Why was the decision made to have 2 boosters burning solid fuel alongside the liquid fueled SSME? Other designs of the time were pushing for all liquid fueled rockets (Russian Soyuz, Chinese Long March). Why didn't NASA take the same SRBs and make them have LOX/LH2 engines? They offer a higher ISP not to mention are completely environmentally friendly.
In googling for an answer, I have come up with a few arguments-
- So they could be easily recovered and reused.
Boosters with liquid fuel tanks could be recovered as well. In fact SpaceX's self-recovering 1st stage is probably the best argument for an all liquid fueled design.
- Classic rocket engines are only efficient at a certain altitude. SSMEs are ignited but throttled down at launch to be throttled back up to full power after SRB separation, where they are more efficient.
Engineer the rocket engine to be efficient at sea level.
Other things to consider:
- SRBs are difficult to control once ignited
- Challenger disaster occurred due to o-ring failure in the SRBs.
- According to wikipedia astronauts who have flown on multiple spacecraft report that Shuttle delivers a rougher ride than Apollo or Soyuz. [Due to SRBs].