An open source, online launcher calculator provided by a small rocket space launch startup, Launcher. Provides an "approximation" of performance for conventional rockets but seemingly a pretty comprehensive at that. The author is Igor Nikishchenko who previously worked at the Yutzhnoye Liquid Propulsion Department, Ukraine, as a Deputy Chief Designer. Comes with good documentation also.
launchercalculator.com
launcherspace.com

Launcher Calculator
An open source orbital launch vehicle (rocket) payload calculator by Launcher


Launcher
Launcher, a Vast company, is developing the E-2 liquid rocket engine, expected to be the world’s most efficient rocket engine for small satellite launch vehicles.
