Sunday, January 30, 2011


Today I finished primary assembly of the Extreme DarkStar (right) and there's the final Nike Smoke my parents bought for me (left). [Update: While I was authoring this entry I figured out they'd shipped the wrong DarkStar fins to me. Awesome now that's it's friggin' built! The Nike Smoke is fine, however. Details below...]
I still need to foam the fin slots, fillet the fins, drill vent holes and static port, attach the rail guides, and layout the electronics board but it'll fly next weekend on a CTI Pro75 6162M1675 Pink. Based on KO's previous flights this rocket seems to get about 3 feet per Newton-second of total impulse. As such I would estimate about 17,000-18,000' from that motor. I'll also fly it dual deploy but plan to run both a Beeline GPS transmitter and my workhorse tracker beacon to assure recovery. Here are some build details from previous weeks:
On the above motor assembly I elected to use the SlimLine 75mm motor mount so no screws are required (unlike the AeroPac which is a very tight fit). I also left off the Kevlar strap as I prefer to link the recovery system to a drop-forged screw eye in the motor's forward closure. That longitudinal masking tape serves as a shim for the centering rings as the tube is tapered and they would have been loose during the cure otherwise.
For some reason Wildman provided WAY too much tab depth on those fins but my new carbide-coated bandsaw blade cut through the 3/8" high stack like butta. That blade was spendy at over $100 but there really is no other way to cut fiberglass on a bandsaw with any reasonable blade life.
I went retro and simply taped each fin pair on with manual alignment. This was a bit difficult because the slots were about 50% too wide for those fins. I chose to datum to the left side of each slot and they all look perfectly aligned to my eye. I'm now wondering if Wildman had intended to ship 3/16" thick fins with this kit? Hmmmm. Low and behold they WERE supposed to ship 3/16" fins as indicated in the kit specs. That's strange because the fin tabs matched the slot length correctly so I assumed they'd just messed up on the fin template. Maybe mine won't fly on any 75mm motor made then... Dammit.
Above is another view of the simple yet effective fin attachment.
Above is the electronics bay. Today I JB Welded those aluminum tubes to the sled so, apart layout out the board positions and switches it's all set to go. Wow I'm really bummed about that fin thing now. This was too much money and work to be uncertain of fin strength at this point.

Thursday, January 20, 2011

Lagger: Plaster City 1/1/11 Launch Report

The coolness of the date of this launch was exceeded only by the complete lack of wind and excellent flying conditions all day. The primary reason for this delayed report is that I was trying to improve the video I shot that day to no avail. My pictures sucked too so that's awesome.

Fiberglass Nike Smoke 4" - This is the kit mis padres purchased for me for Xmas (bah humbug :). It's designed like a tank with a weight to match -- 81 oz dry!! I purchased a Pro54 4-grain 1597K400 Green^3 reload from Jack. The liftoff was slower than I'd expected but, like all Nike Smokes, the thing flew straight as an arrow. I trimmed the delay to 11 seconds and ejection was about 1 second after apogee. I used a 48" parachute and the descent seemed slow until it got nearer the ground then it seemed speedier than I'd like. In any event it landed at the edge of the 'bowl' free of damage.
I think this kit needs some serious thrust so I purchased another 4-grain motor, the 1633K940 White Thunder, for its next flight. At the same time Jack offered me a mystery 4-grain motor for half-price. Others reportedly shied away earlier in the day but I wasn't skerd and bought it anyway. When I got it home I opened the package to find "VM" written on the ejection cap so I'm forced to assume it's the 1408K2045 Vmax. Score!! So I'll reserve that for the third flight of my new Smoke and it should be scootin'.

.38 Spatial - I last flew this minimum diameter, carbon-fiber-over-phenolic design on an I284 to ~9100'. The next logical step on the way to the insane 1115J530 was the J350 I've had sitting in my motor box for many years now. As always these minimum diameter flights take way too long to prep and there never seems to be enough internal room to fit everything despite previous successes there. Once on the pad I had trouble locating the Raven's arm button so I pulled the rocket, slightly rotated the electronics board, and tried again. The second time was sort of a charm. I was able to arm the Raven but, upon 'ignition', the motor pressured up very slowly as old motors will. After a sizable chuff the thing lifted off but was marginally stable. This is about the 10th photo of a 5 fps sequence and the motor took about 5-6 seconds to pressure up:
The resultant wiggle sent it NW and way out there. I had a radio tracker signal the whole time and figured everything was intact. After walking out toward the wash for over half an hour and circling about I finally found the transmitter.... just the friggin' transmitter!! It looked like someone had wrenched the thing off the shock cord, left it there, and kept the rocket; or so I initially thought. I ventured out on the original line and asked a camp of moto dudes if they'd seen anything. One fella seemingly sketchily answered "With a green 'chute?" Yes I answered to which he replied "Oh it's back from where you came about a quarter mile out." His answer didn't sit right with me but what was I gonna do? Ask to search their motor home? I bit my tongue and walked back toward the launch site only to find the rocket about 30 feet east of where I'd found the transmitter. I immediately felt guilty about my suspicions so I need to offer the benefit of the doubt in the future.n Here's roughly where I found it:
OK so the payload section and booster were still mated but the nosecone was off and the Kevlar™ cord had ripped through the payload tube about 1.5". Man that aramid fiber is tough!!
As I walked I pondered failure analysis... The apogee charge had blown but the motor and delay had already burned up so the delay hole in the motor must have vented too much of the gas and failed to pop the payload section off. As the rocket came in ballistic the main popped but I had only loosly knotted the 'chute lines the shock cord (no knot on the cord itself). So the parachute, mounted above the taped-on radio beacon transmitter, slipped up the cord, stripped off the transmitter, zippered the payload section, then promptly shredded off. As I walked back I even found the 'chute!:
I'm constantly destroying those TopFlight 'chutes at every single launch. The Raven survived and here's the flight graph:
Despite the serious flight arc I still attained about 8100' so I'm convinced it would have exceeded 10k were it straight. Also the velocity wasn't too bad at roughly mach 1.5. You can also see the blip after apogee where the charge failed to eject and then where the main ejected at ~800'. I wanted to swap nosecones with an extended design anyway so .38 Spatial will fly again soon. Lessons learned:
  • Burn those motors soon after you buy them! Chuffing (motor pressure surge before actual ignition) is dangerous. In the early 80s motors employed progressive thrust curves and weren't nearly as reliable as today's motors. I saw a few rockets chuff, pop off the launch rod, then fire off horizontally into the desert. If a motor is older and you still choose to fly it always use a long launch rod/rail as an insurance policy against chuffing.
  • RockSim's CP calculation is way too aggressive hence the metastable liftoff. I'm now relying more on OpenRocket's CP calculation as it's typically between RockSim and Barrowman.
  • [Just added] Plug those forward closures!! I'm certain I would have attained apogee ejection were it not for the unintended pressure vent offered by the motor itself.
  • In the unlikely event of a similar failure in the future place the transmitter above the parachute connection point or, even better, always tie a knot in the shock cord for parachute attachment. That way if the parachute strips off it won't take anything with it.
  • Offer the benefit of the doubt to those sharing the desert with us. I have a bad history with this so I have some trauma to overcome. When I was a kid I flew a Composite Dynamics 29mm F45 motor in my prized AeroRoc. It was a high flight on a windy day and the rocket landed on the side of the highway in the bay area. I ran toward it only to see someone pull over and drive away with it when I still had a quarter mile to run. Damn them! Epic recovery FAIL.