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.
The most dangerous part of a spaceplane flight is returning from orbit. 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, or rolling off of the runway and into the ocean.
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 30-35 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 designed for higher speeds than other aircraft and may not be able to fly level at speeds low enough for an easy landing (<50 m/s). When dealing with high-speed landings, you may touchdown too quickly and cause the front of the plane to smack into the runway. After a successful touchdown, high-speed motion on the runway (let alone uneven ground) can be unstable, causing the aircraft to careen to one side or the other, potentially resulting in loss of a wing and sometimes the entire aircraft. Landing also often requires rapid deceleration to avoid running off the end of the runway or crashing into a slope when landing on open terrain.
All of these problems can be exacerbated or reduced by adjusting the amount of fuel in your tanks during landing. Close to empty tanks will allow you to fly slower, decelerate faster, and reduce touchdown strain. Ideally, you ought to test landing the spaceplane with full fuel tanks and with nearly empty fuel tanks prior to taking your spaceplane to orbit. If your spaceplane is difficult to land when full of fuel but easy to land when empty, then it may be helpful to burn off or transfer out most of the excess fuel before landing to make the aircraft lighter. In subsequent missions, you may want to launch with less fuel to cut down on cost and make landing a quicker and easier process.
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.
Landing speed (minimum speed for level flight) can be reduced by adding components to increase maneuverability, by using larger wings, by increasing wing angle of attack on the fuselage (3-5 degrees is the recommended range for a spaceplane to achieve the best lift-to-drag ratio [source]), and by decreasing the weight of the aircraft. When landing, you can achieve the highest lift for a given speed by raising the total angle of attack of your wings to 30 degrees (although this induces a great deal of drag). Be aware that while this angle will provide the lowest possible landing speed for your spaceplane, it will create problems if your center of gravity is too far ahead of your rear landing gears (causing a high-speed nose collision on touchdown) or if your rear landing gears are too far ahead of your engines, causing them to strike the runway first.
If your spaceplane is able to fly and land steadily at low speeds but just you're having difficulty slowing down as you approach the runway, try to reach your desired speed first and then approach the runway in almost level flight. You can slow down either by deploying landing gears (and airbrakes if you have them) and by repeatedly pitching up and then back down to increase your drag.
If you have trouble pitching up enough to land at a reasonable speed, you can increase your maneuverability by toggling your flaps, canards and ailerons to greater than 100% control authority.
These occur at their worst when your center of gravity is far ahead of your rear landing gears and you have a heavy plane at high speeds and a high angle of attack on landing, resulting in your front landing gear rapidly striking the runway after your rear landing gears touchdown. Consider placing your rear landing gears close to the spaceplane's center of mass, but be careful to avoid an engine collision with the runway. It helps to have a center of gravity which is close to your engines so that the landing gears can be close to both. Alternatively, you can try landing at higher speeds with your nose pointed further down, but this increases challenges with stability and deceleration while on the runway.
When your spaceplane rolls shortly after touchdown, veers to one side and then explodes on the runway, you have a problem with landing stability. This is generally an issue of not spacing your landing gears out far enough apart. For an example, see the A-10 Warthog's landing gears: link. Notice how the landing gears are placed out on the wings. For a Mk1-based aircraft, your rear landing gears should not be tightly tucked together on the fuselage. Mk2 aircraft may be able to get away with it, but even then it's often best to space them further out. However, make sure to use struts when placing landing gears on the far edges of a multi-part wing because they may sag enough to cause a fuselage collision with the runway during landing.
Make sure that all of your landing gears are pointing in exactly the same direction. Even a small deviation can cause serious instability, making your aircraft bounce and jolt left and right even during takeoff.
You may also need to turn off or reduce the strength of the front brake on your aircraft. An active front brake can cause your aircraft to rapidly and uncontrollably pivot left or right during landing.
Even with a stable landing, you may find that you don't have enough room on the runway to come to a stop before you reach the end. To minimize the risk of such a situation, try to land on a patch of flat ground leading downwards a downhill slope. A good example of this is at the KSC runway, when landing on a 90 degree bearing. If you can't slow down in time, you can simply flick your engines back on to take off and turn around for another try as you pass over the coast. If you attempt a landing at KSC on a 180 degree bearing, you run the risk of colliding with the upward slope shortly beyond the runway. Similar principles apply when finding suitable landing sites away from the KSC.
To slow down faster, you can increase the breaking strength of your rear wheels. You can also use parachutes on landing, but care must be taken to ensure that an adequate length of runway remains since you'll only get one chance to use them. If you have a very short length of runway remaining and your parachutes can't slow you down fast enough, you'll be forced to cut the chutes and attempt a second landing without them. That said, parachutes are an exceedingly effective means of reducing your stopping distance. Note that a Wheesley or Goliath engine can reverse its thrust to allow rapid deceleration during landing, but these are not recommended for a spaceplane due to being unreasonably heavy and inducing excessive drag when attempting to transition to orbital velocities.