatile branches of industry, including those other than aviation.
Illustrations and photos are courtesy of the Tupolev Public-Stock Company, the Baranov Central Institute of Aviation Motor building, the Gromov Flight Research Institute, and the Research and Industrial Corporation “Molniya”.