Proceedings of the X-15 First Flight 30th Anniversary Celebration

X-15 contributions to the X-30

Robert G. Hoey
Robert G. Hoey

Much of the technology benefit of a research airplane like the X- 15 is gained before the first flight of the airplane;not the paper tradeoff studies, but meeting the challenge of designing, manufacturing, and integrating real hardwarethat works. The remaining lessons are learned during flight test. Many of the technology benefits of the X-15 havebeen extolled in past years as they apply to space flight (high altitude, 0 g) and lifting reentry Oow L/D landings).

As an introduction to the X-30 presentation, I will highlight some of the less publicized flight test results fromthe X-15 program that might relate to sustained high-speed flight in the atmosphere.

Figure 1 - X-15 list of contributions to the X-30
Figure 1. X-15 contributions to the X-30.

I will cover four topics (fig. 1):

1. Energy management and range considerations,

2. The advantages of pilot-in-the-loop and redundant-emergency systems,

3. A summary of some of the aerodynamic heating problems that were encountered, and

4. Some comments on the advantages of an early flight test program and gradual expansion of the flight envelope.

Figure 2 - variation of range capability during a typical X-15 flight
Figure 2. Variation of range capability during a typical X-15 flight.

The energy management for a typical X-15 flight is shown in figure 2. Most of the flights were conductedessentially in the vertical plane. It was Most important to establish the proper heading toward Edwards during thefirst 20 sec after launch, very much like aiming in the direction of the target when firing a gun. The remainder of thepowered portion of flight was used to establish the proper pitch angle and engine shutdown velocity, which again isakin to establishing the correct elevation and muzzle velocity of a gun. Once the X-15 engine was shut down, thetrigger was pulled and the ballistics were pretty well established for the next few minutes of flight.

When we began to fly research flights in the airplane, the heating engineers wanted to obtain data at constantangle of attack, constant q, constant Mach, constant everything. Using the simulator, we determined that if we usedthe speed brakes, reduced thrust, and entered a 4-g turn, we could keep the airplane from accelerating. When webegan looking at emergency considerations for these flights, we began to appreciate the enormous complexity ofadding that third dimension, azimuth, to the flight plan. It was like swinging the gun wildly in azimuth just beforepulling the trigger.

Figure 3 - flightpath of the X-15 aircraft for heating test
Figure 3. Flightpath of the X-15 aircraft for heating test.

For the flight shown in figure 3, the initial heading was 30 off of the heading towards Edwards and the turnHAD to be completed pretty much as planned in order to return to Rogers lakebed. An early shutdown or flightcontrol problem would have dictated a landing at Silver Lake, Three Sisters, or Cuddeback Lake. The turn washeld for about 20 see at about Mach 5, 80,000 ft altitude, and about 4 g. This produced a turn radius of about 36 nm.These heating research flight plans were severely constrained by the geography of the available emergency lakebeds;thus, only a few were flown. They probably caused as much uneasiness among the flight planners and control roompersonnel (not to mention the pilots) as many of the high altitude flights.

Obviously, the X-30 missions will involve much longer periods of sustained hypersonic turning flight with theattendant large radii and geography considerations.

In 1962, a very comprehensive, but little known, study was initiated by Bob Nagle at AFFTC to quantify thebenefits of having a pilot and redundant--emergency systems on a research vehicle. Each individual malfunction orabrionnal event that occurred after B-52 takeoff for the first 47 free flights of the X- 15 was analyzed. The outcome ofeach event was forecast for three hypothetical models; one with only the pilot but no redundant-emergency systems,one with only the redundant--emergency systems but with no pilot, and one with neither the pilot nor redundantemergency systems (i.e., single string, unmanned).

Figure 4 - bar chart of total pilot-in-the-loop and R/E systems benefit
Figure 4. Total pilot-in-the-loop and R/E systems benefit.

The results are summarized in figure 4. The unmanned, single-string system would have had 11 additionalaborts and resulted in the loss of 15 X-15's. Not surprising is the fact that the pilot is of little value in a systemwithout redundant-emergency systems. He must have some alternate course available in order to be effective. Theredundant--emergency systems were also found to be of little value in an unmanned system primarily because thefault detection and switchover logic must presuppose the type of failure or event. For example, few designers wouldhave built in a capability to handle an inadvertant nose gear extension at Mach 4.5.

Figure 5 - bar chart showing comparison of the X-15 and BOMARC pilot and R/E system aspects
Figure 5. Comparison of the X-15 and BOMARC pilot and R/E system aspects.

Of more than academic interest was a parallel, but independent, study conducted by Boeing on the first 60 flightsof their BOMARC missile, an unmanned, single-string, ramjet-powered interceptor. The authors collaborated on theground rules for the study but not on the actual analysis. The similarity of the results as shown in figure 5 is striking,especially when considering that the X-15 study was projecting from a piloted, redundant design to an unpilotcd,nonredundant design, and the BOMARC study was the reverse. The X-30 will have a crew on board, and missionsuccess should be significantly enhanced if the appropriate levels of redundancy and emergency systems and propercrew integration are designed into the system.

The next series of figures depict sequentially some of the aerodynamic heating events that occurred during theinitial envelope expansion of the X- 15.

Figure 6 - inconel deflector strip riveted forward of the canopy joint
Figure 6. Inconel deflector strip riveted forward of the canopy joint.

Our first "hands-on" awareness of the effects of aerodynamic heating occurred just above Mach 3 while theairplane was still flying with the -11 engines. The canopy lifted slightly at the front edge (due to differentialpressure at altitude), allowing stagnation air to bum the rubber canopy sea] with a resulting loss of cabin pressure.The fix was a narrow Inconel deflector strip which was riveted to the skin just forward of the canopy joint (fig. 6).

