Creamy Robot Goodness

Well the launch is done, and it had a measure of success with a flavor of annoyance. The rocket launched and all the new pieces worked perfectly. Unfortunately the cannon had a slow leak that we didn’t discover until the last minute (not enough teflon tape on the joints? loose gauge? not sure yet), and by the time we got the rocket launched the pressure had dropped lower than it should have. It shot up out of the cannon going far slower than it was supposed to. The rocket engine ignited but I think it was already traveling too slow to be stable. The result was about 3 loops in the air, and it actually went almost 100 feet LOWER than the non-rocket-boosted Jujubee launch from 2 months ago. Entertaining in retrospect, certainly, but it was far less impressive than it could have been. The latest predictions (for the rocket stage alone) were almost 9,000 feet up and close to the speed of sound.

The carbon fiber (unidirectional prepreg) was really great. It machined much better than I thought (I was expecting splinters everywhere, and had virtually none), and the rocket came down without a ding on it (which is good considering how fast it comes down — it’s a very dense rocket and parachute space isn’t easy to come by). My custom accelerometer-based ignition circuit worked perfectly, as did my low-current igniters staging the I200 motor. Very nice.

At this point I think it’s too cold to use my PVC pneumatic launcher again this year. I’m thinking of installing some temporary launch lugs and launching the rocket off without the cannon at the next SoJARS launch — the simulations all say the rocket is overstable but I want to know for sure.

Next projects? Not sure. I’ve been thinking a lot about the next generation cannon, big enough for a more powerful 38mm rocket motor, and made entirely out of metal (aluminum for the barrel, steel for the breech/combustion chamber?). I’d power it on either multiple pneumatic butterfly valves (with a burst disc to synchronize them, perhaps?), or just straight up black powder. I’d also like to get my Level 3 at some point, but I’m not sure if that should be sooner or later.

I recently acquired a small amount of round magnesium stock in order to try and experiment with machining it. I’m very impressed with how light it is (something like 2/3 the weight of aluminum) and it machines great, I’d love to start using it all over the place.

Below is a link to a video that Justin took with his digital camera. Unfortunately you can’t see the rocket at all, but over the wind you can make out the countdown, the “poof” of the cannon, me exclaiming “Oh no!”, and Katie laughing hysterically. You can also see the stream of smoke begin mid-air, which is sort of cool in and of itself.

Jujubee video, first launch with high-power motor ignition

If anyone is interested in the small staging timer I built, I’m considering polishing it up and maybe selling a few, so let me know.

So it’s 2:40am and I’m eating a sammich before hitting the bed. Haven’t had lots of free time for food the last couple of days.

So is Jujubee done? Of course not! I still have to lathe up a coupler in the morning. The electronics are all wired up and tested (mostly). Besides the coupler, all that needs to be done is to tie everything together (the chutes, couplers, and airframe sections), pack the chutes, make up some charges for the electric matches, and attach the igniter. Most of that will happen at the field anyway.

I think I just might pull it off. There are some compromises I’ve had to make that, while imperfect, helped me make good progress. For example I cut the fin slots in the airframe but didn’t have time to cut out (and then carbon-fiber over and cure) the fins themselves… so I just cut the fin can from the last Jujubee and that should work ok. I also tried embedding the copper tape underneath the carbon fiber (with a layer of insulation), making it seamless. Unfortunately by the time I’d squared up the tube, the copper situation didn’t look very good. The compromise is to go with outer copper “racing stripes”. They’ll tarnish, but they’ll take about 2 minutes to replace.

With all the extra weight of the new couplers and electronics, the latest (crude) simulation is putting Jujubee at just under 8700 feet up, and 1043 fps peak velocity (about Mach 0.93). That is without the cannon, though.

I hope I have the presence of mind to get some good pictures of this rocket before I send it to its fate. Even with the (medium-grade) RF locator, I still bet it’s a crap shoot whether I ever see this thing again or not.

Well it’s Friday. “Red Glare” (the big launch) started today. I’ll probably end up working on Jujubee all day tomorrow. I wish everything was done, but I do think I’m in ok shape for Sunday. In just a couple of days I designed and built my staging timer. It uses a 250g accelerometer, an AVR, and a mosfet. It lets you, using a serial port, set the acceleration threshold for launch detect, and to set the ignition delay in 100ms increments.

I decided to run copper tape down the side of the rocket to carry the current from the staging timer. I ran a number of tests and it seems very reliable in its current configuration. I can power it up, pull the reset jumper to start the safety lockout timer, and then (after waiting for the timer) a quick smack on the side of the board simulates the high acceleration needed to trigger ignition. After the ignition delay, the igniter burns beautifully. I did, however, end up having to make my own low-current igniters because a 9v alkaline battery wasn’t able to provide enough oomph. Below is a simple explanation of how I made them, for posterity:

Materials:
- 26 gauge shooter wire (aka zip cord, etc), the first 1/4″ or so stripped
- soldering iron, solder, paste flux
- 40 gauge nichrome wire
- Magnelite dip

The concept is that, unlike the pre-soldered igniter wires that often come with Magnelite (see http://www.publicmissiles.com / Webstore / Igniters), we only want a very small loop of nichrome to bridge between the two copper wires (rather than wrapping it multiple times around the insulated section of wire). Using a small piece of nichrome wire (1.5″ is plenty, but most will go to waste), wrap one of the copper conductors several times, very close to the insulation. Then make a bridge, looping halfway around the insulation, to the other copper wire. Apply some paste flux to the two places where the nichrome wraps around the copper. Put a blob of solder on the tip of your iron, and touch it to the flux and copper. This should coat all the nearby copper in solder instantly, and tack the nichrome down adequately. Trim up the wires, carefully bend them into shape if you need very small igniters, and dip in Magnelite. I double-dipped mine after a few minutes to increase the amount of pyrogen per igniter, but I haven’t tested these yet. [Update: I tested most of the double-dipped igniters and one of them had a pretty violent pop. Guess that answers that.]