Thrust to weight for clusters

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Well-Known Member
Jul 31, 2003
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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
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!

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.
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.