Peter, Brian, Machaca & Gums,
Thanks for the discussions and information on the J58 engine, it has all been enlightening to an old jet engine guy. There have been some posts that have been both confusing and humorous which I intend to address here. There have been four types of jet engines in this discussion:
1. A turbo-ramjet engine, better known as a P&W J58 engine (main topic)
2. A turbojet engine, better know as a GE J79 engine
3. A turbofan engine, better known as a P&W F100 or GE F129
4. A high by-pass turbofan engine, better known as a GE90
All are different and one should not be confused with another. So bear with me here.
A turbo-ramjet engine is exactly that, a turbojet working together with a ramjet to power an aircraft to Mach 3+ that could not be achieved independently of one another. To simplify this, A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The turbojet is the booster. Ramjets generally give little or no thrust below about half the speed of sound, and they are highly inefficient until the airspeed exceeds 1000 km/h (600 mph) due to low compression ratios. The turbojet is very efficient in this regime. Ramjets work by ingesting relatively low speed air and expelling the air at a higher speed. The difference in speed results in a forward thrust. The burning fuel creates higher pressures inside the engine, causing higher exhaust speeds. But the thrust of the engine depends entirely upon how much air flows through it. No matter how hot the burning air-fuel mixture is, and how high the pressure, if not much air flows into the front of the engine not much thrust is produced. So the trick to improving ramjet efficiency is to increase airflow through the engine. This is accomplished by the spike or obstruction called an innerbody. It is pointed on both ends and thick in the middle and fits inside the intake tube. Air passing into the tube must flow around the innerbody, and the area around it is less than the area of the intake opening. Consequently the air is compressed as it flows around and reaches a maximum pressure in the narrow throat between the innerbody and the intake tube. The same amount of air flows into the engine, but it is raised to a higher pressure. This increases the pressure that the burning gasses must push against, causing the overall pressure inside the tube to increase. Higher internal pressures mean greater amounts of air in the engine, so more fuel can be burned. The result is still higher pressures, increased exhaust gas speed, and greater thrust. But there is a problem that must be dealt with in the turbojet compressor area. When the pressure becomes too high in compressor, the blades tend to flutter, may break, the compressor can stall and the high temperatures can result in mechanical failures. So the P&W designers cleverly bled off air from the compressor to lower the pressure and temperature and fed it back to the burner in the afterburner section, it worked well.
Ram means 'to compress'. For "Ram" to apply to the J58, the engine would have to be able to shut down its compressors, and rely on inlet air only to propagate its motive power.
Not true, P&W solved that problem.
I do not deny the J58 satisfies the description of TurboRamjet. That makes it a hybrid, not a Ramjet.
Not true. You are thinking of an automotive description of some modern cars and SUV, a turbo-ramjet work together as a team, both at the same time once the speed becomes great enough.
In a turbojet engine such as the J79, all the air ingested passes through the core of the engine and upon exit may, be mixed with fuel and relighted to obtain additional thrust for acceleration needs in AB mode. In particular, the two engine F-4 phantom jet had the capability to achieve Mach 2.3 at 40,000 feet in level flight, Mach 2.5 minus external stores and fuel tank. It was designed to accelerate rapidly during climb to 40,000 feet and dash even more rapidly to a target. But it did have limitations.
A friend flew the F4 and recounted his experience at M2. It had to be done in a dive. He tells of pulling the engines out of AB, and it felt like he had hit a brick wall. Until he decelerated to some lesser Mach, the extreme deceleration was unnerving, indicating the drag had been there all along. He explained it in a way similar to that above, that the engine mounts were driven back into the frames, not pulling forward on them. Even in full burner at 2+.
Although this description is somewhat sketchy, the aircraft, being of aluminum exterior was painted with heat resistant paint good to 400℉. The friction caused by the Mach 2 speed in a dive from 40,000 feet or less if continued long enough would be disastrous and the abrupt slowdown was in effect lifesaving for the pilot, the wings and the engines. The panels on either side of the fuselage just forward of the engine intakes were not for decorative purposes. This plane was simply overpowered for its construction.
The F100 and F110 engines, being turbofan engines take most of the air through the core but the excess from the fan is funneled around the core engine exterior and dumped back into the afterburner where it is mixed with the core exhaust or where fuel is added and ignited in the AB mode. This is much more efficient than a normal turbojet AB engine system.
The GE90 high by-pass turbofan engine develops 60% of its cruise thrust from the fan by-pass. The air is ducted by the nacelle around the engine core and dumped into the air but does not mix with the core exhaust air.
If a GE 90 captured some airflow aft of the fan and directed it back into the engine ( it does), is it a partial "Ramjet"?
No, because a ramjet doesn't produce hardly any thrust at all at 0.83 Mach and the air passing through the fan is only lightly compressed.
If Einstein called a five a four, is he wrong?
Yes...
If I put a cucumber in brine and vinegar for 48 hours, is it a cucumber or a pickle?
It is a dill pickle
TD