@apsidia

Code, physics, and things that orbit. Building Apsidia — a real-physics space mission simulator — and explaining how it all works. In public. 🚀

 Low Earth Orbit
Joined May 2026
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Apsidia is a space mission simulator where the physics is real — not faked, not a toy. Real Newtonian gravity, atmospheric drag, staged rockets — a whole mission, from the launchpad to orbit. Building it in public. 👇
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A rocket engine is weakest right where it needs to be strongest: on the pad. At sea level, air pressure pushes back on the exhaust. Climb into thinner air and that back-pressure fades — the same engine gets ~10% more efficient. Falcon 9's booster: 282 s of Isp at sea level, 311 s in vacuum. That's why upper stages wear those huge bell nozzles — shaped for the vacuum they fire in (348 s). In Apsidia this falls straight out of the atmospheric pressure profile. Not a fudge factor — physics. 👇
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For 50 years, Mercury's orbit quietly broke Newton's gravity. A planet's orbit is an ellipse — and Newton says that ellipse should stay locked in place forever. Mercury's doesn't. The whole ellipse slowly rotates, creeping around the Sun. Most of that drift Newton could explain: the gentle tug of the other planets. But 43 arcseconds per century were left over — a gap pure gravity simply could not account for. Astronomers were so certain a hidden planet was pulling Mercury that they named it: Vulcan. They never found it. Because it was never there. The real answer came in 1915: Einstein. Space near the Sun is curved, and that curvature nudges Mercury's orbit by exactly 43 arcseconds a century. It was the first proof relativity was right. Apsidia is built on Newton — and pure Newton gets Mercury wrong, by design. Add the relativistic correction, and the sim lands on 43.8″ per century. The famous number, straight out of the physics. Building a physically honest space simulator in public👇
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To put 13 tonnes into orbit, this rocket leaves the pad weighing 531. The other 518 never reach space — and that's not waste, it's the only way the trip works. Orbit isn't about height, it's about speed: ~9.4 km/s of it, sideways. And the speed a rocket can build comes down to one brutal ratio — full mass over empty mass. It's logarithmic, so piling on fuel gives less and less extra speed. A single stage would have to be ~95% propellant — a tank too thin to hold its own engines. You can't build it. So you cheat: throw your empty mass away the instant it stops helping. That's staging. This rocket burns 396 t of fuel, then drops the 25.6 t empty first stage at T+144s — dead weight it won't drag any higher. A lighter second stage finishes the climb and delivers the 13 t payload. Real Newtonian physics, simulated — not drawn by hand. Building Apsidia in public. 👇
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This is the whole thing. From the launchpad to a stable circular orbit — 200 km up, computed from real physics, not an animation curve. Straight up out of the thick air, tip over, drop the empty first stage, then a long sideways push until the rocket is going fast enough to fall around the Earth and keep missing it. That's an orbit. Final orbit: 200 × 207 km, eccentricity 0.0005 — near-perfectly circular. Apsidia flew it.
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