MALEO: MODULAR ASSEMBLY IN LOW EARTH …jleslie48.com/db_wishlist/MALEO - Final Report.pdfModular assembly in low Earth orbit (MALEO) is an alternate strategy for lunar-base buildup

M A L E O : MODULAR ASSEMBLY IN L O W EARTH

ORBIT: ALTERNATE STRATEGY FOR LUNAR

BASE DEVELOPMENT*

By Madhu Thangavelu1

ABSTRACT: Modular assembly in low Earth orbit (MALEO) is a new strategy for building an initial operational-capability lunar habitation base, the main purpose of which is to safely initiate and sustain early lunar base buildup operations. In this strategy the lunar base components are brought up to low Earth orbit (LEO) by the Space Transportation System (STS), and assembled there to form the complete lunar base. Specially designed propulsion systems are then used to transport the MALEO lunar base, complete and intact, all the way to the moon. Upon touchdown on the lunar surface, the MALEO lunar habitation base is operational. The strategy is unlike conventional concepts, which have suggested that the components of the lunar base be launched separately from the Earth and landed one at a time on the moon, where they are assembled by robots and astronauts in extravehicular activity (EVA). The architectural drivers for the MALEO concept are, first, the need to provide an assured safe haven and comfortable working environment for the astronaut crew as safely and as quickly as possible, with the minimum initially risky EVA, and secondly, the maximum exploitation of the evolutionary benefits derived from the assembly and operation of space station Freedom (SSF-1). Commonality and inheritability from the space station assembly experience is expected to have an advantageous impact on both the space station program as well as the MALEO lunar base.

INTRODUCTION

The recent political and economic trends in national and global affairs make this a most opportune moment in history for advancing the space frontier for peaceful purposes (Paine 1989). In reaffirming our commitment to leading humanity in manned space exploration, on the 20th anniversary of the Apollo 11 mission, President Bush charged NASA with developing plans for a permanently manned lunar base, and extending the human domain on to Mars. NASA had been working on lunar return concepts all along (Aired et al. 1988, 1989; Aired and Bufkin 1988) and the recent reports from the NASA Office of Exploration suggest several case studies for both manned lunar and planetary missions under consideration, which are viable, using state-of-the-art or near-term technologies (Exploration 1988; Beyond 1988; Von Puttkamer 1985; Lineberry 1988). Assembly in low Earth orbit (LEO) has been suggested for the manned mission to Phobos and Mars (Bell et al. 1988). From these studies it is evident that, in order to sustain permanent manned presence in cislunar space, an evolutionary architecture must be adopted (Jones 1990).

A phase-1 lunar habitation base (LHB-1) is conceived as the second building block of this evolutionary architecture, the first one being the space sta-

aPresented at the April 23-26, 1990, Space 90: 2nd International Conference on Engineering, Construction and Operations in Space, held at Albuquerque, New Mexico.

'Res. Assoc, Inst, of Aerospace Systems Arch, and Tech. School of Engrg. and the School of Arch., Univ. of Southern California, Los Angeles, CA 90089-1191.

Note. Discussion open until December 1, 1991. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on March 30, 1990. This paper is part of the Journal of Aerospace Engineering, Vol. 4, No. 3, July, 1991. ©ASCE, ISSN 0893-1321/91/0003-0256/$1.00 + $.15 per page. Paper No. 25995.

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tion in LEO, for spacecraft assembly operations. LHB-1 is also the first permanently manned facility envisaged on the lunar surface. The facility is intended to safely initiate the assembly operations associated with LHB-1 establishment. It will provide a test bed for extended habitation, exploration, and scientific investigation. It is considered somewhat analogous to a forward base camp in the Antarctic or on Mount Everest or a typical site office set up during a conventional terrestrial civil engineering project from which major construction activity is supervised. LHB-1 could also form the nucleus, if needed, for further expansion and experimentation, which might be necessary during the evolution of what might become the first fully self-sustained, permanently manned lunar colony. This lunar colony will support spacecraft operations in cislunar as well as interplanetary space by providing propellants and other material manufactured on the lunar surface (Pioneering 1988).