Figure 7 - small spanwise buckles and local scorching observed in X-15's skin
Figure 7. Small spanwise buckles and local scorching observed in X-15's skin.

At about the same time, small spanwise buckles and local scorching were observed in some areas of the thinskinned side tunnels. The fix was to segment the side tunnels fore and aft and insert expansion slip joints betweeneach segment (fig. 7).

Figure 8 - heat damage to the X-15's aluminum tubing
Figure 8. Heat damage to the X-15's aluminum tubing.

After a flight to about 4.5 Mach number, these aluminum instrumentation pressure lines in the nose wheel well(fig. 8) were observed to be melted and severed. The cause was a small gap in the nose wheel door seal whichallowed a torchlike stream of hot boundary layer gas to enter the wheel well. The paint on the bulkhead behind thetubes (a cockpit pressure bulkhead) was badly burned and scorched but the bulkhead remained undamaged.

Figure 9 - wing skin buckle during flight
Figure 9. Wing skin buckle following flight to Mmax=5.28.

At 5.28 Mach number, the upper surface wing skins were locally buckled immediately behind the expansionslots in the leading edge. Flow through the slots and the tripping of the boundary layer had created a local hot spoton the wing skin. The fix was a thin Inconel cover over the slot which was welded to one side only (fig. 9).

Figure 10 - X-15 glass failure
Figure 10. X-15 glass failure.

On the maximum speed flight to Mach 6.04, the outer canopy glass shattered shortly after burnout (fig. 10). Asmall buckle in the retainer ring had created a local hot spot producing high stresses on the retainer as well as theglass itself. A redesign of the retainer ring with larger tolerances resulted.

Figure 11 - ablative wear lower fuselage and wing
Figure 11. Ablative wear lower fuselage and wing.

Figure 12 - ablative wear wing leading edge
Figure 12. Ablative wear wing leading edge.

The decision to attempt to expand the envelope to Mach 8 created many aerodynamic heating redesigns andsurprises. The ablator-insulator material, although adequate for the job at hand, was time consuming to apply,difficult to handle, and created a measurable increase in drag after charring had started (obviously not a candidatefor use on the X-30). Figures 11 and 12 show typical wear patterns.

Figure 13 - pylon heat damage, left side
Figure 13. Pylon heat damage, left side.

The severe damage to the ventral (fig. 13) that occurred on the flight to 6.7 Mach number was the result of local shock interference. The solution to this problem would not have been of the "quick-fix" variety. The program wasterminated before a redesign could be completed.

The lesson is NOT that the X-30 might encounter a broken windshield or buckled wing skin, but rather thataerodynamic heating problems tend to be localized effects and are often difficult to predict before flight. They alsotend to be sclf-propagating. Although the X-15 was heavily instrumented, none of the acrothermo events describedwas evident from the instrumentation, real time or otherwise. The nature of an X-15 flight was that it was highlytransient and the flight time at each new Mach condition was momentary. Each of the events described would havebeen much more severe if the flight condition had been sustained even for a few more seconds.

Figure 14 - scattered plot of X-15 envelope expansion for first 45 flights
Figure 14. X-15 envelope expansion for first 45 flights.

The chronology of the initial envelope expansion of the X- 15 is shown in figures 14(a) and 14(b) for the first2 1/2 years of the flight test program. The altitude envelope for the - I I -powered airplane reached 136,500 ft in aI ittle over a year. The first - 99-powered flight was flown 18 months after the first glide flight and the design altitudewas reached 16 months later. The - 11-powered airplane exceeded Mach 3.0 within a year. The max speed flight to6.04 was flown I I months after the first flight with the -99 engine. About half the people on the program thoughtthat this pace was too slow and that we should be more aggressive. The other half thought it was much too fast andthat we should do more research along the way. The real benefit was the luxury to choose whatever pace we thoughtwas right.

It is important to notice that the philosophy of the X-15 program was to let flight test distinguish between the"real" and the "imagined" problems. It was felt that in many cases a detailed preflight analysis of a potential "worryitem" would have been unnecessarily expensive, time consuming, and possibly erroneous or misleading. The photosof heating damage were "worries" that turned out to be "real"; however, many of the "worries" never materialized.For example, the sharp comer at the inboard leading edge of the horizontal stabilizer was expected to reach very high temperatures if it extended beyond the fuselage boundary layer. The gaps at the inboard and outboard edge of the wing flap might have created severe local hot spots.

Unlike the space shuttle program where the reentry envelope had to be expanded from the top down on a single flight, the X-30 should be able to expand its flight envelope gradually from the bottom up very much like theX-15 program. Given the proper mix of pilot-in-the-loop, redundancy, system integration, and a flexible envelopeexpansion plan, the X-30 flight testing should be able to start sooner and with lower risk than might be projected byspace-type systems planning.

Figure 15 - simultaneous development of a new airframe and a new propulsion system
Figure 15. Simultaneous development of a new airframe and a new propulsion system.

The last figure (fig. 15) is merely a reminder that the X-15 and X-30 have another thing in common-simultaneousdevelopment of a new airframe and a new propulsion system. The odds that both will reach maturity at the sametime are very slim. The decision to install interim engines in the X-15 allowed the test team to gather valuable dataand experience with the airframe and its subsystems before encountering the unknown environmental effects of highaltitude and high-speed flight. I recognize that the propulsion system and the aerodynamics for the X-30 are muchmore closely integrated than on any previous vehicle. Nevertheless, history shows that there is both technical andpolitical value in getting SOME type of flight hardware into the air as quickly as possible.

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