The hydrogen in the Space Shuttle Main Engine creates embrittlement in the alloys of the engine. [1][2] The launch sequence also involved running the engines at 104.5% of rated capacity. [3] Before Block II of the engines, the turbopumps needed to be disassembled and rebuilt between flights. [4]<p>I have no real idea how significant coking of the Merlin engine is, but the SSMEs were hardly &quot;clean&quot;.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hydrogen_embrittlement" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hydrogen_embrittlement</a>
[2] <a href="http:&#x2F;&#x2F;www.tms.org&#x2F;Superalloys&#x2F;10.7449&#x2F;1991&#x2F;Superalloys_1991_749_760.pdf" rel="nofollow">http:&#x2F;&#x2F;www.tms.org&#x2F;Superalloys&#x2F;10.7449&#x2F;1991&#x2F;Superalloys_1991...</a>
[3] <a href="http:&#x2F;&#x2F;www.interspacenews.com&#x2F;FeatureArticle&#x2F;tabid&#x2F;130&#x2F;Default.aspx?id=2130" rel="nofollow">http:&#x2F;&#x2F;www.interspacenews.com&#x2F;FeatureArticle&#x2F;tabid&#x2F;130&#x2F;Defau...</a>
[4] <a href="http:&#x2F;&#x2F;www.nasa.gov&#x2F;sites&#x2F;default&#x2F;files&#x2F;174534main_ssme.pdf" rel="nofollow">http:&#x2F;&#x2F;www.nasa.gov&#x2F;sites&#x2F;default&#x2F;files&#x2F;174534main_ssme.pdf</a>