First Lunar Outpost Surface Habitation Phase Crew Time Analysis

Lockheed Engineering and Sciences Company
Contract NAS9-17900
Job Order K4-H13

Prepared by:
P. D. Campbell, Principal Engineer
Flight Crew Support Department

Approved by:
J. D. Harris, Operations Manager
Flight Crew Support Department

For:
Flight Crew Support Division
National Aeronautics and Space Administration
Lyndon B. Johnson Space Center
Houston, Texas

December 1992

LESC-30401

Foreword

This document was produced by the Lockheed Engineering and Sciences Company, Flight Crew Support Department for the NASA Johnson Space Center, Flight Crew Support Division, Human Factors Project Office. Questions and comments concerning the document should be directed to Paul D. Campbell, Lockheed, (713) 483-9948.

Section

Abstract iv

1.0 Introduction 1
1.1 Background 1
1.2 Purpose 1
1.3 Scope 2
1.4 Approach 2

2.0 Model Development 3
2.1 Reference Information 3
2.1.1 Reference Mission 3
2.1.2 Requirements 5
2.1.3 Additional Assumptions 8
2.2 Model Content 11
2.2.1 Structure 11
2.2.2 Inputs 12
2.2.3 Algorithms 15
2.3 Output Formats 18

3.0 Analysis Results 21

4.0 Conclusions 29
4.1 Programmatic 29
4.2 Mission 29
4.3 Future Work 30

5.0 References 31

Figures

Number Title Page

2.2.1-1 Crew Time Model Structure 11
2.3-1 Crew Time Model Graphical Output Sample 20
3.0-1 Time on Lunar Surface Versus Crew Size 23
3.0-2 Work Times Versus Crew Size 24
3.0-3 Sensitivity of Total Science Time to Model Inputs 25
3.0-4 Science Times versus Number of Days Between EVAs 26
3.0-5 Science Times versus Length of EVA Period 27
3.0-6 Science Times versus Number of Crewmembers on EVA 28

Tables

Number Title Page

2.1.3-1 FLO Surface Habitat Maintenance Parameters 10
2.1.3-2 FLO Surface Habitation Phase Health Care Crew Times 10
2.3-1 Crew Time Analysis Model User Interface. 19

Abstract

The First Lunar Outpost (FLO) mission has been defined by the NASA Office of Exploration as the potential return of Americans to the moon for the first time since the Apollo program. A primary resource provided by the FLO crew is useful work in pursuit of the mission objectives.

This resource was analyzed to determine how much crew time may be available for useful work on the Lunar surface. An automated tool was developed to support sensitivity analysis in order to examine the effects of varying operational parameters of the mission.

1.0 Introduction

1.1 Background

The Space Exploration Initiative (SEI) has been defined by the United States government as a long-term effort to explore the Earth's moon and the planet Mars. The SEI spans multiple programs and missions to send robotic and human spacecraft to the moon and Mars. The First Lunar Outpost (FLO) mission has been defined as an initial return of humans to the moon. The baseline for FLO includes a crew of four and a Lunar stay time of two Lunar daylight periods and one Lunar darkness period.

The Human Support function of the FLO study team has been established to perform integration of all aspects of the FLO mission which affect the crewmembers. The Human Support area includes medical, life sciences, and human factors engineering aspects. The NASA JSC Human Factors Project Office has participated in the FLO definition study, and as part of that participation a model of the FLO crew time resource was developed.

1.2 Purpose

Human factors analysis of the FLO is essential during the mission/systems definition phase to quantify and mitigate the uncertainties related to the crew's ability to perform the desired mission. A significant part of this analysis is validation of the availability of crew time to perform useful work such as mission science. This analysis integrates the operational aspects of the crew's mission with the design aspects of the systems which support them.

Crew size and Lunar stay time are important mission parameters in determining the sizing and design of systems and the ultimate cost of a flight program. Modeling of crew time provides insight into the effect of varying these parameters on accomplishment of the mission objectives.

