This was almost immediately preempted by the 90-Day Study issued in response to President Bush's Human Exploration Initiative speech in November 1989. The original plan foresaw establishment of a lunar base at Mare Tranquilitatis by 2005 using Shuttle-C boosters, spacecraft assembly at Space Station Freedom, and modifications of the Orbital Transfer Vehicle.
The Lunar Observer spacecraft, then planned to be launched in 1997, would support site selection and resource mapping of the lunar surface. For purposes of case study analysis, a site was chosen at 0 degrees latitude and 24 degrees east longitude, just north of the crater Moltke in the southernmost part of Mare Tranquillitatis. This site was selected because the study constrained the outpost to be located on the equator in order to ensure daily access to a low-lunar parking orbit. In addition, the hydrogen reduction of ilmenite technique was used as the lunar oxygen production process in the study, which limits potential sites to those in which high ilmenite basalts were found. Only two such areas exist on the lunar equator: a small one in Oceanus Procellarum and a larger one in southern Mare Tranquillitatis. The latter was chosen because of its proximity to the Apollo 11 landing site.
Installation of the initial lunar habitation facilities would begin in late 2003. From 2003 through 2005, three piloted missions and two unmanned cargo missions would be flown to the lunar surface. The first crew tour of duty was 30 days, during which time the crew completes the deployment and began operation of the initial support and habitation systems. A 6-month period of unmanned testing and verification of the surface facilities follows. Permanent habitation of the outpost would begin in mid-2004, when four crewmembers stay for 6 months, and the facilities would be delivered to the surface to maintain the lunar excursion vehicle for personnel transfer. Habitation facilities would be enhanced to provide more living and pressurized laboratory space. The number of permanent crew would grow to eight in 2008. Crew tours of duty would increase to 1 year in 2005 and to 2 years in 2008. This increase provided the opportunity to obtain valuable data in the areas of human adaptation to reduced gravity environments and long-term isolation from Earth, human performance degradation countermeasures technology, and Earth-independent outpost operational experience.
The outpost evolution continued as the number of permanent crew increased to 12 in 2012, and the outpost could support up to three lunar excursion vehicles (LEVs). Oxygen production, initiated in 2012, provided lunar liquid oxygen (LLOX) for the environmental control and life support systems as well as for propellant for LEV. Science capabilities would be expanded to include large pressurized laboratories for life sciences research and human biomedical studies, long-range pressurized rovers and ballistic excursion vehicles for human access to distant locations on the lunar surface, and large astronomical arrays.
Earth-to-orbit transportation of mission cargo, vehicles, and propellant would be accomplished by a combination of Shuttle-C and Block II Shuttle-C vehicles. Large payload shrouds would be required for delivery of the reusable space transportation vehicles, whereas smaller shrouds would be sufficient for cargo and propellant deliveries. In addition, delivery of the space transportation vehicles to low-Earth orbit would occur less frequently, as compared to cargo and propellant delivery, due to the assumed 10-year mission life of the reusable space transportation vehicles. A combination of Shuttle-C and Block II Shuttle-C vehicles, instead of a larger heavy lift launch vehicle, was chosen to provide the ETO transportation for the Lunar Evolution case study. The Shuttle-derived vehicles provided a suitable and efficient mix of vehicle, propellant, and cargo delivery while reducing the initial development costs and reaching a balance of payload size and on-orbit operations. The Shuttle-C payload capability to Space Station Freedom was assumed to be 71 t with a large-diameter payload shroud (10 m in diameter by 30 m in length) for lunar transfer vehicle/ lunar excursion vehicle delivery. A smaller-diameter payload shroud (4.6 m by 15 m) was used for propellant and cargo delivery. The payload capability to Space Station Freedom for the smaller Block II Shuttle-C was assumed to be 61 t per flight. Six Shuttle-C launches per year were assumed to be available for the ETO delivery requirements. Support and mission crews would be transported to Space Station Freedom and returned to Earth by the Shuttle.
Space Station Freedom was the staging location between Earth and the Moon for the lunar transfer vehicles (LTVs). Payloads and propellant would be stored at Freedom, and Freedom concurrently provided the servicing facilities for the LTVs. In addition, Freedom houses lunar mission crews in transit, and provided housing for the LTV/LEV/payload processing support personnel. Both cargo and piloted missions used LTVs for transfer from low-Earth orbit (LEO) to low-lunar orbit (LLO), insertion into LLO, and return to LEO. LEVs would be used to transport cargo and crew to the lunar surface and from the lunar surface for rendezvous with the waiting LTV. Both vehicles would be capable of being operated in an unmanned mode. The LTVs used an aerobrake on Earth return, arriving in an orbit from which the crew could be retrieved and transferred to Space Station Freedom. In addition, the translunar trajectory design permitted a lunar flyby and free-return abort to LEO if necessary prior to lunar orbit insertion.
