Sorry, I think I fixed it while you were typing out your post? Atmosphere conditions are usually a bit better at 9km up than at sea level. If the numbers were

Sorry, I think I fixed it while you were typing out your post? Atmosphere conditions are usually a bit better at 9km up than at sea level. If the numbers weren't quite so bad there would be a case for detachable JATO units to get a solar plane off the ground, which then fly back to the airport to launch the next plane. ... and I forgot the speed of sound gets slower at altitude. It would need to do mach 1.6 to stay under the sun.