One approach that SSTL uses builds on its philosophy of "affordable access to space" by flying new  developments as back-up or experimental units on missions. Hence, flight qualification can be achieved  without the need for lengthy and costly ground qualification. The market SSTL is addressing is changing  though and there are now fewer opportunities to qualify in this manner with customers demanding proven  reliability and heritage.  To achieve the rapid development and qualification with wheels a different approach needs to be taken  from the typical wet (oil or grease) bearing lubricated systems. This simplifies the design, not requiring  reservoirs for the oil and anti-creep barriers but allows for an accelerated life test. Due to the properties of  wet bearing lubricated systems preventing acceleration of life tests with either speed or bearing preload,  wet lubricated systems have to perform real time life tests (at flight speed). This leads to very long,  potentially costly wheel qualification programs, which can lead to launch occurring before wheel  qualification is complete.  All of SSTL early spacecraft were microsats approximately 50-kg spacecraft 360-mm square by 750-mm  long in launch configuration. The AOCS system of these microsats did not rely on reaction wheels to  provide attitude stability. The spacecraft design used a gravity gradient boom to keep it nadir pointing.  Yaw motion was controlled using magnetorquers. Augmenting the AOCS with wheels allowed much  better control of yaw and the potential to off point from nadir to increase the imaging opportunities of a  particular target. Early SSTL wheels were flown as experiments providing enhanced performance above  the baseline mission requirements. This allowed development cost to be kept down by reducing ground  testing to a minimum.  All SSTL wheels to date and the current EMRW, SMRW and SRW use self-lubrication in the form of a  PGM-HT cage. Fortunately for such self-lubricating bearings, there is an increasing body of life test data,  which shows there to be a quite strong correlation between wear rate (i.e. bearing lifetime) and peak  Hertzian ball- raceway contact stress. Much of this data was recently generated by ESTL (European  Space Tribology Laboratory) as part of an ESA-funded campaign to source, fully characterize and qualify  a replacement material for the Duroid 5813 material at the time it ceased production. Taken as a whole,  this data includes both in-vacuum long lifetime application data with PGM-HT and Duroid 581 3 materials  and an in-air test campaign of approximately 3000 bearings aimed at generating design guidelines for  industrial applications of this type of bearing. Further information can be found in [I], [2]. The graph  presented in Figure 2 shows the relationship between preload and life which comes from experimental  data from tests and predictions from design analysis,  Test terninatiozls .  NOT end of life  Figure 2. Preload V's life graph  Use of solid lubricants in reaction and momentum wheel bearings is clearly quite rare. One reason for this  is the higher torque noise of some solid lubricated bearing systems, their perceived short lifetime and  intolerance to misalignment, competing against the long-established heritage of liquid lubricated solutions