Integrated Design

Integrated Design Analysis: Structural, Thermal, and Optical
 

STOP and Go

How do you keep an electro-optic (EO) sensor in focus when even slight temperature changes of the lenses, mirrors, and support structure in an optical bench can adversely affect image quality? The problem is even more severe in a space environment, where thermal environments are extreme and no final adjustments are permitted if the design is flawed.

The answer is STOP: structural/thermal/optical integrated design analysis. Actually, this is not the answer so much as the goal, since many obstacles lie in the path of truly integrated design evaluation.

Many attempts have been made to achieve this goal. One early approach was to create a single design tool that could do a little optical analysis, a little structural analysis, and a little thermal analysis. “Little” meant that the unique aspects of each engineering discipline were ignored in order to create an all-in-one tool, where as those same aspects are the bread and butter of COTS (commercial off-the-shelf) tools targeted at each discipline. So while the all-in-one approach was useful for preliminary design by systems engineers, it could not be extended into later design phases where the skills of each discipline must be exploited, not oversimplified.

In the late 1990’s, CRTech led a NASA SBIR pathfinder project (“OptiOpt™”) that sought to overcome model translation hurdles between structural, thermal, and optical disciplines while respecting the unique talents and favorite tools of each engineering specialty. In addition to C&R’s Thermal Desktop®, Sigmadyne’s Sigfit® and ORA’s Code V® were therefore included in the software design. Significant successes were achieved, including the first automated STOP optimization using COTS tools (see publications: Integrated Analysis of Thermal/Structural/Optical Systems and Automated Multidisciplinary Optimization of a Space-based Telescope). Some very popular features of today’s Thermal Desktop, including automated mapping to independently-generated structural models and externally commanded parametric manipulations, were first developed as part of that project.

However, the OptiOpt project assumed that each discipline would start from the same CAD model, and then build independent models (the data from which must later be interchanged with other models). There was no centralization of model construction, and no attempt was made to capture the knowledge of each discipline such that it can be exposed to others on the design team. This meant that anything other than minor dimensional changes to the optical bench design had to be propagated manually by the team, and that the ability to intercommunicate design data also needed to be manually re-verified with each change.

A project by the Aerospace Corporation and Comet Solutions has made a significant advance in the ideal of integrated STOP analysis. While respecting the unique skills and tools of each discipline (significantly, the same ones used for OptiOpt: Thermal Desktop, Sigfit, and Code V), the Comet software also uniquely enables the ideal of that earlier “all-in-one” modeling environment: centralized model development.

Screenshot of Integrated Thermal, Structural, Optical Analysis Using Comet
Screenshot of Integrated Thermal, Structural, Optical Analysis Using Comet
 

Comet allows each discipline to participate in a common CAD environment, marking up a central drawing as needed to guide the generation of thermal, structural, and optical models. The experienced engineer's unique skills and tools are not lost or shoe-horned into a one-size-fits-all solution. Instead, the capturing of each disciplines’ methods into the central project means that design changes are readily accommodated. Team-level multidisciplinary design activities are thereby not only enabled, they are encouraged: each discipline can easily explore the ramifications that changes to their subsystem (e.g., materials selection, heater locations, strut sizing) have on the key mission objective: image quality.

The Aerospace team reported over two-fold increase in productivity, and good agreement between test and prediction.

Publications

For More Information

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.