RSB low TWR upper stages

[Jony76]

Hello there, 

so this is kinda a modding question. I currently play the RO/RSS version installed via CKAN(1.3) and i love the realscalebooster mod. Now my problem is that with the Delta III/IV upper stages and their low TWR(0.5-1.0) i just can’t make it into orbit. How do I need to adjust my flight profile so I get into a stable orbit before I drop back into the atmosphere? Don’t mind to ask further questions to get more infos. Thanks in advance

[magnemoe]

The way I do this in stock with low twr upper stages is is to lift your nose of second stage above prograde, some of you trust will maintain attitude, walk Ap ahead of you, as you go faster you lower your nose.
One tricks they even do with the centaur is to aim high, the Ap of the suborbital trajectory from the first stage put the second is higher than the intended orbit, this give you longer time to circulate. 
This is why the manned version will use an two engine centaur, the steep climb angle will give an too steep reentry angle if you abort. 

[Pecan]

4 hours ago, Jony76 said:

 How do I need to adjust my flight profile so I get into a stable orbit before I drop back into the atmosphere? Don’t mind to ask further questions to get more infos. Thanks in advance

I take it then that it's the circularisation burn that's causing problems, rather than reaching an adequate apoapsis?

Flatten your ascent path so the lower stage(s) spend more of the gravity-turn accelerating horizontally rather than gaining height.  "Up" happens almost automatically with horizontal velocity and the lower-TWR engines will be able to finalise any altitude needed.  Circularisation should then need comparatively little dV, so doesn't take long.

[Hydrothermalventclam]

The RSS wiki has a fairly good intro to launch profiles.  In my experience, low TWR upper stages are totally doable with the right profile.  I usually try to burn approximately prograde until my Ap is 20-40 km higher than it needs to be, then just burn sideways until I make orbit.  During this process you need to adjust the pitch of the rocket a bit to make sure you don't fall too low after you pass the Ap.  When I launch a heavy payload with my Centaur upper stage it will often burn for 6+ minutes before I make orbit (which I think is fairly realistic).

 

Mechjeb also implemented a guidance algorithm for RSS that deals really well with low TWR upper stages.