In aerodynamics, hypersonic speeds are speeds that are highly supersonic. In the 1970s, the term generally came to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic regime is a subset of the supersonic regime.
Supersonic airflow is decidedly different from subsonic flow. Nearly everything about the way an aircraft flies changes dramatically as an aircraft accelerates to supersonic speeds. Even with this strong demarcation, there is still some debate as to the definition of "supersonic". One definition is that the aircraft, as a whole, is traveling at Mach 1 or greater. More technical definitions state that you are only supersonic if the airflow over the entire aircraft is supersonic, which occurs around Mach 1.2 on typical designs. The range Mach 0.8 to 1.2 is therefore considered transonic.
Considering the problems with this simple definition, it should be no surprise that the precise Mach number at which a craft can be said to be fully hypersonic is even more elusive, especially since physical changes in the airflow (molecular dissociation, ionization) occur at quite different speeds. Generally, a combination of effects become important "as a whole" around Mach 5. The hypersonic regime is often defined as speeds where ramjets do not produce net thrust. This is a nebulous definition in itself, as there exists a proposed change to allow them to operate in the hypersonic regime
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A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic. At higher speeds it is necessary to combust supersonically to maximize the efficiency of the combustion process. Projections for the top speed of a scramjet engine (without additional oxidiser input) vary between Mach 12 and Mach 24 (orbital velocity), but the X-30 research gave Mach 17 due to combustion rate issues. By way of contrast, the fastest conventional air-breathing, manned vehicles, such as the U.S. Air Force SR-71, achieve approximately Mach 3.4 and rockets achieved Mach 30+ during the Apollo Program.
Like a ramjet, a scramjet essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the exhaust jet leaves at higher speed than the inlet air. Also like a ramjet, there are few or no moving parts. In particular there is no high speed turbine as in a turbofan or turbojet engine that is expensive to produce and can be a major point of failure.
A scramjet requires supersonic airflow through the engine, thus, similar to a ramjet, scramjets have a minimum functional speed. This speed is uncertain due to the low number of working scramjets, relative youth of the field, and the largely classified nature of research using complete scramjet engines. However, it is likely to be at least Mach 5 for a pure scramjet, with higher Mach numbers 7-9 more likely. Thus scramjets require acceleration to hypersonic speed via other means. A hybrid ramjet/scramjet would have a lower minimum functional Mach number, and some sources indicate the NASA X-43A research vehicle is a hybrid design. Recent tests of prototypes have used a booster rocket to obtain the necessary velocity. Air breathing engines should have significantly better specific impulse while within the atmosphere than rocket engines.
However, scramjets have weight and complexity issues that must be considered. While very short suborbital scramjet test flights have been successfully performed, perhaps significantly no flown scramjet has ever been successfully designed to survive a flight test. The viability of scramjet vehicles is hotly contested in aerospace and space vehicle circles, in part because many of the parameters which would eventually define the efficiency of such a vehicle remain uncertain. This has led to grandiose claims from both sides, which have been intensified by the large amount of funding involved in any hypersonic testing. Some notable aerospace gurus such as Henry Spencer and Jim Oberg have gone so far as calling orbital scramjets 'the hardest way to reach orbit', or even 'scamjets' due to the extreme technical challenges involved. Major, well funded projects, like the X-30 were cancelled before producing any working hardware.
During and after World War II, tremendous amounts of time and effort were put into researching high-speed jet- and rocket-powered aircraft. The Bell X-1 attained supersonic flight in 1947, and by the early 1960s, 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 North American X-15 and other rocket-powered spacecraft, aircraft top speeds have remained level, generally in the range of Mach 1 to Mach 3.
In the realm of civilian air transport, the primary goal has been reducing operating cost, rather than increasing flight speeds. Because supersonic flight, using conventional jet engines, requires significant amounts of fuel, airlines have favored subsonic jumbo jets rather than supersonic transports. The production supersonic airliners, Concorde and the Tupolev Tu-144, operated with little profit for the French and Russian airlines but British Airways flew Concorde at a 60% profit margin over its commercial life.Military combat aircraft design has focused on maneuverability, more recently combined with stealth. These features are thought to be incompatible with hypersonic aerodynamics because of the very high speeds and temperatures of hypersonic flight.
