ThatSpacePlaneGuy

Well, I finally discovered one particularly massive issue: The plane COMPLETELY lacked reaction wheels (aside from those in the cockpit, which are woefully inadequate for a plane of this size). I thought I installed a cluster of three heavy-fuselage reaction wheels near the back of the ship, but apparently that was a different version. I shuffled the fuselage parts a bit to add in three reaction wheel pieces without altering CoM/CoL relation. Another issue (as pointed out by Murph) was my orbital transfer altitude: It was far too low. I was getting flameout-prevention thrust loss at around 20 km - where the air is too thick to execute the nose-up maneuver without losing control. As it turns out, the centerline Sabre had NO air intakes whatsoever, and the two inboard ramjets also had no air intakes - so I had a HUGE air intake deficit. I added several radial-mount air intakes, and the plane can now begin orbital insertion at ~25 km, and burn air-breathing ramjets all the way up to ~55 km. The combination of higher orbital insertion altitude with extra intakes and reaction wheels for extra rotational authority has fixed the problem. The spaceplane is now quite a pleasure to fly. To answer your question, those other engines are not turbojets - they are ramjets, from the mod "Taviero's Pizza and Aerospace". They perform quite well at high speed, and similar to Sabre engines, their thrust does not taper sharply to zero as you approach a vaccuum. Sequentially switching Sabres from air-breathing to rocket mode as altitude increases conserves a lot of rocket fuel during orbital insertion, and ramjets give statistically significant thrust almost all the way up to main engine cutoff. The spaceplane weighs 184 tons fully fueled, with no cargo. Carrying an orange tank, it weighs 218 tons. (975 delta-V remaining upon establishing circularized 300 km orbit, before consuming any of the orange tank's fuel.) Since the cargo bay is right on the CoM, the flight profile of the spaceplane is NOT altered by hauling an orange tank. The thrust-to-weight ratio just goes down a bit. In fact, thanks to careful design and shuffling parts around on the airframe, the CoM when fuel-full and CoM when fuel-empty are almost identical, so the spaceplane doesn't even fly differently upon re-entry - just lighter.

Mechjeb ascent guidance generally does a good job of getting the spaceplane to go where it needs to, if one uses the slider to adjust the ascent angle. It's infinitely more steady and precise than trying to use a keyboard to fly. Occasionally it requires some manual input to remove unwanted roll, but this particular spaceplane only requires a couple periodic Q or E taps during ascent. It's remarkably roll-stable. Pitching up slow & steady works, but only up to a point. No matter how slowly I raise the nose, it still spins well before I get to prograde-20-degrees-up.

I sometimes had the issue of monopropellant tanks spontaneously exploding as soon as they are sneezed on - which would then cause the rest of the ship to disintegrate in a spectacular, awesome, and eventually annoying manner. Remember that in the case of rockets, thrust-to-weight ratio increases steadily during ascent as the stage burns fuel, so the amount of force applied to your parts will slowly rise. That's why it blows up predictably at some altitude. Make sure your monopropellant tanks are somewhere on your rocket/spaceplane where they are subjected to VERY LITTLE force. Right behind an engine = terrible. Mounted with a radial attachment point to the fuselage of something = perfect. Sometimes, no amount of struts will help. Just put the squishy stuff where it won't be subjected to the forces of takeoff and maneuvering so much.

So I'm in the final phases of testing a heavy-lift SSTO spaceplane (it takes off without disintigrating, flies reasonably well, and has awesome enough TWR to get to orbit...theoretically). Once I hit 1700 m/s at around 20km up, I adjust my ascent trajectory with Mechjeb to point the nose about 20-25 degrees up. The plane pitches up VERY violently and spins out of control, well before the engines flame out. When flying level, my CoL is slightly behind my CoM, so it's nice and stable. However, when pitching up about 20-25 degrees, the CoL moves forward, which causes the sudden and violent orbital insertion spin of doom. At more normal speeds and altitudes, Mechjeb can use the canards and elevators to compensate without much trouble. How do I prevent/reduce the CoL's forward movement as the plane pitches up? Wings are large and delta-ish, for low supersonic drag and plenty of room between the CoM and elevators. I would really rather not decrease wing size, as that would severely limit the amount of stuff the ship can carry. (Design goal is 1 orange tank and able to take off before the runway's end, and no stalling issues when transiting from low-altitude sharp ascent to supercruise.) EDIT: Canards removed & wings resculpted. Screenshots: Notice here, everything is great. When pitched up...not so much. Note that the fins near the front of the fuselage are mapped to provide yaw authority, not pitch. Without them, the plane has some yaw-stability problems. 2ND EDIT: Problem has been fixed. The spaceplane lacked reaction wheels (I thought they were installed), and suffered from a relative air intake deficit (less than 1:1) which limited maximum air-breathing altitude and forced the orbital insertion window much lower than normal. Both design flaws have been fixed, and the aircraft now meets or exceeds all of my design criteria.

Okay, so I downloaded this, since I was tired of 'orbit' being a mere 2 Km/s...and none of my designs work. So I downloaded KIDS, and started buffing specific impulse. Now the Mainsail has specific impulse on par with a space shuttle main engine, and the toroidal aerospike rocket's specific impulse is somewhere between spectacular and godlike. ...And even the FAR Darkhammer II (included with the FAR mod), which is basically a giant pile of fuel and rockets, STILL fails to get close to a 300 Km orbit. Looking at the rockets here...is 2.5 meter stuff even a thing? I'm used to working with 1.25 meter things - even using a 2.5 meter tank feels like a clunky and unwieldy abomination. But these rockets, practical sun-launchers if used in stock KSP, are the bare minimum necessary to establish orbit? We need some 3.75 meter air-breathing jet engines and parts, if spaceplanes are ever going to approach the land of practicality. Or a linear rescale of masses, fuel, and thrust values so that 1.25 meter parts aren't useless.