MechJeb 2 Wiki Outdated

[Krahazik]

wiki.mechjeb.com drastically outdated and missing information.

Using MechJeb 2 version 2.7.0.0
KSP version 1.3.1.1891
Windows 64it

As a result of a lack of information I have no idea if some of the issues I am having are a result of a mod bug, or incorect settings for the craft im flying. There are whole sections missing completely (Aircraft Autopilot window for example) and some sections are incomplete with settings missing. This leads to lots of questions and not sure where to post questions.

In Aircraft Autopilot under PID, what are those settings? What is "Kp", what is "i" and "d". How does the "VertSpeed" fields under PID effect "Vertical Speed Hold" if it effects that at all.
I have one aircraft where vertical speed hold fails completely and results in dratic and fata pitching ocillation. No idea why?

There are settings on the Assent Guidence window that have no informations. What is "Q"?
What are the force roll settings for?

With an aircraft, I had Autoland completely fail to make touchdown. Was doing good untill the point where it should have moved the craft to make contact with the runway but instead just hovered there untill the plane lift stalled and crashed.

[artwhaley]

You are correct.  The wiki is out of date.  And has been... I think... since I bought the game a few years ago.  Reading through the Mechjeb release or development threads are a good start... though of course they're miles long by this point, so sometimes you just have to post your questions there even if they might have been hashed out 20 pages earlier already.   

I've been out of the game a while - so this may be inaccurate - but atmospheric flight has not been a priority for the Mechjeb dev team most of the time...  so I always expect the air plane bits to fail some.   That said, when I reloaded the game the other day, I tried the autoland and it DID work for me on my unscientific single point of data test.   The aircraft autopilot sure didn't used to do anything...  if it does something now, that's an improvement.  The Mechjeb thread is where I'd ask about what does work and for some tips on getting started with it.   

Your other specific questions...  Mechjeb... and a lot of 'machine controlling values based on measurements' systems uses a "PID" controller - where those stand for proportional, integral and derivative.   To super simplify - let's pretend we're talking about throttle for a mun lander.   We'd measure  descent speed and control the throttle to get it where we want it right?    A PID controller makes three pretty good assumptions about the behavior.   

1.  The more wrong the speed is, the harder we should correct the throttle.   That's the P  = proportional.   When you set kP you're telling Mechjeb how hard you want it to adjust for instantaneous errors.  The higher the value the more Mechjeb tries to correct for these instantaneous wrong values.

2.   The LONGER we've BEEN WRONG... the harder we should correct the throttle.   That's what the I term does.   setting kI is telling Mechjeb how much weight you want to give to this function.   The higher the value, the harder it will try to correct for errors that have existed a while.

3.  If we're getting closer to correct, ease back on the correction.   That's the D or derivative term.  This keeps us from overshooting because it softens the correction if the values are already getting closer to right.   So when the P and I start to get results, the D term weakens the correction so that hopefully everything evens out around zero instead of oscillating wildly.   So the higher you set the D term, the stronger this resistance to over correction is.   

There are whole BOOKS on how to tune a PID system, but what I just explained is enough to start playing with it!   If it takes to long to start correcting, increase the P.   If it gets CLOSE... but never gets right on the money...  increase the I...  if it over corrects...  increase the D.   If it is behaving really weird...  turn them all down and start back at the P.   

 

Q is short hand for 'dynamic pressure.'   Think of it as the force of the atmosphere trying to slow or break your rocket.   The faster you're going and the lower you are in the atmosphere, the higher Q is.   If your Q is too high, you're wasting delta-v fighting your way through the soupy lower atmosphere.   So you limit Q during ascent so that Mechjeb will throttle back at a certain speed and keep throttle low until you're high enough that the air resistance losses are acceptable.  When you watch real rocket launches you'll hear them call out Max-Q as they pass that point... and then shortly after they'll call a "Throttle Up" Where they ramp back to maximum power.  

The Force roll bits have two numbers - one for the vertical ascent and one for the pitchover of the gravity turn... and they do what they sound like they should do - set the roll angle of the rocket.   This is handy for situations like the space shuttle where it needed to roll a certain way for gravity to counteract the off center thrust....  but in game...  it's more useful for things like making sure the SRB's are perpendicular to the ground before you separate them...  which decreases the chance of one falling back into your main engine.