Title: PVG launches to wrong azimuth for retrograde orbits (>90° inc)

[easonyujiang]

Environment

• MechJeb2 version: 2.15.3.0
• KSP version: 1.12.5.3190
• RSS/RO: Yes (RSS Earth, launch site 19.81°N)
• OS: Windows 11

Summary

PVG mode pitches due east (heading ≈ 90°) during the atmospheric phase regardless of target inclination. For retrograde orbits this is completely wrong — the rocket ends up on a prograde plane and PVG cannot recover in the vacuum phase. This is a regression — older versions of MechJeb2 handled the same mission correctly.

Steps to Reproduce

1. RSS Earth, launch from 19.81°N latitude
2. Select Primer Vector Guidance (RSS/RO) ascent mode
3. Set target orbit: Ap = 220 km, Pe = 220 km, Inc = 116.6° (retrograde)
4. Do NOT use "Launch to LAN" (leave it at default)
5. Engage autopilot and launch

Expected Behavior

• Rocket pitches toward the correct retrograde azimuth (~208° southwest)
• Atmospheric gravity turn establishes inclination close to the 116.6° target
• PVG vacuum phase handles final orbital insertion with minor corrections

Actual Behavior

• Rocket pitches due east (heading ≈ 90°) throughout atmospheric phase
• Atmospheric phase ends with inclination ≈ 19.6° (= launch site latitude, natural prograde value)
• PVG inherits a state on the wrong orbital plane (off by ~97°)
• PVG solver iterates 156+ times attempting to fix the plane in vacuum
• Final orbit: Pe ≈ 182 km, Ap ≈ 2665 km — circularization completely failed

Data from the Failed Launch

Target: Ap=220km Pe=220km Inc=116.6°
Launch site: 19.81°N

Stage data:
  Stage 0: TWR 0.21, 792 m/s vacuum
  Stage 1: TWR 0.25→1.53, 5480 m/s vacuum
  Stage 2: TWR 0.73→2.13, 3157 m/s vacuum
  Stage 4: TWR 2.61→4.93, 1895 m/s vacuum
  Stage 5: TWR 2.11→6.08, 2960 m/s vacuum

Result:
  Orbit: 182.4 km × 2.665 Mm
  rT: 439.0 km  vT: 8.120 km/s  FPA: -5.6°
  inc: 19.6°  lan: 91.0°
  heading: 90.0  pitch: 51.5
  vgo: 9673  tgo: 927.290
  converges: 156  staleness: 2s

Also Affects Moderate Inclinations

Target inclination 60° shows the same pattern: rocket pitches east during atmosphere (~40° achieved), PVG must dog-leg the remaining ~20° in vacuum. Less catastrophic than the 116.6° case (the Δv exists to fix 20°), but still wastes significant fuel.

Additional Context

• Older versions of MechJeb2 handled retrograde PVG launches correctly from the same RSS install
• ForceRoll checkbox and roll parameters do not affect this behavior
• "Launch to LAN" button was NOT used

[lamont-granquist]

Target: Ap=220km Pe=220km Inc=116.6°

Orbit: 182.4 km × 2.665 Mm

That doesn't match. That looks like the long-standing bug where fiddling with the settings while the optimizer is running confuses the target settings, and a disengage/engage usually fixes it (or exit the flight scene and come back).