How cheap can guidance go?

(John Carmack) :


I would note that we have had quite a few issues with our electronics.
Building rocket engines is easier than the mythology, but all of the
issues and problems with electronics are for real -- we have had
connector problems, electrical noise problems, thermal problems,
vibration problems, RF interference problems, pretty much the whole
spectrum of classic issues. There are a lot of mil-spec things at the
component level that are a really good idea to buy -- using good
tefzel wire and double crimp terminals and splices, for instance.

Question, does it have to really be mil-spec or will high end commerial
equipment work as well? And even if it does is the cost diffirence worth it
or would you say go mil-spec always to be sure as the money savings just is
not there for reliable equipment?

The most expensive part of the system is the fiber optic gyros. We
use an integrated system from Crossbow that costs $11k, because it
gets the readings digitized away from our noisy pc104 bus. Before
that, we used three discrete analog KVH FOG's and a separate triaxial
accelerometer that were quite a bit cheaper ($5k), but our PC104 A/D
board introduced quite a bit of noise. There are a few more overseas
companies selling FOGs in the last couple years, so there are probably
cheaper solutions available now. Of course, this is all dirt cheap
compared to military / commercial aviation IMUs...

Sounds like you think FOG is the way to go for now.

John Carmack
www.armadilloaerospace.com


Earl Colby Pottinger


--
I make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos,
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the time? http://webhome.idirect.com/~earlcp

Earl Colby Pottinger wrote:
(John Carmack) :


I would note that we have had quite a few issues with our electronics.
Building rocket engines is easier than the mythology, but all of the
issues and problems with electronics are for real -- we have had
connector problems, electrical noise problems, thermal problems,
vibration problems, RF interference problems, pretty much the whole
spectrum of classic issues. There are a lot of mil-spec things at the
component level that are a really good idea to buy -- using good
tefzel wire and double crimp terminals and splices, for instance.

Question, does it have to really be mil-spec or will high end commerial
equipment work as well? And even if it does is the cost diffirence worth it
or would you say go mil-spec always to be sure as the money savings just is
not there for reliable equipment?

A lot of Johns problems sound like problems resulting from bolting
hardware together.
At least some of the problems would go away if you designed it from
the ground up.

However, this probably isn't cheaper if you'r building ones.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
'Where subtlety fails, we must simply make do with cream pies' -- David Brin

For a rocket, yes. Cheaper things will work for things like RC
planes, but non light based strapdown rate gyros will have
acceleration biases and other issues to fret about. Some of them can
be compensated for, but it is really really really nice to know that
those issues just don't exist for your sensors.

There are, of course, more expensive options, like RLG's (ring laser
gyros), but rocket flight times are so short (relative to, say, a
submarine trip) that they would be pointless for flight control.

John Carmack
www.armadilloaerospace.com

I have a publication that I wrote on guidance and reliability issues.

http://www.vtol.net/guidance.htm

I don't believe wiring should be used at all. Wiring is obsolete for
anything but the shortest connections.

I have designed processor boards, motor driver control electronics,
fiber optic links etc. After a design is complete it costs about 200
dollars for a processor board and another 300 dollars worth of parts
on it. For a complete guidance system I would estimate at least 10k in
addition to the cost of the FOG of which I would want to have maybe 3.

Zoltan