Challenge: Design and model a fully-reusable PSTO (parallel stage to orbit) shuttle, suitable for taking a crew to orbit and returning them safely to Earth. People get excited about SSTOs (both of the VTVL and winged varieties), but for Earth, using multiple stages is almost always more cost-effective. However, we've never once flown a fully-reusable human-rated shuttle, and there's no obvious configuration for a fully-reusable shuttle that uses staging. In particular, I like the idea