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Principia

Principia is a mod that provides N-body physics to KSP. My personal take is that after using principia, it’s hard to go back, even when you’re not using fancy N-body maneuvers.

In stock KSP, we have the concept of a sphere of influence. When you get close to Kerbin, the navball shows Kerbin and your speed is shown relative to Kerbin. When you’re close to Duna, your navball shows Duna, and your speed is relative to Duna. We get to choose three frames of reference: surface, orbit and target. Surface mode shows your speed and direction from the point of view of someone on the surface of the body, Orbit mode shows your speed and direction relative to the body without accounting for surface rotation, and Target mode shows your speed and direction from the point of view of someone in another vessel.

Principia does away with the concept of a sphere of influence: we are always being influenced by all bodies at the same time. Therefore, alongside choosing Surface, Orbit, and Target reference frames, it also makes sense that we can choose which body our Surface, Orbit, or Target frame refers to. Inertial reference frames e.g Kerbin-Centred Inertial (KCI), or Mun-Centred Inertial (MCI) are the equivalent of Orbit Reference frames in the stock game. Fixed reference frames e.g. Kerbin-Centred Kerbin-Fixed (KCKF) or or Mun-Centred Mun-Fixed (MCMF) are the equivalent of Surface in the stock game. Target reference frames are … well, they’re the same.

TODO image showing window

Not only can we select any reference frame from anywhere, we can also show any reference frame in the map view. For example, if we have a satellite orbiting Kerbin, we can see where it will pass over the ground using the Kerbin-Centred Kerbin-Fixed reference frame. This is useful for those survey contracts (conduct temperature reading 18,000m above Kerbins despair etc.) in the early game, as it shows exactly where we will pass without any deviation. When we select the target frame, we see the path of our vessel from the point of view of the target.

Getting encounters is easy. In stock KSP, we had to guess where the body would be, and

notes

  • In Stock KSP, when you timewarp, the craft will stop rotating, with Principia, this bug is fixed.
  • Sometimess things just get yeeted from Duna/Ike.