Concepts

This page is a bit of a wall of text. Why should you read it? Especially when you have just downloaded Principia and want to go play with it and have fun?

Well, it turns out that N-body celestial mechanics is rather more complicated than the 2-body simplification offered by stock KSP. In order to help planning and understanding elaborate missions that take advantage of N-body physics, Principia provides a number of tools that let you look at trajectories in different frames, use more advanced navballs and plan complex flight paths.

So if you want to become a senior rocket pilot, to execute horseshoe orbits, Lissajous orbits, heliosynchronous orbits and ballistic captures, please read on. Oh, and you'll be able to say these six words you never say at NASA.

User Interface Changes

Principia changes some of the ways that you interact with the map view compared to the stock game. Specifically:

• To select a target vessel in map view, check Select target vessel... in the main Principia window, and click on the target.

• To select a target celestial in map view, open "KSP Features and check 'Select target celestial' in the main Principia window, and click on target. If playing in career, you will need to have upgraded your Tracking Station to the appropriate level.

• To switch to a vessel from map view, target it then press Switch To in the main Principia window.

• To centre the camera on a celestial or vessel, left-click on it.

Trajectories

A trajectory is a line depicting the positions of a vessel. In stock KSP, all trajectories are conics so there is not a lot of complexity to displaying or understanding them. With N-body physics, though, trajectories may become more complex (and are certainly not conics in the interesting cases) so Principia comes with additional tools for plotting trajectories.

For the active vessel, Principia may display three trajectories:

• The history, painted in lime , shows where your vessel was in the past.
• The prediction, painted in fuchsia , shows where your vessel will go if you just sit on your hands: no engine burns, no separations, no nothing.
• The flight plan, painted in alternating segments of solid pink and dashed periwinkle blue , shows where your vessel will go if you execute the planned flight plan with no errors (more on flight planning later).

If there is a target vessel and the selected frame is not centred on the target, the trajectory of the target is also displayed:

• its history is shown in goldenrod .
• its prediction is shown in light mauve .

The main Principia UI let you control the length of the history (Max history length) and the settings of the prediction and flight plan (Tolerance and Length under Prediction Settings). There are trade-offs in using these parameters: keeping a long history is aesthetically pleasing but it may also be confusing and may consume a lot of memory. Computing a long prediction (or flight plan) with a low tolerance may end up consuming lots of CPU and make the game feel unresponsive. Use your judgement: you probably don't need a 1 metre tolerance when planning a multi-month trip to Jupiter, but you may need that when entering orbit.

Plotting frame

Once Principia has computed a trajectory (maybe the prediction showing where your vessel will go based on its current position and velocity) it must display it. It is important to understand that the same trajectory will look very different depending on the frame in which it's plotted.

Let's make the following thought experiment: you may have a parabolic TV antenna on your roof. This antenna points towards a satellite in geostationary orbit. You cannot see the satellite with the naked eye but if you could you would notice that it virtually doesn't move in the sky. In truth it wiggles a bit around its average position, but it's as good as fixed. That's why you can have your antenna installed once and for all and never need to touch it again. The Sun, on the other hand, moves around you once every 24 hours, that's the day-night cycle. The following picture illustrates this situation:

Now transport yourself to the surface of the Moon, and bring a telescope with you. You can see your house and of course it rotates with the Earth every 24 hours. The antenna is bolted to your house and also rotates every 24 hours. The satellite is not bolted to anything but, by virtue of being geostationary, it also rotates every 24 hours. In addition, both the Sun and Earth rotate around you (or rather, the Moon) once every month. The bottom line is that the trajectory of satellite seen from the Moon is the result of the composition of its own circular motion around the Earth and of the circular motion of the Earth around the Moon. It looks a bit like a crushed spring. The following picture illustrates the new situation:

Which one of these is the real trajectory of the satellite? Well, both. They are both correct, but they are not both equally useful. Imagine that you are a flight controller trying to adjust the satellite so that it remains exactly on top of your house. The second representation is useless because a small drift that would cause you to completely lose TV reception would just manifest itself as a minor change in the shape of the crushed spring. The first representation is what the flight controller wants to use: they can use precision thrusters to nudge the satellite a bit so that TV reception stays optimal in the desired region.

