THE X-15 PROGRAM IN RETROSPECT

Introduction



The X-15 program was the first major investment of the United States in manned aerospace flight technology. During the long 15-year lifetime of the program, hundreds of people have contributed importantly to its success. It is a great privilege for me to represent this outstanding team at this meeting of so many distinguished members of your society, including Frau Dr. Sänger-Bredt. We sincerely appreciate the award of the Sänger medal.

Professor Sänger's pioneering studies of long-range rocket-propelled aircraft had a strong influence on the thinking which led to initiation of the X-15 program. Until the Sänger and Bredt paper (ref. 1) became available to us after the war we had thought of hypersonic flight only as a domain for missiles. The concept of manned rocket aircraft flying efficiently at hypersonic speeds for very long ranges was new and highly stimulating. The remarkably detailed analyses of many aspects of their new concept which Sänger and Bredt undertook in their paper gave real substance to the idea. From this stimulus there appeared shortly in the United States a number of studies of rocket aircraft investigating various extensions and modifications of the Sänger and Bredt concept. These studies provided the background from which the X-15 proposal emerged.

By 1954 we had reached a definite conclusion: the exciting potentialities of these rocket-boosted aircraft could not be realized without major advances in technology in all areas of aircraft design. In particular, the unprecedented problems of aerodynamic heating and high-temperature structures appeared to be so formidable that they were viewed as "barriers" to hypersonic flight. Thus no definite requirements for hypersonic vehicles could be established or justified. In today's environment this inability to prove "cost-effectiveness" would be in some quarters a major obstacle to any flight vehicle proposal. But in 1954 nearly everyone believed intuitively in the continuing rapid increase in flight speeds of aeronautical vehicles. The powerful new propulsion systems needed for aircraft flight beyond Mach 3 were identifiable in the large rocket engines being developed in the long-range missile programs. There was virtually unanimous support for hypersonic technology development. Fortunately also, there was no competition in 1954 from other glamorous and expensive manned space projects. And thus the X-15 proposal was born at what appears in retrospect as the most propitious of all possible times for its promotion and approval.

The broad objective of the X-15 was to take a long step forward in developing the new technologies needed for rocket-boosted hypersonic aircraft (ref. 2). Fortunately, it was not proposed as a prototype of any of the particular concepts in vogue in 1954, which have since largely fallen by the wayside. It was conceived rather as a general tool for manned hypersonic flight research,, able to penetrate the new regime briefly, safely, and without the burdens, restrictions, and delays imposed by operational requirements other than research. The merits of this approach had been convincingly demonstrated by experiences with the previous research air planes, notably the X-1 series.

Figure 1 - typical X-15 research flight paths
Figure 1. Typical X-15 research flight paths.


The plan called for two different types of flight profile. The first consisted of a variety of constant angle-of-attack, constant altitude, and maneuvering flights within the altitude corridor of interest to hypersonic gliders. In the second type of trajectory it was proposed that the vehicle should explore for the first time some of the problems of manned space flight by making long leaps out of the sensible atmosphere (fig. 1). Our analyses of 1954 (ref. 2) showed that such excursions into space were feasible provided that an Inconel X heat-sink structural concept was used together with employment of high lift and low L/D during the reentry pull-up maneuver. This latter maneuver in itself was recognized as a prime problem for manned space flight from both the heating and the piloting viewpoints.

While the hypersonic aeronautics aspect of the X-15 proposal enjoyed virtually unanimous approval, it is interesting to note that the space flight aspect was viewed in 1954 with what can best be described as cautious tolerance. There were'few then who believed that space flight was imminent, and even the most sanguine believed that manned space flight was many decades in the future, probably not before the 21st century (ref. 3). Several of the senior research consultants who reviewed the proposal counseled that the space flight maneuver was premature and should be eliminated. Fortunately it remained in the program as a prime objective, not because of superior vision on the part of the X-15 planners, but rather because of their recognition that the problems involved were basic and would have to be solved before true mannea space flight could be achieved, and that it was now possible in the X-15 Project to take the first steps toward their solution.

Figure 2 - scattered plot graph of the velocity and altitude of the X-15 flights
Figure 2. Velocity and altitude of the X-15 flights.


An idea of the scope of the program is obtainable from figure 2. The dots show the speed and altitude of all of the flights. Of the 199 flights accomplished in the program 109 exceeded Mach 5, and four exceeded Mach 6. The highest speed, Mach 6.7 (2020 meters/sec), was reached in 1967. In altitude, it can be seen from figure 2 that the space-trajectory type flights constituted a major part of the program with a peak altitude of 108,000 meters, well above the 82,000 meters vhich was the goal set in the original prospectus.

The results of the program have been widely disseminated and previous reviews have been given periodically (refs. 4, 5, and 6, for example). There is no need for a detailed review here. It will be of interest, however, now that the program has been completed, to examine in retrospect its principal accomplishments against a background of the original hopes and aims of 1954.


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