To this end, the first priority in a phase-1 extended duration mission, is the provision of an assured safe haven for the astronaut crew, to alleviate astronaut anxiety associated with buildup operations, followed by a comfortable environment within the facility, to enhance crew productivity.

Space-station-like modules are considered the major components that need to be assembled in order to form an LHB-1 (Duke et al. 1985; Duke and Aired 1988; Hoffman and Neihoff 1985; Cohen 1989). Conventional strategies suggest launching these modules separately from the Earth, landing them one at a time on the lunar surface, and assembling them there, using robots and astronauts, in extravehicular activity (EVA) (Duke et al. 1985; Duke and Aired 1988; Hoffman and Neihoff 1985). Precursor missions are employed to land assembly equipment, and a rather complex satellite telecommunication network is envisaged, in the event that Earth-based robotic teleoperation is used, for assembly operations (Iwata 1988) (Fig. 1).

At the 1988 inaugural session of the International Space University (ISU), at the Massachusetts Institute of Technology in Cambridge, Massachusetts, the writer proposed the idea of assembling the modules of a lunar base in LEO, then transporting the entire assembly to the lunar surface for immediate occupation by the astronaut crew, thereby avoiding the cost and risk associated with manned extravehicular activity on the lunar surface, which had been proposed in earlier concepts (Duke et al. 1985; Duke and Aired 1988; Hoffman and Neihoff 1985; Cohen 1989). The writer and G. E. Dor-rington from Cambridge University presented a paper entitled "MALEO: Module Assembly in Low Earth Orbit. Strategy for Lunar Base Build-up" at the 39th Congress of the International Astronautical Federation, in October 1988 (Thangavelu and Dorrington 1988) (Fig. 2). Subsequent papers on the subject were presented at several conferences (Thangavelu 1990a, 1990b, 1990c; Thangavelu and Schierle 1990).

Modular assembly in low Earth orbit (MALEO) is an alternate strategy for lunar-base buildup envisaged for about the year 2000. In this strategy, the components of the lunar base are brought up to LEO by the Space Transportation System (STS) and assembled there. The LHB-1 is suspended within a truss superstructure that is employed to uniformly absorb the forces incurred by the MALEO assembly during transit and landing on the lunar surface. A specially designed modular propulsion system that is assembled in LEO is employed to transport the LHB-1, as a completely configured lunar base, directly to the lunar surface. The base is operational upon touchdown.

The lunar surface assembly strategy consists of (Fig. 1):

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FIG. 1. Lunar Surface Assembly Strat- FIG. 2. MALEO. Strategy egy

• Several Earth-to-orbit (ETO) launches. • Several component-wise translunar injections (TLIs). • Several component-wise lunar orbit insertions (LOIs). • Several component-wise lunar landings. • Assembly on lunar surface. • Substantial precursor missions.

MALEO strategy consists of (Fig. 2):

• Several Earth-to-orbit (ETO) launches. • Assembly in low Earth orbit. • One or two TLIs. • One or two LOIs. • One lunar landing. • Minimum precursor missions.

The strategy is envisaged to make full use of the U.S. Space program infrastructure that would be in existence by the year 2000. First element launch (FEL) for the deployment of a MALEO based LHB-1 is envisaged for the year 2000. The support elements of the MALEO strategy include the STS, space station Freedom (SSF-1) and the heavy lift launch vehicle (HLLV). A cryogenic propellant depot is also assumed to be operational in LEO.

LHB-1 will consist of space-station-like modules and nodes containing crew quarters, laboratory and logistics modules, and all the systems required for environmental control and life support, as well as adequate facilities for | power generation and storage, and communication and data-handling sys- I terns, all of which are integral to, and critical for a phase-1 extended-du- i ration mission of this nature. I

MALEO SYSTEMS

The MALEO systems for the deployment of LHB-1 essentially consist of the following elements:

1. A structurally strengthened lunar habitation base (LHB-1). 2. A chemical /electric modular orbital transfer vehicle (MOTV). 3. A lunar landing system (LLS).

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CONFIGURATION OF LUNAR HABITATION BASE-1 (LHB-1)

Several configurations are possible for the design of LHB-1, and two or more modules might be employed, as required by the mission objective (Fig. 3).

