Project Gateway: An Earth orbiting transportation node
Project Gateway: An Earth orbiting transportation node

Todd McCusker, et. all

May 6, 1988


University of Texas Mission Design (UTMD) has outlined the components that a space-based transportation facility must include in order to support the first decade of Lunar base buildup. After studying anticipated traffic flow to and from the hub, and taking into account crew manhour considerations, propellant storage, orbital transfer vehicle maintenance requirements, and orbital mechanics, UTMD arrived at a design for the facility. The amount of activity directly related to supporting Lunar base traffic is too high to allow the transportation hub to be part of the NASA Space Station. Instead, a separate structure should be constructed and dedicated to handling all transportation-related duties. UTMD found that Gateway would need a permanent crew of four to perform maintenance tasks on the orbit transfer and maneuvering vehicles and to transfer payload from launch vehicles to the orbital transfer vehicles. In addition, quarters for four more persons should be allocated for temporary accommodation of Lunar base crew passing through Gateway. Six orbit transfer vehicle(OTV) missions (2 vehicles per mission) are expected each year during the first ten years of Lunar base operation. Enough propellant must be kept on board to accommodate the OTV fuel requirements generated by this mission scenario. UTMD specified the amount of fuel storage needed and the number of remote manipulator arms to accommodate the refueling process and payload integration. An orbital inclination of 28.5 degrees was selected to allow the Space Shuttle access from Kennedy Space Center. The structure should be placed in a circular orbit between 240 and 260 nautical miles altitude. A Delta Truss structure was recommended as the framework for the individual components, primarily because of its resistance to damage after heavy docking activity and its ability to make orbital maneuvers such as a possible plane change or altitude boost. UTMD was careful to recommend and expandable structure that can grow to meet the growing demands of the American space program as it moves toward the twenty-first century.

The Advanced Programs Office at Johnson Space Center in Houston, Texas has decided to begin the construction of a permanent Lunar Base around the year 2000. The facility will require the transport of hundreds of metric tons of habitats, laboratory equipment, rover vehicles, and many other components to the Lunar surface. Rather than launching all of this mass directly from the Earth to the Moon using one vehicle, the payload will be transferred from the booster vehicle to another while in Earth orbit. Several designs for orbit transfer vehicles which can take the payload from Earth orbit to Lunar orbit have been proposed. These orbital transfer vehicles will require a maintenance and refueling facility in Earth orbit. University of Texas Mission Design (UTMD) studied the requirements of such a facility to best serve the needs of the Lunar base, while allowing expandability to accommodate future demands of interplanetary missions.

Project Statement

The operation of the Lunar base will not be possible without a transportation node in Earth orbit. A facility of this type should be tailored specifically to accommodate the needs of the base in terms of fuel storage, vehicle maintenance equipment, vehicle hangars, payload transfer equipment, and crew habitats. It should be designed to allow expandability as traffic to and from the Moon increases, particularly if the production of Lunar liquid oxygen becomes economical. The orbit of the transportation node should allow a large number of departure opportunities to possible Lunar destinations and be within reach of the launch vehicles, while not requiring excessive fuel to maintain altitude.

Problem Background

One of the primary goals of UTMD during the course of the research was to design the transportation facility such that the American space program could benefit as much as possible from the recommendation. This required UTMD to explore such options as making the transportation facility a part of the NASA Space Station structure, or designing the node to be a free-flyer in the proximity (less than 2 kilometers) of the Space Station, or perhaps building a new structure that is dedicated to transportation duties. The latter option offers the added advantage of freeing the Space Station from the frequent docking disturbances that heavy traffic flow will introduce into the microgravity research environment. UTMD has been careful in selecting the components for the transportation facility so that the anticipated growth of the space program will be accommodated.

Major Assumptions

Before undertaking the task of determining the orbit and outlining the growth process of Gateway, UTMD made several assumptions to ensure the feasibility of building Gateway. The primary reason for making these assumptions was to exclude any dependence on nonexistent technologies that may not exist before the beginning of the Lunar base. The assumptions were made as follows:

The United States will build a Lunar Base. The Lunar Base will not differ greatly from those proposed in 1987.

