AMTD - Advanced Mirror Technology Development In Mechanical Stability

Analytical tools and processes are being developed at NASA Marshal Space Flight Center in support of the Advanced Mirror Technology Development (AMTD) project. One facet of optical performance is mechanical stability with respect to structural dynamics. Pertinent parameters are: (1) the spacecraft structural design, (2) the mechanical disturbances on-board the spacecraft (sources of vibratory/transient motion such as reaction wheels), (3) the vibration isolation systems (invariably required to meet future science needs), and (4) the dynamic characteristics of the optical system itself. With stability requirements of future large aperture space telescopes being in the lower Pico meter regime, it is paramount that all sources of mechanical excitation be considered in both feasibility studies and detailed analyses. The primary objective of this paper is to lay out a path to perform feasibility studies of future large aperture space telescope projects which require extreme stability. To get to that end, a high level overview of a structural dynamic analysis process to assess an integrated spacecraft and optical system is included. Motivation NASA Marshall Space Flight Center engineers supporting AMTD are evolving engineering methodologies and tools with which future space telescope programs can perform quick turnaround parametric studies of proposed mirror designs. The efforts documented herein are focused on outlining a high level plan to perform structural dynamic analyses to address mechanical stability facet of such programs. The intended audience includes space telescope program managers, principle investigators, and perhaps systems engineers. The primary motivation of this paper is to facilitate meaningful mechanical stability feasibility studies associated with proposed future programs. Mechanical Stability General Discussion The barrage of tasks imbedded within predictions of what is herein labeled “mechanical stability” is complex. Although, at the highest level the task is that of “simply” determining the motion, with all things considered, of the optical system. As simple as that may sound, doing so entails predictions of both rigid body motions as well as flexible body motions of the mirror itself, its immediate support structure, and the spacecraft. Any vibratory motion, rigid and flexible, applied at the optical system to spacecraft interface is a dynamic excitation and the optical system’s response to that is a perturbation to stability. Sited stability requirements for a future large aperture space telescope are on the order of 10 pm RMS over ten minutes. In most other, not space optics, engineering efforts it is commonplace to proactively err on the side of conservatism. By and large it is reasonable and categorically “good engineering” to use the minimum envelope of pertinent damping data, for example, in structural loads analyses. Doing so will result in structural design loads that are acceptably conservative. That is, predicted dynamic loads that hardware would be sized to would be known to be higher than what would be experienced in service, but not too much so. It is always categorically imprudent to be un-conservative. However, in this arena with pm level stability requirements, with no known conservatism in mechanical stability analyses it will be challenging to demonstrate feasibility and thereby secure resources for a future project. For this reason, it is recommended that extreme caution be exercised prior to incorporating some common engineering philosophies. It is also recommended that care be taken to not underestimate assumptions utilized in feasibility studies and thereby lose credibility. These conflicting philosophies are always present but with the order of magnitude of target requirements being what they are, perhaps selected disturbance data, for example, used in feasibility analyses be given more thought than would be typical. With the challenge of future large aperture space telescope performance requirements in hand, all dynamic characteristics and excitations have to be accounted for and considered as well as any quasi-static loads. Ultimately, https://ntrs.nasa.gov/search.jsp?R=20150021394 2017-09-14T05:52:32+00:00Z