Duna Mission Timings

[Das_Sheep 11]

So since I've started playing I've somehow never launched a proper mission to other planets (other than the odd probe). I got back into KSP a while ago thanks to adding a pile of mods that add the depth I was after (mainly everything and anything USI and Near Future) and I've now decided that a massive mission to Duna sounds like a plan to step up from my multiple 20 kerbal colonies on the Mun and Minmus.

Currently I'm figuring something along the idea of a Martian style mission - probe dropped supplies ahead of the mission with a minimum duration on the surface of 30 days (just cause).

Basically this all boils to a rather simple question - assuming the stock solar system - roughly how long sounds like a good budget for life support supplies / habitation?

I reckon I need to allow at least 300 days for the transfer there plus surface mission time (30 days) but when is the best launch window for return? Is this going to be the expeditionary mission I'm planning on or should Jeb and co be planning for a year or more sat there?

[OhioBob 3,461]

@Das_Sheep, if we simplify the problem by assuming circular and coplanar orbits, and by using Hohmann transfers for both the outbound and return trips, then it's pretty straightforward to figure out.  You depart when Duna is 44 degrees ahead of Kerbin.  It takes 302 days to get there.  When you arrive at Duna, Kerbin is now 75 degrees ahead.  For the return trip you must wait until Kerbin is 75 degrees behind Duna.  This means that Kerbin has 210 degrees of catch up to do, which takes 530 days.  When Kerbin has reached the right spot, you begin the Hohmann transfer back home, which again takes 302 days.  Therefore the entire journey takes 1,134 days, or 2.66 years.
 

[Geonovast 6,543]

I don't know the math for you, but since I started using this, my interplanetary stuff has gotten easier... just have to plan for while you're there!

 

[Das_Sheep 11]

Conveniently I already have this installed for a "planned" Jool mission. To be honest I'm hoping to keep them awake for the whole trip and possibly researching as CTT needs some serious science points for the last few.

[The Aziz 2,089]

Think about it like this: optimal transfer window is when Duna is a bit ahead from Kerbin. For the return, Kerbin must be ahead. So at least few hundred days, return windows from Duna are separated by more than a year. You're gonna wait, either on the surface or in orbit.

Also if you use transfer window planner, you can set earliest departure time from Duna by the time you get there and see when is next window.

Edited October 9, 2017 by The Aziz

[Das_Sheep 11]

1 minute ago, The Aziz said:

Think about it like this: optimal transfer window is when Duna is a bit ahead from Kerbin. For the return, Kerbin must be ahead. So at least few hundred days, return windows from Duna are separated by more than a year. You're gonna wait, either on the surface or in orbit.

Welp, looks like more than one presupply mission then. If their going to be on the surface that long might do rover mission(s) to other biomes.

Think I shall run a practice on the Mun first.

[Spricigo 1,131]

Alternatively you may put the crew in the 'express' more expensive ejection maneuver and more trouble to capture but also less travel and permanence time.

Play a bit with the transfer planner and see if you find transfer times that suits your plans.

[Das_Sheep 11]

Ooooh, somehow I was unaware of this rather awesome tool

[Snark 14,288]

9 hours ago, Das_Sheep said:

Ooooh, somehow I was unaware of this rather awesome tool

It's a nice tool, but I've always preferred http://ksp.olex.biz myself, mainly because I like the graphical display of angles and such.  Matter of taste. 

As @Spricigo points out, you don't have to go with a Hohmann transfer.  You can send supplies and such on a slow transfer, get everything ready, then put the crew in a low-mass, high-dV ship that travels quickly to Duna where it finds supplies (and/or return fuel, and/or mining equipment, whatever) all ready and waiting.  You can get to Duna a lot faster if you're willing to spend some extra dV.  And the only components that actually need to go fast are the kerbals, who don't mass all that much.

[Das_Sheep 11]

4 hours ago, Snark said:

It's a nice tool, but I've always preferred http://ksp.olex.biz myself, mainly because I like the graphical display of angles and such.  Matter of taste. 

I used to use that myself, but since I got comfortable with the how/why and started using Mechjeb to remove the fiddling with manoeuvre  nodes malarkey I seem to have fallen away.

[Blaarkies 367]

15 hours ago, The Aziz said:

Think about it like this: optimal transfer window is when Duna is a bit ahead from Kerbin. For the return, Kerbin must be ahead. So at least few hundred days, return windows from Duna are separated by more than a year. You're gonna wait, either on the surface or in orbit.

Also if you use transfer window planner, you can set earliest departure time from Duna by the time you get there and see when is next window.

I believe for the return, Kerbin should be behind Duna (not ahead as you suggest). Think about it: a Transfer** from Duna to Kerbin has a trajectory/orbit that is bigger(greater SMA) than a Kerbin Orbit, therefor it should take longer to reach the intersection point for the craft...and less time for Kerbin to reach it. So you need to be ahead of Kerbin to match this.

** Assuming Hohmann transfers

[FancyMouse 392]

There's the fun part - if you want to minimize life support resources, you actually could catch two consecutive windows (to and from Duna) and you won't even need a full year of resources for the round trip. I barely remember that the key is you just need some extra fuel so that you arrive at year 1 day 400 or so. 30 days of activity is actually pretty tight - IIRC it's reasonable if the activity is controlled under 10 days or so - you'll need to play with TWP and find a good spot for both going to and coming from Duna.

Tip for using TWP in this scenario: set both x and y axis to be the same scale (meaning trip time max-min equals departure time max-min). Then you'll know that each 45 degree main diagonal line is corresponding to an arrival date. For any desired arrival date, you could find the diagonal line, then find the best transfer by inspecting that line.

This is how I send my elite 4 to Duna and return to Kerbin as early as possible (back on year 2 day 200'ish).

Edited October 10, 2017 by FancyMouse

[Das_Sheep 11]

47 minutes ago, FancyMouse said:

 if the activity is controlled under 10 days or so -

That can be done, 30 days was just a number pulled out of thin air anyways -although I suspect the habitat sent down will end up being over-specced. But that can just be the start of a longer term base