Friday, 15 August 2008

Join the Jet Set!

Gosh - it's been nearly a whole year since my last post. Gulp. What have I been up to? Well, I'm nearly at the end of my first book (my publishers won't be happy that if they find that I've been writing this instead of the final chapter...) Things have been incredibly busy on the tutoring and training front as schools in the UK gear themselves for the new 14 to 19 Diplomas...

So, that's the excuses over with. What about exciting experiments? Here is one that Jess and I attempted this Wednesday - the jam jar jet. Except we didn't have a jam jar. So we had to make do with a beer bottle. And it all got a bit explosive. Here's how it went...


When Francois Reynst was a young boy he discovered that if you poked a hole in the lid of a glass jar, put a small amount of neat alcohol in the bottom, let the alcohol exapourate a little, and then light the top of the jar, flames shoot out of the top of the jar and then get sucked back into the jar again. Then flames get ejected out of the jar. And then get sucked back in again. And this cycle repeats until all the fuel is used up. In this post I tell you how one of my students and I tried to reproduce Reynst's experiment. Many thanks to Jess for helping me out with this one.


We didn't have a jar. And we didn't have any alcohol. We had:

  1. lighter fluid
  2. matches
  3. assorted glass bottles (from the recycling bin)
  4. blue tac

What we needed was a vessel that would allow the air and lighter fluid vapour to circulate easily inside it. After careful consideration and detailed mathematical analysis (ok, I squinted one eye and guessed) we decided on a small beer bottle, which had a fairly large cross-section compared to its height (see picture - courtesy of Jess). What we are about to make is tchnically called a Reynst Combustor...


  1. Squirt a little lighter fluid into the bottle. The amount you need is really a matter of trial and error. A covering across the bottom of the bottle around 3mm deep seemed to work best.
  2. Seal the top of the bottle with blue tack.
  3. Put bottle in the freezer for 5-10 minutes.
  4. Take bottle out of freezer and shake vigourously.
  5. Unseal the bottle. It should hiss as air is drawn into the bottle. Light the top of the bottle now.
  6. The inside of the bottle burns with a vibrant blue glow for around 5 seconds and howls as it draws air into the bottle.

*DON'T* use a match or a lighter to light the top of the bottle in case it explodes (see the end of this post)

How it works

Air is drawn into the bottle and mixes with the lighter fluid vapour. When the vapour is ignited a burst of hot gas is ejected from the top of the bottle. This leaves a slight vaccuum inside the bottle which then sucks air back into the bottle. More lighter fluid vapour mixes with that air and, because it's hot in there, the mixture re-ignites and the cycle begins again. Besides being called a Reynst combustor, this is also called a relaxation oscillator.

How it doesn't work

If you get the ratio of oxygen to vapour wrong it does one of two things:
  • nothing
  • it explodes
We didn't manage to catch on film the jet working well, and there plenty of times when it didn't work at all. But we did record it exploding...

Further investigation

Check out YouTube for more videos of homemade pulse jets (that worked). The pulse jet was the engine that powered the Nazi V-1 rocket - called a "doodlebug" because of the distinctive sound a pulse jet makes...

Also check out the following web resources...