322 Structural Details 

Most of the Tri-ATHLETE components are made from 7075-T6 aluminum. Due to the large thermal  extremes of the lunar environment this material is likely not appropriate for a flight system, but the low  cost, low mass, high strength combination led to  its use in the protot ype vehicle. 7075-T6 is  approximately 1.8X as strong as 6061- T6, but can be up to three times the raw material cost. Due to the  complexity and machining time of many of t he Tri-ATHLETE parts, the use of 7075-T6 added  approximately 20-25% to the total cost per part. Giv en the added strength to weight ratio of 7075-T6 this  trade was an economical way to keep the total mass down.  To further reduce cost and mass, bonded and rivet joints are used whenever pos sible. These joints act  similar to monolithic parts in their strength and weight  characteristics, but with out the high cost associated  with machining large complicated geometries. The out put tubes of all the joints are manufactured with  bonded and riveted end fittings as well as a bonded and rive ted interface that bolts to the circular spline of  the harmonic. Hysol EA 9394 adhesive is used as a liquid shim between concentric f eatures. Radial rivets  are then used to join the parts. In analysis, it is assumed that the rivets take the entire load while the  adhesive contributes no shear strength, only transmi tting compressive load. The body of the rover is  made from a similar riveted structure. Custom structur al ribs are joined to an aluminum sheet metal skin  using a combination of blind as well as solid aluminum rivets. This allows the body, shown in Figure 6, to  be a closed box structure, which gives a high stre ngth-to-weight and stiffness-to-weight ratio.   

Figure 6.  ATHLETE rover chassis, implemented as a riveted box structure   With the exception of the riveted tubes and the ri veted body frame, all structural interfaces are  implemented using a bolted joint in friction. This allows for interchangeable components without the need  for matched and pinned joints. Each structural joint was analyzed using a clamping force equal to the bolt  preload multiplied by the number of bolts with an assum ed coefficient of friction of 0.2. The bolt preload is  based upon the minimum load of either the bolt or the internal threads. For the bolt, the maximum load is  50% of the fastener ultimate tensile strength using the tensile stress area . For the internal threads, if an  insert is used, the preload capability is 50% of the pullout load for in serts. If the bolt threads directly into a  tapped hole, then the maximum preload is 57% of the load capability of the tapped hole. The application torque is then  T=KPD.                                                                           (1)  where P is the preload, D is the nominal diameter and K is the nut factor which depends on the lubrication  and surface finish. The nut factor that was used was based on tension versus torque testing performed at  JPL using Solithane applied to the threads. In the ATHLETE vehicle Loctite 243 was used instead of  Solithane, but it was considered an equivalent lubricant  for fastener installation and a similar fastener  staking method once it had cured. For all critical fa steners, 1200 MPa (180 ksi) steel black oxide socket  head cap screws were used. For non-structural fasten ers, such as external covers, stainless steel  fasteners were used. This makes for easy identificati on of fasteners that require precision assembly with  a torque wrench.  NASA/CP-2010-216272