mach delay or not? Shear pins on drogue.

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    Greg Mackey

    At what feet/sec do you guys start using mach delay function on your altimeters? IE what is the rule of thumb?

    Also, is there some kind of general rule you are using to decide on shear pins for the drogue deploy?



    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.


    John A. Wilke

    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.


    Warren B. Musselman

    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.


    Greg Mackey

    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.


    Chris LaPanse

    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.

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