U.S. Developing Jets That Fly Five Times the Speed of Sound
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 slightly more than Mach 3.2 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.
Contents[
hide]
1 History2 Simple description3 Theory4 Advantages and disadvantages of scramjets4.1 Special cooling and materials4.2 Half an engine4.3 Simplicity of design4.4 Additional propulsion requirements4.5 Testing difficulties4.6 Lack of stealth5 Advantages and disadvantages for orbital vehicles5.1 Lower thrust-weight ratio5.2 Need additional engine(s) to reach orbit5.3 Reentry5.4 Costs6 Applications7 Recent progress8 Scramjet in the movies9 Scramjets in other media10 See also11 References12 External links//
[
edit] History
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.
[1] 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 consortiums 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 Defense 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. No scramjet powered vehicle has yet been produced outside an experimental program.
[
edit] Simple description
A scramjet is a type of engine which is designed to operate at the high speeds normally associated with
rocket propulsion. It differs from a classic rocket by using air collected from the
atmosphere to burn its fuel, as opposed to an oxidizer carried with the vehicle. Normal
jet engines and
ramjet engines also use air collected from the atmosphere in this way. The problem is that collecting air from the atmosphere causes drag, which increases quickly as the speed increases. Also, at high speed, the air collected becomes so hot that the fuel no longer burns properly.
Diagram illustrating the principle of scramjet operation
The scramjet is a proposed solution to both of these problems, by modifications of the ramjet design. The main change is that the blockage inside the engine is reduced, so that the air isn't slowed down as much. This means that the air is cooler, so that the fuel can burn properly. Unfortunately the higher speed of the air means that the fuel has to mix and burn in a very short time, which is difficult to achieve.
To keep the combustion of the fuel going at the same rate, the pressure and temperature in the engine need to be kept constant. Unfortunately, the blockages which were removed from the ramjet were useful to control the air in the engine, and so the scramjet is forced to fly at a particular speed for each altitude. This is called a "constant dynamic pressure path" because the wind that the scramjet feels in its face is constant, making the scramjet fly faster at higher altitude and slower at lower altitude.
The inside of a very simple scramjet would look like two kitchen
funnels attached by their small ends. The first funnel is the intake, and the air is pushed through, becoming compressed and hot. In the small section, where the two funnels join, fuel is added, and the combustion makes the gas become even hotter and more compressed. Finally, the second funnel is a nozzle, like the
nozzle of a rocket, and thrust is produced.
Note that most artists' impressions of scramjet-powered vehicle designs depict
waveriders where the underside of the vehicle forms the intake and nozzle of the engine. This means that the intake and nozzle of the engine are asymmetric and contribute directly to the lift of the aircraft. A waverider is the required form for a hypersonic lifting body.
[
edit] Theory
All scramjet engines have fuel injectors, a combustion chamber, a thrust nozzle and an inlet, 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.
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 is the dynamic
pressure of the gas
ρ (
rho) is the
density of the gas
v is the
velocity of the gas
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 kerosine 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 down 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,
[2] 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).
[3] 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,
[4] CHSTF
[5] 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.
[
edit] Advantages and disadvantages of scramjets
[
edit] Special cooling and m
aterials
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.
Therefore studies often plan on "active cooling", where coolant circulating throughout the vehicle skin prevents it from disintegrating. Active cooling could require more weight and complexity. There is also safety concern since it's an active system. Often, however, 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. Both scramjets and conventional rockets are at risk in the event of a cooling failure.
[
edit] Half an engine
The typical waverider scramjet concept involves, effectively, only half an engine. The shockwave of the vehicle itself compresses the inlet gasses, forming the first half of the engine. Likewise, only fuel (the light component) needs tankage, pumps, etc. This greatly reduces craft mass and construction effort, but the resultant engine is still very much heavier than an equivalent rocket or conventional turbojet engine of similar thrust.
[
edit] 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.
[
edit] 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.
[
edit] 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.
[
edit] Lack of stealth
There is no published way to make a scramjet powered 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 extremely difficult because of its high speed.
[
edit] Advantages 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:
[
edit] Lower 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.