FLO system effectiveness is influenced by the hardware/software system design and by inherent human capabilities and constraints. Significant crew constraints exist in the use of their time; therefore, crew time modeling can be an important input to overall system effectiveness analysis.

Operational planning for the FLO mission includes the allocation of mission functions to both crew and systems. In that allocation, it is vital that crew time is managed to prevent over-allocation of this limited resource.

1.3 Scope

The analysis described in this document is focused on the Lunar surface habitation phase of the FLO human mission. It is further constrained to the nominal planned mission, and does not investigate abort scenarios. It attempts to encompass all aspects of crew activities during the nominal surface habitation phase.

1.4 Approach

This study was approached as an analysis of the effects of varying FLO mission parameters. The method of study consisted of the following:

Gathering of information on crew operational activities and times,
Integration of the information in the form of a parametric model,
Performance of sensitivity analyses by varying model parameters.

2.0 Model Development

2.1 Reference Information

2.1.1 Reference Mission

The FLO Design Reference Mission (DRM) is currently documented in the First Lunar Outpost Requirements and Guidelines (FLORG) document1. The following information is extracted from the DRM:

The DRM identifies the automated delivery, activation, and deployment of the crew surface habitat prior to crew launch from Earth. The crew of four is able to move into the habitat and start science and exploration activities as soon as possible after they land on the moon.

The crew vehicle lands around Lunar dawn within walking distance of the outpost. The crew lives in the surface habitat for up to a Lunar day-night-day cycle (45 days including contingencies).

The crew's first priority is to complete habitat activation and checkout. They then perform science and engineering activities including environmental characterization, deployment of experiments, in-situ resource utilization, and life sciences research.

The crew uses an unpressurized rover to support extravehicular activity (EVA) traverses on the Lunar surface. The capability is provided for daily EVAs. During Lunar night, EVAs will be performed when lighting is sufficient. The bulk of the intravehicular activity (IVA) laboratory research will be performed when EVA is not possible.

When the second Lunar sunset approaches, the crew will configure the outpost for their departure and leave the habitat for Earth. As an option, a second crew of four will revisit the outpost for a second 45 day stay, performing activities similar to the first crew's.

A second document, the FLO Conceptual Surface Mission2, has been produced to describe additional details of the FLO crew surface operations phase. The following statements are drawn from this document:

The optimum times to conduct EVAs on the Lunar surface are shortly after sunrise and shortly before sunset.

Laboratory IVAs include activities such as basic analysis, sorting, and packaging of samples for return to Earth; gravitational biology experiments and additional physiological experiments; and teleoperation of deployed instruments or scientific rovers.

Support IVAs include activities such as outpost maintenance and enhancement, EVA suit maintenance and repair, and training.

The primary monitoring of an EVA crew is done from Earth, allowing the IVA crew to devote their time to other tasks.

Support EVAs include activities such as outpost maintenance and enhancement, rover maintenance and repair, and engineering/operations detailed test objectives.

Exploration EVAs include activities such as geologic field work, emplacing astronomical and space physics instruments, and emplacing and operating resource utilization equipment.

The first week of the surface mission is characterized by transfer, move in, and start up activities...

The final week of the surface mission... is characterized by transitioning the assets that have been emplaced on the moon to operate in an unmanned mode and preparing items for return to Earth.

During the 42 day surface mission, 29 EVAs are planned with roughly 22 EVAs dedicated to exploration and seven EVAs allotted for support.

2.1.2 Requirements

The FLORG contains several specific statements which are useful in developing a crew time model. Guidelines and assumptions include:

Science and engineering goals include initiation of environmental characterization, exploration, instrument operations, demonstration of the use of local materials, enhancement of human capabilities, EVA technology testing, human performance evaluation, and life science research.

Life cycle cost will be minimized.

Activities which do not provide direct science and exploration returns will be minimized.

