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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].
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    $\begingroup$ possible duplicate of Can a solid first stage compete with a liquid fueled first stage? $\endgroup$ – Nathan Tuggy Aug 13 '15 at 0:58
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    $\begingroup$ High specific impulse is nice to have, but you also want enough thrust to lift you off the ground. So you'd want high flow rate on top of having high fuel efficiency. $\endgroup$ – TildalWave Aug 13 '15 at 0:59
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    $\begingroup$ SpaceX's self-recovering 1st stage is probably the best argument for an all liquid fueled design. Remember the space shuttle program began in 1972, with the first working orbiter delivered in 1979. They couldn't just wait for 30+ years of technological advance needed for a self-recovering 1st stage. $\endgroup$ – DepressedDaniel Mar 12 '17 at 17:34
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Thrust and cost. Each SRB produced as much thrust as 6.5 of the shuttle orbiter's main (hydrogen fueled) engines (SSMEs) with a much simpler design.

A high powered liquid fuel engine would not come through a hard water landing like the SRB in reusable or repairable condition.

A winged reusable liquid fueled booster strategy was considered, but I suspect it would have probably doubled development costs.

An incremental strategy of developing a powered booster return, like SpaceX's, wouldn't have been considered; reusability was a hard requirement.

If I've done my math right, a pair of liquid-fueled boosters using either 5 SSMEs or 2 F-1s each could have worked as a replacement for the SRBs. The F-1 based booster -- a shorter version of the Pyrios booster being proposed for SLS -- might even have had enough performance headroom to do a Falcon 9-style powered descent, but a lot of very expensive engines would have been dropped in the Atlantic before that was accomplished.

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    $\begingroup$ This is the correct answer. Shuttle development budget was capped at $4 billion and the original, fully reusable booster + orbiter couldn't be developed for that. The "Thrust Augmented Orbiter System" TAOS that became STS was genned up to fit within the budget, thereby increasing operational costs for the lifetime of the system :( $\endgroup$ – Organic Marble Aug 13 '15 at 2:09
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    $\begingroup$ That '7 tons' figure is when you use the Soyuz rocket as a satellite launcher, I suspect. When you launch with a Soyuz manned capsule, payload is much less. $\endgroup$ – Hobbes Aug 13 '15 at 6:18
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    $\begingroup$ The Soyuz spacecraft is a 7 ton payload. $\endgroup$ – Russell Borogove Aug 13 '15 at 11:45
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    $\begingroup$ You're comparing apples to oranges. If the Shuttle has a payload of 24 tons plus 7 astronauts, a Soyuz has a payload of 3 astronauts plus whatever fits in the capsule, which is a few hundred kg at most, not 7 tons. $\endgroup$ – Hobbes Sep 18 '15 at 9:43
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    $\begingroup$ When I say "Soyuz puts about 7 tons of payload into LEO" I mean the Soyuz launcher, not the spacecraft. I consider the shuttle orbiter to be launcher, not payload. $\endgroup$ – Russell Borogove Sep 18 '15 at 13:16

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