Fully modded trip to Duna

[me68]

I've been to Duna before, but this is the first time i'll be going with a fully modded game. Not including parts packs and some other minor mods, i'll have FAR, RemoteTech2, Deadly Re-entry, Kerbal Alarm Clock, Real Chutes, and most importantly, Tac Life Support.

According to Kerbal Alarm Clock, i have 9 days to my launch window to Duna. I've accounted for what i'll need to bring for all the other mods, but the part i'm stuck on is how much life support to send up. Of course i could just stick some ridiculous amount of life support on there, but i want to be more precise and also keep the bulk and weight down. How long the mission will take will determine how much i bring, but i'm unsure how long it will take. Of course with the launch window, i'll be making a very efficient trip there, but what about back? I can leave Duna anytime i want, but i'll want to feather that line between how quick i return, and how efficient it is to return.

I have a basic three part ship. They dock together in high Kerbin orbit, and then leave during the launch window. The Orbiter stage has an Interstellar reactor and generator, as well as most of the life support and the scrubbers and splitters for the water and oxygen. Attached to the Orbiter stage will be the Transfer stage, which is just fuel and nuclear engines. This stage will be jettisoned after putting the craft on an aerobraking trajectory with Duna. The Landing stage has everything needed to land and do science, as well as something like 20 days of life support, more than enough to collect science from the surface and return to the orbiter stage. I'll then leave for Kerbin when it is optimal.

The most obvious thing to do if you don't have to worry about life support is to leave Kerbin during the ideal launch window, and then leave Duna for Kerbin during the ideal launch window. Am i also right in assuming that would take the longest amount of time (as far as sensible trajectories go)? Whereas the quickest and least efficient way to return would be to leave during the launch window, and then, in order to come back to Kerbin, burn so your Pe is close to Kerbol and rendezvous on the other side with Kerbin?

I'm not going to have the Delta V necessary to make inefficient burns back to Kerbin, so how many days of life support am i looking at needing? Does anyone know the number of days if i leave during both transfer windows, and does anyone know how quick you could possibly make a trip to Duna and back? With those two figures, i should be able to determine a happy medium between life support and Delta V.

How long does it usually take you, and what is your usual trajectory?

[Kryxal]

Are you looking at the Year 1 Day 59 window? If so, your best return window is at day 273, trip time each way is about 66 days. Maybe 150 extra delta-v would let you cut 15 days or so off that flight or leave early with similar flight times. I'm assuming aerobraking on both ends, if not then your extra delta-v needs will be higher since faster transits mean you have a significant radial component to your velocity ... in any case, 300 days would leave a 20-day margin, more than enough with the windows being what they are.

[LethalDose]

Remember, that's 300 days/kerbal. If you take a 3-man crew, you'll need 900 days of support (3 kerbals x 300 support/kerbal). A 270-day trip sounds about right, but I'd have to check the math for the phase angles and transits.

Anyway, all the interplanetary trips are good candidates for the water filters (waste water -> water) and the carbon extractors (CO2 -> O2). I wouldn't take the splitters, which turn water into O2 and waste. Water is the heaviest of the resources by far, so you plan to turn water into O2, you're being really wasteful. i see those hydrolysis units as kind of a last resort for O2.

The carbon extractors and water filters in TAC LS basically reduce the consumption to 0.1 units/kerbal/day (they intake 1 "used" unit and return 0.9 "fresh" units), so you'd need 30 water/kerbal & 30 O2/kerbal with a water purifier and carbon extractor for a 300-day mission. You still gotta pack a full 300 units of snacks/kerbal.\

As far as the transfer, I've usually just used typical Hohmann Transfer orbits. These lead to rendezvous with the target body 180 degrees from where they left the originating body. A Kerbin-to-Duna transfer takes 72-80 days (based on Duna being closer or further from Pe at arrival) and ~ 1750 m/s dV (doesn't include dV for escape velocity, plane changes, aerobraking). A return Hohmann transit takes the same amount of time, and I think about the same dV (again, not accounting for parameters above, but aerobraking at Kerbin is more effective and escape velocity from Duna is lower, so getting back is easier).

Edited March 23, 2014 by LethalDose

[Random Tank]

Not trying to argue, but I thought you can do an IP transfer to Duna in about 65-70 days with about 1500dV? That's what mine usually are...

[LethalDose]

Not trying to argue, but I thought you can do an IP transfer to Duna in about 65-70 days with about 1500dV? That's what mine usually are...

/Shrug, I don't see that as arguing and I think you stated your counterpoint in a perfectly diplomatic way, so I'm not offended. The numbers I provided are calculated. It assumes starting at Kerbin's velocity in kerbin's orbit and ending in Duna's orbit at Duna's velocity. Like I said, I didn't include aerobraking and escape velocity (which is really never the same as kerbin's orbital velocity). My previous IP trips have been in stock, and I didn't pay a LOT of attention to travel times and calculated dV.

You say ~1500 m/s, I say ~1750 m/s, thats about a 15% difference. To me, that's not so different. To you, it's different enough to point out. /shrug thats cool.

The differences between the travel times I'd attribute to the difference between making orbit (my calcs) and entering/exiting an SoI (which is where I assume your numbers are coming from, e.g. Duna encounter in 65 days).

Given the way I design my vessels, I would prefer to over estimate the dV I need, and this estimate has worked fine for me. I'm aware of the assumptions that I used, and I thought I was clear when I stated those caveats about escape velocity & aerobreaking.

I'm sure the differences between my calculations and your experiences are valid, and your experiences are probably more valid in reference to the way you do your interplanetary transfers. If you want to provide the details of your transfers to provide some context, we can discuss it further. Personally, I'd enjoy that and it may help the OP design his Duna mission.

[technion]

When I did this a while ago...

I packed two of the small radial "life support" hexcans on the lander. That was plenty sufficient for the lander to land, get into orbit, and rendezvous with the main ship.

The main ship, once you have a carbon extractor and water purifier, used two of the small oxygen and two small water tanks. Really what this trip is about is food, I packed two standard tanks. Mind you, I only bought one Kerbal on this mission.