RC Aerotech Pheonix - Thrust spike

[MaxQ]

Flew my Aerotech RC Phoenix at Culpeper/Battlepark today.

This was my fourth flight, first one with a G-12, previous three were on the F-13

The G-12 has a 8.5 second burn time, almost twice the burn time of a F-13.
The peak thrust in the first second is the same as a F-13...about 7-8 lbs, but twice he burn time.

As usual I put in some down trim for launch as recommended as this rocket glider tends to arc slightly backwards on launch.
First several seconds were ok, it was flying almost straight up and quite high, then it started to lean backwards, so I put in a little more down elevator and aileron to straighten it out....
Then something weird happened.

The glider suddenly accelerated with a burst of speed - seemed it abruptly started flying twice as fast.
I had to back off the sticks quickly as the increase in speed seemed to make the small control input excessive, it almost did a complete loop before I got it nosed over and into level flight.

The motor assembly instructions mention putting grease on the end of the grain to avoid a thrust spike at burnout.

I put some grease on the end of the grain but I was wondering if it wasn't enough - and what may have caused this burst of speed.
(I have about 24 years of large RC sailplane and powered RC experience (40 -91 size) so I'm pretty sure my control inputs were not an issue).

 

[tab28682]

Could have been a void in the motor grain. The sudden increase in burn surface area can really increase the thrust.

A couple of the regulars around here have experienced exactly this.

 

[burkefj]

Yes I've had that happen twice with g12s, void causing sudden thrust increase, both caused a shred in my applications, I don't trust them now and prefer g25s. Slightly heavier but use the std casing.

Frank

 

[JoeG]

I believe that's what happened to me a few years ago at MWP. The G12's are a slotted motor and the sudden burst acted much like a cutting torch on the side of the liner/case, burned through the case, and the rocket ended up catching on fire and burned itself in half during the flight. Weirdest thing I ever saw. We didn't know it was on fire until the the tail boom fell off.

I have flown a lot of these motors (G12) in the last twenty years and this was the only time it ever happened to me.