<font color="yellow">....the "Irish-Italian friend, armchair rocket-designer, trapped in the body of an Uplink contributor" thinks that it is a crazy idea and suggest to use a multi-engines design with extra-thrust (to work also if one engine will fails) <br /><br />but probably you (and CLV designers) are confident that one engine is sufficient... ok <br /><br />we (me and you) don't have a good (your) and a bad (mine) opinion... we have (both) "an opinion"... </font><br /><br />Suppose we design the CLV 2nd stage with 2 engines, let's visit the scenario of what happen when one of the 2 engine failed during boost?<br /><br />If you 'design' the thrust of each engine such that you need <i>both</i> engine thrust to give the stage a positive acceleration then, with one engine out, you're out of luck. The only option is to abort, which means the CEV plunges back from ~ 300K feet back. No different as if the CEV reentry from orbit. The crew's life is not endangered.<br /><br />Suppose you 'design' so that the thrust of each engine alone will get the CEV to orbit the, if both engine work as expected, you'll have to require engine can throttle to 50% of its power at the beginning of the burn, and down as much as to 20% near at end of burn. This is beyond the technology level of today's liquid rocket engines. So you'd spend another billions of $$$ with another 10+ years of delay before launching CLV?<br /><br />Another consideration, if you 'insist' on putting 2 SSMEs on CLV, with each engine weigh at 7,500 lbm, that's the weight you'll take away from the CEV as payload (more if you add other support equipment). That extra half-ton weight could mean additional safety/life support equipment on the CEV instead. For every pound of "extra' weight you put on the stage in the name of "safety", you'd take the same weight away from the payload. Eventually you'll reduce the CEV capability such that you'll need MORE launches just to make a Moon mission.<br /><br />So Mr. irish <div class="Discussion_UserSignature"> </div>