KWRRCG:The Kansky Water Rocket Radio Controlled Glider Experiment
After spending way too much time looking at various water rocket sites on the web, I decided I had to build a water rocket launching system for my Chrysalis radio-controlled hand launch glider.
Thanks to Clifford Heath's Water Rocket Simulation web site I was able to calculate that an open nozzle 2 liter soda bottle would result in 50+ G's of acceleration with my 10 oz glider strapped on the side. I knew that my poor plane wouldn't be able to handle it, so I decided to go with a reduced nozzle diameter. The most convenient way to do things was with the LEGO garden hose quick disconnect garden hose fittings you can find at your hardware store. Here's a picture of the whole assembly on the launch pad:
If you have any questions, email me at kansky "at" ll.mit.edu. The glider is held onto the bottle by music wire hooks that I taped onto the bottle with strapping tape. These hooks latched on to the finger pegs used for throwing the hand-launch glider. To keep the nose attached I added a balsa wood slot to the bottom of the front fuselage of the glider. The bottle is intended to separate after the thrust phase is over, but I found that because of friction I needed a rubber band to help pull off the bottle. Unless the CG is the same with the empty bottle attached, make sure your bottle comes off in any conceivable orientation!!!
To solve the separation problem, I took some strapping tape and taped a #64 rubber band to the bottle forward of the finger pegs. After I attach the glider to the bottle I stretch the rubber band onto the very end of one of the finger pegs. The rubber band should be stretched just enough so that the bottle will separate, and not so much that it would try to pull the plane off the hooks before the launch.
I also used a section of fluorescent light protector tube to provide initial guidance to the plane, and to keep it from getting wet, but I found that the plane doesn't get wet at all and the dry mass of the rocket is less without it, so I wouldn't bother. I don't have a picture of the launcher and plane without the long tail tube, but without the tail the plane has a tendency to pull the bottle off vertical because of play in the garden hose quick disconnect. To solve this problem I made a cradle out of a section of 2-liter bottle, and thick music wire to support the 2-liter bottle as it is pumped up with air. The music wire was taped to the pvc pipe below the quick disconnect, and then bent to support the ring of 2-liter bottle material that makes up the cradle.
How Does it Work?:
Calm Sunday morning. Moderate inflation pressure, approx. 50 PSI. Possibly not enough venturi vents in the the fluorescent tube protector tail. (I only had 4 hole-puncher sized holes near the nozzle.) No rubber band. Half-full 2-liter bottle.
The plane rocketed off as expected but didn't climb too high. Approximately 30 feet up. The 2-liter bottle didn't separate, so the plane glided like a brick. The flight lasted maybe 20 seconds total. Once the plane touched down, the bottle separated, and the plane glided for another 10 feet! The plane was in one piece, not a bad start but disappointing altitude off launch.
Light Breeze. Still using the fluorescent protector tube tail, with no rubber band. As I was inflating the bottle the plane suddenly blasted off!!! I hadn't been careful in pushing the bottle onto the quick disconnect and only two of the three clamps engaged! I dove to grab my radio, and then searched the sky for the plane! I found it right towards the sun (Murphy's law), and about 60 feet up pointing straight up! The plane stalled, but again the bottle did not separate. I pulled full up elevator, and the plane just leveled off as it hit the deck. The tail splintered as Chrysali are prone to do. Repairs took about 45 minutes. The launch height was encouraging. I only pumped about to very low pressure when it took off, and I estimate that it went about 60 feet up.
I made 4 more hole puncher size venturi holes in the fluorescent tube for this launch. Unfortunately, I neglected to snap on the fluorescent tube onto the bottle using the Schedule 40 PVC coupler, so the tube stayed on the launch rod:-)! Some down trim on the plane kept it going straight up as far as I could tell.
I decided to get rid of the the fluorescent light protector guidance tube, and I build a cradle to hold the bottle vertical as described above. This time I pumped up the bottle as hard as I could with my little bike pump (I'm guessing 60 PSI). I also filled the bottle with a little bit more water. About 3/5th full. I didn't dial enough down trim into the plane, so on launch the plane launched like the space shuttle. It went inverted from the beginning to the horrific end. Yes, another plane is dead. At the end of the boost the Chyrsalis wing folded and the v-tail sheared off. The damage looks doesn't suggest a simple fold like a wing that breaks on a hard winch launch. Instead it looks like the wing folded due to fore/aft acceleration. I can't decide if it was due to the air burst that results when the water is all gone and the pressurize air exhausts, or if it was simply the sudden change in acceleration when the water ran out. Anyway, the wing folded, the v-tail snapped off, and the entire rear fuselage splintered from the acceleration!! The whole mess floated down to the ground like a falling leaf. At least I know there's enough power to make things interesting!
Needless to say, I'm designing a new version as we speak, and I'll keep you posted. To promote this idea I'm defining two new water bottle competition classes to drive your design criteria:
H2O 2-Liter Class:
Unmodified 2-liter bottle pressurized to 80 PSI, 551 KPa. Garden hose quick disconnect nozzle. (Approx. 8.6 mm diameter) Boost glider configuration. (Bottle separates and falls to earth separately) Multiple glider simultaneous launch with a thermal duration task with preset maximum time. No off field landings.
H2O Open Class:
Any PETE plastic material allowed, unlimited volume (e.g. 3-liter, spliced soda
bottles, fluorescent tube protector). Competitors use the same agreed upon air pressure,
(e.g. 80 PSI, 551 KPa) No nozzle restrictions. Multiple stages permitted.
Keep me posted of your successes and failures!
Jan Edward Kansky