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A big selling point of the ramjet is its absence of moving parts (hence why it can be translated as "statojet" from other languages). However, an even bigger drawback is that a ramjet cannot produce static thrust, thus a ramjet-powered aircraft cannot take off by itself.

The early Leduc 0.22 and Griffon prototypes solved this by adding a coaxial turbojet. This was further refined with the Pratt & Whitney J58 turboramjet that powered the SR-71 Blackbird.

The problem is that by solving the no-static-thrust drawback, adding a turbojet negates the no-moving-parts selling point.

There are missiles that use solid fuel rockets to solve the problem, using the exhaust of the rocket to compress the air in the ramjet, which is sometimes referred as a ramrocket. However, there doesn't seem to be aircraft using this kind of engine, and missile engines have quite different constraints.

Is there an aircraft engine, working or theoretical, that works as a ramjet in cruise mode, can take off on its own power (without requiring ground installations or a detachable first stage/mothership), and doesn't have the many complicated moving parts of a turboramjet? What are its expected characteristics? If not, what are the showstoppers?

And why can't we simply inject some oxidiser (for example WFNA) to produce static thrust?

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  • $\begingroup$ The problem of injecting oxidizer is that it has to come from somewhere, i.e. you have to carry it around, i.e. you have more fuel weight and less payload available. $\endgroup$ – Federico Feb 15 '18 at 15:52
  • $\begingroup$ @Federico The idea would be to have a small oxidizer tank to help at takeoff and landing (and in case of emergency). The mass penalty would be a trade-off for using ramjet in the first place. $\endgroup$ – Eth Feb 15 '18 at 15:56
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    $\begingroup$ I don't think the rocket exhaust was fed into the engine, the rocket was used to get the whole thing up to speed where the ramjet could start functioning. Or did I misunderstand you? $\endgroup$ – zeta-band Feb 15 '18 at 16:33
  • $\begingroup$ @zeta-band Both seem to exist: some ramjet missiles are brought up to speed by a rocket first stage, acting like the solid rocket boosters of heavy satellite launchers. Others have a rocket in the front part, or even solid fuel directly inside the ramjet. The MBDA Meteor missile seems to use the latter. $\endgroup$ – Eth Feb 15 '18 at 16:59
  • $\begingroup$ Perhaps interesting to know is that a liquid fuel rocket engine is generally powered by a turbopump, so the no-moving-parts argument doesn't work for liquid fuel powered rockets either. $\endgroup$ – Sanchises Feb 15 '18 at 17:13
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Well the Pratt & Whitney J58 engines which powered the SR-71 worked in a similar fashion, using a bypass system which allows the engine to operate as a ramjet at high speeds and a turbojet engine at lower speeds.

By the very nature of a ramjet, it relies of stagnation pressure at the intake and inlet throat to pressurize the airflow prior to combustion. It is just not possible to achieve these kinds of stagnation pressures at speeds under Mach 2 or so. There was an article in Air & Space magazine a while back about two enterprising Canadian teenagers who tried to fashion a homemade ramjet from iron sewer pipes to power a sled. They couldn’t get the design to work for this reason.

As such, you would unfortunately need another type of engine to accelerate an aircraft to these fast speeds where a ramjet could work. Turbojets work well for this application. So do rocket boosters. Injecting oxidizer would not be very useful here as airflow through the engine is not fast enough to ensure sustained combustion.

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Another way to realise a liquid fuel ramrocket engine, in theory, would be to incorporate the rocket motor into the ramjet air intake centerbody. As the aircraft approached ramjet operating speed, the ratio of fuel to oxidiser being fed into the rocket motor would be increased so that some of the fuel would be expelled from the rocket motor and mixed with air before being burnt. Once at full ramjet operating speed, the supply of oxidiser would be cut off, at which point the engine would be functioning as a ramjet. Ram jet fuel is basically similar to aviation turbine fuel, but with a slightly higher specific gravity, higher viscosity and lower flash point to suit typical ramjet operating conditions. Concentrated hydrogen peroxide would be one potential oxidiser.

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