Orbit Builder is a free spaceflight simulator that runs in your browser. Pick a real rocket (Falcon 9, Soyuz-2.1a, Long March 2D, H3, PSLV, Ariane 62, Vega-C, Nuri or Electron) or design your own from real engines made in seven countries, then fly it from a real launch site to orbit and on to the Moon.
The flight uses real physics: Earth's gravity, drag in the thick lower atmosphere, the dynamic pressure called Max-Q, the speed bonus you get from Earth's rotation (up to about 465 m/s at the equator), and fuel burn from each engine's thrust and specific impulse. Stages drop away when empty and the Moon pulls on you once you leave Earth.
Missions: reach space at 100 km, reach a stable orbit (periapsis above 80 km), fly to the Moon, orbit it and land on it. A low Earth orbit needs about 7.8 km/s of sideways speed, so most of the work is going sideways, not just up. Public data is used where it exists; estimated stage masses are marked with an asterisk.
Climb straight up through the thick air, then tilt gently toward the horizon and keep burning until your periapsis (Pe) is above 80 km. The green line shows your path and the map view (M) helps.
Either the dynamic pressure got too high, or the rocket was tilted too far while the air was still thick. Throttle back near Max-Q and tilt slowly.
TLI stands for trans-lunar injection, the burn that sends you toward the Moon. The button finds the next good moment and the speed change needed, and shows a countdown. It is approximate, so check the orange dotted line afterwards.
Thrust, specific impulse, liftoff mass and published payloads come from public data. Some stage masses are estimated and are marked, and rockets with side boosters are simplified into stages.
Yes. Your hangar choices and three design slots are stored in your browser, so they are still there next time on the same device.