Spacecraft Materials, Outgassing & Atomic Oxygen

Under PocketQube Standard Issue 1 (clauses PQ-Mech-01 through PQ-Mech-07), a 1P body measures 50.0 ± 0.1 mm and interfaces with its deployer exclusively through a 58.0 × 64.0 × 1.6 mm sliding backplate (128 mm long for 2P and 192 mm for 3P) clamped along two 2 mm lateral strips (256 mm² bearing area). Under a 20 g quasi-static launch load, a 250 g 1P exerts 49 N—a mean bearing stress of only 0.19 MPa, more than two orders of magnitude below structural yield strengths—making the backplate an interface-critical rather than strength-critical component.

Alloy temper governs intergranular stress-corrosion cracking (SCC) resistance under ECSS-Q-ST-70-36C. All wrought 6000-series Al–Mg–Si alloys—including Al 6061-T6 (ρ = 2.70 g/cm³, E = 68.9 GPa, σ_y = 276 MPa, k = 167 W/(m·K)) and Al 6082-T6 (σ_y = 260 MPa, k = 172 W/(m·K))—belong to Class 1 Table 5-1. Conversely, peak-aged Al 7075-T6 (σ_y = 503 MPa) falls into low-resistance Table 5-3 under short-transverse tension and requires formal SCC approval, whereas over-ageing to 7075-T73/T7351 (σ_y = 435 MPa, ~10–15% debit) restores Class 1 immunity.

With slot thickness fixed at 1.6 mm (5.94 cm³ solid volume), backplate mass scales strictly with density: 11.0 g in FR-4 (flexural rigidity D = 7.6 N·m), 16.0 g in Al 6061-T6 (D = 26.4 N·m, 6.4% of a 250 g budget), 16.7 g in Al 7075-T73, 26.3 g in Ti-6Al-4V (ρ = 4.43 g/cm³), and 45.4 g in 17-7PH steel. Although FR-4 saves 5 g and leads on free-thickness strength index σ_f^(1/2)/ρ, its 3.5× lower rigidity and resin creep favour Al 6061-T6, while LPBF AlSi10Mg (σ_y = 233–270 MPa) lacks a de-rated ECSS-Q-ST-70-80C path below 100 g.

In atmospheric air, native oxides and adsorbed moisture prevent bare metallic asperities from bonding. During launch vibration, small-amplitude oscillatory sliding (fretting) abrades these films, and once ambient pressure drops below ~1 Pa in orbit the oxide cannot regrow, allowing clean asperities to cold-weld at room temperature. In vacuum fretting campaigns (ESA STM-279), self-mated silver, copper, and 17-7PH exceeded 12,000 mN adhesion, while sputtered MoS₂ on 17-7PH against Ti-6Al-4V wore through in roughly 40 fretting cycles.

To prevent metal-to-metal contact inside P-POD, ISIPOD, QuadPack, and AlbaPod deployers, CDS Rev. 14.1 and clause PQ-Mat-03 mandate MIL-A-8625F Type III hard anodizing (25–50 µm typical within 25–75 µm nominal, Ra ≤ 1.6 µm). Grown in cold sulfuric acid, this porous Al₂O₃ coat blocks adhesion but introduces five effects: ~50% outward growth (~25 µm/face, eating 50 µm of a 58.0 ± 0.1 mm slot tolerance unless machined undersize), lower hardness on LPBF AlSi10Mg (~280 HV), a 50–62% fatigue debit on Al 6082, outgassing checks on PTFE sealants, and chassis electrical isolation.

Cold welding also governs non-pyrotechnic antenna latches (PQ-Gen-02). Coiling a 170 mm UHF tape (t = 0.10 mm) onto a 3 mm hub imposes strain ε = t/(2R_h) = 1.67%—within the 4.0% superelastic NiTi plateau (0.20 J stored at −20 °C, 0.42 J at 25 °C) whereas spring steel (0.6% limit) thins to 0.036 mm (0.04 J). Because 1 N of cold-welded adhesion across 170 mm absorbs 0.17 J (87% of NiTi energy at −20 °C), dissimilar/anodized latches and dual-path 0.15 mm one-way NiTi (0.23 J at 1 W in 0.23 s) plus UHMWPE burn wires are preferred.

