<blockquote><font class="small">In reply to:</font><hr /><p>That study was done and it had one very difficult problem. Where to land the reusable first stage. <p><hr /></p></p></blockquote><br />At the pad. IMO, VTVL isnt worth doing if it doesnt land back at the pad.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>Minimum user cost occured when the staging velocity was at 70% of orbital velocity. The down range distance for that puts you in the ocean or in another country. <p><hr /></p></p></blockquote><br />I dont get the "minimum user cost" point. How can you calculate the user cost when such operational details as recovery are left open ? Please do explain this a bit.<br /><br />I havent done a similar study of course and im not much of a rocket engineer at all, but my gut feeling would tell the ideal staging velocity to be much lower. So the system basically operates like this : you have a upper stage with vacuum-optimized engines, high ISP, it has a task to get up to orbital speed, starting above the atmosphere at relatively low speed. And of course, reentry. <br />Then you have the high-thrust lower stage whose only sole purpose is to lift the upper stage above the atmosphere and to impart perhaps 20% of orbital velocity only. And this big guy absolutely has to return to the pad or else you kill any prospects for sufficiently fast turnaround. If necessary, oversize it, so it has enough delta-V left to come back and land.<br /><br />You could operate such a system with only one lower stage and several upper stages, because the flight profiles for upper stages would often require staying at the orbit for several days, and you want a very high flight rate of launch a day or so.<br /><br />The catch here of course is that your upper stage has to have a near-SSTO performance and everybody knows that SSTO is tough, _but_ this one would start above the atmosphere, no launch aerodynamics issues, purely vacuum-optimized engines which makes it a lot simpler.<br /><br />This is basi