Space entrepreneur Elon Musk made history last week with the successful launch of his Falcon 9 rocket, but he has acknowledged that he worried a few years ago that his project would have to shut down for lack of cash.

The story of how Musk's start-up, Space Exploration Technologies, managed to buck financial turbulence - at one point even flirting with Northrop Grumman about a possible partnership - underscores how risky privately funded space endeavours can be.