Like any good Geek Dad, I have been exposing my children to MythBusters, and over the last year we have exhausted every episode on Netflix. Sometime last spring my oldest son told me he wanted to have a Mythbusters birthday party (later he tried to retract this, but his mother explained to him that dad was too far gone and this simply wasn't an option). So we started dreaming things up - what kind of science fun could I expose my soon-to-be 5 year old to?
- Explosions? Maybe in a few years...
- Death-defying stunts? I couldn't risk damage to the neighborhood kids...
- Mentos & Coke? Not bad, but messy...
Then I remembered this from Make Magazine, one of my all-time favorite Geek guides. So - we set about to build the air-powered rocket launcher from Make, but there were some problems we had to work around:
- The guide in Make suggested doing this with a hand-pump. My son and I wanted something for a birthday party, that would be shot a LOT - 5 year olds aren't going to stand and pump up a launcher more than once, and neither was I.
- We needed a way-cool launcher control. I could have done better in the end, but I like what we came up with.
- In order to blow some tiny minds, we needed some serious power behind our design.
Since this thing runs on compressed air, the answer to #1 and #3 was an air compressor. Before you start worrying about safety you should know that I checked all the numbers...
- 3-inch Schedule 40 PVC Pipe has a maximum operating pressure of 158 PSI and a burst pressure of 840 PSI, according to this site, which is so poorly formatted is must be accurate.
- The guide in Make suggests using a max of 75 PSI. Personally I've found a good rocket can easily fly at 100
- The air compressor I bought won't go above 100 PSI, and it can be adjusted lower
- I wrapped everything in several layers of duct tape, just in case something went horribly wrong.
So - with an air compressor I can (1) avoid burning my arms pumping this thing up for 100 shots and (2) reduce the time between shots significantly. It also ads an air of danger that makes my kid go nuts and take it seriously, so that's a bonus.
Adjusting the design to use an air compressor was simply a matter of changing the fitting from a bicycle pump style to an air-compressor quick-release fitting - so we were off to Home Depot.
As an aside - it's a lot of fun to go to Home Depot and tell the guys in plumbing exactly what you're doing. I've never had as much help trying to find that one plumbing reducer that was stuck in the wrong box as I did when I explained that we were building a rocket launcher for a birthday party.
By connecting the right reducer to the right fitting to the right threaded reducer to the right quick-connect part, we walked out of Home Depot 20 minutes later with a dry-fit design. All that was needed was some PVC cement and thread-tape to complete it. If you are new to compressed-air tools, I'd suggest getting the air-compressor first and taking a tool that fits it to the store with you - this way you can make sure you get the right quick-connect size.
Next up was the rockets themselves. Keep in mind that this is for a group of 25 or so 4-6 year old kids, so working with exacto-knives, glue guns or anything else that could burn, mame, or otherwise disfigure was out of the question - the rockets had to be nearly pre-made. As much as I wanted the kids to learn about rocket-engineering by building their own, I'd have to settle with letting them decorate blanks.
First thing I tried was pipe-insulation. This was inspired by a stomp-rocket toy that we had bought, used, broken, fixed and lost some time ago. This worked nicely at low pressure, but (a) split on the launch pad at high pressure and (b) went in crazy directions without fins. When I tried to solve (b) by hot-glueing some fins on to one of the prototypes, the insulation melted. Pipe insulation was grounded.
Next we tried some thick cardboard tubing. It was a lot bigger than the pipe (1.5 inch tube against a 3/4 inch launcher) but physics didn't seem to mind. For the prototype we wrapped the end in electrical tape and dropped it on the tube - pumped the launcher to 50 PSI and it flew clear across the yard. After a few shots we hot-glued on some fins and it flew straight and smacked the big oak in my back yard with a very satisfying THUNK. We had a design. Here's the material list in the end:
- Cardboard tubes. Find a local fabric shop and call them up - the one near me was thrilled to let me come take as many as I could carry. Up-cycled and free material - double win.
- Electrical Tape. 1 roll should make about 10 rockets.
