News Column

Patent Issued for Spacecraft Transfer Orbit Techniques

July 16, 2014



By a News Reporter-Staff News Editor at Defense & Aerospace Week -- Space Systems/Loral, LLC (Palo Alto, CA) has been issued patent number 8763957, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventors are Higham, John (Mountain View, CA); Tilley, Scott (Los Altos Hills, CA).

This patent was filed on October 8, 2012 and was published online on July 1, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "The assignee of the present invention manufactures and deploys spacecraft for, commercial, defense and scientific missions. On board propulsion systems of such spacecraft are frequently required to perform orbit raising (or transfer). For example, there is frequently a requirement for commercial spacecraft to perform orbit raising from a launch vehicle transfer orbit (or 'parking orbit') to, for example, a geosynchronous orbit. Following separation from the launch vehicle, the spacecraft then performs transfer orbit operations to transfer the spacecraft from the parking orbit to the geosynchronous orbit. The transfer orbit operations conventionally include firing a liquid apogee motor (LAM) and/or firing low thrust electric thrusters. After the spacecraft has completed its transfer orbit, the spacecraft may be reconfigured into an 'on-orbit' three axis stabilized configuration.

"Transfer orbit operations following launch vehicle separation typically include initial spin up about a spin axis, LAM and/or electric thruster firings, coast periods, spin rate changes, and reorientation of the spin axis during and between firings

"Alternatively, orbit raising operations may be conducted in a three-axis stabilized configuration, but such transfer orbit operations entail accumulation of momentum in reaction wheels, and periodic offloading, (or 'dumping') of the momentum by thruster firings during and between LAM firings. The above mentioned operations of changing the satellite spin rate, dumping momentum, and reorienting a spinning satellite consume, undesirably, an appreciable amount of propellant"

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventors' summary information for this patent: "The present inventors have appreciated that transfer orbit operations may, advantageously, be carried out, while avoiding spin-stabilization of the spacecraft. More particularly, following launch vehicle separation, and throughout a series of LAM and/or electric thruster orbit raising maneuvers, and/or intervening coast periods, the satellite may be continuously in a three-axis stabilized mode wherein rotation rates about all axes are zero or very small (less than 0.1 degree per sec). Advantageously, dumping of momentum accumulated in a momentum storage system during each respective intervening coast period is avoided.

"While in such a three-axis stabilized mode, attitude knowledge may be provided by one or more star trackers. Advantageously, reliance on an earth or sun sensor may be avoided.

"The star trackers may be configured to provide continuous inertial sensing over a 4.pi. steradian field of view. Control authority for maintaining satellite orientation may be provided by on board reaction wheels. The on board reaction wheels may be commandably configured to provide a spacecraft momentum bias, or a zero momentum bias. While in three-axis stabilized mode, the spacecraft may be oriented to a 'sun-safe' orientation to provide, for example, a desired power and/or thermal balance. The 'sun-safe' orientation may be selected, for example, before or between orbit raising maneuvers. During orbit raising maneuvers, while still in the three-axis stabilized mode, the spacecraft may be oriented to align a thrust vector of the LAM and/or electric thruster along a desired steering profile. Reorientation from a sun-safe orientation to an orientation desired for an orbit raising maneuver may be performed by a slow slew of the spacecraft, using reaction wheels. As a result of the disclosed techniques, a requirement to use propellant for such reorientation, for spinning or despinning the spacecraft, and for momentum dumping during coast periods is substantially eliminated.

"In an embodiment a method for performing spacecraft transfer orbit operations may include separating a spacecraft from a launch vehicle, while in a launch vehicle transfer orbit, the spacecraft including a propulsion subsystem and a spacecraft controller; controlling the spacecraft in a three-axis stabilized mode; and performing, with the spacecraft continuously in the three-axis stabilized mode, one or more orbit raising maneuvers. Each orbit raising maneuver may include either or both of a chemical thruster firing and an electric thruster firing. Any two consecutive orbit raising maneuvers may be separated by a respective intervening coast period. While in the three-axis stabilized mode, the spacecraft does not rotate about any axis at a rate greater than 0.1 degree/sec and dumping of momentum stored in a momentum storage system during each respective intervening coast period is avoided.

"In an embodiment, a solar array may be deployed prior to performing the one or more orbit raising maneuvers.

"In another embodiment, the momentum storage system may include one or more reaction wheels and control authority for maintaining spacecraft orientation may be provided by one or both of the reaction wheels and thrusters. In some embodiments, control authority for maintaining spacecraft orientation may be provided only by the reaction wheels. During the orbit raising maneuvers, the spacecraft may be in a maneuver orientation wherein a thrust vector of either or both of the chemical thruster and the electric thruster is aligned along a steering profile. During at least one of the intervening coast periods, the spacecraft is in a sun-safe orientation.

"In an embodiment, reorientation from the sun-safe orientation to the maneuver orientation may be performed by a slow slew of the spacecraft, using reaction wheels. The reorientation may be performed without use of propellant.

"In another embodiment, at least one orbit raising maneuver includes a chemical thruster firing and not an electric thruster firing.

"In an embodiment, a spacecraft includes a propulsion subsystem and a spacecraft controller. The spacecraft controller may be configured to control the spacecraft in a three-axis stabilized mode, wherein, while in the three-axis stabilized mode, the spacecraft does not rotate about any axis at a rate greater than 0.1 degree/sec and dumping of momentum stored in a momentum storage system during each respective intervening coast period is avoided. The spacecraft controller may cause the propulsion subsystem to perform, while the spacecraft is continuously in the three-axis stabilized mode, one or more orbit raising maneuvers, each orbit raising maneuver comprising either or both of a chemical thruster firing and an electric thruster firing, any two consecutive orbit raising maneuvers being separated by a respective intervening coast period."

For the URL and additional information on this patent, see: Higham, John; Tilley, Scott. Spacecraft Transfer Orbit Techniques. U.S. Patent Number 8763957, filed October 8, 2012, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8763957.PN.&OS=PN/8763957RS=PN/8763957

Keywords for this news article include: Aerospace, Spacecraft, Space Systems/Loral LLC.

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Source: Defense & Aerospace Week


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