What fraction of launch payload mass for a manned (or animal'd) mission is attributable to life support equipment?
The fraction of launch payload of a ECLSS (environmental control and life support system) depends on a number of factors. The two most important factors to consider are:
- What type of systems make up the ECLSS? Are your systems completely regenerative or do they require consumables? Most likely the system is a combination of the two. A fully regenerative system wouldn't require extra consumables and thus would result in a lower overall subsystem mass.
- What is the mission profile and how does this influence the safety margin of the ECLS system? All human spaceflight missions are designed within a certain safety margin and thus require redundant systems, extra margins with regard to the consumables, and spare parts to fix the systems if/when they break. This is all to say that a mission to LEO which has the ability for a quick abort might allow for a lower safety margin. Such a mission could quickly abort if the ECLSS fails irreparably using an escape vehicle (i.e. Soyuz on the ISS). A mission to Mars does not have the ability for a quick abort and thus the safety margin and the ECLSS mass must increase.
Larsen and Pranke's text Human Spaceflight gives a rule of thumb of 8% dry mass for the ECLSS. This includes pressurizing, oxygen supply, controlling and revitalizing the atmosphere, and active thermal control. This was found via extrapolation from NASA mission data.