Off-world Power Generation Air-Bearing Testbed

The air-bearing testbed is the ground analogue for the project's in-space self-assembly work: identical robotic modules float on a near-frictionless planar surface (3 degrees of freedom — x, y, yaw) and rehearse relative navigation, docking, and growth logic at hobby cost. It does not reproduce orbital relative dynamics; it validates the planar kinematics, docking, and autonomy. The shared apparatus below underlies Lab 3 (propulsive navigation), Lab 4 (moment control), and Lab 5 (robotic self-assembly).

The running surface

Modules float on a sheet of float glass (flat to tens of microns by manufacture) — the smooth running surface. The glass is supported continuously by a stiff, flat bed; the bed is leveled on 3 adjustable points. Full stack:

stock 4-leg table (rough-leveled, no drilling)
  -> 3 leveling adjusters (top-side, at working height)
    -> torsion-box bed (stiff + flat)
      -> float glass (continuously supported)
        -> air-bearing puck (the module)

Glass: ¼″ (6 mm) clear annealed float, cut to ~20″×35″ to sit just inside the 20.5″×36″ table (no overhang — a cantilevered annealed sheet cracks). Ask for seamed edges (safe to handle); don't pay for polished — the bearing never runs on the edge. Specify annealed, NOT tempered (tempering's roller-wave distortion ruins the ~10 µm fly height). Source locally and pick up — don't ship glass. Get ≥2 quotes; small custom cuts vary widely on shop minimum + edgework.

Leveling — the #1 build variable

Residual tilt projects gravity into the plane as phantom gravity: a = g·sin θ. At 0.10° that is 17 mm/s² of drift; the target is < 0.05° (8.6 mm/s²), ideally 0.01°. Use kinematic 3-point support — three points define the plane exactly (no rock, both tilt axes trimmable); 4 supports over-constrain and rock.

Level the surface, not the table. The table has fat 4×4 legs; drilling and turning feet under a heavy table is awkward, and table-leg leveling only fixes overall tilt — not the wood top's local waviness. Instead, stand 3 adjustable bolts on top of the table, under the bed, and trim there at working height beside the digital level. A stiff bed on 3 points also bridges over the table top's waviness. Rough-level the table once with shims (~1°), then trim < 0.05° on the 3 bolts, and re-check loaded (glass + module shifts it). The air bearing exerts negligible horizontal force, so gravity holds the bed; optional cone / V-groove / flat (Kelvin) seats under the 3 bolt tips make the mount non-sliding and repeatable.

Torsion-box bed — build plans

A torsion box is two thin skins glued to an internal rib grid: stiff and dead flat for low weight and ~$40 of materials. A plain ¾″ panel on 3 points would sag at the overhangs (0.3 mm over 10″ ≈ 0.07° slope — over budget); the box stays flat. It is only as flat as the surface you glue it on — so assemble it on the float glass itself (a micron-flat reference), covered with wax paper so glue won't stick. Target box ≈ 20.5″×36″ × ~2.5″ thick.

Buy

Skins: ¼″ MDF or tempered hardboard — two pieces 20.5″×36″.
Ribs: ¾″ × 2″ strips, ~16–20 linear ft (rip from ¾″ stock, or 1×2 furring).
Glue: PVA wood glue (Titebond); brad nails/staples or clamps + even weights.
3× threaded inserts (fine-thread 3/8″-24 or M10×1.0) + 3 bolts / leveling glides.

Buy raw and cut yourself, have a store make the straight panel cuts, or order the panels cut-to-size online (e.g. SendCutSend, Cherokee Wood Products) and glue at home. A light box (~10–15 lb) is what makes the 3-bolt leveling easy — avoid heavy ready-flat plates (granite ~100 lb, MIC-6 aluminum) here, since you hand-turn the leveling bolts.

Build

Cut the two skins flat and square (20.5″×36″). Cut the rib grid: a perimeter frame + internal ribs forming ~5–6″ cells (≈5–6 cross ribs along the 36″ length + 2–3 long ribs across the 20.5″ width). Butt joints are fine.
Lay the float glass on the table, cover with wax paper / plastic → your flat assembly jig. Lay the bottom skin on it.
Glue the rib grid to the bottom skin; tack with brads or weight down. The flat skin forces every rib into one plane.
Glue the top skin on. Weight the whole sandwich evenly and let it cure on the flat reference (overnight). It cures locked-flat.
Seal / edge-band the edges (humidity stability); wipe the top clean.
Install the 3 threaded inserts in the bottom skin, each directly over a rib (so the adjuster load goes into structure, not an unbacked ¼″ skin) — two along one long edge, one centered on the opposite edge, inset ~3–4″. Thread in the 3 bolts; their tips stand on the table top.
Set the box on the table, drop the glass on top (continuous support), and level on the 3 bolts against the digital level. Re-check loaded.

Key gotchas: assemble on the flat reference (step 2 is the whole game); the inserts must land over ribs (step 6); rough-level the table first so the bolts only do the fine trim.

Lazy fallback (no box): doubled ¾″ MDF (1.5″) on the 3 adjusters — stiffer than single ¾″ but marginal vs the 0.05° budget at this span; build the box if you can.