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Travail in the Thin Air

Sixty-five years ago this month, USAF Major Charles E. Yeager set an unofficial world speed record of 1,650 mph (Mach 2.44) in the Bell X-1A flight research aircraft. In the process, the legendary test pilot very nearly lost his life when the aircraft departed controlled flight shortly after rocket motor burnout.

The USAF/Bell X-1A was a second generation X-aircraft intended to explore flight beyond Mach 2. It measured 35.5 feet in length and had a wing span of 28 feet. Gross take-off weight was 16,500 pounds.

Like its XS-1 forebear, the X-1A was powered by an XLR-11 rocket motor which produced a maximum sea level thrust of 6,000 lbs. The XLR-11 burned 9,200 pounds of propellants (alcohol and liquid oxygen) in roughly 270 seconds of operation.

Departing Edwards Air Force Base, California on Saturday, 12 December 1953, Yeager and the X-1A (S/N 48-1384) were carried to altitude by a USAF B-29 mothership (S/N 45-21800 ). X-1A drop occurred at 240 knots and 30,500 feet. Within ten seconds, Yeager lit off three of the XLR-11’s four rocket chambers and started to climb upstairs.

Yeager fired the 4th chamber of the XLR-11 passing through 43,000 feet and initiated a pushover at 62,000 feet. The maneuver was completed at 76,000 feet; higher than planned. In level flight now and traveling at Mach 1.9, the X-1A continued to accelerate in the thin air of the stratosphere.

Yeager quickly exceeded Scott Crossfield’s briefly-held Mach 2.005 record set on Friday, 20 November 1953. However, he now had to be very careful. Wind tunnel testing had revealed that the X-1A would be neutrally stable in the directional channel as it approached Mach 2.3.

As Yeager cut the throttle around Mach 2.44, the X-1A started an uncommanded roll to the left. Yeager quickly countered with aileron and rudder. The X-1A then rapidly rolled right. Full aileron and opposite rudder failed to control the roll. After 8 to 10 complete revolutions, the aircraft ceased rolling, but was now inverted.

In an instant, the X-1A started rolling left and then went divergent in all three axes. The aircraft tumbled and gyrated through the sky. Control inputs had no effect. Yeager was in serious trouble. He could not control his aircraft and punching-out was not an option. The X-1A had no ejection seat.

Chuck Yeager took a tremendous physical and emotional beating for more than 70 seconds as the X-1A wildly tumbled. Normal acceleration varied between plus-8 and negative 1.3 G’s. His helmet hit the canopy and cracked it. He struck the control column so hard that it was physically bent. His frantic air-to-ground communications were distinctly those of a man who was convinced that he was about to die.

As the X-1A tumbled, it decelerated and lost altitude. At 33,000 feet, a battered and groggy Yeager found himself in an inverted spin. The aircraft suddenly fell into a normal spin from which Yeager recovered at 25,000 feet over the Tehachapi Mountains situated northwest of Edwards. Somehow, Yeager managed to get himself and the X-1A back home intact.

The culprit in Yeager’s wide ride was the then little-known phenomenon identified as roll inertial coupling. That is, inertial moments produced by gyroscopic and centripetal accelerations overwhelmed aerodynamic control moments and thus caused the aircraft to depart controlled flight. Roll rate was the critical mechanism since it coupled pitch and yaw motion.

The X-1A held the distinction of being the fastest-flying of the early X-aircraft until the Bell X-2 reached 1,900 mph (Mach 2.87) in July of 1956. Yeager’s harrowing experience in December 1953 would be his last flight at the controls of a rocket-powered X-aircraft. For his record-setting X-1A mission, Chuck Yeager was awarded the 1953 Harmon Trophy.

Posted in Aerospace, History

To the Moon and Back

Fifty years ago today, three American astronauts departed Earth to become the first men to orbit the Moon during the flight of Apollo 8. This epic mission also featured the first manned flight of the mighty Saturn V launch vehicle as well as history’s first super-orbital entry of a manned spacecraft.

Following the Apollo 1 tragedy in January of 1967, the United States would not fly another manned space mission until October 1968. That flight, Apollo 7, was a highly successful earth-orbital mission in which the new Block II Apollo Command Module was thoroughly flight-proven.

Notwithstanding Apollo 7’s accomplishments, only 14 months remained for the United States to meet the national goal of achieving a manned lunar landing before the end of the 20th century’s 7th decade. The view held by many in late 1968 was that an already daunting task was now unachievable in the narrow window of time that remained to accomplish it.

