Which do you think is more efficient?

[EdFred]

Using the same exact craft, (I can post up the file if anyone wants) I hit all the available science in the Duna system in a single launch. It consists of a lander and an orbiter. Which (for me) means landing on Duna and Ike, and obtaining an orbit around each of them.

Do you think it is more efficient to visit Ike on the way into the Duna system, or to visit Ike on the way out of the Duna system? Assume that you are aero-braking at Duna rather than injection burning.

[Red Iron Crown]

Ike first would seem to be more efficient to me because you can use more aerobraking. The two profiles:

Aerobrake in Duna's atmosphere to get Ike intercept, Ike capture, Ike landing & ascent, Duna transfer, aerobrake to Duna deorbit, Duna landing & ascent, transfer burn to Kerbin.

vs.

Aerobrake to Duna deorbit, Duna landing & ascent, transfer to Ike, Ike capture, Ike landing and ascent, "Oberth maneuver" to lower periapsis close to Duna, transfer burn to Kerbin.

The additional aerobraking and already being in a good position to harness Oberth for the return transfer will save dV in the first scenario.

[r4pt0r]

getting Duna to slow you down without burning fuel would always be more efficient I would think.

[SunChaser]

I think you want to orbit Duna first. Aerobraking is useful, but won't get you very far--you will likely have to do a periapsis burn on top of the apoapsis burn to bring your periapsis out of Duna's atmosphere. Duna has a better Oberth effect due to its higher gravity. Ike is easy to encounter from Duna and easier to escape.

At least that's the way that seems easier intuitively. Ultimately, unless you're using Ike to do a gravity assist, I don't think it matters too much.

[Alshain]

I'll add to Red Iron Crown's first scenario. If you hit Ike first, then Duna, then you will be a in prime location to get a Gravity assist from Ike to throw you out of Duna's SOI. It costs a lot less to burn to Ike than it does to burn to Kerbol SOI.

[EdFred]

I've been doing it this way :

Aerobrake into a Duna 50km-60km orbit. Land at Duna, ascend, rendezvous, then head to Ike. Ike Orbit at around 25km. Land, ascend, rendezvous. When leaving Ike, burn to 40km Duna periapsis, placing it at the correct spot for an ejection burn to return to Kerbin.

It often seems I never get a good Ike encounter on the way in.

[Red Iron Crown]

I'll add to Red Iron Crown's first scenario. If you hit Ike first, then Duna, then you will be a in prime location to get a Gravity assist from Ike to throw you out of Duna's SOI. It costs a lot less to burn to Ike than it does to burn to Kerbol SOI.

Both of my scenarios are using a low Duna transfer burn, both should theoretically be able to leverage a gravity assist from Ike. Though I usually don't take gravity assists into account in my planning because I'm bad at them.

It often seems I never get a good Ike encounter on the way in.

Heh, I have the opposite problem, Ike has messed up many carefully aligned aerobraking approaches for me.

[EdFred]

I change my conics patch limit from 3 to 6. Helps a lot more with gravity assists. But I've found that while Ike is at about 195º from prograde to Duna's orbit, and while the craft is at about 315-330º from prograde in Ike's orbit is where the first burn needs to take place. Then burn at Duna PE and I get back to Kerbin for a lot less than the ~600dV Alexmun's planner says.

[Red Iron Crown]

I change my conics patch limit from 3 to 6. Helps a lot more with gravity assists. But I've found that while Ike is at about 195º from prograde to Duna's orbit, and while the craft is at about 315-330º from prograde in Ike's orbit is where the first burn needs to take place. Then burn at Duna PE and I get back to Kerbin for a lot less than the ~600dV Alexmun's planner says.

Well yeah, you're starting from a much more energetic orbit. Alexmoon's planner assumes a circular initial orbit, usually at low altitude if you don't adjust the presets.

[Alshain]

Both of my scenarios are using a low Duna transfer burn, both should theoretically be able to leverage a gravity assist from Ike. Though I usually don't take gravity assists into account in my planning because I'm bad at them.

Well, you don't have to be perfect with a gravity assist. The idea is to get headed in the general direction of Kerbin and then make a correction burn to get the capture. All of that should cost less than the burn to go straight to Kerbin.

[EdFred]

Well yeah, you're starting from a much more energetic orbit. Alexmoon's planner assumes a circular initial orbit, usually at low altitude if you don't adjust the presets.

Oh, I know why I use less dV. My orbital velocity is much higher at my PE vs a circular orbit.

It's a matter of does

Ike -->Duna (orbit) --> Ike Slingshot --> Kerbin cost more or less dv than

Duna --> Ike (orbit) --> Duna slingshot --> Kerbin

I suppose there is one way to find out. Go play moar KSP.

[Red Iron Crown]

I don't think you can use Duna for a slingshot in this scenario, only for added Oberth effect.