Piloted landings will occur within four days following Lunar sunrise at least one kilometer from the outpost.

EVAs will not exceed ten hours in duration.

A crewmember will not be scheduled to perform EVAs on consecutive days.

Related system requirements include:

The lander shall provide the capability to deliver five metric tons of cargo with a nominal crew to the Lunar surface.

The surface segment shall provide the capability to support 34 EVAs (estimated) per mission inclusive of contingency.

The habitation subsegment shall provide the capability to remain dormant for up to To Be Determined (TBD) months.

The habitat shall provide the capability to monitor crew health.

The habitat element shall provide crew sleeping accommodations.

The habitat element shall provide the capability for the crew to maintain habitat hygiene.

The habitat element shall provide the capability for the crew to maintain personal hygiene.

The EMU element shall provide the capability for 17 (estimated) EVAs per nominal mission inclusive of contingency.

The science and engineering subsegment shall provide the capability to perform research activities on the Lunar surface.

The science and engineering subsegment shall provide the capability to perform engineering demonstrations on the Lunar surface.

The integrated logistics subsegment shall provide an integrated capability for maintenance and supply of equipment and materials to support FLO systems.

Crew health maintenance and medical care shall be provided for all crewmembers during all mission phases.

Crew accommodations required to support life and contribute to mission success shall be provided.

The First Lunar Outpost Detailed Assumptions document3 provides additional, lower level requirements which were used in formulating the FLO crew time model:

The crew will visit the habitat as soon as possible after landing to verify its integrity and initiate any necessary corrective procedures.

A method of logistics resupply of the habitat shall be provided.

A method shall be provided for preparing trash for disposal.

It shall be possible for the habitat to be placed in a stand-by mode by one crew person in TBD hours.

It shall be possible for one person to operate the habitat.

The habitation subsegment shall include the capability to service and maintain all habitation subsegment components, both internal and external.

The storm shelter shall be capable of supporting a crew of four for three to seven days (estimated).

At least two crewmembers will perform nominal EVAs.

The crew will include at least two trained scientists.

A pressurized laboratory area shall be provided for science activities.

The outpost may be in storage mode for up to 12 months between crew visits.

The crew follows a regular Earth circadian rhythm following one work shift.

Three one-hour meal periods per day are provided, with exceptions.

Crew sleep periods last eight hours, and pre-sleep and post-sleep activity periods last one hour each.

EVA periods last no more than ten hours.

Each crewmember receives one day off out of seven mission days.

All crewmembers have their days off at the same time.

The four crewmembers are paired into two two-person teams: all four are EVA qualified.

After a team performs a full EVA, they are not permitted without waiver to go EVA again for two days.

Typical habitat-based EVA operations have no less than two EVA personnel at a time, with one IVA in a support role (this contradicts other assumptions that the IVA crew will not provide continuous support to EVAs).

EVA preparation and post-EVA procedures last no more than one hour.

EVAs may be performed on consecutive mission days utilizing alternate pairs of crewmembers.

During the first several days of the surface mission, every crew transit from the lander to the habitat involves cargo transfer, with the most critical items being carried first.

Limited EVAs are planned on the days surrounding local noon, EVAs occurring then will be foreshortened to accommodate the increased demand on consumables.

The EMU element shall require no more than one hour maintenance for every ten hours of nominal operation.

The PLSS element shall require no more than one (estimated) hour maintenance for every ten hours of nominal operation.

2.1.3 Additional Assumptions

Other assumptions which were useful in this analysis are contained in this section of the document.

The outpost will contain a number of maintainable components. It is assumed that beginning of life failures (infant mortality) will occur and be repaired during integrated system verification testing on Earth prior to outpost launch. Therefore, outpost maintenance will reflect steady-state failure rates during the nominal crew mission. Based on a comparison to Space Station Freedom4, the FLO habitat is assumed to include maintainable units as described in Table 2.1.3-1.

