[roc-chat] Re: BP for Deployment of Main Question
- From: "Rick Dickinson" <rtd@xxxxxxxxxxxx>
- To: "roc-chat@xxxxxxxxxxxxx" <roc-chat@xxxxxxxxxxxxx>
- Date: Thu, 8 Dec 2011 16:55:05 -0800
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
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 :-)
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