Duna and Return Transfers - Timing?

[G'th]

So I'm getting ready to make headway to Duna again (and I think I'm going to actually make it this time woo) but I'm wondering what I should expect as far as how long a mission should be on the surface.  Is it going to be a case of I get there and end up having to wait the entire year or two for the transfer window to open back up, or would I be able to stay on the surface for a relatively short amount of time (Lets just say a 30 day period for example) before transferring back with no issue? 

Playing with Life support mods, so of course this logistical issue is a very important part of the planning. Last time I tried to go to Duna I just planned to stay for the whole year and sent a whole base as a result (course I then proceeded to crash the descent vehicle into the transfer vehicle and screw it up but oh well), but I'm wondering if I can simplify for my first landing and return sans cheating.

[qzgy]

Yup, you'll have to plan to wait 2 to 3 years there, kinda depending on the game time. Here's a handy calculator for that.

https://alexmoon.github.io/ksp/#/Duna/100/Kerbin/100/false/ballistic/false/2/69

[Foxster]

You don't have to wait but you will not have an optimal transfer back to Kerbin, which is fine if you have a good dV margin. 

[mk1980]

the transfer to duna and the transfer from duna back to kerbin both take about 270 days when using a reasonably efficient transfer window, so your mission will need supplies for about 1.5 years anyway - just to cover the actual travel time. and you'll have to stay on duna (or in orbit) for about 1.5 years waiting for the transfer window home.

i guess you could leave (most of) the life support supplies/equipment on the "mothership" in orbit rather than sending it down to the surface. do the landing, plant some flags, collect some stones or whatever, then return to the mothership and wait for the transfer home. 

[Palaceviking]

Or just save a butt ton of fuel for a very direct return?  Oh and you'll need extra to slow down at kerbin if you do this..

[Tyko]

I use the transfer window planner mod but this would also work in the web version - set your outbound transfer, note your arrival date, set up a return transfer and change the earliest starting date to your Duna arrival date - this will ensure you're only seeing transfers that would work for your mission.

Edited September 27, 2017 by Tyko

[Zhetaan]

On 9/1/2017 at 8:02 PM, G'th said:

Is it going to be a case of I get there and end up having to wait the entire year or two for the transfer window to open back up, or would I be able to stay on the surface for a relatively short amount of time (Lets just say a 30 day period for example) before transferring back with no issue?

You can always take extra fuel and say, 'Why wait for a window when I can kick in the door?'

In your case, the issue is what the issue always is:  weight.  In stock KSP, it's best to wait at Duna (or wherever) for the most fuel-efficient window because fuel is the only thing you have to budget.  When you play with a life-support mod, you have to budget supplies for this trip too, but they also have mass.  Take more supplies and go on a longer mission, but you also have to take more fuel to lift that rocket and send it on that mission.  If you take enough supplies, you can use those same minimum-fuel transfers, but the sheer mass of the craft will put the minimum very high.  Take fewer supplies, and perhaps the amount of fuel you save by doing that is more than the amount you lose by needing to use a faster transfer.

Remember, the tyranny of the rocket equation is that the amount of fuel needed increases exponentially with the mass.  If you take twice the fuel you do not get twice the delta-V.  However, life support tends to increase time available for the mission only linearly with the mass:  if you take twice the LS supplies, you can go for twice as much time.  This means that overall, fuel concerns will still win out over LS concerns unless there's another mechanic such as habitation or recyclers involved.  Even then, habitation only increases the minimum size of your rocket, making it a weight problem, and recyclers decrease the mass of LS resources you must take and replaces that mass with the recycler and the power generators to run it, which still puts the trade-off in terms of weight.

[G'th]

1 hour ago, Zhetaan said:

You can always take extra fuel and say, 'Why wait for a window when I can kick in the door?'

In your case, the issue is what the issue always is:  weight.  In stock KSP, it's best to wait at Duna (or wherever) for the most fuel-efficient window because fuel is the only thing you have to budget.  When you play with a life-support mod, you have to budget supplies for this trip too, but they also have mass.  Take more supplies and go on a longer mission, but you also have to take more fuel to lift that rocket and send it on that mission.  If you take enough supplies, you can use those same minimum-fuel transfers, but the sheer mass of the craft will put the minimum very high.  Take fewer supplies, and perhaps the amount of fuel you save by doing that is more than the amount you lose by needing to use a faster transfer.

Remember, the tyranny of the rocket equation is that the amount of fuel needed increases exponentially with the mass.  If you take twice the fuel you do not get twice the delta-V.  However, life support tends to increase time available for the mission only linearly with the mass:  if you take twice the LS supplies, you can go for twice as much time.  This means that overall, fuel concerns will still win out over LS concerns unless there's another mechanic such as habitation or recyclers involved.  Even then, habitation only increases the minimum size of your rocket, making it a weight problem, and recyclers decrease the mass of LS resources you must take and replaces that mass with the recycler and the power generators to run it, which still puts the trade-off in terms of weight.

Indeed. Hence why most of my Mars/Duna plans hinge on the flotilla concept, with several craft making up one mission, many of which precede the crew. Ground supplies gets launched with precursor ground equipment, return supplies launched at some point (usually with mapping/comms probes, though it could also just be a straight separate launch in of itself), and of course the journey to supplies are launched with the crew or the habitat.

But, as much of my habitats would be using large ion engines dV wouldn't be an issue, just burn times. 

[Helmetman]

I'm not that much of a fan of ion engines because of their burn time also. But when used only for the return mission it's probably doable concerning the burning times. By that time you probably only have a command pod(s) for the return home. In that case you'll have acceptable TWR for 4x physics timewarp.

Unless your "mothership" has the weight of a actual mothership. But I suspect a return ship (mothership) isn't all that heavy even with life support added. It gives you plenty of Delta-V to go directly back to Kerbin. But as is said, you'd need to decelerate when entering kerbin SOI.

[Palaceviking]

Throw us some numbers (ls weight per day, mass of lander etc) and somebody will be able to help, although the flotilla approach does seem best in my experience.