As a first approximation, it was further assumed that there would be no false maintenance (unneeded crew maintenance actions) and that all corrective maintenance actions are successful when the crew performs them.

It is assumed that the number of EVAs is to be maximized by means of crew scheduling. It was also assumed, however, that for odd crew sizes, EVA teams would be of equal sizes and the single (odd) crewmember not assigned to an EVA team would perform IVA every day.

No Lunar thermal or lighting constraints on the number of EVAs were assumed, although it is known that these will be site-dependent factors in determining when EVAs can be accomplished.

Health care time estimates5 which were used to generate the crew health care time allocation are shown in Table 2.1.3-2.

Table 2.1.3-1 FLO Surface Habitat Maintenance Parameters
Habitat intravehicular maintainable units:	1061
Habitat extravehicular maintainable units:	197
Average mean time between failure of all units:	50000 hr

Table 2.1.3-2 FLO Surface Habitation Phase Health Care Crew Times
Operational Health Care:

Exercise: 112 mhr, assuming 2 hr/day, 6 day/wk for crewmembers when not performing EVA or physiologic monitoring; 45 days, 32 EVAs of 2 crew and 2 EVAs of 4 crew

Private medical conference: 11.25 mhr, assuming one 0.25 hr call per day, rotating among crewmembers

Physical exam: 52 mhr, assuming a 2 hr exam with medical crewmember attending 3 times per mission and a 0.5 hr exam prior to leaving the habitat for the return to Earth

Physiologic Monitoring and Countermeasure Investigations: 168 mhr, assuming a 3 hr session with each crewmember and a monitor every 5 days

Total: 112+11+52+168=343 mhr per surface mission

Average time per crewmember per day on the Lunar surface: 2.0 hr

Table 2.1.3-2 FLO Surface Habitation Phase Health Care Crew Times
Operational Health Care:
Exercise:	112 mhr, assuming 2 hr/day, 6 day/wk for crewmembers when not performing EVA or physiologic monitoring; 45 days, 32 EVAs of 2 crew and 2 EVAs of 4 crew
Private medical conference:	11.25 mhr, assuming one 0.25 hr call per day, rotating among crewmembers
Physical exam:	52 mhr, assuming a 2 hr exam with medical crewmember attending 3 times per mission and a 0.5 hr exam prior to leaving the habitat for the return to Earth
Physiologic Monitoring and Countermeasure Investigations:	168 mhr, assuming a 3 hr session with each crewmember and a monitor every 5 days
Total: 112+11+52+168=343 mhr per surface mission
Average time per crewmember per day on the Lunar surface: 2.0 hr

2.2 Model Content

The following subsections of the document address the automated tool developed to support crew time sensitivity analysis. They discuss the model's structure, inputs, algorithms, and outputs.

2.2.1 Structure

Crew time was structured as shown in Figure 2.2.1-1 for the purposes of developing the model. The most basic distinction made is that of crew self-maintenance versus crew work.

Self-maintenance was defined to include the IVA activities necessary for the crew to survive, along with other non-work activities such as off-duty recreation.

Work was defined as being composed of science, maintenance, and overhead activities. Each of these includes both an EVA and an IVA component. Maintenance was defined as all the activities required to service and repair the outpost elements. Overhead was defined as all the activities required to conduct the mission which are not specifically science or maintenance. This includes items such as crew monitoring and control of the outpost elements, planning and scheduling of their activities, communication with other personnel, logistics, housekeeping, and IVA support of EVAs.

