Barrowman by hand!!!

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graylensman

Well-Known Member
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!

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?

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

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!

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.

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

powderburner

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

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:

And come on back here to TRF if you have qstns

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

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.

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?

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.

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

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.

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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