Graham C. Williams wrote:
Mark wrote:This sketch of a Marconnet struck me as being short for a valveless. Interesting how the valveless pulsejet and ramjet have this gray, vaguely defined separation. And in Foa's book, to reiterate, he says the line of demarcation is blurred when analyzing a sub-sonic ramjet in which combustion benefits from pulsating combustion.
I guess though short valveless designs have been on helicopter blade tips for some time, optimized to run at speed and utilizing ram air. There is that trade-off if you want or need a certain thrust when stationary. Imagine all the degrees of separation there are in that spectrum.
I posted this sketch because a subtopic was how Nick's jet was somewhat shorter than the classic designs, (in case you think I drifted off topic again). ; )
You hit the nail on the head. Forward motion or forced induction air moves the massflow balance point of the motor towards the rear and performance drops for a number of reasons. Designs like these attempt to address this problem. The strange sub wave structure of the design allows you to take liberties with the conical half angles. The one design allows you to play with at least 3 modes of operation just by changing the proportions, so I would not be surprised if the early French designers had many long and short variants of the design.
Mark and Graham -
Good one! This is something often missed in talking about the exact angles of cones and "sharpness" of transitions. Without variable geometry, it is probably impossible to design an engine that is BOTH optimal for static running AND optimal at some practical forward speed. The ramjet is just a carrying of this idea forward to the ultimate extreme, really - it won't even function below a certain forward velocity but is highly efficient at some point well above the speed where it starts working.
This should not seem strange. For about a century now, engineers have been designing gas turbine combustor nozzles and turbine buckets ["vanes"] to have a satisfactory angle and curvature for optimal efficiency at speed
, not stationary (as they are at startup). The difference is large, not subtle, because the relative speed between the nozzle and the turbine buckets is such a high value when the engine gets up to speed. Even the design of the stationary vanes is affected by the working rotational speed.
This is another example of why it's futile to try to measure some "maximum" static thrust as a comparison between engines [unless the only purpose of both engines is static running]. You have to be able to measure what the engines will do at some reasonable speed
. The ultimate goal of most engines, after all, is to get something moving.