Hot air balloon carrying a crate ?

Suggestions on modeling the physics objects for a hot air balloon carrying a crate ?

I am not concerned about the string. Idea being is if the character hits the crate the balloon tilts and floats slightly astray, but if they hit the balloon it pops and the crate falls…

I have tried all kinds of parent → child relationships with varying types of spring joints etc…

So in the end my physics methods were correct, there is a bug with recursively turning off nodes that blasts my physics objects for some reason.

The physics model I ended up using: Crate is a rigidbody with a hinge joint connected to the balloon. The balloon has same mass but a constant up force and the two objects are sibling objects. The balloon has an increased drag and angular drag and they both use gravity.

Now here is the unity bug: If they are both childed to GameObject A, and I do A.setActiveRecursive(false), then turn it back on later with A.setActiveRecursive(true)…

Then the physics gets completely blasted. This is a hard deal for me to figure out as I have tons of objects on different level nodes that get recursively shut down and turned back on.

When I say physics gets blasted I refer to things like the balloon appears 1000 units to the right, and the crate is at world 0 jiggling back and forth, and other symptoms. The references on the joint are still correct, but they seem to be ignored. However, if I don’t turn them on or off, it works fine. Ugh…

Provided you have some joint between both, you can simulate a baloon physics with the following:

var massCrate: float = 1; // adjust for the crate mass, if needed

function FixedUpdate(){
  rigidbody.AddForce(-Physics.gravity * (rigidbody.mass+massCrate));
}

This will keep the baloon and the crate at a stable condition. If you kill the crate, the force will become too excessive and the baloon will fly. If you kill the baloon, the crate will fall since nothing more sustains it.

You may have to tweak the baloon mass to make it go up reasonably slowly when the crate is killed - the most massive, the slower it will go up.