Thrust to weight for clusters

[JeffS]

Hi folks,
Does anyone know how I can arrive at a thrust to weight ratio for a 3 motor cluster rocket? Do you use the peak thrust of the three motors added together? I need to make sure I'm over that 5:1 thrust to weight ratio. The rocket in question weighs approximately 15.7 oz. with D12's and 17.50 with E15's (I'm using the demo version of RocSim - no E9's).
Is there a formula for this?

Many Thanks

 

[Ryan S.]

add the numbers. 3 d 12s (3x12) 36Ns of force.

 

[Elapid]

most important, since that's what's getting it off the pad.
at least for motors with high initial thrust, some start low and build toward the end of the burn, etc.

i found the link i read...
great cover shot, too!
https://stephensonline.ca/carweb/archive/Earthrise_vol32.pdf

check out the RSO training manual portion that gives an in-depth discussion of progressive, regressive, or neutral burn motors and determining safe liftoff weights.

 

[KermieD]

The key with clusters though is to make sure you're well over the normal ratio. While D12s are not difficult to light, if you've put a lot of work into the bird, you want to make sure it will survive. As such, I would attempt to make sure you have at least marginal stability with 2 lit motors. That's just me though, and I've been known to screw up a lot.

 

[JeffS]

Thanks all. Good advice all around.