[EdFred]

I don't think you can use Duna for a slingshot in this scenario, only for added Oberth effect.

True, as I'm starting withing Duna's SOI.

[Frank327]

The way I did it, I first established an elliptical orbit around duna (periapsis 60km, apoapsis halfway to Ike). Then I released the lander and landed. Then I established orbit with the lander again and docked, then the whole thing was already most of the way to Ike so it was just waiting for it to pass around. That way the biggest part of your ship doesn't have to get apoapsis around duna all the way down to 100- km and after that back up again towards Ike. If your orbiter has the whole habitation module, science labs, nuclear engines etc. you'll save yourself lots of fuel.

I didn't aerobrake at duna though, but there's no reason why you can't. I don't think you save an enormous amount of fuel with aerobraking.

[magnemoe]

I'll add to Red Iron Crown's first scenario. If you hit Ike first, then Duna, then you will be a in prime location to get a Gravity assist from Ike to throw you out of Duna's SOI. It costs a lot less to burn to Ike than it does to burn to Kerbol SOI.

Duna aerobrake put your Pe high so you will get an Ike intercept for free, you might want to tweak you orbit while focusing on Ike to get an good intercept.

Get into Ike orbit and land, get back to orbit and do an second aerobrake around Duna to get into orbit.

Land on Duna.

Note that your lander should have enough dV to do this alone if it can land safely on Duna, leaving the heavy mothership in Duna orbit.

[EdFred]

I didn't aerobrake at duna though, but there's no reason why you can't. I don't think you save an enormous amount of fuel with aerobraking.

Aerobraking into a Duna 60km orbit saves about 600dV - assuming you are on an optimal intercept. A non-optimal intercept would save even more.

[Levelord]

What I'd usually do is to encounter Ike in a retrograde approach where its gravity will slow my craft down into a Duna insertion. It's less guesswork compared to aerobraking and less dangerous.

[cantab]

I think it's going to depend on the exact ship a lot. For example if I was designing a "disposable" mission trying to minimise overall mass, I'd be minded to hit Duna first, because I don't need chutes or as much fuel capacity for Ike so I could ditch those.

[FacialJack]

Hi, i saw that today:

http://forum.kerbalspaceprogram.com/threads/93685

Which seams to be a mod that display the new trajectories after aerobraking, or the reel trajectory of a plane, as it take count of drag

But i didnt test it in situation, but will do now

[EdFred]

OK, I cannot get a good Ike gravity assist to return to Kerbin, but here's what I found in terms of dV (ignoring small tweaks to get things just right)

Kerbin --> Ike --> Duna --> Kerbin (no Ike gravity assist)

Duna aerocapture into Ike 25km capture = Free

Ike 25km circularization burn = 125dV

Ike --> Duna = 120dv

Duna aerocapture AP drop to 60km = Free

Duna circularization burn = 30dV

Kerbin transfer burn = 625dV

Total dV = 900

Kerbin --> Duna --> Ike --> Duna Oberth assist --> Kerbin

Duna aeocapture to 60km AP = Free

Duna circularization = 30dv

Duna --> Ike 25km transfer 290dv

Ike circularization burn = 125dV

Ike --> Duna 40km PE for Oberth assist = 120dv

Burn at Duna PE transfer to Kerbin = 335dV

Total dV = 900dV

All values were rounded to the nearest 5dv. Looks like a wash. The issue is, Ike has to be in perfect position for the first scenario. The last 15-20 times I've gone to Duna Ike has not been "in the way" for an aero capture from Duna.

[Red Iron Crown]

Interesting results EdFred, good to put theory to the test.

It looks to me like the additional Oberth effect in the second scenario's transfer burn makes up for the lost aerobraking.

As for intercepting Ike in the first scenario, I think you could just aerobrake your Ap to Ike's orbital altitude, then when you get to that Ap perform a small burn to get the encounter. Definitely more fiddly than the second scenario any way you cut it.

[EdFred]

I think it's going to depend on the exact ship a lot. For example if I was designing a "disposable" mission trying to minimise overall mass, I'd be minded to hit Duna first, because I don't need chutes or as much fuel capacity for Ike so I could ditch those.

Which is how I build a lot of my craft. I also leave perfectly good lander cans floating about the Kerbol system.

[cantab]

Ike --> Duna 40km PE for Oberth assist = 120dv

Burn at Duna PE transfer to Kerbin = 335dV

According to this delta-V map, http://i.imgur.com/Wfyf9l2.png , it's slightly cheaper to burn for Kerbin transfer directly from Ike. But the difference is within error and variation tbh.

[EdFred]

That map is confusing, and it shows way less to go from Duna to Ike. My transfer from Duna --> Ike was 290dV, that map says 30dV? Huh?

Nevermind, read it again. Still confusing though

Edited September 16, 2014 by EdFred