Tutorial: Fighter Plane Design
Basic fighter design
Fighter jets are among the hardest to build, and each will have its own characteristics (and purposes!). So, this tutorial will only cover basic design aspects, and all the things you should think about while designing/testing your fighter.
In the early days of fighter design, maneuverability was valued higher than speed. However, as WW2 broke out, higher speeds were preferred (they allow quick pounce-and-run attacks from high altitudes). This new form of combat is usually called "energy fighting". In KSP, energy combat is most effective at high altitudes, in the range of 10–15 km. There are multiple ways to achieve high speeds. First is acceleration. For more acceleration, you need more engines and also more intakes. Note that more intakes DO help your plane even if you oversaturate at sea level. At high altitudes (where energy fighting is probably going to happen) intakes will help keep your engine power. Beware, too many intakes will increase drag. (But if you go in higher atmosphere, this starts mattering less and less) Another way to increase speed is to lower drag. In vanilla KSP drag is determined by adding up all the drag values of the parts - so it is preferable to have as few parts as possible. If you use FAR, aerodynamic profiles start mattering. Other than arrow shaped fuselage, delta wings have the best drag/lift ratio (which is why modern fighter jets use them!) Note: rocket engines are very powerful but have a limited range
Maneuverability is essential in low-flying fighters (specifically with a usefulness ceiling below 8 km). Smaller wings will usually allow your fighter to turn faster and more tightly. However if they are too small they will not generate enough lift or will be too unstable. The speed of your craft will also be an issue here as a slower speed bi-plane can usually make tighter corners than a high speed jet.
Basically how far the fighter can travel. A fighter that has a larger range can go further and fight for longer, and in real life wars often genuinely affects the outcome of a battle. A jet designed with normal intakes and engines will get better range lower in the atmosphere than a jet with ramjets and turbojet engines, which are designed to work better higher up and vice/versa. The amount of fuel carried will make a difference, a jet fuel tank contains less fuel than a rocket fuel tank. However this is where weight comes into play, as a lighter jet can fly with its engines at a lower speed and get better efficiency. A fighter with a large amount of lift could also glide while traveling to save fuel. This tactic works best for a high altitude jet. Rocket engines provide great speed and power but will decrease fuel efficiency sharply due to their large fuel consumption and weight.
How heavy your jet is. A heavier jet will have severe reductions to manoeuvering and usually the same applies for it's range. To keep weight down you can use jet fuel tanks instead of rocket fuel, or structural fuselages if extra fuel is not needed. Removing extra features can also keep weight down.
Esssentially what a jet can do, and what it can't. A pure bomber cannot usually beat a fighter, a high altitude stealth bomber is probably not built to land on water and an air superiority fighter will probably beat a jet designed to be an SSTO space fighter. Multiple engines and intakes designed for different purposes will help, allowing a jet to have fair performance in fights at any altitude. Also, consider what you want your jet to do. What's it trying to fight? If it's ground targets, maybe you should build a bomber to operate at low speed and altitude for accuracy. If it's trying to fight other planes it needs to be fast and manoeuvrable. If it needs to go long distances add dropable fuel tanks, give it and efficient engines and such. And if it needs to go into space, it will need RCS and a specialised setup. VTOLS are horribly difficult to built but can often be superb fighters, if you intend to build one it is strongly recommended you look at a specialised tutorial, as they are often very different to normal fighters.
Which features you have to keep your jet airborne and your Kerbal alive. Armour can be used though is not usually very effective and will contribute to the weight of the fighter. The cockpit/s can have their own release systems and parachutes to eject in an emergency. Also, having the engines on an action key to turn them off can help in a flameout/stall situation, to prevent the jet going into a spin as so often happens during a flameout.
Kerbal space program was never designed as a war game, so there are no inbuilt weapons. However there are many modded weapons on the internet, so finding weaponry such as homing missiles or lasers should not be hard. As for creating your own, there are several things to remember (this is using weapons you create out of parts and not using mods):
1: The type of fighter affects the weapon. A pure fighter will want something accurate that flies straight. Your best choice is a light missile with its own propulsion that is designed to reach top speed as fast as possible, and to have the jet stable and fast enough to get in close to the target. A relatively long stack of Oscar-B Fuel Tanks with a 48-7S "Spark" Liquid Fuel Engine and Small Nose Cone strapped to it will do, as long as it can retain enough fuel on impact to do some damage. A bomber however can carry a much heavier bomb that could use SAS and a probe core to create a guided bomb, or at least fly in a straight line, or just lots of light bombs. Even better, a bomb could use a J-20 "Juno" Basic Jet Engine to become a low-power precision missile, or a flying bomb. Remember that often, making the bomb fall parallel to the ground to increase drag can be valuable as high enough speed can make the bomb warp right through the ground, rendering it grossly ineffective against targets (especially buildings).
2: The plane should have a similar balance when carrying the weapon and once the weapon is fired. Looking at the centre of lift/balance while the weapon is not attached helps to keep this constant. If you do not do this there is a good chance the jet will become unflyable either from the start or once the weapon/s are fired.
3: Space weaponry will behave very differently to weapons built for the atmosphere, and will deviate towards the centre of mass, though if the centre of mass is lined up straight, it will have perfect aim (providing the plane doesn't tilt or pitch while firing).
4: Recoil is not an issue for the jet but the decouplers will have an effect on the missile, rocket or bomb when it is deployed, often sending them spiraling. Without a probe core and SAS this is hard to avoid, but having the weapons undock instead of decoupling often solves this (however, if your weapon has its own propulsion it cannot be fired at the same time as undocking which has its own set of problems and solutions)