MechJeb Ascents

NathanKell edited this page May 11, 2016 · 6 revisions

Please first see the False KSP Lessons page regarding what Gravity Turns actually are. MechJeb can be used to fly an actual gravity turn: launch, pitch over to 85 degrees at 100m/s, and use MJ's attitude controls to stay fixed to Surface Prograde.

This guide will teach you how to use MechJeb to fly a pitch program that works decently well for RSS.

  1. In the Ascent Guidance window, enable the "Limit AoA" control. It will keep your deviation from surface prograde to within that many degrees while in the thicker atmosphere. A value of 3 to 5 should be fine, depending on how much control authority you have.

  2. Next, Click "Edit Ascent Path" from the Ascent Guidance window.

  3. Set your "turn start" to Velocity: 100 m/s (or lower if your SLT is higher than 1.3 or so; the higher your pad TWR, the lower speed you want to start your turn).

  4. Next, you want to set your "turn end" altitude and your "turn shape" such that deviation from your velocity vector (the surface prograde indicator) is minimized. For your average two-stage rocket with a decent second-stage TWR (i.e. starts around 1, ends at maybe 3 or 4), a turn shape of 45% and turn end of about least 150km will do just fine (your apogee will be rather higher).

  5. Because of limited ignitions, you'll want to set the target altitude (in the main ascent panel) to much higher than you want, say 5000km. You'll turn it down later when you approach orbital insertion and you know what your final insertion altitude will be.

  6. You can vary the "ending pitch" during flight to tune your actual insertion altitude, and don't be afraid to circularize after apogee. Just pitch up to kill the negative vertical velocity. A deviation of ten to twenty degrees is not uncommon, and at the later stages of flight, won't affect the gravity losses all that much. Your goal is to get your stated apoapsis at about where you want your desired (parking/transfer/final, depending) orbital perigee to be. Remember, if you want to insert into orbit with a set perigee, you need to have zero vertical velocity at that perigee; if you want a circular orbit, you need to insert at zero vertical velocity and cut off your burn when perigee reaches apogee.

  7. Once you near orbital velocity, finally set the target altitude to the desired target apogee. If you're aiming for a circular orbit, set it for a few km higher than your current stated apogee, since by this point your apogee should, if it is rising at all, be rising only a few meters per second.

  8. If you want to fine tune things, fly a test ascent with "show ascent path" turned on. Watch how close the purple target (and your rocket's nose) is to the velocity vector. Adjust your turn shape and turn end to taste so you get the pitch program you want with the desired apogee and lowest steering losses.

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