There is increasing interest in employing Solar Electric Propulsion (SEP) for new missions requiring transfer from low Earth orbit to the Earth-Moon Lagrange point, L1. Mission architecture plans place the Gateway Habitat at L1 in the 2011 to 2016 timeframe. The Gateway Habitat is envisioned to be used for Lunar exploration, space telescopes, and planetary mission staging. In these scenarios, an SEP stage, or "tug," is used to transport payloads to L1--such as the habitat module, lunar excursion and return vehicles, and chemical propellant for return crew trips. SEP tugs are attractive because they are able to efficiently transport large (>10,000 kg) payloads while minimizing propellant requirements.
To meet the needs of these missions, a preliminary conceptual design for a general-purpose SEP tug was developed that incorporates several of the advanced space power and in-space propulsion technologies (such as high-power gridded ion and Hall thrusters, high-performance thin-film photovoltaics, lithium-ion batteries, and advanced high-voltage power processing) being developed at the NASA Glenn Research Center. A spreadsheet-based vehicle system model was developed for component sizing and is currently being used for mission planning. This model incorporates a low-thrust orbit transfer algorithm to make preliminary determinations of transfer times and propellant requirements. Results from this combined tug mass estimation and orbit transfer model will be used in a higher fidelity trajectory model to refine the analysis.
Solar Electric Propulsion (SEP) tug for the Revolutionary Aerospace Systems Concepts' Hybrid Propellant Module project. Nine 50-kW gridded ion thrusters are mounted on the deployable boom to enable the base pallet, arrays, and payload vehicle to remain in a solar inertial attitude while maintaining a constant thrust along the center of mass. The
deployable boom is conformable to enable it to fit around the payload vehicle. In this case, the xenon propellant is on the payload vehicle, which is not shown. For scale, the coilable center mast section of the deployable boom (in yellow) is 20 m in length.
Long description
The SEP tug system model was used in two design studies in 2001. First, the conceptual design of the Hybrid Propellant Module under the Revolutionary Aerospace Systems Concepts program at the NASA Langley Research Center required an SEP tug to transfer it to L1. This SEP tug, shown in the preceding illustration, would deliver a 36-metric-ton (MT) payload to L1 in 272 days. The second conceptual design activity was for the Lunar L1 Gateway mission architecture study at the NASA Johnson Space Flight Center. The SEP tug in this case would be able to deliver the required 30 MT payload to L1 within the time constraint of 180 days. The tug's performance is shown in the following graph, where the transfer time as a function of power on the tug is shown, along with the xenon propellant required.
Solar Electric Propulsion (SEP) tug performance for Johnson Space
Center's Gateway mission. Transfer time from low Earth orbit to Earth-Moon L1
and xenon propellant mass required for transfer are shown as functions of
the power generated. At 584 kW, the tug would be able to deliver the 30
MT payload in 179 days.
Long description
For both studies, the SEP tug performance satisfied the requirements of the respective mission designers, and the SEP concept was incorporated into the mission architectures. Further refinement of the system model, including structural and dynamic analysis and incorporation of the results of the improved orbit trajectory, is planned. The development of the SEP tug system model has been a joint effort by Glenn's Power and Propulsion Office, Power and On-Board Propulsion Technology Division, and Systems Engineering Division, through Glenn's Systems Assessment Team.
Glenn contact: Timothy R. Sarver-Verhey, 216-977-7458, Fax 216-433-2995, Timothy.R.Verhey@grc.nasa.gov
Authors: Timothy R. Sarver-Verhey, David J. Hoffman, Thomas W.
Kerslake, Steven R. Oleson, and Robert D. Falck
Headquarters program office: HEDS (Advanced Programs), OSF (Advanced Projects)
Programs/Projects: Advanced Power and On-Board Propulsion,
Systems Engineering, Power and Propulsion
Engineering and Technical Services
Last updated: June 2002
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