Thanks for the great thoughts, guys. This helps a lot. I didn’t have time to inspect the chute before I left for work this morning, but I took another look at the external structure. One of the fins does have a cracked fillet that I didn’t notice before. The fin still feels firmly in place though. The chute was small on this, so it might have happened on landing. Tough to tell.
The rocket is a flying case design with the bulk of the structure being a Loki 38/1200 case. The fin can is a carbon fiber tube with the solid carbon fins attached to it. The fin can assembly is epoxied directly to the Loki case. On the front end, a short body tube section is epoxied to the Loki case with about a 2” band of Hysol. That’s the avbay section & the only place I see for it to leak is out the static ports. I guess it’s possible it leaked out the plugged forward motor closure, but that was all intact & felt good when I disassembled it. For the rocket to slow down that quickly, it would have taken something fairly serious going on. How about a fish tail/slide type motion during the “something” time? I ran RasAero II on it, but maybe the predictions there were more optimistic than reality. It may be possible that something strange happened with the smoke/delay grain. The nozzle still looked good, but maybe there was some low volume gas escaping the nozzle at a weird angle because of something partially blocking it. Perhaps that along with being at the lowest stability point in the flight was enough to make the rocket hunt. The main reason I was curious about the drogue is that if you replay the flight, I thought it called out “drogue” and showed drogue as the status before apogee. The graphed data showed drogue right at apogee though. I’ll replay it though - I’m probably remembering that wrong. It continuing on after the something doesn’t make sense if the laundry is hanging. The static ports/vents are roughly 3” back from the nose cone on the flat part of the avbay body tube. They’re about an inch forward of a transition that reduces from the body tube down to the motor casing. That transition shouldn’t matter for those ports while the rocket’s supersonic though. I’d really love to have 3-axis gyro and accelerometer data. :) I’m hot & heavy into making rockets to go after the F & G records now. I’m aiming to fly them at Tripoli Colorado’s Springfest in a week and a half. Those will have a TeleMini in them. I didn’t have time to do the external GPS mod I wanted to with the TeleMini, so I’ll have to hunt those rockets down with a Yagi. Should be fun! I’m busy with work until tomorrow night. I’ll take a better look at my “something” rocket then. Thanks! -Bryan > On May 30, 2018, at 11:29 AM, Keith Packard <[email protected]> wrote: > > Bryan Duke <[email protected]> writes: > >> Hello List- >> >> I flew a rocket a few days ago in search of the J altitude record. The >> rocket teleported off the pad & quickly got to over 3000ft/s. Very >> quickly after that, the telemetry reported the drogue firing (at >> around 10k ft) and then the rocket peaked a few thousand feet >> later. The accel data show a deceleration momentarily maxing out the G >> meter. So, about a 100G accel followed by a 100G decel. Ouch. > > Ouch indeed. It looks like you were just shy of maxing the > accelerometer, which is an impressive feat. > >> I’m still trying to figure out exactly what happened. I think the >> motor made it all the way through burnout, then some “event” >> happened. > > I think the pressure curve is probably the most interesting part. At > about 0.8 seconds after boost, the pressure starts *dropping* rapidly, > then "something" happens, which generates a tremendous amount of noise > on both pressure and acceleration curves, then at about 1.8 seconds, > things return to "normal" and the flight carries on with what looks like > a completely nominal decelleration curve (starting at about 10g, then > tapering down to 1g as the speed decreases). > >> The rocket and all the recovery gear are still in great >> shape, so it’s tough to point a finger at anything from just looking >> at the rocket. Can I please get some thoughts on what happened based >> on the data? Is the “drogue” callout on the flight replay matched to >> the TeleMetrum firing the drogue? If so, why did it fire it so early? > > I don't see it firing early; looks like it's right on top at 24 seconds. > >> I’m definitely not ruling out the rocket tumbling, but the motor >> exhaust looked straight until we couldn’t see it any more. > > It definitely didn't tumble -- it would have come apart and then the > rest of the trip to apogee wouldn't have looked clean. > >> FWIW, this was the first flight on a new TeleMetrum. The telemetry >> page showed that v1.8.5 is loaded. I’ve flown an EasyMini before to >> about Mach 2, but not a TeleMetrum and not anything approaching Mach >> 3. This was the maiden flight on this rocket. The chute was in the >> nosecone & that’s the only thing that gets ejected (single chute at >> apogee was the plan). My drag & decel calcs show that the nosecone >> should have plenty force holding in place throughout the flight. It >> was a fairly tight fit and held on by a single 0.060” styrene rod as a >> shear pin. I’m not ruling out a drag separation either. > > There's no way the rocket came apart or did anything 'weird' externally; > it wouldn't have been aerodynamic enough to survive to apogee at 12600'. > > There's clearly something 'weird' going on with the pressure data. Are > the static ports in a nice smooth part of the airframe, far back from > any edges or diameter changes? If I were guessing, I'd guess that > there's something evacuating the ebay, and that eventually that caused > some turbulence, which generated enough drag to slow the airframe down > rapidly, but not enough to tear it apart. > > The fact that it happened after motor burn-out makes me kinda wonder if > there's some air leakage around the base of the rocket (which seems odd > for a min-diameter airframe). Imagine the sudden low-pressure area > pulling air from inside the airframe. That doesn't explain how the > pressure returned to normal a second later though. > > That's one weird data set, that's for sure. > > -- > -keith _______________________________________________ altusmetrum mailing list [email protected] http://lists.gag.com/mailman/listinfo/altusmetrum