In the past, both horizontal and vertical configurations have been proposed for lunar habitation bases (Johnson and Leonard 1985; Kline et al. 1985; Lowman 1985). In the MALEO strategy, the horizontal configuration was preferred to the vertical one for the following reasons.

1. Commonality with SSF-1 enhances design and engineering economy. 2. Commonality with space-station design enhances crew adaptation and pro

ductivity. 3. Better, larger work spaces for extended-duration missions. 4. Wider spacecraft footprint for better landing stability. 5. Ease of expansion during lunar base evolution by attaching additional hor

izontal modules.

Issues of fire control and cabin contamination dictated a dual-egress design, and this requirement was satisfied by linking the modules and nodes in a continuous circulation loop with hatches at regular intervals, which may be secured in order to isolate any of the endangered modules or nodes. A crew rotation period of three months enabled a lunar-base design that did not require added regolith radiation protection since the nominal dosage during this period on the lunar surface is within the NASA limits (Natchwey 1988). However, a separate solar storm shelter (S3) will have to be constructed and maintained for protecting the astronaut crew in the event of a solar particle event. The three-month rotation period also has an advantageous impact on consumables and the environmental control and life support system (ECLSS), which would need replenishment from time to time during the initial operational capability (IOC) and expansion phases, before complete cycle closure of the ECLSS is achieved (Hypes and Hall 1988).

Using space-station-like modules as a base line (Cohen 1989), assuming certain slight configurational modifications that are imposed by lunar gravity, it may be possible to contain all the necessary systems required for the

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phase-1 LHB-1, for a crew of four astronauts/mission specialists, in three modules and three nodes.

COMPONENTS OF LUNAR HABITATION BASE-1 (LHB-1)

The major components of LHB-1 are the modules and the nodes. The three modules and the nodes that constitute the manned core of LHB-1 are described in the following (Fig. 4).

• Module 1. The habitation module will house the four astronauts/mission specialists, and will contain crew sleeping quarters, a gymnasium/recreation facility, and a galley. •

• Node 1. The sanitation/hygiene node is conveniently located adjacent to the habitation module. Besides serving this primary function, parts of the ECLSS like the solid-waste management system and the water-reclamation system are located in this node.

• Module 2. The laboratory module is also adjacent to the sanitation/hygiene node. This module is equipped with interchangeable racks that are partially outfitted so that modifications and new setups are possible, as the base evolves. Subsequent mission specialists could bring along special equipment when they arrive at LHB-1, and replace it with the equipment that is not required anymore.

MALEO PAYLOAD SUMMARY

1. HABITATION MODULE

2. LABORATORY MODULE

3. POWER / LOGISTICS MODULE

4. PRIMARY EVA NODE

5. AIR REVITALIZATION NODE

6. SANITATION / HYGIENE NODE

7. TRUSS SUPERSTRUCTURE

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9. SOLAR ARRAYS / COMMS.

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POWER AND LOGISTICS MODULE

PRIMARY EVA NODE

ATTITUDE CONTROL SYSTEM PALLET (ACSP)

LANDING GEAR

FIG. 4. Components of Three-Module MALEO Lunar Habitation Base and LHB-1 Mass Summary

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• Node 2. The primary EVA node is the airlock that is used for all of the extrabase activity. It contains the EVA suits, and equipment to prevent regolith backtracking and contamination.

• Module 3. The power and logistics module contains the power generation, storage, and regulation equipment. The module contains solar power panels that are deployed externally, can accomodate a nuclear power source that could be deployed externally, and will sustain the crew and essential equipment and communication through the long lunar night.

• Node 3. The ECLSS node is also the command center of LHB-1 and is designed so that the base may be extended by attaching additional modules, if necessary. These additional modules could be delivered initially as logistics/consumables modules, which, when depleted, may find other uses in the evolving base. The strategy is similar to the logistics module proposed for SSF-1 (Kline et al. 1985), except that the depleted modules are not brought back to Earth, but used for the expansion of the lunar base.

The components that integrate the modules together as a complete spacecraft are the following:

1. The truss superstructure. 2. The landing gear/airbag deployment system. 3. The attitude control system pallet (ACSP). 4. External storage pockets for EVA equipment/solar arrays. 5. Storage pockets for lunar rovers.

