Dual Augmentors and Air Cooling

[cbromano]

A friend of mine builds hovercraft as a hobby. We want to make a jet propelled version but safety has become a real issue. If the thing catches an edge at high speed it will most likely flip. The problem is that we would have to mount the engine low for balance reasons and far too close to the driver for comfort. I've been toying with ways to decrease temperature and sound from the jet here's what I came up with. Weight is not that important and the second augmenter is so I have an easy way to mount fins to vector the thrust.

1) Does this stop the jet even running properly?
2) Any significant amount of extra thrust caused by two augmenters?

CB Romano

[Guest]

Too complex. Start with a single augmentor because the shell around the engine will not work since it *draws* in air instead of ramming it down to a tight cone where it *rapidly* expands to create more thrust.

[cbromano]

I realize that the air "drawn" through the front would have little (read: no) acceleration due to thermal expansion like what you are talking about in normal augmentors. BUT here's what I was thinking-

As the air bypasses the jet it will absorb quite a bit of heat it would by no means be as hot as the exhaust but still very hot. It would be accelerated as it merges with the exhaust. Would that extra hot air create more power through the real augmentor due to extra mass being shot out at high temp?
I was thinking also of heat diffusing fins on the intake head like the Dynajet and also fins running the length of the exhaust tube to increase heat transfer (yes I realize that just welding fins down the length of the tube is a bad idea).

Never the less the main point is too cool the jet to where at least it is not red hot, extra thrust is a bonus. I’d just be happy if it worked.

CB Romano

"I don't pretend to have all the answers. I don't pretend to even know what the questions are. Hey, where am I?"
~Jack Handy

[Hank]

Hello, CB- I tried using an engine shroud in one of my earlier experiments.
This does not work and here's why not. The pulsejet requires the exaust end to be open to the ambient atmosphere for the admission of the reversion wave (compression). Any disparity in flow or pressure will throw things off, possibly to the point of disfunction. It doesn't take much.
For an enlightening bit of information regarding Augmentors (Ejectors)
check the NASA Technical Report Server. There is a recent paper authored by D. Paxton on "Ejectors". Hank

[Guest]

Hank,
you better call that ignition, because the "compression" is barely above atmospheric. The main idea there as I see it is to let the column of hot air enter the engine to ignite the mixture without any resistance, so that it happens faster.
Shrouds would work as a halfway ramjet, so I`ll go for simplicity - a single large augmentor. If you want more, perhaps a dual-stage augmentor would work, kind of like a gluey intake just schrunk in length and a bit wider openings.

[Mark]

"I don't pretend to have all the answers. I don't pretend to even know what the questions are. Hey, where am I?"
~Jack Handy[/quote]

Deep thoughts. I like platitudes like that, he always had something ironic to say. Too bad about his untimely demise.
Mark

[dynajetjerry]

Hi, Guys,
While reading some of the comments in this thread, I recalled the design of the Dyna-Fog generator. The pulsejet powered products of Curtis Dyna-Fog, Ltd. and the original DFG from Aeromarine employed discharges that introduced ambient air into the jet's exhaust. The DFG's discharge was about 4 in. ID and the jet outlet was about 1.5 in., extending very slightly into the discharge duct. It served to cool the outgoing gasses and help the insecticide-laden oil to condense into a fog; it also sucked out the air that surrounded the engine (inside the shroud,) and helped cool the engine and whole package.
This was NOT the case with Aeromarine's smoke-screen generators for the military. All discharge air went through the engine, the smoke oil serving to keep internal temperatures from rising too high. In fact, a thermocouple-driven thermometer served as the generators' oil rate indicator, having a special dial that registered a high rate at the cool end and a low or no rate at the hot end. Of course, it was useless until the machine reached normal running temperatures.
Jerry

[Eric]

I have tried a lot of things with the dynajet, and there are all kinds of things that you can do to it like this.

As long as airflow isnt too disrupted or the pipes and augmenter start resonating in really bad ways, then the engine will run. If you run it on propane you can do even more abusive things that would normally cause problems with a fuel venturi setup.

I would put the end of the pulsejet pipe at the very end of the engine shroud that way it can pull in air from the shroud/augmenter opening, and the shroud wont act as a bottleneck for the jet exhaust.

I wouldnt count on much or any thrust being made by the shroud, but like Jerry said it will help keep the engine cooler, as well as whatever you put the engine next to.

Eric