This article has a good summary: https://www.apogeerockets.com/Peak-of-Flight/Newsletter533 https://www.apogeerockets.com/Peak-of-Flight/Newsletter533 The key

This article has a good summary: https://www.apogeerockets.com/Peak-of-Flight/Newsletter533 https://www.apogeerockets.com/Peak-of-Flight/Newsletter533 The key challenges called out are a) fuel, b) robust airframe, c) active guidance to handle varying conditions, d) funding for testing and robust components. The article goes into a good amount of detail on each.