ISS rotated 180 degrees successfully by non-propulsive maneuver

by Chris Bergin

The International Space Station (ISS) was successfully rotated by 180 degress today, without the use of propellant.

The rotation of the station – now flying in orbit on with the -X-axis in Velocity Vector (-XVV) was performed by scheduling a series of commands to the ISS Control Moment Gyro (CMG) attitude controller – a new method developed by Draper Laboratory.

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The 180 degree no-prop maneuver was completed successfully at GMT 062:19:26 (1:26pm CST), lasting around 2 hours, 45 minutes. The maneuver utilized three CMGs with 80 attitude/rate commands.

The new method was developed by Draper Laboratory for performing large angle spacecraft maneuvers. This is the second non-propulsive maneuver that has been executed, although the first only involved a 90 degree maneuver, which was successfully performed on November 5, 2006.

By scheduling a series of commands to the ISS Control Moment Gyro (CMG) attitude controller, the ISS was maneuvered through a pre-planned trajectory, referred to as the Zero Propellant Maneuver (ZPM). The ZPM method does not require modification to the flight software nor use of thrusters.

Prior to the development the ZPM method, large angle re-orientation of Station was performed using thrusters as the CMGs do not have enough capacity to control along a nominal trajectory.  The ZPM has the benefit of saving valuable propellant by not using the station thrusters, and also avoids solar array plume impingement and contamination issues associated with thruster firings.

According to Dr. Naz Bedrossian and Sagar Bhat of the Draper Laboratory, the team that developed the ZPM, The pre-planned trajectory is optimized to take advantage of naturally occurring environmental torques in a coordinated manner in order to maintain CMG capacity within operational limits while transitioning the spacecraft between specified initial and final rotational states without firing thrusters.

If the CMGs were used without the ZPM, they would have rapidly reached capacity at which point control would have been switched to thrusters in order to complete the rotation.

During the maneuver, the Solar Array Rotary Joint was still enabled, tracking the sun – as were all Arrays. The CMG momentum only reached a peak of 75 percent capacity during the maneuver.

The new method allows the ISS to conserve its propellant/comsumable levels, whilst saving NASA a fair amount of money. The same maneuver was performed in January, using 50kg of propellant via thrusters – and given propellant costs more than $20,000 per kilo, the ZPM saved around $1m.

The next no-prop maneuver is tentatively scheduled for March 14, when the ISS is rotated back 180 degrees.

Draper has a long history of contributions to manned space missions, starting with the Apollo Program and continuing through operational support for the Space Shuttle and ISS.  Draper currently is working with NASA to develop the next generation of spacecraft for Space Exploration.

Headquartered in Cambridge, Mass., Draper is a non-profit engineering research and development laboratory dedicated to providing technological solutions in areas including guidance, navigation, and control; highly reliable embedded software; autonomous systems; miniature, low-power electronics; and biomedical engineering. Draper serves the national interest through applied research, engineering development, and technology transfer.

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