I know I'm late to the game (been on vacation), but wherever possible, I use
motor ejection as backup to electronic deployment. As recently as last year,
it saved my Punisher from certain death when the altimeter unexplainably lost
power during the flight (both drogue/main e-charges unfired, replaced battery,
then worked fine, hmmm...)
When I do use motor backup, I try to make sure it is "truly back-up" in the
sense that I choose a delay element for ejection timing 4-5 seconds past
apogee, as determined by simulation. My first L2 attempt used a motor that
chuffed on the pad, then flew perfectly, but the delay element had already been
lit with the first chuff. The motor "backup" then became primary, about 4
seconds early and the resulting zipper was not pretty.
Typically, I use the longest delays available (RDK 8+ for 29mm, RDK-16+ for
38mm, RDK-32+ or RDK-33+ for 54mm), trying to guarantee that motor deploy will
occur several seconds past apogee.
Chris Kobel
-----Original Message-----
From: roc-chat-bounce@xxxxxxxxxxxxx <roc-chat-bounce@xxxxxxxxxxxxx> On Behalf
Of Terry McKiernan
Sent: Tuesday, July 16, 2019 7:58 AM
To: roc-chat@xxxxxxxxxxxxx
Subject: [roc-chat] using motor ejection charge as backup?
Polling the ROC crowd for opinions here ... I'm wondering if it's recommended
to use a motor ejection charge as a backup to the altimeter on high-power
flights.
Specifically this Saturday I'll be flying my Firestorm 54 "stretch" on a CIT
J360 Skidmark. It has a Raven3 altimeter to fire the drogue and main ejection
charges. I'm wondering if I should leave the J360's charge in place as a
backup to fire the drogue just in case.
The OpenRocket sim says time to apogee after motor burnout is 14.8. The
J360 conveniently comes with a 15-second delay grain. So that seems about
perfect as a backup.
My concern is that OpenRocket sims are just sims after all, and may not be very
accurate. Plus delay grains are not very high-precision either. I'm mostly
worried about the flight being longer to apogee than the sims says, and the
delay grain burning through faster than expected, and thus releasing the drogue
a few seconds before apogee while the rocket is still moving pretty fast.
What do you think? Leave the motor charge in as a backup, or avoid it due to
the possibility of early ejection, and just count on the Raven to do its thing?
Thanks!
Terry
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