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How a Small Wheel Helped to Stabilize Philae's Bumpy Landing

The genesis of Philae's momentum wheel and the emotions of comet-landing day in the words of a Surrey Satellite Technology engineer who worked on the project.

Richard Williams, our colleague at Surrey Satellite Technology Ltd. (SSTL), is an engineer who worked on the momentum wheel supplied by SSTL in 2001 for the Rosetta mission. Space Blog asked Richard about the project, and how it felt to see the mission come to such a fantastic conclusion last week…

Last week’s comet landing event was definitely one of the high points in my space engineering career – and my family are amazed that something that I have had a hand in designing and building has survived a long sleep through space, functioned brilliantly, and is now parked on a comet!

Looking back, I remember that at the time that the Rosetta mission was being devised, I was a lead engineer here at SSTL working mainly on small reaction wheels that we had started developing for our own satellites. Our reaction wheels were spun up to change the pointing of the satellites, so that the camera onboard the spacecraft could take targeted images of Earth.

The request for the Philae Lander was for another type of wheel – a momentum wheel. Momentum wheels work in a different way from reaction wheels, spinning at a constant speed to “lock” the spacecraft to point in one direction. However, our small reaction wheels were just the right size for the Lander. So we adapted our design to perform as a momentum wheel and ensure that the feet on Philae always pointed towards the comet’s surface after being released from the Rosetta “mothership.” It obviously worked as Philae stayed the right way up during its initial landing and the subsequent “bounces”!

We were awarded the contract and our wheel designs were modified to take account of the slightly different environment and requirements of the Rosetta mission. One of the first decisions we made was to use a solid lubricant to “oil” the bearings, rather than the traditional liquid lubricant. We based this choice on the fact that Philae would be spending ten years dormant on the mothership as it made its journey towards the comet, and we couldn’t run the risk of an oil lubricant evaporating over the years.

Philae's momentum wheel motor
We also quickly realized that the motor bearings would be overloaded during the launch from the mothership, and so we included a third bearing. This, together with a stiffer main shaft, resolved that issue.

During testing we discovered that the current required by the wheel drive electronics to “power up” (called “inrush current”) was too high and modifications to the electronics were necessary.

The circuit board attached to the flywheel housing. The motor protrudes through a central circular hole in the PCB. There are two D-type 9-way connectors mounted on the housing at the top of the image, and to their left is the large inductor in series with the 28-volt input added to control the inrush current.
Finally we put the completed momentum wheel through environmental validation testing to ensure it would survive launch and operate in space.

The flywheel is mounted on an adaptor prior to thermal vacuum testing. The adaptor enables the unit to be tested with the motor shaft horizontal and attachment plane vertical.
Once all the testing was over, we delivered the wheel and sat back to wait for thirteen years!

After such a long wait, last week’s comet landing attempt was highly charged, exciting, and nail-biting, all at the same time. For me personally, getting confirmation that Philae had landed was an emotional moment, because it wasn’t just a great result for ESA, it was also a great result for the small SSTL team who had been involved in the project.


18 November 20140 Comments1 Comment

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