The truss superstructure has three functions. It is the structure employed to support the thrust structure of the MOTV and to distribute the forces transmitted from the MOTV uniformly through the entire LHB-1 during TLI, LOI, and lunar landing. It also offers the primary attachment points for the ACSP, and the landing gear/airbags systems. Upon touchdown, the superstructure could be dismantled and used for phase-1 buildup operations. Though the modules might be capable of handling the forces during transit and landing all by themselves, it was decided on safety issues not to stress the skin of the modules during transit and landing.

The landing gear/airbag deployment system is designed to absorb the shock on impact and may be conventional lunar-excursion-module-type shock absorbers or controlled gas-escape airbags or a hybrid employing both systems.

The three attitude control propulsion pallets are assembled and fueled on Earth, brought up to LEO by the STS, attached to the three corners of the LHB-1 after the truss superstructure has been built around the modules, and they stabilize the attitude of the spacecraft during transit and landing operations.

The storage racks may be configured as required and are placed symmetrically about the MALEO truss superstructure in order to maintain the thrust structure symmetry and balance. They contain the rovers, the solar panels, and other EVA equipment essential for the effective operation of LHB-1.

MODULAR ORBITAL TRANSFER VEHICLE (MOTV)

The MOTV is used to transport the fully configured MALEO lunar base, complete and intact, to the prescribed lunar parking orbit. The MOTV provides the required delta V for TLI, LOI, and midcourse impulse and cor-

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rection maneuvers. Two current or near-term options are available for the design of the MOTV propulsion system. They are:

1. Chemical cryogenic propulsion. 2. Electric propulsion.

In the first option, LH2/L02 engines are employed to make up the MOTV cluster. Engine redundancy and gimballing are employed for handling engine-out emergencies. The fully fueled units are brought up by the HLLVs, one at a time, and clustered at the teleoperated cryogenic propellant depot. The propellant depot has enough reserves to top off the tanks of the clustered MOTV just before the LHB-1 + MOTV docking operation and consequent translunar injection. Advanced space shuttle main engines (SSMEs) may be used as the propulsion system for the MOTVs and the advanced space engine (ASE) is also a candidate for this purpose. The MOTV cluster is docked with the LHB-1 and TLI is effected. The maximum accelerations occur at TLI and are estimated at about 2 g using modified SSMEs. On arrival at the prescribed lunar altitude, the MOTVs fire again for LOI. After this maneuver, the MOTVs are jettisoned and the LHB-1 remains in lunar parking orbit.

In the second option, the MALEO LHB-1 is gradually spiraled out of LEO using the low but continuous thrust of the ion engines until lunar gravitational capture occurs, when the engines are used to circularize the LHB-1 into a predetermined lunar parking orbit. Upon achieving LPO, the ion engines are jettisoned and the LHB-1 carries on with lunar orbital operations till the lunar landing system arrives. This option drastically reduces the total mass of the assembly as well as the thrusting forces encountered by LHB-1 during TLI and LOI and may be employed if rapid deployment is not a priority. The recent advances in SP-100 nuclear space reactor power system (SRPS) using an ammonia arcjet system holds promise not only for MOTV applications, but also for power generation at the base, after touchdown (Deininger et al. 1989).

If rapid deployment is not a priority, electric propulsion offers several advantages over the conventional chemical MOTVs for TLI and LOI of the MALEO LHB-1. They are:

1. The propulsion systems are at least 30-50% lower in mass than the conventional chemical MOTV {Exploration 1988; Cohen 1989).

2. The thrusting forces during TLI and LOI are much lower and continuous, which make the TLI and LOI operations much more controlled and therefore much less critical operations than with chemical propulsion.

3. The truss superstructure may be redesigned for the more critical lunar landing operation using the LLS chemical propulsion system and the nominal acceleration for this operation (deorbit and lunar surface touchdown) is less than 2g.

The disadvantages of an electric propulsion orbital transfer of the MALEO LHB-1 are:

1. The slow spiraling trajectory exposes the LHB-1 to a long period in the LEO orbital debris field.

2. LHB-1 equipment will have to be shielded against the Van Allen radiation belt in which it will spend considerable time. Ion engine plasma interaction with

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EARTH TO MOON DELTA V BUDGET

I. EARTH TO LEO LAUNCH 9500 m/s

FIG. 5. Cislunar Orbital Dynamics

the radiation belt and associated impact on both the spacecraft and the environment need to be assessed (Chiu 1979).

