Terry,
I would use the full two grams for a 54mm airframe. Rule of thumb is that too
much is far better than too little. Don't try to cut BP amounts too close to
the fail/not fail line.
Kurt
Sent from my iPad
On Jul 18, 2019, at 9:45 PM, Terry McKiernan <terry@xxxxxxxxxxxxx> wrote:
I looked around online to see how much BP that CTI puts in their motor
ejection charges, and found a posting saying 2.0g for 54mm on up. Then it
occurred to me I could just take the charge out and weigh it ... duh! And it
came out to 1.97g. Probably a few grains still in there.
A few BP calculators tell me 1.1g is plenty for this skinny airframe (54mm
minimum diameter rocket) so the charges I wired up to my Raven are 1.25g.
For now I've put 1.25g of the CTI BP back into the charge well.
I'm still on the fence on this and may yet change my mind and take the motor
charge out before I fly. The aerospace engineer in me (a) likes having a
backup system and thinks it's unlikely both will fire at the same time, but
(b) hates the motor delays since they are based on the imprecise burnthrough
of some material.
Or maybe I'm just overthinking this. Two quotes to keep in mind:
"Let's light this candle" -- Alan Shepard
"What, me worry?" -- Alfred E. Neuman
Of course there's also "What could possibly go wrong?" and "The road to Hell
is paved with good intentions" :)
Terry
On 7/18/2019 8:59 AM, R Dierking wrote:
I don’t know about the timing. I haven’t seen damage from this that I know
of, but I have seen damaged airframes from deployment charges. Most of the
time, the damage is concentrated around the location of the charge, and it
was a big one. On the other hand, there are issues with the sections
deployment velocities. We have seen lots of separations for various
reasons. So, instead of like 15 psi, it’s 30 or 40 psi. What could happen?
So... let’s potentially find out. Hey Terry, you are good to go! JK of
course, but for us the primary thing is that after the button is pushed it’s
safe and spectacular.
Yes, success is in the eye of the beholder. ;-)
Richard
On Jul 18, 2019, at 8:41 AM, Andrew Wimmer <xenonrocket@xxxxxxxxx> wrote:
How close to each other in time would the charges need to fire to cause an
issue? Have you ever seen an airframe blowout from too much charge?
-Andrew
On Thu, Jul 18, 2019 at 8:32 AM R Dierking <applerocketry@xxxxxxxxxxx>
wrote:
I don’t think there’s a good way for Terry to test this because it’s the
timing he’s most concerned about. He would need to static fire a similar
motor and, even then there’s variance in the delay grains within lots.
But, every launch and flight is a test. Just would be sorry to see him
blow apart his airframe because both charges went off simultaneously.
And, hey, it’s rocketry - it could happen! You are being soooo careful,
and then, what the heck!
I’m wondering about the charge size for the Saturn V Estes model rocket
I’m launching on the 20th. It’s electronic deployment using a
Perfectflite CF. Charge calculator says 0.2 grams BP. Kurt farts
stronger than that. LOL. I’m going with 0.4 grams and hope the thin
airframe holds along with the plastic model rocket parachutes, etc.
Richard
On Jul 18, 2019, at 8:14 AM, Rich Silva <richsilv@xxxxxxxxxxx> wrote:
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>
wrote:
Nope. You get what you get...
Sent from my iPhone
On Jul 16, 2019, at 4:13 PM, Terry McKiernan <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>
Date: 7/16/19 8:37 AM (GMT-08:00)
To: 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 <roc-chat-bounce@xxxxxxxxxxxxx> on
behalf of Terry McKiernan <terry@xxxxxxxxxxxxx>
Sent: Tuesday, July 16, 2019 7:57 AM
To: roc-chat@xxxxxxxxxxxxx <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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