In-orbit demonstration of a triply periodic minimal surface heat sink with phase change material aboard the Matilda payload on Waratah Seed-1 CubeSat

Miniaturised, high-power satellite avionics require effective waste-heat management, especially in low-Earth orbit (LEO), where sunlight–eclipse transitions impose high-amplitude thermal cycles. Phase change material (PCM) heat sinks can mitigate these fluctuations through latent heat storage, while additive manufacturing enables integration with triply periodic minimal surface (TPMS) architectures to enhance heat transfer. However, experimental in-orbit operation of TPMS-PCM heat sinks has not yet been demonstrated. This work presents the first 1 in-orbit demonstration of a TPMS heat sink with 7.0 g of paraffin wax PCM. Flown aboard the Matilda payload on Waratah Seed-1 (WS-1), the TPMS-PCM module was benchmarked against a conventional radial-finned heat sink without PCM. Terrestrial high-grade thermal vacuum tests closely matched in-orbit heating profiles, suggesting no dominant module-level deviation in PCM melting behaviour for the investigated conditions, geometry, and scale. The TPMS-PCM module extended the operational time to reach 85 °C by 85% on Earth and 92% in orbit, with a mass increase of 16% (PCM mass relative to metallic mass). During idle operation, peak-to-valley temperature fluctuations were reduced by up to 2.9 °C, demonstrating effective thermal buffering of orbital temperature cycles. The influence of orbital position and solar exposure was quantified at multiple power levels, revealing stronger sensitivity to illumination conditions at lower power inputs. Overall, the results demonstrate the potential of TPMS-PCM heat sinks for mass-efficient CubeSat thermal management and support the use of terrestrial thermal-vacuum testing as a useful ground-based reference for module-level performance assessment.