In the United States, from 1986-1993, a reasonably serious attempt to develop a single stage to orbit reusable spaceplane using scramjet engines was made, but the Rockwell X-30 (NASP) program failed.
Hypersonic flight concepts haven't gone away, however, and low-level investigations have continued over the past few decades. Presently, the US military and NASA have formulated a "National Hypersonics Strategy" to investigate a range of options for hypersonic flight. Other nations such as Australia, France, Russia, and India have also progressed in hypersonic propulsion research.
Different U.S. organizations have accepted hypersonic flight as a common goal. The U.S. Army desires hypersonic missiles that can attack mobile missile launchers quickly. NASA believes hypersonics could help develop economical, reusable launch vehicles. The Air Force is interested in a wide range of hypersonic systems, from air-launched cruise missiles to orbital spaceplanes, that the service believes could bring about a true "aerospace force."
There are several claims as to which group were the first to demonstrate a "working" scramjet, where "working" in this case can refer to:
• Demonstration of supersonic combustion in a ground test
• Demonstration of net thrust in a ground test
• Demonstration of supersonic combustion or net thrust in a ground test with realistic fuels and/or realistic wind tunnel flow conditions.
• Demonstration of supersonic combustion in a flight test
• Demonstration of net thrust in a flight test.
The problem is complicated by the release of previously classified material and by partial publication, where claims are made, but specific parts of an experiment are kept secret. Additionally experimental difficulties in verifying that supersonic combustion actually occurred, or that actual net thrust was produced mean that at least four consortia have legitimate claims to "firsts", with several nations and institutions involved in each consortium (For a further listing see Scramjet Programs). On June 15, 2007, the US Defense Advanced Research Project Agency (DARPA) and the Australian Defence Science and Technology Organization (DSTO), announced a successful scramjet flight at Mach 10 using rocket engines to boost the test vehicle to hypersonic speeds, at the Woomera Rocket Range in Central Australia.
A scramjet is a type of jet engine designed to operate at the high speeds typically associated with rockets. Its main difference from a rocket is that it collects air from the atmosphere to burn its fuel, rather than carrying an oxidizing substance on board. More conventional jets (turbojets, turbofans and ramjets) share this characteristic but are unsuitable for the high speeds at which scramjets can operate.
FIGURE 1.X-43A with scramjet attached to the underside
Turbine-based engines, while efficient for flight at subsonic and supersonic speeds, quickly lose their efficiency at higher Mach numbers. As air enters the compressor, its pressure and temperature increases, with high Mach numbers resulting in high temperatures. High temperatures are undesirable because they can cause melting or structural failure of the engine, and because the energy released from combustion reduces as the temperature of the fuel-air mixture increases. As the available energy decreases, the drag increases with Mach number squared. The maximum operating speed of a turbine-based engine can be increased by cooling the air in the inlet, and by combining the turbine with other thrust-producing technologies like afterburners or ramjets .
Ramjets are easier to build for higher operating temperatures than turbojets, and produce less drag. They are thus capable of flight at higher speeds than turbojets (but with the drawback that they cannot usefully operate below about 400mph). However, ramjets must slow intake air down to subsonic speed for fuel mixing and combustion by compressing it at the inlet. At conventional supersonic speeds with subsonic combustion this is more efficient than using a bladed compressor, but at higher speeds a problem develops. The shock wave which forms during the compression process causes a high drag on the engine. The drag on the engine is eventually more than can even theoretically be compensated for by the thrust produced. Similarly to the turbojet, the compression at high speeds causes high temperatures which reduce the combustion efficiency.
For an engine to be efficient, it must have low drag and good combustion efficiency. The theoretical upper operating limit for engines with subsonic combustion is not a hard line, but lies somewhere between Mach 4 and Mach 8 depending on the fuel used.
3.1 Diagram illustrating the principle of scramjet operation
The scramjet is intended to avoid the high drag and low combustion efficiency of other types of engine at high Mach number by maintaining supersonic airflow through the whole engine. The lack of a strong shock, as in a ramjet, significantly reduces the drag of the engine. Because intake air is decelerated less than with a ramjet, it is also heated less and fuel can be burned more efficiently. The difficulty is that at these higher airflow velocities, the fuel must be mixed and burned in a very short time, and that any error in the geometry of the engine will result in a high drag.
