Re: Ariane5 FAQ

["Robert I. Eachus" <rieachus@attbi.com>]

Hyman Rosen wrote:
> Mike Silva wrote:
> 
>> What evidence do you have that if the behavior (I refuse to call it a
>> limitation) had been better documented, anybody would have noticed,
>> and realized that it would be a problem for the Ariane 5?
> 
> 
> The findings of the investigatory board, which say that.


Where? As far as I can tell there are no FINDINGS like that. Especially 
in an accident investigation, finding has a limited and well defined 
meaning. The findings (section 3.1) related to the failure are:

13. The inertial reference system of Ariane 5 is essentially common to a 
system which is presently flying on Ariane 4. The part of the software 
which caused the interruption in the inertial system computers is used 
before launch to align the inertial reference system and, in Ariane 4, 
also to enable a rapid realignment of the system in case of a late hold 
in the countdown. This realignment function, which does not serve any 
purpose on Ariane 5, was nevertheless retained for commonality reasons 
and allowed, as in Ariane 4, to operate for approx. 40 seconds after 
lift-off.
14. During design of the software of the inertial reference system used 
for Ariane 4 and Ariane 5, a decision was taken that it was not 
necessary to protect the inertial system computer from being made 
inoperative by an excessive value of the variable related to the 
horizontal velocity, a protection which was provided for several other 
variables of the alignment software. When taking this design decision, 
it was not analysed or fully understood which values this particular 
variable might assume when the alignment software was allowed to operate 
after lift-off.
15. In Ariane 4 flights using the same type of inertial reference system 
there has been no such failure because the trajectory during the first 
40 seconds of flight is such that the particular variable related to 
horizontal velocity cannot reach, with an adequate operational margin, a 
value beyond the limit present in the software.
16. Ariane 5 has a high initial acceleration and a trajectory which 
leads to a build-up of horizontal velocity which is five times more 
rapid than for Ariane 4. The higher horizontal velocity of Ariane 5 
generated, within the 40-second timeframe, the excessive value which 
caused the inertial system computers to cease operation.
17. The purpose of the review process, which involves all major partners 
in the Ariane 5 programme, is to validate design decisions and to obtain 
flight qualification. In this process, the limitations of the alignment 
software were not fully analysed and the possible implications of 
allowing it to continue to function during flight were not realised.
18. The specification of the inertial reference system and the tests 
performed at equipment level did not specifically include the Ariane 5 
trajectory data. Consequently the realignment function was not tested 
under simulated Ariane 5 flight conditions, and the design error was not 
discovered.
19. It would have been technically feasible to include almost the entire 
inertial reference system in the overall system simulations which were 
performed. For a number of reasons it was decided to use the simulated 
output of the inertial reference system, not the system itself or its 
detailed simulation. Had the system been included, the failure could 
have been detected.

Incidently, note that in finding 17, it only speaks about Ariane 5 
programme reviews, not the ones done for Ariane 4.  Similarly findings 
14-16 should be considered as a group, or you get the wrong impression.

-- 

                                            Robert I. Eachus

�In an ally, considerations of house, clan, planet, race are 
insignificant beside two prime questions, which are: 1. Can he shoot? 2. 
Will he aim at your enemy?� -- from the Laiden novels by Sharon Lee and 
Steve Miller.