Figure 2.2.1-1 Crew Time Model Structure

2.2.2 Inputs

The current FLO crew time model includes 26 inputs, which are structured according to the time breakdown described above. Each input is defined below along with a range of possible values and a default value:

Stay time (Nstay): the number of days the crew inhabits the outpost (range: 1 to 90, default: 42)

Revisit interval (Nrevisit): the number of months between crew arrivals (range: 3 to 12, default: 6)

Number of crewmembers per habitat (Ncrewperhab): the maximum number of individuals who may occupy a single outpost habitat element (range: 1 to crew size, default: 4)

Work days per week (Nworkdaysperweek): the number of days during a seven day period on which the crew follows the normal work schedule; the remainder being crew rest days on which the crew performs only light duty (range: 1 to 7, default: 6)

The following items are grouped as IVA self-maintenance:

Meal time per day (Tmeals): the number of hours allocated to crew meals, including preparation, eating, and cleanup (range: 1 to 5, default: 3)

Post-sleep time per day (Tpostsleep): the number of hours allocated to crew personal activities immediately following the sleep period, not including the morning meal (range: 0.5 to 1.5, default: 1)

Pre-sleep time per day (Tpresleep): the number of hours allocated to crew personal activities immediately prior to the sleep period, not including the evening meal or dedicated off-duty time (range: 0.5 to 1.5, default: 1)

Personal hygiene and waste management time per day (Tpershyg): the number of hours allocated for crewmember hygiene during the day (range: 0.25 to 1, default: 0.5)

Off duty time per day (Toffduty): the number of hours allocated to crewmember recreation following the normal work day (range: 0 to 2, default: 1)

Health maintenance time per day (Thealthmaint): the number of hours allocated to crewmember exercise, health monitoring, and medical care (range: 1.5 to 2.5, default: 2.0)

The following items are grouped as IVA overhead:

IV Planning and scheduling time per day (Tivplan): the number of hours allocated to each crewmember to plan activities (range: 0.25 to 1, default: 0.5)

IV resupply time per day (Tivresupp): the number of hours allocated to moving consumables into habitat storage from logistics carriers (range: 0.25 to 1, default: 0.25)

IV housekeeping time per day (Tivhk): the number of hours for each crewmember to perform cleaning of the habitat interior (range: 0.25 to 1, default: 0.5)

IV overhead per EVA per EVA crewmember (Tivpereva): the number of manhours dedicated to supporting an EVA crewmember, including both real-time IVA support and pre- and post-EVA support and EMU servicing (range: 2 to 10, default: 2)

The following items are grouped as corrective maintenance:

Number of EV replaceable components per habitat (Nevhaboru): the total number of maintainable units on the exterior of each habitat (range: 500 to 1000, default: 197)

Number of other EV replaceable components (Nevoru): the total number of maintainable units on the exterior of each habitat (range: 500 to 1000, default: 197)

Average mean-time-between-failures of all replaceable components (Tmtbf): the mean number of hours between maintenance actions for the entire set of replaceable units on each habitat, assuming steady-state failure rates (range: 50,000 to 100,000, default 75,000)

Average time per IV maintenance action (Tivmaintaction): the mean number of manhours required to perform an IV maintenance action (range: 0.5 to 2, default: 1)

Average time per EV maintenance action (Tevmaintaction): the mean number of manhours required to perform an EV maintenance action (range: 1 to 4, default: 2)

The following items are grouped as EVA inputs:

Length of an EVA (Teva): the number of hours defined as a single EVA period (range: 4 to 10, default: 8)

Number of crewmembers involved in each EVA (Nevacrew): the number of individuals on an EVA team (range: 1 to crew size, default: 2)

Number of days between EVAs for each crewmember (Ndaysbetweeneva): the number of IVA days a crewmember experiences between any two EVAs (range: 0 to 2, default: 1)

2.2.3 Algorithms

The model includes a variety of output parameters which are calculated based on the previously described inputs. The algorithms which generate the outputs are described below:

Number of habitats required to house the crew: the quantity of equal-size habitable elements required for the given crew size and habitat crew size. This is an intermediate result needed to determine the total number of outpost components which the crew must maintain.

Nhab=Integer((Ncrew/Ncrewperhab)+0.999)

Crew work time per day: the number of hours in a normal work day, excluding all crew self-maintenance activities.

