Category Hypersonics Before. the Shuttle

The XLR99 was the first large man-rated rocket engine that was capable of being throttled and restarted in flight. This com­plexity resulted in many aborted missions (approximately one-tenth of all mission aborts) and significantly added to the devel­opment cost of the engine. When the X-15 program ended, many felt that the throt­tleable feature might have been a needless luxury that complicated and delayed the development of the XLR99.

But in the mid-1960s these attributes were considered vital to the development of a rocket engine to power the Space Shuttle. At the time, Shuttle was to consist of two total­ly reusable stages—essentially a large hyper­sonic aircraft that carried a smaller winged spacecraft much like the NB-52s carried the X-15s. The same basic engine was going to power both stages; the pilots therefore need­ed to be able to control its thrust output. At some points in the early Shuttle concept development phases, the same engines would also be used on-orbit to effect changes in the orbital plane. So the original concept for the Space Shuttle Main Engines (SSME) included the ability to operate at 10 percent of their rated thrust, and to be restarted mul­tiple times during flight.11

In the end, the production SSMEs are throt­tleable within much the same range as the XLR99—65 to 109 percent, in one percent increments. In actuality about the only rou­tine use of this ability is to throttle down as the vehicle reaches the point of maximum dynamic pressure during ascent, easing stresses on the vehicle for a few seconds on each flight. Even this would not have been necessary with a different design for the solid rocket boosters.12 So the complexities required to enable the engine to throttle may, again, have been a needless luxury. Nevertheless, the development pains experi­enced by Reaction Motors provided insight for Pratt & Whitney and Rocketdyne (the two main SSME competitors) during the design and development of the SSMEs.

Coming at a time when serious doubts were being raised concerning man’s ability to han­dle complex tasks in the high-speed, weight­less environment of space, the X-15 became the first program for repetitive, dynamic mon­itoring of pilot heart rate, respiration, and EKG under extreme stress over a wide range of speeds and forces. The Bioastronautics Branch of the AFFTC measured unusually high heart and breathing rates on the parts of the X-15 pilots at points such as launch of the X-15 from the NB-52, engine shutdown, pull­out from reentry, and landing. Heart rates averaged 145 to 160 beats per minute with peaks on some flights of up to 185 beats per minute. Despite the high levels, which caused initial concern, these heart rates were not associated with any physical problems or loss of ability to perform piloting tasks requiring considerable precision. Consequently, theo­retical limits had to be re-evaluated, and Project Mercury as well as later space pro­grams did not have to be concerned about such high heart rates in the absence of other symptoms. In fact, the X-15’s data provided some of the confidence to go ahead with early manned Mercury flights—the downrange bal­listic shots being not entirely dissimilar to the X-15’s mission profile.’3

The bio-instrumentation developed for the X-15 program has allowed similar monitor­ing of many subsequent flight test programs. Incorporated into the pressure suit, pickups are unencumbering and compatible with air­craft electronics. The flexible, spray-on wire leads have since found use in monitoring car­diac patients in ambulances.

Another contribution of the X-15 program was the development of what John Becker calls the “first practical full-pressure suit for pilot protection in space.”14 The David Clark Company had worked with the Navy and the HSFS on an early full-pressure suit for use in high-altitude flights of the Douglas D-558- II; the suit worn by Marion Carl on his high – altitude flights was the first step. This suit was made of a waffle-weave material and had only a cloth enclosure rather than a hel­met. It should be noted that Scott Crossfield was heavily involved in the creation of this suit, the success of which Crossfield attrib­utes to “… David Clark’s genius.”15

The David Clark Company later developed the A/P-22S-2 pressure suit that permitted a higher degree of mobility.16 It consisted of a link-net material covering a rubberized pres­sure garment. Developed specifically for the X-15, the basic pressure suit provided part of the technological basis for the suits used in the Mercury and Gemini programs. It was later refined as the A/P-22S-6 suit that became the standard Air Force operational suit for high altitude flight in aircraft such as the U-2 and SR-71. However, it should be added that the space suit for Project Mercury underwent further development and was pro­duced by the B. F. Goodrich Company rather than the David Clark Company, so the line of development from X-15 to Mercury was not entirely a linear one, and security surround­ing the U-2 and Blackbird programs have obscured some of this history.17

X-15 pilots practiced in a ground-based sim­ulator that included the X-15 cockpit with all of its switches, controls, gauges, and instru­ments. An analog computer converted the pilot’s movements with the controls into instrument readings and indicated what the aircraft would do in flight to respond to con­trol actions. After a flight planner had used the simulator to lay out a flight plan, the pilot and flight planner worked “for days and weeks practicing for a particular flight.” The X-15 simulator was continually updated with data from previous flights to make it more accurate, and eventually a digital computer allowed it to perform at higher fidelity.18

Much has been made of the side-stick con­troller used on the X-15. Although the con­cept has found its way onto other aircraft, it has usually been for reasons other than those that initially drove its use on the X-15. The X-15 designers feared that the high g-loads encountered during acceleration would make it impossible for the pilot to use the conven­tional center stick; such worries are not the reason Airbus Industries has used the con­troller on the A318-series airliners. And although the side-stick controller has proven very popular in the F-16 fighter, it has not been widely adopted. Nevertheless, the X-15 experience provided a wealth of data over a wide range of flight regimes.

Some phases of X-15 flight, such as reentry, were marginally stable, and the aircraft required artificial augmentation (damping) systems to achieve satisfactory stability. The X-15 necessi­tated the development of an early stability aug­mentation system (SAS). The first two X-15s were equipped with a simple fail-safe, fixed – gain system. The X-15-3 was equipped with a triple-redundant adaptive flight control system; the pilot flew via inputs to the augmentation system. Although a point of continuing debate, the X-15 did not incorporate a “fly-by-wire” system if meant to denote a nonmechanically linked control system. Nevertheless, the SAS system did “fly” the X-15-3 based on pilot input rather than the pilot flying it direcdy. This basic concept would find use on an entire generation of aircraft, including such high performance fighters as the F-15. The advent of true fly-by­wire aircraft, such as the F/A-18, would advance the concept even further.