With that #24 closed, would it at least be possible to make ascent AP able to ascent to non-circular orbits? Besides the general usefullness (I have circular orbits in KSP as they tend to jump back and forth in map view), this would allow me to get to like -10x100 orbit and get rid of ascent stage safely.

Thank you!

This sounds like a pretty good idea, and could be useful (especially in combination with launch to plane) to establish an orbit for performing a rendezvous, or for a properly timed launch to go directly into a trans-munar or Kerbin escape trajectory.

The use case you described — dropping the ascent stage into the atmosphere — is reasonable. Lots of people like to keep their orbits clean of ascent stages. But since some manual input from you is already necessary, why not just disable the autopilot during the "coast to apoapsis" phase?

Or, alternatively, should this be a new "Deorbit ascent stage" option? You'd have a checkbox as well as a number specifying the last ascent stage. If enabled, the autopilot would de-stage up to this stage number just prior to periapsis passing a certain altitude. (This altitude would be determined automatically on a per-planet basis: 0 km for anything without an atmosphere, or something higher for any planet with an atmosphere. I think on-rails ships disappear at the altitude at which pressure is 1% of the base pressure, but I'm not sure of that.)

I'm not sure it makes sense to combine ascent and rendezvous burns (except to ascend with a given inclination, which you can already do). For a rendezvous, telling the autopilot to ascend to an eccentric orbit wouldn't be enough — you'd also need to specify the direction in which your orbit should be eccentric.

No, this is useful feature on it's own right. Personally I hate circular orbits and never use them because they tend to jump around in map view making them harder to use. Besides there are many useful eccentric orbits (for example Molniya).
Disabling ascent AP after gravity turn is exactly what I do now, but I have to then raise Pe manually to make it an orbit I want. I usually launch spacecraft into somewhat of intercept orbit for rendezvous - Ap at target's altitude, Pe is in atmosphere, then I detach ascent stage thus ensuring it's disposal and raise Pe with main S/C's propulsion to 70 km. This way has a benefit of maximal amount of fuel left in S/C itself.
But "deorbit ascent stage" option would also be very useful, while ascent to non-circular orbits covers wider spectrum of use cases.

just set up a manver node and the burn to make it curc again

use the curi option in the manover node planer

check the MechJeb causing game crashes it is more important

@jannikstarwars, are you referring to an already-filed issue on game crashes? If not, please file one (see these instructions). We need bug reports to be very specific, because we can't really solve a bug that we can't observe, and we can't observe a bug unless (1) we get (un)lucky or (2) someone tells us how to make the bug occur.

For all the time I play KSP (~6 months) it didn't crash a single time, and I've got shit ton of mods installed.

@gmcnew under the open issues there is one that i also commented in and it is called MechJeb causing game crashes also i had that. I only had KSP version 0.19.1.54 and the only mode was Mechjeb 2.0.2 and when i was in the VAB and had the Vessel info open it made a loud error noise and the game crashed always after the same amount of time and i have the payed version so.

On-topic:

"Deorbit ascent stage" option - This would be nice for cases where you don't want to bring that "final stage" with you. I've been working around this problem in two ways: Designing my rockets in such a way that all booster stages drop before periapsis rises above zero, and putting a radial MechJeb component and an RTG on the final stage so that after circularizing and dropping the payload (whether it has its own engines or not), I can then switch to it, turn it around, and use the last of its fuel to drop it back into the planet.

Having another option would be great, although its use would seem to be limited to cases where your payload has engines to finish raising its periapsis (and possibly apoapsis if you're like me and tend to use flight paths that get Pe > 0 before Ap >= target!)

However, I do think it could be useful to have an option to specify periapsis as well as apoapsis for a non-circular orbit, and simply raise periapsis only that far when performing the "circularization" burn. If you get your launch phase angle right so you're on a direct ascent to your desired orbit, you could even combine this with use of "deorbit ascent stage" so that instead of having to first get your periapsis up, then raise your apoapsis, warp there, drop payload, and then have your launcher deorbit itself, you could simply drop the launcher just before it would fail to reenter, and finish the orbit off using the payload's engine.

Closing this as an old MechJeb issue. Many MechJeb issues have had little interest in years, have been fixed for years, do not include adequate replication steps, refer to old problems which are no longer applicable, or are difficult to determine what the problem is. This issue is being closed for one of those reasons. We apologize for any inconvenience, but keeping the TODO list tidy helps the developers.

If this bug/issue is still a problem, please open a new issue. For bugs please try to include a Minimal, Complete, Verifiable Example that explains all the steps required to replicate the issue. A link to the KSP.log file should be ideally included, but is often not sufficient information. Screenshots or short videos are often the best way to show a bug.