Originally Posted by
B Fraser
The exhaust from the SRBs seems to be a little dark at the point of ignition, was that a concrete rich exhaust plume ?
Could be from the ignition device they use - those ammonium-perchlorate based solid boosters are not easy to ignite, so some sort of highly energetic pyrogen is used to light the propellant.
Read somewhere that the solid boosters need to light off withing a few hundreds of a second of each other or the asymmetric thrust can cause catastrophic structural damage (at least that was the case of the Space Shuttle).
Using ammonium-perchlorate in my high-power hobby rocketry, trust me that getting both solid boosters to ignite within such a short time period is highly non-trivial...