Actuators and Hydraulics

TL;DR

Purpose: Drives and mechanisms for Gen-2 robots

Heat transfer fluid: Sodium-Potassium (NaK) instead of water

Drives: Cycloidal, direct-drive, tendon, NaK hydraulics

Bearings: MoS₂ journal bushings (local) + imported ball bearings for motors

Localization: ~90% local

Overview

Actuators are the drives of Gen-2 robots, responsible for limb, joint, and manipulator movement. On Mercury, conventional water-based hydraulic systems cannot be used (water freezes and evaporates in vacuum), so special fluids and lubricants are employed.

Types of Actuators

Hydraulic Actuators (Primary)

Parameter	Value
Working fluid	Sodium-Potassium (NaK)
Pressure	200-300 bar
Temperature	−12°C … +400°C
NaK source	Distillation

Advantages of NaK: - Liquid across a wide temperature range (−12°C to +785°C) - Does not boil in vacuum at operating temperatures - Produced from local regolith (electrolysis)

Disadvantages: - High chemical reactivity (reacts with water) - Requires hermetic circuits

Electric Actuators

Parameter	Value
Drive types	Cycloidal, direct-drive, tendon
Power	100-500 W
Localization	~70% (Al windings, Fe housings — local; NdFeB magnets — import)

Cycloidal drives (Mole-M, Crab-M, and Centaur-M wheels, WAAM robots):

Eccentric + cycloidal disk + roller pins
All parts: WAAM + grinding ±0.1 mm
Gear ratio up to 87:1 (single-stage), efficiency 75-90%
100% local production
Precedent: Nabtesco — largest manufacturer of cycloidal drives for robots

Direct-drive (Centaur-M joints 1-4):

High-torque motor directly on shaft — no gearbox
Zero backlash, zero gearbox maintenance
Import NdFeB magnets ~1-2 kg/motor
Precedent: KUKA iiwa, Kollmorgen

Tendon drives (Centaur-M wrist, joints 5-6):

Motor on robot body, wire through pulleys to remote joint
100% local production (Fe/Al wire)
No backlash with proper tensioning

Drive summary table:

Robot	Task	Drive
Mole-M wheels	1:20, high torque	Cycloidal
Mole-M bucket	Linear, 50 kN	NaK hydraulic cylinder
Crab-M wheels	1:20, medium torque	Cycloidal
Crab-M platform	5 t lift	NaK hydraulic cylinder
Centaur-M wheels	1:20, light	Cycloidal
Centaur-M joints 1-4	Precision, 1:1	Direct-drive (NdFeB)
Centaur-M wrist 5-6	Light, precise	Tendon drive

Earth analog: Serial integrated servo drives for humanoids are already being manufactured in automated production: EYOU (100K joints/year, Shanghai, 2026). The basic kinematics of Centaur-M are analogous to a 6-axis humanoid manipulator. See TRL assessment.

Hydraulic System

NaK Circuit Design

flowchart LR
    subgraph CIRCUIT["HYDRAULIC CIRCUIT"]
        PUMP["Pump<br/>NaK"]
        ACTUATOR["Actuator<br/>(cylinder)"]
        RADIATOR["Radiator<br/>(cooling)"]
    end

    PUMP --> ACTUATOR
    ACTUATOR --> RADIATOR
    RADIATOR --> PUMP

    style CIRCUIT fill:#d4edda

Component	Material	Mass	Source
Pump	Al housing + Fe rotor	5 kg	Local
Tubing	Stainless steel	3 kg	Local
Cylinders (actuators)	Steel + Al	10 kg	Local
Radiator	Aluminum	2 kg	Local
TOTAL	—	20 kg	100% local

Hydraulic Seals

NaK destroys PTFE (Teflon) and all organic polymers — standard hydraulic seals are not applicable.

O-FLEX metal seals (Technetics Group):

Parameter	Value
Pressure	vacuum … 500+ bar
Temperature	−273°C … +980°C
Material	Stainless steel / alloy 718, silver coating
Leakage	10⁻⁴ … 10⁻⁹ std·cm³/s (He)
Principle	Self-energizing — medium pressure increases seating force

NaK compatibility: stainless steel 316 — 32,600 hours continuous operation (DTIC).

