Ion engine dv help

[maceemiller]

Hi all. So I'm using 8 ion engines to push my tug along but I've noticed something I can't understand. 

I've plenty of electric charge. I create a manouver node with a dv of say 315.

When I start my burn I actually use nearly 1000dv?

This only happens using the ions. Why is this?

Thankyou in advance

 

Edited May 25, 2017 by maceemiller

[Zhetaan]

The first answer that comes to mind is that the delta-V you see is an idealised amount that assumes your burn is instantaneous.  Reality spreads that burn over time and causes you to use more delta-V.  This effect will be magnified by longer burn time, which in turn relates to available thrust from the engines.  Therefore, the ion engines, being the weakest, suffer the most from it ... but that's a lot of loss.  If you'll tell me the orbital parameters, I can run it through the equation and let you know whether your delta-V readout is bugged.

Edited May 25, 2017 by Zhetaan

[tomf]

Pictures will probably help with the diagnosis.

If you are using KER for your readout of delta-V it can definitely sometimes get confused with complicated arrangements of stages, particularly with ion engines so it could be that the delta-v was wrong to begin with.

[Ohm is Futile]

To be honest, Zheetan's assumption is likely correct. It's an issue common to most low-thrust vehicles. It's not a bug, not even a display bug, it's the Oberth effect. Possible fixes include splitting the burn at periapsis on multiple passes. For instance, burn for a minute before and after periapsis for a total of two minutes every orbit until you reach the desired orbit or escape trajectory. Another fix is to make your orbit bigger (much bigger) so that the time you spend accelerating is less significant compared to your orbital period (but that's gonna waste dV either way, but probably less than say, doing your burn from LKO).

For a maneuver that requires more than 2000m/s of dV, you may feel like swearing as you will reach escape velocity from Kerbin long before your maneuver is done and ask yourself why people use low-thrust high-Isp engines. I know I did.

[maceemiller]

2 hours ago, tomf said:

Pictures will probably help with the diagnosis.

If you are using KER for your readout of delta-V it can definitely sometimes get confused with complicated arrangements of stages, particularly with ion engines so it could be that the delta-v was wrong to begin with.

Have only just got in from work so can't do piccys till later however I feel you've all made me see why this is happening.

Thaks again guys/gals

[Spricigo]

3 hours ago, Ohm is Futile said:

 It's not a bug, not even a display bug, it's the Oberth effect.

Be careful there!

The Oberth Effect its the fact that if on object change its velocity there will part of the change in kinetic energy will depends not only in the change in velocity but also on the initial velocity of the object.

Spoiler

OP's issue is probably the fact the burn its not instantaneous, resulting in the maneuver happening along a curved trajectory. Doing the maneuver at a fixed direction means velocity and change in velocity are not aligned at all times, so part of this change in velocity is change in direction rather than change in intensity. On the other hand doing the maneuver following prograde / retrograde part of the final burn will be in opposite direction of part of your initial burn, cancelling each other.

Its really a lose-lose situation, in any way the maneuver will be less efficient than the theoretical instantaneous change in velocity predicted. If you go higher this will be less pronounced, but also less Oberth.

[Ohm is Futile]

2 hours ago, Spricigo said:

If you go higher this will be less pronounced, but also less Oberth.

That's what I meant by "It's the Oberth effect". Since you are unable to burn only or mostly at the most efficient point as per the Oberth effect, you end up needing a lot more fuel to achieve the target maneuver than the UI reports. That's due to low TWR.

[Red Iron Crown]

As someone who has spent way too much time doing low TWR burns I say: It's both. Reduced benefit from the Oberth Effect plus way larger steering losses. You can reduce both through periapsis kicking, though.