When Blue Streak was cancelled as a weapon, the then Minister of Defence, Harold Watkinson, announced that the project would be continued as a satellite launcher. I strongly suspect that this done to lessen the political trouble the cancellation brought, for although there was quite some interest in a British space programme, there was no money. The military were not interested in satellites, and the science budget could not afford satellites and a launcher.
The obvious design, which had been in many minds at Farnborough for the past few years, was to use Black Knight as a second stage for Blue Streak. But it turned out not to be as simple as that: Black Knight was too small to be an effective match for Blue Streak, and its 3 foot diameter would lead to problems with satellites that were bigger. However, the brief given to the design team was to come up with a satellite launcher that used as many existing facilities as possible, and could be produced with the minimum of extra development.
The limiting factor turned out to be the static testing beds at High Down (whose remains can still be seen today). These limited the diameter of the second stage to 4 foot 6 inches.
The design was soon finalised, but the question of cost again reared its ugly head. Thorneycroft, who was then Minister of Aviation, tried involving the Old Commonwealth - Canada and Australia - and although Australia was prepared to provide Woomera, there was little other interest, even though all concerned knew that this meant handing the Americans a monopoly.
Efforts with the Commonwealth having failed, Thorneycroft went to the French, and two years further down the line from the original Blue Streak cancellation, ELDO started to emerge. The price for this, though, was the second stage going to the French. Europa One was little more than a redesigned Black Prince ... and rather less successful - although Blue Streak worked every time.
Here is the design:
In contemporary terms, it would have as good a performance as, say, Atlas Agena, and would have been equal to any U.S. launcher until Centaur. But, sadly, it was never to be.
This is a composite photograph to give an impression of what Black Prince might have looked like.
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