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SYSTEM DESIGN NOTE

SDN0007.11 - Tangent Bar Thermal Issues

 1. Introduction

 This design note addresses shielding of the tangent bars. The tangent bars are three bars of G-10 that are attached to the bench in the center and to the bulkhead at their ends (2 locations for each bar). We want to minimize heat input through these bars.

 2. Conduction

 The basic geometry was originally laid out using a simplified conduction calculation, which ensured that heat conducted along the bar into the bench would not be excessive.

The bars are not quite rectangular - they are curved so that the "hot" ends have somewhat smaller cross-section - and the conductivity of G10 is a function of temperature as well. These two effects cancel out somewhat. The cross-section ranges from 7.3 cm² to 12.9 cm², while the conductivity ranges from 0.0032 W/(cm-K) at 60K to 0.008 W/(cm-K) at 300K. (This is conductivity along the fabric fibers; perpendicular conductivity, which is not relevant here, is somewhat less.)

 If the bar conduction is taken as kA/L (in the usual units), it makes sense to use an average value of the kA  product, which is about 60 mW-cm/K for the bar dimensions and the conductivity of G-10 described above. The path between attachment points is about 33 cm, so the flow from each end of a bar is roughly 0.4 W for a 233K temperature difference.

 3. Radiation

 The G10 is, however, a fairly efficient emitter; the emission from a cm of length near 300K is close to 0.5 W. It is possible to do a calculation similar to that done for wires (SDN012.01), considering the case of a bar of semi-infinite length attached at one end and bathed in a radiation field. The tangent bar is either inside the floating shields only (radiation temperature around 200K) or inside the active shield (radiation temperature assumed around 80K). For simplicity, and average cross-section and conductivity are used.

 Relative to a hot (300K) attachment point, the bar reaches equilibrium (heat flow <0.1W) at about 16 cm for the first case and about 30 cm for the second case. The heat flow into the bar is 2.0W and 2.5W respectively. This power is dissipated in radiation; in the first case it ends up in the shields, but in the second roughly half is absorbed by the bench.

 For the first case, we also need to consider what is happening at the cold end; the analogous calculation indicates that equilibrium is reached about 16 cm out, but that there is enough 200K radiation absorbed to provide a heat flow of 1.3W into the cold end.

 3.1 Solutions

 Thus in either case we end up with about 1.3W, for each half of the tangent bar, for the amount of heat coming into the bench. This implies that the bar should be shielded (wrap with aluminized Mylar - aluminum foil is too conductive for the purpose). A slight refinement would be to leave the warm end exposed to radiate if it is outside the active shield, down to a point where the thermal gradient equals what it would be with conduction alone. This is about 7K/cm, and occurs at about 5 cm length. This appears to reduce heat flow by about 10%, so it is not critical to do this or get it exactly right.

 An alternative approach is to "cold-station" the bars where they penetrate the active shield; if this can be done efficiently the heat flow into the bench is then nearly zero. In practice, this may be quite difficult given the space constraints around the tangent bars. Also, if the attachment point on the tangent bar is close to the 300K end, the heat flow into the active shield will be substantially larger than if the cold station point is close to the 60K attachment point. A reasonable approach is probably to examine the situation during assembly in order to see whether some simple cold station approach can be used for a given bar; trying to ensure feasibility during design appears difficult and is not required.

 3.2 Other Trusses

 The trusses attaching the rear of the bench to the bulkhead also need to be considered. The cross-section and area are both slightly less than those of the tangent bars, and the typical length is somewhat greater. Heat conduction will be about 2/3 that through the tangent bars. The same considerations apply in terms of shielding and possible cold-stationing.

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