Planning Missions Using Life Support

Missions requiring life support for kerbals on board is something that needs to factor into your mission planning. Life support resources create extra mass for your ship, and that mass changes as the resources are used up/recycled. Especially on long-duration missions extending months or years, this change in mass can significantly impact the amount of Delta-V your ship is capable of, as well affect the length of your burn times for accurate maneuvers. Even short-duration missions of a few days can have an effect on maneuvers if you are looking to make very precise adjustments to your orbit.

Although this article focuses on TAC Life Support, the general concepts apply to other life support mods as well.

Basic Mission Planning

The first thing you need to do when planning for life support is determining how much you need to carry on the mission. This depends largely on your mission objectives and the life support system being used, but it all boils down to one word: consumption. Okay maybe two words: consumption rate. The amount of resources a Kerbal consumes per second works up to how much they consume per hour, per day, per week, etc etc. With TACLS, we can get these consumption rates from the LifeSupport.cfg file in the ThunderAerospace\TacLifeSupport\PluginData\TacLifeSupport directory (also accessible and configurable through the in-game GUI in the Space Center). The consumption (and production) rates are defined in units per second (u/s).

Alright so now we know how much a kerbal consumes per second of resources with mass - water, oxygen and food. How that affects the mass of your ship then depends on the mass in tonnes per unit (t/u) of these resources, which you can find via the density property of the resource as defined in the TacResources.cfg file in the ThunderAerospace\TacLifeSupport directory.

Let's take an example: Water - the densest and thus heaviest (by far) of the three main resources. By default, a kerbal will consume 0.000011188078704u/s of water, which when multiplied by the 0.001t/u density of water means that your ship is losing 0.000000011188079t per second, per kerbal. So if you have 3 kerbals in a capsule on life support for 3 Earth days (a free-return from Mun, say):

0.000000011188079 * 259200 * 3 = 0.00869985023t

0.009t worth of mass lost over three days for a small crew isn't really a significant amount, so this shows there are definitely missions in which you do not need to fully account for the affects of life support. You should always run these calculations during the early stages of your mission planning, however, to decide whether or not consideration for life support resource mass consumption should be taken into the full mission plan.

Accounting for Resources in Mission Architect

If you decide it's worth keeping track of your mass loss/gain due to resource consumption/production, Mission Architect provides the Mass Dump event, which can be used for this purpose. Specifically, entering a value into the Dry Mass field will subtract that amount of mass from the ship. Since Mission Architect does not handle resources natively, they are considered part of the ship's Dry Mass, which you should have set properly for the Initial State event (the first event) of your mission plan.

A Mass Dump event should occur prior to any Delta-V Maneuver event with a suitably long coast time between it and the last maneuver, and cover the amount of time that has passed since the previous Mass Dump event. You can get this delta time by selecting the previous Mass Dump event, right-clicking and selecting Copy UT at Start of Selected Event. Paste the value somewhere you can get it later (the Output window at the bottom works, or the Edit->Mission Notes pad, or just an external text editor). Now select the Delta-V event and get the UT time for the start of that event, or if you haven't created the Delta-V event yet, get the end UT time for the event prior to the Mass Dump. Subtract the first time from the second.

When using this delta time to determine the mass change, if you are not recycling waste material there is no need to factor them into these calculations. Waste material can be completely vented prior to the maneuver to negate any mass gained from their production.