50 mph ramjet???

[redneck]

OK, Jim Berquist, as I hope you are reading this... remember back when in the post the great ramjet challenge, you were asking about a 50 mph ramjet? Have you done anymore thought to this, if so I think i have an idea for you. Now forgive me because i had typed a book already for this posting, then went to send, hit save on accident, hit backspace to go bak and click send and gasp!!!!! it was all gone, so this will probably be a shorter version... here goes...

the problem with a 50 mph ramjet is that there is almost no way to get winds moving at 50 mph to do enough work to compress themselves enough to creat any useful thrust of of a ramjet of any proper deminsions. if you have an intake large enough to gather the amount of air needed, then what will happen is the amount of drag created by the intake will overpower the thrust generated and slow the jet down to much to work. How to remedy this????

Spin the jet. If the jet is rotating with the outsidewalls moving at about 400 mph, then you have a vortex of 400 mph winds moving through the jet at 50 mph. to design a flame holder, just create a vortex generator to spin the air in the opposite direction just enough to slow down the spinning air for a suffient presure increase, but not entirerly stop the rotation of the air. ( it is easier to think of the air as spinning and not the jet ) then make the exhaust portion of the jet in such a way as the rush of exhaust air will spin the jet to creat the effect of spinning air in the jet. what you have done is increased the apparent lenght of the difusser and apparent speed and preasure inside the jet. the air doesnt HAVE to move fast, it only has to THINK it does...


ok guys.... blow my idea to pieces!!!!!!!!!!

[Mike Everman]

Wellllll, 400mph on a 4" diameter is hmmm 34,000 rpm for starters. So they counter rotate for 17,000. fun fun, run!

[Najm]

Good idea for a static thrust producing ramjet...
How to rotate the ramjet though??

[redneck]

mike, you said : Wellllll, 400mph on a 4" diameter is hmmm 34,000 rpm for starters. So they counter rotate for 17,000. fun fun, run!

you lost me on what you are talking about where you say they counter rotate for 17,000.... what is they that counter rotates???? and why????? i sugest slowing down the spinning air by having a vortex generator...... hmmmmm wait.... 17 is half of 34..... are you sujesting that we slow the air down by half the rotation using reverse rotating veins? I was thinking of only having blades that double as the flame holder that would have a pitch opposite to the rotation of the air and these blades are simular to stator blades, they are fixed to the jet and do not move independantly of the jet. This I THINK will boost the pressure and slow the velocity of the inner air without having to have any moving parts....

As to HOW to spin the jet, well I was thinking along the lines of stator blades in the fast moving exhaust air to cause the jet to spin, or have outside veins to the outside airflow around the jet to rotate it. or just hook it up to a 1 hp model airplane motor, and turn it into a type of backwards inside out and sidways motor jet, having it spun by the 1 hp motor. Larry might like that idea.... modify Ol' Maggie for the application.... but I am NOT saying NO foward air speed will be needed, I do think 50 mph will be good enough though, sooo.... hook up a ducted fan in line with the ram, on a common shaft, and spin that sucker up. that would be at static, then add foward air speed as the model flies, and we could have some serrious thrust..... i think it could work.... even if we have to insert some tesla veins in the intake of the ram jet to help with air induction......

[Najm]

The ramjet will be moving btw...
Nice idea, will require some calculation if you are proposing stators in a ramjet, but then the ramjet will not remain a ramjet... It will become something else, dunno what...

[redneck]

an inside out turbine????? naw.... it'll stay a ramjet, if the stators double as a flame holder.... and if this works, we can hook it up to a model airplane engine to spin it at high rpms, then we have something that could actually fly! And boy would it move!!!!

[larry cottrill]

I'd like to encourage this once again. I am convinced that a low-speed ramjet is possible. Its basic problem will be that the pressure obtainable will always be limited, so it will need to be fairly bulky to provide reasonable T/W ratio. Even so, your plane will need very lightweight construction. But, I think there is absolutely nothing "impossible" about doing this -- you just have to get past the inherent inefficiencies in a low-speed design with its serious compression limitations.

How about using a "water rocket" as a "JATO" launch system? If you're going to be the first on your block, why not take it all the way? Ha.

Good luck! Get one flying, and amaze everyone!

L Cottrill

[redneck]

Larry, you dont think we can get higher pressure by spinning the jet? if you had a 400 mph vortex of air spinning through a jet at a foward speen of 50 mph, you dont think it could be compressed to sufficient pressure to make some real thrust and do some major propulsion?

Out of curriosity, take said 400 mph spinning vortex at a foward speed of 50 mph, how would you use it to make a ramjet work? I do have a few ideas, but I would like some other takes before i put mine out there. never mind on how to spin the jet at all, just make that a given, maybe a few people could present a few ideas, and togeather we can actually put togeather a design that will really do the job and do it well. I would like to see what anybody else can think of?

[larry cottrill]

Sorry, sir -- I completely forgot that part of the discussion! It's great to be getting old. Ha.

The only problem I can see is the difficulty of making the front end behave like a good diffuser while it's spinning at high speed. In basic priciple, the action is something like a rotor tip jet, but much more compact in radius. I think there would need to be a multi-duct design to the diffuser part. Then, the sections would be carefully designed for proper diffuser flow, and each could have its own little flameholder associated with it, even if they had a common chamber. I don't see how you'd get it to work without "compartmentalizing" it in that way.

