I realize that my perspective is totally different from most fliers, and I hope I don't come off as bragging, but here is how I handle multiple rockets/motor selection. I have over 50 rockets that were built with repeatability foremost in my mind. Some have dozens of flights, some only a few, depending on how much I like them, or how easy they are to prep. Each rocket has a folder in a 3 ring binder, with several binders for low power, high power, inactive and destroyed/lost. When I prep for a launch, I decide which rockets I want to launch based on the launch. Waiver altitude, ground conditions and host club are all factors in this. When I have a list, I pull out the folder and go over my previous flights with each rocket, which is recorded after each flight. A lot of my favorite rockets have favorite motors, like my dual deploy Argent, which has 15 flights on an H115 dark matter. If I want to change that, I look at the thrustcurve printout in the folder and determine if another motor will work. Sharon and I stockpile motors, we have a Google doc. with a list of them. It's a bit embarrassing when people come up to us at a launch and ask us if we are vendors. Most of you go to a launch and buy a motor for a rocket you want to fly. You could use the same thrustcurve printout to see if the motor your vendor has will fly in that rocket. Or, if you have a smart phone or Ipad, bring up thrustcurve and check it that way. All my rockets are on there, except for clusters, which thrustcurve won't support. When I go to a launch, I'm there to fly rockets, not prep rockets. I usually bring 15 rockets ready to fly for a weekend launch, and fly 12 of them. Sharon is slightly more conservative and might only fly 10. All those rockets are prepped here at home. We buy motors based on what we flew last year, with some additions based on what we plan to build in the off season. If I didn't log my flights, none of this would work. Plus, at my age, I can't remember what I had for lunch yesterday, so how am I gonna keep track of all this other stuff?So, here's some advise from an old guy that flies a LOT of rockets. 1.) Take notes, your learning curve will be a lot shorter, and you can claim to be a rocket scientist! 2.) Use Thrustcurve.org, it does most of the science for you.