On TRF, Performance Nut asked Estes about how they find a models Center of Pressure or CP:
I received Estes's response:
"Hello,
There is no "calculated" CP for the rockets - glue, paint, engine use, etc....these all affect the CG of each rocket. However, there is a system we use here at the rocket ranch. First, measure the overall rocket length and place a mark at half the length. Next, load the rocket with a motor, stuff wadding/chute in the tube and mark the current CG. Ideally the CG will be forward (toward the nose cone) of the center of the rocket that you measured. For stable flight, the CP needs to be forward of the CG. You can add some weight (modeling clay works good) to the nose to move the CG forward."
Performance Nut added:
"So it seems that Estes assumes CP is somewhere below the center of the rocket so they play it safe and say the CP is the center. So my next step I guess is to perform a swing test and keep adding weight to the rear until it gets unstable. I'll measure Cg and call that my Cp. Will not be precise but might be better than center estimate."
For the record, the Estes letter has an error. The statement: "For stable flight, the CP needs to be forward of the CG", should say "For stable flight, the CG needs to be forward of the CP".
ReplyDeleteI am shocked to hear that Estes does not use Rocksim or some program of there own design. They need to move into the 21st century with the rest of their customers.
Hi Eero,
DeleteYou're right. There is a big mistake in the letter! Just a typo, but a big one.
I would assume Estes would use simulators now, I don't know. But they did always inform you about C/G and C/P relationships in the catalogs and tech reports.