Credit: © Mark Wade
Status: Design 1974. Thrust: 99.14 kN (22,288 lbf). Gross mass: 20,600 kg (45,400 lb). Unfuelled mass: 2,550 kg (5,620 lb). Specific impulse: 460 s. Height: 10.80 m (35.40 ft). Diameter: 4.27 m (14.00 ft).
The Tug could be outfitted with a variety of kits to serve in many roles, including as a manned lunar lander. Aerobraking for recovery in low earth orbit was considered for further study, but the baseline used RL10 engines to brake into earth orbit for refurbishment and refueling at a space station. All further work was cancelled by NASA in 1972, but resurrected as the aerobraking Orbital Transfer Vehicle in the 1980's.
Space Tug Systems had to be compatible for both utilization as (1) upper stages and payload components for the Saturn V vehicle and its derivatives and (2) as upper stages and payload components for the Earth-to-Orbit Shuttle (EOS). Primary applications for the Space Tug/Saturn V Systems would be for transportation of large payloads to lunar orbit and interplanetary missions. The Space Tug systems would be utilized as payload components for the above missions when used in conjunction with the nuclear shuttle. The majority of the Space Tug missions would, however, be in conjunction with the EOS. The baseline EOS considered for selection of the compatible Space Tug inventory was one with a 4.57 m diameter by 18.29 m long cargo bay. The maximum capability of this baseline EOS was specified as 24,500 kg to a 28 deg 185 km circular earth orbit. Later EOS design criteria, however, established the EOS capability to the 185 km. 28-1/2 deg inclination orbit at 29,500 kg. This larger EOS would allow utilization of a larger Tug propulsion module. The study had shown that the desirability of a larger propulsion module was generally questionable unless the size could be increased to on the order of 40,900 kg. However, if the aerobraking mode was proven feasible, this larger EOS capability could allow either placement or retrieval of 4500 kg of payload to or from geosynchronous orbit with a single EOS launch.
Considering the overall mission requirements and the required compatibility of the Space Tug with the other elements of the Space Transportation System, an inventory of Space Tug elements was selected. This inventory could accomplish, when assembled into the proper configurations, the overall mission spectrum. The selected Tug inventory consisted of the following components:
The primary propulsion module was designed for earth orbit and planetary missions. This module would use LOX/LH 2 propellant at a nominal mixture ratio, by weight, of 5 to 1, LOX to LH2. The primary thrust would be provided by an uprated RL-10 engine which would provide a maximum thrust of 10,600 kgf at a specific impulse of 460 seconds. The engine was throttleable over a range of from 10 percent maximum thrust to maximum thrust. It was equipped with an extendible nozzle section which could be retracted 1.6 m to minimize length for transport in the EOS or to minimize the interstage length when tandem stages were required or desired. Utilization of this stage for lunar missions would require application of increased insulation and micrometeoroid shielding; a reaction control system booster kit; and auxiliary power kit; and a landing leg kit.
Expendable drop tanks for very high energy missions would be desirable to minimize the size of the required Space Tug configuration. The expendable drop tank consisted of the same tankage arrangement and pressurization systems as that of the primary module. The insulation was the same as that provided to the primary propulsion module. The items deleted from the primary propulsion module to provide drop tanks include the reaction control system, engine, reaction control system, thrust structure, electrical actuation system for engine gimballing and some of the micrometeoroid protection systems.
The Space Tug's basic characteristics included:
Primary Propulsion Module (45,000 lbs Weight)
Drop Tank Module
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Space Tug. This illustration (from 1984) depicts a manned space tug returning to a space station from geostationary or lunar orbit. The vehicle passes through the Earth's atmosphere to slow down; its aeroshell is heated to thousands of degrees by kinetic friction. The small cylinder is the crew module.
Credit: NASA via Marcus Lindroos