Barrowman by hand!!!

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graylensman

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Well, I did it. Working up a design for a D egglofter based on some other plans I've seen kicking around (most notably George Gassaway's TwoMinuteEgg). Sat down at the Mac, cracked open my <i>Handbook</i>, fired up Excel, and started pounding away at the keys.

One hour later, I had my CP!!!

My brain was buzzing for an hour afterward! :p
 

powderburner

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Now the secret is out. Everyone will want to try it.
And it's not hard to do.

Do you want to add a matching spreadsheet calculation for center of gravity?
 

powderburner

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may not be as satisfying as using a paper and pencil (and chopping down another forest?) but you still have to know what the equations are doing
 

graylensman

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Originally posted by powderburner
Now the secret is out. Everyone will want to try it.
And it's not hard to do.

Do you want to add a matching spreadsheet calculation for center of gravity?
The hardest thing to do was keeping track of what portion of the formula I was doing at any given time. I'm sure an Excel expert could find easier/quicker ways to do this; and I'll probably take a stab at it for future use.

Do you *have* a spreadsheet for CG calculations? That would be handy too!
 

rstaff3

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I think everyone should go through this exercise at least once. I even made a spreadsheet prior to getting vcp and then RockSim. There was a fair amount of reconfiguration for each design, but it still helped.
 

Mike

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Little problem...where can I get the formula???
 

graylensman

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powderburner

your spreadsheet is perfect for my needs. I'm not designing anything exotic - just sport models. Thanks for posting that.
 

powderburner

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Originally posted by Mike
Little problem...where can I get the formula???
If you are looking for the fabled Barrowman eqns, do a quick google and you will find sites like this one:

https://my.execpc.com/~culp/rockets/Barrowman.html
where the eqns are presented in summarized form

Or, go look at our Apogee friend's website for the complete original report:
https://www.apogeerockets.com/education/downloads/Barrowman_report.pdf

And come on back here to TRF if you have qstns


Update:
Oops----just saw that someone already posted one of those sites.
 

BlueNinja

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I did it for teh Tethys by hand and on a TI-30XIIS calc, came within .8 inches of the PML recommendation of 41 inches.
 

GL-P

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I did it for teh Tethys by hand and on a TI-30XIIS calc, came within .8 inches of the PML recommendation of 41 inches.
What does PML use for its calculations?
 

rstaff3

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Since they provide Rsims for all their designs, I assume that's what they use. Thie file for the Tethys says 41.276 using the Barrowman method.
 

BlueNinja

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ok, then I was within .6" :) I rounded some of teh numbers however.
 

graylensman

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Originally posted by rstaff3
...You can laso examine the spreadsheet that graylensman mentioned.
Well, the only thing I used a spreadsheet for was the actual CP calculations... and to hold results of one part ready to mix with the results of another.

I don't have one set up where you can plug in your variables and spit out the answer. I'm intending to consult with the company Excel Expert to try to get a sheet set up that way.

Powderburner's CG sheet with attendant explanation was quite useful. Looking forward to testing the results empirically.
 

rstaff3

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Mine wasn't that snazzy either but it automated the acutal math. Now I'd much rather play with cuter toys.
 

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