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G.B) Stomp Rockets

Our son got a set of Stomp Rockets for Christmas just after his fourth birthday. These lightweight plastic missiles are about 200mm in length and are set in motion when a bladder is jumped upon. The compressed air leaves the bladder, travels up a flexible tube and through the hollow 'launcher'. The standard version provided many hours of fun, but Daddy thought there was room for improvement...

Firstly, the flexible hose tended to collapse and kink, so that was lined with some garden hose, which gave a distinct improvement.

To give the rockets some real boost, more energy had to be passed to the rocket than could be achieved by a four-year-old jumping on a plastic bladder. This is how:

Firstly, a three-litre drinks bottle had to be emptied. These can: take a lot of pressure; are free; are easily obtained; involve drinking lots of cider. In the interests of science, a cider bottle was emptied over the course of a weekend, and fitted to a 3/4" BSP black-iron tee that I happened to have. Just to be awkward the bottles don't have BSP threads, so the top of the bottle was carefully heated and screwed into the tapering BSP thread whilst malleable - some Milliput epoxy putty ensured that there were no leaks. A 15mm ball-valve with lever arm was threaded into the other end of the tee. A lot of 15mm plumbing fittings have 1/2" BSP threads but are untapered, so some PTFE tape had to be carefully wound to effect a secure and gas-tight joint. At the other end of the valve a short length of 15mm copper pipe allow the launching hose to be connected.

[At this point, I feel that I should point out that I don't generally consume cheap bulk-buy alcoholic products that are targeted at tramps & teenagers - my tastes are generally more refined. However, on this occassion I made the sacrifice in the interests of research, although I did find that after drinking it I had to overcome an urge to purchase a Burberry baseball cap]

To fill the bottle a schraeder valve from a car-tyre was inserted into a 15mm copper x 1/2" male iron fitting. When fitted into the limb of the tee (again with more PTFE tape and a 1/2" x 3/4" bush) this allowed the bottle to be pressurised with a car footpump.

Fifty pumps of the footpump takes the bottle to 30 psi. As we have taken the pressure from 14.7 psi to 30 + 14.7 psi then we have more-or less trebled the pressure (and volume) in the bottle. So, when the valve is opened and ten litres of air is given the chance to escape it sends the projectile off like a you-know-what - accompanied by suitably impressive noise. In fact, I am equally impressed by the cloud that forms in the bottle when the pressure drops so quickly (given the previous contents of the bottle it might even be a cloud of cider!)

The received wisdom seems to be that these bottles can be taken to 120 psi, but I am not sure how the joint at the tee-piece would stand up to that sort of pressure and with small children around it may not be a good idea to test it. As it is, the bottle seems to relax back to its original state after the pressure is released - if the valve is closed and then opened again thirty second later more air can be heard escaping.

The energy in this bottle is approximately:

= V1.(P2-P1)

= 0.003m3 * 206,842 Pa

= 620 J.

If this was completely and perfectly exchanged with the projectile then it would have a speed of 335 metes per second: however, it has probably left the launcher after a few milliseconds. Using the volume of the rocket, the velocity works out at about 60 mph.

Eighty pumps takes the bottle to 52 psi!

Using the copper-pipe outlet as a 'spud gun' by pushing into a potato, this 52psi managed to send a 15mm diameter x 70 mm long piece of potato into the air for just over six seconds. This works out (by my reckoning) to an exit velocity of approx 30 m per s.

The projectile needs to be lightly secured with sellotape so that it is released when the pressure in the launcher has built up sufficiently to break the tape. If this is not used, it is merely displaced from the launcher by the first few cc of air which isn't necessarily at the peak pressure.