How to get to Duna outside of optimal transfer window?

[RX2000]

So in my current 1.2 game I'm trying to get up the nerve to go interplanetary for the first time in KSP. (Yea I know, I'm such a n00b )

Anyways, KAC is telling me that the optimal transfer window to Duna is over a year away. I really want to go ahead & set up a relay sat in polar orbit of Duna BEFORE that time, so my other stuff I send when that window finally does come around will be able to have good comms back to Kerbin even if I dont stick the best antennas on them.

I have watched several Youtube videos & read some tutorials about how to get to Duna, but they pretty much all say to just wait for the optimal transfer window & then go. Well lets say I dont want to wait & dont mind spending the extra dV to get there by leaving at a non-optimal time. How do I go about setting up an encounter?

[bewing]

If your optimal xfer window is a year away, then you are probably on the wrong side of the Sun to even be thinking about it. But if that's wrong and Duna is basically in front of Kerbin, then you can just set up a normal maneuver node. Adjust the position and deltaV to get your Close Approach as close as you can. You'll still miss by 10s of billions of meters on the close approach. But then you do a mid-course correction to adjust your trajectory quite a bit. On the other hand, if this is a relay sat that you're sending, then it doesn't actually need to get captured at Duna. You can get it halfway and then circularize at that point, and it'll be pretty well positioned to do relay stuff.

[Streetwind]

There's no way around it... you'll need throw dV at the problem until it goes away.

There are two possible ways to do this. The first is a high energy transfer. It is the more straightforward of the two solutions, but it only works (when going to an outer planet) when you're somewhere between "just missed the window" and maybe at most half a Kerbin year past that. To do this, you burn past Duna's orbit in such a way that Duna happens to be where you cross Duna's orbit at the same time you get there - and then, you hit the brakes like mad. This may involve making your ejection burn at a radial-out angle, and dV costs vary wildly depending on where exactly Duna is at the time and how aggressive you want to be. There's a minimum solution for the current planetary alignment, but there's also an infinite fuel torchship solution, and everything in between So you might want to pull up a porkchop plotter for this.

The second way works regardless of Duna's position, and gives you more predictable dV costs (though not necessarily lower, depending on the situation), but it results in a longer flight time. What you do is, while Kerbin may be one year out from being in the ideal position for a transfer window, you can make your spacecraft "skip forward" by assuming a solar orbit that's lower than Kerbin's, thereby creating its own transfer window. First, you escape Kerbin retrograde, lowering your orbit - as if you wanted to go to Eve or Moho instead. The lower you go, the faster you get to Duna, but the more dV you must expend. Once you get to your low solar periapsis, you circularize there. Then you can just make a maneuver node with a classic Hohmann transfer to Duna, and drag-and-drop it around your low solar orbit until you get a Duna encounter. The combined trip time of getting to solar periapsis plus getting to your node plus getting up to Duna from there will be significantly shorter than the time it would take Kerbin to reach its transfer window plus the time for a Hohmann transfer.

And, for what it's worth: you can also elect to perform a high energy transfer instead of a Hohmann transfer for the final intercept, saving even more time (at the cost of even more dV).

Edited October 25, 2016 by Streetwind

[Starhawk]

Here's an image from my favourite KSP transfer planner. ( KSP Planner )

The two dark blue areas represent optimal transfers with the dV for these around 1657 m/s.  The least optimal transfers are those represented by red areas.  In the info pane to the right you can see that the transfer selected by the crosshairs (just about exactly a year away from either optimal transfer) requires more than ten times as much dV. (18 483 m/s)

It's certainly possible to go at a suboptimal time.  But you pay an enormous price to go at the least optimal time.

Hope this helps.

Happy landings!

[John Cochran]

Since you want the relay to make certain that the comms for future probes work without problems, why not launch the relay and the other mission at about the same time. Say within a couple of days of each other? There's very few times where you need to make multiple maneuvers in short succession, so start one craft on it's way, figure out the next maneuver node, then switch to the next craft. Wash, rinse, repeat.

[Plusck]

In any event, it's probably completely pointless to try.

As @bewing said: if the optimal transfer time is a long way away, then Duna is on the wrong side of the sun or far behind Kerbin. Getting to it quickly would require near-unplayable amounts of dv.
What you'd essentially have to do is reduce burn radially-out and retrograde from Kerbin so fast that your sun orbit is a tight ellipse which has you leave Kerbin heading straight out into space, then come falling back towards the sun just as Duna passes by.
At a minimum, this means leaving Kerbin at 9300* m/s +"X" (where "X" is whatever is needed to get you the right height above the sun), then capturing at Duna with ≈7500* m/s +"Y" (where Y is less than "X" but not all that much, depending on how far Duna is behind Kerbin).
This could be a reasonably fast transfer, but extremely expensive. You'll essentially be at the top end of a highly elliptical orbit around the sun, floating slowly around waiting for Duna to catch up.

* 9300m/s = Kerbin's orbital velocity; 7500 m/s = Duna's average orbital velocity

I just looked again at @Starhawk's post and this is more or less what is going to be the case in that "least optimal" case.

Any other option is just going to be a variation on the same theme: you burn outwards past Duna's orbit and catch it on the way back, but you take longer to get there by nullifying less of Kerbin's orbital velocity when you leave. The cheapest option (above the upper limit of the porkchop plot that Starhawk posted) is simply to push your orbit up somewhere near Dres and then catch Duna on the way back down, but you'll be catching Duna nearly a whole Duna orbit (800 days) later and that makes it fairly pointless: you'd be quicker waiting a whole Kerbin year before leaving.

