John Newman at FAR builds many AP motors and says that creating the delay
grains is the most difficult thing about making solid motors. I asked about
the age of the motor because age seems to affect the delay grains more than the
motor grains. Usually, you can see a color difference as the delay grain ages.
And, I think the delay becomes shorter with age. Perhaps someone with more
experience can correct if that’s wrong.
Anyway, I appreciate redundancy and have seen people use motor ejection as a
back up but it’s more when the timing is separated by like 4 or 5 seconds.
And, I’ve been told that you don’t want the primary and back-up firing at the
same time, so I’ve never done that. :-). I guess if you thought the timing to
apogee was going to be 15 seconds, you could actually have the Raven be the
back up and delay it’s firing a couple seconds. On the other hand, the Raven
is a reliable piece of electronics, and with all the verification it goes
through when it gets in flight mode, do you need a back-up? Tough call, and of
course you’ll know more about the delay grain timing on July 21st.
Richard
On Jul 17, 2019, at 11:16 PM, Terry McKiernan
<terry@xxxxxxxxxxxxx<mailto:terry@xxxxxxxxxxxxx>> wrote:
Richard,
Right now yes the ejection charge has the full amount of BP. I could certainly
change that before flying, or take it out entirely. That's what I'm trying to
decide on. I'm using 1.25g of FFFFg for the charges attached to the Raven,
same amount as a normal motor charge, so it would be double the force if both
went off simultaneously. What would happen ... don't know. Of course it would
be more force but would it be enough to damage things, I can't say. This is a
fiberglass rocket with a GLR Hardpoint shock cord attachment, and 3/8" Kevlar
shock cord, so it's pretty tough and I expect it would be OK but that is just a
gut sense.
I don't have shear pins at both ends of the drogue bay since it's down in the
lower section of the fuselage. That is, the drogue is between the motor and
the payload bay. There are 3 2-256 shear pins at the top of that section where
it meets the payload bay.
Yes, I already ran the Raven through test mode with matches. All is well
there. I'll test again at the site plus of course the Raven gives continuity
beep status codes as a final preflight check.
Terry
On 7/16/2019 6:08 PM, R Dierking wrote:
So, what would happen if both charges went off at the same time? Are you using
the full amount of BP for the ejection charge?
Do you have shear pins at both ends of the drogue bay?
Did you run the Raven through test mode with a match?
Glad you are thinking this through Terry. Only trying to help.
Also, just asking, how old is the motor and delay?
Richard Dierking
TRA 11366, L3
On Jul 16, 2019, at 4:56 PM, Cris Erving
<cris.erving@xxxxxxxxxxx<mailto:cris.erving@xxxxxxxxxxx>> wrote:
Nope. You get what you get...
Sent from my iPhone
On Jul 16, 2019, at 4:13 PM, Terry McKiernan
<terry@xxxxxxxxxxxxx<mailto:terry@xxxxxxxxxxxxx>> wrote:
Thanks Cris. Is there any way to lengthen the delay? The J360 comes with a 15
second delay grain and it sounds like I would want 17 seconds.
Terry
-------- Original message --------
From: Cris Erving <cris.erving@xxxxxxxxxxx<mailto:cris.erving@xxxxxxxxxxx>>
Date: 7/16/19 8:37 AM (GMT-08:00)
To: roc-chat@xxxxxxxxxxxxx<mailto:roc-chat@xxxxxxxxxxxxx>
Subject: [roc-chat] Re: using motor ejection charge as backup?
For motor eject as a backup, you want the adjusted delay to be about two
seconds past what you expect your apogee to be, so if it's a bit short it
shouldn't be a problem. Motor delays almost never run long. If your
projected time to apogee is 14 seconds and your maximum delay is 15 seconds,
that's a bit close... don't be surprised if the motor eject happens before the
altimeter fires.
Eggtimer Cris
________________________________
From: roc-chat-bounce@xxxxxxxxxxxxx<mailto:roc-chat-bounce@xxxxxxxxxxxxx>
<roc-chat-bounce@xxxxxxxxxxxxx<mailto:roc-chat-bounce@xxxxxxxxxxxxx>> on behalf
of Terry McKiernan <terry@xxxxxxxxxxxxx<mailto:terry@xxxxxxxxxxxxx>>
Sent: Tuesday, July 16, 2019 7:57 AM
To: roc-chat@xxxxxxxxxxxxx<mailto:roc-chat@xxxxxxxxxxxxx>
<roc-chat@xxxxxxxxxxxxx<mailto: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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