US begins hypersonic weapons program

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Actually the issues goes far beyond the control. there is also safety issue. The crack a size of a hair in the solid fuel could be very fatal. The solid fuel vehicles is pretty much pack, use and discard type like a missiles. There is no fuel replacement. once there is a problem in the solid fuel integrity, the whole system have to be dispose. Replacing fuel will be far to risky by current tech standard.

the other problem with solids is that its inherently unstable compred to liquid. If you don't get a uniform burn, then it will disrupt flight. Its one of the reasons why germany, russia, UK, US and even australia (we did lots of rocket and missile testing for about 30 years) abandoned them in favour of liquid propulsion.

solids were favoured for short throw and targeted at massed formations where an errant rocket was less of an issue.

the other thing with solids is that they discovered that shaped propellant gave better thrust, but shorter up time. In the scheme of things there were better results handling liquid - even if it was more volatile in the pre launch stage