Unfortunately, this is the typical case where our transparency system will work poorly (2 transparents with one inside the another), the current workaround are :
- in the skfb editor, select "single sided material" for the outer shell (to be honest, it exploits a minor bug in our rendering system but in your case it will fix your issue)
- separates the spheres into several geomtries/patches (~10 patches?), I didn't try it might reduce the popping
- just use one sphere, or make the inner shell opaque
Maybe one day we'll have an order-independent system (simple additive blending can do the trick sometimes).