The Jujubee Project -- Update
So the Jujubee project is my attempt to create a fully recoverable system by which a pneumatic cannon can boost a rocket into the air. So far I have built 4 such rockets and 5 launchers. The first rockets were small Estes-sized things just to get the idea. With each successive rocket I found things that could have improved. The current version, dubbed "Jujucee", is the first high-power-capable rocket, and it fires from by far the most serious pneumatic cannon so far -- the cannon stands 8' tall, and can shoot a tennis ball 300 feet per second without even trying (at max pressure it should be able to hit over 500 feet per second). The rocket is a minimum-diameter phenolic airframe and is the first of these rockets to have an altimeter. (I had to modify my G-Wiz MC2 in order to get it to fit in the airframe. I can post details if you're interested.)
Last weekend I tried to launch it down in Maryland on a small H238 motor (which should have taken it to Mach), but had not yet made one of the modifications to the altimeter so the electronics wouldn't quite fit. The new plan is to attempt to launch it tomorrow at a more local field. Unfortunately due to the cold weather I can't pressurize the cannon to a very high pressure (PVC doesn't do well in the cold), and due to the small field I will probably be limited to the D rocket motor. We'll see what happens.
Anyway, the project is drawing to a close and I've found some interesting things along the way, so I thought I'd share them.
Picture of Jujucee and launcher, prior to painting
The black section at the very top of the launcher is the homebrew chronograph I built for testing. I ended up running out of time and didn't get a chance to gather any data from it.
I ran lots of simulations and calculations for this project. One interesting thing I found (assuming the simulations are correct) is that while using a heavier projectile in the cannon does lower the maximum speed it will attain, the total impulse imparted by the cannon actually increases. (With rockets, any given rocket engine basically has a fixed total impulse built-in, and that's what you get.) For anyone that's interested, the tools I've been using are: RockSim for the rocket side, GGDT for the gas gun side, and Excel for anything else. I basically modeled my cannon in GGDT, and entered the weight of the rocket as the projectile weight in the cannon. That allowed me to use RockSim's EngEdit program to model a rocket engine that (in theory) behaves just like the cannon for this particular rocket. That allowed me to model the entire cannon->rocket launch and flight in RockSim by simulating it as a 2-stage rocket. There are, unfortunately, some places where GGDT and RockSim disagree -- namely in the acceleration of the rocket during the boost (cannon) phase. GGDT reports very high accelerations (such as, say, 200 gees) while RockSim reports much lower (perhaps 20 gees). I don't know why that is.
If anyone's interested in seeing my RockSim files or the internal ballistics graphs or anything, let me know. By the way, at the highest pressure (~100psi) and using my particular rocket, the cannon alone should have the total impulse of an F motor, with an average thrust comparable to some M motors (over 1000 Newtons, albeit for less than a tenth of a second!).
So what's next, besides launching it off and gathering some results? I'm not sure. I'm sort of thinking of going back to do another robot project. I'm thinking about making a robot similar to Johan (my last Big Frickin' Project), using what I learned from Johan... and adding a turret that it can aim, complete with a small pneumatic gun (perhaps it could shoot unpopped corn kernels or something) and a rack of rocket launchers. No practical purpose at all, but wouldn't it be cool? Take it out to one of the small local launches, and program it to drive out to a spot, raise up the rack, and ripple-fire like 9 little rockets into the air... How cool would that be?
Last weekend I tried to launch it down in Maryland on a small H238 motor (which should have taken it to Mach), but had not yet made one of the modifications to the altimeter so the electronics wouldn't quite fit. The new plan is to attempt to launch it tomorrow at a more local field. Unfortunately due to the cold weather I can't pressurize the cannon to a very high pressure (PVC doesn't do well in the cold), and due to the small field I will probably be limited to the D rocket motor. We'll see what happens.
Anyway, the project is drawing to a close and I've found some interesting things along the way, so I thought I'd share them.
Picture of Jujucee and launcher, prior to paintingThe black section at the very top of the launcher is the homebrew chronograph I built for testing. I ended up running out of time and didn't get a chance to gather any data from it.
I ran lots of simulations and calculations for this project. One interesting thing I found (assuming the simulations are correct) is that while using a heavier projectile in the cannon does lower the maximum speed it will attain, the total impulse imparted by the cannon actually increases. (With rockets, any given rocket engine basically has a fixed total impulse built-in, and that's what you get.) For anyone that's interested, the tools I've been using are: RockSim for the rocket side, GGDT for the gas gun side, and Excel for anything else. I basically modeled my cannon in GGDT, and entered the weight of the rocket as the projectile weight in the cannon. That allowed me to use RockSim's EngEdit program to model a rocket engine that (in theory) behaves just like the cannon for this particular rocket. That allowed me to model the entire cannon->rocket launch and flight in RockSim by simulating it as a 2-stage rocket. There are, unfortunately, some places where GGDT and RockSim disagree -- namely in the acceleration of the rocket during the boost (cannon) phase. GGDT reports very high accelerations (such as, say, 200 gees) while RockSim reports much lower (perhaps 20 gees). I don't know why that is.
If anyone's interested in seeing my RockSim files or the internal ballistics graphs or anything, let me know. By the way, at the highest pressure (~100psi) and using my particular rocket, the cannon alone should have the total impulse of an F motor, with an average thrust comparable to some M motors (over 1000 Newtons, albeit for less than a tenth of a second!).
So what's next, besides launching it off and gathering some results? I'm not sure. I'm sort of thinking of going back to do another robot project. I'm thinking about making a robot similar to Johan (my last Big Frickin' Project), using what I learned from Johan... and adding a turret that it can aim, complete with a small pneumatic gun (perhaps it could shoot unpopped corn kernels or something) and a rack of rocket launchers. No practical purpose at all, but wouldn't it be cool? Take it out to one of the small local launches, and program it to drive out to a spot, raise up the rack, and ripple-fire like 9 little rockets into the air... How cool would that be?
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