Connectors, Seals and Materials

Seals, barriers, fasteners and structural materials selected by flown and qualified planetary robotics programs, with the dust and vacuum test results published for them.

The strongest result in this class is negative. IPEx removed the electrical charging connector entirely and charges through an antenna carried on the radiator cover, because a dust-tolerant electrical connector is the component most likely to fail in a regolith environment [2]. Where a rotating interface cannot be removed, the IPEx answer is a three-stage stack: labyrinth outermost, then felt, then a PTFE lip seal [1][2].

Seals, fasteners and materials selected and flown

Part	Manufacturer	Used by	Source
Nomex felt seal	Not named	ipex (all drive actuators)	[1][2]
Spring-loaded PTFE rotary seal	Not named	spirit, opportunity (arm)	[3]
Aeroflex rotary shaft seal	Aeroflex	spirit, opportunity (steering)	[3]
Beta cloth	Not named	Boots and bellows over mechanisms	[3]
Chromel-R fabric, RTV-630 facing	Not named	Apollo suit boots and gloves	[4]
Velcro hook and pile on Kapton	Not named	Apollo 17 receiver thermal bag	[5]
A286 screws, Nitronic 60 inserts	Not named	ipex	[1][2]
Zirconia and silicon nitride balls	Not named	Lunar mechanisms, candidate	[3]
Sapphire window, fused silica optics	Optimax	perseverance (SHERLOC ACI)	[7]

Ratings and qualification results

• Nomex felt seal, vendor not named. Three-stage rotary dust seal: a labyrinth path, then felt, then a PTFE lip. Ratings: Layered outward from a PTFE lip seal to a Nomex felt seal to a labyrinth path [1][2]. Qualification: Tested in dirty vacuum under the TC4 test-as-you-fly profile with a dust cup conveying simulant over the rotating output. A single felt seal gave sufficient protection in the worst case, with minimal dust found beyond it. A large quantity of dust penetrated the labyrinth stage, attributed to the test submerging the lower section of the running actuator rather than to the seal design [1]. Measured seal drag torque across the Generation 1 campaign peaked at 0.03 N m, under one percent of the overall load [2].
• Spring-loaded PTFE rotary seal, vendor not named: a spring-energized lip seal for outer diameters above about 1.5 inch, flown on the MER Instrument Deployment Device [3]. Ratings: Flight heritage on the MER arm; tested in vacuum in the presence of lunar simulant [3]. Qualification: Durable and effective in simulant vacuum testing, and scored joint highest of three seal types in a weighted trade study, Table 3. Long-term stability under lunar temperature extremes and radiation is recorded as unresolved [3].
• Aeroflex rotary shaft seal, Aeroflex. Dry running rotary shaft seal, on the MER wheel steering actuators. Ratings: Simple mechanism with no lubricant [3]. Qualification: Successfully limited regolith ingress in vacuum simulant testing. Requires high machining tolerances, is susceptible to misalignment during installation, and is affected by vibration; lunar material selection is unresolved [3].
• Beta cloth, vendor not named. Fiberglass fabric used as a flexible dust barrier in boots and bellows over lunar mechanisms. Ratings: Compared against thermoplastic polyurethane and Tyvek for the same application, scored on mass per square meter, complexity, TRL, manufacturability into a boot, durability and 40 to 400 K temperature sensitivity [3]. Qualification: Scored highest of the three barrier materials in the weighted trade, the gap driven by higher TRL and by durability and effectiveness. The result is a trade score, Table 5, not a life test [3].
• Chromel-R fabric, RTV-630 facing, vendor not named. Abrasion-resistant metallic fabric in the extravehicular mobility unit boots and gloves. Ratings: Woven into the boots and gloves; RTV-630 used for soles and finger tips [4]. Qualification: Introduced in flight because the super beta cloth outer covering was not abrasion resistant against sharp lunar rock. The rest of the dust control scheme did not work: a single nylon bristle brush removed coarse grains but not fines and may have transferred nearly as much dust as it removed, wrist bearings and rotational connectors had only fabric covers that were not effective, and from Apollo 15 Velcro-attached dust covers were added to the front connectors [4].
• Velcro hook and pile on Kapton, vendor not named. Fastener system bonded with polyurethane FR-127 A and B, on the Apollo 17 lunar surface receiver thermal bag. Ratings: Pile pads bonded to the Kapton bag, hook straps bonded to the Kapton flaps [5]. Qualification: Flight failure. Both flap bonds had failed before the end of the first EVA, letting dust settle on the mirror surface under both flaps. The pads came away leaving no adhesive trace on the Kapton. FR-127 A and B remains recommended for bonding Velcro to Kapton; the failure was attributed to bonding preparation and procedure, with mixing, timing and freedom from surface contamination named as the controlled parameters [5].
• A286 screws, Nitronic 60 inserts, vendor not named: a standard 4-40 fastener set with locking helical inserts [2]. Ratings: Used throughout the vehicle for commonality, with 2-56 fasteners as a deliberate exception at the harmonic drive flex cup and wave generator. Qualification: Carried through the KSC actuator qualification campaign, including the accelerated life test that ran the wheel actuator to 6,547,036 input revolutions and repeated disassembly and inspection between runs [1][2].
• Zirconia and silicon nitride balls, vendor not named. Ceramic rolling elements as a dust-tolerance strategy, a candidate for lunar surface mechanisms. Ratings: Very high hardness against abrasion, and both run dry, which removes lubricant from the failure set in lunar vacuum [3]. Qualification: Scored highest of four dust-tolerant bearing types in a weighted trade, with a clear gap to stainless steel and to hybrid superconducting magnetic bearings, driven by durability and dry running. The result is a trade score, Table 9 [3].
• Sapphire window, fused silica optics, optics by Optimax. Fluorescence-free optical train of uncoated UV-grade fused silica and magnesium fluoride behind a sapphire window. Ratings: Four-element telecentric objective behind a sapphire window, all elements uncoated, made from UV-grade fused silica or fluorescence-free magnesium fluoride; the camera lens group moves on a linear rail [7]. Qualification: Materials were chosen to be fluorescence-free because the instrument measures deep ultraviolet fluorescence and Raman scatter, so any optic that fluoresces becomes a background signal. The dust cover is solid aluminum and opaque, carrying the internal AlGaN calibration target on its inner face [7].