Tworkperday=24-Tsleep-Tmeals-Tpostsleep-Tpresleep-Tpershyg-Toffduty-Thealthmaint

Total crew time on Lunar surface per flight: the number of manhours spent by the crew during the surface habitation phase of the mission.

Ttotal=24*Ncrew*Nstay

Total crew self-maintenance time per flight: the number of manhours spent by the crew on self maintenance activities during the surface habitation phase of the mission.

Tselfmaint=Ncrew*Nstay*(Tsleep+Tmeals+Tpostsleep+Tpresleep+Tpershyg+Toffduty+Thealthmaint)

Total crew rest day time per flight: the number of lost work manhours spent by the crew on rest days during the surface habitation phase of the mission.

Trestday=Ncrew*(7-Nworkdaysperweek)*Integer(Nstay/7)*Tworkperday

Total available crew work time per flight: the number of manhours spent by the crew on work activities during the surface habitation phase of the mission.

Ttotalwork=Ncrew*Tworkperday*Integer(Nstay/7*Nworkdaysperweek)

Number of corrective maintenance actions per flight: the total number of intravehicular and extravehicular corrective maintenance actions during the surface habitation phase of the mission.

Nmaintactions=((30*24*Nrevisit))/Tmtbf*(Nivoru*Nhab)+((30*24*Nrevisit))/Tmtbf*((Nevhaboru+Nevoru)*Nhab)

Total intravehicular and extravehicular corrective maintenance time per flight: the total crew time spent on intravehicular and extravehicular corrective maintenance actions during the surface habitation phase of the mission.

Ttotalmaint=((30*24*Nrevisit))/Tmtbf*Nivoru*Nhab*Tivmaintaction+((30*24*Nrevisit))/Tmtbf*(Nevhaboru+Nevoru)*Nhab*Tevmaintaction

Available IVA work time per flight: the total number of manhours available for IV crew work during the surface habitation phase of the mission.

Tivwork=Ttotalwork-(Ntotaleva*Teva*Nevacrew)

IVA corrective maintenance time per flight: the total number of manhours required to perform intravehicular maintenance during the surface habitation phase of the mission.

Tivmaint=((30*24*Nrevisit))/Tmtbf*Nivoru*Nhab*Tivmaintaction

IVA overhead time per flight: the total number of manhours required for the crew to perform intravehicular systems operations during the surface habitation phase of the mission, specifically excluding science activities.

Tivoverhead=((Ttotalwork-(Ntotaleva*(Teva*Nevacrew)))*(Tivccc+Tivplan+Tivresupp+Tivhk)/(Tworkperday))+(Tivpereva*Nevacrew*Ntotaleva)

Available IVA science time per flight: the total number of intravehicular manhours available to perform science during the surface habitation phase of the mission.

Tivscience=Tivwork-Tivmaint-Tivoverhead

Available number of EVAs per flight: the total number of EVAs which can be performed based on crew schedule constraints during the surface habitation phase of the mission.

Ntotaleva=Integer(Nstay/7*Nworkdaysperweek)*Integer(Ncrew/Nevacrew)/(1+Ndaysbetweeneva)

Available EVA work time per flight: the total number of manhours available based on crew schedule constraints during the surface habitation phase of the mission.

Tevwork=Ntotaleva*(Teva*Nevacrew)

EVA corrective maintenance time per flight: the total number of manhours required to perform extravehicular maintenance during the surface habitation phase of the mission.

Tevmaint=Nmaintevas*(Teva*Nevacrew)

Number of corrective maintenance EVAs per flight: the number of dedicated EVAs required to perform the extravehicular corrective maintenance actions.

Nmaintevas=Integer(((30*24*Nrevisit))/Tmtbf*Nhab*(Nevhaboru+Nevoru)*Tevmaintaction/(Teva*Nevacrew)+0.999)

Number of logistics EVAs per flight: the number of dedicated EVAs required to perform the extravehicular logistics activities during the surface habitation phase of the mission.

