Some Basic Questions Regarding Ramjets

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Some Basic Questions Regarding Ramjets

Post by Röör » Thu Feb 04, 2016 6:19 pm


As the topic emerged again, I would like to try building some "flat" ramjets as discussed in another topic.

But a few questions emerged, which are actually basic questions relating to ramjets, where I couldn't really find any answers.
Maybe some of you people can help on these questions.

Position of fuel injection nozzle:
In the Decker manual as well as in the Hill patents for his tip jets the fuel injection happens very early still in the diffusor area.
But in other patents and pictures I have found they do that much later. Sometimes about in the middle of the engine, and sometimes even behind the flameholder.
I know that in turbojets they inject inside the flame can, so basically behind the flameholder. Wouldn't that also be better for a ramjet?
As stated in the decker manual, the wall air (boundary layer) is basically for cooling. So IMHO it would make sense to have no fuel distributed there, or not?
So injection after the flameholder would make sense in this way. Maybe one could even introduce a swirler like in a turbojet combustor?

E.g. in the Fleissner patent mentioned before, it seems they injected about half-way
Also in this design of a HR144 T helicopter (see pic) they inject very late and also the flameholder is very late. Why that???
Why wouldn't one do the flameholder right after the diffusor or already in the diffusor like in the Hiller Ramjets (which I again do not understand anymore, why one would advance the flameholder that much)?
OTOH I anyway think the HR144T ramjet violates basically all design rules from the Decker manual...(e.g. the inlet area is equal to the outlet area)

Flameholder shape:
I think it is interesting that in the decker manual they clearly state to not have wall connections (or as few as possible) with the flameholder, as this would result in a cooling problem behind them.
But basically all used known practical tip ramjets (e.g. Hiller) had exactly that. They had many thick wall connections...
??? Any thoughts ???
I also wondered why it seems that noone used flame cans for a subsonic ramjet design like in a turbo jet combustion chamber.
Wouldn't that be also beneficial there, or would the possible increased drag counter the positive effects of a better burning and wall cooling?

OTOH there's this A023471 report ( from the airforce which concludes that the flamecan is a good thing, also for subsonic ramjets.
But maybe this only makes sense if the ramjet has a certain size? So for small tip jet ramjets it wouldn't be beneficial???
Any thoughts?

Diffusor Physics:

Please don't laugh...But as I understood it, the job of the diffusor is to increase the pressure and slow down the air.
Unfortunately I couldn't understand any explanation I read so far, as to why it increases the pressure.
I can certainly understand why it slows down the air. But at best I would even expect a little pressure drop, not a pressure increase.
I would understand it, if the diffusor first would get smaller and then wider. This would IMHO certainly increase the pressure.
But why does it increase the pressure if the diffusor just gets bigger???

Flame location:
As I understood it, the flame should have ended before the nozzle starts, so it should never exit the engine. Am I right? For as I understand it, it wouldn't make much sense burning fuel after the engine in free air.
But OTOH there are plenty of Youtube Ramjets videos where the flame is exiting the my question is: Flame only inside the ramjet or also outside?

If I saw it correctly, then afterburners also seem to burn still outside the engine. But why does this make sense?
HR144T Ramjet.jpg
HR 144 T Ramjet (Tip Jet)

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