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Walker’s Wild Ride


Fifty-five years ago this month, the USAF/Douglas X-3 Stiletto research aircraft exhibited a then little known dynamic instability mode during a flight test with NACA test pilot Joseph A. Walker at the controls.

The X-3 was designed to fly at speeds up to Mach 2.  The aircraft was approximately 67 feet in length and had a wing span on the order of 23 feet.  Gross weight was 23,840 pounds.

A pair of Westinghouse J46-WE-1 turbojets were intended to power the X-3.  However, protracted developmental problems and installation issues with these powerplants would eventually prevent their use in the aircraft. 

The X-3 was ultimately outfitted with a pair of Westinghouse J34-WE-17 turbojets.  The result was that the X-3 was now underpowered and could barely fly supersonically.  Maximum achieved Mach number was 1.21 and that was in a 30-deg dive!

Notwithstanding the above, the X-3 took to the air 54 times between October 1952 and May 1956 for the purpose of conducting transonic flight research.  It would be on its 43rd flight that the X-3 would make its most important contribution to aviation.

On Wednesday, 27 October 1954, Joe Walker took-off in the X-3 (S/N 49-2892) from Edwards Air Force Base, California.  At Mach 0.92 and 30,000 feet, Walker applied left aileron at fixed-rudder in an effort to develop a rapid roll response.  To Walker’s utter amazement, the X-3 went wild in both pitch and yaw.

Although it seemed to last much longer, Walker was able to recover control of the X-3 within 5 seconds of his initial left aileron input.  In true test pilot fashion, Walker again made an abrupt rudder-fixed left aileron input at Mach 1.05.  The same thing happened.  However, this time the aircraft’s motions were more violent.

Happily, Walker again recovered control of the X-3.  Having had enough of flight test frontiersmanship for one day, Walker uneventfully recovered the aircraft to Edwards.

The phenomenon that Joe Walker and the X-3 encountered that day in 1954 is known as Inertial Roll Coupling.  It is a resonant divergence in either pitch or yaw due to the presence of roll rate.  Aircraft like the X-3, which have low longitudinal and/or directional static stability as well as high pitch-to-roll and yaw-to-roll moment of inertia ratios, are especially susceptible to this phenomenon.

As a postscript to our story, the phenomenon of Inertial Roll Coupling had been hypothesized by the NACA’s William H. Phillips back in June of 1948.  For Joe Walker in October of 1954, engineering theory would become flight test fact in a few terrifying seconds high in the skies over Edwards Air Force Base.

Posted in Aerospace, History

Like a Phoenix Rising


Forty-one years ago this month, NASA successfully conducted the first manned Apollo Earth-orbital mission with the flight of Apollo 7.  This mission was a critically-important milestone along the path to the first manned lunar landing in July 1969.

The launch of Apollo 7 took place from Launch Complex 34 at Cape Canaveral Air Force Station, Florida at 15:02:45 UTC on Friday, 11 October 1968.  The flight crew consisted of NASA astronauts Walter M. Schirra, Donn F. Eisele, and R. Walter Cunningham.  Their primary goal was to thoroughly qualify the new Apollo Block II Command Module (CM) during 11 days in space.

Apollo 7 was not only the first flight of the Block II CM, but in fact the first manned mission in the Apollo Program.  Apollo 7 also featured the first use of the Saturn IB launch vehicle in a manned mission.  Apollo 7’s critical nature stemmed from the tragic Apollo 1 fire that took the lives of Virgil I. (Gus) Grissom, Edward H. White II, and Roger B. Chaffee on Friday, 27 January 1967.

The Apollo 1 fire was attributed to numerous deficiencies in the design, construction, and testing of its Block I CM.  The Block II spacecraft flown on Apollo 7 was a major redesign of the Apollo Command Module and was in every sense superior to the Block I vehicle.    However, it had taken 21 months to return to flight status and the Nation’s goal of a manned lunar landing within the decade of the 1960’s was in serious jeopardy.

The Apollo 7 crew orbited the Earth 163 times at an orbital altitude that varied between 125 and 160 nautical miles.  In that time, they rigorously tested every aspect of their Block II CM.  This testing included 8 firings of the Service Propulsion System (SPS) while in orbit.  Apollo 7 splashdown occurred in the Atlantic Ocean near the Bermuda Islands at 11:11:48 UTC on Tuesday, 22 October 1968.  

The Nation’s Lunar Landing Program overwhelmingly got the unqualified success that it desperately needed from the Apollo 7 mission.  The Apollo Block II CM would provide yeoman service throughout the time of Apollo.  The spacecraft would also go on to see service in the Skylab and Apollo-Soyuz Test Project programs.

While the technical performance of the Apollo 7 crew was unquestionably superb, their interaction with Mission Control at Johnson Spacecraft Center (JSC) in Houston, Texas was quite strained.  The crew suffered from head colds through much of the mission and the food quality was poor.  Coupled with Houston’s incessant attempts to cramp more tasks into each moment of the mission, Apollo 7 Commander Schirra took control of his ship and made the ultimate decisions as to what work would be performed onboard the spacecraft. 

The flight of Apollo 7 would be Wally Schirra’s last mission in space as he had announced prior to flight.  Schirra holds the distinction of being the only astronaut to have flown Mercury, Gemini, and Apollo missions.

Interestingly, Apollo 7 was not only Schirra’s last time in space, but it was Donn Eisele’s and Walt Cunningham’s first and last space mission as well.  That there is a direct connection between this historical fact and the crew’s insubordinative behavior during Apollo 7 is obvious to the inquiring mind.

Posted in Aerospace, History

Sound Barrier Breakthrough


Sixty-two years ago this month, the legendary USAF/Bell XS-1 experimental aircraft exceeded the speed of sound when it reached a maximum speed of 700 mph (Mach 1.06) at 45,000 feet.

