[roc-chat] Re: BP for Deployment of Main Question
- From: Lakestake Rocketry <lakestake@xxxxxxxxx>
- To: roc-chat@xxxxxxxxxxxxx
- Date: Fri, 9 Dec 2011 14:02:36 -0800
Yes, in flight you also have aerodynamic resistance to overcome. I
remember one of Dirk's big rockets coming in ballistic. The main charge
went off, the nose popped out, then seated itself right back.
If you are in a flat spin the loading will be flexing your rocket so it may
take more force to separate the halves.
I was surprised the topic of shear pins didn't surface earlier in the
conversation.
One note I observed in my high altitude testing; the gasses shrink
considerably as they cool. This is an effect that may come into play if
you are using a metal mesh baffle such as a brillo pad to protect your
recovery. In the test chamber the ejection curve looked a lot like a
thrust curve with a large spike then a much lower plateau quickly following.
Matt
On Dec 9, 2011 9:19 AM, "Richard Dierking" <redierking@xxxxxxxxxxx> wrote:
> I apologize for not giving an update yesterday; it was crunch time
> getting ready for this weekend. You probably all saw the video Bryan
> posted. After all the discussion, he decided to ground test with 3.5 g
> BP. The deployment was clean, so he decided to go with 4.0 g BP and a
> single canister. Bryan can tell you more about his rocket, but I know it's
> fully redundant for the drogue (switch, battery, altimeter, e-match, etc.),
> and has one system for the main. The drogue chute is relatively large
> because he's not planning for the rocket to go really high.
>
> Regarding the amount of BP, shock cord length, and redundancy, I think
> this was a good conversation. Bryan doing a ground test for the main was a
> good call. It's best to do ground test but you must keep in mind that if
> it's successful, you still don't know if the charge was marginal (unless
> the rocket blows up). My experience with the deployment out of a side
> hatch showed me that! I used 0.5 g BP for the ground test and it worked
> fine. When I used the same amount for the flight it failed. So, I suggest
> using a little more BP for the flight to keep on the positive side if the
> ground test was marginal.
>
> Anyway, thanks for all the comments and suggestions.
>
> Richard Dierking
> ------------------------------
> Date: Thu, 8 Dec 2011 20:22:17 -0800
> Subject: [roc-chat] Re: BP for Deployment of Main Question
> From: wedgeoldham@xxxxxxxxx
> To: roc-chat@xxxxxxxxxxxxx
>
> 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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> ****
>
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