Maneuver Planner Question

[merlyn63]

First, let me say hats off and many thanks to sarbian for creating a great mod! I love simulating the space program, but don't necessarily want to be a literal joystick pilot. MechJeb allows me to enjoy space and leave the flying to a computer.

At first I thought I had a problem with the Maneuver Planner, specifically with the Hohmann Transfer feature.  

During a Mun mission, I would launch into Kerbin orbit, do CSM separation, turnaround, and dock with the LEM/3rd stage of rocket. Since the 3rd stage still had some fuel, I wanted to keep it and use it for the lunar injection (Hohmann Transfer). But when executing the transfer node, the rocket would turn to retrograde, fire a short burst, spin around to pro-grade, then spin again to retrograde and fire another burst, continuing the cycle until I dropped back down into the atmosphere. I clicked on the CSM and told it to 'control from here', but to no avail.

After consulting the flight path diagram of an historical Apollo lunar flight, I realized my error. I had to start the Hohmann transfer with the rocket with 3rd stage and CSM/LEM intact, THEN do the separation, turnaround and docking. 

But I am curious. Why can't the Maneuver Planner correctly execute the Hohmann Transfer with the CSM turned around and docked with the LEM/3rd stage? Is it because Mechjeb is confused on which end of the rocket is in control? Because now you have a stack with an engine at both ends, one pro-grade, the other retrograde? Even though only one engine is active, the one pushing the 3rd stage.

Please advise.

 

Thanks!

 

[Capt. Hunt]

Your control point is pointing the wrong way.  It is expecting the thrust to be pointing in the same direction as the point of control, when it in fact thrusts backwards relative to the control point, MJ2 is confused.  If you do want to do something like this on purpose, make sure to set the point of control to be something that is inline with your direction of thrust.

 

Also, there is a reason it's called Lunar Orbit Rendezvous

Edited March 22, 2018 by Capt. Hunt

[merlyn63]

Thanks for the reply, Capt. Hunt.

I figured as much, but couldn't figure out how to remedy it. Finally, I realized all I had to do was click on the docking ring of the 3rd stage and control from there. No more crazy oscillations

 

I had a successful mun mission with this Saturn V from the Curse site. I used Mechjeb, but find I have to overestimate for "fine tune closest approach to target". I used 100km to Mun orbit and it gives me a periapsis of around 30-50km. On return flight to Kerbin, i gave it a fine tune value of 200k to get a periapsis of 80-90km. Any thoughts on this? Is this just how Kerbal computers work?

 

That particular Saturn V doesn't have a mun rover stored on the lander. So I tried this one. For some reason this rocket wont circularize on the mun orbit, no matter what I tried. So I could never get into mun orbit to attempt a landing. So I tried swapping the lander into the first Saturn V I had success with. I can get into mun orbit,  when I start my autoland, Mechjeb reports its "executing a low orbit plane change" but the rocket never fires, I just orbit endlessly. And the engine is activated. I am using KSP 1.3 and this Saturn V mod states its for 1.3

 

Using This  rocket, I was able to get to the mun, but the lander wouldn't auto land. The engine just kept firing til it ran out of fuel and it went to a higher orbit. This mod is for 1.0.2

 

Any reason why Mechjeb has trouble flying these particular landers? 

Thanks!

 

[Hotel26]

3 hours ago, merlyn63 said:

Any reason why Mechjeb has trouble flying these particular landers? 

Imagine you have various thrusters on your craft active and opposing each other, possibly even at odd angles.  Would you expect MechJeb to figure out how to get the most thrust in the direction the maneuver requires?

It naturally assumes that net thrust will be in the direction of the selected Control From Here point and that you've taken responsibility for setting that up correctly.   MechJeb is a tool with a man/machine interface.  You still have to do your part -- or it wouldn't be any fun.

[Snark]

Moving to Add-on Discussions, since this isn't and add-on release.