This past weekend at our club launch, I launched my minimum diameter scratch-built 54mm fiberglass rocket on a K185. I've launched this rocket countless times over the last several years on various motors. I've probably only launched a K185 maybe once before but I've launched it's baby brother, the J90W, at least 3 times in the past.
Anyway, the flight was nominal for about 1000 ft before it started sky-writing. Rocket separated at the lower apogee section (dual deployment) while still under boost, which is why it started sky-writing (I also had this section pinned with a shear pin to avoid drag separation, which has been working for years). Shock cord broke but upper section came down on main once it tumbled to deployment altitude, lower section spin recoveed and was not seriously damaged.
Upon investigation, the shock cord was burnt through and the lower motor section was clearly charred, paint was even bubbled. I removed the motor casing and expected to see the fwd closure blown through but it wasn't. The only strange thing was that there was baked-on soot on the fwd closure into the charge well. The liner was burned through but at the aft end, not fwd and this is normal, especially for a long-burn. It did it's job. I first thought that there could be blow-by at the fwd threads but it was tight. Aft closure was tight too (I checked both before the flight). It almost seems like there was flame or pressure escaping through the charge well hole (I left the delay grain undrilled since I was using motor ejection as a backup to electronics). Either the pressure of whatever happened to the motor caused the airframe to separate under boost, or the apogee charge was activated by this anomaly.
Has anyone had a similar thing happen? Have there been cracked or defective smoke grains that have allowed hot gases out of the forward charge well, on this or other motors? The only other thing that I recall strange mis that when I built the motor (purchased approx 3 months ago), the delay grain, o-ring, and spacer was already assembled which I thought was strange, but that may have had not bearing on this. This motor is a long burn with slotted grain and medusa nozzle. Uses forward seal disk which I had firmly seated in liner with new o-ring.
Motor casing is fine, rocket has minor, repairable damage. But I'm out a $130 K motor. Would like to know what happened. Sorry, I didn't think to take pics. Wish I did.....
Any ideas/thoughts?
Jim
Anyway, the flight was nominal for about 1000 ft before it started sky-writing. Rocket separated at the lower apogee section (dual deployment) while still under boost, which is why it started sky-writing (I also had this section pinned with a shear pin to avoid drag separation, which has been working for years). Shock cord broke but upper section came down on main once it tumbled to deployment altitude, lower section spin recoveed and was not seriously damaged.
Upon investigation, the shock cord was burnt through and the lower motor section was clearly charred, paint was even bubbled. I removed the motor casing and expected to see the fwd closure blown through but it wasn't. The only strange thing was that there was baked-on soot on the fwd closure into the charge well. The liner was burned through but at the aft end, not fwd and this is normal, especially for a long-burn. It did it's job. I first thought that there could be blow-by at the fwd threads but it was tight. Aft closure was tight too (I checked both before the flight). It almost seems like there was flame or pressure escaping through the charge well hole (I left the delay grain undrilled since I was using motor ejection as a backup to electronics). Either the pressure of whatever happened to the motor caused the airframe to separate under boost, or the apogee charge was activated by this anomaly.
Has anyone had a similar thing happen? Have there been cracked or defective smoke grains that have allowed hot gases out of the forward charge well, on this or other motors? The only other thing that I recall strange mis that when I built the motor (purchased approx 3 months ago), the delay grain, o-ring, and spacer was already assembled which I thought was strange, but that may have had not bearing on this. This motor is a long burn with slotted grain and medusa nozzle. Uses forward seal disk which I had firmly seated in liner with new o-ring.
Motor casing is fine, rocket has minor, repairable damage. But I'm out a $130 K motor. Would like to know what happened. Sorry, I didn't think to take pics. Wish I did.....
Any ideas/thoughts?
Jim