beowulf in space

Jim Lux James.P.Lux at jpl.nasa.gov
Fri Apr 18 13:56:31 EDT 2003


I have to say that while this discussion is straying a bit from the usual 
enjoyable stuff on the list about switches, interconnects, and how fast one 
processor or another is, I find it gratifying that folks are coming up with 
creative ideas, and thinking about other applications for Beowulves than 
just computer rooms full of rackmounted computers.

Look back on the growth of cluster computing in the overall supercomputing 
business. Did anyone think, back in 1995, that there would be the 
penetration there is today?  My hope is that the same can occur for space 
applications, which, while different in details, have   many of the same 
drivers.

And on to my comments interspersed within...


At 04:54 PM 4/17/2003 -0400, Robert G. Brown wrote:
>On Fri, 18 Apr 2003, Mof wrote:
>
> > Ok excuse my ignorance, but what is involved in rad harding hardware ?
> > Is the cost really necessary, in that couldn't you put the unprotected
> > hardware into some sort of shielded container ?
> >
> > Or am I just being silly ? :-)
>
>Not really silly, but IIRC shielding is both difficult and expensive and
>sometimes actively counterproductive in space.  I'm sure the NASA guys
>will have even more detail, but:
>
>   a) Difficult, because there is a very wide range of KINDS and ENERGIES
>of radiation out there.  Some are easy to stop, but some (like massive,
>very high energy nucleii or very high energy gamma rays) are not.

Precisely the case. There's two aspects: total dose, which gradually 
degrades the components, and single event effects (SEE), which come from 
the "one big fast (high energy) particle" kind of things. SEEs can be 
either transient (upsets) or permanent (Gate rupture).

Total dose is talked about in terms of kiloRads or MegaRads (Yeah, I know 
the real units are Grays and Sieverts, but we work in rads for historical 
reasons). And, of course, taking dose and trying to collapse it into a 
single number ignores important things like the energy spectrum and dose 
rate effects (some degradation processes are enhanced and others reduced at 
low dose rates)

Single events are usually talked about in terms of Linear Energy Transfer 
(LET).. typically some number of MeV per cm, etc.  A neutrino may have high 
energy, but because it won't hit anything, it doesn't transfer any energy 
to the victim circuit, hence have low LET. On the other hand, a big old 
heavy ion, moving slow, has a very high collision cross section, so the LET 
might be quite high. LET is sort of a way to represent a combination of 
particle energy and reaction cross-section.



>   b) Expensive, because to stop radiation you basically have to
>interpolate matter in sufficient density to absorb and disperse the
>energy via single and multiple scattering events.  Some radiation has a
>relatively high cross section with matter and low energy and is easily
>stopped, but the most destructive sort requires quite a lot of
>shielding, which is dense and thick.  This means heavy and occupying
>lots of volume, which means expensive in terms of lifting it out of the
>gravity well.  I don't know what it costs to lift a kilogram of mass to
>geosynchronous orbit, but I'll bet it is a LOT.

$100K/kg is a nice round number...
Of more significance is that launch capability comes in chunks. You might 
have 300kg of lift, and if your box winds up being 320kg, you have to buy 
the next bigger rocket, at a substantial cost increase. At an early stage, 
your mass budget gets set according to your dollar budget. The mission 
designer divvies up the mass budget among all the folks clamoring for it 
(so many kg for attitude control, so many kg for thermal management, so 
many kg for instruments, etc.) holding a bit back in reserve (because 
systems ALWAYS get heavier), so that when the inevitable happens, they can 
still buy the cheaper rocket.



>   c) Counterproductive, because SOME of the kinds of radiation present
>are by themselves not horribly dangerous -- they have a lot of energy
>but are relatively unlikely to hit anything.  So when they hit they kill
>a cell or a chromosome or a bit or something, but in a fairly localized
>way.  However, when they hit the right densities of matter in shielding
>they can produce a literal shower or shotgun blast of secondary
>particles that ARE the right particles at the right energies to do a lot
>of damage (to humans or hardware).  So either you need enough shielding
>to stop these particles and all their secondary byproducts, or you can
>be better off just letting those particles (probably) pass right on
>through, hopefully without hitting anything.

Scattering is one of those horrible things.. adding shielding might make 
things worse. And the real problem is that it is very, very difficult to 
model accurately. So we make approximations (spherical shells, etc.), add a 
bit of margin, and go from there.

Think of this.. high energy neutrons are actually safer than thermalized 
neutrons, for human exposure (looking at RBE numbers), because the cross 
section is much higher for thermalized neutrons... they're slower. Same 
kinds of things apply to electronics.


>Basically, we are pretty fortunate to live way down here at the bottom
>of several miles of atmosphere, where most of the dangerous crap hits
>and showers its secondary stuff miles overhead and is absorbed before it
>becomes a hazard.  Our computer hardware is similarly fortunate.  Even a
>mile up the radiation levels are significantly higher -- even growing up
>in subtropical India I was NEVER sunburned as badly as I was in a mere
>two hours of late afternoon exposure in Taxco, Mexico, just one mile up.
>A single six hour cross-country plane ride exposes you to 1/8 of the
>rems you'd receive, on average, in an entire year spent at ground
>level.  God only knows what astronauts get.  Maybe they bank gametes
>before leaving, dunno...

IBM did a bunch of studies a while back comparing DRAM error rates at 
computers installed at sea level and in Colorado and in a mine in Colorado, 
and found significant (in a statistical sense) differences.



>So definitely not silly, but things are more complex than they might
>seem.  I'm sure that if a cost-effective solution were as easy as just
>"more shielding" the rocket scientists (literally:-) at NASA would have
>already thunk it.

All it takes is money...



James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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