Steve's ramjet revisited

[metiz]

I was going through the ramjet forum (after dusting it off) and found this viewtopic.php?f=4&t=1881 it's an old topic about "Steve" and his sweet ramjet.

I'm thinking about trying the ramjet behind pulse-jet thing again but I have a couple of questions.

Steve has his injector behind the flameholder (intake>> flameholder>> injector) shouldn't it be in front of it?
Can I scale this engine to any desired size (i.e. x2?)
would it be a good idea to create an intake spike for this engine to difuse the high speed air from the pj? And if so, how?
The last attempt with my "Talon 4" failed because I couldn't light the engine: can I use a sparkplug? any other suggestions? should I partially block the ramjet intake during starting to prevent an imediate blow-out?

[Irvine.J]

Hi Metiz,
I believe steve's injector was behind the flameholder but directed forward into it, not ideal but it worked for him.

Honestly ramjets IMO are a bit of a black art, truely only quantified by proper testing and development.
However a few basic principals and parts remain true.

A ramjet can be built to any size but I would still follow guidelines set out by Becker, but then as your target speed increases things would need to change, your ramjet will need to be longer to allow more time to slow oncoming air etc.

Inlet spikes are always fun but hard to manufacture, but the biggest problem i've found is generating enough air flow to run the things Maybe get a solid block of balsa, and cover that with an epoxy with a high temperature hardener like 2404 (Araldite product) or 5163, and add some talc to it as a gelcoat. Because its behind a pj, its gonna burn. Only metal will really hold up to it, but have fun forming a nice curve with it. Maybe you can find some pre made stuff online that will do the job? The balsa will give you something to screw into to hold the thing in place.

On my large engine, I welded a bolt to the flame holder, so the spike would thread on, then could be located with some bolts going through welded on nuts up the front.
you made a Talon? Link me? A spike is an aid to change air pressure on its path to the flameholder, also provides a nice way to get fuel injectors in there cleaney. Helps break down a shock front a few other factors. Beyond my understanding, but not beyond my fascination.

[metiz]

you made a Talon? Link me?

Trade ya for some injector info.

I've made a scaled up, slightly modded version of this ramjet, drawing below. Tell me what you think

[metiz]

No realy, tell me what you think. Anyone?

[ganuganu]

the ramjet sequence will be like this

inlet - fuel injection - flameholder (this is the right sequence)

placing a spike in front is a good thing,but i think its a tricky job because they convert the incoming kinetic energy of air into pressure energy..To which engine you are going to fix this,because the inlet dia of the ramjet depends upon the exhaust dia of the pulsejet engine,and in my opinion try to use an convergent divergent nozzle..i think there is no need to block the inlet of the ramjet during starting..And best of luck with your design..

[metiz]

This will be powered by the M25. Nozzle dia on both of the engines is about 100mm. Why would I go convergant divergant?

[ganuganu]

i have explained in the picture,in my opinion CD nozzle is good to use..

[metiz]

Ok that explains it, but how does a divergant nozzle get me mach 1+ speeds if at the divergant point it isn't yet. there's no added energy at that point, if anything I would think the gas slows down with a divergant nozzle at the end. Like a straight pipe pj vs a cone exaust. the straight pipe will have higer speeds and temps

[ganuganu]

valveless pj's are different from ramjets the only similarity between them is both are hollow no valves required..

[metiz]

still doens't explain the speed increase. And I doubt that all ramjets have a mach 1+ exhaust speed.

[ganuganu]

its not that all ramjets produce mach 1+,i just did that diagram for explanation purpose..

[PyroJoe]

The RJ43 presents a nice example.
http://home.earthlink.net/~chadslattery/id5.html
(1st picture)


Most high speed ramjets fall close to a Sears–Haack body
http://en.wikipedia.org/wiki/Sears%E2%80%93Haack_body
(2nd picture)

from the ARLA site:
http://home.earthlink.net/~altaccel/tri ... -basic.htm

Thrust
The ramjet net thrust is the difference between two opposing forces, drag and thrust.

Drag comes from the intake air being slowed down (for compression), flow around internal components like the flame holder, and flow along the skin (internal and external). Thrust comes from heating the air to high temperaures and accelerating it. To maximize the net thrust you need to minimize the drag and maximize the thrust.

This means designing an inlet that flows the right amount of air smoothly, finding the lowest drag flameholder that works, keeping the air smooth through the engine, and designing a nozzle of the right size.

To maximize the thrust you need the highest temperature you can achieve. This comes when the fuel to air ratio is stoichiometric. That is, every atom of oxygen in the air has the right number of fuel atoms to go with it.

These may sound daunting but need not be.

A visual review of pictures of several ramjet engines shows that the inlet and nozzle throat are typically the same size. Further, their cross sectional areas are about half that of the combustion chamber. Typically these are circular, because that is the most efficient, but are often oval, square, rectangular, or other shapes.

Some flameholders, like the can type, have very low drag and are very efficient at maintaining a flame. They are also fairly simple to build. The rule of thumb on can-type flame holders is to have enough holes so that the sum of their cross sectional areas is at least 50% more than the cross sectional area of the combustion chamber. There are probably equivelant rules of thumb for other types of flame holders.

Operating the ramjet a little off of stoichiometric does not lose much thrust. Most operational ramjets are operated a little lean to keep the combustion temperatures down. For short flights, such as are discussed here, a little excess fuel may ensure complete combustion of the air and helps keep the flame going.

[metiz]

Thanks for the heads up Pyro, but can you give some comments on my design as posted here? I think I'm ok with most of the components (bar a few). Is the inletspike nesessary? I'm not realy looking for super performance (well, I am but lets stay realistic:P) but I do want the engine to be able to maintain a flame even behind a pj

[PyroJoe]

what I say here as not whole cloth, for I have not built these types.
(at least not on purpose)
If the core gas from the PJ is reasonably close to the spike it will likely melt it. (even in stainless) I would remove it from the design.

What you have there is close to Deckers subsonic jet, although the rear cone angle is just slightly steep. Also the intake could be increased slightly to give an area near 3990 sq.mm

The idea of placing a ram jet behind a PJ is somewhat self defeating I think. Most engines require a considerable change in temperature to reach reasonable efficiency.

The pulse jet has already consumed oxygen from the gas and heated it.

About the worst 2 things you can do to a ram jet is feed it low density heated gas with depleted oxygen content.

The intake would need to be designed to haul in as much or considerable more flow than the PJ produces, sadly the velocity decreases when this is done. Sad because ramjets don't get interesting until they reach high velocities.

It makes considerable more sense to place the PJ within the ram jet so the inflow air is not preheated and the highest change in temperature can be achieved within the ramjet body.

The heat radiating off the PJ will add to the change in temperature and reinforce the ram jet operation. Unfortunate in that this becomes either a wind tunnel, or "in motion" condition.

[metiz]

Good points al around. I didn't realy take into consideration the heat comming from the exaust, all I was concerned about was the oxygen. I now doubt if it would make any thrust difference. I would like to argue your comment on a melting inlet spike though. There is no way in hell that a pj exhaust, when exiting the exaust, is more than 1200 degrees celcius, that's just absurd in my oppinion.