Flight Question IMFD "Delta Velocity" and ion engines

[EnDSchultz]

So, the documentation doesn't elaborate very much on how to use this, and I was wondering if someone could fill me in...

I'm having a little trouble getting my ship to where I want it to go (imagine that), as well as simply getting the targeting information to show up at all.

After I've broken Earth orbit and have switched to the Perseus ion engine (dummy craft) and start configuring the Delta Velocity program, all the targeting information (apoapsis, periapsis to the target, etc) just disappears. The manual says that this will happen if you have under 3 seconds until burn, but I set it to a couple thousand and this still happens. I'll change the delta V parameters and it won't show any updated hypothetical orbit on the display...

It feels like the flight computer's just trying to spite me now that the Perseus is up and running.

Secondly, how valid is the Delta Velocity program for use with low, constant thrust engines? I got it working once, and it told me my periapsis would be within 100M. Never got anywhere near my target. Does it not take into account how gravitational forces change over the extremely long burn times?

 

[Tommy]

I suspect part of the problem is that the ion engines are "dummy craft". This could make it impossible for IMFD to make the required calculations - it can't get the info from the "dummy craft" when the focus is another vessel, or visa versa.

Also, all of IMFD's programs make calculations based on an "impulse" burn, where the entire delta-V is delivered in an instant. While it uses "burn intregration" to help account for non-instantaneous burns, that only goes so far. It simply adjusts the burn timing and attitude to try to "average" the burn around the "burn point". Trajectory calculations are still made using instantaneous burns.

While I haven't experimented with low thrust vessels myself, I've heard reports that P-30 guidance mode works much better than Velocity Frame for this purpose.

After planning the burn, you might try stopping the burn after a while, and essentially using several smaller burns, rather than one long one. This would allow IMFD to recalculate and may improve accuracy.

 

[perseus]

Open IMFD in one display. Select "Course" -> select "Delta Velocity"
In the "Delta Velocity" screen select "Vel. Frame". Then enter the time for the start of your burn and the required dV along three directions - your prograde velocity vector, perpendicular to the orbit plane and orthogonal to these directions (outwards). Open IMFD in another display. Ensure it is in shared mode with the other IMFD display (click on [PG] and enter 0). Select "Map".
To show the result of your planned "Delta Velocity" burn, click on [PG] -> select "Plan"
ISP change operation ion engine

Your trajectory is now shown in blue and you can see what your closest approach to your target will be.
ion engine is a ship docked to simulate the change in Vs ion engine, it can be connected or disconnected by the scenario editor.
Simulation of the Variation of ISP can be done in different methods:

a) Activating ion engine by Vessel Remote control.

b) Activating "I.E.A.T.MFD” ENGINE CONVERSION: you can change the thrust and ISP of any spacecraft's Main Engine while the simulation is running. "I.E.A.T.MFD" is displayed, otherwise the vessel will revert to its original default values the next time the simulation is started.
c) Press F3 and select ion engine.

Given the low to high ISP Thrust acceleration time is high.
Using iMFD once targeted the ship, auto-burn off AB accelerate time until a next to the desired orbit.
Enced finally the autopilot for the final corrections