Materials

• Empty Soda Bottles
• Fruit & Vegetables for Detonation (Cabbage (Korean, large), Watermelon, Pumpkin, etc...)
• Fruit & Vegetables for Launching [Apples, Pineapples – Note: Apples should have a diameter larger than that of the PVC pipe to be used (and > 3 cm min.)]
• Water
• Dry Ice

Procedure

1. Planning is key! – This is quite a popular spectacle and people will want to see this. It is ideal to schedule this event and have shows every hour (or so) and have them coordinate with big events such as the Magic Show so people leaving one can head to the next.
2. Ensure the lawn area of the Davenport Garden is TAPED OFF this will prevent people from wandering on the lawn either during the shows or before/after them and thus complicating clean-up and resetting the event. The fruit can be set up anywhere on the lawn but ideally not closer than 10-15 ft. from the fountain and not further than the end of the walkway. Spectators will be safe on the other side of the fountain and as well will be safe along the walkway – this is a popular area (with a small amount of fruit shrapnel for the kids). No one under any circumstances should be watching from the lawn unless it is near the walkway towards the front door of MP (physics), other than the demonstrators of course.
3. In order to make it worth people’s time each show should consist of (at minimum) 3 large fruit and 2 small fruit. This will keep people satisfied (largely)! Encores should always be performed if the crowd desires so and the fruit supply permits it.
4. There is significant benefit for digging small holes in the ground in which to place the bottles that will be put inside cabbages – this ensures the planted charge will not roll or fall to one side and has been shown to improve the success rate and immobilize the charge (the holes need not be very deep, just enough to put the bottle in a earthy groove). While it should go without saying in order to “plant” the PVC pipe in the ground and provide reasonable structural stability when a fruit is hurriedly placed atop it one must ideally dig a hole with the same diameter as the pipe about 2 to 3 inches deep and place the pipe in – ensuring a tight fit; this can be supported further by packing the pipe in with soil removed to make the hole.
5. You will find that as the demonstrations progress the hole will fill with water, though this has no effect on the demonstration. Additionally it is always a toss up as to whether the fruit will be launched and leaves the pipe in the ground or if the pipe will be blown out of the ground, in this case all that must be done is the pipe must be re-planted.
6. An additional noteworthy point is that the pipe can be of varying length with negligible effect on the outcome. Finally, users should always insert the primed charge [bottle] cap down.

Background Science

From our past experience the following points should be clarified.

Bottles:

The bottle should be ideally shaped based on the fruit. For watermelons that are longer than they are tall water bottles work best with pop bottles (750 mL) making suitable substitutes. The bottles that can hold higher pressures (pop bottles) are especially ideal for launching of fruit from the PVC tube. Bottles were obtained by “dumpster-diving” and while this is economically ideal since it allows for many bottles to be obtained quickly and without expense (except the students dignity) the integrity of the bottles is not guaranteed by this method and can lead to the occasional “dud” resulting from the use of a bottle that had been previously structurally compromised. In general pop bottles are more reliable than water bottles… If this is to be avoided the ideal situation is that the people involved save a number of bottles over a period of time that they have themselves used. This will guarantee them to be structurally sound and clean. The required spherical coke bottles (Fig 1) should be purchased to ensure you will have them – they are not trivial to find, but can be found at Costco/WalMart – especially at Christmas time.

Fruit:

Fruit are ideally obtained from a grocer in China Town as undoubtedly a deal can be struck if the fruit are all being purchased in one lump transaction. Fruit can be obtained either the afternoon prior or the morning of to ensure freshness (partly rotted fruit or old fruit lead to unfavorable odors). Watermelons are prepared by hollowing a hole through the center of the fruit using a knife and a spoon. This hole should be water bottle sized (in diameter) and can go all the way through the fruit or not. By placing the bottle in the exact center of the fruit the best explosion is obtained. The criticality for having the bottle thoroughly within the fruit ensures sufficient room for the bottle to swell prior to rupture (pretty important for watermelons). If the fruit is compromised structurally by the blast but still remains intact the detonation can be attempted again and usually operates successfully. With cabbages a similar pocket is to be carved (ideally removing the stem or trunk since this does not explode well). This should be sized such that a custom bottle (spherical coke bottle) fits inside the vegetable but is slightly sticking out on the bottom. Apples require no preparation and pineapples should be prepared simply by cutting off the bottom 1 inch of the fruit (or about 2 cm) to give a flat bottom. This is then placed on the pipe and forced down with a rotating force to engrave a groove in the fruit that matches the pipe. This will ensure the pineapple remains tightly affixed on the pipe after the charge has been set. If the pineapple is placed on crooked the odds of it exploding rather than launching vertically become greater. Apples almost always can be recovered unless a bottle of exceptional pressure tolerance is used. Pineapples can sometimes be re-used, though this is not easily predictable.

Note: The PVC pipe required for launching fruit should be available through ChemClub – this is of course assuming no one lost it after the 2011 Science Rendezvous; ensure this is kept every year to make your life easy! 😛

BLOWING STUFF UP!

The protocol is simple. You need a bottle, water (the fountain will suffice) and a supply of dry ice. While quantified values cannot be provided the following operating instructions and empirical observations will be shared. The volume of water in the water bottle generally controls the time it takes for the bomb to go off. The more water, the shorter the fuse, period. However, the more water that is used, generally leads to a less spectacular explosion.

WARNING: Too little water (or no water) will be extremely problematic. If too little water is used the water will freeze and the bomb will not go off leading to an armed and dangerous bomb that must be dealt with appropriately. One can either attempt to sequester the bottle between two blast shields and try to release the pressure, or simply wait until the bomb goes off on its own (this can take 20 min or more!) In the mean while it is extremely unsafe to approach. The optimal volume of water that allows for a reasonably quick detonation and a strong blast involves filling water bottles approximately to the bottom of the label (roughly 1/3 of the bottle). TOO MUCH WATER CAN LEAD TO RAPID DETONATION! OPTIMAL RANGE: ⅓ through to ½ the volume of the bottle.

With the appropriate volume of water in each bottle, one can prime the charge by adding dry ice to the bottle. The optimal quantity is about 5-6 standard sized pieces per bottle. More will either not make a difference or freeze the water. Too little will simply cause the detonation to take longer. While the gas is evolving from the bottle and the cap is readied all that remains is to seal the bottle and put the fruit on top. To prevent shooting off the cap one should always place the bottles cap down and then put the fruit on top. Typically detonation requires 30-60 seconds and as such upon sealing the bottle you have time, but should still operate swiftly to prime everything and clear the area. The only piece of advice for this is that the operator must be confident that they have fastened the cap of the bottle tightly when planting the charge – under no circumstances should you approach to check after. When launching fruit – a partner is required as one person will fasten the cap and drop the bottle in cap down while the other partner places the fruit on the pipe.

Hope these instructions have been helpful, please contact Science Rendezvous should you require any further information, and happy explosions!