In this AVweb VODcast, former NASA lead flight director and KITPLANES editor at large Paul Dye explains why this week’s launch of the Crew Dragon spacecraft requires split-timing. It’s a little like driving a really fast car with only a few quarts of gas in the tank.


  1. Thank you, Paul and Paul, for an explanation of the orbital mechanics that was technically sophisticated, not dumbed down to blurriness, yet still accessible to your average pilot. First-rate work.

    And, from the backgrounds we can see how lavish is the AVweb budget for location shoots. 🙂

  2. For space missions, the launch window is defined, as the first time the launch can occur to the last time the launch can occur and meet the mission objectives. For Demo 2 the launch window was only one second long to reach the ISS. For missions like Mercury flights where there was no other orbiting object to reach, the launch window could be longer based on such things as crew fatigue, being in daylight, etc. For Apollo, the ability to get to the moon depended on the moon’s orbit, lightning for lunar landing visibility, etc. The launch windows for Gemini, Apollo, and the Space Shuttle were indeed longer than one second. How could this be? Like the Dragon, the Gemini and Apollo spacecrafts sat upon a booster to get to orbit. Only for abort situations would the spacecraft propulsion systems be used to get to a safe orbit and then would probably miss the mission objectives. The Shuttle was actually part of the booster, as after the solid rockets were separated, the main engines on the Shuttle continued to achieve orbit. The Shuttle Orbital Maneuver System provided the last bit of push to get to orbit leaving the external tank to be disposed of safely. Where this leads is that the launch window depends on the booster reserves or total energy available. As Paul noted, the earth’s rotation moves the launch pad out of the ISS orbit plane even for one second (the launch pad is moving east about 1300 feet every second). As the Shuttle and the Saturn V had a bit more push than just enough to get to orbit, they could steer out of plan, a “dog leg” if you will, and during launch phase, match the plane of the ISS and the conditions to get to the moon. The extra reserve was a design feature to give a tangible launch window to allow time to fix vehicle problems, account for weather, etc. I don’t know how much extra reserve the Falcon 9 has, but if there is any, it could be used to “dog leg” and have a longer than one second launch window.
    Paul well done, just a bit of additional information for folks.
    Retired JSC Flight Dyanamics Team

    • I would guess that Falcon 9/Dragon would have a little larger launch window for ISS intercepts if the booster were not recovered. The booster recovery requires extra fuel and a specific launch trajectory to ensure that it can return to the planned landing location.