In every endeavour. Yet the viability of the whole can not always be deduced from them.
The Armstrong-Whitworth Ensign, Bristol Brabazon and Saro Princess all flew but none operated for real. Yet machines like DC-3, Lockheed Constellation or even the Boeing 707 were successful. What made these differences? Is a certain amount of failures always needed? Could the waste be avoided beforehand? Interesting questions.