applyCentralForce() in simpleUpdate or prePhysicsTick

Physics should run framerate independent. Bullet should scale force by physicsDelta automatically. Gravity, thrusters, lift. Impulse is a instant force. Jumping/collision response. I have the feeling that you are mixing game tpf/framerate with physics update. If you set the phyiscs rate at 60hz and the renderer is limited to 60hz the physicstick listener is executed exact 1 time per frame, independently of the substeps you set. Those come into play if the renderer cannot keep up. If you render faster then physics then some renderframes should be skipped. At least that is my understanding on how bullet works. The tpf value from the ticklistener should be constant i think and unrelated to jme’s tpf