'''초음속 연소 램젯'''('''S'''upersonic '''C'''ombustion '''Ramjet''') 또는 약어로서 '''스크렘젯'''(SCramjet)은 램젯의 한 변형된 형태이다. 초음속 연소 램젯 엔진은 항공기용 동력 기관으로서, 제트 엔진의 한 형태로 분류될 수 있다.

== 구성 ==
초음속 연소 램젯 엔진은 램젯 엔진과 마찬가지로 공기 흡입구, 연소실 및 노즐로 구성된다. 그러나 이러한 구성은 초음속 연소 램젯 엔진의 각 부분에서 발생하는 공기역학적 현상에 따라 구분하여 설명하는 것일 뿐이다. 실제로 스크램젯 엔진은 하나의 긴 관과 같은 단순한 형상으로서, 길이 방향에 따라 그 단면적만이 달라질 뿐이다. 

== 터보젯 및 터보팬과의 비교 ==
thumb|265px|램젯의 원리
thumb|265px|초음속 연료 엔진의 원리
터보젯 및 터보팬과 비교하면, 스크램젯은 공기를 압축시키는 데 압축기와 같은 회전하는 구성품을 사용하지 않는다는 특징이 있으며, 이러한 점은 램젯과 동일하다. 램젯과 초음속 연료 엔진은 모두 공기를 압축시키는 데 충격파를 사용한다.

== 로켓과의 비교 ==
로켓은 비행체 내부에 연료를 저장하고 있을 뿐만 아니라, 연료를 연소시키는 데 필요한 산화제도 저장하고 있다. 따라서 로켓은 공기가 희박하거나 없는 우주 공간에서도 연료를 연소시켜 추력을 발생시킬 수 있다. 반면에 스크램젯을 포함하여 램젯이나 모든 가스터빈엔진은 산화제를 따로 저장하지 않고, 대기를 흡입하여 그 속에 존재하는 산소를 산화제로 사용한다. 따라서 이러한 엔진은 공기가 없는 곳에서는 연소를 할 수 없다.

== 램젯과의 비교 ==
램젯에서, 공기는 흡입구 내에서 경사충격파 및 수직충격파를 형성하게 되며, 결과적으로 흡입구를 지나면 아음속으로 감속되어 연소실에 유입된다.
그러나 스크램젯 엔진에서는, 공기가 흡입구 내에서 경사충격파 만을 형성하며, 흡입구를 지나면서 감속이 되기는 하나 초음속으로 유지된 채 연소실에 유입된다.

== 특징 ==
공기가 흡입구에서 충격파를 형성할 때에는 압력 손실이 동반된다. 흡입구에 의한 공기 감속의 폭이 크면 클수록 압력 손실이 커진다. 압력 손실은 엔진의 추력 감소로 이어진다. 램젯 엔진의 경우, 공기가 연소실에 유입될 때에는 아음속으로 감속되어야 하기 때문에, 램젯 엔진을 장착한 비행체가 실질적으로 낼 수 있는 속도는 이론적으로 마하 6 정도이고, 현실적으로는 마하 4를 넘기 어렵다.

초음속 연료 엔진은 램젯 엔진의 이러한 속도 한계를 넘을 수 있는 대안으로서 고안되었다. 초음속 연료 엔진은 흡입되는 공기를 엔진 내부에서 음속 이하로 감속시키기 않아도 되기 때문에, 램젯 엔진에 비하여 더 빠른 비행 속도에서도 흡입구에서의 압력 손실을 줄일 수 있다. 결과적으로 초음속 연료 엔진을 장착한 비행체는 이론적으로 최대 마하 15 혹은 그 이상의 속도를 낼 수 있다.

thumb|265px|[[X-43A with scramjet attached to the underside]]

A '''scramjet''' ('''s'''upersonic '''c'''ombustion '''ramjet''') is a variation of a Mach 12 and Mach 24 (SR-71, achieve approximately Mach 3.4 and rockets from the Apollo Program achieved Mach 30+.

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 (between 7 and 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.

==History==
During and after jet- and rocket-powered aircraft.  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 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<ref>[http://www.concordesst.com/retire/faq_r.html Concorde Sst : Faq<!-- Bot generated title -->]</ref> (though this does not include government-subsidized initial costs).  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.

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 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 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.

==Simple description==
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.

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 (See for example SABRE).

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.

thumb|left|300px|Diagram illustrating the principle of scramjet operation. This version uses an [[inlet cone]]
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.

==Theory==
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.

thumb|265px|Computational fluid dynamic (CFD) image of the [[X-43A with scramjet attached to the underside at Mach 7]]

A scramjet is reminiscent of a 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 Mach 25 is around 10% of the total enthalpy of the working fluid. Depending on the fuel, the 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

:<math>q = \frac{1}{2}\rho v^2 </math>

where<br/>
:''q'' is the dynamic pressure of the gas <br/>
:''ρ'' (rho) is the density of the gas<br/>
:''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,<ref></ref> 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).<ref>Voland, R.T., Auslender, A.H., Smart, M.K., Roudakov, A.S., Semenov, V.L., Kopchenov, V. "CIAM/NASA Mach 6.5 scramjet flight and ground test", AIAA-99-4848.</ref> 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 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,<ref>[http://wte.larc.nasa.gov/facilities/hypersonic/arc-heated.cfm?field=11&id=2&fac=1 The Langley Arc-Heated Scramjet Test Facility Brochure<!-- Bot generated title -->]</ref> CHSTF<ref>[http://wte.larc.nasa.gov/facilities/hypersonic/combustion.cfm?field=12&id=2&fac=1 The Langley Combustion-Heated Scramjet Test Facility Brochure<!-- Bot generated title -->]</ref> and  HTT.

numerically stiff requiring reduced reaction schemes.

Much of scramjet experimentation remains US Navy with the SCRAM engine between 1968-1974, and 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.

==Advantages and disadvantages of scramjets==
===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 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.

===Engine weight===

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:

:<math>\Pi_e+\Pi_f+\frac{1}{\Gamma}=1</math>

Where :
*<math>\Pi_e=\frac{m_{empty}}{m_{initial}}</math> is the empty mass fraction
*<math>\Pi_f=\frac{m_{fuel}}{m_{initial}}</math> is the fuel+oxidiser mass fraction
*<math>\Gamma=\frac{m_{initial}}{m_{payload}}</math> is initial mass ratio, and is the inverse of the payload mass fraction