Super Simple Heat-Recovery Ram Air Augmentors

[larry cottrill]

James and James, you gentlemen will especially like this one!

Here is a 'recipe' for really simple heat-recovery shells that give augmentation via 'heat-recovery ramjet' action as well as engine cooling. In reference to the drawing below:

All augmentors are single cones of circular section with small front-edge flares. The upper drawing is how it would be done on a "flashlight" style engine such as a Dynajet or the smaller Tiger Jet. The only driving power for the airflow in static operation is the shear forces of the exhaust leavng the tailpipe. The dashed-line portion at the rear indicates the option of cutting off the tail end flare. Speed flyers often make this modification, which weakens the engine only in static running, but improves performance at speed.

This "Dynajet" drawing is the ideal place to point out the main design principle involved: To develop the slight pressure needed as a means to increased thrust, we must have the right proportioning of the three NET cross-sectional areas between the engine shell and the augmentor shell. What we must have is:
- Net area at Station A significantly SMALLER than at Station C
- Net area at Station B significantly LARGER than at Station C
The first rule guarantees that we will have net forward thrust force; the second guarantees that the expansion due to heat pickup will create a speed increase in rearward air flow. Both will contribute to net thrust. These rules apply to all the examples shown in the attached drawing.

Even on the Lady Anne Boleyn Rev 06 engine, a single cone shell should be capable of effective augmentation - but in this case, of course, we can have not only the heat-recovery effect but an exhaust-driven augmentor effect as well! We shorten the cone to permit adequate wave de-coupling between the rear-facing intake flare and the front-edge flare of the shell.

The simplest, symmetrical form is shown in the middle drawing. This geometry is flawed in that it fails to take full advantage of the intake blast outflow, since a large portion of this flow will pass over the front edge of the shell.

The lower drawing shows how much of an improvement can be had just by tilting the shell slightly - about enough to make the top surface line up parallel to the engine centerline. The rear of the shell remains centered on the engine tail cone rear edge. It might be argued that this will throw things off by forcing a higher speed flow at the bottom (meaning the area on the opposite side from the intake) where air enters the shell, but I think this will be offset by the blast phase flow from the intake, which will be a very high speed flow. This geometry takes practically full advantage of the intake blast flow during static running. It is less advantageous at speed, when all the flow around the engine will be pretty fast.

Note how easy it would be to set up these simple augmentation shells for testing - basically, you'd just need three radial screws at the front and rear to hold the shell. The shell could be the lightest possible material - thin galvanized ductwork steel, even aluminum if you don't care about welding it on. Note that to work, this MUST be just as leak-proof as the engine shell, since we ARE theoretically developing a slight internal pressure. It would even be possible without too much trouble to have such a shell that would rotate from the the asymmetrical to the symmetrical geometry for high-speed flight and back again for low operating speeds!

There will naturally be a temptation to turn this shell into a "real" ramjet by adding fuel, afterburner style. This would probably work, BUT remember that if you do that, you no longer have cooling for the engine. You would have an uncooled pulsejet surrounded by a half-cooled ramjet! Doesn't sound good to me ...

So, what do you think?

L Cottrill

[ZSartell]

Nice work Larry. I have wanted to test this type of augmenter for a while, but never got around to it. I am just wondering how well it would work on a valveless engine. This will increase the airflow at the lip of the exhaust, hopefully supplying a stronger breath of fresh air during the intake phase and helping the exhaust gases get the heck out of the way. I wonder if you are able to increase the exhaust gas speed if the operational dimensions of the original engine would change?
Adding a little fuel and an ignition source would be very tempting and I think it needs to be tried. Just another project I never got to.

[larry cottrill]

James D -

Here's a revision of the simple cone heat-recovery ram augmentor designed just for your Lady Anne 'Twin Stacker'. You would make a simple cone with small flare, then flatten the front just behind the flare to the width that will contact the sides of the middle cone just ahead of the choke cone. It could be plug-welded there, or whatever works. A small plywood disk could be used to keep the rear edge round as the front is flattened.

L Cottrill

[James D]

I like it Larry, I'll try something like that once I've got it running stronger. I'd quite like to try a full length shell also at some point, to see how much it cools the combustion chamber walls while running static.

James D

[larry cottrill]

I like it Larry, I'll try something like that once I've got it running stronger. I'd quite like to try a full length shell also at some point, to see how much it cools the combustion chamber walls while running static.

James -

Good deal! Keep in mind, though, that the shells I have shown in this thread are not designed as ideal static mode augmentors - the basic idea is something that will improve thrust once we are in motion. That's why the heat recovery aspect is so important.

If you're interested in cooling, shells like this should do the job well. Just don't expect huge thrust improvements that can be measured in static testing, though there should be some effect in the right direction, of course. I just wanted to make sure everyone's clear on this point.

L Cottrill