IMFD IMFD 'time to burn' suddenly slows down

[IronRain]

I have a question about the IMFD 'total burn time' timer. I want to do the Rosetta mission and the first thing to do after launch it to burn the second stage so that the Rosetta spacecraft is on a path for an earth fly-by. when I've set up IMFD it shows me that the burn should take about 1040 seconds and burn time MFD shows me that the Ariane second stage has enough fuel for 1100 seconds. Everything goes well until the ~800 second mark. then the timer goes down by 1 second every 2 seconds in real time.
I don't have nonspherical gravity sources, radiation pressure or Gravity-gradient torque turned on since I'm still learning interplanetary traveling.

Why is this happening and what can I do about it?

Thanks,
IR

 

[dgatsoulis]

Why is this happening and what can I do about it?

I think that this happens because of the duration of the burn. I assume that you start from LEO (orbital period T~5500 secs), so a burn that takes about one fifth of a full orbit (~70 degrees) is a bit too much. By the time you reach near the end of your burn you are so far away from Pe that IMFD tries to remedy this by burning longer (and probably more to the "outside").

A way around that is to brake the burn into two or more Periapsis "kicks".

Hope this helps

 

[Tommy]

I suspect dgatsoulis has it right. IMFD calculates it's course trajectories as a "point impulse" burn - all the dV is imparted instantaneously. Then it uses burn integration techniques to provide a burn that closely resembles the plan. Extremely long burns just don't integrate well - and will result in some strange behavior.

This is why IMFD doesn't work well for low thrust vessels, like those using ionic propulsion.