THE X-15 PROGRAM IN RETROSPECT
Operational Problems With Subsystems
In order to achieve maximum penetration into the realms of hypersonic
and space flight, the X-15 necessarily had to use many newly ccnceived,
partially developed subsystems fabricated of new materials by new processes.
The XLR99 rocket engine, the auxiliary power units (APU), and the stability
augmentation (damper) system are examples. A host of new problems were
encountered with the subsystems after the start of flight operations, and
flight schedules were subject to innumerable delays (refs. 5, 7, and 21).
In retrospect, a great deal of money and lost time could have been saved
through more imaginative and more elaborate ground testing of all major
subsystems. This sounds like a serious general criticism. But one must
remember that the only precedents available in the mid-fifties were those
based on experience with ordinary aircraft and with the much simpler previous
research airplanes. As it was, the initial environmental testing of the
X-15 subsystems far exceeded that of any previous vehicle. In this area
perhaps more than any other, the X-15 brought to light innumerable "real"
problems which could not have been foreseen by any means other than involve
ment with an actual flight vehicle. The program revealed the true nature
of the difficulties encountered in an aerospace environment and identified
many specific new requirements for developmental testing that were of value
in subsystem development for the space program