Spin-Stabilized Solar Sail Deployment

Deployment Controls

Initial spin rate 12 rpm

Corner tip mass 1.0 kg

Sail areal density 12 g/m²

Reel-out rate 0.4 m/s

0% deployed

Live Readout

Spin rate ω

12.0 rpm

Boom radius r

0.00 m

Moment of inertia I

— kg·m²

Angular momentum L

— kg·m²/s

How it works

Boom-less sails such as JAXA's IKAROS (2010) are not held taut by rigid struts. Instead the spacecraft spins up before launch, and four corner tip masses are released to reel outward on tethers; centrifugal tension alone pulls the folded membrane into a flat square, one triangular quadrant at a time.

No external torque acts during deployment, so angular momentum is conserved — exactly the "spinning skater" effect:

L = I(r)·ω = const

I(r) = I_hub + 4·m_tip·r²  +  (2/3)·σ·r⁴
        ↑hub bus    ↑tip masses   ↑membrane
                                  (4 right-triangle
                                   panels, legs = r,
                                   I_panel = m_panel·r²/3
                                   about the hub vertex)

ω(r) = L / I(r)

Initial spin rate — sets ω₀ and, together with the fixed hub inertia, fixes L at the instant deployment starts (r = 0, sail still folded).
Tip mass / areal density — heavier corner masses or a heavier film both grow I(r) faster, so the same L produces a steeper spin-down as r increases.
Reel-out rate — how fast the tethers pay out; it changes how quickly deployment happens, not the final spin rate, which depends only on L and the fully-deployed I.

Real sail film areal densities run roughly 7–20 g/m² (aluminized polyimide/CP-1); IKAROS's 14 m-per-side membrane massed about 2 kg for ~196 m² of sail — the same order of magnitude modeled here with a 10 m boom radius.

Stowed — press Start Deployment

Membrane panel Corner tip mass Central bus

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