543 Thermal Vacuum Testing Lessons Learned for Small Stepper Motors and a  CubeSat Translation Mechanism  

Alex Few*, Lynn Albritton*  and Don McQueen*  

Abstract  

Near Earth Asteroid (NEA) Scout is a deep space satellite manifested on NASA’s ARTEMIS 1 as a  secondary payload. The spacecraft meets the CubeSat 6U standard (about 300 x 200 x 100 mm) and is  designed to travel 1 AU  (150,000,000 km)  over a 2- year mission t o observe a NEA 1. Once dispensed from  SLS, the NEA Scout will use an 85- m2 solar sail to maneuver from lunar orbit to the asteroid. One of the  critical mechanisms aboard NEA Scout, the Active Mass Translator (AMT), serves as a trimming and  momentum managem ent mechanism for the sail system as it balances the sail center of pressure and the  vehicle’s center of mass . The AMT produces 150 x 68 mm of translation at sub- millimeter precision and  accommodates a shielded wire harness and coax cables during operation. The system has strict power,  mass and data budgets and must survive operation in a shaded deep- space environment . The AMT system  has recently completed and passed environmental testing. This paper will discuss lessons learned through  three consecutive thermal vacuum tests spanning nine months and includes insight from the NASA Marshall  Space Flight Center (MSFC) design/test team, NASA MSFC Subject Matter Experts in DC motors and  electronics and the NASA Engineering and Safety Center (NESC) Mechanical Syst ems Discipline Team  (MSDT). Important points of discussion will include (1) failure modes of a micro stepper gear motor in  vacuum and destructive analysis findings, (2) instrumentation of a TVAC test to determine stepper motor  health in near -real time, (3)  determination of the duty cycle at a given operational environment, and (4) the  design of the TVAC test profile to discover thermal capabilites of the micro stepper motors in vacuum.  Papers were previously presented at the 44th Aerospace Mechanisms Sympos ium entitled “Testing and  Maturing a Mass Translating Mechanism for a Deep Space CubeSat” and the 43rd Aerospace Mechanisms  Symposium entitled, “Development of a High Performance, Low Profile Translation Table with Wire  Feedthrough for a Deep Space CubeSat ”.  

Introduction  

The AMT was added to the Near Earth Asteroid Scout project late in the design cycle. So, the volume for  the device had to be carved from other subsystems already under design and development. This late  addition caused the design team to make design decisions  that would not normal ly be recommended in  order to have a chance at meeting volume, cost and schedule targets. One such decision was to use a  commercially available space rated stepper/gear motor arrangement. This paper describes t he problems  encountered in using this motor design in vacuum. and the steps taken to complete qualification testing of  the AMT design.  

The AMT test phase was originall y scoped as a one week activity but grew to nine months due to back -toback stepper motor failures during TVAC testing. Prior to further discussion, It should be noted that the  motor s chosen for the AMT design were the smallest motor s commercially available, and had been used  in a flight design befor e. However, the previous flight proj ect app lication was in atmoshere (in cabin)  promoting convective heat flow . Since this was a first -time in -vacuum  application for these motors,  significant design changes (discussed in detail in previous papers cited in the abstract) were made to create  a more thermodynamically and mechanically robust design. The mechanical interfaces were revised to  include an indium and aluminum clamshell design. This clamshell approach used all surface areas available  on the motor and transmission casing as thermally conductive paths . Even so, both failures  proved the 

* Marshall Space Flight Center, Huntsville, AL  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020