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 avoid 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've got it, check the Aeris 4A tutorial mission to learn how to get into space easily. Note: This tutorial was last updated for version 1.0.5.
While having the same stock parts as in the VAB, spaceplane design is quite different from pure rocket design. First of all, since the launch happens horizontally, you will have to include landing gears, and you will most likely want to include jet engines for the first stage. While it's true that they don't work in space, they offer one large advantage over 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 Mk2 and Mk3 fuselages 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 Mk2 and Mk3 cockpits match the Mk2 and Mk3 fuselages, greatly simplifying the process of designing a plane with these cockpits.
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 enough lift to keep your fuselage approximately prograde during your ascent to orbit. 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.
Note that the lift rating does not mean that the wings will automatically lift a spaceplane into the air when it's moving forward. To takeoff and land at low speed, it's helpful to rotate the wings so the front is slightly above the back of the wing. It can also be helpful to design your landing gears so that your fuselage points slightly upward, thereby increasing the angle of attack of your wings during takeoff.
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 intakes and the absence of any thrust. This most likely indicates that your spaceplane's center of lift is too far forward compared to its center of gravity. You can resolve the emergency by transferring fuel from rear fuel tanks to forward fuel tanks, but you should alter your design to bring your wings further back to prevent such incidents in the future. Note that the drag produced by the intakes will not be reduced if they are closed. Once your jet engines have shut 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. 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.