Bell Aircraft Corporation of Buffalo, New York built three copies of the XS-1 under contract to the United States Army Air Forces (USAAF).  The aircraft were designed to approach and then fly beyond the speed of sound.

The Bell XS-1 was 31-feet in length and had a wing span of 28 feet.  Gross take-off weight was around 12,500 lbs.  The aircraft had an empty weight of about 7,000 lbs.  Propulsion was provided by a Reaction Motors XLR-11 rocket motor capable of generating a maximum thrust of 6,000 lbs.

On the morning of Tuesday, 14 October 1947, the XS-1 (S/N 46-062) dropped away from its B-29 mothership (S/N 45-21800 ) as the pair flew at 220 mph and 20,000 feet.  In the XS-1 cockpit was USAAF Captain and World War II ace Charles E. Yeager.  The young test pilot had named the aircraft Glamorous Glennis in honor of his wife.

Following drop, Yeager sequentially-lit all four XLR-11 rocket chambers during a climb and push-over that ultimately brought him to level flight around 45,000 feet.  The resulting acceleration profile propelled the XS-1 slightly beyond Mach 1 for about 20 seconds.  Yeager then shutdown the rocket, decelerated to subsonic speeds, and landed the XS-1 on Muroc Dry Lake at Muroc Army Airfield, California.

The world would not find out about the daring exploits of 14 October 1947 until December of the same  year.  As it was, the announcement came from a trade magazine that even today is sometimes referred to as “Aviation Leak”.

Today, Glamorous Glennis is prominently displayed in the Milestones of Flight hall of the National Air and Space Museum located in Washington, DC.  For his efforts in breaking the sound barrier, Chuck Yeager was a co-recipient of the 1948 Collier Trophy.

Posted in Aerospace, History

The Fastest X-15 Flight


Forty-two years ago this month, USAF Major William J. “Pete” Knight piloted the fabled USAF/North American X-15A-2 hypersonic research aircraft to a record speed of 4,520 mph – about a mile and a quarter per second.

North American’s original X-15 production run consisted of three (3) aircraft.  The X-15A-2 was a rebuild of the 2nd airframe (S/N 56-6671) which had been severely damaged during an emergency landing at Mud Lake, Nevada in November of 1962. 

The rebuilt aircraft was configured with a pair of droppable propellant tanks that allowed the type’s XLR-99 rocket engine to operate 60 seconds beyond the stock X-15’s 80-second burn time.  Among other modifications, the aircraft also carried a pylon-mounted dummy ramjet in the ventral region of the aft fuselage.

With the addition of the external propellant tanks, the X-15A-2 was really a three-stage vehicle.  The first stage was the NASA NB-52B mothership which launched the X-15 at Mach 0.82 and 45,000 feet.  The second stage consisted of the propellant-laden external tanks which were jettisoned at Mach 2.0 and 70,000 feet.  The third stage was the X-15A-2 with its entire internal propellant load.

Due to the increased speed of the X-15A-2, the aircraft was covered with Martin MA-25S ablator to protect it from the higher aerodynamic heating loads.  The baseline ablator was pink in color and gave the X-15A-2 a rather odd appearance.  Fortunately, application of a white wear/sealer over the ablator gave the aircraft a more dignified look. 

On Tuesday, 03 October 1967, Pete Knight and the X-15A-2 dropped away from the NB-52B (S/N 52-008) at the start of the X-15 Program’s 188th mission.  Knight ignited the XLR-99 rocket engine and excuted a pull-up followed by a pushover to level flight at a little over 102,000 feet.  Aircraft speed at XLR-99 burnout was 4,520 mph (Mach 6.7).

As the aircraft decelerated following burnout, Knight executed a series of pre-planned flight maneuvers to acquire vital aerodynamics data.  However, passing through Mach 5.5, he received an indication in the cockpit that a high temperature condition existed in the XLR-99 engine bay. 

Knight attempted to jettison the aircraft’s remaining propellants, but to no avail.  The jettison tubes were welded shut by whatever was happening in the engine bay.  This meant he would land heavier and faster than usual.  Fortunately, Knight’s piloting skills allowed him to get the X-15A-2 on to Rogers Dry Lake  in one piece.

As flight support personnel inspected the X-15A-2 airframe following Knight’s emergency landing, they were alarmed at what they found.  The aft ventral region of the aircraft had incurred significant thermal damage.  Further, the dummy ramjet was gone. 

As reported in the classic NASA document, TM-X-1669, higher-than-expected aerodynamic heating levels were responsible for the damage to the X-15A-2. 

First, shock wave/boundary layer interaction heating on the lower fuselage just ahead of the pylon (1) completely destroyed the ablator in that region and (2) penetrated the Inconel-X airframe structure.  This introduced very high temperature air into the X-15 engine bay.

Second, impingement of the dummy ramjet nose shock on the detached bow shock coming off of the pylon produced a shear layer that  focused on the pylon leading edge.  The resulting heating rates were of sufficient magnitude and duration to both burn away the pylon ablator and burn through the pylon structure.  The weakened pylon attachment eventually failed and the dummy ramjet departed the main airframe.

Pete Knight will forever hold the record for the fastest X-15 flight.  However, the X-15A-2 never flew again.  Only 11 more flights remained in the X-15 Program at the time.  A lack of time and funding meant that little was to be gained by repairing the thermally-damaged aircraft.

As for the final disposition of the X-15A-2 (S/N 56-6671), the aircraft’s remaining ablator was removed with its external surface cleaned-up and original markings restored.  The aircraft now resides in a place of honor at the National Museum of the United States Air Force located at Wright-Patterson AFB in Dayton, Ohio.

Posted in Aerospace, History