Advice needed for dV charts - Low orbit burns

[Kowgan]

Hello,

I've looked around a lot before finally deciding to make this thread.

I've been using delta-V charts, but this thing has always bugged me off. The "low orbit" required delta-V.

While intercepting bodies whose I can't use aerobrake, I follow the chart's dV needed to kill my velocity until I get into the desired low orbit.

But no matter what, everytime I set a maneuver node the body's periapsis to create the orbit, it will always require the double, or even triple of the dV specified in the charts; as shown on the pictures below.

Considering Minmus in this example, almost all charts I've checked will tell me that the required ammount of dV to set a low orbit around Minmus is equal to 80m/s, in a 10km altitude. But as you can see on the picture, my maneuver will require almost the double m/s of that.

In my intercept, I'm passing in front of the planet, so that should reduce my speed, am I right?

So, what am I doing something wrong?

Thanks in advance! This is getting fairly frustrating.

Edited October 15, 2014 by Kowgan
Answered. Thanks for the infos, everyone!

[Taki117]

dV charts offer the OPTIMUM UNDER ABSOLUTELY IDEAL CONDITIONS dV required for an orbit. Always pack at least another couple hundred m/s of dV to account for variations.

Here is a slightly more accurate dV map.

[Kowgan]

Taki117,

Thanks for your quick answer. I'm aware of that, and I always pack some hundreds - even thousands of reserve dV for those missions. But the minmus mission posted above is just an example, and the difference of 80m/s isn't much. But my concern goes especially for flights like to Eeloo, where, according to those charts, the required dV to acchieve a low orbit is of 2100m/s, and we can't aerobrake.

In any case, I'm not asking for a solution to go around this issue. I'm asking for an answer on either I'm doing something wrong (as you said, not optimum), and what would be the optimal situation to achieve said dV efficiency. The chart is there, and I want to use it as a base, if that's what it was created for.

Edited October 14, 2014 by Kowgan

[Taki117]

I suggest using the chart I linked, there really isn't a solution other than I believe that the dV map on the wiki (The one you are currently using) needs to be rechecked or replaced. With the one I linked, there is a capture/escape, and that will put your Apoapsis at the edge of the SoI, with an additional value to circularize at low orbit.

[Kowgan]

Thank you for the answers, Taki117.

I've compared both Charts. In general, the major changes I noticed on the one you've posted are the great dV reduction to low orbit some planets like Moho and Eeloo.

Still, the same problem persists according to my example below. Both the first and the second charts pasted in this thread have a dedicated field for a capture/escape and an additional value to circularize at low orbit. In the Minmus example, the second chart have even a smaller value for the low orbit maneuver, of 70m/s, unless I'm reading it wrong.

So, even if the first chart is outdated at this point, many values still match between those two charts, including the ones concerning the parcel regarding the problem I'm trying to focus in this thread.

Again, thank you for the advice. Even with my problem not solved, it's good to know there are more accurate charts out there.

[Taki117]

Thank you for the answers, Taki117.

I've compared both Charts. In general, the major changes I noticed on the one you've posted are the great dV reduction to low orbit some planets like Moho and Eeloo.

Still, the same problem persists according to my example below. Both the first and the second charts pasted in this thread have a dedicated field for a capture/escape and an additional value to circularize at low orbit. In the Minmus example, the second chart have even a smaller value for the low orbit maneuver, of 70m/s, unless I'm reading it wrong.

So, even if the first chart is outdated at this point, many values still match between those two charts, including the ones concerning the parcel regarding the problem I'm trying to focus in this thread.

Again, thank you for the advice. Even with my problem not solved, it's good to know there are more accurate charts out there.

I think you are reading it wrong. You are trying to go from Minmus Transfer to Low orbit, according to the chart I posted that's 90+70 m/s which is 160m/s which is about right for your situation.

[Laie]

In my intercept, I'm passing in front of the planet, so that should reduce my speed, am I right?

So, what am I doing something wrong?

For starters, you're passing in front of the moon, which reduces your speed. But Minmus is quite a bit faster than you are! Check the numbers: Minmus is moving at something like 250-300m/s; if you look at your orbital speed just before the SOI change, I guess it will be on the order of 100m/s or less.

It's the same with the outer planets, btw: when you're aerobraking in Jool's atmosphere, you aren't braking at all -- on the contrary, Jool is bringing you up to speed (relative to the Sun, at any rate).

The dV maps I know are pretty accurate; bringing 10% extra is plenty in most cases. Notable exceptions are bodies on very eccentric or inclined orbits (like Moho or Dres); for these, dV maps are next to useless.

[Taki117]

The dV maps I know are pretty accurate; bringing 10% extra is plenty in most cases. Notable exceptions are bodies on very eccentric or inclined orbits (like Moho or Dres); for these, dV maps are next to useless.

Not true, the one I linked takes into account the maximum dV needed to match inclination.

[cantab]

As mentioned, the maps show the reasonable best-case scenario. Just looking at your trajectory in Minmus's SOI there, it looks like you've come in fast.

[Kowgan]

Thank you very much, guys. That was very clarifying. I haven't paid any attention at all to those speed differences, so far.

This brings another aspect into the gameplay, for me. For that, thank you again.