Nlogevas=Integer((Tevlog/(Teva*Nevacrew))+0.999)

Available EVA science time per flight: the total number of manhours available for EVA science based on crew schedule constraints during the surface habitation phase of the mission.

Tevscience=((Ntotaleva-Nmaintevas)*(Teva*Nevacrew))-Tevlog

Available number of science EVAs per flight: the number of dedicated EVAs available for science based on crew schedule constraints during the surface habitation phase of the mission.

Nscienceevas=Integer(Ntotaleva-Nmaintevas-(Tevlog/(Teva*Nevacrew)))

Total science time per flight: the number of crew manhours available for both intra- and extra-vehicular science activities during the surface habitation phase of the mission.

Ttotalscience=Tivscience+Tevscience

2.3 Output Formats

The crew time analysis model provides a tabular spreadsheet interface as shown in Table 2.3-1. The user inputs are arranged across the top of the page and the model outputs are at the bottom. Ten rows of inputs and outputs are displayed simultaneously to allow the user to perform sensitivity studies on any parameters.

Graphical display of model inputs and outputs is also provided as illustrated in Figure 2.3-1.

Table 2.3-1 Crew Time Analysis Model User Interface

Figure 2.3-1 Crew Time Analysis Model Graphical Output Sample

3.0 Analysis Results

A sensitivity analysis was performed to illustrate the variation in available science time based on changes in several model input parameter values. The inputs were varied from the default values by a factor of two in each case. Figures 3.0-1 through 3.0-6 show the results of this analysis.

Figure 3.0-1 details the results of an analysis of the variation in crew time available based on changes in crew size. The curves are defined as described in section 2.2. Default values for all model inputs, described in section 2.1, were used while varying only crew size. The figure illustrates the fact that a small fraction of crew time is available for useful work on the Lunar surface and that an even smaller fraction is available for scientific work.

Figure 3.0-2 shows a further breakdown of crew work time. In this figure, the topmost curve is identical to the bottom curve of the previous figure. Total science time is composed of two constituents: IVA science time and EVA science time, which are plotted here. It is interesting to note the variations in these two parameters based on changes in crew size. EVA science time increases with increasing crew size except in two cases. The first is when an additional habitat element is added to accommodate crew sizes of five to eight and crew sizes of nine to ten. The additional maintenance time requirements of these added habitats contributes to the decrease in EVA science time. The other contributor is the fact that the model always uses evenly sized EVA teams, composed of two crewmembers each in this analysis. When an odd number of crewmembers is present at the outpost, IVA science time increases due to the fact that one additional IVA crewmember is present every day of the surface mission. EVA science time does not benefit from this additional crewmember.

Other components of crew work time which are plotted in Figure 3.0-2 are IVA overhead, IVA maintenance, EVA maintenance, and EVA logistics.

Figure 3.0-3 shows the results of an analysis of the sensitivity of total science time to various model input parameters. Each input listed on this figure was varied by a factor of two, and the resulting percent change in total science time was plotted, beginning with the most influential parameter and proceeding to the least influential parameter. Default model input values were used for all inputs other than the one being varied in each case. The amount of time allocated to crew sleep was the most influential parameter analyzed, because sleep takes up a large amount of overall crew time. The model assumption is that less time allocated to sleep translates into an equal increase in crew science time, which may be an over-simplification of the real case. Stay time and crew size are shown to be highly influential determiners of crew science time, as well. It is interesting to note that the number of days between EVAs for each crewmember is not highly influential on available science time. This is true if, as assumed in the model, crewmembers who are not performing EVA science are performing IVA science. In this case, total science time, the parameter analyzed here, is not greatly affected by the proportion of EVA and IVA science times.