Cislunar orbital dynamics is well understood and has been applied very efficiently in past missions (Woodcock 1985; Woodcock 1986). Fig. 5 illustrates schematically, the various velocity changes and their approximate points of application.

LUNAR LANDING SYSTEM (LLS)

The LLS is used to deorbit and soft-land the MALEO LHB-1 on the lunar surface. The thrusting points on the LHB-1 truss superstructure are the same for both the MOTV as well as the LLS. Chemical cryogenic propulsion is favored for the descent and landing maneuver. Advanced RL-10 technology would be applicable for the development of the LLS. The LLS is assembled and fueled in LEO from components brought up by the STS/HLLV fleet and a similar chemical MOTV is used to quickly transport the fueled LLS to the same LHB-1 lunar parking orbit where the LLS and the LHB-1 rendezvous in lunar orbit. After the LHB-1 and the LLS are secured together to form an integral landing craft, the LLS fires briefly and the entire spacecraft deorbits and descends to the lunar surface.

MALEO ASSEMBLY AND DEPLOYMENT OF LHB-1

At least four options exist for the MALEO assembly of LHB-1. They are:

1. Free space assembly using the STS as the assembly platform. 2. Free space assembly using STS as the primary assembly platform with as

sistance from Freedom and her crew (Fig. 6). 3. MALEO assembly attached to Freedom. 4. MALEO assembly connected to the manned core of SSF-1 (Fig. 7).

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FIG. 6. STS-Based, Freedom-Assisted Free-Space Assembly of Four-Module MALEO LHB-1

JEM EXPOSED FACILITY

MALEO LHB-1 STACK

FIG. 7. Four-Module MALEO LHB-1 Stack Being Assembled Connected to Manned Core of Space Station Freedom

The fourth option utilizes the space station infrastructure most effectively by providing the following benefits:

• The inertia and structure of Freedom provide the stable platform/scaffolding required for MALEO assembly of LHB-1.

• Options 1 and 2 require a separate reaction control system (RCS) to maintain orbit and orientation during the several ETO sorties by the STS, needed to bring up all the components for LHB-1 assembly. Options 3 and 4 utilize the Freedom RCS effectively.

• Once connected with the manned core of SSF-1, much of the EVA as-

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sociated with outfitting MALEO LHB-1 might be carried out as intra vehicular activity (IVA), in a shirt-sleeve environment.

• The manned core of the LHB-1 would offer added utility and space for Freedom and her crew, till it departs for the moon.

Two options are possible for the deployment of MALEO LHB-1.

1. Single-phase direct lunar descent and landing option. 2. Two-phase lunar orbit rendezvous (LOR) option.

The MALEO single-phase direct lunar descent and landing option deployment sequence of LHB-1 is as follows:

• The STS is employed to bring the modules of LHB-1 to LEO, where they are assembled to form the lunar-base configuration. The space station and her crew would be employed for this purpose. If free space assembly is employed, the modules are assembled using the space shuttle as the work platform, with assistance from Freedom (Fig. 6). In the next major assembly operation, the truss superstructure is built around the modules. An attitude control system pallet (ACSP) is added next, landing gear attached, and the entire truss superstructure is pretensioned so that the stresses might be more efficiently distributed within the structure.

• The STS/HLLV then brings up a partially assembled lunar landing system (LLS), which is integrated with the LHB-1 at the thrust distribution points within the truss superstructure of the LHB-1. The LLS + LHB-1 is fueled up at the cryogenic propellant depot and is ready for translunar injection.

• The STS is finally employed to carry the modular orbital transfer vehicle (MOTV), unit by unit, to LEO, where they are clustered and fueled at the orbiting cryogenic propellant depot. The MOTV docks with the LLS + LHB-1 configuration and the entire assembly is thrust into TLI.

• On approaching the prescribed lunar orbit, the MOTV fires again in order to circularize the orbit, and then the MOTV is jettisoned.

