Trying to salvage Duna mission/go to orbit

[Rainshine]

My planned Duna mission seems to have gone awry.  Here's the current map of intersects.  Not having done any successful interplanetary missions in a very long time, this looked fine-ish.  But now when I go to plot my burn to orbit, I'm having a little trouble.  Burning retrograde at Pe, it will take ~2260m/s to enter orbit, which would drain my tanks, my lander, and about half my return fuel.  I had heard 250 for the number from flyby to orbit.  I've tried playing around with going radial or such, but nothing seems to help.

Am I missing something, or did I just pick an atrocious angle to enter at; and if so, how should I adjust for my next missions?

Edited June 23, 2020 by Rainshine

[bewing]

Considering how your craft's orbit line goes right through Duna's orbit, what it means is that your craft's solar Ap is way too high. You just want it to touch Duna's orbit, not go through it. Going through it means that you added way too much deltaV to your craft to get it going, and now you have to take it all back out again to come to a stop.

 

[Streetwind]

If you are plotting your maneuver node with a retrograde burn at periapsis, as you describe, that is the most efficient orbit insertion possible. I'm afraid you are simply coming in with that much of a difference in velocity that you must cancel out. This typically indicates that you have not flown a Hohmann transfer, but rather something else. Either you launched well out of the transfer window, or you plotted a high energy transfer (burning really hard when leaving Kerbin to make the trip as short as possible).

You might however be able to at least salvage your mission to the point of being able to insert into orbit and later return to Kerbin. Because luckily, Duna has an atmosphere. That means you can aerobrake.

Make a maneuver node just a few minutes ahead of where your ship is right now, and play with the radial and normal sliders to try and move your PE closer to Duna. The atmosphere ends at 50km, so you want it to be closer than that. Of course, depending on how sturdy your ship is, and how insanely fast you come screaming past Duna, your ship may be torn apart by shock heating. So, after you have executed your maneuver to push the periapsis into the atmosphere, press F5 to quicksave. Now timewarp to your arrival and observe what happens. If you get torn apart, go a little higher next try. If you feel like you have ample breathing room, try going deeper.

You should probably align retrograde and fire your engines even while aerobraking. If you have to bleed off two km/s just to capture, a single aerobraking pass without engine support will not do it, especially if your craft is fragile.

Your goal should be to capture into any form of orbit. Even if it is crazy elliptic with the apoapsis way out near the edge of the SOI. Additional aerobraking passes will let you bring that down without spending any fuel.

Note that even a slight grazing of the atmosphere at these kinds of speeds is liable to rip off any extended antennas and non-fixed solar panels. If this would result in a loss of mission... well. You're kinda screwed. Your best bet in that scenario would be to not insert into orbit, but rather to burn at periapsis so that you end up in a solar orbit from which you can later get a Kerbin encounter, so at least you can return home.

Edited June 23, 2020 by Streetwind

[AHHans]

39 minutes ago, Rainshine said:

Am I missing something, or did I just pick an atrocious angle to enter at; and if so, how should I adjust for my next missions?

Your orbit around the sun is crossing to Duna's orbit at quite some angle, instead of just coming up to it. Because of that you enter Duna's SOI at quite some speed that you then need to counteract in order to brake into a Duna orbit. So there is at least a good portion of the latter in there. Another issue might be that your PE around Duna is quite high. I can be that you actually need less dV to capture around Duna with a lower PE. (But I don't know what your PE is, so it's hard to say.)

For the next time: the AP of your orbit around the sun should only get up to Duna's orbit. (I.e. so that you do a  Hohmann transfer.) If that means that Duna wouldn't be there when you reach Duna's orbit, then you are not starting at a transfer window and thus need to wait for one.

I personally use Kerbal Alarm Clock to figure out transfer windows. But you can also use other tools e.g. the Launch Window Planner webpage, or the Interactive illustrated interplanetary guide and calculator for KSP. I suggest to at least have a look at the latter because it also includes some diagrams that illustrate the position of the planets during the launch window.

P.S. O.K. Two other answers right now. So I won't explain aerobraking here.

Edited June 23, 2020 by AHHans

[Zhetaan]

8 hours ago, Rainshine said:

Am I missing something, or did I just pick an atrocious angle to enter at; and if so, how should I adjust for my next missions?

I don't know that I would call it atrocious.  It's not a great angle, but at least you haven't completely ruined the mission.

Given the state of your intercept angle, you either boosted too much or left too late--or both.  While your idea to use radial adjustments to fix your encounter isn't a bad idea, you're far too close to Duna for orbit corrections to have much effect.  You can manipulate your Duna-sphere trajectory from where you are presently, but you're not going to fix the intercept.  An item to note is that in addition to an atmosphere, Duna also has a big moon that you can use for some (albeit not a lot, given your speed) gravity braking and a minor boost to the Oberth effect in addition to the aforementioned aerobraking.  The key to that is to chase Ike (or, more accurately, leave Ike's sphere of influence in Ike's retrograde direction) and get as low of a periapsis as you can without splatter, if possible.

Alternatively, you can accept the flyby and use the next encounter with your solar periapsis to get a resonant orbit with Kerbin so that you can get back there within the next few years.  You ought to have more than enough fuel for that.  If you want to salvage an orbit for the World's First rewards but can't keep enough fuel for a landing, then you can try landing at Ike instead of Duna, but otherwise, you should seriously consider draining the lander's fuel tanks into your main vessel before jettisoning it for the delta-V.

[Rainshine]

Thanks for the help and suggestions!  I know I was in a good vicinity for the transfer window, but I'm sure I just overburned and didn't play with my maneuver nodes enough for setting up the initial intercept.  This was originally actually an Ike mission, the lander was a lightweight one that I've been using for lunar missions.  But by spending some of my lander budget (I figured I'd end up leaving it there in orbit anyways and just hauling some fuel for it next time) I managed to get a flyby on Ike, and dropped my Pe to 28k on Duna, which let me aerobrake down to a stable orbit (- the ~50 m/s to boost the Pe up to 60k afterwards).  So I got some Ike science, some Duna atmo science, some Duna space science, and hopefully I can figure out my transit back to Kerbin orbit with my remaining fuel