TransX Question about course correction dv after flying by Jupiter

[NOMAD]

This question confused me for a long time.
I made a journey from Earth to Neptune,using voyager 2 flight plan, departure time is Aug. 20th,1977.
After flying by Jupiter ,I found that the mid-course correction dv is
at least 60m/s.But when I fly by Uranus,the dv is less than 20m/s,this made confused.Why?

I also found this :http://www.orbiter-forum.com/search.php
BrainJ said his method was simply to keep "one step ahead".

My method was simply to keep "one step ahead" - i.e. when setting up a TransX maneuvre to target the Jupiter flyby, I was watching the distance of the Saturn intercept.

Can somebody explain this in detail? Thanks .

 

[insane_alien]

there could be a couple of reasons, the happy reason: your entry was more accurate so needed less correction

the realistic reason: the lower speeds and greater distances to the next point of interest means you don't need to change your velocity much to have a large impact on arrival as you position is the time integral of velocity.

BrianJ was reffering to the fact that when you are preparing for entry into a gravity assist you should use transX to calculate where you will end up at the next gravity assist. Armed with this information you can tweak your entry vectors so that you come out in exactly the right direction and you don't need a large mid course correction.

basically, if you fix a course error early, you don't need a large dV to fix it. the later you leave it, the larger the dV you'll need. so, really the advice is, fix errors as soon as you can detect them, pay close attention to how you'll exit a gravity assist BEFORE you enter it and adjust as necessary.

 

[NOMAD]

But I was confused by a problem. When I was about to enter Jupiter's sphere of influence, i adjusted the orbit to make the PE ratio to 1 and RInc down to 0,but after leaving Jupiter's influence,i attemped to make another mid course correction,but this time the dv is 95m/s. I thought that in reality the voyager 2 spacecraft didn't use too much fuel to correct the orbit,so i always tried to reduce the dv but haven't got any good results. How to explain my problem?

 

[agentgonzo]

Nasa's guidance programs are a lot more accurate than TransX. TransX makes many assumptions that mean that it's not perfectly accurate (but is very accurate). These innaccuracies mean that you need to do a course correction burn after encounters and your figures are fairly normal.