[time-nuts] Space mission comes to an end becuase of a "computer time tagging" problem

[Chris Albertson]

On Sat, Sep 21, 2013 at 11:03 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>

> We are talking about running computers and radio gear rather than motors.
>
> I can see running motors in bursts from batteries, but computers and radios
> generally run continuously so they shouldn't need a battery when the sun is
> up.
>
> Am I missing something?  Can't the computers and radios run off solar power
> when the batteries are dead?  Does the receiver take more than (ballpark)
> peak solar-cell output under nasty conditions?
>

I jst looked up some specs.  The electronic is housed in a heated box.
 Apparently if the heat goes away some parts fail.

Also it is a close call if the radio and computer could run under
solar power alone.  It can if the communications path uses a Mars
orbiting relay but it takes 55W to transmit directly to Earth and the
computer burns 15W.  The computer can't run full time.  It uses to
much power so the solar charger has a built-in timer that can wake the
computer.   I looks like normal operation is to let the battery charge
while the computer and everything thing else of powered down, then
after so much time the charger wakes the computer so it can do
something, then back to charging.    This lets them do work with very
degraded solar cells.    But at some point the battery freeze and the
heated electronics box can't be kept warm any more.    This was a
reasonable design for a system that had a 90 day planned operational
lifetime.

There are a lot of things they could have done.  For example why not
push the arm into the ground hard enough to left the front wheels and
tilt the panels square to the sun?   I guess the added weight of the
more robust arm was not worth it.  They had a weight, volume and cost
and time budget to work with.



-- 

Chris Albertson
Redondo Beach, California