Dave VanHorn wrote:
>>Space Shuttle:  
>>What's the purpose of the spark shower around the main engines during launch?
>>Is this to prevent gas build up or does this actually light the thing off?
> 
> Both, I think.  You wouldn't want a pool of H2/O2 mixture to suddenly ignite.
> 
> The scary failure is one SRB failing to light. 

Fortunately they do a have safety mechanism for that.
The on board flight control computers (majority vote)
can blow off the top of the active booster and it
then sits on the pad as a roman candle, having no effective
thrust. I imagine that the gantry doesn't melt because
most of the energy is then expended upwards.

I found this in an FAQ on the NASA site shortly
after the Challenger stupidity. Humans can't react fast enough
to a SRB failure.

http://encyclopedia.thefreedictionary.com/Space%20Shuttle%20Solid%20Rocket%20Booster
http://travel.howstuffworks.com/space-shuttle2.htm

R

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