[
edit] 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.
[
edit] Reentry
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.
[
edit] Costs
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).
[
edit] Applications
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.
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 4000 s whereas a rocket provides less than 600 s while in the atmosphere,
[6][7] potentially permitting much cheaper access to space. However, a scramjet's specific impulse decreases rapidly with speed, and the vehicle also exhibits increased drag.
One issue is that scramjet engines are predicted to have exceptionally poor thrust to weight ratio- around 2.
[8] This 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 would be reusable or not is still a subject of debate and research.
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.
There are also questions as to how realistic such a proposal is that revolve around costs (capital and maintenance) of technology that is yet to be developed.
[
edit] Recent progress
Main article:
Scramjet ProgramsIn recent years, significant progress has been made in the development of hypersonic technology, particularly in the field of scramjet engines.
US efforts are probably the best funded, and the
Hyper-X group has claimed the first flight of a thrust-producing scramjet with full aerodynamic maneuvering surfaces. The first group to demonstrate a scramjet working in an atmospheric test was a project by an Australian team at the
University of Queensland. The university's
HyShot project demonstrated scramjet combustion in
July 30,
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.
On Friday, June 15, 2007, the US Defense Advanced Research Project Agency (
DARPA), in cooperation with 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 least the following nations have active scramjet programs (by alphabetical order):
AustraliaFranceGermanyIndiaItalyJapanRussiaSouth KoreaSwedenUnited KingdomUnited States of America[
edit] Scramjet in the movies
The
1983 television movie "
Starflight: The Plane That Couldn't Land" explores the concept of a hypersonic
jetliner for passenger transportation, developed by the fictional company Thornwall Aviation. The jetliner uses scramjet engines to reach a point high in the
stratosphere for a quick two-hour jump from
Los Angeles to
Sydney, and the engines are powered with hydrogen. NASA is accustomed to handling this fuel, and a NASA
space shuttle handles a refuelling job while the jetliner is (accidentally) stuck in orbit.
In the
2005 movie
Stealth both the
F/A-37 Talon and UCAV EDI are powered by
pulse detonation engines with scramjet boosters.
[
edit] Scramjets in other media
The Mave fighter in
Sentou Yousei Yukikaze has an option within its performance range called "RAM-AIR," which is treated as a ramjet but has performance which more closely resembles a scramjet. It was used by Rei Fukai, the main character, to chase after and catch up to an enemy fighter mere moments before it did a kamikaze attack on a friendly ship.
One of the engine types available for the customizable aircraft in
Ace Combat X is called the "SCRAMjet."
In the episode "Pandora's Box" of the CBS television show
NUMB3RS, a crashed plane was carrying a ScramJet engine prototype as undeclared cargo for testing.
The
V-Wing from the
Star Wars comic book series
Dark Empire features a scramjet as a booster engine.
The X-43 is an unmanned
experimental hypersonic aircraft design with multiple planned
scale variations meant to test different aspects of highly
supersonic flight. It is part of
NASA's Hyper-X program.
A winged booster rocket with the X-43 itself at the tip, called a "stack", is launched from a carrier plane. After the booster rocket (a modified first stage of the
Pegasus rocket) brings the stack to the target speed and altitude, it is discarded, and the X-43 flies free using its own engine, a
scramjet.
The initial version, the X-43A, was designed to operate at
speeds greater than
Mach 7, about 5,400
mph (8,050
km/h) at
altitudes of 100,000
feet (30,000 m) or more. The X-43A is a single-use vehicle and is designed to crash into the ocean without recovery. Three of them have been built: the first was destroyed; the other two have successfully flown, with the
scramjet operating for approximately 10 seconds, followed by a 10 minute glide and intended crash.
The first flight in June
2001 failed when the stack spun out of control about 11 seconds after the drop from the B-52 carrier plane. It was destroyed by the range safety officer, and it crashed into the Pacific Ocean. NASA attributed the crash to several inaccuracies in data modeling for this test, which led to an inadequate control system for the particular Pegasus used.
