According to Wedge, Bryan should use 75' of shock cord. With that
much you could use kite string and it should still be safe. ;-)
The question is...should it be nylon or Kevlar kite string.
Kurt
-----Original Message-----
From: Richard Dierking
Sent: Dec 9, 2011 9:30 AM
To: roc-chat@xxxxxxxxxxxxx
Subject: [roc-chat] Re: BP for Deployment of Main Question
Bryan's currently in school otherwise he would answer this himself.
Yes, he used a parachute for the test and a shorter/lighter shock
cord (10', 1000 lb vs. 15', 2300 lb). The cord was attached to both
parts. He might also be extending the shock cord more than 15' for
the flight. The ground test was energetic and the parachute was
deployed nicely.
Richard
> Date: Fri, 9 Dec 2011 09:22:19 -0800
> From: dmccue@xxxxxxx
> To: roc-chat@xxxxxxxxxxxxx
> Subject: [roc-chat] Re: BP for Deployment of Main Question
>
> Yay! I hope you also try with the parachute in there as well - it
would
> suck to have the sections separate, but no parachute ejection. (I
don't
> know if the 'chute is attached to both parts, or only one. If
attached to
> both airframe sections, the bridles will puil it out for sure...)
>
> -Dave McCue
>
> On Thu, 8 Dec 2011, Bryan Dierking wrote:
>
> > Richard and I just did a ground test with three 2-56 sheer pins
and 3.5 grams in one of the canisters - worked like a charm.
> >
> > Bryan Dierking
> >
> >
> > Le Dec 8, 2011 à 16:07, David.P.Smith@xxxxxxx a écrit :
> >
> >> Using the table on the web page I listed it came out to 4
grams. His table has worked well for me in the past, but I haven't
used it on a 7.5 inch tube.
> >>
> >> Using the bottom formula on that same page and shooting for 10
psid I came up with 5.87 grams. Intuitively this seems closer. Of
course there are various assumptions made that can only be validated
with testing.
> >>
> >> One assumption in all these equations is 100% conversion of the
solid black powder into gas. That seems very optimistic. With
tightly wrapped charges as described by Dave McCue used with shear
pins to hold the parts together a bit longer, you can produce a
pretty substantial pressure in the tube before things start to
actually move apart.
> >>
> >> Ground testing seems like a very good idea to me...
> >>
> >> David P Smith
> >> NAR 78668 L2
> >> Amateur Extra, W6DPS
> >>
> >> The opinions expressed in this email are my own and do not
necessarily represent the positions, strategies or opinions of
Southern California Edison, its parent company Edison International,
or any of their affiliates.
> >>
> >>
> >>
> >>
> >>
> >> From: Richard Dierking <redierking@xxxxxxxxxxx>
> >> To: "roc-chat@xxxxxxxxxxxxx" <roc-chat@xxxxxxxxxxxxx>
> >> Date: 12/08/2011 12:14 PM
> >> Subject: [roc-chat] Re: BP for Deployment of Main Question
> >> Sent by: roc-chat-bounce@xxxxxxxxxxxxx
> >>
> >>
> >>
> >> Let me ask this first: did you come up with 4 grams per
cartridge or 4 grams total?
> >>
> >> Sent from my iPhone
> >>
> >> On Dec 8, 2011, at 12:07 PM, David.P.Smith@xxxxxxx wrote:
> >>
> >> Large areas can generate very large force if you keep the
pressure constant.
> >>
> >> Think of it this way. For the same length of tube, the diameter
increases the area and volume by the square of the increase in
diameter.
> >>
> >> Round off a 4 inch rocket tube to 12.5 square inches times 24
inches in length, the volume is 300 cubic inches.
> >>
> >> Round off an 8 inch rocket tube to 50 square inches times 24
inches, the volume is 1200 cubic inches. Four times the volume of
the 4 inch.
> >>
> >> So, if a 4 inch diameter body tube 2 feet long is pressurized
to 20 psid with a given charge, then an 8 inch body tube 2 feet long
would be pressurized to 5 psid.
> >>
> >> 12.5 times 20 psid comes out to 250 pounds of ejection force.
50 times 5 comes out to 250 pounds of ejection force.
> >>
> >> So for a given ejection charge, in a range of body tube
diameters, you get the same ejection force.