For the same reason, Principia offers a choice of plotting frame that you can use to display your trajectories. Each of these plotting frame is most useful in specific circumstances. You can choose between several kinds of plotting frames using the Plotting frame selection... panel.

• Reference frame fixing the surface of a celestial (e.g., Earth-Centred Earth-Fixed, ECEF)

  Selecting this reference frame as the plotting frame sets the navball speed display to surface mode, and conversely setting the navball speed display to surface mode sets this reference frame as the plotting frame. Setting the navball speed display to orbit mode switches the plotting frame back to the last non-surface reference frame. If the active vessel is in an atmosphere, the Body-Centred Body-Fixed frame can only be selected for the body in whose atmosphere the active vessel lies.

  You can use the panel to pick the celestial (a planet or one of its natural satellite) to use as the centre. This plotting frame is pinned to the centre of the celestial and rotates with its surface. For instance, if you use such a frame centered on Jupiter, Jupiter will look like it doesn't move, and if you look at its surface the atmospheric features (e.g., the Great Red Spot) will appear immobile. On the other hand the distant stars like Sirius, the Sun and the natural satellites of Jupiter will appear to move around.

  This frame is useful when taking off or landing: because the celestial looks immobile you can tune your approach, see where you'll land (note however that atmospheric effects are not predicted), and make sure that you don't come in too fast or too steep. It is also useful for synchronous orbits (or semisynchronous orbits), since they will be periodic in this reference frame.

• Non-rotating reference frame fixing the centre of a celestial (e.g., Earth-Centred Inertial, ECI)

  Selecting this reference frame as the plotting frame sets the navball speed display to orbit mode.

  A frame similar to the previous one, except that it doesn't rotate with the celestial and stays fixed with respect to distant stars. For instance, if you use such a frame centered on Jupiter, Jupiter will look like it doesn't move, and if you look at a distant star like Sirius it will also appear immobile. On the other hand the Sun and the natural satellites of Jupiter will appear to move around.

  This frame is useful for any manœuvre that happens in low orbit (think International Space Station for instance).

  This frame is the one in which the stock KSP displays its conics (the trajectories are only conics in this particular kind of frame).

• Reference frame fixing the centre of a celestial, the plane of its orbit around its parent, and the line between them (e.g., Moon-Earth-Orbit, MEO)

  Selecting this reference frame as the plotting frame sets the navball speed display to orbit mode.

  This plotting frame is pinned on the centre of the orbiting celestial and the line between the celestials appears immobile. In the case of the Earth and the Moon, the Moon will appear immobile and the Earth will stay in a fixed direction, although its distance will vary a bit over time (due to the eccentricity of its orbit). The distant stars, the Sun and all other celestials will appear to move around.

  This frame is useful for when your vessel needs to come close to a celestial, but the direction of another celestial is important. An example is a transfer orbit to the Moon. The direction can also be important for reasons independent of your trajectory: for instance, if you are in low Earth orbit but must orient your solar panels towards the Sun.

• Reference frame fixing the centre of two celestials and the plane of their orbit (e.g., Moon-Earth Lagrange, MEL)

  Selecting this reference frame causes equipotential lines around the Lagrange points of the two celestials to be drawn, using the colour of the primary celestial.

  A frame that rotates to keep the line between the two celestials fixed and their distance constant. This is the only frame that "pulsates", i.e., doesn't display actual physical distances but adjusts the unit of length to keeps celestials fixed irrespective of the eccentricity of their orbit. This is useful because, in first approximation, the Lagrange points are fixed in that frame (in reality they move a bit because of the influence of distant celestials, so the situation is not as clean as in textbooks). Doing physics in such a frame is difficult, and therefore it cannot be used as a manœuvre frame, but only as a plotting frame to display trajectories. Technically this frame is conformal, meaning that, although it doesn't represent physical distances accurately, it does represent angles accurately. Note that, if the second celestial has satellites, the point that is maintained fixed is the barycentre of that system; this makes a difference in systems like Pluto-Charon where the barycentre lies outside of Pluto.

