It is true that Modern Smart phones have much more Computing power when compared to Apollo mission computers but computing power alone is insufficient to put your experimental rocket in space. You need to find a way to integrate all the rocket’s other control and sensory systems to your smartphone which is sadly not possible. In addition, you would also need to do away with all the flashy GUI based apps which would otherwise be heavily taxing the processor just to provide that rich user interface instead of helping out your rocket.
And of course, you would also need to be super genius – say to the level of an expert rocket scientist and a super pro computer programmer combined.
I guess we’ll have to grow brains all over our bodies first for that!