They'd be better off uprating the RS-68s ISP for fuel efficiency if used for the CaLV heavy lifter. Because, as shuttle_guy has pointed out before, the RS-68 is a much heavier and thirstier engine (higher thrust and lower ISP will do that) than the RS-25 (SSME) and so the use of the RS-25 for the CaLV is a trade off for the most payload. For a corestage with RS-68s, 3x RS-68s is the optimum amount, otherwise for say, 4x RS-68, you'd have to stretch the corestage tankage far more for those thirsty motors, pushing the height of the CaLV taller than a Saturn V!!<br /><br />1): Cluster of 5x RS-25:<br /> Weight -- 37,500 pounds.<br /> Thrust Sea Level -- 1.96 million pounds.<br /> ISP (vacuum) -- 450 seconds.<br /><br />2): Cluster of 3x RS-68s:<br /> Weight -- 42,000 pounds.<br /> Thrust (sea L.) -- 1.95 million pounds.<br /> Vacuum ISP -- 420 seconds.<br /><br />See what I mean? 5x RS-25s weigh less, are more efficient, and give slightly more thrust. If they were to use 4x RS-68s, sure the thrust would go to 2.6 million pounds but the corestage would run out of fuel far sooner, so you'd want to "stretch" the corestage to compensate, which as I said would make the CaLV taller than a Saturn V.<br /><br />Also, the RS-68 in it's current configuration needs to burn off a big "belch" of hydrogen before liftoff, which is what gives the Delta IV Heavy a scorched look after liftoff. Ugh! Scary!!<br /><br />Also, 5x RS-25 gives you more engine-out capability in Abort-To-Orbit scenario during the loss of an engine. With 3x RS-68 you'd lose ONE THIRD of your thrust during an engine-out with much greater chance of loss of mission.<br /><br />The only way to use the RS-68 is to try find the money to develop an 1-million pound thrust uprated version with no burn-off characteristics, and then only in the long run would you save money over disposable RS-25s, for a negligible amount more payload. Seems to be little point (at least in the short term).<br /><br />It's all in h <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!! LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>