Spaceplane design - busting through the operational ceiling

[Jovus]

I've been inspired to try creating my own spaceplane for cargo lifting to LKO (I need to supply my orbital station with stuff and kerbals), and I have a pretty decent thing going so far. The problem is, it tends to hit ~23km and then the prograde vector slowly dives back down below the horizon, no matter what I do. I'm using FAR, and when I run the simulations, it claims that my L/D ratio is >1 at all Mach numbers, but I suspect it's not accounting for altitude.

I get the impression this is a somewhat common problem for first designs, and I'd like to know what the solution is. Is this, as I suspect, a symptom of not having enough lift? Or is it something else, like a problem with my flying profile? (Generally speaking I take it up 45 degrees to 10km, then drop to 20 degrees until the problems start, and achieve a velocity somewhere around 700 m/s).

[Borogove]

It's usually a thrust problem rather than a lift problem. Air's pretty thin up there at 23km; typically that's where you want to fly level to get to maximum speed, then pitch up and switch over to rockets. You might try adding a couple more intakes to get you a little higher.

[Hodo]

I rarely trust the simulation numbers. I like to use them as a base to see if it will reach some speeds, then I go out on a series of test flights. The craft will be tested on take off and landing never going above 10km. Then tested for supersonic flight then hyper sonic flight. Then finally I will take it to space to its test alt of 100km by 100km orbit, the final test is a full operational mission of some type. Usually at this point I am happy with the design and it may make it into service into my fleet.

But pictures of the craft would be helpful in diagnosing any issues with it. The best pictures are the ones in the SPH with CoM, CoL and CoT icons on, taken from the side and from the top, a third angle is helpful usually a 45deg angle from the front top is a good angle for a final pic but not required.

This is a sample picture of how a SSTO in FAR using stock parts should have its CoM and CoL.