Tutorial:Basic SSTO Design
This tutorial explains how to build a single-stage-to-orbit (SSTO) craft that is able to launch into space and return without jettisoning any parts.
Hello, I am Suzaku a dedicated SSTO enthusiast. In the ideal world all spacecraft are launched via mass driver or any kind of launch system that does not rely on costly fuel and materials in order to reach orbit, however until that is implemented into KSP craft will have to rely on internally contained power for the launching of spacecraft into orbit and beyond.
So lets get started!
SSTO are by leaps and bounds far harder to construct than a vertical launch space vehicle in KSP. Not only do you need to concern yourself with Center of Gravity, Center of Thrust, and structural integrity, now you also need to focus on Center of lift, aerodynamic drag, Angle of attack, and Horizontal structural integrity.
Lets get started.
Just to note, this tutorial does not cover the vertical take off vertical landing SSTOs, only the Horizontal Space Plane type SSTO.
Why build an SSTO
SSTOs are hailed as the future of Spacecraft, not only are they far more cost effective than multi-stage rockets, they are also safer and capable of landing and taking off at traditional runways.
Is it impossible to build an SSTO?
Nonsense, many people say it is in version 0.16, but I think they don't understand the nuances of building an SSTO and then blame it on the game. SSTOs are capable of reaching orbit, the Mun, Minimus! and beyond! (I even broke orbit of the Kerbin Sun with a SSTO)
Spaceplane SSTOs by nature are not symmetrical, the tail, landing gear, etc. all will change the aerodynamic drag profile of an SSTO. Many people enjoy building a SSTO that looks similar to a typical aircraft, however many fail to consider that aircraft are designed to thrust and operate inside of an atmosphere fighting against gravity and therefore the center of gravity for such craft does not need to be in line with the center of thrust. If the center of gravity is not in line with the center of thrust, the vehicle will either spin constantly when attempting to thrust outside of the atmosphere or will have a tendency to pitch up and or down.
Liquid Fuel or Solid Fuel?
There is basically only once choice for a type of fuel for a SSTO. Liquid. A SSTO is exactly as it is described, a single stage to orbit craft which must not jettison any part of it in order to reduce operating cost and increase passenger safety. Anything once drained is dead weight on an SSTO and therefore power to weight must be put into consideration. Solid fuel boosters can be used for take-off, however it must be noted that they are quite heavy and do affect your center of gravity quite a bit. And then there is the matter of Jet Engines. They use, as opposed to Liquid Fuel and Oxidizer, Liquid Fuel and Intake Air. Now, although more efficient and just as powerful as Rocket Engines, (At least in KSP) Intake Air cannot be obtained in space. So you use a R.A.P.I.E.R. Engine. These hybrid engines are slightly underpowered as a jet and more-than-slightly underpowered as a rocket, they can run either as a jet or as a rocket. These may prove quite useful for SSTO vehicles.
There is nothing else more important in SSTO design than center of gravity. As your vehicle consumes fuel it cannot jettison stages into a smaller and smaller stage that has its center of gravity maintained at a close to center position. The center of gravity for a SSTO will rapidly move forwards and backwards as fuel tanks are expended and this applies without changing the aerodynamic profile of the craft and therefore the fuel tanks must be positioned in such a way that they will not rapidly shift from the front to the rear after a tank is drained.
The same that applies to all other spacecraft, however once again center of thrust should be in line with the center of gravity for any spaceship, even SSTOs.
This is the Killer issue for most players when it comes to making a SSTO. Most players have the tendency to design the coolest looking ship they can possibly design and then try to fly it. Some even try to copy real-life craft and attempt to fly it and then blame the game when the craft fails miserably. Once again, remember you are building a SSTO out of rocket parts which weigh 4 times more than aircraft components. There is no guarantee that the parts used in KSP are of the same mass of the parts used on the craft you are trying to imitate and therefore don't expect too much out of copying a craft.
Traditionally the center of lift should be slightly in front of your center of gravity for lower speed craft or slightly behind it for higher speed craft. If the center of lift is in front of the center of gravity it will promote the tendency to pitch up and enhance maneuverability, a positive attribute for low speed low altitude craft.
For high speed craft if the center of lift is on the center of gravity (a very bad idea; we'll explain why in a second) or in front, it will cause aerodynamic instability. Typically a SSTO should have its center of lift slightly behind the center of gravity as it provides aerodynamic stability during high speeds as well as the resistance to turning on its axis. Center of lift can be changed by aerodynamic resistance as well.
Angle of Attack During Takeoff
Angle of attack is nothing more than the angle of the lifting surfaces in comparison to the surrounding winds (like riding a bike and getting a breeze where there was none). The greater the angle the greater the lift that will be generated as the vehicle is moving forwards off the runway. The energy required to lift the vehicle off of the runway if the wings are perpendicular to the runway or at a negative angle is exponentially greater than if the wings themselves were generating some lift as the vehicle moves forwards as the craft will be relying on nothing but the lift generated by the far smaller control surfaces.
On a side note, if the angle of attack is too steep and the craft does not have enough thrust to take off it will generate instability for the craft where one wheel might lose contact will the runway and cause a tendency to yaw to one side. Also, if the craft too great of an angle of attack for the engines to compensate then the vehicle will stall and fall back down.
The angle of attack can be adjusted on any craft by moving the rear landing gear so that it sits higher up into the fuselage than the forwards landing gear.
SAS is almost required for any spaceplane. A computer that can stabilize your craft faster than you can react is invaluable for a vehicle with a constantly shifting center of gravity.
RCS and Vectoring Engines
RCS is useful in space only, however due to pod torque mechanics it is not entirely necessary for smaller SSTOs unless the need to dock arises
Advantages of a SSTO
- 1/100th the operating cost of a traditional multi-stage spacecraft
- The ability to land at any runway and take off after a simple refuel
- No required re-assembly or hoisting vertical required after each mission
Disadvantages of a SSTO
- Must keep all original components from launch
- Poor thrust to weight ratio when compared to typical multi-stage rocket designs
- Cannot be built too heavy due to horizontal structural weaknesses
- Extremely complicated design process
- Must be designed to operate as an aircraft as well as a spacecraft