Metal choice and welding.

[Mike Everman]

...but if it's a self sustaining engine, the faiure mode is to collapse as Bruno says. The cycle alternates between partial vacuum and pressure, and the weakest the duct can be is buckling inward due to the vacuum. It will fail that way before it gets a chance to fail the other way, in which it is orders of magnitude stronger.

[Mark]

I remember Don Laird telling me a funny story of a Tigerjet that he was running and the spark plug vibrated loose from the nut on the inside of the combustion chamber, like the Dynajet has to hold the spark plug in place. He had his head above the Tigerjet when it happened and the little spark plug lifted off and plunked him in the forehead. It's a little 1/4 32 plug that is adjustable, one that you can take apart. I replaced mine on my Tiger with an American made one, for the adjustable one was not as trustworthy and made a lesser spark. The center piece wasn't even straight but more of a banana ceramic, very cheaply made, but did have that feature of adjustability, although mine had the propensity to creep instead of arc to the outer jacket/threaded wall.
Mark

[ed knesl]

Man, not trying to be a smart-ass, and I don't mean any disrespect at all, but A friend of mine welds pipes for a DOW Chemical plant here in Georgia. I'd be willing to bet that Dow is as strict as Boeing when it comes to weld inspections, most of these pipes he welds are meant to carry large amounts of very volatile chems, & gases. Everything he does is x-rayed and inspected by 3 different inspectors. He does most all his work with an arc-welder.. Also, weaker weld joints may not be such a terrible thing in p-jets, especially larger ones. From what I understand it is possible to make one explode, whereas a weak spot might just save your #%@, spltting a seam would be a heck of alot less violent than an actual metal shredding, shrapnel slinging explosion

... No comment ... !

Ed

[El-Kablooey]

Whoops, I thought I had read somewhere, (possibly on this site) about a large pulsejet, and the possibility of it exploding during the starting sequence, and that the builder had anticipated this and assembled the pj so that it would come apart rather easily, this payed off when the pj did pop and the easy breaks aslo made for easy repair and no injuries, but apparently I was mistaken or simply misread the document.

but on another note, wasn't someone on this forum simply spot welding their pj's??? that would be the easiest method for me to attack, having a home-made spotwelder that I built from a couple of old microwave oven transformers.. I could also weld with oxy-acetylene, but I have never used my torch for anything but cutting, and some brzing/silver-soldering, I will have to practice welding with it, I'm a fair welder with a stick but i'm afraid thats no help to me when dealing with such thin sheet..

[ed knesl]

but on another note, wasn't someone on this forum simply spot welding their pj's??? that would be the easiest method for me to attack, having a home-made spotwelder that I built from a couple of old microwave oven transformers.. I could also weld with oxy-acetylene, but I have never used my torch for anything but cutting, and some brzing/silver-soldering, I will have to practice welding with it, I'm a fair welder with a stick but i'm afraid thats no help to me when dealing with such thin sheet..[/quote]

Yes, I am the proponent of PJ spot welding.
It is the easiest and cheapest method. Spacing of spots ~ 1/4 - 3/8 in.,
overlaps ~ 1/4 in.
I TIG weld only, where spots are not possible at all.
It provides the best structure without heat distortions and also looks the
best of all welding methods. ( That may be individual judgement )

If you need any further infos, just ask.
You may take a look at my older posting EDK0401B Engine.
All spot welded, except the intake to CC connection.

Good Luck !

Ed

[El-Kablooey]

That RULES! I'm all for cheap and easy BJ's, errrr.. I mean PJ's :)
I'm gonna go get some steel right now and see what i can whip up, I'll bet with a little elbow grease a man could spotweld an entire lockwood!

[El-Kablooey]

Ok. makes sense to me. I shoulda saw that. Hell, as a youngster experimenting with "firecrackers" :) of the homebrew variety I had more than one steel pipe become a one blast rocket engine instead of the desired uniform disentigration, even when filled with explosives like AP.. so at the moment i feel like a bit of a moron thinking one could detonate a steel pipe with an open end with a shot of propane and a leaf blower....

I am thinking that the initials B.S. must have a double meaning huh????????

[Bruno Ogorelec]

I am thinking that the initials B.S. must have a double meaning huh????????

Well, you will not find many opponents of that view in this forum :o)

[Mike Everman]

Hey, El-K, what's your name and story, dude? We usually ask this sooner, seeing as how we endlessly discuss the best way to make F/A explosions very fast... It's nice to know who you're talking to. ;-)

[El-Kablooey]

Man, you know what these pj's make me wanna do for some reason, build a fully auto spud-gun, of course it would have to shoot golf balls (thinking about the driving range covered with balls down the road, I would send them all back of course ;) and would be nothing like a pulsejet, more like a two-stroke shooting pistons.. just a dream I guess..

Oh yeah, name's Joey, and no story here, just a country boy, creeping up on 30, with a faithful 7 year-old sidekick named Zach. I have always had a fascination with things that go BOOM, as safely as possible of course. I'm addicted to tinkering, always think I can build anything, I'm often wrong but I always learn something cool trying.. My role model growing up was a split between MacGyver and the dude who's gotten filthy rich making "Girls Gone Wild" videos (bastard has a freakin Leer jet!) man why didn't I think of that one........ anyway I'm trying to learn as much as I can about PJ's here lately, these are very cool in my opinion.. I need a big one on my canoe.. and one on my kids old "Power Wheels" Humvee.. and most of all I need one because I have some hick neighbors who believe 3 Blue Tick hound dogs make great backyard pets, they apparently can't hear these stupid mutts howling at nothing 24/7, but I'll just bet they'll hear a big pulsejet engine running wide open 20 feet from their bedroom window :)..

Well, anyway, good to meet you guys. some of you seem to be even nuttier than myself!

Later!

[Bruno Ogorelec]

some of you seem to be even nuttier than myself!

Well put, Joey. You latched onto the fact very quickly.

[Rossco]

Somehow, we seem to attract the most amaizing characters in here!

Ha, you sure are one Joey! Great to have you on board.

Ive got a auto spud gun design in here by the way somewhere! I was thinking of shooting ping pong balls tho. A little less leathal i would think than golf balls.

Rossco

[El-Kablooey]

you're kidding me!?!? where can I find those plans?? and yeah, ping pong balls would be smarter, but the golf balls would be free :) not to mention they are tuned to fly, you might could even achieve something faintly resembling accuracy firing golf balls.. just watch out for ricochets!!

[larry cottrill]

I would think that golf balls would give very poor accuracy because they react so strongly to spin. A golfer has a lot of control over the applied spin via club face angle and technique, but it seems to me that coming out of a smoothbore gun, the final spin would be essentially random. In that case, surely a perfectly smooth ball would be better, wouldn't it?

L Cottrill

[El-Kablooey]

I totally agree with you, if shot from a smoothbore gun there would be a pretty random pattern of shots, but it wouldn't be TOO difficult to give the barrel a primitive rifling. I've also seen 'airsoft' guns that apply a backspin to the plastic BB's to help with accuracy. I'm not sure how they work, but I'll investigate