Monday, December 21, 2009

Kick-off: 7.5" Der Red Mix

This will be my first scratch-built, upscale design. As a kid I built the 1.6" diameter Estes kit "Der Red Max" and found the design appealing and the decals amusing. I'm not a fan of huge fins so on my design I've taken the liberty of scaling them down to just enable a static margin of 1.16 with the heaviest motor array (L935, 2xJ530, 4xI224). While 1.16 might sound low for a cluster I've learned to trust RockSim's stability equations and find the original Barrowman equations to be too conservative. Also I'm not likely to fire the cluster on the ground and will probably always start with a powerful 54mm motor to send the rocket up and away before igniting outboard motor pairs. Here are some screengrabs from RockSim 9:
I'm still working out the details of the electronics bay but I plan to seat a removable trap door just above the fin tops to conveniently seat two multi-function altimeters and a Flip MinoHD video camera. The onboard video should rock! I've slowly been buying components for this design from Jack at What's Up Hobbies as well as Loc Precision. I also ordered my first set of laser-cut fins and centering rings from Balsa Machining Services and an AMAZING set of decals from Mark Hayes at Here's the array of parts pre-assembly:
The fins are only 1/4" thick so I'll reinforce both sides of each with one layer of 6 oz carbon fiber to improve rigidity. Having just cleaned my garage I plan to start on this build ASAP!

Sunday, December 6, 2009

Plaster City Launch, 12/5/09

I had three goals for this month's launch:
  1. Test my Beeline GPS on a lower altitude flight to visually verify the position data. I also wanted to test a G10 fiberglass support I'd cut to stabilize the otherwise fragile electronics board/antenna.
  2. Test thermite ignition on an older Redline motor like the I366 I've had for seven years as these are notoriously slow starting.
  3. Fly my Viciously Mean Machine on a CTI J360 Skidmark (Pro 54, 3-grain).
I was mildly breezy and very cloudy when I arrived at about 10:15AM. If we weren't in the desert it probably would have been raining. I headed over to Jack Garabaldi's trailer to buy some components and a CTI 55F29 Imax motor for my GPS flight. I set up the GPS unit in the payload of my AT Arreaux and Frank Hermes helped me set my radio up to receive the APRS packets to monitor the flight's position every 5 seconds. I trimmed the F29 motor's delay from 12 seconds to 9 seconds forgetting how weak an F29 was. I really haven't flown mid-power for about a year now. Basically I should have used a 5 second delay on this cold day because 9 seconds was way too long. I came painfully close to crashing my $300 GPS unit on it's first otherwise underwhelming flight! I stripped two parachute strings but the whole thing came in tethered and fluttering. Everything was intact and I was able to download the flight profile successfully:
At first I was trying to figure out why the rocket only flew to 269 feet then realized that GPS altitude is recorded in meters. As such I actually attained 882 feet. Much better. :) I'm also confused by the region marked "What's this?" That's about where the 'chute finally ejected but it certainly didn't shock upward or sideways. Besides this isn't an accelerometer-based device and should only be storing actual GPS coordinates every second. Weird.
Then I spent another two hours prepping the Viciously Mean Machine for a dual deploy flight using the Parrot2 altimeter and the predictably spectacular J360 Skidmark. I finally got it out to the pad to discover a) I had managed to misalign the rail guides so there's no way I could fly it and b) looking up the 8+ foot skinny rocket that the payload section was warped. I had used Blue Tube 1.0 for the payload section and read that it was subject to warping at higher humidities. I had never noticed the curve while building so perhaps it happened afterward. I threw that part in the trash today as it's a total loss. I need to decide now if I want to try Blue Tube 2.0 for the rebuild. I feel pretty burned right now because I spent a great deal of time and money reinforcing the tube and coupler with two wraps of carbon fiber each. I disassembled everything and packed it away.
Finally I quickly prepped my horribly battered and thrice repaired LOC Bullet with a CTI 660J381 Skidmark. I really can't get enough of these loud and impressive motors. The flight was cool and uneventful except the ~1.5 mile walk east since I'd used a 45" parachute. I was able to track it visually but I'm still glad I put my radio tracking beacon in there as an insurance policy. I left the AVCHD camera on it's tripod during my roughly half-hour walk and captured some other flights before the wind wall kicked up effectively shutting down the launch for the day:
The song is Tenori by Francois Dubois. I never got around to the thermite ignition test but I vow to git 'er dun one of these days!