General Ramjet Info Please

[thecheat]

actually, ramjets become more efficient at speed, to a point where they're more efficient than a turbine... (that's at a few mach though...)

Also, a ramjet is just a "standalone" afterburner. But, it's the pulsations that make it hard to incorporate in a PJ.

PS: as far as I know that's how you measure thrust, the leaf blowers don't produce much at all, so that's not MUCH of a problem. You do just deduct it from the overall equation though.

[Zippiot]

Ramjet goods and bads

GOOD
lightweight
virtually maintenance free
high thrust/weight ratio
realitively simple is design and operation
can run almost any fuel (but so can a turbine)


BAD
fuel efficiency below mach
noise
minimun airspeed


A ramjet is a great little guy when design properly. Ramjs do work best above mach but there may be something to using them below. A ramjet is cheap simple and requires almost NO maintenance. Bad thing is that fuel efficiency below mach is pretty bad comapred to turbines, but at less than 1/1000 the cost it is a small comparison. A big issue is minimum speed, possibly in UAV's a little rocket booster can be used to get it up to speed, that adds to runnign costs (but is still butt loads cheper than a turbine).

My 2 cents: RAMBOFAN
Design a ramjet-turbofan combo. At low takeoff speeds the turbofan feeds high speed air into the ramjet. While the turbofan and ramjet can be run at the same time, maintenance of the turbine can be cut in 1/2 if the ramjet is the sole provider of thrust once up to cruise. Would work fine in a UAV but in a passenger jet may cause too much noise. The ramjet gets pretty fuel efficient around 500mph, so the big comparo to turbines becomes quite blurred on the fuel efficieny line. Also a smaller turbine can be used, saving money. If I had a lathe/mill I would be working on a rambofan right now...need a lathe/mill...

Picture afterburning the air from the turbofan at first, then removing its blades and shutting down the turbine.

[jthompso]

Otherwise there's the tried-and-true method that was used on the SR-71 blackbird. The Pratt & Whitney J58 was essentially a turbojet inside of a ramjet--as air speed increased the turbojet was throttled back so more air would bypass it and be fed into the afterburner. Something along the lines of the J58 on the hobby scale would be very impressive, and maybe even possible. I think that at mach 3.2 about 80% of the thrust of the blackbird was provided by the afterburner.

[Jim Berquist]

I agree! the SR-71 did the conversion from jet to ram jet.

It's initial intake orifice was like 12" around the intake cone was ended something like 1/4 inch.

It then converted the intake trough the engine and used the after burner as a Ram Jet. I don't know if they by passed the jet or passed the stream through it. It worked as a Ram Jet! Something held back the pressure from the C.C. and caused thrust! Thinking they was running on the after burner ! That leads to Ram Jet.

don't Know!

Jom

[jthompso]

Air that wasn't used by the turbo-jet was ducted around it to the afterburning ram-jet engine. Here's two sites with some good pictures of the the operation and operating procedure of the engine:

http://www.wvi.com/~sr71webmaster/j-58~1.htm
http://www.personal.psu.edu/cxc11/24_ID ... AMJETS.PPT

[Zippiot]

The problem with using a model sized jet is that most are centrifugal, which are short and fat. A short fat ramjet is kinda tricky, the entire device would be huge!
I think a side by side would be ebtter than a all in one...

The turbojet can be inside of the jet itself and the ramjet can be strapped on to of it, outside where the passing air will cool it.


Alcohol is another good liquid fuel its not too expensive evaps very quickly and gives a pretty blue flame. Denatured works great, add some boric acid to make pretty green flames!

[jthompso]

Assuming that you use a "traditional" sub-sonic ramjet (such as one designed from the information on this site) the flameholder occupies a large percentage of the cross-sectional area of the ram-jet which means lots of drag. I've never done any model flying myself but I would imagine that the extra drag incurred with an external ram-jet engine while it was not operating would make it difficult to get it up to operating speed.

As a side note: has anybody put an afterburner onto a model jet engine yet?

[multispool]

Yes, I made this one last Spring.
Did some preliminary test runs on Jet A1 in the summer. but haven't done much since due to pressure of other interests.
It has a two ring V Gutter stepped Flame holder and six injectors.

[Mike Everman]

Whoa, Multispool!
When will you play with it again and post some vids???
Impressive.

[Dave_G]

Yes, I made this one last Spring.
Did some preliminary test runs on Jet A1 in the summer. but haven't done much since due to pressure of other interests.
It has a two ring V Gutter stepped Flame holder and six injectors.

