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Pulse jets

Well recently I saw one of the few good things on TV, it was channel 4’s blitz street in which a set of houses were built to the classic 1930s design on a remote military site, using the materials used in those day and to the typical standard of the time. Next the TV company got a group of explosives experts and defence scientists to plant sensors all over the site and high speed cameras. Finally they subjected the street to a series of simulated German WWII bombs. Starting with a 25 kg TNT charge in a SC50 casing, then they went to 250 kg of TNT (not in a metal casing) to simulate a SC500. Then they went to 500 kg of TNT to simulate the SC1000, then they did it with RDX (scaled down charge) to simulate a V1 and then lastly they went for a subsurface charge of 1000 kg of ANFO to simulate a V2.

Here is the overview of the series

As you can guess the houses were in a bad way after this campaign of bombing. But you might ask what has this got to fo with pulse jets ? The V1 had a pulse jet power plant to create thrust, it was a very clever design but it did make a nasty noise which stopped when the bomb started to fall.

I looked into the pulse jet design and I discovered that it is possible to build a much weaker pulsejet using a jam jar and some ethanol/isopropanol. I found that a jam jar with a 10 mm hole in the lid with the alcohols inside can be made to pulse a few times. I found that if a second smaller jam jar is placed inside the larger jar then it tends to be able to pulse for much longer. Sadly when I find a good sized jar which pulses for a long time the heat tends to make the inner jar break. When the geometry changes the system stops pulsing.

Here is a film of one of my pulse jets in operation soon, but a word of warning. If you try to make a pulse jet then make sure you wear eye protection and take great care regarding fire safety. If you look at some of the films on youtube you will see how sometimes the jar can break creating a fire.

Also be careful of the fact that in direct sunlight it is hard to see an alcohol flame. very little light is created by an alcohol flame. Make sure that the pulse jet is not still burning before you open the jam jar ! If the mixture in the jar goes too rich then it tends to create a steady and small flame at the hole. If you open the jar at this point it is likely to set fire to all the alcohol in the jar. Myself and others have noticed that the jam jar pulse jet can have a overheating problem, I suspect that as it warms up the mixture changes from lean to rich. This change in mixture can stop the pulse jet working correctly. I hope to be able to experiment with altering the size of the hole to get the perfect pulse jet.

By the way I have tried acetone free nail varnish remover and it does not work as well as alcohol.

Disclaimer, while Dr Mark Foreman takes all reasonable care to comment on interesting chemistry and create safe and enjoyable experiments I accept no responsibility for any death, injury, disease, property damage, prosecution, criminal court case, tort, civil court case or other adverse outcome resulting from you trying out an experiment of mine or using any information I provide. All experiments are done at your own risk. If you are not an adult you must consult your parents and/or science teacher before trying an experiment. If you are doubt about your ability to do the experiment in a safe manner then do not attempt it. Also I accept no responsibility if your mum/dad/teacher/policeman/(insert some other authority figure) tells you off for doing the experiment. Also I am not responsible for your moral, social and spiritual wellbeing. Also if you hate the experiment and do not enjoy it or are left with an intense feeling of self disgust then again I am not responsible. If you read this disclaimer or anything else I write and have some adverse reaction to my words such as going insane, laughing yourself to death, pushing your blood pressure through the roof or some other effect then again I accept no responsibility. Also if you use information in my blog to answer an exam question then I offer no warranty to how good the information is. If you take material from my blog and try to pass it off as your own when you hand in your home work then I take no responsibility for you or offer any sympathy when your teacher tells you off (I hope that the teacher is reading this blog also)


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