clarification for thrust/weight calc

Just a real quick clarification - when doing thrust to weight calculations, should I be looking at average thrust, or peak thrust? I'm assuming its the former, but want to be certain.

RSO's use average thrust (i.e., the motor designation) when they decide whether or not to let you fly. If you are using a motor with a higher initial thrust (i.e., off the pad), be prepared to shoe them that the motor can safely launch the rocket. For initial thrust, you're concerned with the first tenth of a second or do.

Also, if you use average thrust, always check the curve to make sure you are getting enough initial thrust, too.

Bat-mite said:

be prepared to shoe them that the motor can safely launch the rocket.

Click to expand...

Know it's just a typo, but I think there might be some RSO's out there a little nervous now!

Both.

1.) BP motors usually have peak thrust during the first 0.3 to 0.5 seconds. You want to make sure that the average lift-off acceleration during rod/rail time is sufficient so the fins will have aerodynamic authority before the rocket leaves the rod/rail. Same lift-off criteria applies for all rockets.

2.) TRA safety code requires a 3:1 average thrust to weight ratio of engines ignited on pad divided by the pad weight of the rocket.

Bob

OK, thanks. I ask because my recently built Astron Cobra upscale came out a little heavier than planned. I want to verify that I'm still in the clear for what I should be doing with three E16 or F15 motors (which looks favorable still). The sim predicts it to be acceptable, but sims aren't 100%...