SCRamjet stands for S
. This is an aircraft
engine for atmospheric
use. A ramjet
engine is one which does not have compressor
fans, but rather uses the speed of the aircraft's passage to force (ram) air into a combustion chamber
. This air is then heated (both through compression
and fuel burn) and released out the back of the engine to produce thrust
The major limiting factor with a 'standard' ramjet is that airflow through the engine must be slowed to subsonic speeds for fuel mixing and combustion. Once this has occurred, energy gleaned from fuel burn is used to accelerate the airflow back up to supersonic speeds by passing high-pressure air through a contricting nozzle; this supersonic airflow is then ejected out the rear of the engine and used to drive the craft. This slowdown-and-speedup is highly wasteful of energy at high speeds, however; in addition, the engine inlet must withstand the incredible stresses of decelerating the airflow, and the engine itself (combustor, constrictor and nozzle) must be able to withstand extremely high internal pressures in order to generate enough energy to overcome the concomitant losses. Remember, the useful thrust derived from the engine is the energy represented by the difference in velocity between the exhaust and the intake airflow - this represents energy added by fuel consumption. Hence, all the energy used to simply accelerate the airflow back up to the craft's airspeed is essentially 'wasted.' This is why turbojets and turbofans become less efficient at higher airspeeds - it becomes harder to generate those useful differences in exhaust velocity.
The scramjet is intended to deal with these problems by allowing for supersonic airflow through the entire engine. However, physics dictates that a scramjet will not be effective until a minimum of approximately Mach 3; hence, either an additional means of propulsion must be supplied to accelerate the engine up to its minimum operating airspeed, or the engine itself must be capable of multiple modes of operation. The reason for this is as follows. In a ramjet, the inlet is designed to form a stable 'inlet shockwave' which cushions the structure of the engine from incoming air and performs initial compression and heating of the airflow; however, that shockwave (essentially, a static sonic boom) is intended to only form at the front of the engine. The airflow is reduced to subsonic before entering the combustor, and hence the shockwave vanishes. For a scramjet to operate, fairly precise (and known) airflow conditions must exist throughout at least the inlet and the combustor in order to allow for the difficult problem of mixing fuel and air at supersonic speeds - speeds at which air tends to perform more like a solid than a fluid, hence the static shockwaves. This means that the standing shock must extend back into the combustor. For true scramjet operation, the shockwave must, in fact, exist in a stable form all the way through the engine and back out the rear into what is called the 'external nozzle.' This is usually a region comprising both the end of the engine and the exernal fuselage behind the engine which allow the high-energy airflow to expand and convert additional heat energy to kinetic energy - providing additional thrust - in much the same manner as the bell of a rocket engine.
The requirement for this standing wave is what limits the scramjet to Mach 3 and up; a shockwave powerful enough to stand up to the pressures and stresses created by burning jet fuel will not occur until roughly Mach 3. Hence, to date, all operational scramjets have depended on external 'low speed' systems to reach Mach 3+, forming the required wave pattern before becoming operational. One exciting possibility is that of a dual-mode engine; once which operates as a ramjet at lower speeds but is able to reconfigure itself to operate as a scramjet as the requisite airflow speed is achieved. In this case, the engine would have to be carefully designed and operated to remain inside a realm of operation that lies between two boundary conditions while in ramjet mode. The first danger region is called an 'inlet unstart' and is similar to a compressor stall in a turbojet or turbofan. In an inlet unstart, the airflow into the engine becomes disturbed and/or the airflow velocity drops far enough that the inlet wave pattern is disrupted. When this occurs, the standing wave which would normally protect the combustor from the supersonic airflow collapses, and supersonic air rushes through the engine, extinguishing the burners. The SR-71 Blackbird was highly vulnerable to a form of this problem; rather than standing waves, it used physical spikes to shape and direct the inlet shockwaves. If they moved out of the proper position, the shockwave would strike the inlet at the wrong spot, and the engine would unstart (pilot-ese for 'flame out.') Hence, the ramjet must maintain a minimum airflow through the engine for a given velocity at all times, and avoid disturbing the shockwaves which serve to compress and direct the incoming air.
At the other end of the spectrum is a phenomenon called 'boundary layer separation.' The boundary layer, in this case, is the point in the engine where the standing wave devolves into turbulence; i.e. the 'back edge' of the stable wave pattern. As the boundary layer passes the combustor, the engine can be considered to be in scramjet mode; supersonic flow past the combustor indicates supersonic combustion. However, if this happens too early (something that, admittedly, is difficult to make happen) then supersonic airflow might enter the combustor when the system is not configured for it, and again, an unstart might occur.
In 'normal' operation, a ramjet will continue to operate as a ramjet up to the point where (again) the trailing edge of the standing wave passes the combustor; at that point, the engine will need to reconfigure itself to operate as a scramjet. How this will happen is a matter of a great deal of engineering, physics and math; however, in general, we can assume that the region behind the combustor which had (up to this point) been fairly narrow in order to convert higher pressures to high velocity will need to expand so as to allow the supersonic airflow full passage; the aft part of it will need to reconfigure into the forward part of the external nozzle. Assuming all this happens properly, the engine should pass into scramjet mode around Mach 3, and then around Mach 5 or Mach 6 (as maximum pressure in the engine is reached) the trailing edge of the shockwave should finally pass fully out the back of the engine as boundary layer separation occurs. At that point, a complete supersonic standing wave will exist all the way though the scramjet - it will be operating more like a rocket than a jet engine, albeit one taking in its oxidizer through the nose. The actual thrust will be occurring aft of the combustor, just behind the external nozzle.
One variant of this is the external combustion supersonic ramjet. In this engine, the airflow and combustion actually occur outside the airframe of the aircraft! The aircraft is shaped in such a way that a flow pattern is formed against (usually just beneath) the hull, and fuel is ejected into the slipstream and burned. The standing shockwaves which in the enclosed scramjet are used mostly to direct the airflow and compress the air are used to form the entire 'structure' of the engine. This, however, cannot be used in a 'dual-mode' ramjet/scramjet, since it will only operate at hypersonic speeds.
As scramjet engines achieve higher and higher velocities, frictional heating becomes a concern. One solution to this is to utilize liquid hydrogen for fuel, since in addition to readily burning for thrust, liquid hydrogen fuel can serve the dual purpose of cooling the engine and airframe structure. Although hydrocarbon fuel (jet fuel) can serve the same purpose, liquid hydrogen offers much better heat transfer properties. The SR-71 Blackbird did, in fact, use JP-7 fuel to cool the airframe while at the same time using the same process to preheat the fuel for combustion. As an added plus, if the craft utilizes dual modes of propulsion, hydrogen is a tried-and-tested rocket fuel and could serve to fuel booster engines as well, allowing for a single common fuel system.
Although this would seem to make hydrogen an attractive alternative, one drawback to using hydrogen is its lower energy content per unit volume when compared to hydrocarbon fuel as well as its extremely high volatility. The former makes it less efficient in terms of fuel storage volume, and the latter makes it more dangerous to utilize. The associated requirements of having the fuel system be cryogenic increase the complexity and weight of onboard systems as well. Finally, although hydrogen can easily be used in rocket propulsion, it is not nearly so easily adapted for use in air-breathing low-speed engines; turbine-based engines would be difficult to adapt.