[SOLVED] Why does not my gravitational simulation work well?


I am doing a gravitational simulation of n-bodies where there is an attractor point at (0, 0, 0). But the orbits of rigid bodies change a lot for each period. What am I doing wrong? Is it a problem of the simpleUpdate? Or in the calculation of the distance to the attractor point?

    public void simpleUpdate(float tpf) {

        for (PhysicsRigidBody rbc : bulletAppState.getPhysicsSpace().getRigidBodyList()) {
            rbc.applyCentralForce(rbc.getPhysicsLocation().mult(-50f / rbc.getPhysicsLocation().distanceSquared(ZERO)));


JME physics uses single-precision (32-bit) floating point arithmetic. For astronomical simulations, you’d probably need double precision (64-bit). Bullet Physics has double-precision capability, but accessing it from JME would require massive changes.

In the mean time, you could try a smaller physics timestep. The default timestep is 1/60 second, and you can alter it by invoking PhysicsSpace.setAccuracy(timeStep). However, since JME physics runs in real time, you’ll encounter performance issues if you make the timestep too small.


Are you actually using bullet’s collision detection? If not then it’s a waste to use it at all. The “physics” part of a physics engine is dead-simple.

v = v + a * t
p = p + v * t

Apply forces as needed… which isn’t that much harder.

Edit: (and then you could pretty easily use double precision.)


Thank you very much for the advice, I have already tried calling PhysicsSpace.setAccuracy (timeStep) but the bodies follow exactly the same path if timeStep is worth 1/60, 1/60000 or 0.2. I am a beginner and I would like to study where the error comes from. Interestingly, if I replace what is inside mult (…) by -5f it works very well and the orbit is stable, so I suspect that the error may come from the calculation of the distance.


Thank you very much, but I would like to have the things that jBullet has like collisions between the surfaces of the bodies. I am a beginner and I would die to know why the orbits unexpectedly change and I am disappointed not to know the cause. I have noticed that if I replace what is inside mult (…) by -5f (always having an attractive force to the ZERO vector, but that remains constant with the distance) the orbits are perfect and do not alter with time . That’s why I suspect that the problem is in the calculation of distance.


I notice you’re using a 1/x law for the central force instead of an inverse-square law. Also, physics forces should be applied before each timestep, not during the frame update.


distance is squared in the code. How would you implement the second thing you say?


Because you have the distance in the numerator, you are computing a force proportional to distance/(distance squared) which is 1/distance, not 1/(distance squared).

To apply forces before every physics step, either add a PhysicsTickListener to your PhysicsSpace or else override the prePhysicsTick method of your BulletAppState.


ok, thanks, i gonna try it.


I finally solved it, but I still lack the physicsTickListener. Now the orbits are stable and works well.

rbc.applyCentralForce(rbc.getPhysicsLocation().normalize().mult(-200f / rbc.getPhysicsLocation().distanceSquared(ZERO)));

I used the normalize. I would not have realized the error without your help. Thank you.