• Once the landing beacon acquisition is achieved, the LLS fires briefly, and the LLS + LHB-1 spacecraft deorbits and begins to descend to the lunar surface. The LLS fires again at the proper altitude, and on approaching the landing site, conducts a hovering orientation maneuver followed by main engine cutoff (MECO). The LLS + LHB-1 drops gently to the lunar surface and lands on conventional LEM-type shock-absorbing landing gear or controlled gas-escape multicellular airbags or a hybrid system.

• Upon touchdown, the residual propellant is vented, the solar arrays and communication equipment and a solar storm shelter are deployed. The phase-1 lunar habitation base (LHB-1) is operational shortly thereafter.

The MALEO two-phase LOR-option deployment of LHB-1 is as follows:

• The LHB-1 is first assembled in LEO-like option 1 and transported to the predetermined lunar parking orbit by the MOTV which is jettisoned after the LOI maneuver.

• Next, the LLS is transported to the same lunar parking orbit as the LHB-1 where they rendezvous in lunar orbit. After securing the LHB-1 to the LLS, the LHB-1 + LLS MALEO spacecraft descends to the lunar surface and soft-lands at the landing site.

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The two-phase LOR option effectively reduces the TLI mass of the MALEO operation in half and might be advantageous for cryogenic pro-pellant management during the MALEO operation.

The sequence of illustrations (Figs. 8-10) help to visualize the MALEO strategy two-phase option for the deployment of a three-module lunar habitation base.

MALEO LHB-1 LUNAR LANDING OPERATIONS

Two modes of landing are being considered for the MALEO deployment of LHB-1.

1. The manned deployment mode. 2. The unmanned deployment mode.

In the first option, the landing is manned from the control center located in one of the nodes of the MALEO LHB-1. The crew either TLI in the MALEO if the single-phase direct descent option is employed or they accompany the LLS in the two-phase LOR option and help to secure the LLS to the LHB-1 in the lunar parking orbit, and then descent to the lunar surface, controlling the spacecraft from the command center in the node 3.

In the unmanned landing option, two points of remote telerobotic control are considered.

• Real-time teleoperation from lunar orbit. A command and control center coorbits the MALEO LHB-1 in lunar parking orbit. The orbital periods are so matched that a line-of-sight descent and landing operation is effected from the command module. After the MALEO has been safely landed by remote, real-time, line-of-sight telerobotic control, the crew can land be-

FIG. 10. MALEO Deorbit, Descent, and Touchdown

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side it and start LHB-1 operations, or they could return to LEO. • Real-time teleoperation from the lunar surface. An LEM-type command

and control center is landed close to the landing site. The crew might prepare the landing site and establish the landing aids/beacon. The LHB-1 is again brought down using a line-of-sight, remotely controlled procedure from the control center. The crew take over LHB-1 operations after touchdown.

LHB-1 STRUCTURAL SYSTEM

From the operations listed above in sequence, it is evident that the truss superstructure of the LHB-1 and the complementing structures of the lunar landing system (LLS) and the modular orbital transfer vehicle need to be highly efficient, lightweight, and reliable. Historically, spacecraft have used few, if any, members in tension (Cohen 1987). Inherently stiff tension members are suggested in the MALEO truss superstructure pretensioning system in order to conserve mass. The configuration is structurally strengthened by suspending the LHB-1 modules within a truss superstructure so that LHB-1 will be able to uniformly absorb the stresses induced on it during translunar injection (TLI), lunar orbit insertion (LOI), and lunar surface touchdown. The thrust structure, which includes the truss superstructure and the lunar landing system and MOTV interconnections, is so selected that the forces are applied symmetrically about the truss superstructure. Though a stressed-skin module configuration could be designed to take the stresses arising from TLI and LOI, which are estimated to be about 2 g maximum, module safety and other factors dictated that a separate truss superstructure be employed for the thrusting load distribution. Vibration and resonance characteristics of the large assembly (20 m on each side and about 5 m deep) during the critical translunar injection when the acceleration might be about 2 g for a period of about 5 min, are being studied. Attitude control during descent and landing will also need quick and effective response. Since these structures are heavy in comparison to existing spacecraft (LHB-1 weighs about 100 MT, LLS weighs about 100 MT and the MOTV weighs about 600 MT) it may be more convenient to adopt a two-phase transfer from LEO to LPO and then attempt the LLS + LHB-1 rendezvous in lunar orbit. This two-phase option would reduce the TLI mass by half and might be the more manageable way for propellant management also. Further, if electric propulsion is employed for the orbital transfer, the thrusting forces become negligible and then the critical forces encountered are the descent and landing impact forces. These forces are expected to be typically less than the TLI forces suggesting an advantageous redesign of the truss superstructure for the landing operation.

