Designing for a particular speed

[Hawking]

There is a wealth of information available on the net in regards to basic ramjet design. That being so, I've been doing a lot of reading lately. I've learned that ramjets can, in fact, can operate at modest airspeeds (~200mph), and that their upper speed limit is about Mach 6-7. With such a wide speed range possible, what is the speed where they are most efficient? I suppose this means most fuel efficient, or the most thrust output for the least fuel input.

Also in my research, I found that there are basic design criteria for ramjets. What I haven't found is what makes a good low speed engine vs what makes a good higher speed engine. I'm guessing that the flameholder has the most impact on this, but the inlet seems to be critical as well. Are there guidelines to follow based upon the specific design speed of a particular engine? Obviously, standard aerodynamic rules apply, and as speed increases, the interior boundry layer plays an increasingly important part.

Lastly, and unrelated to the above; with the exhaust tempuratures being as high as they are for ramjets (and pulsejets as well), what is the best construction material? Stainless steel is the most common as far as I can tell.

-Don

[Stuart]

Glenn used to have a website where he had a nifty graft that showed the ramjet efficiency peaking at about mach 2.4

[tealster]

Why doesn't someone try extending the augmentor intake to a point forward of the ramjet intake so the incoming air would act like a high forward speed...? A ducted ramjet! Centrifugal flameholder? Static ramjet?

[Bruno Ogorelec]

Why doesn't someone try extending the augmentor intake to a point forward of the ramjet intake so the incoming air would act like a high forward speed...? A ducted ramjet! Centrifugal flameholder? Static ramjet?

Are you talking of a pulsejet augmenter used as an afterburner? [The afterburner is just a species of ramjet.]

It's been done, but I have no idea of the thrust or consumption figures. The one case I know of is Bruce Simpson. It was a very primitive arrangement.

On a slightly different tack, why do people often think of putting an afterburner after a pulsejet. I'd rather do the opposite -- put a pulsejet after a jet engine.

Why hasn't anyone thought of a pulse afterburner?

Ordinary turbojets with reheat waste fuel by burning it in this huge inefficient ramjet-like thingy. Why not use a pulsejet instead of ramjet? Pulse combustion is more efficient. there's no need for a flameholder. Given the great inlet pressure, making the thing pulsate in self-sustaining fashion would be a doddle.

What I am saying is, make a dump combustor, with discontinuity instead of a diffuser, have the turbojet blow into it and inject fuel. There's plenty of oxygen left in the turbojet exhaust, and it's heated, making ignition easier.

Forget about noise -- reheat produces huge noise whatever you do. Forget vibration -- reheat is only used for very short periods. A few rubber rings and/or hydraulic couplings will do it.

We are talking of pulsejets as inefficient, but ramjets are rather worse than that, yer it is the ramjet that's used for reheat of high performance turbojets. Switching to pulsating combustion would be a big improvement. Instead of burning $ millions per minute, this will only burn hundreds of thousands.

[Mark]

One of those subsonic-ramjet-valveless-pulsejet-chimeras which would probably fit Foa's comment that the line of demarcation is blurred, ...subsonic ramjets benefitting from pulsating combustion.
Mark

[tealster]

No.. The reason for bringing the intake of the augmentor forward of the intake of the engine is that the incoming rush of air through the augmentor and out the exhaust is that a well placed inlet to the engine would benefit from that rush of air by way of a "shearing effect" similar to what happens to the exhaust end, but in reverse. If you educt a fast enough movement of air, some of it is bound to impinge into the intake
tract so that the engine becomes self feeding without any other external device needed. It also seems reasonable to me, since a flame holder is needed and some flywheel effect is helpful as well as compression, that a torous chamber would benefit from the swirl of gases by entrapping and using centripedal force to achieve some benefit from, at least, the mixing of gases if not the compression part! I will send you a microlathe concept of one of the design ideas that help explain it.
Thanks[/list]

[Zoch]

On a slightly different tack, why do people often think of putting an afterburner after a pulsejet. I'd rather do the opposite -- put a pulsejet after a jet engine.

Why hasn't anyone thought of a pulse afterburner?

Ordinary turbojets with reheat waste fuel by burning it in this huge inefficient ramjet-like thingy. Why not use a pulsejet instead of ramjet? Pulse combustion is more efficient. there's no need for a flameholder. Given the great inlet pressure, making the thing pulsate in self-sustaining fashion would be a doddle.

What I am saying is, make a dump combustor, with discontinuity instead of a diffuser, have the turbojet blow into it and inject fuel. There's plenty of oxygen left in the turbojet exhaust, and it's heated, making ignition easier.

Forget about noise -- reheat produces huge noise whatever you do. Forget vibration -- reheat is only used for very short periods. A few rubber rings and/or hydraulic couplings will do it.

We are talking of pulsejets as inefficient, but ramjets are rather worse than that, yer it is the ramjet that's used for reheat of high performance turbojets. Switching to pulsating combustion would be a big improvement. Instead of burning $ millions per minute, this will only burn hundreds of thousands.

Afterburning turbofans have supersonic exhaust, and thus pulsejets would be unsuitable.

