Autopilots

Autopilots

The bridge has its own autopilots for things MechJeb doesn't do well (planes, propeller craft, helicopters, suicide burns) and hands everything else to MechJeb when it's installed. Only one autopilot flies at a time: if one is running, the others answer "Busy... say 'abort' first". abort autopilot always stops them all.

Sidry: One pilot at a time. Two is how you get a cockpit argument at 3000 meters.

Planes

The holds (AICS -> Aircraft Autopilot)

Classic airliner-style holds, each with its own checkbox and target field. Chat orders like climb to 3000 or heading 270 set the same targets, and changes apply live without restarting anything.

• Altitude hold: meters above sea level, or AGL (above the runway or ground below you, never sea level).
• Vertical speed hold: in a climb, the throttle chases the target climb rate while pitch stays in a sensible band.
• Heading hold: shows the signed heading error (+ right, - left).
• Roll hold: bank angle, capped at 20 degrees at low speed and 10 above 250 m/s (see overrides).
• Speed hold: true airspeed, throttle only. It never trades altitude for speed.

If you engage the holds while lined up on a runway, the plane takes off on the runway heading first.

Flight rules it sticks to

These came from a lot of crashed test planes, so they're on purpose:

• Speeds are equivalent airspeed: 200 m/s target and a 220 m/s cap down low, rising with altitude. Supersonic dashes fly Mach 1.5 above 8 km.
• Throttle moves in 5% steps with a few seconds for jets to spool up. It's never 0 in flight and stays at 30% or more in the climb and cruise.
• Climbs hold 5-15 degrees of pitch. The nose only goes down on final or a planned descent.
• Safe ceiling: the autopilot estimates how high your craft can safely go from its engines and intakes (3-18 km for jets, 20 km with rocket engines, 3 km with nothing to breathe through, 5 km for props) and asks before going above it.
• Max speed estimate: from the measured drag and available thrust it works out roughly how fast the plane can go at the target altitude, and clamps requests above that with a note like "Best estimate max is ~326 m/s here; you asked 400. Flying 326."

Takeoff

The starting throttle comes from your thrust-to-weight ratio (full power for weak planes, about a third for very powerful ones), then it adds power in steps only if the plane isn't accelerating enough. Ramjets start at full power. At the rotation speed a rotation assist eases in extra up-elevator until the nose comes up.

The bridge learns each craft's real liftoff speed from every takeoff, hand-flown or not, and uses about 92% of it as the rotation speed next time. Without a learned value it uses 1.25 x the stall speed.

If it realizes it can't reach flying speed before the end of the runway while it can still stop, it aborts: idle, brakes, and a chat message saying why. It never cuts the throttle once airborne.

Autoland and fly-to

land at KSC, land at Island, the Landing window, or AICS -> Approach & Autoland:

• Picks runway 09 or 27 by the side you approach from (or the approach heading you set), and flies to a long final (11 km by default, 5 km for short final) on the extended centerline. If it arrives pointing the wrong way, it flies a teardrop turn to line up.
• 3 degree glideslope, gear down late (about 2.5 km or 30 s out), flare at about 20 m, touchdown on the main wheels nose-up.
• Stall speed: the first landing of a new craft includes a short slow-flight test at altitude. Approach is flown at 1.3 x stall and touchdown at about 1.15 x stall. The result is remembered per craft name.
• Flaps only on dedicated flap parts, stepped with speed limits. Elevons are never used as flaps.
• Touch and go count: 0 = full stop, -1 = keep going until told.
• Go around at any time.

fly to <runway> does the whole trip from wherever you are, including from the ground: takeoff, climb, cruise, a planned descent that bleeds speed early, then the autoland. It checks the fuel range first.

While the landing or fly-to autopilot is flying, you can still say speed 180 (en-route cruise speed) or climb to 4000 (cruise altitude). On final, an altitude change asks you to go around first.

Rollout: gentle brakes and reversers

On a full-stop landing the brakes wait until every wheel is on the ground and the plane has settled, then pulse gently, ramping up over a few seconds. A nose dip, a wing lifting or a wheel coming off the ground releases them until things calm down again. (This exists because a tail-heavy prop plane flipped onto its back under hard braking. Bill still talks about it.)

Engines that can reverse thrust (an engine mode called "reverse", or a reverser action on the part) go to 60% reverse once the mains and nose are down, then back to forward below 30 m/s. A bounce or a swerve of more than 15 degrees means idle. Turn this off with /reversers off.

[COMMS] Bill (Eng): Gentle brakes. GENTLE. I'm not rebuilding that tail again.

Propeller planes (Breaking Ground rotors)

Breaking Ground rotors with propeller blades count as real propulsion: no more "this craft has no engines" for a prop plane. A rotor is the motor (RPM Limit, Torque Limit(%), Motor, Brake), and its blades' Deploy Angle is the blade pitch.

