Systematic study to reduce the effects of cracks in multilayer insulation Part 2: experimental results

Abstract

A series of cracks with different widths and shapes was cut in a multilayer insulation (MLI) blanket. The measured data shows that the incremental heat load per unit slot area has a maximum of ≈ 135 W m −2. The heat load increment is essentially independent of the preparation of the cold surface under the crack, i.e. its emissivity, if the slot width is sufficiently small. The temperature distribution and the equivalent thermal conductivity near the cracks are quite different from that in a system without cracks. The dependence of the heat load and temperature distribution on the vacuum pressure was also observed. A systematic study of a crack-covering ‘patch’ method to reduce the heat load to a 77 K surface through cracks in a MLI blanket was conducted. The following patch materials were used to determine the optimum distribution of the patches in a 30 layer blanket: 300 Å ‡ single aluminized crinkled Mylar (NRC-2) and 1000 Å double aluminized flat Mylar. The experimental results indicated that the use of a patch every few layers is almost as effective as using a patch every layer. Placing the patches in the upper half of the blanket is much better than in the lower half and can reduce the heat load essentially to that without cracks. Putting the same number of patches on top of a crack is much less effective. The data suggest that 1000 Å material is preferable for patches. All of the experimental results are generally in agreement with the enhanced black cavity model.