1. Duct tape and ingenuity at a distance, again
Andy Weir’s The Martian returns because a mechanism in orbit cannot be repaired with duct tape either. The choice of lubricant and the design of the mechanism must anticipate every failure mode before launch. The repair kit is on the ground, which is too far away.
This entry reads about lubrication choices and mitigation strategies for spacecraft mechanisms.
2. Choosing between liquid and solid lubrication
The choice depends on duty cycle, speed, temperature, contamination sensitivity, and required life:
- Liquid lubrication is preferred for high-speed, continuous-motion mechanisms such as reaction wheels and momentum wheels. It provides lower torque noise and better heat conduction. The trade is evaporation, creep, and the need for seals or reservoirs.
- Solid lubrication is preferred for intermittent, low-speed mechanisms such as latches, deployment hinges, and release mechanisms. It avoids evaporation and contamination but has finite wear life.
- Hybrid lubrication combines a sputtered MoS₂ film with a small amount of PFPE fluid. The fluid provides low torque noise and cooling; the MoS₂ provides a backup if the fluid migrates or degrades.
A paper from the 44th Aerospace Mechanisms Symposium describes this trade-off in detail.
3. Extending solid lubricant life
For solid lubricants, life is measured in cycles or sliding distance. Strategies to extend life include:
- Thicker films: more material to wear through, but with risk of poor adhesion.
- Doped or composite films: MoS₂ combined with Sb₂O₃, carbon, or metals to improve durability.
- Run-in under controlled conditions: initial cycles distribute the lubricant and establish a stable transfer film.
- Limiting contact stress: lower stress reduces wear rate and delays film failure.
An MDPI review of solid lubricants emphasizes that application method — sputtering, burnishing, or bonding — strongly affects adhesion and life.
4. Preventing cold welding and galling
Cold welding is prevented by keeping metal surfaces separated:
- Lubricant films: oil, grease, or solid film between contacting metals.
- Hard coatings: TiN, DLC, or other coatings that reduce adhesion and increase hardness.
- Dissimilar metals: some metal pairs are less prone to adhesion than others, though galvanic compatibility must be checked.
- Avoiding unnecessary metal-to-metal contact: use bushings, inserts, or non-metallic separators where possible.
A recent ESMATS paper on cold welding recommends tracking cold-welding risk early in the design process and validating surface treatments after simulated launch conditions.
5. Qualification and life testing
Mechanisms are usually qualified by a combination of:
- Life testing: operating the mechanism for the required number of cycles in vacuum and at temperature.
- Vibration and shock testing: proving the mechanism survives launch without damage to surfaces or coatings.
- Thermal cycling: verifying clearances and coatings survive repeated temperature swings.
NASA lessons learned recommend real-time life testing rather than accelerated testing when possible, because accelerated tests can miss temperature-dependent failure modes.
6. What this changes
- Lubricant selection is a trade-off between duty cycle, life, contamination, and temperature.
- Solid lubricants are the natural choice for the desktop’s intermittent mechanisms.
- The next entry will translate these findings into requirements for the desktop.