<i>propforce, <br /><br />How do the state-of-the-art solid rocket motors compare to state-of-the-art liquid fueled motors in terms of ISP, dry mass fraction, and other measuring sticks? Isn't it possible to scale up a liquid fueled booster a lot easier than a solid fueled? How close do kersene and lox motors come to hydrogen and lox? Would a first stage using solids and a second stage using liquids work? How about vice-versa? </i><br /><br />Not addressed to me, but I will give my answer.<br /><br />The ability to "scale up" is irrelevant until we have a robust presence out there. Thiokol SRBs are in production, have a 99.5% + success rate, and are subject to commodity pricing, meaning we know EXACTLY how much a 5 segment SRB will cost.<br /><br />Using composites rather than aluminum for dry mass will be an incremental improvement rather than a clean sheet fresh start because the solid fuel shapes can remain essentially unchanged.<br /><br />A 5 segment Thiokol SRB plus a 3 RL-10 upper stage should cost $50 million (give or take) and lift 50,000 pounds (give or take) for about $1000 per pound. With minimal upfront design costs. <br /><br />Besides, two 5 segment SRBs with a big tank and a cluster of RS-68s and we are scaled about as big as we can possibly use for the foreseeable future.<br /><br />= = =<br /><br />No need to man-rate the SRB + RL-10 idea. Just buy Klipers. ;-) <br /><br />= = =<br /><br />As a non-technical observer, it seems to me that the Russians and Ukrainians have us licked on efficient low cost medium lift. Compare the cost to per pound to LEO for Delta and Atlas with Proton and Zenit (not be mention Soyuz-2) and how can we compete except for government funded projects?<br /><br />But going big, really big? Ironically, the US can build much bigger rockets than those guys, if we choose to. A big shuttle derived will dwarf even Energia.<br /><br />Why continue a race - - US EELV versus Zenit/Proton - - when we are clearly so far behind?