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Advanced Life Support Research and Technology Development Metric – Initial Draft Introduction Basic Metric Formulation The Advanced Life Support Research and Technology Development Metric, or Metric, is one of several measurement devices, or metrics, which will be employed by the National Aeronautics and Space Administration (NASA) to measure the Advanced Life Support (ALS) Program’s progress. Because any measure must have a baseline, whether explicitly defined or implied, the Metric is a comparison between a selected advanced life support system (ALSS) and an equivalently detailed system based on technology from the Environmental Control and Life Support System (ECLSS) for the International Space Station (ISS). More specifically, the Metric is the ratio of the equivalent system mass (ESM) for a life support system using the ISS ECLSS technologies divided by the ESM for an equivalent life support system using the “best” ALS technologies. As defined, the Metric should increase in value as the ALS technologies become less massive, less power intensive, and less voluminous. Here “best” is defined as the ALSS which, at the time of the Metric evaluation, provides the Metric with the highest value. Also, the ALSS technologies used here are not flight-certified; rather, they are in many cases much less mature. The uncertainties associated with less mature equipment add additional complicating factors to the ALSS technology selection process. While this may seem arbitrary, this process actually encourages the ALS Program to research more than a single technology for each life support function and then select the most appropriate for a particular mission, which is similar to the actual process used by mission planners. Also implied in this Metric formulation is an underlying mission for which each life support system might be evaluated. Currently the Design Reference Mission (DRM) of the NASA Mars Exploration Study Team (Hoffman and Kaplan, 1997, and Drake, 1998) provides an appropriate mission scenario both because of its relevance and as an appropriate challenge for the ALS community. For those life support functions which a particular set of life support technologies cannot provide, an open loop approach is assumed. Future versions of the Metric will consider various life support approaches, including supply from Earth, physicochemical and biological regenerative technologies, and in situ resource utilization, on their own merits. Description of Equivalent System Mass Equivalent system mass (ESM) is a technique by which several physical quantities which describe a system or subsystem may be reduced to a single physical parameter, mass. The primary advantage is to allow comparison of two life support systems with different parameters using a single scale. This is accomplished by determining appropriate mass penalties or conversion factors to convert the non-mass physical inputs to an equivalent mass. For systems which require power, for example, the power system can yield an appropriate power-mass penalty by dividing the average power plant output by the total mass of the generating power system. Thus, for a nuclear power system on an independent lander which, on average, delivers 100 kW of electrical power and has a overall mass of 8,708 kg (Mason, et al., 1992) 1 the power mass penalty is 11.48 W/kg. This power-mass penalty effectively assigns a fraction of the power system mass to a power-using subsystem in place of that subsystem’s power requirement. In like manner, mass penalties to account for heat rejection and volume within a pressurized shell are defined. Work is also in progress to define a crew-time-mass penalty to convert maintenance time to mass, but the derivation for this conversion factor is not as obvious. A form developed by Drysdale (1998) is used in this work. The definition of equivalent mass for a system is the sum of the equipment mass plus the power, volume, thermal control, and crew time as masses. Background Mars Reference Baseline As per the DRM (Hoffman and Kaplan, 1997, and Drake, 1998), a vehicle with a rigid shell, the Mars Transfer Vehicle (MTV), transports the crew from Earth orbit to the surface of Mars. The MTV, once on the surface of Mars, is incorporated into the surface habitat. In addition to the MTV, the surface habitat includes an additional dedicated lander which provides some additional living volume for the crew during the mission’s exploration phase. The additional pressurized volume used solely on the surface employs an inflatable structure, as proposed for the TransHab 2 program, as per the DRM. A separate Earth Return Vehicle (ERV), which is initially deployed ahead of the crew, transports the mission personnel back to Earth. Thus, two long-duration life support system hardware sets were assumed for each mission. The life support systems employed during descent to Mars, ascent from Mars, which uses a separate vehicle in the current DRM, and within the Earth-return capsule are omitted. Such systems, especially within the ascent vehicle and Earth-return capsule, are expected to operate for no longer than a few days and, therefore, are likely to use expendable, open-loop technologies. Alternate assumptions might be made, including use of a reusable transit vehicle, and a single shipset for the surface. However, the annual resupply mass (consumables) is about a third of the fixed mass, so this might not be economical. The DRM defines the overall mission as: 1 The actual mass quoted here has been adjusted slightly to account for some differences between the work listed in the reference and the desired system. 