A very simple scramjet would look like two kitchen funnels attached by their small ends. The first funnel is the intake, into which air is forced, compressing and heating in the process. At the narrow section where the funnels join and compression is greatest, fuel is added and combusted which heats the gas further. The gas expands and exits through the second funnel, like the nozzle of a rocket, and thrust is produced.
Note that most artists' impressions of scramjet-powered vehicles depict waveriders, on which the underside of the vehicle forms the intake and nozzle of the engine; the two are asymmetric and contribute directly to the lift of the aircraft. A waverider is the required form for a hypersonic lifting body.
3.2 Scramjets integrate air and space
The world's first scramjet engine to demonstrate operability at Mach 4.5-6.5 using conventional fuel.
As the 21st century unfolds, a revolutionary engine technology is aiming to fly craft at high Mach speeds and seamlessly integrate air-to-space operations. The supersonic combustion ramjet, or scramjet, uses no rotating parts, will power vehicles hundreds of miles in minutes, and will make rapid global travel and affordable access to space a reality.
These goals drew closer to achievement this spring when the first scramjet-powered aircraft flew on its own. On the afternoon of March 27, an unpiloted X-43A, a National Aeronautics and Space Administration (NASA) craft mounted on a Pegasus booster rocket, dropped from a B-52 flying at 40,000 ft off the coast of California. The rocket sent the experimental aircraft soaring to its test altitude of 95,000 ft, where the X-43A separated from its booster, and its scramjet engine fired for a planned 10-s test, achieving an incredible Mach 7, or 5,000 mph.
Data from that flight helped validate the concept of a hypersonic craft with an airbreathing engine. More flights during the next several years will expand on the engine and aerodynamics data obtained in March, and could put some scramjet vehicles in service in less than a decade. Scramjets will enable three categories of hypersonic craft: weapons, such as cruise missiles; aircraft, such as those designed for global strike and reconnaissance missions; and space-access vehicles that will take off and land like airliners.
Scramjets have a long and active development history in the United States. On the basis of theoretical studies started in the 1940s, the U.S. Air Force, Navy, and NASA began developing scramjet engines in the late 1950s. Since then, many hydrogenand hydrocarbon-fueled engine programs have helped scramjet technology evolve to its current state. The most influential of these efforts was NASA’s National Aerospace Plane (NASP) program, established in 1986 to develop a vehicle with speed greater than Mach 15 and horizontal takeoff and landing capabilities. The program ended in 1993, but the original NASP engine design, significantly modified by NASA, provided the foundation for the power plant used during the X-43A’s recent flight.
3.3 Scramjet propulsion
Thrust is the force which moves any aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton's third law of motion. For every action there is an equal and opposite reaction. In any propulsion system, a working fluid is accelerated by the system and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. Engineers use a thermodynamic analysis of the scramjet to predict thrust and fuel flow.
In the early 1900's some of the original ideas concerning ramjet propulsion were first developed in Europe. Thrust is produced by passing the hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust. To maintain the flow through the nozzle, the combustion must occur at a pressure that is higher than the pressure at the nozzle exit. In a ramjet, the high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. The external air that is brought into the propulsion system becomes the working fluid, much like a turbojet engine. The combustion process in a ramjet occurs at subsonic speeds in the combustor. For a vehicle traveling supersonically the air entering the engine must be slowed to subsonic speeds by shock waves generated in the aircraft inlet. Much above Mach 5, the performance losses from the shock waves become so great that the engine can no longer produce net thrust.
In the 1960's an improved ramjet was proposed in which the combustion in the burner would occur supersonically. In the supersonic combustion ramjet, or scramjet, the losses associated with slowing the flow would be minimized and the engine could produce net thrust for a hypersonic vehicle. Tests were begun to design the supersonic burner and to better integrate the inlet and nozzle with the airframe. Because the scramjet uses external air for combustion, it is a more efficient propulsion system for flight within the atmosphere than a rocket, which must carry all of its oxygen. Scramjets are ideally suited for hypersonic flight within the atmosphere.
