They want multiple launch vehicles for a couple of reasons.

1. Competition drives the cost down a little.

2. Redundancy. If one launch vehicle gets grounded, the other can be used in its place. I haven't kept up and don't know if it has been replaced yet, but the Atlas V was using an NPO Energomash RD-180 main engine from Russia. That supply could be cut off at any time.


https://spaceflightnow.com/2017/06/...eplane-mission/



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