Tiny pulsejet, oh so small

Mark · Post by **Mark** » Wed Jul 28, 2004 11:42 pm

There's a little pulsejet for sale on eBay. Very small, the smallest I have ever seen.
Mark

Tom · Post by **Tom** » Thu Jul 29, 2004 12:04 am

eeedy bitty pulse jet heh.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 79911&rd=1

Tom

Mark · Post by **Mark** » Thu Jul 29, 2004 1:11 am

Yes, it's cute. I wonder what frequency it runs at at 6.5 inches, actually less the head it would be even shorter.
Mark

Post by **Mike Everman** » Thu Jul 29, 2004 2:05 am

Mein Gott! How can that work??????????????????????????????????????

Mark · Post by **Mark** » Thu Jul 29, 2004 2:24 am

Seems we should be making pulsejets that size. Just for the fun of it. At least it doesn't take a lot of tubing!
Mark

Post by **Mike Everman** » Thu Jul 29, 2004 2:33 am

I bet it makes a sound like: "wreeeeereeeeereeeee" LOL

Mark · Post by **Mark** » Thu Jul 29, 2004 2:44 am

Exactly my guess too.
Mark

steve · Post by **steve** » Thu Jul 29, 2004 2:48 am

I had considdered building a single valve engine like that for a while but decided against it in light of potential complications resulting from its diminutive size. After seeing this and after my experience with the acetlyne fueled mini logan (aka. "pipejet") it is starting to seem like a good idea again. how thick do you think the valve is? .002 maybe?

Mark · Post by **Mark** » Thu Jul 29, 2004 3:15 am

The thinnest blue tempered spring steel I have used is .003ths, but I bought a roll of .002ths if the need ever arose. .002ths is thin stuff and kind of delicate to take a beating. I think the Cox 049 engines use .002ths for the X-shaped copper/beryllium reed held in place by a spring in the gas tank region. It flexes very little and it is not really stressed. I guess if you were patient, you could find a way to use the foil-like thickness.
I wonder if you could unscrew that spark plug on the eBay micro pulsejet and put a side port on it and go Logan?
I've got some thin/light titanium tubing in the .75, .5, and .25 inch diameters. If only I had a way to transition the sizes with light-weight bell reducers.
Mark

Post by **Mike Everman** » Thu Jul 29, 2004 4:00 am

SMash, smash, smash!

Mark · Post by **Mark** » Thu Jul 29, 2004 4:18 am

It crossed my mind. My brother has some small perfect pulsejet shapes made out of brass tubing, they are foot long prototypes he made using the smash technique, it leaves flanges on either side of the exhaust tube, but I suppose you could use the flanges for mounting or something like mounts for little wings.
Mark

skyfrog · Post by **skyfrog** » Thu Jul 29, 2004 6:23 am

Since pulsejet is an acoutic engine, there must be a fast way estimating the resonant frequency from the length of its tailpipe. The first lowest mode gives the largest contribution, right ? Let me get my vibration analysis book first...

larry cottrill · Post by **larry cottrill** » Thu Jul 29, 2004 1:04 pm

skyfrog wrote:Since pulsejet is an acoutic engine, there must be a fast way estimating the resonant frequency from the length of its tailpipe. The first lowest mode gives the largest contribution, right ? Let me get my vibration analysis book first...

Horace -

One problem here is that you have to know [or realistically, assume] the gas temperature and density, since these profoundly affect the speed of sound [and therefore, the frequency].

I am convinced that the gas temps in these really small pipes are a lot cooler, on average, than the temps attainable in large engines, because in the little ones there is so much more radiation heat loss per unit mass flow of gas, because of the ratio of surface area to flow volume being so high. Maybe someone else would like to comment on this idea. But, I think the wave action in tiny engines must be quite a bit slower than in big ones because of the radiation losses.

I have often thought while looking at the tremendous size of the Argus engine [the mill for the V-1 "bizz bomb"] that I'll bet nobody ever saw that pipe run red hot. Does anyone know whether this was true or not?

L Cottrill

Mark · Post by **Mark** » Thu Jul 29, 2004 1:43 pm

I can remember using some anodized aluminum arrows I had in the garage, tubing which is slightly larger than a quarter inch diameter. I cut short ~6 inch lengths of it and perfume-like atomized a fine mist of methanol into the lengths and held one end closed with my finger while lighting the opposite end. It's very challenging to get anything more that a loodle sound, but it can make a high pitch sudden ruffff sound.
I suppose making a 6 inch pulsejet would separate the men from the boys. Either that or it would put everyone in a mental institution. I've done an 11 inch long pulsejet with a single flapper .003ths reed and a washer for the valve plate. I used some 3/4 inch plumbing pipe for the chamber but it is more like 7/8ths in actuality I think if you measure the inside diameter.
There is also little copper tubing fittings that you could use for this project. If you flare one end of the exhaust tube and slip it in the bell fitting it will lock air tight against the walls of the reduction. That gives you the basic pulsejet shape nicely.
Mark

skyfrog · Post by **skyfrog** » Thu Jul 29, 2004 6:03 pm

Larry Cottrill wrote:One problem here is that you have to know [or realistically, assume] the gas temperature and density, since these profoundly affect the speed of sound [and therefore, the frequency].

Yes, you're right, and the fact that I know nothing about gas dynamics has stopped me from analyzing it via this way.

If it wasn't mistaken, when self-sustained state reached, the pulsation frequency of working gas and one of the natural frequencies of engine body is in synch, right ? This gives me a clue to solve this problem using solid mechanics, instead of fluid mechanics.