CHAPTER – IV
All scramjet engines have fuel injectors, a combustion chamber, a thrust nozzle and an intake, which compresses the incoming air. Sometimes engines also include a region which acts as a flame holder, although the high stagnation temperatures mean that an area of focused waves may be used, rather than a discrete engine part as seen in turbine engines. Other engines use pyrophoric fuel additives, such as silane, to avoid such issues. 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.
Computational fluid dynamic (CFD) image of the X-43A with scramjet attached to the underside at Mach 7
A scramjet is reminiscent of a ramjet. In a typical ramjet, the supersonic inflow of the engine is decelerated at the inlet to subsonic speeds and then reaccelerated through a nozzle to supersonic speeds to produce thrust. This deceleration, which is produced by a normal shock, creates a total pressure loss which limits the upper operating point of a ramjet engine.
For a scramjet, 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 is 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 minimizing drag as maximizing thrust.
This high speed 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. Usable dynamic pressures lie in the range 20 to 200 kPa (0.2-2 bar), where
Where q =.5 ρ v^2
q Is the dynamic pressure of the gas
ρ (rho) is the density of the gas
v is the velocity of the gas
To keep the combustion rate of the fuel constant, the pressure and temperature in the engine must also be constant. This is problematic because the airflow control systems that would facilitate this are not physically possible in a scramjet launch vehicle due to the large speed and altitude range involved, meaning that it must travel at an altitude specific to its speed. Because air density reduces at higher altitudes, a scramjet must climb at a specific rate as it accelerates to maintain a constant air pressure at the intake. This optimal climb/descent profile is called a "constant dynamic pressure path".
Fuel injection and management is also potentially complex. One possibility would be that the fuel is pressurized to 100 bar by a turbo pump, heated by the fuselage, sent through the turbine and accelerated to higher speeds than the air by a nozzle. The air and fuel stream are crossed in a comb like structure, which generates a large interface. Turbulence due to the higher speed of the fuel lead to additional mixing. Complex fuels like kerosene need a long engine to complete combustion.
The minimum Mach number at which a scramjet can operate is limited by the fact that the compressed flow must be hot enough to burn the fuel, and of high enough pressure that the reaction is finished before the air moves out the back of the engine. Additionally, in order to be called a scramjet, the compressed flow must still be supersonic after combustion. Here two limits must be observed: Firstly, since when a supersonic flow is compressed it slows down, the level of compression must be low enough (or the initial speed high enough) not to slow the gas below Mach 1. If the gas within a scramjet goes below Mach 1 the engine will "choke", transitioning to subsonic flow in the combustion chamber. This effect is well known amongst experimenters on scramjets since the waves caused by choking are easily observable. Additionally, the sudden increase in pressure and temperature in the engine can lead to an acceleration of the combustion, leading to the combustion chamber exploding.
Secondly, the heating of the gas by combustion causes the speed of sound in the gas to increase (and the Mach number to decrease) even though the gas is still travelling at the same speed. Forcing the speed of air flow in the combustion chamber under Mach 1 in this way is called "thermal choking". It is clear that a pure scramjet can operate at Mach numbers of 6-8, but in the lower limit, it depends on the definition of a scramjet. Certainly there are designs where a ramjet transforms into a scramjet over the Mach 3-6 range (Dual-mode scramjets). In this range however, the engine is still receiving significant thrust from subsonic combustion of "ramjet" type.
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 (2.4 m) HTT.
Computational fluid dynamics has only recently reached a position to make reasonable computations in solving scramjet operation problems. 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 makes severe demands on computing resources. Reaction schemes are numerically stiff, having typical times as low as 10-19 seconds, requiring reduced reaction schemes.
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. Since these results have not been published openly, they remain unverified and a final design method of scramjet engines still does not exist.
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 propulsion with a scramjet, allowing for additional oxidizer to be added to the scramjet flow. RBCCs offer a possibility to extend a scramjet's operating range to higher speeds or lower intake dynamic pressures than would otherwise be possible.
Figure 4. Propulsion efficiency decreases with speed as we progress through turbojets to ramjets and scramjets to rockets; hydrogen is more efficient than jet fuel.
