Roll-Out Solar Arrays (ROSA)

Most spacecraft solar arrays open like a folding screen, snapping rigid panels into place. Roll-Out Solar Arrays do something different: they unspool from a tight cylinder, much like a tape measure unwinding from its case.

Quick facts

• What it is: A lightweight, flexible solar array (a structure that converts sunlight into electricity) that deploys by unrolling instead of unfolding.
• Who made it: Developed by Deployable Space Systems (DSS) with NASA Space Technology Mission Directorate funding starting in 2009; now produced by Redwire Space, which acquired DSS.
• Key advantage: Roughly 20 percent lighter and about one-fourth the stowed volume (packed-up size) compared with rigid arrays of equal power output.
• How it opens: Motor-free, driven by stored “strain energy” in two rolled composite booms.
• Demonstrator mass: 325 kg (717 lb).
• First flight test: Deployed, retracted, and jettisoned from the International Space Station in June 2017.

How it works

For launch, a ROSA wing is rolled up tightly. Its thin, flexible solar-cell blanket wraps around a central spindle (a rod that works like the core of a paper-towel roll) to form a compact cylinder. This sharply reduces both weight and the space it takes up inside the rocket.

To deploy, the array simply unrolls. Running along the two outer edges of the blanket are slender booms made of composite material (a strong, lightweight engineered material). While stowed, these booms are coiled. As they spring back from their coiled shape to a straight one, they release the elastic energy stored inside them, called strain energy. That stored energy alone pushes the whole array open, the way a coiled tape measure shoots outward when released.

The booms do two jobs at once: they are both the thing that opens the array and the main support arms that hold it rigid afterward. Because of this, ROSA needs no deployment motor and no complicated system of hinges. The result is a high-power wing that is mechanically simpler and far more compact than a traditional accordion-fold rigid panel array.

Why it matters

Every kilogram launched into space is expensive, and rocket interiors have limited room. By delivering more power per kilogram and per unit of packed-up volume, ROSA makes launches cheaper and more flexible, and it gives high-power missions in deep space or orbit the energy they need. The motor-free deployment also removes mechanical parts that could jam or fail.

One trade-off comes with that simplicity: because the booms power the deployment themselves, rather than a motor an operator can speed up or stop, the array opens at a pace set by the booms’ own mechanics. In exchange, the design is modular and scalable, spanning the small 2017 demonstrator up through much larger wings that all use the same core technology.

Where it is used

International Space Station (iROSA): Scaled-up roll-out wings, each about 63 ft by 20 ft (19 m by 6 m), now upgrade the station’s power. Rather than replacing the aging legacy arrays, each smaller new wing is mounted in front of a degraded older one, shading about half of it while the rest keeps producing. Each new wing generates more than 20 kW (up to about 28 kW when new). Six wings were installed via spacewalks across the CRS-22 (2021), CRS-26 (2022), and CRS-28 (2023) missions, with two final wings planned for fall 2026. Once all are in place, the station will have up to about 215 kW of power available, a 20 to 30 percent increase over the legacy arrays alone.

DART (Double Asteroid Redirection Test): NASA’s asteroid-deflection spacecraft, launched in November 2021, used ROSA to power its solar-electric propulsion on the way to the Didymos and Dimorphos system.

Lunar Gateway Power and Propulsion Element: Two ROSA wings, supplied by Redwire under prime contractor Maxar, are sized to generate about 60 kW using Z4J solar cells to power the future lunar station’s electric propulsion.

Ovzon-3: The first commercial geostationary (GEO) satellite to fly Redwire ROSA, launched January 3, 2024, with its two arrays deployed in orbit by January 10, 2024.