We have to always remember that it's the diffuser section that makes or breaks a ramjet design. Get that wrong by just a little bit, and you have a non-performer. Of course, good diffuser design might be easier to get right at high flow speeds than at very low ones.

L Cottrill

[Najm]

uhh wont thrust be directed in the direction of the rotation??
just a thought....
would have to angle the thrust in the chamber a bit if you want some forward motion....
the stators are the key....

[larry cottrill]

Stepping back to the basic premise, that we should try to achieve a simple low-speed ramjet:

At one point in my old (1948) design book, they show a turbojet combustion chamber designed by BMW. Although this is a turbo combustor, I think it has some useful points we can learn from. Unfortunately, I don't know if I can ever find a way to scan it in, but I will try to describe it to see what you think:

The front end of the chamber is a fairly wide-ratio diffuser, much more "blunt" than we would normally expect. Embedded entirely within the rear half of its length is a smaller diffuser. The outside contour of this element is bullet-shaped, apparently to maintain laminar flow between it and the outer diffuser. Its interior forms a fairly conventional conical diffuser shape, though again rather short. Entirely embedded within the forward half of this element is what appears to be a fairly small annular flameholder structure, a kind of "half toroid" oriented in the usual direction, so its cross section resembles two little textbook ramjet flameholders. The fuel is sprayed from a point just slightly ahead of this flameholder, so the 50-degree spread of the spray almost fills the "donut hole" in the center as it expands rearward into the center of the second diffuser and then onward into the combustion zone.

Now here's what makes this fairly cool: The inlet speed for the outer diffuser is 350 ft/sec (106 m/sec), obviously much faster than our 50 mph target; BUT, by the time we get to the full diameter of the chamber (just beyond the rear of the secondary diffuser), we are down to only 90 ft/sec (27 m/sec !!!) -- an incredibly slow flow at the flame front! So what this proves is that it is possible to stabilize a flame front at a VERY low flow speed if it's done properly. Of course, this is a fairly draggy setup, but at the low speeds we're talking about, that doesn't matter; what matters is being able to stabilize the flame at the front of the chamber where we want it.

Now, this still doesn't give us a 50 mph inlet speed -- 90 ft/sec = 61.4 mph if I've calculated rightly, but at least we're in the ball park. We could certainly get a little compression driving a 90ft/sec flow if we worked from a 100 mph forward speed (146.6 ft/sec or 44.7 m/sec), for example. This is an easily attained speed for a model aircraft; the problem of course would be how to achieve it fairly quickly via a practical launching mechanism (while arranging for engine startup at the same time - ha).

It may be that the most practical thing to do would be to design the ramjet, add a big electric ducted fan to the front end so we'd have a motorjet for takeoff, and then retract or jettison the fan out of the way after adequate flight speed is attained. Such a scheme is not exactly hassle-free, but at least it would give you an engine that you know is running before you attempt to launch.

If there is any interest in this, I could try to draw up a sort of "slow ramjet version" of the design I've just described.

L Cottrill

[Mike Everman]

To my mind, the neat thing about a motor jet at flight speed would be that you could run the fan at whatever speed you want to get the diffusion you want through it. ie, running it slowly when you are going fast(?) Skip the whole diffuser grid and let it be the fan blades.

[larry cottrill]

To my mind, the neat thing about a motor jet at flight speed would be that you could run the fan at whatever speed you want to get the diffusion you want through it. ie, running it slowly when you are going fast(?) Skip the whole diffuser grid and let it be the fan blades.

To me, the trouble here is that you're boosting velocity, not pressure (I mean, if you eliminate the diffuser by just substituting the fan for it). If your speeds are slow and you have a really well-designed flameholder for it, you might get stable combustion, but what would that do for you without measurable pressure gain? (I grant you that in a low-speed engine, any gain will be small, but it seems to me you have to have SOME pressure gain to make the expansion in the chamber mean anything.) I guess another way to say it is that you don't want to end up with a ducted fan followed by a thrustless burner! Recall that even in turbojets, you have to have a heck of a diffuser behind that big "compressor" section -- it really isn't a compressor without it! (I suppose in some cases, the diffuser is considered the front end of each combustion chamber, but it's still there in some form.)

Secondarily, I would also worry about the roughness coming off the fan, even if it's just idling in the wind. It seems to me that any roughness is going to create paths for the flame front to break up and push forward, which can't be very good.

Then again, maybe I'm misunderstanding what you're saying. A slow ramjet has to be thought through in a lot different way from a fast one. In a fast ramjet, the problem is keeping the flame front forward in the chamber where you want it, so you can take full advantage of the gas expansion. In a slow ramjet, the problem is going to be keeping the flame front back where you want it, i.e. prevent it from surging forward into the diffuser section (however you happen to configure it).

L Cottrill

[Mike Everman]

well, it's all about slowing the air down to get compression, right? an adaptive fan (read: controllable rpm) gets you as much or little diffusion (read: compression) as you need at flight speed. Whether it has enough guts to get you going from static is another thing.

[Ghrey]

If there is any interest in this, I could try to draw up a sort of "slow ramjet version" of the design I've just described.

L Cottrill

Yes there is interest. (!)


At least for my part...