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Example: Mission to Duna
Your kerbals have gotten bored with exploring the Mun and Minmus and they want to visit another planet. Wernher determines that Duna is the best choice. He calculates that the round trip will last 1150 days and asks you to plan the life support requirements for a crew of three.
Each kerbal will consume 10.8 kg of Supplies per Kerbin-day. (Supplies represent all of the water, oxygen, and food that kerbals need to survive.) That means the trip will require 10.8 kg of Supplies per kerbal per day * 3 kerbals * 1,150 days = 37,260 kg of Supplies, or 37.26 metric tons (not counting the containers to hold them)! Surely we can do better than that!
If we bring along a science lab, we can borrow some of its chemistry equipment to recycle most of our life support materiel, greatly reducing the Supplies we need to bring.
The MPL-LG-2 includes a life support recycler rated for 50% efficiency for up to four kerbals. That means it reduces our three kerbals’ consumption of Supplies by 50%. We now need only 50% of what we needed before: 37.26 tons * 50% = 18.63 tons.
Since the lab itself weighs 3.5 tons, we’re now planning to haul 22.13 tons of equipment and Supplies to keep our crew alive. That means we’re already saving 37.26 - (18.63 + 3.5) = 15.13 tons! Just remember to switch on the Life Support function for the lab once the kerbals are aboard - it consumes 0.26 ElectricCharge per second.
We can still do better, though…
If we bring along a greenhouse, our kerbals can grow their own Supplies during the trip using the Mulch they produce (from consuming Supplies) and a little bit of fertilizer. This will allow us to reduce our mass requirements even more.
The Nom-O-Matic 5000 can produce up to 2.8512 kg of Supplies per day from 2.59 kg of Mulch and 0.26 kg of Fertilizer. The Mulch we get for “free” since our kerbals produce it as waste. That means that bringing along Fertilizer to grow Supplies reduces our mass by a factor of 2.8512 / 0.26 = 11. So instead of bringing along 18.63 tons of Supplies, we can instead bring along 0.89424 tons of Fertilizer.
Our total life support mass is now:
- 3.5 tons for the science lab, for its recycler
- 0.154 tons for the greenhouse
- 0.89424 tons of
Fertilizer
You’ll also need to bring along a handful of Supplies so that the crew can make some Mulch for the greenhouse to start working. And don’t forget to to turn on the greenhouse, either! (The button is labeled “Agroponics”.) It will consume 0.26 ElectricCharge per second.
The crew has reluctantly accepted that they’ll be eating food farmed from their own waste, but they’re not willing tolerate cramped quarters for the trip - it’s more than two and a half years long, after all. They’ll need some space to stretch their legs.
A kerbal can sit in her crash couch for 30 days before getting grumpy. The PPD-10 Hitchhiker Storage Can has additional habitation space above and beyond just its seats: 21 kerbal-months (or 630 kerbal-days) per can plus four kerbal months (120 kerbal-days) for its four seats, yielding a total of 750 kerbal-days . Our crew requires 3 kerbals * 1,150 days = 3,450 kerbal-days of habitat space to avoid getting grumpy. That means we’d need 3450 / 750 = 4.6 or five hitchhiker cans to give our crew enough space.
Again, we can improve on this…
Some activities really help kerbals relax and forget about the stresses of being cooped up in a spaceship. They seem to love staring out the window, for example - the bigger the better. Happily, we have some very big windows in the Cupola Module, which acts as a multiplier for our habitat space.
The PPD-12 cupola has a hab multiplier value of 1.65. Hab multipliers are additive, and the vessel has an intrinsic multiplier of 1; so if a vessel has one cupola, its hab space is 2.6 times more effective. That means that for each cupola we include, we reduce the number of hitchhiker cans we need.
Here are some configurations that will give us at least 3,450 kerbal-days of total habitation time:
| Cupola | Hitchhiker | Kerbal-Days | Mass(T) | % Days Over or (Under) |
|---|---|---|---|---|
| 0 | 5 | 3,750.0 | 12.5 | 300.0 |
| 1 | 2 | 3,975.0 | 6.8 | 525.0 |
| 2 | 1 | 3,225.0 | 6.1 | (225.0) |
| 3 | 1 | 4,462.5 | 7.9 | 1012.5 |
In this case, having one cupola and two hitchhiker cans will save us the greatest mass (6.8T compared to 12.5T). You may prefer another configuration - for example, having one hitchhiker but three cupolas provides 29% extension in habitation for still less mass.
The formula for the winning solution above is
(Kerbal-Days for base part) * sum of all Kerbal multipliers
(2 hitchhikers * 750 days per hitchhiker) * (1 for base part + (1 cupola * 1.65 multiplier per cupola))
(1500 days) * (2.65) = 3,975 days
Using the two-can-one-cupola configuration above, the Kerbals will start with 3,975 kerbal-days of Hab time and Home time. Home time will decrease by 3450 kerbal-days over the course of the mission.
If the mission plan calls for a separate lander, note that the lander will need to provide enough Hab time to carry the kerbals down to the surface and return to the main ship in orbit. Home time will be unaffected by the time spent on the lander. Hab time will reset upon returning to the main ship.