The high-speed air-induction system consists of the vehicle forebody and internal inlet, which capture and compress air for processing by the engine’s other components. Unlike jet engines, vehicles flying at high supersonic or hypersonic speeds can achieve adequate compression without a mechanical compressor. The forebody provides the initial compression, and the internal inlet provides the final compression. The air undergoes a reduction in Mach number and an increase in pressure and temperature as it passes through shock waves at the forebody and internal inlet. The isolator in a scramjet is a critical component. It allows a supersonic flow to adjust to a static back-pressure higher than the inlet static pressure. When the combustion process begins to separate the boundary layer, a precombustion shock forms in the isolator. The isolator also enables the combustor to achieve the required heat release and handle the induced rise in combustor pressure without creating a condition called inlet unstart, in which shock waves prevent airflow from entering the isolator. The combustor accepts the airflow and provides efficient fuel–air mixing at several points along its length, which optimizes engine thrust. The expansion system, consisting of the internal nozzle and vehicle aftbody, controls the expansion of the highpressure, high-temperature gas mixture to produce net thrust. The expansion process converts the potential energy generated by the combustor to kinetic energy. The important physical phenomena in the scramjet nozzle include flow chemistry, boundarylayer effects, nonuniform flow conditions, shear-layer interaction, and three-dimensional effects. The design of the nozzle has a major effect on the efficiency of the engine and the vehicle, because it influences the craft’s pitch and lift.
Figure 5. As the vehicle speed increases from Mach 3 to Mach 8, the isolator pressure ratio passes through a peak at Mach 6. As the shock train and boundary layer retreat, the modes change from dual-mode ramjet to dual-mode scramjet to pure scramjet mode.
An air-breathing hypersonic vehicle requires several types of engine operations to reach scramjet speeds. The vehicle may utilize one of several propulsion systems to accelerate from takeoff to Mach 3. Two examples are a bank of gas-turbine engines in the vehicle, or the use of rockets, either internal or external to the engine. At Mach 3–4, a scramjet transitions from low-speed propulsion to a situation in which the shock system has sufficient strength to create a region(s) of subsonic flow at the entrance to the combustor. In a conventional ramjet, the inlet and diffuser decelerate the air to low subsonic speeds by increasing the diffuser area, which ensures complete combustion at subsonic speeds. A converging– diverging nozzle behind the combustor creates a physical throat and generates the desired engine thrust. The required choking in a scramjet, however, is provided within the combustor by means of a thermal throat, which needs no physical narrowing of the nozzle. This choke is created by the right combination of area distribution, fuel–air mixing, and heat release.
During the time a scramjet-powered vehicle accelerates from Mach 3 to 8, the airbreathing propulsion system undergoes a transition between Mach 5 and 7. Here, a mixture of ramjet and scramjet combustion occurs. The total rise in temperature and pressure across the combustor begins to decrease. Consequently, a weaker precombustion system is required, and the precombustion shock is pulled back from the inlet throat toward the entrance to the combustor. As speeds increase beyond Mach 5, the use of supersonic combustion can provide higher performance (Figure 3). Engine efficiency dictates using the ramjet until Mach 5–6. At around Mach 6, decelerating airflow to subsonic speeds for combustion results in parts of the airflow almost halting, which creates high pressures and heat-transfer rates. Somewhere between Mach 5 and 6, the combination of these factors indicates a switch to scramjet operation. When the vehicle accelerates beyond Mach 7, the combustion process can no longer separate the airflow, and the engine operates in scramjet mode without a precombustion shock. The inlet shocks propagate through the entire engine. Beyond Mach 8, physics dictates supersonic combustion because the engine cannot survive the pressure and heat buildup caused by slowing the airflow to subsonic speeds.
Scramjet operation at Mach 5–15 presents several technical problems to achieving efficiency. These challenges include fuel–air mixing, management of engine heat loads, increased heating on leading edges, and developing structures and materials that can withstand hypersonic flight. When the velocity of the injected fuel equals that of the airstream entering the scramjet combustor, which occurs at about Mach 12, mixing the air and fuel becomes difficult. And at higher Mach numbers, the high temperatures in the combustor cause dissociation and ionization. These factors—coupled with already-complex flow phenomena such as supersonic mixing, isolator– combustor interactions, and flame propagation—pose obstacles to flow-path design, fuel injection, and thermal management of the combustor.