The pessimism about reaching the Moon before the end of the decade was easy to understand. The Saturn V moon rocket had not been man-rated. The Lunar Module had not flown. Lunar Orbit Rendezvous (LOR) was untried. Men had not even so much as orbited the Moon. Yet, history would record that the United States would find a way to accomplish that which had never before been achieved.

George Low, manager of NASA’s Apollo Spacecraft Program Office, came up with the idea. Low proposed that the first manned flight of the Saturn V be a trip all the way to the Moon. It was something that Low referred to as the “All-Up Testing” concept. The newly-conceived mission would be flown in December 1968 near Christmas time.

While initially seen as too soon and too risky by many in NASA’s management hierarchy, Low’s bold proposal was ultimately accepted as the only way to meet the national lunar landing goal. Yes, there was additional risk. However, the key technologies were ready, the astronauts were willing, and the risk was acceptable.

Apollo 8 lifted-off from LC-39A at the Kennedy Space Center in Florida on Saturday, 21 December 1968 at 12:51 hours UTC. The crew consisted of NASA astronauts Frank Borman, James A. Lovell, Jr. and William A. Anders. Their target – the Moon – was 220,000 miles away.

After a 69-hour outbound journey, Apollo 8 entered lunar orbit on Tuesday, 24 December 1968 – Christmas Eve. The Apollo 8 crew photographed the lunar surface, studied the geologic features of its terrain, and made other observations from a 60-nautical mile circular orbit. The spacecraft circled the Moon 10 times in slightly over 20 hours.

For many, the most poignant and memorable event in Apollo 8’s historic journey occurred on Christmas Eve night when each of the flight crew took turns reading from the Book of Genesis in the Holy Bible. The solemnity of the moment was evident in the voices of the astronauts. They had seen both the Moon and the Earth from a perspective that none before them had. Fittingly, they expressed humble reverence for the Creator of the Universe on the anniversary of the birth of the Redeemer of mankind.

Apollo 8 departed lunar orbit a little over 89 hours into the mission. Following a nearly 58-hour inbound trip, Apollo 8 reentered the earth’s atmosphere at 36,221 feet per second on Friday, 27 December 1968. The first manned super-orbital reentry was performed in total darkness. It was entirely successful as Apollo 8 landed less than 1 nautical mile from its target in the Pacific Ocean. The USS YORKTOWN effected recovery of the weary astronauts and their trustworthy spacecraft. Mission total elapsed time was 147 hours and 42 seconds.

The year 1968 was a tumultuous one for the United States of America. Martin Luther King and Robert Kennedy had been assassinated. American military blood flowed on the battlefields of Vietnam and civilian blood was let in countless demonstrations taking place in the nation’s cities. The ill-posed sexual revolution continued to erode the country’s social and moral moorings.

But, as is so often the case, an event from the realm of flight, now newly extended to lunar space, reminded us of our higher nature and potential. For a too brief moment, Apollo 8 put our collective purpose for being here into sharp focus. Perhaps a short phrase in a telegram sent to Frank Borman from someone he had never met said it best: “You saved 1968!”

However, looking through the lens of history, we now know that Apollo 8 did much more than end the penultimate year of the 1960’s on a positive note. Indeed, it may be said that Apollo 8 saved the entire Apollo Program.

Posted in Aerospace, History

Final Flight: USAF NF-104A

Fifty-five years ago today, USAF NF-104A (S/N 56-762) crashed to destruction following a rocket-powered zoom to 101,600 feet above mean sea level (AMSL). The pilot, USAF Colonel Charles E. “Chuck” Yeager, was seriously injured, but survived when he successfully ejected from the stricken aircraft approximately 5,000 feet above ground level (AGL).

The USAF/Lockheed NF-104A was designed to provide spaceflight-like training experience for test pilots attending the Aerospace Research Pilot School (ARPS) at Edwards Air Force Base, California. The type was a modification of the basic F-104A Starfighter aircraft. Three copies of the NF-104A were produced (S/N’s 56-0756, 56-0760 and 56-0762). It was the ultimate zoom flight platform.

In addition to a stock General Electric J79-GE-3 turbojet, the NF-104A was powered by a Rocketdyne LR121-NA-1 rocket motor. The J79 generated 15,000 pounds of thrust in afterburner and burned JP-4. The LR-121 produced 6,000 pounds of thrust and burned a combination of JP-4 and 90% hydrogen peroxide. Rocket motor burn time was on the order of 90 seconds.

Around 1400 hours PST on Tuesday, 10 December 1963, Colonel Yeager took-off from Edwards Air Force Base to attempt his second zoom flight of the day. That morning, he had zoomed NF-104A, S/N 56-760 to an altitude of 108,700 feet. Four days earlier, Yeager had zoomed the same airplane to the highest altitude he would ever achieve in the type; 110,500 feet AMSL.