Because a 250 g 1P PocketQube has a lumped thermal time constant of τ_th ≈ 56 minutes—roughly half the 104 minutes of a 1U CubeSat—its temperature tracks the 95.1-minute sunlit/eclipse cycle closely, accumulating ≈5,500 thermal cycles per year. Single-node orbital balances predict 30–37 K bulk eclipse swings for a 1P (against 16–23 K for a 1U), while thermally decoupled exterior solar panels swing wider still (+15 °C to −26 °C modeled on SMOG-1, and up to 75 K observed on the 3P Delfi-PQ).

Repeated thermal excursions drive cyclic shear strain Δγ across rigid solder joints whenever adjacent materials differ in coefficient of thermal expansion (CTE): triple-junction solar-cell germanium substrates expand at ~6 ppm/K and ceramic chip packages at 6.5–7 ppm/K, versus 14–17 ppm/K in-plane for FR-4 (50–70 ppm/K out-of-plane), 17 ppm/K for copper, and 23.6 ppm/K for Al 6061-T6. Screening models show rigid Ge-on-FR-4 joints can fatigue near ~5 × 10³ cycles (~1 year in LEO), motivating compliant silicone attachment, flexible interconnects, and eutectic Sn–Pb or de-golded finishes that avoid pure-tin vacuum-arc whiskers, <13.2 °C tin pest, and brittle AuSn₄ intermetallics.

Polymeric laminates, adhesives, wire jackets, and additively manufactured parts (PEEK, Windform XT 2.0, or FDM ULTEM 9085, which loses ~40% tensile strength along its Z build axis) must pass vacuum outgassing screening at 125 °C for 24 h below 7 × 10⁻³ Pa with a 25 °C collector: Total Mass Loss TML ≤ 1.00% under ASTM E595 (or Recovered Mass Loss RML = TML − WVR ≤ 1.00% under ECSS-Q-ST-70-02C for hygroscopic nylons and aramids) and Collected Volatile Condensable Material CVCM ≤ 0.10%, preventing volatile films from fogging solar coverglasses or neighbouring payloads inside shared deployers.

Solar extreme-ultraviolet photodissociation of molecular O₂ makes monatomic oxygen (AO) the dominant neutral atmospheric constituent between 200 and 600 km. At a circular LEO speed of v ≈ 7.6 km/s, ram-facing collisions impart translational kinetic energy ½·m_O·v² ≈ 4.5 eV per oxygen atom—high enough to break C–C and C–H covalent bonds and convert exposed hydrocarbon or polyimide surfaces into volatile CO and CO₂ reaction products.

In the repository's thermospheric AO model (lib/physics/atomic-oxygen.ts), number density scales exponentially about a 400 km reference as n_O(h) = n_400 · exp(−(h − 400)/H_AO), with n_400 = 1.2×10⁸ · exp((F10.7 − 120)/85) atoms/cm³ and scale height H_AO = 46 + 0.14·(F10.7 − 70) km. A randomly tumbling convex spacecraft intercepts one quarter of fixed ram flux (f_ram = 0.25), whereas attitude-stabilized ram faces receive f_ram = 1.0; surface recession depth d equals cumulative atomic fluence F multiplied by the material's volumetric erosion yield E_y.

Flight-calibrated MISSE-2 PEACE yields span more than two orders of magnitude across spacecraft polymers. Unprotected Kapton HN polyimide (E_y = 3.00 × 10⁻²⁴ cm³/atom, ρ = 1.42 g/cm³) and PEEK structural spools (2.99 × 10⁻²⁴ cm³/atom; receding ~60 µm/year in fixed ISS ram) erode rapidly below 450 km, while FR-4 epoxy matrix (2.1 × 10⁻²⁴ cm³/atom) strips down to a matte E-glass weave. Fluorinated FEP/PTFE lowers yield to 0.20 × 10⁻²⁴ cm³/atom (though synergistic VUV + AO promotes embrittlement), and depositing a 130 nm SiOx silica barrier onto Kapton drops effective yield to 0.02 × 10⁻²⁴ cm³/atom—cutting recession by >98% outside pinhole defects.