[roc-chat] Re: BP for Deployment of Main Question
- From: Wedge Oldham <wedgeoldham@xxxxxxxxx>
- To: roc-chat@xxxxxxxxxxxxx
- Date: Thu, 8 Dec 2011 20:22:17 -0800
I agree with Dickinson, that a backup charge 50% greater than the primary;
set one second after apogee. Not sure if I intuitively agree with 4.5
grams; I'd do 6 grams.
I agree with Kurt; that a backup altimeter is a good idea.
I also agree with Kurt "use a lot of shock cord. My rule of thumbe is 10
feet of shock cord for every inch of body tube diameter. 7.5" airframe
would get 75 feet of shock cord.
Other things not mentioned that I do: Make sure your primary charge is well
seperated from your backup charge....you must not have both go off at the
same time.
Wedge Oldham
On Dec 8, 2011 5:06 PM, "Kurt Gugisberg" <kurtgug@xxxxxxxxxxxxx> wrote:
> I agree with Rick here. If you have a back up altimeter, set it for a
> second or two later and use the 7 gram charge. Use plenty of shock line
> also just in case.
>
> Kurt
>
> -----Original Message-----
> From: Rick Dickinson **
> Sent: Dec 8, 2011 4:55 PM
> To: "roc-chat@xxxxxxxxxxxxx" **
> Subject: [roc-chat] Re: BP for Deployment of Main Question
>
> Now, when you set it up for the flight, use at least 4.5g, with 7g as your
> backup charge a second after apogee....
>
> - Rick "belt & suspenders" Dickinson
>
>
>
> -- Sent from my Palm Pre
>
> ------------------------------
> On Dec 8, 2011 4:49 PM, Bryan Dierking <bryandierking@xxxxxxxxx> 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*<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*<redierking@xxxxxxxxxxx>
> >
> To: "*roc-chat@xxxxxxxxxxxxx* <roc-chat@xxxxxxxxxxxxx>" <*
> 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*<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*<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* <David.P.Smith@xxxxxxx>
> To: *roc-chat@xxxxxxxxxxxxx* <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*<roc-chat-bounce@xxxxxxxxxxxxx>
> ------------------------------
>
>
>
> I use the "rule of thumb" on this site. *
>
> **http://www.vernk.com/EjectionChargeSizing.htm*<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*<redierking@xxxxxxxxxxx>
> >
> To: <*roc-chat@xxxxxxxxxxxxx* <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*<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 :-)
>
>
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