Which physics engine should Gazebo use by default for real‑time simulation?

Goal: Identify the physics engine that should be set as Gazebo’s default so that typical simulations maintain a real‑time factor (RTF) of at least 1.0 without sacrificing numerical stability in multi‑body contact scenarios.

Constraint: RTF measurements can be inflated by GUI rendering, making headless execution preferable for isolating physics bottlenecks, and the default engine differs between Gazebo releases (e.g., ODE in Gazebo 9, Bullet in Gazebo Harmonic), which adds version‑specific uncertainty.

Uncertainty: It is unclear whether switching the default from ODE to Bullet yields a measurable RTF improvement for complex contacts, how the trade‑off between stability and CPU load varies across versions, and what RTF threshold should guide the selection decision.

Does changing the default physics engine from ODE to Bullet improve the RTF for typical contact‑rich worlds? Which version‑specific trade‑offs between stability and computational load should be considered when choosing the default? Should the selection be based on a headless‑measured RTF target of 1.0 or higher?