2 The TransHab is a concept to create an inflatable module with greater volume than a standard International Space Station (ISS) rigid module while using no more volume during launch than is currently available within the Space Transportation System (STS) Orbiter payload bay. Effectively the TransHab has a non-rigid outer structure which is inflated or deployed once the vehicle reaches orbit. Internal outfitting is accomplished by deploying folded structures and/or by moving appropriate equipment into the shell’s volume once it deployed. Drake (1998), page 43, provides a picture of a transfer vehicle with a TransHab crew compartment. Number of Crew per Mission 6 Transit Duration 180 days (nominal) Surface Duration 600 days (nominal) Number of Missions to One Site 3 Metric Baseline Assumptions For both life support systems, a habitable volume of 50 m³ per person has been included. This is primarily a pressure-vessel cost, and there is a difference between ISS technology (0.015 m³/kg), and an inflatable structure such as TransHab, (0.43 m³/kg). An ISS pressure vessel is assumed for interplanetary transit while inflatable structures are assumed for the additional habitable volume provided on the surface. Makeup gas for leakage and the mass for filling the volume once have been included for ISS. For the advanced life support system (ALSS) case, all gas masses are assumed to be included in the mass of gas provided. Crew time is assumed to be 50 hrs of time devoted to mission goals per crew member per week. Additional time will be required for base operations. Assuming the most significant mission work is done on the surface of Mars, crew time here is only counted for days spent on Mars. Thus, a crew of six will provide 76,932 hours over three missions. No provision has been made for contingencies. Further, radiation shielding is omitted, as the requirement is ill-defined at present and the penalty is expected to be similar in both cases. Finally, support for extravehicular activities (EVA) and their associated airlock operations have also been omitted. While EVA loads on a life support system (LSS) are far from negligible, further study is necessary to define the actual EVA loads. Thus, while this is a useful estimate of technological capability, it does not directly represent a good estimate for the LSS initial mass in low Earth orbit (IMLEO). Waste disposition is limited to overboard jettison for the ALSS scenario. ISS returns waste to Earth, using either a soft landing on-board the Space Transportation System (STS, or Shuttle) or incineration during re-entry. However, this is believed to be a relatively small item, and is not addressed in these estimates. International Space Station Environmental Control and Life Support System Technology Baseline The numerator of the Metric is an ESM estimate of a LSS for a mission to Mars based on technology from the ISS ECLSS and the DRM. Background ISS ECLSS Technology Information The ISS ECLSS technology is defined in various documents (such as Carrasquillo, et al., 1997), often at a high level of detail The values here have been adjusted to remove any overcapacity as this is a form of contingency. EVA and airlock related items have also been removed. Appropriate duration and infrastructure equivalencies have also been applied. The assessment presented is based on the ECLSS of the U. S. On-Orbit Segment of the ISS (USOS). Fixed masses, mostly equipment, and time dependent masses, mostly consumables, are identified separately and applied as appropriate to the mission. Note that the ISS baseline for food is taken from Bourland (1998), though Hanford (1997) provides a similar value. ISS ECLSS Mission Definition Data Number of Crew 4 USOS only Nominal Duration 3,650 days at the same site, or 10 Earth years. Location LEO ISS ECLSS Infrastructure Costs Factor Value Units Comments Mass Delivery Factor 2 kg packaged For components requiring packaging /kg unpackaged (food and clothing) Pressurized Volume 0.015 m³/kg ISS common module; No shielding or secondary structures Power 11.4 W/kg Nuclear power; Based on SP100 Program Heat Rejection 25.4 W/kg Crew Time 2 person•hr/kg A rough estimate Derived Costs for Mission Energy 492.48 kWh/kg Heat Rejection 3,950.2 kWh/kg References: Hanford (1997) and Hanford and Ewert (1996).

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