The X-43A's successful second flight made it the fastest free flying
air-breathing aircraft in the world, though it was preceded by an Australian
HyShot as the first operating scramjet engine flight. While still attached to its launching missile, the HyShot flew in descending powered flight in
2002.
The third flight of a Boeing X-43A set a new speed record of 12 144 km/h (7,546 mph), or Mach 9.8, on
November 16,
2004. It was boosted by a modified
Pegasus rocket which was launched from a
Boeing B-52 at 13,157 meters (43,166 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.
The most recent success in the
X-plane series of aircraft, the X-43 is part of
NASA's Hyper-X program, involving the
American space agency and contractors such as
Boeing,
MicroCraft Inc,
Orbital Sciences Corporation and General Applied Science Laboratory (
GASL). MicroCraft Inc., now known as ATK GASL built the X-43A and its engine.
The Hyper-X Phase I is a NASA Aeronautics and Space Technology Enterprise program being conducted jointly by the
Langley Research Center,
Hampton, Virginia, and the
Dryden Flight Research Center,
Edwards, California. Langley is the lead center and is responsible for hypersonic technology development. Dryden is responsible for flight research.
Phase I is a seven-year, approximately $230 million, program to flight-validate scramjet propulsion, hypersonic
aerodynamics and design methods.
Contents[
hide]
1 The craft2 The engine3 Tests4 Further development4.1 X-43B4.2 X-43C 
4.3 X-43D5 The Future of the Scramjet6 FALCON7 Boeing X-518 External links9 References10 Related content//
[
edit] The craft
NASA's B-52B launch aircraft takes off carrying the X-43A hypersonic research vehicle (
March 27,
2004)
The X-43A aircraft was a small unpiloted test vehicle measuring just over 12 feet (3.7 m) in
length. The vehicle was a
lifting body design, where the body of the aircraft provides a significant amount of
lift for flight, rather than relying on
wings. The aircraft weighed roughly 3,000
pounds or about 1,300
kilograms. The X-43A was designed to be fully controllable in high-speed flight, even when
gliding without
propulsion. However, the aircraft was not designed to land and be recovered. Test vehicles crashed into the
Pacific Ocean when the test was over.
Traveling at Mach speeds produces a lot of heat due to the
compression shock waves involved in supersonic
drag. At high Mach speeds, heat can become so intense that
metal portions of the airframe melt. The X-43A compensated for this by cycling
water behind the leading edges of the aircraft, cooling those surfaces. In tests, the water circulation was activated at about Mach 3. In the future, fuel may be cycled through such areas instead, much like what is currently done in many
liquid-fuel rocket nozzles and high speed planes such as the
SR-71.
[
edit] The engine
Full scale model of the X-43 plane in
Langley's 8 foot, high temperature
wind tunnel.
The craft was created to develop and test an exotic type of engine called a supersonic-
combustion ramjet, or "
scramjet," an engine variation where external combustion takes place within air that is flowing at supersonic speeds. The X-43A's developers designed the aircraft's airframe to positively affect
propulsion, just as it affects
aerodynamics: in this design, the forebody is a part of the intake airflow, while the aft section functions as a nozzle.
The engine of the X-43A was primarily fueled with
hydrogen. In the successful test, about two pounds (or roughly one kilogram) of the fuel was used. However, because hydrogen poses certain difficulties in storage, transport, and even production, further X-43 versions were planned to use more commonly available
hydrocarbon fuels instead. Unlike rockets, scramjet-powered vehicles do not carry oxygen onboard for fueling the engine. Removing the need to carry oxygen significantly reduces the vehicle's size and weight. In the future, such lighter vehicles could bring heavier
payloads into
space or carry payloads of the same weight much more efficiently.
Scramjets only operate at hypersonic speeds in the range of Mach 6 or higher, so rockets or other jet engines are required to initially boost scramjet-powered aircraft to this base velocity. In the case of the X-43A, the aircraft was accelerated to high speed with a
Pegasus rocket launched from a converted
B-52 Stratofortress bomber. The combined X-43A/Pegasus vehicle was referred to as the "stack" by the program's team members.