Fastener and structure notes

RASSOR 2.0 records a manufacturing result rather than a materials one [6]. The shoulder actuator housings were 3D printed in titanium with extra material left on the bearing, stator, rotor and sealing faces for later machining. With no printed datum to work from, the machinist had to build fixtures for each of six parts and assume an axis of revolution in each, to bring the motor, bearings and harmonic drive coaxial [6]. Where a printed housing carries a bearing bore and a seal face, the print does not remove the machining operation.

Seal and surface materials with a measured or normative limit

The rows below are the level at which a seal, a connector or a surface coating stops meeting its requirement. Solder joints, package interconnects, platings and the printed wiring they sit on are on Packaging and Board Reliability.

Part	Manufacturer	Used by	Source
Sample tube seal SN282	Not named	perseverance	[8]
TR01447 Kapton with Kevlar	Not named	viper (dust sock)	[9]
Cr3C2-NiCr HVOF coating	Not named	Langley Taber abrasion	[10]
M83513-03-E03C connector	Glenair	astrobee payload interface	[11]

Ratings and requirements

• Sample tube seal SN282. Ratings: one tube, measured by static-mode noble gas mass spectrometry over accumulations of 23 to 29 hours [8]. Qualification: helium leak rate 1.7e-15 scc/s at -51 C and 1.4e-13 scc/s at +42 C, over eight average run temperatures on that one tube, rising with temperature and linear against inverse absolute temperature [8]. The demonstrated range is -51 to +42 C and nothing outside it.
• TR01447 Kapton with Kevlar, heavyweight Kevlar reinforcement, selected as the VIPER dust sock baseline. Ratings: Martindale abrasion testing at the Johnson Space Center softgoods laboratory [9]. Qualification: no increase in air permeation, 7.8 percent thickness loss and 1.2 percent weight loss at 100 percent completion of the run [9]. The candidate MO01503 reached 200 percent completion but lost 23.1 percent of thickness and 3.9 percent of weight, and DE355 lost 49.5 and 21.7 percent, so the selection traded abrasion endurance for retained thickness.
• Cr3C2-NiCr HVOF coating on aluminum 6061, as deposited and unpolished [10]. Ratings: ASTM D4060 Taber abrasion with CS-17 wheels, eddy current thickness at 400, 1200 and 5000 cycles. Qualification: just under 2 micrometers of thickness lost after 5000 cycles, the best of seven commercial coatings, with mass loss too small across every composition to compute a wear index [10]. Tribaloy T-800 and Cr3C2 coatings lost under 5 micrometers, an air plasma sprayed alumina-titania coating lost over 20 and also had the highest as-processed roughness at 1.55 micrometers Ra, and an air plasma sprayed yttria-stabilized zirconia coating gave a wear index of 9.20 mg per 1000 cycles, roughly twice the alumina rate. Under a 20 kHz sonic wand screen with LHS-1D milled highland simulant, the alumina, Cr3C2-NiCr and Tribaloy T-800 coatings retained the least residual dust of the seven, judged semi-quantitatively from optical images [10].
• M83513-03-E03C connector, Glenair. Thirty-one pin blind mate connector carrying the Astrobee payload interface. Ratings: 14.4 V at a 3 A limit, with three USB 2.0 data pinouts routed to different processors [11]. Qualification: no environmental or life result is published for it.

Aluminum against itself is the worst uncoated cold-welding pair measured in the corpus, at 1775 mN of adhesion under vacuum impact, above titanium [12]; the full set of contact-pair results is on Bearings and Lubrication.