MechJeb Ascent settings for FAR?

[Box of Stardust]

So recently I downloaded FAR because I was getting tired of the weird stock aero behavior. Now I'm dealing with real aero issues, and this one is baffling me.

I'm trying to launch a rocket, and these are the two iterations of it (in reverse order of development, compared to how the album is ordered):

I know CoL is generally supposed to be behind CoM, but hey, I'm trying out 'realistic' designs, so no fins. I feel like logically, it should still work, but MJ's autopilot always ends up losing control somewhere between 3km and 7km, depending on the settings. I've tried changing AoA, toggling Corrective Steering, modifying the Ascent Path (currently I've got it set to begin gravity turn at 0.8km and 75m/s, ranging from 50-80% shape in my tests)... and I still can't get this thing to space. Either aero forces drag it way down or drag it back up, which then ends in the rocket spinning out of control and breaking up.

Help. MJ autopilot makes this game playable for me, because I've been through enough manual launches that I'm not really in the mood to even supervise the launch stage.

Edited April 24, 2016 by Box of Stardust

[Harry Rhodan]

2 hours ago, Box of Stardust said:

I know CoL is generally supposed to be behind CoM, but hey, I'm trying out 'realistic' designs, so no fins.

In that case you really have to watch out for your weight distribution and might want to use one short but heavy 2.5m upper stage instead of two long and light 1.25m ones. And you have to remember that even if you use a more sophisticated aerodynamic model, the fuel flow rules are still extremely basic and stacking multiple small tanks will always result in a CoM that will drop during the first phase of the flight.

[Vegetal]

That's gonna be hard. MJ is great, but it's generally quite aggressive at the beginning of the gravity turn, which won't help you at all.

You can try severely limiting the AoA allowed and changing the starting point for the turn to be very early. Real rockets have more sophisticated control systems than MJ, so trying to make totally realistic rockets doesn't make all that sense, especially with Kerbin's small size. Your rocket it's quite short compared to real life, mainly because of that fact.

[numerobis]

If you can keep to a nearly zero angle of attack, you might get lucky -- maybe use the gravity turn mod?

Your design is inherently highly unstable, as you seem to know. If you want to fly it, you'll need much more control authority -- elevons or powerful vectored thrust -- or you'll need to make it more stable with either fins to move the aerodynamic center backwards, or moving the mass forwards by making your second stage fatter.

[NathanKell]

Well, early on there were plenty of fins (see Thor, and Saturn IB, and Saturn V, and R-7...). It's just that in the modern day, with highly sophisticated computer avionics and quick-gimbaling engines, you can get away with very statically unstable craft. Also note that in real life engines gimbal much, much more than in KSP, and the rockets have a much smaller fairing in relation to their total size.