John:
Per your suggestion, I have examined your data for the U band Receiver. I
shall first examine the power supplies for the receiver. The data that I
have elicited from your information is as follows, powers in Watts:
Device Dissipation Delivered
U16 0.340 0.850
U17 0.411
U18 0.384 1.200
U19 0.108 0.090
U20 0.040 0.224
------------ ------------
Totals 1.283W 2.364W
For a grand total power dissipation of the power system of 3.647W. Of the
delivered power you have identified two devices that suck up (U2 = 0.490W
and U3 = 0.388W) a total of 0.878W. It is not clear where the remaining
1.486W is dissipated. Your help in identifying where this thermal energy is
dumped in the module would be helpful, just to make sure that no stone
remains unturned in this matter of handling these dissipations. Hot
resistors could be an issue of concern......
You talk (somewhat glibly) of sinking regulator devices to PCB copper pads
and such. For at least five devices, U2, U3, U16, U17, & U18 I consider that
such mounting is not acceptable as such concentrated dissipations,
exacerbated by the vacuum environment, must be treated more specifically in
achieving heat sinking to the module housing. The module Base Plate, drawing
E05 21, being of somewhat robust 1.6mm thick aluminum should help
significantly in dissipating these module powers. More direct thermal
coupling methods are going to be needed in this module to get that heat to
the Base Plate. PCB thermal conductivities, even enhanced by added copper,
are not good at all and cannot really be depended upon in these situations.
With this module "pushing" the envelop of module power rating, baring a full
heatsink module, it is clear that the coatings of the module will not be
bare aluminum. This raises the ante in the picture of keeping command-level
modules from getting too cold in the eclipse situation. The original concept
for maintaining full control of the spacecraft through a 3 hour eclipse,
where the spaceframe can be as low as -130°C or lower, low power,
command-sensitive modules would not become dangerously cold, provided that
their dissipations are indeed low and the thermal coatings highly
reflective. If on the other hand we depend upon higher dissipation modules,
such as this URx, then the module temperatures may be a concern in the
eclipse situation. This is all subject to detailed analytic examination upon
the construction of a full spacecraft analytic model, which we do not have
at the moment, we first need a detailed mechanical design before having that
analytic model.
In any event, this URx presents a significant thermal challenge. Solving
this challenge first needs an understanding of the physical bodies of these
devices of the forenoted concern. Can you provide me with the physical
information on these devices, either directly or by URL? Can you also
respond to my earlier question on where the rest of the power dissipation
resides? By one means or another we need to engineer a more direct thermal
connection between these devices and the E05 21 Base Plate, which may
require some inventiveness of our collective crania.
'73,
Dick Jansson, KD1K
(ex: WD4FAB)
kd1k(a)amsat.org
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