Hypersonic Aircraft Engines

[KJ_Lesnick]

I've read about a variety of high performance engines which have been proposed for hypersonic designs, including the following.

1: Supercharged Ejector Ramjet Engine
As I understand it, the engine incorporated a small number of rocket engines to blast air out the back and, in turn, draw air through the front of the engine as to allow a ramjet to work at low speeds; it also featured a small gas-generator (which looks like a gas-turbine) located on the side of the engine which drove a fan which would draw in air and feed it into the ramjet duct, further augmenting performance.  Some designs which were for hypersonic performance even had the means to have the fan rotate out of the airflow allowing the gas-generator to be shut-down.  From what I've been told the engine was compact and probably lightweight. 

Questions
1: Though looking at drawings, it looked to be quite compact and I assume it was lightweight: Does anybody know what the projected T/W ratios, and SFC would be at low-speeds if it's not classified?
2: Though the design used ejector rockets in the design, were they shut off at high-mach or used throughout the whole flight?


2: Hydrogen Expander Engine
The engine basically utilized the energy from the outside air as well as combustion to actually drive the engine.  Liquid hydrogen would be routed from the fuel tanks through the engine's combustion chamber to a small turbine located deep-inside the engine; the heat from the outside air, several hundred degrees hotter than the LH2 introduced would cause it to vaporize which would provide the force necessary to drive the small turbine which would spin at a high RPM; a gearbox would reduce the RPM and increase the torque to a necessary level to drive a multiple stage fan at the front of the engine which operates in the traditional style, drawing in and compressing the air which is admitted into the combustion chamber along with the now fully vaporized hydrogen gas; the compressed air and fuel mix is then burned producing thrust; the heat from the combustion accelerates the effect and eventually an equilibrium is achieved. 

The design appears to offer a number of advantages over a conventional gas turbine in that it's innately subjected to lower temperatures than a traditional turbine which would ride in the hot engine exhaust-gas; the exhaust velocity would probably be more energetic as the gases would not have to blow through a turbine and be slowed-down; all of which would probably yield overall better performance, particularly at supersonic speeds where high exhaust-velocities are useful; the lower turbine-inlet temperatures would allow one to fly faster and probably allow to some extent a higher pressure-ratio for the given mach-number. 

The engine doesn't need to be dependent on LH2 as LCH4 can do the job as well though it's less cold, it's still plenty cold enough.

Questions
1: The P&W 304 was stated to produce a relatively small amount of thrust relative to it's weight even despite the fact that the physics should produce contrary results -- was this because the engine was inefficient, the cycle was inefficient, or the numbers were fudged if not classified?
2: How much noise would this engine produce if it's not classified (I'm thinking of creating a WHIF Hypersonic Transport concept)
3: Could one use a toroidal combustion-chamber and aero-spike nozzle to squeeze extra efficiency out of the engine


3: RBCC Strut-Jet
This design was mostly for wave-riding designs that could either cruise in the atmosphere or accelerate into space and appeared to be a design that appeared into being in the early 1990's.  It basically featured a traditional ramjet duct with a small auxiliary inlet duct above it which featured a turbo-compressor in it.  The engine design would be driven off a cryogenic gas, probably LH2 or LCH4, which when exposed to the air temperature, would vaporize and drive the expander turbine, which in turn would drive the compressor, which would be piped into a combustion chamber placed inside a streamlined strut located inside the main-duct along with the fuel and would be burned; the combustion chamber is based on a rocket design and would be designed to produce high chamber-pressure and a supersonic exhaust.  The exhaust would produce thrust in and of itself, as well as induct air through the front of the engine, which could allow a successful ramjet light (the use of LOX, pre-burners to drive-up chamber-pressures, hell, theoretically an aero-spike could help produce more thrust and force more air through the duct). 