EM pumps for cooling circulation — no moving parts or seals. Precedent: SNAP-10A reactor (1965), NaK circulation via EM pump (NASA NTRS).

Manufacturing: WAAM body + grinding of seating surfaces. Metal O-rings: forming press (2 m², 5 kW). Silver coating — Ag import (grams per seal).

Working Fluids

Sodium-Potassium (NaK)

Parameter	Value
Composition	Na₂₂K₇₈ (22% Na, 78% K) — eutectic
Liquid range	−12°C … +785°C
Density	0.87 g/cm³
Viscosity	0.7 cP (at 100°C)
Application	Hydraulics, cooling, heat transfer

Source: Extracted from local regolith via electrolysis.

More details: Distillation

Coolant

NaK is used not only for hydraulics but also for cooling: - Robot solar panels (Crab-M, Centaur-M) - Electric motors - Power electronics

Cooling circuit: - NaK flows through radiators facing away from the Sun - Heat is radiated into space - Temperature maintained at +50-100°C

NaK Thermal Management

Problem: NaK freezes at −12°C, but Mercury’s shadow reaches −180°C.

Solutions:

Robots operate in the terminator zone (+50…+150°C) — NaK always liquid
During operation: waste heat from motors keeps NaK above −12°C
In sleep mode: electric heating 0.5 kW (from cable or battery) — autonomy: Crab-M 100 h (~4 days), Centaur-M 40 h (~1.7 days), Mole-M unlimited (cable)
Emergency (total power loss in shadow): NaK freezes but recovers upon heating without circuit damage

Bearings

MoS₂ Journal Bushings (Low-Speed Applications)

Molybdenum disulfide (MoS₂) is the standard vacuum lubricant for space mechanisms. Applied via magnetron sputtering, coating thickness 0.5-5 µm.

Parameter	Value
Friction coefficient in vacuum	0.03–0.06
Temperature range	−200°C … +1200°C
Allowable PV (pressure × velocity)	0.5–2.0 MPa·m/s
Coating life	3-40 million cycles

Application by robot:

Component	RPM	PV, MPa·m/s	Status
Crab/Mole wheel	60 RPM	0.05	40× margin
Centaur joint	oscillation ±45°	<0.01	100× margin
Rollers, guides	<30 RPM	<0.01	100× margin

Precedent: Mars Perseverance — GGB bushings in the drill system.

Ball Bearings (High-Speed Shafts)

For electric motors >1000 RPM, imported ball bearings from 440C stainless steel are used:

Parameter	Value
Sizes	6205/6207
Mass	~200 g each
Service life	20,000-100,000 hours (replacement every 5-10 years)
Import for entire fleet (60,000 robots)	0.8-1.6 t

Part of “vitamins”.

Precedent: Mars Perseverance — Timken ball bearings in wheel drives.

Lubrication (Tribology)

Critical rule: Conventional oils evaporate in vacuum, so special lubricants are used.

Lubricant Types by Environment

Environment	Lubricant type	Material	Source
External (vacuum)	Dry solid	Molybdenum disulfide (MoS₂)	Import Mo from Earth
External (vacuum)	Soft metals	Lead (Pb), aluminum (Al)	Local
Internal (dome)	Vacuum synthetic oils	Perfluoropolyethers (PFPE)	Import from Earth

Molybdenum Disulfide (MoS₂)

Applied via magnetron sputtering (DC sputtering) at a magnetron station.

Parameter	Value
Coating thickness	0.5–5 µm
Mo per bearing	~0.01–0.1 g
Mo per fleet/recoating	~10-100 g
Mo import	<1 kg/year
Coating life (60 RPM)	~38 days
Recoating interval	3-6 months (heavy duty), 6-12 months (light duty)

Maintenance at scale (1000 robots): ~25,000 bearing points, ~70 recoating operations/day, 10-13 Centaur-Ms (~1% of fleet) on maintenance duty.

TiS₂ alternative (local): Titanium disulfide — layered structure analogous to MoS₂. Ti (0.3% of regolith) + S (up to 4% of regolith). Synthesis: Ti + 2S → TiS₂ at 400-800°C. Status: TRL 2, requires experimental verification of lubricating properties.