  This frame is useful for planning trajectories to the Lagrange points or for low-energy transfers.

• Reference frame fixing the target vessel, the plane of its orbit around a celestial, and the line between them (e.g., Target Earth Orbit, Tgt EO)

  This reference frame is only available if you have a target vessel. You can select it using the plotting frame selector or by setting the speed display of the navball to target mode. This reference frame is only used for plotting and cannot be used to specify a manœuvre. Take the case where the selected vessel is the International Space Station and the celestial is the Earth. The Space Station will appear immobile and the Earth will stay in the same direction, wobbling up and down below the Space Station by about 10 kilometers over the course of an orbit.

  This is useful for performing rendezvous, since the target vessel doesn't move (see our guide on rendezvous for more). The points of closest approach to the target vessel and the nodes where your trajectory intersects the orbital plane of the target vessel are displayed on your prediction and flight plan.

Some of these reference frames fix the centre of a celestial body. Since that body is fixed, the closest approaches (periapsides) and furthest separations (apoapsides) between the vessel and that body are visible on the trajectory, and are labelled with the Pe and Ap markers; the speed in the frame and the altitude are shown.

Displaying the trajectory in a reference frame that fixes the centre of a body allows you to check (visually, as well as with the apsis markers) that you don't enter the atmosphere or (horror!) crash on the surface. If a periapsis is below the mean surface (0 m altitude) of the celestial body, it is shown as an orange impact marker. No such warning is shown if the periapsis is merely inside the atmosphere or inside a mountain, so check the altitude!

Similarly, for the reference frame that fixes the target vessel, the closest approaches are visible on the trajectory, with separation and speed in the Target Earth Orbit frame.

When the reference plane of the plotting frame is an orbital plane (that is, when the plotting frame is the X-Y-Orbit frame, where X is either a celestial or the target and Y is the celestial around which X orbits), the nodes intersecting the orbital plane are shown. They are labelled with the AN and DN markers, for ascending node and descending node. The component of the velocity that is orthogonal to the orbital plane is shown (this corresponds to stock's angle display on nodes, but works for arbitrary trajectories).

Navball

In stock KSP your vessel is always within the sphere of influence of a single celestial. The navball is defined by that celestial: the blue/brown color separation is parallel to the surface and the line from the N marker to the 180 marker is parallel to the North/South axis.

In Principia things get more complicated because (in the interesting missions at least) your vessel is not under the influence of a single celestial, but of several (or many) celestials. This is for instance the case when navigating to a Lagrangian points or when orbiting a complex system like Jupiter and its natural satellites. Having the navball align on the celestial that currently has the largest influence is not very helpful because it would appear to flip semi-randomly depending on who pulls the stronger.

The navball also changes color to give you a visual clue of its meaning. The exact meaning of the navball in each plotting frame is as follows:

• Reference frame fixing the surface of a celestial

  The navball is similar to that of the stock KSP: the ground is brown and the sky is blue. The separation between the colours is parallel to the surface of the celestial below your vessel. The line from the N marker to the 180 marker is parallel to the North/South axis.

• Non-rotating reference frame fixing the centre of a celestial

  The navball is painted light and dark grey. When using the unmodified stock game, the separation between the colors is parallel to the equator of Kerbin. When using RealSolarSystem the separation between the colours is parallel to the equator of the Earth and the line from the ♈︎ marker to the 12 marker is parallel to the line joining the Spring and Autumn equinoxes. In all cases, the navball is immobile with respect to distant stars. (Directions on this navball are given as right ascensions in hours.)

• Reference frame fixing the centre of a celestial, the plane of its orbit around its parent, and the line towards another celestial

  The navball is painted light and dark orange. The separation between the colours is parallel to the orbital plane of the two celestials (remember that this plane is pinned) and the line from the filled circle (●) to the open circle (○) marker is parallel to the line from the pinned celestial to the other celestial.

