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
