Test it.
If you are worried about damage, then mock something up using left
over bits and pieces.
The other suggestion, KISS.
From: roc-chat-bounce@xxxxxxxxxxxxx <roc-chat-bounce@xxxxxxxxxxxxx> On
Behalf Of Terry McKiernan
Sent: Wednesday, July 17, 2019 11:16 PM
To: roc-chat@xxxxxxxxxxxxx
Subject: [roc-chat] Re: using motor ejection charge as backup?
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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