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Air Rockets

Public summary: 

5, 4, 3, 2, 1, Lift Off! With a lemonade bottle and a bike pump, we'll show you how to make a rocket! How high can you get it to fly?

Launching lemonade bottle air rockets along a rope
Useful information
Kit List: 

* Pump
* Bung with hole or with valve
* Rope
* Lemonade (or other fizzy drink) bottle with tube attached for the rope to run through (something with low friction so that the bottle will come back down again and fly further).
* Launch stand

Frequency of use: 

Launching lemonade bottle air rockets along a rope (going up at an angle of 20-30 degrees) - this is a less messy version of the water rockets experiment which can be done inside in a suitably large hall.

Set up the rope running from the launch area to a higher point (raising the launch area, e.g. blocks, can keep the rope above head height) - run the rope through the tube attached to the bottle. Jam the bung in the bottle and get kids to pump until the rocket launches up the rope. Hopefully it should come back down to somewhere you can grab it. Keep people out of the area where the rope is so that they don't get hit by the rocket or walk into the rope!

As a further point - it can very occasionally be done without the guide rope pointing vertically upwards, or without a guide rope at all. This is suitable in large sports halls or outside only. Here care needs to be taken that a large enough area is cordoned off so that it does not hit anyone when it returns to Earth. If a guide rope is not being used, it can still be helpful to attach a rope to the rocket, in order to limit its range (such as to avoid hitting people if you can't cordon off a large enough area) - however, care must be taken to ensure the rope itself does not pose a hazard in such cases.

Explanation and Demonstrating

Like the water rockets, everyone likes to play with the air rockets... and they can't fail to notice the experiment since it makes a bang every time the rocket goes off!

If you want to explain the experiment, you'll probably find it best not to let the kids get onto the launch platform until you've talked to them a bit. It's generally best to ask a question first to see what they know already.

There's quite a lot of physics you can explain with the experiment. Firstly the idea of pressure - you can talk about the pump - put your finger over the valve and get them to push down on the pump - they'll find it gets harder as they push down and increase the pressure. So we have the idea that as we pump the bottle up, we increase the pressure and this pushes the bung out. If they understand (ask them to explain it again!) you can talk about molecules. The molecules move at about 500 ms-1, so even though they are light give a good push on the walls of the bottle when they hit (plus obviously there are > 10^23 of them!). So, as you pump up the bottle, adding more molecules, there is a harder push against the wall and the bottle gets harder.

So what happens when the rocket launches? We've said that the bung gets forced out by the pressure in the bottle (what pressure it reaches depends on how hard you jam the bung in each time, and varies, and thus does the height the rocket gets to). You probably don't want to talk about conservation of momentum with the kids - the bung flies out backwards with a certain momentum, and thus to give the overall system zero momentum the bottle gets equal and opposite momentum, sending it up the rope. You can talk about equal and opposite reactions, though. Think about leaning on a wall - does it push back on you? Most kids probably think not. However, if you get one of them to push on your hand, and you push back, you both stay where you are. If you ask them to stop pushing, then you fall forwards (or at least pretend to!) - so leaning against the wall, the wall must be pushing against you! Going back to the rocket - when you pump up the bottle the air is pushing against the bung - when the bung pops out it's like stopping pushing against your hand, so the rocket flies forward. You can show this with throwing a bag of rice; as you throw it it pushes you back. Another example that kids might know about is cannon/ gun recoil which is essentially the same physics as this. The cannon moves backwards and fires the shot forwards and in the same way the bung moves backwards and fires the bottle forwards.

You can also talk about energy - where does the energy for launching the rocket come from, and where is it stored? The more pumping you do, the higher the rocket goes (all else being equal).

Kids will keep coming back to have another go, so each time ask them something about it to see what they've remembered!

N.B. A good trick with families or other groups is to challenge them to get the rocket as far as possible along the string (maybe with only one attempt!) little do they know that the distance pretty much only depends on how tightly you put in the bung, and thus the meekest among them can be the mightiest!

Risk Assessment
Date risk assesment last checked: 
Mon, 16/12/2019
Risk assesment checked by: 
Date risk assesment double checked: 
Sun, 05/01/2020
Risk assesment double-checked by: 
Risk Assessment: 
Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Bottle (projectile) Hitting people with the bottle All 4 2 8 Keep people away from the rope (which the rocket travels on) by blocking off any area where the rope is below head height with chairs, hazard tape or similar. Make sure the person launching the rocket doesn't lean into its path. Also locate the experiment sensibly, bearing in mind walkways that people will want to use in the venue.
If done without a guide rope, cordon off sufficiently large area to ensure it lands within it. Check prevalent wind direction as well by test firing the rocket before people come.
If using the rope to limit range, the areas in which the rocket might land (accounting for prevalent wind direction and lean of the launch stand) must still be cordoned off as there are two hazards - both from the rocket falling on someone and the trip hazard due to the rope.
2 2 4
Bottle (pressurised) Bottle exploding (slim risk) All 2 2 4 Check bottle for cracks and other damage before use.If a cracked or damaged bottle is found, it should be cut or marked to show that it is not for use if it cannot be disposed of immediately. Only fizzy drinks bottles should be used, as still drink bottles may not be suitable for pressurising. 1 2 2
Rope Walking into/tripping on the rope which the rocket travels on All 4 2 8 The provisions for ensuring people aren’t hit by the bottle keep visitors away from the rope. Keep rope ends tucked away and off the ground if possible. 2 2 4
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Publicity photo: