Winter fun with the Scouts

[Bingo]

   For the past 3 years I have run the winter camp for my troop. I have come up with a few fun projects for the kids to build and use at camp.
   The first year was a 1/2 in. PVC dog sleds that the boys built and raced. The second year was a 1/2 in. PVC catipults for throwing snow balls. Last year was 3 in. PVC cannons using innertubes to propell the snow ball.
   Now what can I do? I have been racking my brain to find a new Idea and have come up empty.

1. We need to build 3 or 4 for team compitions. I give small prizes to the winners.
2. The boys have 3-4 hours to build the contraption with little adult guidence and no             measurements. This make the boys think and read a ruler.
3. NO FLAMIBLE LIQUIDS OR COMPRESSED AIR MORE THAT A BICYCLE PUMP.
4. Keep it cheap.I have to keep costs down that is why I love PVC.

   My boys call me the Assistant Fun Master because I come up with  crazy Ideas and things to build.

[Dweezil]

Given your time limitation...3-4 hours, you might want to be careful with compressed air.  PVC cement can take as much as 24 hrs to reach full cure and may fail catastrophically if you try to use it as a pressure vessal.  This takes even longer in the cold.

BTW...frozen cylinders of ice make GREAT air cannon projectiles.  I just cut short sections of PVC the same diameter of my barrel, debur and stretch 2 layers of Cling wrap over the bottom, fill with water and freeze overnight.  Pull out of cooler a few minutes before firing so they unmold easily.  Perfect fit for the barrel and no mess to recover. A little food coloring in the water makes it a bit easier to track the flying projectile.