> This makes air-breathing hypersonic devices a much bigger challenge than air-breathing supersonic ones. Indeed, and the Virgin Galactic vehicle isn't even ei

> This makes air-breathing hypersonic devices a much bigger challenge than air-breathing supersonic ones. Indeed, and the Virgin Galactic vehicle isn't even either of those; it uses a hybrid rocket engine instead of breathing air. Hybrid rocket engines are fairly simple compared to liquid fueled rocket engines and are fairly safe compared to solid fuel rocket engines (although the fatal V.G. explosion some years ago should perhaps challenge this wisdom?) However they're probably a dead end technology and I don't think V.G. will ever get to orbit with them.