which offer high lifetime (though often this is achievable only with the addition of a wheel re-lubrication  system). The disadvantage of the conventional approach for an accelerated wheel development program  is that to qualify a liquid-lubricated wheel, no strict tribologically valid method exists which can accelerate  the life test without modification to the lubricant regime in a manner which risks significant under-test. For  a solid-lubricated wheel however testing can be relatively straightforwardly carried out at high speed  without significant impact on the lubricant wear behavior.  SSTL's approach with such bearings can be thought of using the cage as providing a lubricant reservoir  with solid lubricant being transferred from cage to balls and ultimately onto the raceways as a so-called  "transfer film". Bearing lifetime is limited by ultimate wear-out of the cage pockets or by excessive or  lumpy transfer, which may in some applications create unacceptable torque noise.  For SSTL's first generation of reaction / momentum wheels, which were successfully flown, the lifetime  requirement was not particularly high and the self-lubricating bearings were adopted and used in a  relatively commercial manner. However, for this latest generation of wheels, there was a requirement to  demonstrate a much higher lifetime nominally 2.2 billion revs for a 7-year DMC type mission (including a  factor of 2) with low torque noise, combined with a programmatic need to demonstrate the life within an  approximate 12 month testing window.  With this change in requirements for lifetime there is an on going investigation into extending life by using  novel hybrid lubricant combinations, which are not presented in this paper.  MRW  Backaround / Development  The first experimental MRW flew in 1995 on FASat-Alpha which unfortunately failed to separate from the  launch vehicle upper stage. FASat-Bravo was built to replace FASat-Alpha and was successfully placed  into orbit in 1998. The experimental reaction wheel worked well and encouraged development and use of  similar reactions wheel on future missions.  Between 1997 and 1999 three reaction/moment wheels were developed: a nanosat momentum wheel, a  minisat reaction wheel and a modified microsat reaction wheel. All of these wheels were based on  commercial motors and with the exception of the nanosat momentum wheel, which used vacuum grease  lubrication, all used dry lubricated bearings.  The most successful of these wheels was the modified microsat reaction wheel, which flew on Thai Phutt  (TMSat), Tiungsat-1. TMSat and Tsingsat-1 each had a single wheel that was used for experimental  operations only. Tsinghua-1 had 3 reaction wheels and was operated in three-axis reaction wheel control  for months before its gravity gradient boom was deployed, during this time one wheel biased at 500 rpm  conducted over - 6.59~108 revolutions.  The modified microsat reaction wheel used for Thai Phutt, Tiungsat-1, Tsinghua-1 was the basis of the  EMRW. The EMRW is a primary attitude control component on six SSTL spacecraft currently in orbit. The  first of the spacecraft to use the EMRW was AISat-1 this was the first spacecraft of the DMC constellation,  launched in 2002. This spacecraft used two wheels in combination with a gravity gradient boom. Since  the lifetime requirements for the AISat-1 wheel were largely met by the on orbit operation of the Tsinghua-  1 wheels and the design of the wheels were similar, extensive ground testing and requalification was not  carried out.  UK-DMC, NiSat-1 and BilSat-1 were the next spacecraft to use the EMRW. These three were launched  together in 2003 and also form part of the DMC constellation. The core platform design of UK-DMC and  NiSat-1 was the same as AISat-1. However, BilSat-1 was first of a new generation of SSTL spacecraft  designed to be more agile and have a longer mission life (seven years instead of five). It was not  conclusive from the available data at that time that the EMRW design would have sufficient life to meet  the mission requirements of BilSat-1. Hence, as a contingency, Bilsat-1 also included a deployable boom  to allow gravity gradient ADCS operation in the event of a wheel failure.  189