Aerotech K185 - Have you had this problem?

[jdeveau]

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

 

[David Schwantz]

I've had the delay grain and O ring assembled before. The spacer almost always is in place also.

 

[CoyoteNumber2]

Did the motor take more than one attempt to light? And was it a quick ignition, or did it smolder on the pad before coming up to pressure?

 

[COrocket]

If I had to speculate, this would be my theory on the sequence of events based on your description

1. Motor burn nominal until hot gasses start leaking past delay O-ring and prematurely ignite ejection charge.

2. Rocket separates/skywrites due to ejection charge.

3. At the same time, force of ejection charge dislodges and removes partially burning delay grain from the delay well and into the combustion chamber, where it is either burned up or ejected through the nozzle.

4. Motor is still burning, but without a delay grain present, starts blowtorching the inside of the rocket from motor gasses being pushed through the delay well hole.

5. Motor burns out.

Was there any evidence that the delay well hole is enlarged? I’d imagine the aluminum hole would experience some melting/erosion if this is the case.

 

[mikec]

You were using the extended forward closure, not the standard forward closure, per the instructions at https://d11fdyfhxcs9cr.cloudfront.n...ages/54_852-1706wl_in_20071_1642718053427.pdf ?

I think this must be true because otherwise the assembly wouldn't fit, but I just thought I'd check.

 

[kjm]

Hi Jim --

I had the exact same thing happen at BALLS this year flying my 3" CF rocket on an AT K185W. Dual deploy, redundant altimeters and shear pins in the booster and NC. Around seconds into the 8 second burn, the booster separated from upper section, deployed the drogue at high speed and proceeded to skywrite over the range.
Luckily everything held together and one of the altimeters popped the main out at 1000' on the way down, saving the vehicle. Other than a shredded drougue and scorched shock cord, there was no damage.

Upon removing the motor I saw the same thing you did. I expected to see the classic signs of a forward closure failure - a grossly enlarged touch hole. But the touch hole looked fine. The ejection well and exposed shoulders of closure were covered in a thick layer of black soot. So rather than a classic 'complete/catastrophic' fwd closure failure, it seems to me that a some amount of gas was able to leak around the o-rings and seals, pressurizing the drouge bay causing separation while under thrust.

I've flown many J90s and J135s and a few K185s, so am confident in my ability to correctly assemble a 54mm extended fwd closure. Just to make sure, I brought the unopened motor case over to the Aerotech table. Gary watched as I carefully disassembled the motor. We could find no obvious assembly errors, and a close examination of what was left of the delay liner, delay insulator, delay oring and rubber washer was inconclusive since they were well scorched and burnt.

So it's been a mystery as to what happened to allow hot gas to escape. I wonder if anyone else has had the same problem as you and me?

Kevin.

 

[manixFan]

Just a point of reference, I use plugged forward closures whenever possible for exactly this reason – I’ve had and seen a couple of similar failures over the years with these long burn motors, which nearly always use an altimeter for deployment anyway. At a club launch last year everyone was thinking it was the usual forward closure blow-by, but there was no discernible damage to the case or closure. I don’t think it’s common, but it is easy enough to avoid the risk.

Sorry about the flight and glad no serious damage.


Tony

 

[reddrock]

I like COrocket's speculation.

This reminds me of blow by problems many years ago when AT motors used delay O-rings that compressed against the fwd end of the delay grain instead of the current design where the O-ring seals between the fwd closure and O.D. of the delay grain. Back in the day, Oddmanrockets taught me to wrap tape around the delay insulator tube to get a snug fit against the fwd closure, and the same for the delay grains for a snug fit against the delay insulator. Grease both well without getting grease on the ends of the delay grain. Fixed the blow by problem back then. If COrocket is correct, this should fix this problem too. No reason not to do it other than the time it takes.

 

[Buckeye]

This past weekend at our club launch, I launched my minimum diameter scratch-built 54mm fiberglass rocket on a K185.

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).

Is motor backup even viable on min dia 54mm rocket with K185? The max delay of the motor is 14s, but I am guessing that you probably need around 20s for optimum delay. Even if your motor burn was successful, it seems like 14s would blow way too early. Did you sim the optimum delay?

I agree with @manixFan that a plugged closure and no ejection charge is the best way to go with this motor.

 

[Donnager]

I don't believe my failure was as you say, but it may be worth mentioning. I had trouble with a J460 that had a similar failure, but it was to my knowledge, self-inflicted. I installed the delay element for one rocket with no delay (14s). I pulled the delay element for a different rocket and removed a couple of seconds (12s). But, weather and opportunities changed and I moved the motor to another rocket, which needed a couple more seconds removed, now 10s.

The delay element was very difficult to remove on the third time and I believe I delaminated the delay element from its tube or cracked it on the third disassembly/assembly.

I'm fairly sure I made a gas bypass crack that allowed a forward burn through. It burned the touchhole to 3/8 dia and made a flight to about 150-200' before it came apart. Lots of burned carnage and a nice zipper.

 

[jdeveau]

Thanks all for your replies! Sounds like I’m not the only one who has seen this. KJM seems to have had exactly the same issue. To answer one of the questions, I did NOT see an enlargement of the charge hole, same as KJM.

I like the idea of just going with a plugged fwd closure (which I have) since I always use electronics. I like using the motor ejection as a backup because my smaller rockets usually don’t have room for redundant electronics.