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.
Yes and yes.
For takeoff, you can
- perform an all-jet VTOL
- use girders or cubic struts; if your plane has a low enough takeoff/stall speeds you can drag them with no damage
- use a very tall launch tower and rev the engine before decoupling to perform a drop takeoff
- perform a rocket-assisted forward-oriented takeoff, using the rockets to get you airborne and your jet engines to keep you that way
- turn the plane 90 degrees upward and perform a stovepipe takeoff by giving it a thrust-to-weight ratio of >= 1, or a rocket assisted takeoff
- drag your girders on radial decouplers; detach to lift off or when one breaks
For landing, you can
- use girders or struts as low-speed contacts
- use parachutes to land the plane on its belly or tail
- all of the above
The simplest way to do this is definitely a stovepipe rocket-assisted takeoff and a parachute landing. The most hardcore is the VTOL just because they're damn near impossible to build or land. (Also, VTOL protip: use action groups to switch modes between vertical engines and horizontal.)
Best Answer
This exact thing happened to me. The problem seemed to be that my career save was lacking the "SPH" subdirectory to save spaceplane designs.
If you hit Alt/Option - F2, you can bring up the game's console. Within the console log, I saw errors popping up for
or something like that.
Once I created that directory in the appropriate place, I was able to save, launch, and load spaceplanes properly. I didn't even have to restart the game.