Variable View Factor Multi-Layer Insulation (MLI)

Project Objective

Analytically and experimentally compare traditional multi-layer insulation (MLI) efficiency to a newly developed variable view factor MLI efficiency. Mechanism design, MLI design and modeling, prototyping, and thermal testing will be completed.

Project Description

The goal of this project is to design a Variable View Factor Multi-Layer Insulation (MLI) that can adjust its effective emissivity and heat flow as needed to either reject more heat or become more insulative. This technology would be ideal for projects that operate heat generating components in a transient manner or projects with changing and diverse thermal environments. The Variable View Factor MLI would work by having each layer of MLI consist of two sheets of mylar with a hole pattern in each one. In an insulative mode, the hole patterns would not align and the Variable View Factor MLI would behave as traditional MLI. However, when in an open mode, the hole patterns would align allowing a higher view factor to space. The increased view factor then allows more heat to transfer through the layers by increasing its effective emissivity. Initial designs of this technology would use active control to adjust the view factors, but the eventual goal is to allow the Variable View Factor MLI to function passively using shape memory alloys. This effort would result in a hole pattern optimization analysis and a vacuum tested prototype.

Project Results and Conclusions

Multi-layer insulation (MLI) is ubiquitous in any space system that must insulate itself against radiative heat loss to space. A typical design approach is to size radiators to accommodate maximum waste heat load in a worst-case hot condition and then apply MLI and makeup heaters to the remaining areas for the worst-case cold condition. With higher-performance spacecraft needing to operate in more extreme thermal environments, new approaches to thermal control are required. An initial investigation has been performed into a variable MLI (VMLI) that provides an adjustable effective emissivity, allowing the spacecraft to vary the required insulation performance for the current mission condition.

A developmental test and analysis campaign, funded by the Center Innovation Fund (CIF), established initial performance of the VMLI and provided many lessons learned to be applied to future maturation of the technology. A Thermal Desktop model, built by summer intern Kermit Booker, proved highly nuanced and more difficult to build than initially expected. By the end of the summer the model was running but useful analysis was unable to be performed. This model will provide a building block for the next intern to establish proper analysis cases to establish baseline performance expectations.

Testing, performed by early career employee Mackenzie Byrnes and summer intern Kermit Booker, resulted in an initial assessment of effective emissivity. Effective emissivity is the primary performance parameter of MLI. A test fixture was designed and built to suspend layers of MLI with 0.5” holes punched out in a grid pattern. Each layer of MLI was initially offset such that no hole was aligned with another in its adjacent sheet. This represents the “closed” position of the VMLI, which is the lowest effective emissivity (most insulating). The layers were then adjusted such that the hole pattern in each sheet was fully aligned, representing the “open” position (highest effective emissivity/least insulating).

Initial test results indicated that the effective emissivity in the closed position was e*=0.180 and e*=0.186 in the open position. The fact that they are very close indicates that the VMLI was not performing as anticipated. Upon review with more experienced thermal engineers, the test design was called into question as a major source of potential error that obfuscated the actual effectiveness of the VMLI. Future work includes performing a full thermal analysis to anchor the expected performance and designing an updated test fixture to significantly reduce the test error and provide accurate results.

This technology seeks to create a novel capability for multi-layer insulation (MLI) that provides inherent turn-down ratio within the system. The Variable View Factor MLI will have intentionally created holes in the material of each layer . In the “insulation” mode, the holes will be misaligned to ensure proper radiative insulation . In the “open” mode, the MLI layers will be adjusted to line up the holes allowing more heat transfer to space. This will enable future payloads to operate through more drastically changing thermal environments, e.g., from lunar day to lunar night and back.

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