Papers Published

Not the least of the technological legacies of the X-15 consisted of the more than 765 technical documents produced in association with the program, including some 200 papers reporting on general research that the X-15 inspired. John Becker saw them as “confirmation of the massive stimulus and the focus provided by the [X-15] program.”51

Other Views

William Dana took time in 1987 to write a paper for the Society of Experimental Test Pilots pointing out some of the lessons learned from the X-15 program.52 Dana should know—he was the last pilot to fly the X-15. Two he cited were particularly appro­priate to the designers of the X-30 and X-33, although neither heeded the lessons. They are included here in their entirety:

The first lesson from the X-15 is: Make

it robust. As you have already seen, the

X-15 was able to survive some severe mistreatment during landings and still came back to fly another day. The X-15 that broke up after a spinning re-entry had self-recovered from the spin prior to break up, and might well have survived the entire episode had fixed, rather than self-adaptive, damper gains been used during re-entry. Another example exists of where the X-15 did survive a major stress in spite of operating with a major malfunction. This flight occurred in June 1967, when Pete Knight launched in X-15 No. 1 on a planned flight to

250,0 feet. At Mach 4 and at an alti­tude of 100,000 feet during the boost, the X-15 experienced a complete electri­cal failure that resulted in shutdown of both auxiliary power units and, there­fore loss of both hydraulic systems. Pete was eventually able to restart one of the auxiliary power units, but not its gener­ator. By skillful use of the one remain­ing hydraulic system and the ballistic controls, Pete was able to ride the X-15 to its peak altitude of 170 or 180,000 feet, reenter, make a 180 degree turn back to the dry lake at Tonopah, and dead-stick the X-15 onto the lakebed. All of these activities occurred without ever flowing another electron through the airplane from the time of the original failure.

There will be a hue and cry from some that the aerospace plane [X-30—NASP] cannot afford the luxury of robustness; that the aerospace plane, in order to be able to get to orbit, will have to be highly weight-efficient and will have to forego the strength and redundancy margins which allowed the X-15 to survive during adversity. And my answer to these people is: build your first aerospace plane with X-15 margins, even at the expense of per­formance; these margins will serve well while you are learning how to make your propulsion system operate and learning how to survive in the heating thicket of hypersonic flight. Someday, with this

knowledge in hand, it will be time to build a no-margins aerospace plane, but for now I suggest that you seize all the margins that you can because you will need them, as did the X-15.

The other lesson from the X-15 is: con­duct envelope expansion incrementally. The typical increment of speed increase for the original X-15 was about half a Mach number. With this increment it was easy to handle the heating damage that occurred in the original speed expansion phase. Again, I would expect to hear protest from the aerospace plane community, because when using one – half Mach number increments it is a long flight test program to Mach 25. Indeed, I cannot specify what size bite to take during the aerospace plane enve­lope expansion, but I can offer you the X-15A experience, in which two con­secutive flights carrying the dummy ramjet were flown to Mach numbers of 4.94 and 6.70. The former flight exhibit­ed no heat damage because of the wake of the dummy ramjet; the latter flight almost resulted in the loss of the aircraft due to heat damage.

Looking at the X-33 program in particular, another lesson jumps out. There will only be a single X-33. The building of three X-15s allowed the flight test program to proceed even after accidents. In fact, each of the X-15s was severely damaged at some time or another requiring it to be rebuilt. Plus, with multiple aircraft, it is possible to have one aircraft down for modification while the oth­ers continue to fly. And should one aircraft be lost, as sometimes happens in flight research, the program can continue. In today’s environment it is highly unlikely that the X-33 program would continue if it exploded during an engine test like the X-15-3 did while ground testing the XLR99. Hopefully the X-33 will not experience such a failure, but is that not part of the reason we conduct flight research—to learn from the failures as well as the successes?