Tutorial: Spaceplane basics
If you're tired of big rockets that use tons of fuel and disintegrate in the atmosphere when coming back to Kerbin, this tutorial is perfect for you! This tutorial will help you with the basics of spaceplane flight, and will help you avoiding the most common errors that could ruin your perfect day as a spaceplane pilot! This is just a general briefing section with lots of "to do" or "not to do" things: when you think you get it all right, check the Aeris 4A tutorial mission to learn how to get into space easily.
While having the same stock parts as in VAB, the way to design spaceplanes differs from the one for spacecrafts. First of all, since the launch happens horizontal, you will have to include gear bays, and you may want to include jet engines for the first stage. While it's true that they absolute hate space, they offer one large advantage over pure rocket engines while inside the atmosphere: fuel efficiency. One FL-T100 tank can't power any rocket into space, yet a Shock cone intake, a Mk 1 inline cockpit, a FL-T100 and J-X4 "Whiplash" Turbo Ramjet Engine aimed in the general direction of "up" will let you laugh your way past the 70km mark at 1100m/s TWICE before running out of fuel.
The Whiplash's ridiculous fuel efficiency allows a spaceplane to climb high into the atmosphere and gain a lot of speed while barely using any fuel at all. It is worth noting that jet fuselages, since they don't carry oxidizer, are lighter than normal liquid fuel tanks. However, the fuselages Mk2 and Mk3 aren't rounded and they have their own adapters (which also act as additional fuel tanks) to connect them to the other rounded stock parts. The Mk3 Cockpit has on the back the same size of a Mk3 Fuselage, greatly simplifying the process of designing a plane with this cockpit.
As you would expect, spaceplanes need wings: they have various shapes and dimensions, and they differ basically in lift rating: you will want to have as much lift rating as possible. It is also advisable to add some control surfaces to your plane to have some extra control in the atmosphere: you can manually add them to the wings or choose winglets with effective control surfaces, like the Standard Canard. Keep in mind that lift rating and control surfaces are not connected: lift rating is basically the capacity of your wings to sustain the weight of your ship, while control surfaces are parts of wing that can be moved to change the flow of the air around the plane and through this change a plane's direction, angle of attack or inclination. This is an important distinction; a plane with great lift rating but without any control surfaces will fly easily but will be almost uncontrollable. Vice versa a plane with lots of control surfaces will be perfectly controllable (maybe even too much) but will have big difficulties taking off.
Finally, keep in mind that jet engines are air-breathing, and this means: first, you need to include Air intakes in your spaceplane, otherwise the engine won't even ignite; second, as you fly higher the intakes will be less effective and you will come to a point when the engines will shut down, due to the lack of air. At that point, the plane could start spinning around due to the very high drag of the open intakes and the absence of any thrust. To avoid that, keep an eye on the "Intake Air" on the resources panel: the engines will shut down when they have less of 0.05 intake air each, so you need to close the intakes and shut the engines basing on how many engines are active (for example, with two jet engines you will need to close the intakes at 0.10 intake air, or you'll risk to screw up the whole flight). After shutting the engines down, you will need to start the second part of your space mission.
When your jet engines stop working, it is time to ignite the old, reliable liquid fuel engine. Note that in versions of KSP prior to 1.0, closing the air intakes would reduce drag. In versions since then, there is no benefit to closing air intakes. Keep in mind that as your altitude increases, your control surfaces and winglets will become increasingly ineffective and will no longer work at all once you leave the atmosphere, so you may need to add alternative control systems like reaction wheels or RCS thrusters.
As long as you're in space, your spaceplane won't differ from any spacecraft: you will probably want to add batteries and generators to prevent the command pod from running out of power. It is also common to add an Inline Clamp-O-Tron, which, unlike all the other docking ports, can be placed in the middle of the spacecraft (a handy solution, since there is not much space at either end of the craft) to allow your plane to dock with space stations or other spacecrafts.
Re-entering in the atmosphere and landing
The most dangerous part of a spaceplane flight is landing. Re-entry heating can destroy parts of your spaceplane, or destroy it entirely. After that, you face the challenge of touching down on the ground and coming to a stop safely without rolling and breaking off a wing or taking a nosedive and blowing your aircraft up on touchdown.
To handle re-entry, it's recommended to start your approach back into the atmosphere at a shallow angle, ideally with a periapsis of around 35 km. Keep your spaceplane pointed about 90 degrees above prograde so that the wings and body of your aircraft slow you down as much as possible. You may also want to deploy your landing gears to increase your drag, as well as airbrakes if you have them. As you approach 35-50 km, your aircraft will most likely level itself out, at which point you can try aiming about five degrees above the horizon line. If your aircraft is burning up during this stage, you may need larger wings to slow you down faster, radiator panels to carry away the heat more effectively, parts with a higher temperature tolerance (like the Mk2 liquid fuel fuselage instead of the Mk1 liquid fuel fuselage), or parts to increase your maneuverability, like RCS thrusters, reaction wheels, or canards and elevons.
Landing can be trickier for spaceplanes since they are often designed for higher speeds and may not be able to fly level at speeds low enough for an easy landing (<50 m/s). This can be helped by adding components to increase maneuverability, or by using larger wings. If your spaceplane is unable to land at low speeds and you're ready to land but have a large quantity of fuel, it may be helpful to burn off most of the excess fuel before landing to make the aircraft lighter. If your spaceplane is able to land at low speeds but you're having difficulty slowing down as you approach the runway, you can try approaching at a lower angle and repeatedly pitching up and then back down to increase your drag as you approach your landing zone.
To recover the most value from your spaceplane, you should try to land on the runway at the Space Center (this tutorial, although it has been written for spacecraft and not spaceplanes, is a great help). If that's not an option, you can still recover some value by landing at any suitable flat place on Kerbin. Be aware that landing on water is possible if your spaceplane can fly level at less than ~40-50 m/s.