Orbital mechanics question

[strider3]

I have a "mother ship" and lander, docked, in orbit around Duna. The orbit is low, 55,000m, which has worked perfectly for "equatorial" landings and science gathering. Now I have to hit the last 5 biomes (poles and etc.). My "mother ship" has fuel and engine to put me into a more polar orbit, to get to these last biomes. My question is this...this requires a VERY large inclination change. Is it more efficient to go to a higher circular orbit, and then make the inclination change? If so, how much higher makes it worthwhile? Or, does the DV required to heighten the orbit and then make the inclination change not pay off?

Keep in mind that I will want to be in a low (55,000 m) orbit again once in that inclination. This makes it easier for the lander and refueler to rendezvous and do another biome.

Thanks to the guru's, as always.

Edited April 21, 2018 by strider3

[HebaruSan]

38 minutes ago, Victor3 said:

Is it more efficient to go to a higher circular orbit, and then make the inclination change? If so, how much higher makes it worthwhile?

Yes. There's a thread about this somewhere, which I wasn't able to find just now, but I believe the cutoff is around 45 degrees; if your plane change is less, you should burn at the ascending or descending node, otherwise it's cheaper to raise apoapsis first.

[KevinW42]

Also, Instead of going to a higher circular orbit, you should use an elliptical orbit and only raise your apoapsis, leaving your periapsis at your current altitude. You'll save some dV by skipping circularization and on the plane change itself as well since you'll be moving slower.

[Reactordrone]

You could you hit Ike for science and then return to Duna orbit in a high inclination to finish off the Duna science.

[Capt. Hunt]

the cheapest inclination change of all is to make the burn when you first enter the Dunian SOI.

[maja]

@Victor3 My spreadsheet with Hohmann transfer and inclination change calculations says, that it will be better to bruteforce it. Changing apoapsis from this low orbit, then inclination and then apoapsis again sums into bigger dV, than changing inclination right away.

Edit: I forgot, that I have both AP changes in inclination calculations already included. Result is, it's better to raise AP and then do inclinitation change for changes bigger then 45 degrees.

Edited April 21, 2018 by maja

[strider3]

I have to go with the man with a spreadsheet (care to share that, maja? ).

<Reactordrone> At this point I'm not sure I have enough fuel to do both Duna and Ike (didn't think that through at the beginning of this mission). I'll most likely send up another tanker or 2 from Kerbin when the Duna launch window opens again, then do Ike, then bring all that science home.

Edited April 21, 2018 by strider3

[HebaruSan]

16 hours ago, HebaruSan said:

Yes. There's a thread about this somewhere, which I wasn't able to find just now

Finally found it:

"Bi-elliptic" refers to the raising of Ap before changing planes:

Edited April 21, 2018 by HebaruSan

[maja]

27 minutes ago, Victor3 said:

I have to go with the man with a spreadsheet (care to share that, maja? ).

<Reactordrone> At this point I'm not sure I have enough fuel to do both Duna and Ike? I'll most likely send up another tanker or 2 from Kerbin when the Duna launch window opens again, then do Ike, then bring all that science home.

Here it is: https://docs.google.com/spreadsheets/d/13Ov2VIV5kecHZGJ-rqd681oONBZhA35DItFOaQq8Nsk/edit?usp=sharing

Make a copy of it, when you will be editing it.

[strider3]

34 minutes ago, HebaruSan said:

Finally found it:

"Bi-elliptic" refers to the raising of Ap before changing planes:

What do the 7 different "bi-elliptic" line represent?

Never mind...found it in the original article.

Edited April 21, 2018 by strider3

[strider3]

26 minutes ago, maja said:

Here it is: https://docs.google.com/spreadsheets/d/13Ov2VIV5kecHZGJ-rqd681oONBZhA35DItFOaQq8Nsk/edit?usp=sharing

Make a copy of it, when you will be editing it.

Thank you!

[Aegolius13]

You could also look at using a gravity assist from Ike to help with the inclination change.   I have not tried this at Duna but it works great in the Jool system. 

[maja]

@Victor3 A little correction. I tested it and I included both apoapses changes in inclination calculations already, so look only on inclination part of the sheet. It was a long time when I played KSP and used this, so I made a wrong assumption now and then wondered, why I'm getting wrong results  It's better to do that AP change and then inclination change.

Edited April 22, 2018 by maja

[strider3]

20 hours ago, maja said:

@Victor3 A little correction. I tested it and I included both apoapses changes in inclination calculations already, so look only on inclination part of the sheet. It was a long time when I played KSP and used this, so I made a wrong assumption now and then wondered, why I'm getting wrong results  It's better to do that AP change and then inclination change.

That's what resulted when I used your spreadsheet. I increased apoapsis 55,000 m to 220 km and made inclination change then back to 55,000 m. Saved quite a bit of dv that way.

[maja]

35 minutes ago, Victor3 said:

That's what resulted when I used your spreadsheet. I increased apoapsis 55,000 m to 220 km and made inclination change then back to 55,000 m. Saved quite a bit of dv that way.

I'm glad to hear that. The Hohmann transfer part of this sheet shows you dV required to transfer from one circular orbit to another with dV for both burns. The first burn at Pe to setup transfer orbit and the second one at Ap to circularize.