Comparing Architectures - Pressurized vs. Unpressurized Rovers

Lunar Architecture and Technology Analysis Driven by Lunar Science Scenarios

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This study examines a hypothetical 14-day mission to the lunar surface in which astronauts use robotic rovers for transportation and as assistants for certain tasks. We compare two mission architectures: one in which pressurized rovers are used and one in which unpressurized rovers are used. Then, we explore the impact on productivity of adding margins to the times required for various activities and augmenting the list of tasks to be performed. Finally, to test the robustness of the analysis, we identify the most important parameters, vary them within a range of uncertainty, and assess the impact on overall productivity.

Site geometry

A specific set of experiments is to be accomplished at each of 5 localities, each of which is about 3 km across. Four geographical sites are covered: Shackleton Crater, Shoemaker Crater, Sverdrup Crater, and de Gerlache Crater, all of which are near the lunar south pole. Each of the 5 localities has 7 science-activity sites, for a total of 35 places where the astronauts and robots stop to conduct experiments.

Numbering of the localities is arbitrary. Site geometry, consisting of 5 localities at 4 geographical sites.

Mission architectures

This study compares the relative cost-benefit ratios of two mission architectures, subject to certain assumptions and constraints. One consists of four astronauts and two small, pressurized rovers (SPRs), and the other consists of four astronauts and two unpressurized rovers (UPRs). Both kinds of robotic rover are capable of performing certain tasks at the science-activity sites. Our study assumes they are monitored by ground control on Earth after the astronauts have set up and begun the operation, freeing the astronauts to perform other useful activities.

Example illustrations of the two robotic rovers considered in this study. The UPR (left) is designed to transport two spacesuited astronauts, with the capacity to also transport two additional astronauts in an emergency. The SPR can transport two spacesuited astronauts "chariot-style," and/or two astronauts inside the pressurized cabin, where they would not have to wear spacesuits. Tools and instruments operated by the robots are not shown.

In the reference problem, the experiments to be conducted at each locality consist of collecting samples via the following activities: (1) collecting rocks, (2) collecting regolith (fine gravel and soil on the lunar surface), (3) digging shallow trenches, (4) hammering a "drive tube" into the soil, (5) drilling soil cores, (6) drilling rock cores, and (7) raking. In addition, a Lunar Environmental Monitoring Station is to be deposited at one of the localities.

Table I. Mass and Times for Science Activities

Activity Value per site* How often performed Total goal for mission Mass per site* (kg) Astronaut EVA time required (hours per site*) Rover time required (hours per site*) Original tasks: Collect rocks 10 At all sites 35 (25 basalt, 10 breccia) Basalt: 0.5 Breccia: 5.0 0.25 n/a Collect regolith 10 At all sites 35 2 0.25 n/a Trench 8 At all sites 35 1 0.25 n/a Use drive tube 8 At all sites 35 0.7 0.25 n/a Drill soil core 8 One site per locality 5 0.9 n/a 2.5 Drill rock core 8 One site per locality 5 0.5 n/a 1.5 Rake 10 At all sites 35 0.5 n/a 0.4 Lunar environmental monitoring station 10 Once 1 n/a 4.0 hrs / 2 astronauts n/a Additional tasks: Microscopic imaging 10 One site per locality 5 n/a n/a 0.25 Lidar mapping 10 One site per locality 5 n/a n/a 1.5 Ground-penetrating radar 8 One site per locality 5 n/a n/a 2.0

* "Site" refers to science-activity site, of which there are 7 at each of the 5 localities.
  "Additional tasks" are described on the next page, "Variations and non-intuitive solutions."

Productivity definition

The objective of this study is to determine the relative productivity ratios of the UPR and SPR scenarios along with the drivers thereof, with productivity defined as the value of the activities performed divided by the relative marginal cost (expressed in equivalent work-hours) of completing those activities. Only operational costs are considered. Since we assume that both unpressurized and pressurized rovers will be developed for NASA's lunar exploration program at some point, we do not take development costs into account here. The cost of transporting the rovers to the Moon is also excluded because that expense is so much greater than the relative operational costs that any difference in operating the two kinds of rovers would be insignificant. Ancillary infrastructure costs such as those associated with communication with Earth are similarly excluded.

Relative operational costs of astronauts working in EVA (Extra-Vehicular Activity -- i.e., working outside in spacesuits) or IVA mode (Intra-Vehicular Activity -- i.e., operating the SPR from within its pressurized cabin without wearing spacesuits), and of robots operating under local control (i.e., control by the astronauts on the lunar surface, which was not used in this study) or ground control (i.e., remote control by people on Earth), are shown below. These relative costs are adapted from estimates of conducting activities on the International Space Station (ISS) which are assumed for purposes of this analysis to be comparable to the activities posited for the lunar mission under study.

A unit of time that would cost $57 in EVA mode would cost $14 in IVA mode, $15 in robot local-control mode, and $17 in robot ground-control mode. The cost weight for "robot under local control" does not include the cost of the astronauts, which is calculated as EVA or IVA, depending on whether the astronauts are outside or inside a pressurized cabin while monitoring the robots.

The much higher cost of EVA mode, in which astronauts work in spacesuits, results principally from the additional ground crew required to monitor the astronauts while in this mode, the requirement that the astronauts break the suits down and refurbish them after every 24 hours of use, and the extra expendables (oxygen, water, etc.) that would be consumed while wearing the suits.

Our optimization tool, HURON, is instructed to maximize completion of all required activities and to maximize productivity of the selected sites and activities, subject to the EVA and rover performance specifications and the constraints on astronaut and rover activities, with an end state of all astronauts in the habitat module (the lander).

Results

HURON calculated itineraries for astronauts working with each of the two kinds of rovers, given the assumptions and constraints, and found that the UPR scenario's value/cost ratio is about 4, whereas that of the SPR scenario is about 13. Thus, in our study's model, the SPR scenario delivers about 3 times the productivity of the UPR scenario. A summary of these results is given below.

It is important to note that the SPR scenario does not fill the 14 days (12 work days plus one day each for arrival and departure) that were envisioned for the mission. It needs only 5 days, 11 hours to complete all of the tasks, and one 2-day rest/recharge period.