This study concentrated on the requirements that a Lunar Base will place on an Earth-orbiting station. The information made available by NASA and Eagle Engineering of Houston will serve as the driver for Lunar Base requirements. The design of Gateway is tailored specifically to accommodate these demands; including crew quarters, OTV service facilities, and propellant storage tanks. The orbit of Gateway will be chosen to economize the Earth to Moon round trip from a propellant savings standpoint.

Launch vehicle capabilities will not change dramatically before Gateway is constructed.

The calculations used to determine the orbit of Gateway relied heavily on specifications of operational or near-operational launch vehicles. The important parameters include: Specific impulse, fuel capacity, crew capacity, and payload vs altitude curves. Primarily, the following vehicles were considered: Space Shuttle, Shuttle-C, and a U.S. HLLV(not operational).

Orbital Transfer Vehicles (OTV's) will be built. The OTV's will perform close to the specifications available today. The maintenance requirements for the OTV's will not differ greatly from the figures available today.

At the present time, no OTV has been tested in space. The Earth to Moon transportation network being considered is dependent on some type of vehicle to transfer crew and cargo between Gateway and a Lunar orbit. The OTV's that have been proposed are reusable and require maintenance while in space.
The United States Space Station will be functioning before Gateway is constructed.

This is primarily a political and monetary issue. If an American research facility is not yet in orbit, it should be given priority over a dedicated transportation node such as Gateway.

Approach to the Problem

Because the Lunar base is likely to produce a high volume of traffic flow before the first manned interplanetary missions are started, UTMD chose to use available Lunar base buildup scenarios to generate the requirements that will be placed on the transportation hub. The analysis of the Lunar base scenarios yielded the expected number of orbital transfer vehicle (OTV) flights per year and the number of Heavy Lift Launch Vehicle (HLLV) and Space Shuttle flights needed to support the base. The number of OTV flights was used to determine the quantity of Orbital Maneuvering Vehicles (OMV's) needed on the transportation node, along with the required amount of hangar space and propellant storage. The total crew capacity and the permanent crew capacity were derived from this data and a study of allowable work loads for astronauts. The number of HLLV and Space Shuttle launches needed to construct the transportation hub was calculated considering standard space structure components (radiation shielding, attitude control, power supply, etc). Finally, such factors as Space Shuttle lift capabilities, atmospheric drag, and Earth launch site were included in an orbit determination computer program. The goal of the program was to select an orbit for the structure that maximized the departure opportunities from the transportation node to several possible Lunar destinations, while placing the structure in an orbit which did not require excessive reboosting. Gateway's orbit also had to remain within the useful range of the selected launch vehicles. From these analyses, UTMD selected both the structural requirements and possible orbit range for a transportation facility.


Because of the anticipated role of the transportation facility in making travel to the Moon and outer planets feasible using current launch vehicle technology, the structure has been named GATEWAY.

UTMD determined that a separate transportation facility from the NASA Space Station should be constructed in order to support the Lunar base scenarios proposed in 1987. A permanent crew of four vehicle and payload integration specialists will be needed onboard Gateway. During the first ten years of Lunar base operation, approximately six OTV missions (2 OTV's per mission) per year will be initiated between Gateway and the Moon. Therefore, only two OTV's need to be housed and maintained at Gateway. In addition, one OMV will be needed for the proximity operations. UTMD elected to store enough liquid rocket propellant onboard Gateway to refuel two OTV's at any time. UTMD found that a Delta Truss will provide an excellent framework for the selected components. A total of four Shuttle flights and one HLLV flight will be needed to boost the structure into orbit and assemble the components. As Lunar base or interplanetary mission traffic demands increase, the Delta Truss can be expanded to accommodate more components than selected in this study. An artist's conception of Gateway is shown on the inside cover page of this report.

UTMD selected an orbit for Gateway based on approximately a 1995 technology baseline for launch vehicle capabilities. The altitude of the orbit is limited to 270 nautical miles because of the lifting capability of the Space Shuttle. The Delta Truss was used as a model to find a minimum altitude for Gateway. A minimum altitude of 210 nautical miles was selected. The inclination should be 28.5 degrees as long as Kennedy Space Center is the only Space Shuttle launch facility. Explanations of the individual topics of analysis that UTMD undertook are presented in the following sections of this report.