Finally flew a "K" impulse motor.  Got a battery low warning tone from the primary altimeter on the pad, decided to risk the flight anyhow since the conditions were so nice.  Beautiful boost, perfectly straight just like a good tube fin rocket should, on a big loud white flame.  Good drogue at apogee, but as it descended I could tell that the upper payload bay (the main chute compartment) had separated as well, with the main chute still inside.  Heard the sharp crack of the main charge go off at 700 feet but it had already been pulled out of the payload bay.  Rocket landed in the mud under drogue with minimal cosmetic damage to one of the tube fins.  Need to adjust the friction fit of the upper payload bay before the next flight (and replace the altimeter batteries).