I do believe that within 25 at the longest we will have one or more fully functional space elevators. At that point, chemical rockets as launch vehicles will be pointless. Thermal Management systems for re-entry will also be meaningless.<br /><br />I don't know, how do space elevators work on the moon? The gravity is lower, but the rotation rate of the moon is much lower than earth. Maybe tethers might be the way to go there. Just enough rocket power to reach an altitude of a mile or two and then tether on up.<br /><br />As for Mars, again, I know the gravity is less, but I don't know what the rotation rate is off hand.<br /><br />Once your in orbit, it's all about ISP baby. Thrust is almost meaningless. That means that we can throw out most considerations of fuel density.<br /><br />In orbit, LH2/LOX would be the low end of the scale in terms of ISP.<br /><br />Lifting bodies and SSTO are a meaningless concept in a world with a space elevator.<br /><br />So, given that, what R&D path should we proceed down knowing that chemical rocket launch vehicles won't be launching anymore in 25 years?<br />