Hopping Planetary Rover

Premise

Wheeled rovers are slow. Rotorcraft are limited by atmosphere. But on the Moon and Mars, the energy budget for a ballistic hop is unreasonably good: low gravity stretches each jump, no atmosphere wastes energy on drag, and a parallelogram leg with a diagonal spring recovers a meaningful fraction of the landing energy on the next cocking cycle.

The result is a rover that covers ground faster than wheels and lighter than wings, with the rough-terrain tolerance of a legged robot. This isn't a new idea — ETH Zurich's SpaceBok demonstrated the mechanism on parabolic flights — but the engineering remains underexplored and the design choices are not obvious.

Architecture

The mechanical heart is a four-bar parallelogram leg with a single diagonal spring (the BD diagonal) doing the energy storage. A small actuator cocks the spring against the ground reaction during a crouch phase, then releases it ballistically; the closed-form kinematics of the linkage do the rest.

DOF

2 (planar)

Linkage

Four-bar parallelogram

Spring

BD diagonal

Calibrated loss

0.66 vs Kolvenbach 2021

The implementation is end-to-end: closed-form four-bar closure equations (no 2D surrogates), the actual diagonal-spring force model, cocking dynamics, jump-phase physics, self-righting analysis, and matplotlib-driven animations + flipbooks for each phase. The first analytical pass calibrated cleanly against Kolvenbach 2021 at a loss fraction of 0.66, which keeps the project grounded in a published benchmark.

Current state

Working prototype in Python. Phase 5 contact simulation is the active front, focused on the discontinuous dynamics at touchdown and the self-righting envelope after an off-nominal landing. Phase outputs live under results/spacebok/ — articulated-contact PNGs, phase-5 motion GIFs, and flipbooks for each subsystem.

This project recently adopted the Design Graph as its digital thread, making it the first internal program to commit to a graph-backed effectivity model end-to-end.

What's next

Bow-leg comparison module — the other leading hopping-leg topology — to bound the architectural trade. After that, a dynamic contact + self-righting validation pass against the SpaceBok flight data, and then a sizing exercise for a Mars-mass vehicle with realistic actuator power density.