The radiator has an area of 1 m^2, has a surface with alpha-s = .2, epsilon-ir = 0.8, and is equipped with internal heat pipes sufficiently effective that it can be considered isothermal at temperature Tr. It is exposed to conditions that can vary from full sun (qs = 1400 w/m^2) to deep space.
An aluminum (k = 164 w/mK) thermal doubler with the dimensions shown runs to an electronics box. Model it as two thermal resistances- one from the radiator to point A, and the other between points A and B.
An electronics box contains equipment that gives off 50 w in waste heat. Structurally, it looks like an aluminum plate 0.5 cm thick; thermally assume that the heat is dissipated by the electronics in such a way that the box is isothermal at temperature Te.
The box is effectively glued down with a layer of RTV silicon rubber 0.5 mm thick.
(RTV: alpha =90 micro-strain/K, k = 0.87 w/mK, E = .02 Msi, nu = 0.3)
- Figure out the temperatures Tr, TA, TB, and Te in for each design, for the worst (most extreme) environments. Clearly state the assumptions used to perform the analysis.
- Figure out the shear stresses and strains in the RTV.
- The thermal doubler is replaced by a composite with approximately the same thermal and mechanical properties as aluminum, but alpha=0. What happens? Assume room temperature assembly.
- At what temperature will the middle layer crack?
- If it is found that the toughness of the material is reduced according
to the relation G(N) = G(0) .94^log(N), how many cycles to a low temperature
of -25°F will crack the middle layer?
El = 20.6 Msi
Et = 1.42 Msi
alpha-l = +0.05 micro-strain/°F
alpha-t= +16 micro-strain/°F
GIc = 150 J/m^2 (=.855 English units: in lb/in^2)
zeta (xi in notes) = 0.6