Dissatisfied with the progress, though, just weeks later the Pentagon canceled the HEF-3-powered J93 engine program and limited the Valkyrie program to the use

Dissatisfied with the progress, though, just weeks later the Pentagon canceled the HEF-3-powered J93 engine program and limited the Valkyrie program to the use of JP-4 fuel only. Testimony by USAF scientists before Congressional committees revealed that the technical hurdles and the operating cost of using HEF-3 as a jet fuel were prohibitive given the defense budget of the day.