Solar Sail II — Ported to Orbiter 2024
02 Jul 2026After almost 20 years, Solar Sail II has been resurrected and ported to Orbiter 2024. The add-on, originally released in 2007 for Orbiter 2006, now runs on the latest Orbiter build with full VESSEL3 API support.
What’s New
The original source code (written for the now-obsolete VESSEL API) has been completely migrated:
| 2006 API | Orbiter 2024 API |
|---|---|
ovcSetClassCaps |
clbkSetClassCaps() as class method |
ovcTimestep(simt) |
clbkPostStep(simt, simdt, mjd) |
ovcConsumeKey(keystate) |
clbkConsumeBufferedKey(key, down, kstate) |
| Global variables | Class members (SolarSail) |
AddExhaust(...) |
AddExhaustStream(...) |
ANIMCOMP sequences |
MGROUP_ROTATE / MGROUP_TRANSLATE + AddAnimationComponent |
The build system has been modernized to CMake + Visual Studio 2022, making compilation straightforward.
Two Vessels
The add-on consists of two modules:
- SS2.dll — The solar sail itself, with 4 deployable arms, 32 sail mesh segments, and solar radiation pressure physics
- tl-ss2.dll — Transfer vehicle that carries the sail to orbit and deploys it (press J to separate)
Physics
The solar radiation pressure model is based on Dr. Bong Wie’s paper Dynamic Modeling and Attitude Control of Solar Sail Spacecraft, with specular coefficient ps=0.9 and diffuse coefficient pd=0.1:
- Normal force: Fn = P·A·[(1+ps)·cos²α + (⅔)·pd·cosα]
- Transversal force: Ft = P·A·[(1−ps)·cosα·sinα]
Auto-navigation modes include Prograde and Kill Rotation.
Animation
The deployment sequence is divided into three phases:
- 0.0–0.3: 4 arms rotate outward
- 0.3–0.8: 22 segments per arm extend
- 0.8–1.0: 32 sail meshes appear sequentially
Press D to deploy/retract, J to separate from the transfer vehicle.
SPAE Solar System
The add-on includes the custom Sol_SPAE solar system, featuring Rapa Nui surface base on Easter Island — a fitting launch site for an interplanetary solar sail mission.
Source Code on GitHub
The full source code is now available on GitHub:
- Repo: fcomparini/solarsail-ii
- License: Open source (preserving the original 2007 copyright)
Built with MSVC 19.44 (VS Build Tools 2022), Orbiter 2024 SDK.
Downloads
- Solar Sail II ZIP — Original 2007 release
- New binaries will be available once Orbiter 2024 reaches stable release
From the original documentation
This was my second add-on for Orbiter. I wrote it very long ago, and I had compiled it for the newest Orbiter version. It simulates the behavior of a Solar Sail of area SAT_AREA, specular coefficient PS and diffusion coefficient PD (this values can be changed on the scenario file, although the size of the mesh visible will be always the same).
— Felipe “ObiWan” Comparini, 2007
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