I set the Mach delay for rockets going better than 200 mph. I use the time the sim says until the rocket has decelerated to ~.5 Mach. This can be quite long sometimes. I do not trust Rocksim - I go with RASAero.

If you are wondering about shear pins use them.

Edward

I agree with Edward. I use mach delays all the time, even on sub-mach flights. On a lot of my minimum diameter stuff that has high velocity I'll set them to 25 seconds. The first time I did that, I had to gulp hard... but it is better than shredding.

Shear pins - like mach delay, I use them almost all the time. Some of my drogue sections may only have one #2 nylon screw, but that is way better than none. Like mach delays -- I have a hard time coming up with a reason that you would NOT want to use them. There seems to be no downside to using them?

I thought John was crazy when he suggested I use a 25 second mach delay for my L3. Turns out that it worked perfectly.

As for shear pins, I generally use them on all my rockets - 2-56 is my standard. For my L3 I used 4-40 on the drogue and 6-32 on the main.

Edward

I too use a mach delay, although I tend to compute the length based on the simulated time above mach plus about 4 seconds. I don't think I've had one longer than 18 seconds, but I'm not sure. I use either 2-56 or 4-40 nylon screws for shear pins. Since all of my rockets are glass or carbon composite, I thread the AIRFRAME and through drill the coupler or nose cone for a pass-fit on the screw. I've also been known to break off the heads of the screws if I'm squeeking for altitude. I've even used 2-56 screws with snapped off heads on 24mm altitude birds.

Warren

Thanks guys.
I'm going to err on the side of caution from now on. I don't think my delay was long enough on the VA flight and it deployed right at burnout. I'd have to go look at the alts again to see what I used, but I know it wasn't anywhere near 12 or more sec, which is should have been based on what you are saying and my sims. I need to start using RASAero some too - I have it but never opened it.

Thanks for the info on shear pins too.

G

I always go on the long side for mach delay. I always set it to at least the length of the motor burn, plus a couple more seconds on a slow rocket. On a fast (near-mach) rocket, I'll usually set the mach delay so the rocket sims at <M0.6 by the time the mach delay expires.