Hey guys I've finally found a good formula for getting into orbit with the Endeavor launching from a runway; Here's the procedures I've made for it:

Rainier Endeavor Runway Launch Procedure Eastbound Trajectory to 100km Orbit:

1. Activate AG-2 [Flight Controls]
2. Max Pitch Up
3. Set Throttle to Max 100%
4. Activate AG-1 [Atmos Rocket Engine Ignition]
5. Set Computer Guidance Pitch to 75°
6. Throttle down to 80%-85% after passing 1,000m
7. Set Pitch to 60° after reaching 5,000m
8. Set 100% Max Throttle after reaching 15,000m
9. Activate AG-5 [Reaction Control System] after reaching 20,000m
10. Set Pitch to Lock Apoapsis
11. Activate AG-6 [2nd Stage Orbital Maneuvering Vehicle Separation & RCS Controls & Orbitz Engine Activation]
12. Set OMV Orbitz Engine Thrust to 100% until Apoapsis reaches at least 100.0km, then cut engine power to 0%
13. At 40 seconds from reaching Apoapsis at 100.0km, re-start Orbitz engine to 100% Max Power and lock Pitch to Apoapsis.
14. Continue until steady orbit is achieved or Kerolox/RP-1 fuel tanks are depleted.
15. Once a safe orbit is confirmed, separate protection fairings.
16. Once fairings clear the OMV, release the payload.
17. Switch to payload for payload operations (solar panel deployement, gyro/RCS systems activation, etc...)

@JohnnyBoythePilot you have to do an orbital turn, aka roll prograde when gaining altitude. It's more efficient than going at a fixed angle all the way up.

@Oski
My typical orbital entry/configuration is to depart the runway at a 75-65 angle, and continue like that all the way until the apoasis reaches 70,000 meters. It's been my best method so far at reaching as high as an orbital height I can get to, but I think it may be inefficient. I'll try a more shallow departure & orbit entry next time I'm on SR2.

Remember, you don't need a twr over 1 on your orbital stage, you need an efficient engine. And also try to roll the first stage after your apoapsis goes above the atmosphere, at this point you need lateral velocity. Put it almost horizontally, you will need less fuel to make an orbit and save it for a higher orbit or small corrections.

Fun fact, I once made a rocket that could reach low (barely over the atmosphere) orbit, so it was technically a ssto. But my monitor broke and I can't play anymore. Thanfully there are lots of space games on mobile.

@CasualCrusade
I changed my 2nd stage "Orbitz" engine from a Gas Generator 1 to a Staged setup, with tuned settings, and now my engine is a lot more efficient while still pushing out a lot of power. On my 2nd try with the redesigned Orbitz engine, I was able to achieve a steady, sort-of-lop-sided orbit, but it was definitely an improvement. Now at this point I think I just have to fine-tune my methods into directing the entire launch into a proper orbit and getting it to be nice and even, not lop-sided. And yes I've been using the performance analyzer. Although I don't expect my Endeavor rocket-plane to launch anything higher than 150km. My next rocket-plane design will be larger and more powerful. For now at least, I've been launching the Endeavor with a runway take-off, and it does eat up quite a bit of fuel; I'd usually be at 75-80% fuel once I cross the end of the runway and climbing. I'm thinking a vertical launch off of a launch pad or an air-launch will get me into a higher orbital height.

That extra fuel is going to weigh you down. You're going to need to change up your engines. Open the performance analyzer and mess with the throat size and nozzle length to increase your delta v. Also, to get the most out of your first stage, keep your twr around 2.0 - 2.25 by throttling down to minimize atmospheric drag -- this won't be necessary at >50km