• Pre-flight check: before takeoff (and continuously in flight) every rotor's Brake must be 0, Torque Limit(%) above 0 and Motor engaged. The bridge fixes what's wrong and tells you, for example Pre-flight: center rotor (M-32S Rotor) Brake was 100 -> released (0).
• Takeoff: motor on, RPM 460 (the Breaking Ground maximum), torque 100%, blades deployed at a fine 8 degree pitch.
• Blade pitch schedule: while an autopilot flies, the blade pitch follows the airspeed so the blades keep a good angle of attack, and the torque limit follows the autopilot's throttle.
• Never in flight: torque below 10%, RPM below 115, motor off, blade pitch below 2 degrees, or reverse.
• Reverse thrust: negative blade pitch on the ground only. The autoland uses it on the rollout and steers with it on twin-prop planes, more reverse on the side it needs to turn toward.
• Sides: rotors are grouped into left, right and center by their position, and counter-rotating pairs are detected. If one side's prop dies in flight, you get a MAYDAY, wings level and a bit of rudder trim toward the good side.

Orders, each optionally prefixed with left, right or center:

prop pitch 15        prop pitch auto
rpm 400              rpm max
torque 80            torque max          torque auto
props on             props off
props reverse        props forward
left props pitch 20  right rpm max       left props reverse

Helicopters (experimental)

Helicopter support is new and still being shaken out in game, so treat it as a test pilot program. Save first.

How it recognizes a helicopter: rotors whose spin axis points up are lift rotors. Only rotors with blades attached count. The layout is detected per craft and logged, rotor by rotor:

• Conventional: main rotor plus a tail rotor (yaw = tail rotor pitch).
• Compound: main rotor plus a left and a right side prop (forward thrust from the side props, yaw from differential side-prop pitch). Small wings are fine.
• Coaxial: two lift rotors stacked on one axis, counted as one main rotor and driven together (counter-rotating, yaw from differential torque).
• Tandem: two lift rotors apart.

A craft with one lift fan and real wings but no side props or tail rotor is still treated as a plane.

How it flies: helicopters fly differently from planes, so they get their own controller and never the plane holds or runway logic.

• Collective (main rotor blade pitch) controls vertical speed. Altitudes under 500 m are above the ground.
• Tilt controls ground speed: nose down to go, nose up to slow and stop. Tilt is capped at 12 degrees (6 on a compound, since its side props push it forward).
• Heading vs. track: where the nose points (heading) and where you're actually going over the ground (track) are separate. Above 15 m/s the nose follows the track unless you've pinned it with face.

Orders:

Say	What happens
take off	Rotor checks, waits for the main rotor to spin up past 60 RPM, then lifts to a 20 m hover.
hover / hover at 30 / hover facing 090	Hold a hover, optionally at a height or nose direction.
climb to 50 / descend / descend to 10	Height above the ground. Bare descend stops at 10 m.
land	Slow vertical descent, under 1.5 m/s at touchdown.
fly 090 / fly 090 at 20 / fly heading 090 speed 20	Track over the ground (and speed).
face 270	Turn the nose only.
sidestep left 5 / back up 10	Small moves (defaults 5 m and 10 m).
hold position	Stay right here.
fly to <place>	Fly there and hold a hover.
heading 180 / turn left	When moving: the track. When hovering: the nose.
speed 15	Forward speed.

pitch up and pitch down are refused for helicopters, since the pitch is busy being the throttle. After landing you can have it apply the rotor brake automatically (setting rotor_brake_park, off by default).

[INTERCOM] Bob (Sci): Why is the floor going sideways? Sidry: That's called "flying", Bob.

Rockets

With MechJeb2 and kRPC.MechJeb installed, the rocket side mostly drives MechJeb's own autopilots:

• Ascent to orbit ("launch to an 80 km orbit"), launch into a target's plane, circularize.
• Maneuvers: set apoapsis, periapsis or inclination, match a target's plane, transfer to another body (Hohmann), course corrections, capture burns and aerocapture setups.
• Docking with your target (experimental): rendezvous, then dock at a free port of matching size. It checks RCS, monopropellant and delta-v first.
• Station keeping over a longitude, antenna lock (nose to nadir), sun lock.

Landings use the bridge's own lander: land here (deorbit if needed, reentry, a suicide burn down to about 1.5 m/s, legs and chutes, then a touchdown report), or MechJeb-guided land at KSC and land at a picked map spot, with the bridge's controller taking over for the final burn. Every landing or deorbit first runs a delta-v check and refuses with "NOT ENOUGH DELTA-V: have ~X, need ~Y" unless you insist. On Kerbin you'll need engines that work at sea level (Swivel or Reliant, not Terrier or Poodle).

The AI asks before orbit-changing burns unless your message clearly says "now". When you're flying a plane, the rocket landing tools are hidden from the AI so it can't try a suicide burn in your spaceplane.

Taxi

AICS -> Taxi drives you between waypoints (runway starts, runway middle, the pad, the Island runway ends, your saved spots), or along a route you type. Planes use a gentle throttle and brakes; rovers use their wheel motors. It drives in straight lines and doesn't avoid buildings, so add waypoints to get around them.