Soft Metals (Pb, Al)

Applications: - Lead bushings in joints (Pb melts at +327°C — sufficiently soft) - Aluminum coatings for non-critical components (µ = 0.3-0.5)

Source: Local (Pb — trace amounts in regolith, Al — primary material).

Vacuum Oils (PFPE)

Application: Internal mechanisms under dome (low pressure 0.1 atm O₂)

Characteristics: - Low vapor pressure (<10⁻⁹ mbar) - Recirculation in closed crankcases (no losses)

Problem: Import from Earth (complex synthesis)

Actuator Manufacturing

Process

Stage	Description	Equipment
1. Cylinder fabrication	WAAM printing of steel tube Ø50 mm	WAAM cell
2. Cylinder grinding	Abrasive processing of inner surface	Grinding cell
3. Piston fabrication	WAAM printing from Al	WAAM cell
4. Piston grinding	Abrasive finishing to ±0.1 mm	Grinding cell
5. Actuator assembly	Piston and O-FLEX seal installation	Centaur-M
6. NaK filling	Vacuum filling of NaK into circuit	Vacuum chamber
7. Sealing	Laser welding	Laser welder
8. Testing	Hydraulic testing at 500 bar	Test stand

Production time: 4 hours per actuator

Critical Component Equipment

In addition to existing WAAM + grinding, bearing and seal production requires additional equipment:

#	Equipment	Area	Power	Mass	Zone
1	Magnetron station (MoS₂ sputtering)	4 m²	10 kW	200 kg	Forming
2	O-ring forming press + dies	2 m²	5 kW	100 kg	Forming
Total		6 m²	15 kW	300 kg

Cycloidal drives, bushings, and hydraulic cylinders are produced on existing WAAM + grinding equipment (Forming zone, 200 m²).

Share of F-R dome: 6 m² = 0.4% of area, 15 kW = 0.013% of power.

Material Balance

For 5 robots/day (average robot 960 kg):

Component	Mass for 5 robots	Source
NaK (hydraulics)	100 kg	Distillation
Steel (cylinders)	50 kg	Iron
Aluminum (radiators)	20 kg	Aluminum
MoS₂ lubricant	0.5 kg	Import Mo from Earth
PFPE oil	2 kg	Import from Earth
NdFeB magnets (Centaur-M)	~3 kg	Import from Earth
Ag for O-rings	~0.05 kg	Import from Earth
TOTAL	~176 kg	~96% local

Consumption from daily production: - NaK: 100 kg from ~5 t/day = ~2% - Fe: 50 kg from 18 t/day = 0.3% - Al: 20 kg from 42 t/day = 0.05%

Typical Actuators by Robot

Mole-M (excavator)

Actuator	Quantity	Type	Force
Wheel drive	6	Electric + cycloidal	5 kW
Bucket drive	2	Hydraulic	50 kN
Manipulator	4	Hydraulic	10 kN

TOTAL: 12 actuators, ~65 kg

Crab-M (logistics)

Actuator	Quantity	Type	Force
Wheel drive	6	Electric + cycloidal	5 kW
Cargo lift	2	Hydraulic	30 kN
Manipulator	2	Hydraulic	5 kN

TOTAL: 10 actuators, ~40 kg

Centaur-M (manipulation)

Actuator	Quantity	Type	Force
Wheel drive	4	Electric	2 kW
Main manipulator (joints 1-4)	8	Direct-drive (NdFeB)	1 kN
Precision manipulator (joints 5-6)	4	Tendon	0.1 kN

TOTAL: 16 actuators, ~25 kg

Maintenance

Operation	Frequency	Performer
MoS₂ bearing recoating	3-6 months	Magnetron station + Centaur-M
NaK seal check	3 months	Automation
O-FLEX seal replacement	1 year	Centaur-M
Actuator replacement	As needed	Assembly jig

Service life: - Hydraulic actuators: 3-5 years - Electric actuators: 5-10 years - Motor ball bearings: 5-10 years - NaK (fluid): indefinite (recirculation)

Quality Control

Parameter	Method	Standard	Action on deviation
Seal integrity	Vacuum test	<10⁻⁶ mbar·l/s	Rebuild
Pressure	Hydraulic testing	500 bar (2× working)	Reject
Force	Bench testing	±5% of nominal	Adjustment
Temperature	Thermocouples	−12°C … +400°C	Check NaK