AT G71R delay length

[Reed Goodwin]

This weekend I launched my PML X-Calibur on a few G motors. I used two G64-7W's and had two G71-7R's ready to go. I noticed that the G71 delays were about 2/3 the length of the delays in the G64 packages. When I flew one of the G71's, I experienced ejection at most 1-2 seconds after burnout, cracking the piston. When I disassembled the motor, I checked for evidence of a blowby, but the outside of the delay insulator was clean. Any ideas? Should there be a difference in the lengths of the delay grains between the two motors? The Redline motor packages hadn't been opened, so no one switched delays on me. Photos of the result. Rocket is still flyable (in fact, I flew it on a G64 soon afterwards).

#1 - forward closure

#2 - delay insulator showing the dirtiest part

#3 - piston

#4 - coupler

Thanks,
Reed

 

[bguffer]

Different types of propellant (ie: white lightning, redline, green mojave) require different lengths of delay elements. I think the differing propellants burn at differing pressures, affecting the burn time of delay elements, thusly the differece delay element lengths.

Below should list the lengths of different delay elements.
https://www.aerotech-rocketry.com/c...tructions/hdk_instructions/29mm_hdk_instr.pdf

Bob

 

[bguffer]

Had a somewhat similar problem and discussion in January:
https://www.rocketryforum.com/showthread.php?t=1928

 

[Reed Goodwin]

Bob, thanks for that link. As I suspected, they use the same delay for both the G64-7 and G71-7 and it's 0.593" long. The ones in these packages are ~0.405" long, so somehow I have HDK-16 delays in a reload that should have an HDK-20 delay (0.593" delay).
Reed

 

[bguffer]

Same HDK for the 7 second G64 and G71, but different HDK's for the 4 second G64 and G71's.

 

[Microspeed]

I used G71's for TARC a couple years ago, and remember having issues with the delays, with slightly varying lengths between batches for the same delay time, and correspondingly variable delay times. We were using 7's too, and figured it might have just been some tuning/adjustment of the delay lengths during early production. Do you know how old the reload was?

 

[als57]

There used to be an issue with delays packed exposed to redline propellant. I've seen in the HPR reloads delays that were 3-4 seconds shorter than indicated.

Just recently purchased a used 29/40-120 case that had a G71-7R loaded in the case for 6+ months. O-rings were OK ; so I decided to fly it. The delay was at least 2 seconds short. Didn't bother the Little Dog any.


Al