Re: Ariane V update

[john@assen.demon.co.uk (John McCabe)]

Ken Garlington <garlingtonke@lmtas.lmco.com> wrote:

<..snip..>

>  o  The flight is called a "qualification" flight, which sounds to me
>     like it was part of the test program and not really a "production"
>     flight.

Weeelll... you could say that. It was basically the first real flight
but the payload (Cluster) was an experimental satellite (4 satellites
actually) funded by the European Space Agency, so it wasn't a
commercial flight.

>  o  The on-board computers are dual-redundant (which amazed me; I would
>     have expected triplex at least).The June 12, 1995 edition of AW&ST
>     apparently had an article on some problems encountered with the
>     development of the fail-operational [!] fault detection algorithms
>     between the two computers, requiring extra manpower to solve.

>  o  "The computers are more powerful than the single [non-redundant!]
>     one in the Ariane 4, but they use the same general logic."

You may be confusing redundancy with something else here. In my
interpretation, Dual-redundant means that each unit is effectively 2
identical units in e.g. 1 box. As far as I understand it, duplex or
triplex is related to having two/three separate units operating in
parallel using a voting system for example to aid fault-tolerance. I'm
not sure if Ariane 5 has this but it would seem reasonable for it to
do so. On the other hand, with Ariane 4, although it has a single
computer, this computer may actually contain 2 redundant halves.

Generally in the equipment we build, dual-redundancy is perfectly
adequate to satisfy most reliability requirements, whereas
triple-redundancy doesn't improve the (calculated) reliability much.
The dual-redundant system I work on at the moment has a calculated
reliability figure of ~0.996, but we had a look at creating a
single-redundant unit with a calculated reliability of ~0.989 or so.
There's always a trade-off though between mass, power and reliability
(and cost of course!).

However, if you can give any more information on the content of the
article you mention, it may prove that I am talking out of my arse :-)

<..snip..>

Best Regards
John McCabe <john@assen.demon.co.uk>