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 over-saturate 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
Tip: Drag will make or break the top speed of a fighter aircraft. Ensure there are no flat surfaces (such as an exposed fuselage) and add a nose cone (the NCS Adapter works particularly well since it cuts drag and adds 80 units of Liquid fuel).
Tip: Almost all air superiority aircraft should use the J-404 "Panther" Afterburning Turbofan. While it's regular mode is as weak as a J-33 "Wheesley" Turbofan Engine, its afterburner mode drastically improves performance. Additionally, it is capable of thrust vectoring, which greatly aids maneuverability (particularly at stall speeds). It is recommended to assign an action group to the engine, allowing one to switch mode at the push of a button.
Tip: If speed is needed, use the J-X4 "Whiplash" Turbo Ramjet Engine. This allows top speeds in excess of 1,400 m/s compared to the 900+ m/s top speed using an after-burning Panther engine.
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.
Tip: Pitch is the most important aspect in KSP maneuverability. Use of both a canard and a elevator can greatly improve an aircraft's ability to pitch, improving maneuverability.
Tip: Most control surfaces's effectiveness can be adjusted with the control authority slider found when right clicking the surface. The lower the authority, the lesser the degree the control surface can operate. Conversely, setting the slider to 150 (max amount) will lead to the control surface being noticeably more effective. However, too much control authority generally leads to the rate of roll being hard to control, so it is recommended to turn off roll for some parts while keeping the control authority high, which helps reduce the turning radius of most planes (especially ones with a canard and elevator).
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 maneuvering, acceleration, and range due to a reduced thrust to weight ratio. 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.
Tip: A liquid fuel tank is in general, very, very heavy and will drastically hinder performance. For cosmetic or structural purposes, it is recommended to use the Structural Fuselage, a structure element which looks almost identical to the Mk1 Liquid Fuel Fuselage while weighing 1/23 as much.
Essentially 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 specialized 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 specialized 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 flame-out/stall situation, to prevent the jet going into a spin as so often happens during a flame-out.
Tip: Safety is overrated.
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 (such as armory), 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 bomber however can carry a much heavier bomb that could use SAS and a probe core to create a bomb that can fly itself, or at least fly in a straight line, or just lots of light bombs.
2: The plane should have a similar balance when carrying the weapon and once the weapon is fired. Looking at the center 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 center of mass, though if the center of mass is lined up straight they will have perfect aim
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)