Several sources contribute to engine heating during hypersonic flight, including heating of the vehicle skin from subsystems such as pumps, hydraulics, and electronics, as well as combustion. Thermal-management schemes focus on the engine in hypersonic vehicles because of its potential for extremely high heat loads. The engine represents a particularly challenging problem because the flow path is characterized by very high thermal, mechanical, and acoustic loading, as well as a corrosive mix of hot oxygen and combustion products. If the engine is left uncooled, temperatures in the combustor would exceed 5,000 °F, which is higher than the melting point of most metals. Fortunately, a combination of structural design, material selection, and active cooling can manage the high temperatures.
Hypersonic vehicles also pose an extraordinary challenge for structures and materials. The airframe and engine require lightweight, high-temperature materials and structural configurations that can withstand the extreme environment of hypersonic flight. These include:
• very high temperatures
• heating of the whole vehicle
• steady-state and transient localized heating from shock waves
• high aerodynamic loads
• high fluctuating pressure loads
• the potential for severe flutter, vibration, fluctuating and thermally-induced pressures
• erosion from airflow over the vehicle and through the engine
With the completion of the successful X-43A flight and the ground-testing of several full-sized demonstration engines, confidence in the viability of the hydrogen- and hydrocarbon-fueled scramjet engines has increased significantly. NASA plans to launch another X-43A this fall and fly it at Mach 10, or 6,750 mph.
ADVANTAGES AND DISADVANTAGES
5.1Advantages and disadvantages of scramjets
5.1.1 Special cooling and materials
Unlike a rocket that quickly passes mostly vertically through the atmosphere or a turbojet or ramjet that flies at much lower speeds, a hypersonic airbreathing vehicle optimally flies a "depressed trajectory", staying within the atmosphere at hypersonic speeds. Because scramjets have only mediocre thrust-to-weight ratios, acceleration would be limited. Therefore time in the atmosphere at hypersonic speed would be considerable, possibly 15-30 minutes. Similar to a reentering space vehicle, heat insulation would be a formidable task. The time in the atmosphere would be greater than that for a typical space capsule, but less than that of the space shuttle.
New materials offer good insulation at high temperature, but they often sacrifice themselves in the process. Therefore studies often plan on "active cooling", where coolant circulating throughout the vehicle skin prevents it from disintegrating. Often the coolant is the fuel itself, much in the same way that modern rockets use their own fuel and oxidizer as coolant for their engines. All cooling systems add weight and complexity to a launch system and reduce its efficiency. The increased cooling requirements of scramjet engines result in lower efficiency.
The efficiency of a launch vehicle depends greatly on its weight. Calculating the efficiency of an engine system is mathematically complex, and involves tradeoffs between the efficiency of the engine (takeoff fuel weight) and the complexity of the engine (takeoff dry weight), which can be expressed by the following:
• is the empty mass fraction
• is the fuel+oxidiser mass fraction
• is initial mass ratio, and is the inverse of the payload mass fraction
A scramjet increases the mass of the engine Πe over a rocket, and decreases the mass of the fuel Πfe. The logic behind efforts driving a scramjet is (for example) that the reduction in fuel decreases the total mass by 30%, while the increased engine weight adds 10% to the vehicle total mass. Unfortunately the uncertainty in the calculation of any mass or efficiency changes in a vehicle is so great that slightly different assumptions for engine efficiency or mass can provide equally good arguments for or against scramjet powered vehicles.
5.1.3 Simplicity of design
Scramjets have few to no moving parts. Most of their body consists of continuous surfaces. With simple fuel pumps, reduced total components, and the reentry system being the craft itself, scramjet development tends to be more of a materials and modelling problem than anything else.
5.1.4 Additional propulsion requirements
A scramjet cannot produce efficient thrust unless boosted to high speed, around Mach 5, depending on design, although, as mentioned earlier, it could act as a ramjet at low speeds. A horizontal take-off aircraft would need conventional turbofan or rocket engines to take off, sufficiently large to move a heavy craft. Also needed would be fuel for those engines, plus all engine associated mounting structure and control systems. Turbofan engines are heavy and cannot easily exceed about Mach 2-3, so another propulsion method would be needed to reach scramjet operating speed. That could be ramjets or rockets. Those would also need their own separate fuel supply, structure, and systems. Many proposals instead call for a first stage of droppable solid rocket boosters, which greatly simplifies the design.