This result is further analyzed in Figure 3.0-4. Here the number of days between EVAs for each crewmember was varied from zero to three. Again it is seen that total science time is not greatly influenced, while the proportion of EVA and IVA science times vary greatly, as expected.

Figures 3.0-5 and 3.0-6 show the results of further analysis of the relationship between EVA and IVA science times. In Figure 3.0-5, the length of an EVA period is varied from four to ten hours, and the effects on science times are shown. Total science time is somewhat affected, while IVA and EVA science times are greatly affected, as expected. The modeling assumption is that shortened EVA periods translate directly into added IVA science time, which again is an over-simplification of the real case.

Figure 3.0-6 shows the results of varying the number of crewmembers on each EVA team. For a crew size of four, it is seen that an EVA team size of three is distinctly disadvantageous when maximizing EVA time, when coupled with the requirement that EVA crewmembers must have an IVA day between each two EVA periods. Total science time is seen to be relatively independent of the EVA team size because IVA crewmembers are assumed to perform science as well.

Figure 3.0-1 Time on Lunar Surface Versus Crew Size

Figure 3.0-2 Work Times Versus Crew Size

Figure 3.0-3 Sensitivity of Total Science Time to Model Inputs

Figure 3.0-4 Science Times versus Number of Days Between EVAs

Figure 3.0-5 Science Times versus Length of EVA Period

Figure 3.0-6 Science Times versus Number of Crewmembers on EVA

4.0 Conclusions

4.1 Programmatic

Definitions of FLO terms are needed, including crew time terminology. A particular crew time structure and set of terms has been used in this study, but other studies have used different structures and terms. Common terminology which is accepted by the program would be useful in future work.

Integrated study of FLO crew time is important to plan and manage this resource. All the elements of crew time must be integrated in order to make informed decisions on what objectives may be accomplished within a given length of surface mission with a given crew size.

Results of crew time sensitivity analysis may be useful in writing FLO operations concepts and plans. Very often, in developing these plans, quantitative data is useful in selecting a particular operational approach. Relative effects of varying operational parameters can be used to develop the rationale for selection of one approach over other possible approaches.

4.2 Mission

Science time is extremely sensitive to the amount of crew time needed for operating the outpost. Operational overhead, logistics, and corrective maintenance may impose a significant burden on crew time. Methods to offload some of this activity from the crewmembers may be useful in maintaining a viable amount of science activity.

The choice of an odd versus even number of crewmembers is important in the FLO EVA context. The desire to maximize the amount of crew EVA time coupled with the idea that EVAs include two crewmembers and that each crewmember have an IVA day between EVAs results in a distinctive advantage in having an even number of crewmembers.

4.3 Future Work

The crew time analysis performed to date has been at a high level and has not considered several aspects of the FLO mission:

transit phases
contingencies and aborts
sequencing of crew activities.

Each of these areas represents potential future work which should be performed when the FLO study reaches the point of needing that information.

Integration of the crew time model into a larger mission/system effectiveness model may be useful in order to optimize not only crew time availability but also development variables such as system mass, cost, schedule, and risk.

5.0 References

1. NASA Exploration Programs Office, "First Lunar Outpost Requirements and Guidelines (FLORG) Fully Annotated Working Draft", EXPO-T1-920001EXPO, June 10, 1992.

2. NASA Exploration Programs Office, "First Lunar Outpost Conceptual Surface Mission", unpublished, May 21, 1992.

3. NASA Exploration Programs Office, "First Lunar Outpost Detailed Assumptions Working Drafts", June 3, 1992.

4. NASA JSC Medical Operations Branch, "First Lunar Outpost Operations: Preliminary Description of Operational Health Care for Complete First Lunar Outpost Mission", draft, July 24, 1992.

5. Erickson, J., Dragg, J., Hack, E., Aucoin, P., "A Preliminary Data Base for Use in Estimating Maintenance Requirements for the First Lunar Outpost (FLO) Habitation Element", revised June 12, 1992.