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How a small wheel helped to stabilise Philae's bumpy landing

The genesis of Philae's momentum wheel, and the emotions of comet-landing day from an engineer who worked on the project.
Richard WilliamsRichard Williams is an engineer here, who worked on the momentum wheel supplied by SSTL in 2001 for the Rosetta mission.  Space Blog asked him about the project, and how it felt to see the mission come to such a fantastic conclusion last week.  

Richard says

" 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 which 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 on-board the spacecraft could take targeted images of Earth. 
The request for the Philae Lander was for another type of wheel – a momentum wheel; these work in a different way to 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 it was 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. This choice was based 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 realised 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 which was added to control the inrush current.

Finally the completed momentum wheel was put 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.  We were running the live feed of the event on the TV in our café here at SSTL’s HQ, and we were all glued to the live updates coming out of Darmstadt.  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. 

However, it was also a bittersweet moment, as a key member of the team had died in tragic personal circumstances during the project. So, last weekend, we raised a glass to both the technical achievement, and to the memory of Mark Tucknott, a talented Engineer (and fantastic frisbee player!) prematurely taken from us." 






18 November 20140 Comments1 Comment

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