 

So rather than doing any of that, you'd be better off simply waiting. By burning a touch more radially-out at the optimal transfer date, you meet Duna much sooner. So you can have your relay sitting in polar orbit weeks before the rest of the expedition (taking the most efficient route) gets there, at a very modest cost. If you don't need weeks, just send them all on the same route (or even all on the same vessel, splitting up in sun orbit), and make a slight mid-course correction to bring the relay's arrival time down to a few days before the rest.

And just to be clear: there is no point in trying to leave earlier to get there earlier, it just doesn't work that way. Sending your relay a few days later would actually let you get to Duna sooner for only a reasonable cost.

Edited October 25, 2016 by Plusck

[ForScience6686]

It is totally feasible, and I do it myself for exactly the same reason you want.  The good news is probes are light weight, which makes it easy to pack on the dv.  The thing to worry about is a reasonable twr.  You will need long burns for the escape and capture.  So I build in as much drop tanks as possible to keep the minimum amount of dead weight on the craft.

[RX2000]

Mmm ok thanks guys.

So it looks like my answer basically is "you dont." Hahaha.

I was thinking that it was maybe doable with just a couple thousand more dV, but over TWICE as much? Sheesh, I guess I'll just wait.  I mean I guess it really doesnt matter if the relay only gets there a few HOURS before the other stuff I plan on sending. I guess I'll just have to be patient & wait. KAC is saying the optimal transfer is 1 yr 47 days away.  I dont really want to just warp to it, so I guess I'll work on some space stations & maybe on perfecting my rovers by practicing with them on the Mun & Minmus in the meantime.

[ForScience6686]

And waiting is what I hate, so I just keep sending out missions.  Optimal is only the cheapest route, but a successful space program won't be lacking funds if done properly.  So I don't see cost as an issue.  I see it more as an engineering problem, which is what this game is all about, and light weight probes are relatively easy to increase range.  

[Streetwind]

I usually check whatever transfer window comes up next, and then launch something thataway.

(Yes, this ends up with me going to Moho a lot )

Considering how small the KSP solar system and its associated dV costs are, you can go everywhere as soon as you unlock the Terrier and static solar panels, if you build it right (and have an upgraded launchpad). Further part unlocks just make the whole thing much easier and comfortable, but they don't really give you capabilities you didn't have before.

...or rather, that's how it used to be. If you play with commnet enabled, this gets harder, because antennas with useful range unlock much higher up in the tech tree. But of course, you could send manned vessels staffed with pilots, then you don't need a connection home.

[RX2000]

57 minutes ago, Streetwind said:

I usually check whatever transfer window comes up next, and then launch something thataway.

(Yes, this ends up with me going to Moho a lot )

Considering how small the KSP solar system and its associated dV costs are, you can go everywhere as soon as you unlock the Terrier and static solar panels, if you build it right (and have an upgraded launchpad). Further part unlocks just make the whole thing much easier and comfortable, but they don't really give you capabilities you didn't have before.

...or rather, that's how it used to be. If you play with commnet enabled, this gets harder, because antennas with useful range unlock much higher up in the tech tree. But of course, you could send manned vessels staffed with pilots, then you don't need a connection home.

Oh yea thats a good point. I've been so focused on Duna because it just seems like the first place you should go after the Mun & Minmus, so I completely forgot to check the other planets hahaha! I think I'll check to see if there are any other optimal transfers coming up & just go there. Looks like Moho is only around 8000 dV or so, & Eve is only 6000. Maybe I'll just go that way.

[Hodari]

1 hour ago, RX2000 said:

So it looks like my answer basically is "you dont." Hahaha.

One does not simply transfer into Duna. 

[Streetwind]

Just be careful with Moho. It's the highest difficulty, highest dV transfer destination in the game. That applies even when you hit the transfer window perfectly... and quickly grows beyond that if you're off by as little as an hour. That's because Moho orbits the sun so rapidly, the window just flashes by. It comes back every couple days, but you better be precise about it! Even the burn itself is sensitive, small errors add up quickly into major correction burns later-on. With Eeloo, by contrast, you can miss the window by days and then burn sloppily in the general direction and still not notice much in the way of correction necessary.

Additionally, Moho windows vary in quality more strongly than those of any other celestial body. There can be a dV cost difference as high as 10% between two windows even if you hit them perfectly. This is because of Moho's high inclination and eccentricity. More than one person has been left stranded in Moho orbit because the same ship that worked last time suddenly doesn't have the dV to return home anymore. Bring a contingency buffer!

For those reasons, many players forgo the in-window Hohmann transfer and opt for the longer-duration but easier to execute out-of-window apoapsis transfer. In which you wait until Kerbin is lined up with Moho's apoapsis, which happens to coincide with its ascending node relative to Kerbin. That means you can easily fold the entire plane change into your ejection burn for extra efficiency. Lower your spacecraft's orbit until it just brushes up against Moho's orbit at its periapsis. Then you let the spacecraft loop around a few times in solar orbit, while making a node at periapsis every loop to check if you can make a cheap burn (prograde works, but retrograde is preferred) that gets you a Moho encounter at a later date.

 

But hey, if you can nail both the Hohmann transfer window and the burn, and stay under 8500 m/s total cost by the time you're circularized in low Moho orbit, you can confidently say that you can fly any mission profile in KSP without breaking a sweat

Edited October 25, 2016 by Streetwind