• Reference frame fixing the target vessel, the plane of its orbit around a celestial, and the line between them

  The navball is painted light and dark red. The separation between the colours is parallel to the orbital plane of the target vessel and the line from the brown circle to the blue circle markers is the vertical of the target vessel (brown toward the ground, blue towards the sky).

Flight planning

In stock KSP, flight planning is done using manœuvre nodes. Manœuvre nodes are only a rough approximation of the real behaviour of your rocket because they assume that any change of velocity is instantaneous. You may notice this when trying to effect a burn planned using a manœuvre node: you never quite get things right because the actual burn doesn't cause an instantaneous change of velocity.

This is good enough for fly-by-the-seat-of-your-pants, cowboy-style piloting, but it's entirely inappropriate for more complex missions where it's important to make the best use of your engines. Thus, Principia includes an elaborate flight planning system which let you plan burns in a much more realistic manner.

While Principia makes it possible to plan complex trajectories, it's important to understand that there are two things that it doesn't do because, well, they are part of the fun of using realistic N-body physics:

• Principia doesn't help you optimise the flight plan, e.g. by minimising the Δv. You've got to look for good flight plans yourself, using intuition and trial and errors. You'll get better over time. And yes, it's hard, that's why they hire PhDs to do this.

• Principia doesn't help you execute the flight plan. Once you are done with the planning, you need to execute the manœuvres by hand by starting and cutting off the engines just at the right time. You will not get this completely right, and in some cases a delay of a fraction of a second may cause significant changes to the trajectory down the road. That's ok, don't try to overdo it, you can always schedule a minor correction to compensate for any inaccuracies. In real life rocket scientists do just that as they learn more about the gravitational fields around unexplored celestials or because of intrinsic inaccuracies in the intensity of the burns.

  • Mechjeb, however, will help you execute the flight plan, and it is configured to work with Principia in doing so.

The controls that let you plan your flight won't allow you to tune your Δv or your burn times extremely precisely (typically no finer than 1 mm/s). That's intentional: even if you were able to construct a flight plan that way, you wouldn't be able to execute it. If a small change of your Δv has a large effect on your future trajectory, you probably want to make smaller burns at different points along your mission.

Flight planning user interface

Now on to flight planning. A flight plan is made of a number of manœuvres, which are segments where your engine is burning. These segments are painted in solid orange red. In between manœuvres, your vessel is coasting, i.e., is just moving under the influence of gravity. The coasting segments are painted in dashed baby blue.

To create a flight plan, click Flight plan... on the main Principia UI. This opens the flight plan editor. At the top you can specify the overall Plan length. No burn can be beyond the end of the flight plan, so in order to add burns in the distant future you may have to increase the plan length. The parameters Maximal step count per segment and Tolerance control the accuracy of the computation of each segment (burn or coasting) of your plan. As explained above, a long flight plan with very accurate computations might make the game feel sluggish.

At the bottom of the flight plan editor you'll find buttons to Delete last manœuvre or Add manœuvre. Adding a manœuvre opens up a subpanel where you can pick the desired Δv along three different axes, as well as the engines to activate for the burn.

The button Manœuvring frame selection... makes it possible to select the frame used for orienting the manœuvre. Just like for plotting, notions like "prograde", "retrograde", "radial" depend on the frame in which you look at your vessel. For instance, "prograde" is always tangent to the trajectory, but of course the shape of the trajectory changes from frame to frame. By picking an appropriate frame for each manœuvre, you effectively define which way is "prograde" and in which direction the burn will move you.

Above the first manœuvre are displayed the total Δv and the count-down to the next manœuvre. Also, if you tick Show on navball, the direction of the next manœuvre is indicated on the navball to help effecting the burn in the right direction (in career mode, this option is only available if patched conics have been unlocked; this may change in a later version).

