How high can I set the beamSpring value before it explodes into an explosion of craziness?

I'm trying to make a ridiculously fast (we're talking tens of thousands of miles per hour) rocket but the problem is that even on sun gravity when the thrusters are on the middle sections bends back and forth causing the rocket to go in midair and flipout. The beamdeform values are aleady set ridiculously high so the rocket never permanently deforms, but I need a very high beamspring value so it doesn't bend in the first place. When I try to set it very high, everything just breaks and all reality is consumed into a black hole. What is the maximum value before it freaks out?
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Alternatively, is there a way to set the gravity higher than sun?