 It was at this point that the first pair of manned models were also proposed, of which "Model A" was selected to proceed with.Development of the craft was not straightforward, having posed several challenges and difficulties that needed to be overcome from the onset.Early on, one criticism of the Model A was that the single annular peripheral jet engine would be incapable of adequate pitch and roll stability while the diagonal stability jets featured on Cockerell's original model had been eliminated; however Saunders-Roe's aerodynamics office were confident that the design was good.