Deployable Solar Array

Fifty square metres of solar cells folded into a cylinder the size of a rolled-up sleeping bag, then unfurled in the silence of orbit to drink sunlight for fifteen years.

Every spacecraft that doesn't carry a nuclear power source runs on sunlight converted to electricity by photovoltaic cells. The problem is that the cells need area — square metres of it — but a rocket's fairing (the nose cone) is only a few metres wide. Deployable solar arrays solve this by folding, rolling, or stacking the cell blanket into a compact package that survives the violent vibration and acoustic loads of launch, and then smoothly extending to full size once in the quiet of orbit. Modern arrays achieve packaging densities of 40 kW per cubic metre stowed — meaning a kilowatt of power capability that occupies roughly the volume of a wine bottle before deployment. They must then withstand tens of thousands of thermal cycles as the spacecraft alternates between the 120°C of sunlit orbit and the −160°C of eclipse, for up to 15 years without maintenance.

The cell blanket itself is a thin flexible substrate — usually a polyimide (Kapton) film — to which triple-junction gallium arsenide solar cells are bonded. These multi-junction cells stack three semiconductor absorber layers tuned to different parts of the solar spectrum, achieving efficiencies of 28–32% in space compared to 15–22% for typical terrestrial silicon panels. For a roll-out array (ROSA type), the blanket is wound around a central spindle and held flat by two composite roll-out booms that extend like a carpenter's tape measure — the stored strain energy in the booms drives deployment with no motors required. For a fan/umbrella type (UltraFlex, MegaFlex), a central hub unfolds outward like an opening umbrella, pulling pie-slice panel segments into place around a circular aperture. For a rigid-panel fold-out (classic type), the panels are hinged and spring-latched into a flat configuration. All designs include a Sun-tracking rotary joint (Solar Array Drive Assembly, SADA) so the panels can track the Sun as the spacecraft rotates and manoeuvres.

In plain terms

Picture one of those party noise-makers that you blow into and a little paper tube unrolls. Now imagine that paper is covered in solar cells, it's the size of a school bus when open, it folds up to smaller than a suitcase, and it has to work perfectly the first time with no second chances — in space, at minus 160 degrees Celsius. That's a deployable solar array.

Also called: ROSA, iROSA, UltraFlex array, MegaFlex array, roll-out solar array, foldable solar panel, blanket solar array, solar array wing, SAW.

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A short history

The first spacecraft solar arrays were fixed rigid panels — Vanguard 1 (1958) used six small silicon cell panels, and early arrays grew by simply adding more rigid panels to a frame. The stowage problem became acute with larger spacecraft in the 1960s: NASA's Orbiting Astronomical Observatory (OAO-1, 1966) pioneered deployable rigid paddle arrays. Through the 1970s and 1980s, most communications satellites used rigid fold-out panel arrays, with the Boeing 702 bus's large rigid wings becoming a benchmark for GEO power. Flexible blanket arrays emerged in the 1990s as a route to higher specific power; the Space Station Freedom design used flexible arrays that eventually became the ISS solar array wings installed from 2000 to 2009. The circular fan/umbrella concept — UltraFlex — was developed by AEC-Able Engineering and flew first on Mars Phoenix Lander (2008), demonstrating 103 W/kg at NASA technology level 6. ROSA (Roll-Out Solar Array) originated from work at the Air Force Research Laboratory and was flight-tested on the ISS in 2017 via a spacewalk, then commercialised by Deployable Space Systems (later Redwire). The first operational ROSA mission was the DART asteroid impactor (2021), followed by the iROSA ISS upgrades (2021–2023).

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