<blockquote><font class="small">In reply to:</font><hr /><p>An example drawback can be found in this statement from propforce: <br /><br />Imagine the space shuttle on top of the 747. Would the shuttle get better or worse performance using 747 air launch (without the 2 SRBs) versus the existing ground launch (with the 2 SRBs)? How would one separate the shuttle from the 747 at 30K feet going at 500 mph? <br /><br />Me: <br />This question however, is easy to answer, The present orbiter cannot be launched from a 747 without additional boost. Its simply too massive. In addition to that, no mention was made of the ET. The 747 cannot carry a fully loaded ET and orbiter. Without ET/SRBs, the shuttle goes nowhere but down. <br /><br />When the Enterprise was airdropped from a 747, it was lighter because among other things. It had dummy SSMEs and no TPS. <br /><br />Air launched concepts from a 747 would have to be smaller vehicles such as the air force 1983 study. And these vehicles could be mounted on ETs although thats aerodynamically dirty if you ask me, or build a vehicle designed for airlaunch with internal tanking. The 747 would have to be extensivelly modified as well to contain the propellant replenish equipment for the spaceplane. <p><hr /></p></p></blockquote><br /><br />Agree. Which goes back to my original point that the size of upper stage depends on the configuration, lift, climb and speed capability of the carrier aircraft. My "problem statement" was meant to elucidate these issues with carrier aircraft and its impact, ultimately, on the payload capability (including humans as payload) for the rocket. <br /><br />Now taking the shuttle/ 747 as an example further, assuming a much smaller shuttle with some ET tank on top of the 747, how would you conduct a "separation" flight test? Would you fire up the SSMEs before separating from the 747, hence risk the burn-up of its tail section? Or would you "free flight" the shuttle upon the separation, then ignite the SSMEs after separati <div class="Discussion_UserSignature"> </div>