Things have been hotting up recently. No I’m not referring to the surprisingly Sahara-like British weather which I’m missing out on as I write this, what I am actually referring to is something rather different, something rather special; The KSP 0.15 update.

I’ve been patiently waiting for this update for several weeks now, especially since the release’s bevvy of new features was announced. Now that the update has arrived I can unequivocally state it was well worth the wait.

We were originally told to expect the long-awaited docking mechanic and parts to make it into this version. Alas, the folks who make up the game’s development crew Squad quickly realised this mega-feature was too complex to implement within one development cycle. So perhaps to placate their fan base they replaced docking with something arguably more fun. Aircraft and space planes.

Some of you may already have experience with KSP’s space planes – they’ve been an unofficial part of the game since C7 introduced his flight pack modification. They were certainly impressive parts and lots of fun to play around with. The difference now being that C7 is an official member of the Squad team and a selection of his best wings, tanks and engines have now become available with the stock version of KSP.

For those who have no idea how to tell CoG from CoL, or what a ‘canard’ is then hopefully this short guide will give you some idea of how to put together a stable aircraft. One that is actually capable of not only taking-off, but also not diving back into the runway five seconds later. I specify a ‘short’ guide since the goodies that have arrived with 0.15 are still likely to change in behaviour as development moves forward (in particular it is rumoured that modifications to KSP’s aerodynamic modelling are a high priority).

 

Always pack at least one laser. Two is better.


Basics

Just like their real-life brethren, the aircraft of KSP are governed by four main physical forces;

  • Weight due to mass of the aircraft/fuel/payload
  • Drag due to air resisting the passage of the vehicle through the air
  • Thrust provided by the aircraft’s engines to accelerate the vehicle
  • Lift provided by any lifting surfaces (wings, tail-fins, canards, ailerons etc.)

In what is a seemingly obvious general rule of thumb, aircraft must have enough thrust to overcome drag, and enough lift to overcome weight to sustain any kind of flight.

Sounds quite simple then? Just bolt an absurd amount of wings, weld a nefarious number of engines onto your creation and watch it tear-ass down the runway and into the sky…

As with many things in this life, I’m afraid it’s just not that simple. Following are several of the more complex parameters that you’ll need a basic grasp of to help you create nice stable designs.

 

Don't be afraid to experiment! Even designs like Kyle's Monstrosity can work well.


CoG

CoG or ‘Centre of Gravity’ (otherwise known in physics as ‘Centre of Mass’) is literally the point on your aircraft where the game assumes all of your mass to be concentrated. In layman’s terms it is simply the point of balance. An obvious analogy is a 30cm ruler. Since the ruler has a uniform shape and mass distribution its CoG would be in the centre next to the 15cm mark. If you were to place an eraser at one end of said ruler, you’d have to move your finger towards the eraser to keep the system balanced. This is because the added mass of the eraser has moved the system’s CoG.

The same works for your aircraft; add some heavy engines to the rear and you’ll move the CoG towards the aft, and vice-versa.

When considering CoG alone as a function of overall stability, a general rule of thumb is to ensure the CoG is somewhere between the aircraft’s centre and its tail.

CoL

CoL or ‘Centre of Lift’ refers to the point on the airframe where all the lifting force generated by any wings et cetera is assumed to act upon. It is really like an upside-down version of CoG. Going back to the 30cm ruler analogy, place a pair of wings at the 15cm mark and your CoL will also line up with the 15cm divide. Place another identical set of wings at the 30cm mark and your CoL will shift to exactly halfway between the two sets of wings. Therefore the new CoL would now be at the 22.5cm mark.

Considering that there are so many different permutations and combinations of lifting surface, complex designs can make calculating CoL accurately outright guesswork. As a result, when pursuing these you’ll have to resort to the good ol’ Kerbal way of a bit of trial and error.

CoT

CoT or ‘Centre of Thrust’ denotes the point at which any engine (or group of engines) can be said to act through. Like the aforementioned CoG and CoL getting this right is another balancing act. The best way to illustrate getting this wrong is a scenario where you place one engine on say, the left wing only. In this case all the aircraft’s thrust is acting to push the left wing forwards. Since we didn’t place an engine on the right wing to balance this force, the aircraft will pull to the right with violence; likely veering off so hard it will tip over and plant itself headlong into the runway.

Not a good look.

The obvious way around this is to place all engines symmetrically to remove and unwanted moments (or spin forces) that would otherwise ensue. Bear in mind that CoT has three-dimensional properties, so you’ll need to balance your design up and down as well as left and right.

 

Right-clicking on certain parts will bring up an informative context menu that allows you to control it.


Considering these factors for a final design

As stated previously, stable designs are difficult to achieve (especially for the novice) and this is because all of the above factors (CoG, CoL etc.) must be all taken into consideration at the same time. Variations in one can have such a large effect on handling that other factors need to be adjusted to suit.

Again due to the nature of the physics model used by KSP, finding the right combination and placement of parts requires a lot of experimentation (unless you get lucky and hit the jackpot first time) before you’ll have any decent success stories.

As a general rule however, the following points should give you a helping hand along your way;

  • Try to place CoG behind the CoL (so more weight behind your wings than in front). This sounds counter-intuitive but seems to work in KSP.
  • Placing the CoL towards the front helps with the above point. It will also help you pull-up (‘rotation’ in aviation parlance) on take-off, since you need a fair bit of lift to get that nose off the ground.
  • Place your rear-most gear as close to the CoG as possible, but always slightly towards the rear. This will also assist you with rotating at take-off (imagine the ruler again; lifting one end of the ruler by pressing the opposite end down is much easier if you place your balacing finger in the centre (15cm) rather than closer to the finger applying the force.
  • For maximum effect, place any control surfaces symmetrically and spaced equally out from the CoG. Any control input will try to rotate your aircraft around its CoG so if you keep moments and how levers work in your mind you’ll be that bit better off.
  • You can increase the lifting power of a surface by angling the front (or leading edge) slightly upwards (usually 5-10 degrees is a good bet). If you have a conventional-looking design (straight wings near the centre, tail plane at the rear) that refuses to lift off the runway, you can angle the main wings’ leading edge slightly upwards. If this spoils the look of your ship, you can always angle the leading edge of the tail plane downwards slightly as this will give the same effect.
  • Building delta-winged designs (e.g. Concorde) is a little different. Since CoG and CoL are both quite far back in these designs they can become rather unstable, especially in the yaw axis. If you have problems with yaw stability then adding more vertical tail fins and/or rudder controls should help you out there.

 

You can even use an aircraft as a platform to launch cargoes into space.

 

As I stated before, this guide only aims to touch on the basics of atmospheric flight in KSP and explain some of the more technical terms that people associate with such endeavours. I consider it a work in progress as such, and expect more changes as the game and its physics engine become more mature.

Another big thank you to the Squad team for all of their hard work and another excellent update;

I cannot wait to see what they have in store for us in the future with 0.16.