The annual mass requirements to LEO fall within the capacity of six Shuttle-C and two Shuttle flights for all years except 2004, averaging 356 t per year for the first 10 years, and 180 t per year for steady-state operations. Mass requirements for 2004 could be slightly adjusted by delivering some cargo in 2003, thereby limiting the Shuttle-C flight rate to a maximum of six per year. This ETO flight rate, combined with 14 Space Shuttle flights per year (NASA's baseline launch rate), appeared to be a natural break point above which major new facilities would be required at KSC.
The transportation system for the Lunar Evolution case study consisted of two classes of vehicles derived from earlier NASA Orbital Transfer Vehicle studies: (1) the lunar transfer vehicle (LTV), used to transfer personnel and/ or cargo between LEO and LLO, and (2) the lunar excursion vehicle (LEV), used to transfer personnel and/ or cargo between LLO and the lunar surface. Personnel vehicles, which could accommodate up to eight crewmembers, were designated LTV-P or LEV-P, whereas cargo vehicles were designated LTV-C or LEV-C. All vehicles were designed to be reusable with the LTVs based and serviced at Space Station Freedom and the LEVs based and serviced on the lunar surface at the outpost. The vehicles had an assumed reuse lifetime of 10 missions.
An important design concept of this case study was the extensive use of vehicle commonality. Common LEVs that could be used for either a pure cargo mission or a combined personnel and cargo mission kept the lunar outpost overhead to a minimum. A single vehicle design required a smaller inventory of spare parts, fewer systems for the crew to understand, and the ability to interchange vehicle parts. Also, a common design eliminated the need for separate backup vehicles for personnel or cargo missions, thus minimizing the number of LEVs on the lunar surface and providing inherent redundancy and mission safety. Commonality of LTVs had the same effects at Space Station Freedom.
The LTV used liquid oxygen and liquid hydrogen propellants. A large foldable aerobrake on the LTV allowed the returning vehicle to use Earth's atmosphere for orbit capture in LEO, thus reducing the total propellant load required for a lunar mission. Since the aerobrake was foldable, it required no on-orbit assembly at Freedom. When an LTV was used for a cargo mission, only unmanned payloads were loaded onto the LTV and delivered to lunar orbit for transfer to the LEV and subsequent delivery to the lunar surface. When an LTV was used for a personnel mission, a 9 t crew module was attached to the LTV, and both crew and payloads were delivered to LLO. The crew module was an ablative, Apollo-style design capable of Earth entry and a soft landing. This design provided a redundant capability for Earth return of the crew in the event of an aerobrake failure. The LTV-P used four ASE engines with a thrust of 266.9 kN, a specific impulse of 481 seconds at an O:F ratio of 6:1. It had a dry mass of 18.7 metric tons (including the 9 metric ton crew module), and a capacity of 150 metric tons of propellant. The LTV-C was the same, except that deletion of the crew module decreased the empty mass to 9.7 metric tons. Actual propellant loading would vary between 106.8 metric tons and 146.6 metric tons depending on the mission, with payload carried varying from 42.1 metric tons to 64.6 metric tons depending on the version (expendable or reusable) and mission.
The LEV used the same engines and propellants as the LTV. The engines on the LEV employed a smaller nozzle than their LTV counterparts, which resulted in a lower engine specific impulse. However, the smaller nozzle allowed the LEV to have short landing legs and reduced dry mass. When an LEV was used for a cargo mission, only unmanned payloads were loaded onto the LEV and delivered to the lunar surface. For a personnel mission, a crew module was attached to the LEV, and both crew and payloads were delivered to the lunar surface. The LEV-P used four ASE engines with a thrust of 266.9 kN, a specific impulse of 465 seconds at an O:F ratio of 6:1. It had a dry mass of 6.4 metric tons (including the 3 metric ton crew module), and a capacity of 25 metric tons of propellant. The LEV-C was the same, except that deletion of the crew module decreased the empty mass to 3.4 metric tons. Actual propellant loading would vary little - between 23.8 metric tons and 24.9 metric tons depending on the mission, with payload carried varying from 15.2 metric tons to 37.0 metric tons depending on the version (expendable or reusable) and mission.
Although the LTVs and LEVs were designed to be reusable and space-based, there was no experience in servicing vehicles in space. Therefore, the initial three LTVs and three LEVs used for the emplacement phase were not planned for reuse; instead, they were expended and used as test-beds to better understand what types of servicing would be required and how this servicing would be done. As operational and maintenance experience was gained, vehicles could gradually be reused to greater degrees until they become totally reusable.