149 Challenges of Designing a 13 -Hz High -Load Vibration Isolation System with Tight  Volume Constraints : Lessons Learnt and Path Forward   

Stella Dearing* and Dale Ruebsamen* 

Abstract  

This paper describes the design of a passive isolation system using D -struts® to isolate an optical  payload from aircraft -borne jitter  with challenging stroke per volume requirements . It discusses the use of  viscoelastic -coated D-struts®  that meet the customer performance and outgassing specification, NASA1124 . The result  was a relatively soft isolation system , (where the first mode was 13 Hz), with each  individual strut capable of withstanding loads on the order of magnitude of 623 N (140 lbf), weighing less  than 910 g (2 lbm), fitting in a volume 5 .1 cm (2 inches ) in dia meter and 12- cm (4.7-inches ) long and  capable of performing up to 1000 Hz without nonlinearities .     

Introduction  

This paper is a result of a contract to develop a passive isolation system using D -struts® for a high  altitude application to isolate an optical payload from aircraft -borne jitter and environmental influences  such as temperature, pressure, vibration, and shock loads . The design challenge consisted in meeting a  13-Hz isolation system 1st mode requirement within a limited volume with  no permi ssible non -linearities in  the operating range of 0- 1000  Hz while experiencing large vibration, shock and static loads. This  precluded the use of soft st ops and led to a design optimized to minimize length and diameter as well as  a strut soft enough to stro ke the required amount. This resulted in D -struts® with stroke per unit length  and component rotations outside Honeywell’s design heritage. It was found that such a soft spring in such  a small package resulted in a surge mode, surge mode harmonics and lateral modes at frequencies lower  and gains higher than typical designs. This was detrimental to performance out to 1000 Hz and meant a  change in modeling approach was required and a low outgassing viscoelastic coating  was applied to the  main spring to meet t he customer specification.      D-Strut ® Background  

D-strut® isolation systems have been used consistently in space for the last 25  years for many  applications, including satellite launch, momentum control , and payload isolation. They  are passive  mechani cal devices made up of a linear -action machined spring and a fluid damper subassembly  including hermetically sealed welded metal bellows . Figure 1  shows  the ‘MTV’ strut with its main   component s labeled. They are 3 parameter isolators, with a spring in parallel with a series combination of  a spring and damper, with 40 dB/decade roll -offs as compared to 20 dB/decade roll -offs from the more  traditional two parameter isolators (a spring and damper in parallel). When D -struts® are positioned in a  hexapod or octopod configuration they provide six degree of freedom support allowing decoupling  between the payload and the supporting structure making them ideal for isolation.  

                                                     * Honeywell Aerospace, Defense and Space, Glendale, AZ   

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016