Blamjet

Ramjets, traditionally, are inefficient below Mach ~1, and produce little to no thrust (and so are not useful) below about Mach 0.5. This is partly because they receive too little ram air at these relatively low airspeeds.

(Sufficient ram air is needed to allow compression in the intake, which is a diverging nozzle. A diverging nozzle slows down the air, because there's the same mass airflow across more cross-sectional area, and slower- moving air has higher static pressure for some reason. The compressed air is then combusted with fuel, and the exhaust is blown out the back to produce thrust.) (The other reason for inefficiency is the nonexistence of shockwaves at subsonic speeds. Supersonic airspeed ramjets—not scramjets—use shockwaves to contain the flame front and shape the internal airflow. Subsonic ramjets, IIUIC, need draggier internal structures to do the same things.)

To increase the amount of air the ramjet receives, the obvious thing to do is blow into its intake. This has been done, in a variety of ways: with integrated electric fans, with electric shop-vacs and leafblowers (as used for the Maggie Muggs travel mug ramjet), and with pulsejets: [links]. The problems with these are readily apparent: integrated fans impede ram air at higher airspeeds and introduce turbulence, and/or increase complexity; shop- vacs and leafblowers are only suitable for test stand use; pulsejets provide pulsatile blowing, and supply air that's already had quite a bit of its oxygen used.

Blowing a ramjet with a pulsejet is effectively putting two separate jet engines in series. My design effectively puts one jet engine in series with itself.

A turbojet, essentially, is in series with itself: it takes energy from its exhaust using a turbine, and uses it to drive a compressor at the intake. In fact, that's the difference between a ramjet and a turbojet: a turbojet is a ramjet with the addition of a compressor to blow into its intake, and a turbine to power the compressor. My design accomplishes the same thing, without the turbine or the compressor.

Now, you might be thinking that I'm planning to simply recirculate the ramjet's exhaust to become its blow air. That would work even at zero airspeed, but only if the air had unlimited oxygen, and no thrust was desired. That's not very practical (so you probably weren't thinking that).

A common addition to hobby pulsejets is the augmentor. This is a duct encircling the exhaust nozzle, somewhat like an afterburner, but without the burning. It's effectively an annular injector/ejector/eductor that, using the Venturi effect, entrains surrounding air into the exhaust jet to increase thrust by increasing mass flow, at the expense of exhaust velocity. (Actually, a surprising amount of air is entrained even without any duct: [link]. So I expect my design to work quite well with one.)

In my design, the augmentor is used only on a portion of the ramjet's exhaust that is split off with a pipe from the main exhaust nozzle (bleed air, basically, but taken post- combustion). This pipe goes around to the front of the engine, where the augmentor is mounted some distance in front of the intake. The split-off exhaust coming through the pipe is released through a nozzle into the front of the augmentor. The augmentor then entrains a lot of ambient air from in front of the engine and blows it into the intake, providing more air to the ramjet. The use of the augmentor minimizes the ratio of recirculated air to fresh air, avoiding asphyxiating the flame with exhaust and avoiding taking too much of the exhaust away from producing thrust.

This is expected to provide sufficient blowing that the ramjet can stay running, and producing thrust, down to much lower airspeeds, possibly even zero, after being started with high airspeed or external blowing.

(This system could also be implemented with a non-augmentor-shaped injector/ejector/eductor, as a separate device that could be mounted inside a wing or something and would not have to be externally aerodynamic. This would then be ducted to the ramjet's intake.)

The blow system should automatically turn off and get out of the way of the actual ram air when the airspeed gets high enough that the ram air is sufficient to blow the ramjet. Probably, it should turn off gradually with increasing speed, balancing artificial blowing with ram air to keep the ramjet efficient.

As a bonus, this system can quite simply be combined with other ramjet variants such as integrated rocket ramjets and air-augmented rockets.