From: stellare@NOSPAMPLEASE.erols.com.retro.com
Lex Spoon wrote:
> rk writes:
>
>> Lex Spoon wrote:
>>
>>> A common reason voiced is that the CPU usage is too much. But this is
>>> bogus, IMO. CPU efficiency is the "launch mass" for computer people.
>>> Fast CPU's are extremely cheap compared to the cost of a single
>>> programmer's salary, and thus economically you should splurge on the
>>> CPU's. The days are long gone when companies have *one* computer plus
>>> a swarm of programmers hovering around it.
>>
>> But on many spacecraft, the cost of an extra computer is "expensive" and
>> it's worth some effort for the designers to spend some effort to
>> eliminate computers that aren't needed.
>
> I wasn't proposing to use more computers, but faster ones.
OK, sorry, you emphasized the "*one*" and I thought you were talking about
quantity.
What computer would you have suggested for this mission, at the time? The one
that they are using is capable of multiple speeds, up to 20 MHz, for a basic
operating clock. Note that during cruise you are subject to the deep space
environment. During EDL and on the surface, you do not get the same
protections that you do on Earth, as their are no magnetic fields and the
atmosphere is relative thin. Roughly, only a factor of 2 reduction in heavy
ion flux, with only low energy stuff blocked.
If you go with faster computers, if available, and all things being equal, you
also have to deal with the extra power consumption and how that fits into the
power profile of the mission, conductive cooling paths (need to keep the
junction temperatures low for reliability, roughly speaking, figure a factor
of two reduction in reliability for each 10 degree C increase in temperature,
and this is single string electronics). Analyze each of these for the CPU and
memories (in this case the RSC6000 and IBM DRAMs; EEPROMs are used for booting
and are inherently slow) and support logic devices. Of course, if you have a
substantial power increase, this can affect the design of the power supplies,
the capacity of the batteries, the thermal design, and the size of the solar
arrays. Working off of that you must analyze the increase of mass that will
likely come with these changes. And, depending on margins, the decrease in
science. Another trade-off, if you have available mass, is to put redundant
electronics on-board, increasing the reliability. Since missions to Mars are
rather expensive (this one costs $820,000,000.02) the trade-off may often be
in favor of spending more on programming and doing more science.
> This example was supposed to be illustrative. A few decades ago,
> computers were expensive compared to programmers, and you tended to
> have lots of programmers hovering around each computer. Try to
> picture that in a modern office -- it's hard, isn't it! Nowadays it's
> the other way around, with computers all over the place. Nowadays CPU
> time is cheap compared to programmer time, and so the appropriate
> design strategy is different: make things easy on the programmers, in
> order to conserve your most valuable resource.
Using your analogy, you can get an office computer for say $1,000. An
aerospace worker costs approximately $100 per hour, with overhead. Thus, for
10 worker-hours, you have the breakeven point for an extra computer. In
general, the electric load as well as the thermal one is not a major driver in
the office environment.
However, the flight to Mars and the surface of Mars is not an office
environment. So, it's not clear that your analogy holds.
--
rk, Just an OldEngineer
"For a successful technology, reality must take precedence over public
relations, for nature cannot be fooled."
-- R. Feynman, Appendix F.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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