Overview

Steel (Scalable Throughput-optimized Engine for Embodied Learning) is a GPU solver for articulated robots and rigid bodies in Newton. It integrates robots in joint coordinates, solves contacts and joint constraints with a projected Gauss-Seidel sweep that does not assemble the world’s constraint matrix, and schedules every world of a batch on its own, so different robots, tasks and scenes can share one batch. Newton is the open-source physics engine hosted by the Linux Foundation.

newton.solvers.SolverSteelexperimental in Newton 1.8

We benchmark Steel against PhysX and MuJoCo Warp on single-task and mixed-task Isaac Lab workloads in the launch post.

What it is for

Steel was built for robot learning at scale: thousands of environments per GPU, fixed memory, every step capturable in a CUDA graph, and batches that mix embodiments. Tree joints hold exactly because the state is the joint coordinates; constraint rows are needed only for contacts, limits, optional drive rows, mimic couplings and loop closures. In the default serial sweep each world’s rows are solved by one GPU warp in sequence, which keeps Gauss-Seidel’s convergence while thousands of worlds run in parallel.

Steel is experimental in Newton 1.8: SolverSteel, its constructor options and its steel: attributes may change without the normal deprecation period.

Newton’s articulated-body solvers

Newton has several solvers that simulate articulated bodies. From their source and Newton’s solver documentation, with Newton’s joint types PRISMATIC, REVOLUTE, BALL, FIXED, FREE, DISTANCE, D6 and ROD:

Solver	Coordinates	Contacts and constraints	Joint types	Loop closures
Steel SolverSteel	Reduced	Hard contacts with Coulomb friction; matrix-free projected Gauss-Seidel	All but ROD; DISTANCE moves as FREE	BALL, REVOLUTE and FIXED loop joints
MuJoCo Warp SolverMuJoCo	Reduced	MuJoCo soft constraints (solref, solimp); CG or Newton solver; pyramidal or elliptic cone	All but DISTANCE and ROD	FIXED, REVOLUTE, BALL and one- or three-axis angular D6 loop joints, as equality constraints
Featherstone SolverFeatherstone	Reduced	Penalty spring-damper contacts	All but ROD; DISTANCE moves as FREE	No
Kamino SolverKamino	Maximal	Hard contacts with friction and restitution as a complementarity problem; proximal ADMM by default	PRISMATIC, REVOLUTE, BALL, FIXED, FREE	Yes
XPBD SolverXPBD	Maximal	Position-based contact and joint constraints	All but ROD	Yes
Semi-implicit SolverSemiImplicit	Maximal	Penalty spring-damper contacts and joints	All but ROD; DISTANCE moves as FREE	Yes
VBD SolverVBD	Maximal	Vertex block descent; compliant augmented-Lagrangian contact and joint constraints	All but DISTANCE	Yes

Solver	Particles, cloth, soft bodies	Gradients	Devices	Worlds with different structure
Steel	Through coupled solvers	No	CUDA; CPU for the core solve	Yes
MuJoCo Warp	Through coupled solvers	No	CUDA (MuJoCo Warp); CPU (MuJoCo C)	No: bodies, joints and equality constraints match across worlds; meshes may differ
Featherstone	In the same solver	Basic (diffsim examples)	CPU, CUDA	Yes
Kamino	Through coupled solvers	No	CPU, CUDA	Yes
XPBD	In the same solver	No	CPU, CUDA	Yes
Semi-implicit	In the same solver	Basic (diffsim examples)	CPU, CUDA	Yes
VBD	In the same solver	No	CPU, CUDA	Yes

Newton marks Steel, Kamino and VBD experimental. Steel can be one entry of Newton’s experimental coupled solvers, next to VBD or XPBD for cloth, cables and soft bodies; see Coupling. Model features Steel rejects at construction are listed in the FAQ.

To see what Steel runs, browse the examples: scenes with a clip of each one’s Steel preset, its presets for the other solvers and the command that runs it.