Claims were made as to the engine being similar in concept to a SERJ, which it is as high velocity exhaust is used to pump a ramjet to produce thrust.  How it's performance compares (inferior, equal, superior) in terms of T/W ratio, SFC, I have no idea, but the RBCC Strut-jet exhibits spectacular performance.  A hypersonic aircraft fitted with this engine would likely have T/W ratios exceeding 1:1, yielding excellent acceleration and climb performance; fuel consumption when cruising at 25,000 feet would probably be similar to a 1960's-era military turbofan (unimpressive for a subsonic engine these days, but for an engine designed for hypersonic performance -- awesome).  As mach number is increased, the ramjet performs more effectively and above a certain speed the turbo-compressor would be shut-down, the doors would cover over the auxiliary duct, and the strut would retract out of the duct allowing the engine to function as a pure-ramjet.  For extreme speeds, the ramjet could undergo a variable-geometry change to a scramjet, and for space-flight, the strut could extend back into the duct and function as a rocket.  You'd need LOX to be routed in but otherwise it would work.


4: Pulse-Detonation Engine
The concept is similar to a pulse-jet except this design works by starting a deflagration which is transitioned into a powerful detonation using a properly shaped tube.  As the exhaust blasts out the back it pulls air in through the front and the cycle is repeated.  It can be used as a propulsive cycle unto itself, or as a booster for a ramjet similar to the strut-jet like above but lighter and simpler.

Questions
1: Though there has been great difficulty in making the PDE work, didn't the V-1 have a combustion chamber that produced an actual detonation?


5: Pre-Cooled Hydrogen-Expander
This engine is basically a hydrogen expander with a pre-cooler located in front of it.  The pre-cooler could be cooled directly by liquid hydrogen, or via another liquid gas (i.e. liquid helium) which would circulate through the pre-cooler; heat would be transferred either from the air to the fuel, or from the air to the coolant, and from the coolant to the fuel, which would vaporize.  The expansion would drive a small turbine located inside the engine which through a gearing mechanism would drive a multi-stage compressor which would draw in air.  The air would flow through the pre-cooler and would go from outside air temperature to around -140 or -150 celsius, drastically increasing it's density, and from there through the compressor; the vaporized fuel and air would mix in the combustion chamber and be burned with the exhaust expanding out the back.  The engine would possess all the advantages that the hydrogen-expander would have with the effect of cooling the airflow down to -140 or -150 celsius at sea-level which also means a massive pressure ratio can be achieved (150:1 IIRC for the SABRE) and very high thrust levels (150 metric tons for SABRE).  As the mach-numbers would increase, the degree of cooling would be increased though due to the rapidly rising ram-compression temperatures, the air temp reaching the compressor would steadily rise; as extra cooling is added, RPM is progressively increased in proportion up to the target cruise speed.

JAXA developed a design called ATREX which fit this design description using LH2 as a coolant and fuel; SABRE and Scimitar worked by using LH2 as a fuel and heat-sink, and LHe as a coolant; ATREX (IIRC) had a rocket-cycle built in as did SABRE; Scimitar did not, but it had a turbofan cycle as well -- some exhaust could be routed through an auxiliary air passage and drive an aft-turbine/aft-fan with a secondary combustion stage behind the fan.

Questions
1: When was it first possible to develop a pre-cooler that could do the job?
2: Could LCH4 be used for the fuel and Liquid Helium as the coolant?
3: Can such a pre-cooler handle FOD?
4: Which has a better power-to-weight ratio, the strut-jet or the pre-cooled hydrogen expander design?

The Big Questions[/b]

1: Could the fan-stage in the SERJ be replaced with one driven off of hydrogen or liquid methane expansion? (That eliminates the need for the gas-generator)
2: Could the SERJ's rockets be replaced with a bunch of small strut-jets or PDE's? (That eliminates oxidizer use)
3: Could a sound suppressor be made to work with such an engine and keep noise from blasting out people's ear-drums?

And: Could a pre-cooler be added to that (adds extra performance)



KJ