> >>
> >> On the other hand, increasing the charge to four time the 4
inch charge, gives us 1000 pounds of ejection force in the 8 inch
rocket. that may be a bit much...
> >>
> >> If you just stick to the formulae published on some sites, you
just keep increasing the charge size proportional to the increase in
volume, which may over stress your shock cord.
> >>
> >> So how much force do you need to make the rocket come apart?
> >>
> >> Do you plan to ground test at all?
> >>
> >> David P Smith
> >> NAR 78668 L2
> >> Amateur Extra, W6DPS
> >>
> >> The opinions expressed in this email are my own and do not
necessarily represent the positions, strategies or opinions of
Southern California Edison, its parent company Edison International,
or any of their affiliates.
> >>
> >>
> >>
> >>
> >>
> >> From: Richard Dierking <redierking@xxxxxxxxxxx>
> >> To: "roc-chat@xxxxxxxxxxxxx" <roc-chat@xxxxxxxxxxxxx>
> >> Date: 12/08/2011 11:36 AM
> >> Subject: [roc-chat] Re: BP for Deployment of Main Question
> >> Sent by: roc-chat-bounce@xxxxxxxxxxxxx
> >>
> >>
> >>
> >> Wow, 4 grams BP is about one half what we were coming up using
the web and a reference book I had.
> >> Richard
> >> Sent from my iPhone
> >>
> >> On Dec 8, 2011, at 11:26 AM, "Chris J Kobel" <Chris.J.Kobel@xxxxxxxx
> wrote:
> >>
> >> I would second David's recommendation of 4 grams, which should
give about 200 lbs of ejection force at 5000 AGL, with the 3 2-56
shear pins requiring about 100 lbf to overcome.
> >>
> >> Chris
> >>
> >>
> >>
> >> From: David.P.Smith@xxxxxxx
> >> To: roc-chat@xxxxxxxxxxxxx
> >> Date: 12/08/2011 11:10 AM
> >> Subject: [roc-chat] Re: BP for Deployment of Main Question
> >> Sent by: roc-chat-bounce@xxxxxxxxxxxxx
> >>
> >>
> >>
> >> I use the "rule of thumb" on this site.
> >>
> >> http://www.vernk.com/EjectionChargeSizing.htm
> >>
> >> So, by that site you would need 4 grams of ffffg black powder.
> >>
> >> Remember that you are looking at a lot of area to apply the
ejection pressure to. A 7.5 inch circle had a bit over 23.5 square
inches of are. So 10 psid will give you 235 pounds of ejection force.
> >>
> >> What is the shear force rating for your pins?
> >>
> >> David P Smith
> >> NAR 78668 L2
> >> Amateur Extra, W6DPS
> >> The opinions expressed in this email are my own and do not
necessarily represent the positions, strategies or opinions of
Southern California Edison, its parent company Edison International,
or any of their affiliates.
> >>
> >>
> >>
> >>
> >>
> >> From: Richard Dierking <redierking@xxxxxxxxxxx>
> >> To: <roc-chat@xxxxxxxxxxxxx>
> >> Date: 12/08/2011 10:10 AM
> >> Subject: [roc-chat] BP for Deployment of Main Question
> >> Sent by: roc-chat-bounce@xxxxxxxxxxxxx
> >>
> >>
> >>
> >> Say your main deployment section is 7.5" diameter, 24" long and
using three (3) 2-56 nylon shear pins. How much BP?
> >>
> >> Also, what's the advantage of using two or more deployment
canisters instead of just one? For multiple canisters, would you
wire in series or parallel? (One 9 volt battery and Perfectflite WD
altimeter with main deployment at 1100'.)
> >>
> >> Richard Dierking
> >> BTW: Kurt, we're not intending to turn the nose into a second
stage :-)
> >>
> >>
> >> -- ROC-Chat mailing list roc-chat@xxxxxxxxxxxxx
//www.freelists.org/list/roc-chat
> >> -- ROC-Chat mailing list roc-chat@xxxxxxxxxxxxx
//www.freelists.org/list/roc-chat
> >> -- ROC-Chat mailing list roc-chat@xxxxxxxxxxxxx
//www.freelists.org/list/roc-chat
> >
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