- 1/4" Foam Board - hit a hobby store, it's about $2 / sheet
- Poster Board - 1 sheet is good for about 25 nose cones.
- Hot glue gun
- Small saw
- Exacto-knife or Box Cutter
- Large Can of beans - or anything else
- Straight-edge and an engineer's square
How to make a bunch of rockets:
- Cut the carboard tubes into 12-18" lengths, using the saw. This will be the rocket bodies
- The foam board will make the fins. DON'T SKIP THE FINS - not only are they important for aerodynamics, but the make the rockets look cool...
- Cut the foam board into strips, approximately 1/3 the length of the rocket bodies you made in the first step.
- Draw rectangles along the strips - keep your angles straight. The width of the recatagle will be the same size as the bottom of the fin - anthing from 2" to 5" is cool.
- Draw lines bisecting the boxes into right-angle triangles. Think Pythagorean.
- Cut every line you drew.
- You will have piles of fins. Put them in a pile.
- Hot-glue the fins to the cardboard tubes. If you have a good gun and good glue this is a simple process, and you don't have to have them spaced exactly.
- Wrap the top in electical tape. Be sure you don't leave any air gaps.
- Make nose-cones from the posterboard:
- Use the large can to draw a bunch of circles on the posterboard.
- Cut out the circles.
- Cut a pac-man mouth into each one
- Overlap the mouth to form a cone and give it a little hot-glue.
- Set the nose-cones aside.
In the end I spent about $0.12 and 5 minutes on each rocket. I didn't pre-mount the nose-cones to make it easier to stack them in a laundry basket, but do as you wish.
Some of the great things about these:
- You can fire them into tree trunks about 20 times at high pressure before they get damaged. Statistically they will be lost or lodged in a tree before won't fly.
- They are made of plain cardboard, so with some stickers and markers, kids can go nuts personalizing them.
- Cheap - seriously cheap - and make a way-cool party favor.
The final key to the experience was a launcher button. There are some requirements here:
- Battery Powered
- Visual Safety. I wanted to know at a glance if the laucher was "hot" or not.
- Distance - I wanted the kids to be several feet behind the launch pad.
- A really big button
We created a simple circuit that ran the power from a pair of 9-volt batteries, wired in serial, to a safety switch, then a launcher switch, then down some wire to aligator clips that fastened to the valve (it's the same launch button as this switch, but with some style). To make the whole thing look cool I bought this switch and this switch from Amazon, and used a clear-plastic box for the enclosure. Every kid took it upon themselves to to do a count-down, flip the safety, then bang down the launch button. If I'd had more time I'd have run this through an Arduino with a really simple sketch that would accomplish the following...
- Drive a 7-segment display to count down from 5 after the safety switch was flipped.
- Make some lights flash and do crazy things
- Engage the valve for a specified time - say 1 solid second - once the launch button was hit. This would avoid the two main launch problems we had - kids frequently either (1) tapped too lightly and therefore their rocket just hopped off the tube or (2) held down the switch too long, de-pressurizing the system entirely, which made for a long re-charge time.
The rocket launcher was a hit. My son's birthday party was held in our back yard and in our pool, parents were told to bring bathing suits and sunscreen. I barbequeued (it's Texas) and after the kids spent about an hour in the pool I pulled out the launcher. My son and I fired a warning shot over the pool - and suddenly I was surrounded by 20 kids chanting "I want to shoot that!". We herded them inside where I had laid out rockets, stickers, markers, crayons and assorted decoration tools. Ten minutes later we had out first customer.
In order to try and keep the rockets we fired them at a low trajectory across my yard, and some of the other dads stood at the end of the range, catching the rockets and handing them back to the kids. I manned the launcher and we had a line of children for about an hour, just circling through and firing rocket after rocket.
The kids each took home their own rocket, and some of the other dads and I spent "cake-time" challenging ourselves to try and get one over the neighbor's house. We also launched an empty beer can 40 yards (again - it's Texas).
Best of all, my son learned a little about air-pressure, aerodynamics, electical circuits - and that the best toys are built, not bought.