Very nice work, multispool!

During your test runs, did you get a chance to measure thrust (afterburning and non-afterburning)? Is there much pressure drop across your flameholder?

By "stepped" flameholder I assume you mean the two v-rings were not in the same axial plane?

Dave

[multispool]

Well, the design criteria was to see if it was possible to get a realistic looking short flame cone at the exit. Extra thrust wasn't a priority, that engine already gives around 130N. The main problem is that it needs about six pairs of hands to test it!

The Flame Holder is the same as on the J-59. At least that was what I think it was lying outside the Hanger doors at the Midland air museum, Coventry. The outside V gutter is located about 1/4 diameter rearward from the inner.

I was getting a substantial rise in pressure between Turbine and Flameholder which was upsetting the engine and nearly had a nice fire with a seepage of Kero out past the Injector holes. Ignition was easy with a blowlamp but difficult using the spark plug. Think it needs a longer reach plug to get the sparks further in near the center.

So as you can see, it needs a lot more development work on it when time permits!

[ZVWOLF]

Hiya guys,

Okay... I am sort of getting my ramjet together... a friend is making biodesiel in chemistry so i though i would also see how that stuff goes in a ramjet compared to propane...

I have attached some pics of my parts so far... not much but still... i am going to be welding the parts together instead of that glue. I found a stainless steal cup in a shop today and i thought it would be great for the exhaust section of the ramjet... In the last attached picture i have put the parts together in a different disign to the usuall ramjets.

I am not sure if it will even work... but i don't really see why it shouldn't but i believe it would create more drag. Anyway check out the attached picture for a better idea.

Also i am creating a 3d animation of how the ramjet works... hopefully it comes together nicely... I will be using it in my assignment but it should be a good visually guide on ramjets for any beginners like myself... i will post it on this forum when i am finished.

Anyway, What do you think of my design... should it work? Also has anyone here experimented biodesiel and ramjets?

Thanks

ZVWolf

[Zippiot]

Flip the front section around.

It wont work without that!!

I know its counter-intuitive but as the air slow down it increases pressure, so the combo of a flameholder and a big expansion creates high pressures inside the jet. But what you can do is make a 3 section ramjet. A big scoop speeds up the air, then chop an expansion chamber shorter than the big scoop (so they dont just ballance out) then attatch exhaust.

You are going to need a flameholder, a decent easy one can be found at any hardware store and many grocery stores. A stainless steel sink drain cover, stops the goldfish from slipping down the drain when you clean its bowl. It gets welded in the middle of the expansion chamber, usually where the 2 cups meet is fine.

[thecheat]

hmm... the real problem I see is that the two mugs don't seem to have the same diameter in the large part.... you may have to get another one...

however, it does look like you're getting closer. But, do you have a welder? (this would have to be a TIG) I know I'd prefer to not use JB weld so you could run the ramjet longer... but, if you don't, it's not a problem. You'll just have to keep your runtimes rather short.

[larry cottrill]

If you really don't want to weld, you could cement it together with fireplace cement. You can get it in caulking gun sized tubes at big DIY stores like Menards, Lowes, Home Depot, etc. It comes in various colors, and will stand any temperature you can produce. The only problem I can see with using it is that it dries rock hard, and might break loose due to the metal's greater thermal expansion. I made some intake flares for pulsejets with it, and it worked all right - but those don't get to really high temps like a combustion chamber. It also is somewhat brittle if dropped or struck with a metal tool or something.

I actually thought about making an entire valveless pulsejet out of it, but abandoned the idea because the thin shell would be so easily broken in handling. One other guy did make an FWE type jet out of something similar, but I don't think it was successful if I remember correctly. So, it's just something else to try.

Welding is by far the best way to go, if you can do it (or hire it done) - the big difficulty is the thinness of the metal.

Here's something I said in another thread - one of the few things I've said about ramjet building that might be worth repeating:

One real problem with us amateur designers is not really appreciating the nature and importance of the diffuser. This makes us too willing to mess with it. ... What we need to remember is that the diffuser is (my opinion here) BY FAR the most important element of the ramjet design!

...

The diffuser is the device which produces the ENTIRE OPERATING PRESSURE of your engine. Period. ... In the ramjet, all you have is what you can wring out of the incoming air by smoothly slowing it to a crawl with that simple cone. What you get is an increase in pressure, density and temperature, with the velocity magically reduced to a speed that will work in the combustion section. Nothing else will do - whatever you do, you must not screw that up!

L Cottrill