Reactor is a physics engine developed by the Irish software company Havok for use in Autodesk 3ds Max.
In 3ds Max 2012, Reactor was replaced by a PhysX-based engine called MassFX.
[1] Reactor was often used for realistic physics simulation that would be difficult or time-consuming to animate by hand.
Reactor can also simulate dynamics of any supported type interacting with a water volume, with adjustable viscosity and depth.
Reactor includes a large number of forces that can be used in simulation, apart from the default gravity: springs, dashpots, motors, wind, fractures (breakable objects), and a "toy car" type, with definable body/axis/wheels.