The engines in the X-43A test vehicles were specifically designed for a certain speed range, only able to compress and ignite the fuel-air mixture when the incoming airflow is moving as expected. The first two X-43A aircraft were intended for flight at approximately Mach 7, while the third flew at approximately Mach 10.
[
edit] Tests
The Pegasus booster accelerating the X-43A, shortly after booster ignition (
March 27,
2004)
X-43A at Mach 7
The X-43A being dropped from under the wing of a
B-52B Stratofortress.
NASA's first X-43A test on
June 2,
2001 failed because the Pegasus booster lost control about 13
seconds after it was released from the B-52 carrier. The rocket experienced a control oscillation as it went
transonic, eventually leading to the failure of the rocket's
starboard elevon. This caused the rocket to deviate significantly from the planned course, so the stack was destroyed by onboard
explosives as a safety precaution. An investigation into the incident stated that imprecise information about the capabilities of the rocket as well as its flight environment contributed to the accident, though no single factor could ultimately be blamed for the failure.
In the second test, the Pegasus fired successfully and released the test vehicle at an altitude of about 95,000 feet. After separation, the engine's air intake was opened, the engine ignited, and the aircraft then accelerated away from the rocket. Fuel was flowing to the engine for eleven seconds, a time in which the aircraft traveled more than 15 miles (24 km). After burnout, controllers were still able to maneuver the vehicle and manipulate the flight controls for several minutes as the aircraft was slowed down by wind resistance and took a long dive into the Pacific. Peak speed was at burnout of the Pegasus but the scramjet engine did accelerate the vehicle in climbing flight, after a small drop in speed following separation.
NASA flew a third version of the X-43A on
November 16,
2004, achieving a speed of approximately Mach 10 and further testing the ability of the vehicle to withstand the heat loads involved.
[
edit] Further development
Other X-43 vehicles were designed, but as of November 2004 appear to have been suspended. They were expected to have the same basic body design as the X-43A, though the aircraft were expected to be moderately to significantly larger in size.
[
edit] X-43B
The next letter down the list, the X-43B, was expected to be a full-size vehicle, incorporating a t
urbine-based combined cycle (TBCC) engine or a
rocket-based combined cycle (RBCC)
ISTAR engine. Jet turbines or rockets would initially propel the vehicle to supersonic speed. A
ramjet might take over starting at Mach 2.5, with the engine converting to a
scramjet configuration at approximately Mach 5.
[
edit] X-43C
The X-43C would have been somewhat larger than the
X-43A and was expected to test the viability of hydrocarbon fuel, possibly with the
HyTech engine. While most scramjet designs have used hydrogen for fuel, HyTech runs with conventional kerosene-type hydrocarbon fuels, which are more practical for support of operational vehicles. The building of a full-scale engine was planned which would use its own fuel for cooling. The engine cooling system would have acted as a chemical reactor by breaking long-chain hydrocarbons into short-chain hydrocarbons for a rapid burn.
The X-43C was
indefinitely suspended in March 2004. The linked story reports the project's indefinite suspension and the appearance of
Rear Admiral (RADM) Craig Steidle before a House Space and Aeronautics subcommittee hearing on
March 18,
2004.
According to a special feature article by Daryl Stephenson in the August 2005 online issue of
Boeing Frontiers the X-43C appears to be funded through 2005. "Thanks to a funding request of $25 million for NASA sponsored by U.S. Rep. Jim Talent (R-Mo.), work on the X-43C program will continue through 2005."
[
edit] X-43D
The X-43D would have been almost identical to the X-43A, but expanding the speed envelope to approximately Mach 15.
[
edit] The Future of the Scramjet
After the X-43 tests in 2004, NASA Dryden engineers said that they expected all of their efforts to cumulate into a Two Stage To Orbit Manned Vehicle in about 20 years. The scientists expressed much doubt that there would ever be a
Single Stage to Orbit manned vehicle like the
National Aerospace Plane (NASP), also known as the "Orient Express", that would takeoff from an ordinary airport runway.
_Mach_7_computational_fluid_dynamic_(CFD).jpg){kind=link}