ADVANTAGES OF MALEO

If space-station-like modules are to be employed in the construction of a phase-1 lunar base, then the MALEO strategy offers the following advantages:

1. The safer LEO radiation environment for EVA, which is less risky and more economical than EVA on the lunar surface (Bufkin et al. 1988).

2. The inheritability of the module assembly and truss-erection techniques acquired while building space station Freedom.

3. Commonality with space station Freedom hardware enhances cost /unit

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economy as well as spares and replacement units for both the MALEO as well as the space-station program.

4. Avoids the risk of the repeated ETO launchings, TLIs, LOIs and lunar landings associated with the typical sequential buildup prescribed by the modular assembly on lunar surface (MALS) strategy.

5. The clustered engines that form the modular propulsion system in the MALEO strategy would be more reliable than small single engines suggested for component-wise launching and landing manuevers.

6. The clean LEO assembly environment avoids dealing with the lunar soil that has undesirable abrasive and cohesive properties. Regolith has interfered with manned EVA systems in the past (Weaver and Laursen 1988; Loftus and Patton 1980; Jones 1990). During the phase-1 buildup, paucity of men and equipment demands high reliability, and therefore, MALEO avoids the initial lunar surface assembly operations entirely.

7. LEO offers STS-based and/or space-station-assisted assembly of LHB-1 as well as the possibility of Earth-based real-time telerobotic assembly operations.

8. Reduces precursor missions, equipment and associated activity. 9. Spares and replacements are easily and more economically flown to LEO

or borrowed from the infrastructure existing in LEO. 10. The tight power constraints encountered in assembly on the lunar surface

is avoided (Fordyce 1988). 11. The LHB-1 is safely configured for habitation upon touchdown. Conven

tional strategies involving lunar surface assembly cannot offer a comparably safe environment during base buildup without additional investment.

12. In the event of an assembly accidentia crew rescue is probably more feasible from LEO using the assured crew-return capability (ACRC) of the space station, than from the lunar surface.

13. It may be possible to develop a MALEO-based assured crew-return capability (ACRC) system for the entire crew of MALEO LHB-1, which might be a vehicle of similar proportions. This vehicle would have the capability to land on the lunar surface, and during an emergency, would have enough propellant for a single-stage launch to LEO that would bring the lunar crew to the safety of the space station.

14. The strategy might be adopted for landing outposts on Phobos, or could be used for lunar or Mars orbiting stations.

15. In this primary extended-duration manned mission, an assured and substantial safe haven at IOC will help to diminish anxiety and enhance productivity among the astronaut crew.

16. The MALEO deployment of LHB-1 will provide the experience required for assembling and operating the interplanetary manned Mars mission vehicle. Cryogenic propellant management in LEO, propulsion system development, LEO assembly of large manned spacecraft, and their testing and certification will help to design the manned Mars mission.

17. The MALEO strategy offers ample opportunity for international cooperation.

CHALLENGES

The risk of losing the entire LHB-1 in the event of an accident during the transportation and landing of the base is a disadvantage of the MALEO strategy but should be evaluated carefully against the repeated component-wise

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ETO launches, TLIs, LOIs and lunar surface landings associated with the conventional strategies in which the operations are arranged more or less sequentially, and hence susceptible to delay and operational collapse in the event of a failed component launch or landing.

The MALEO strategy suggests a prefabricated phase-1 lunar base that deprives the assembly crew of the initial experience of learning to work on the lunar surface (Ivan Bekey, personal communication, March 1988). It is important to realize that a substantial safe haven is the first priority for the astronaut crew on an extended mission, establishing a permanent base on the moon. Lunar surface EVA experience will be gained as exploration and experimentation continue and the assured safety of the MALEO base would also help to relieve astronaut crew anxiety while the base is operating in the IOC mode.

