I want to build a single stage to orbit (SSTO) space plane. I have a design right now that I know is capable of making it to orbit, but only just. There isn't enough fuel or monopropellant to rendezvous with my station in orbit, let alone return it safely to the surface of Kerbin.
Obviously, the thrust to weight ratio of both the air breathing and rocket engines need to be considered, as well as the placement of the centre of lift and centre of gravity, but what other design principles do I need to consider not only when building my space plane, but also while piloting it? What requirements should I prioritize?
Best Answer
There are a few.
Wings provide lift and control, and having not enough makes you more vulnerable to shifts in fuel weight.
Thrust to weight is misleading, what is important is that you have enough force to move your mass not only against gravity, but against your aerodynamic drag. Most efficient, vertically launched rockets don't have this much trouble, but SSTO ascents try to get as much lateral velocity as possible while still in atmosphere.
Air Intake. A major issue with most designs, more air intakes will let you go higher and faster before beginning to burn precious jet fuel for the final ascent. 3 RAM intakes per Turbojet is generally acceptable, but very lightweight designs may be able to get away with 2.
The deathknell of every SSTO design ultimately comes down to finding a magic balance between your final altitude (Air Intake), rocket fuel weight to breach into orbit, and airspeed. Too much mass, your airspeed will suffer, but you may get higher with more intakes. Small SSTO's trade off mass for more speed (which slightly increases final altitude)
The last challenge is highly technical, but unique to every craft: piloting. The timing, ascent angles, and thrust controls make or break any design fit to see orbit. An avionics control module and good structural rigidity (plenty of struts, nothing should wobble, ever) will help immensely.