[tealster]

A near vacuum can produce a supersonic speed on an incoming stream which is only incidental to the outgoing exhaust, I think, due to a narrowing of the flow of the air coming through the leading part of the air inlet. The exhaust flow that educts the incoming air serves to set up the near vacuum on the inlet. The rest should be apparent.

[Bruno Ogorelec]

A near vacuum can produce a supersonic speed on an incoming stream which is only incidental to the outgoing exhaust, I think, due to a narrowing of the flow of the air coming through the leading part of the air inlet. The exhaust flow that educts the incoming air serves to set up the near vacuum on the inlet. The rest should be apparent.

Again, as with the exhaust, local supersonic speeds do not matter. The intake charge is dumped into a larger volume combustor where it slows down dramatically before ignition. That's what removes the need for a flame holder.

[Irvine.J]

Ok so I figured out an idea how one "MAY BE" able to get enough airspeed into the ramjet body without any external air source.... feel free to shoot holes all through this....

So, I was playing with my air compressor, and a piece of wood I had cut while mucking around on boxing day, that looked like an "X" shape, and I had just drilled a hole in the centre and stuck a little piece of an round metal through. Anyway, I had the compressor charged up and for some reason decided to give it max pressure with the little gun nozzle to see how fast I could make it spin, the result was "damn fast." (See where I'm going with this) I then look at the gauge which said around 100psi and then at my gas tank... See where i'm going!

My gas tank reads a massive 3.3mpa or 480 PSI as I believe most would...
henceforth, if one was to inject the gas nozzles directly onto a free spinning impellor blade, or even better had the nozzle ejecting from the sides of the impellor blades themselves, your (Equal and opposite) reaction physics would/should turn one very fast impellor. Not only that, but create a zone that was not only rich with air but excellently mixed with fuel!

Simply, as its free spinning, by turning up or down the gasflow would increase/decrease impellor speed, but at high speed, the airflow would keep the impellor turning extremely fast but you could turn down your fuel flow with little effect... there must (or atleast I hope) be a way of using the pressure difference to suck in more air, what do you think, something like this?

[larry cottrill]

James -

It's a form of "reaction turbine" (as opposed to the usual "impulse turbine"), similar to the action of a "Hero's Engine" - the earliest turbine known! I like your idea of coupling it directly to (actuall, more like making it part of) the impeller. There's no reason not to try this. Of course, like most "great ideas" it is probably fraught with unforeseen difficulties that will become evident later ... ;-)

Intuitively, there should be some advantage to the fuel jets firing "across" the intended airflow. Because each fuel jet IS a jet, keep in mind that the velocity of the fuel coming out will be very high (in fact, it will NEED to be high to make the impeller work efficiently). I think this would be a really good design for tiny nozzles and liquid fuel. One of your difficulties is going to be providing a sealed bearing that will let the fuel into the ports leading to the fuel jets - like the rotortip engine fueling problem.

Reaction turbines aren't used much. It is fairly easy to make toys with them; much harder to make efficient power delivering machines.

L Cottrill

[Glenn Olson]

Don,

I believe your question was not answered in full so I'll give it a try. First, I suggest you take the time to review my web material at http://www.alt-accel.com/arla/3m-arla.htm and http://www.alt-accel.com/ramjet2.htm. I spent a couple thousand hours on research and writing and prefer not to repeat myself.

That said, I admit I was not terribly clear on designing for a particular speed. For most subsonic combustion ramjets the airflow through the combustion chamber is about 200 fps (as I recall, it's been a long time) regardless of the outside airspeed or Mach number. This is, of course, once you've significantly exceeded the 200 fps. Therefore, the flameholder does not play a significant part in the design for a particular speed.

For subsonic ramjets there is no inlet spike. The inlet is similar to that of subsonic jets, and you can peruse these via the internet. Inlets intended for slower speeds will have more rounded inlet edges while those for higher speeds will have sharper inlet edges. It is the inlet that is designed for a particular speed or speed range. Subsonic ramjets have converging exhaust nozzles because the exhaust normally does not go supersonic and therefore does not require choking.

Supersonic inlets typically have a spike, or a wedge like the F-4. Higher speed designs have longer spikes because the angle of the shock wave off the spike is dependent on the Mach number. The most efficient operation is typically when the spike shock wave hits the inlet lip. Some designs, such as the SR-71, have moveable spikes to improve efficiency at various speeds. Others, such as the F-104, have fixed spikes. Some have no spikes, such as the F-16 which will go supersonic but is more efficient at subsonic speeds. The ASALM, tested in the 1980s, had a fixed inlet similar to the F-16, was designed for M 2.6-4.0 but went to M 5.0 when the throttle stuck. Supersonic ramjets also have converging-diverging exhausts which cause a shock wave to form across the narrowing. The exhaust velocity in the combustion chamber is subsonic but the shock accelerates it to supersonic speeds for better efficiency.

If you want to design to a specific speed I recommend you find some jet aircraft designed for the speed you are looking for and try to imitate their inlet shape. For small hobbyist ramjets mild steel should be good enough but stainless would be better. If you plan on going above Mach 2.5 I suggest you coat the exhaust with a ceramic or ablative material.

The information above is somewhat simplistic but should suffice for hobbyist level work.