Bets
The story behind it comes from a drunken bet. Well it wasn't that drunken, but as those of you who know me well will agree, if you give me some utterly geeky or silly or engineeringy challenge I have to accept. It went something like this:Me: "stirling engine blah blah"
Elliot (a righteous longboarder and water polo-er): "Cool...blah... but JET ENGINES"
Me: "Yes, but want to build a stirling engine like so"
Elliot: "CHALLENGE: I bet you £100 that you will not build a working jet engine in 2 years from today."
Me: "... GODDAMMIT!" *shakes on the bet, agrees terms*
And so it was decided. The terms of the bet are pretty simple. I must make, not just purely from a kit, a working jet engine that generates thrust. The bet is off if I build a steam powered longboard. Which is kinda fair, cos either would be totally awesome.
Pickles and Meths
The most basic form of jet is the "pulse jet" as used on the doodlebug bombs during WW2. Valveless are the simplest to make, but harder to make work as efficiently as valves. The simplest one however is like so. I however used a large pickle jar (not the picture, thats just to show you what the concept looks like), and a very crude hexagonal hole I got by jamming a whole screwdriver through the lid then twisting the handle around. Using methylated spirits I managed to get one good 3 second jet, with roughly 5" of jet out the top of the jar. I didn't measure the experiment in any way really, but as a proof of concept it works. I also produced a 2 foot high jet of flame at one point. That was interesting.
The idea here explained on the website but as a quick summary:
Jet works by causing an explosion i.e. a wave that goes out in all directions. Directing the forward chunk of the blast backwards increases forwards thrust. making this redirected blast take a much longer/shorter route compared to the backwards blast means that when the first blast in over air will rush back into the blast chamber faster from one of the jet nozzles, making the other jet much more efficient at providing thrust.
This example is one I chose because thats what I basically did with a coke can and a baked bean tin; the coke can had the ringpull end opened fully to the can walls, the sides crushed so the cross-section of the can was square at the ringpull end to allow air in, and a hole in the bottom, which was going to be the jet nozzle. I then flipped the can uside down, nozzle aimed at the sky, poured meths into the bean tin, and wedged the coke can into the bean tin. Together they formed the chamber, the nozzle was simply the hole in the can, and the gaps between tin and can walls were the gaps around the chamber.
Shaking up and lighting it worked. Not as well as the jar, but it worked for a few gentle revvs. I intend to build a larger, more refined version (from tin cans) powered by butane camping gas, or possibly spray-gunned meths.
Yes I did wear goggles,yes they did do nothing, but thats because nothing tried to asplode my face. Thankyou and goodnight!
Pulse Jets
The next part of the experiment was based on the Tharratt design (thanks to this site, although the pics don't work here anymore):The idea here explained on the website but as a quick summary:
Jet works by causing an explosion i.e. a wave that goes out in all directions. Directing the forward chunk of the blast backwards increases forwards thrust. making this redirected blast take a much longer/shorter route compared to the backwards blast means that when the first blast in over air will rush back into the blast chamber faster from one of the jet nozzles, making the other jet much more efficient at providing thrust.
This example is one I chose because thats what I basically did with a coke can and a baked bean tin; the coke can had the ringpull end opened fully to the can walls, the sides crushed so the cross-section of the can was square at the ringpull end to allow air in, and a hole in the bottom, which was going to be the jet nozzle. I then flipped the can uside down, nozzle aimed at the sky, poured meths into the bean tin, and wedged the coke can into the bean tin. Together they formed the chamber, the nozzle was simply the hole in the can, and the gaps between tin and can walls were the gaps around the chamber.
Shaking up and lighting it worked. Not as well as the jar, but it worked for a few gentle revvs. I intend to build a larger, more refined version (from tin cans) powered by butane camping gas, or possibly spray-gunned meths.
Yes I did wear goggles,yes they did do nothing, but thats because nothing tried to asplode my face. Thankyou and goodnight!
Posts
No comments:
Post a Comment