MALEO is a large manned spacecraft. All the MALEO systems, the LHB-1, the LLS, and the MOTV needs to be studied, their dynamic characteristics examined, and their limitations confirmed. Though the TLI and LOI can be effectively controlled using an electric propulsion MOTV for the orbital transfer maneuver, the vibration and resonance characteristics of such a large structure during lunar descent and a heavy impact or unsymmetric landing are being studied and will require detailed analysis.

CONCLUSION

One method of module assembly in low Earth orbit has been discussed in this paper. The systems described herein need to be designed and built for assembly in space (Mikulas 1988). In an extended-duration primary mission of this nature, crew safety and provision of a safe haven and comfortable spaces for work and rest require the highest priority. The MALEO strategy minimizes the initial risky manned EVA associated with lunar surface assembly operations and provides a safely configured working environment for the astronaut crew upon touchdown. The LHB-1, thus deployed, will alleviate crew anxiety and enhance productivity. The MALEO strategy maximizes the commonality and inheritability from the LEO infrastructure that will be in operation by the year 2000. The experience gained while assembling space station Freedom will be invaluable for the MALEO assembly and deployment of LHB-1. More study is required to confirm the feasibility of clustering existing propulsion systems to build the MOTVs, the modifications that might be necessary, and if these vehicles might be launched from the Earth fully loaded with propellant, so that a cryogenic depot might not be necessary in LEO in order for the MALEO operation. High-power electric propulsion like the SRPS ammonia arcjet offers promise for an MOTV for use in the MALEO deployment of LHB-1.

MALEO LHB-1, is, in essence, a large manned spacecraft and could be used as an orbiting station for the moon and Mars. The lack of aerodynamic contours and for reasons of stability, control, and propulsion requirements, the concept is particularly suited to landing on low-gravity bodies without an atmosphere, and the moon is an ideal first choice. MALEO bases could land on Phobos or could be used for prospecting the near-Earth Asteroids in the future.

The importance of highly reliable structural systems for the truss superstructure of the LHB-1, the lunar landing system, and the MOTV are clearly evident for the success of such a mission. Advances in structural materials, on-orbit assembly techniques acquired while assembling and operating space

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station Freedom, advances in electric propulsion for orbital transfer applications, and the year 2000 time frame for the execution of such a mission all seem to coincide favorably for the successful application of this strategy for a lunar base buildup.

The improving U.S.-Soviet superpower relations, the return to flight of the STS, the imminent United Europe, and the maturing space programs of China and Japan, all show a trend toward accomplishing major near-term achievements in the space frontier. A project like the MALEO deployment of LHB-1 could become a catalyst for the peaceful international development of space.

In the event that a decision' is made for the deployment of a national or an international lunar base about the year 2000, using space-station-like modules, the MALEO strategy holds promise and needs detailed engineering analyses along with other strategies.

ACKNOWLEDGMENTS

This paper is dedicated to my mother Nanoo Saraswathy Thangavelu. I would like to thank P. Diamandis and T. Hawley and the whole crew at ISU for helping me to carry through the idea. Thanks are due to Graham Dorrington of Cambridge University, who helped to hone the MALEO concept further. Pat Rawlings and Ron Schaefer were very helpful in illustrating the early concepts. I would like to thank R. Harris, dean of the School of Architecture, for his financial and moral support to carry on the work at the Master's level, G. G. Schierle, my thesis committee chairman, for his insight into the structural aspects of MALEO, R. F. Brodsky of the Department of Aerospace Engineering for all those engaging lectures in Spacecraft Systems Design and very helpful critiques and for loaning me his library of lunar information, and D. Vergun who guided the study on a uniform first-order systems level. 1 would like to thank Eberhardt Rechtin in the Department of Aerospace Engineering and Industrial Systems Engineering who injected new insight and energy into the MALEO concept and helped me to evolve it further along the direction of the U.S. Manned Space Program, along with Leonard Silverman, Dean of Engineering, R. K. Miller, associate dean of engineering and E. P. Muntz, cochairman of the Department of Aerospace Engineering, all of whom helped me to develop a framework in which to carry out a detailed study of the MALEO concept at the University of Southern California.

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