By checking Show on navball, the next manœuvre is shown on the navball using KSP's manœuvre marker . Note that, unlike stock KSP, the Δv counter next to the navball doesn't count down as you burn. This is because that countdown is only useful as guidance for burns that are modelled as instantaneous (as in stock KSP), but in Principia burns are applied continuously. In order to execute a manœuvre you have to either take a look at the times in the flight plan editor (a countdown to ignition is provided, followed by a countdown to cut-off), or take a look at the shape of the prediction (which should change to become similar to the flight plan) and control your burn accordingly. Once the burn has been executed, you may want to revise your flight plan with trajectory correction manœuvres to account for errors, as in real life.

Note that by default, in Principia burns are non-inertial, meaning the manœuvre node indicated on the navball moves around following the Frenet frame (the tangent/normal/binormal directions in the manœuvre frame). This is more efficient; it however requires adequate attitude control throughout the burn, especially on manœuvres that lead to a large change in orientation of the Frenet frame, such as a plane change.

Checking Inertially fixed causes the direction of the burn to be inertially fixed throughout its duration, instead of tracking the Frenet frame. This is useful for vessels without such attitude control capability—and most importantly for spin stabilized burns, wherein the vessel cannot change attitude at all. This needs to be checked even if the burn is modeled as Instant Impulse, because the latter isn't really instant but just very short.

All the sliders in the flight plan editor are exponential sliders: the farther to the left (or right) you drag them the bigger effect they have on the quantity that you are tuning. This makes it possible to enter large quantities (by dragging far to the left or right) while retaining the option to do fine tuning (by dragging very little away from the midpoint).

Manœuvre optimization

Principia provides tooling to help fine-tune a flyby of a celestial:

• First, a plotting frame must be selected whose centre is the celestial for which you want to find a flyby. For instance, it could be Mün-Kerbin-Orbit (MKO) for a flyby of the Mün. This causes the optimization parameters to be displayed in the flight planner thus:

  

• In the optimization parameters panel, enter the desired altitude of the flyby. Optionally, you can also choose the inclination of the flyby; if you don't care about the inclination, click the On button and it will turn into Off.

• Then, you must adjust by hand a manœuvre that is reasonably close to the desired flyby. What is "reasonably close" depends on the problem at hand, but you must be aware that Principia only performs a local optimization, so if your manœuvre is too far off, it may not find a solution.

• Finally, click the Optimize button on the manœuvre that you want to fine-tune:

  

Principia then tries to adjust the intensity, timing, and direction of the manœuvre to get closer to the desired altitude and inclination. It also tries to minimize Δv subject to the other constraints. This process may take some time, during which the best flight plan found so far is displayed. Note that the entire process is completely best-effort, and Principia may not be able to find a solution (in which case it leaves the manœuvre unchanged) or may find a bogus solution (e.g., one with a very high Δv). When optimization is successful, though, it avoids spending significant time fiddling with minute corrections in the manœuvre.

Terminology

Veteran players of stock KSP will be familiar with the terms prograde, retrograde, radial, and normal. This terminology makes sense for conics: prograde is a standard term, the next vector is the orthogonalisation of the inward radial vector (towards the planet), the third vector is normal to the orbital plane.

In Principia, all three components of Δv depend on the choice of frame. In addition, orbits are arbitrary curves in 3D, which may curve away from the nearest body. Thus, we use a different terminology, closer to mathematical conventions:

Stock KSP	Principia	Symbol
Prograde	Tangent
Retrograde	-Tangent
Radial In	Normal
Radial Out	-Normal
Normal	Binormal
Anti-Normal	-Binormal

Limitations

At the moment, flight planning suffers from the following limitations, which we'll try to address in future releases:

• The engines used for a burn are either the active engines, the RCS or an instant impulse. In practice this means that you'll probably want to have an active engine when doing your planning. Furthermore, solid-state engines cannot be activated without running them at full power, so it's not really possible to use them in planning.

• The mass used for computing the flight plan is the one when you actually create the plan. If you later do a vessel separation, it will change the mass and will effectively make the flight plan useless: you'll need to delete it and re-create it.