5.1.5 Testing difficulties
Unlike jet or rocket propulsion systems facilities which can be tested on the ground, testing scramjet designs use extremely expensive hypersonic test chambers or expensive launch vehicles, both of which lead to high instrumentation costs. Launched test vehicles very typically end with destruction of the test item and instrumentation.
5.1.6 Lack of stealth
There is no published way to make a scramjet powered vehicle (or any other hypersonic vehicle) stealthy- since the vehicle would be very hot due to its high speed within the atmosphere it should be easy to detect with infrared sensors. However, any aggressive act against a scramjet vehicle would be difficult because of its high speed.
5.2Advantages and disadvantages for orbital vehicles
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 holds 616,432 kg of liquid oxygen (LOX) and 103,000 kg of liquid hydrogen (LH2). The shuttle orbiter itself weighs about 104,000 kg (max landing weight). Therefore 75% of the entire assembly weight is liquid oxygen. If carrying this could be eliminated, the vehicle could be lighter at takeoff and hopefully carry more payload. That would be a major advantage, but the central motivation in pursuing hypersonic airbreathing vehicles would be to reduce costs. Unfortunately there are several disadvantages:
5.2.1Lower thrust-weight ratio
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 tankage ratio of ~100:1 also.
Thus a rocket can achieve a very high mass fraction (Takeoff rocket mass:unfuelled rocket mass=fuel+oxidiser+structure+engines+payload:structure+engines), which improves performance. By way of contrast the projected thrust/weight ratio of scramjet engines of about 2 mean a very 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.
5.2.2 Need additional engine(s) to reach orbit
Scramjets might be able to accelerate from approximately Mach 5-7 to around somewhere between half of orbital velocity and orbital velocity (X-30 research suggested that Mach 17 might be the limit compared to an orbital speed of mach 25, and other studies put the upper speed limit for a pure scramjet engine between Mach 10 and 25, depending on the assumptions made). Generally, another propulsion system (very typically 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. Opponents of scramjet research claim that most of the theoretical advantages for scramjets only accrue if a single stage to orbit (SSTO) vehicle can be successfully produced. Proponents of scramjet research claim that this is a straw man, and that SSTO vehicles are exactly as difficult to produce and bring the same benefits to rocket-powered and scramjet-powered launch vehicles.
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, 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, as in scramjets, means that the vehicle itself becomes a much larger percentage of the costs (rocket fuels are already cheap). Indeed, the unit cost of the vehicle can be expected to end up far higher, since aerospace hardware cost is probably about two orders of magnitude higher than liquid oxygen and tankage. 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 are only designed to survive for short periods.
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 in order to be equally as cost efficient (if the scramjet is a non-reusable vehicle).
Seeing its potential, organizations around the world are researching scramjet technology. Scramjets will likely propel missiles first, since that application requires only cruise operation instead of net thrust production. Much of the money for the current research comes from governmental defense research contracts.
• Access to Space Study: Summary Report; Office of Space Systems Development, NASA Headquarters, Washington, DC, 1994.
• Curran, E. T.; Murthy, S. N. B.; Eds., Scramjet Propulsion; Progress in Astronautics and Aeronautics, vol. 189; AIAA: Washington, DC, 2000.
• Faulkner, R. F. The Evolution of the HySET Hydrocarbon Fueled Scramjet Engine; AIAA Paper 2003-7005; AIAA: Washington, DC, 2003.
• Heiser, W. H.; Pratt, D. T. Hypersonic Airbreathing Propulsion; AIAA: Washington, DC, 1994.
• Kandebo, S. W. New Powerplant Key to Missile Demonstrator. Aviation Week Space Technol., Sept. 2, 2002; p. 56.
• McClinton, C. R.; Andrews, E. H.; Hunt, J. L. Engine Development for Space Access: Past, Present, and Future. Int. Symp. Air Breathing Engines, Jan. 2001; ISABE Paper 2001-1074.