The zoom apex altitude for the ill-fated afternoon flight was only 101,600 feet AMSL with rocket motor burnout taking place 5 seconds post-apogee. That is, the aircraft was already on the descending leg of the zoom trajectory and in the early stages of reentry. Yeager later reported that the aircraft angle-of-attack at that point was on the order of 50 deg; a figure that is well past the NF-104A pitch-up angle-of-attack (i.e., 14-17 deg). Yeager had flown the aircraft this way on previous zoom flights and had always been able to lower the nose via reaction control system (RCS) inputs.

Unfortunately, the RCS did not not have sufficient pitch control authority to bring the nose down on the mishap flight. As a result, the aircraft began the reentry in an extremely nose-high attitude. As the dynamic pressure rapidly built-up, the NF-104A departed controlled flight and went through a series of post-stall gyrations between 90,000 and 65,000 feet. These gyrations ultimately led to a series of flat spins occurring between 65,000 and 20,000 feet.

Running out of altitude, Yeager desperately deployed his drag parachute as an anti-spin device. This action indeed stopped the flat spin. Airspeed picked-up to 180 KIAS with the aircraft hanging in the chute, but the pilot was unable to get an airstart on his J79 turbojet which had spooled down to 6% of maximum RPM. At 12,000 feet, Yeager jettisoned his drag chute and the NF-104A immediately pitched-up again into a flat spin. After three-quarters of a turn, Yeager ejected about 5,000 feet AGL. Yeager landed close to where the mishap aircraft had impacted and was in a great deal of pain due to burns he received during the ejection process. Happily, he survived this traumatic event and recovered completely from his injuries.

Objective analysis of the loss of NF-104A, S/N 56-762 reveals that the aircraft simply was not flown in a manner commensurate with the intricacies of the zoom environment. The critical importance of quickly intercepting and maintaining the target inertial pitch angle during pull-up had been repeatedly demonstrated by other test pilots as had proper control of aircraft angle-of-attack during reentry. Colonel Yeager elected not to fly the aircraft in accordance with these dictates.

In all of his NF-104A zoom attempts, Colonel Yeager consistently flew the airplane over the top at angles-of-attack well beyond the pitch-up value. RCS control authority was sufficient to lower the nose to sub-pitch-up angles-of-attack just prior to reentry on all but the mishap flight. Unfortunately, the low apex altitude (101,600 feet AMSL) of that zoom resulted in a higher dynamic pressure that, in conjunction with very high angles-of-attack, produced a nose-up aerodynamic pitching moment that the RCS could not overcome.

The aircraft mishap of 10 December 1963 forever changed the way in which NF-104A pilots would be allowed to fly the rocket-powered zoom mission. Prior to the mishap, the NF-104A had been zoomed to an altitude of 120,800 feet AMSL by USAF Major Robert W. Smith on 06 December 1963. This unofficial United States record still stands today. After a mishap investigation, NF-104A maximum altitude was limited to 108,000 feet AMSL. This restricted performance was mandated ostensibly out of concern for the safety of ARPS student test pilots.

The ultimate and lasting result of the post-mishap restriction on NF-104A flight performance was that it did a great disservice to ARPS student test pilots in that it made their spaceflight training experience something less than what it could and should have been. It is ironic that, although the correct manner in which to zoom the airplane had been repeatedly validated by USAF and Lockheed test pilots prior to the 56-0762 mishap, the decision to restrict NF-104A performance was based on a single flight which clearly demonstrated how not to fly the airplane.

Posted in Aerospace, Final Flight, History

NF-104A Zoom Climb Record

Fifty-five years ago today, USAF Major Robert W. Smith zoomed the rocket-powered USAF/Lockheed NF-104A to an unofficial world record altitude of 120,800 feet. This mark still stands as the highest altitude ever achieved by a United States aircraft from a runway take-off.

A zoom maneuver is one in which aircraft kinetic energy (speed) is traded for potential energy (altitude). In doing so, an aircraft can soar well beyond its maximum steady, level altitude (service ceiling). The zoom maneuver has both military and civilian flight operations value.

The USAF/Lockheed NF-104A was designed to provide spaceflight-like training experience for test pilots attending the Aerospace Research Pilot School (ARPS) at Edwards Air Force Base, California. The type was a modification of the basic F-104A Starfighter aircraft. Three copies of the NF-104A were produced (S/N’s 56-0756, 56-0760 and 56-0762). It was the ultimate zoom flight platform.

In addition to a stock General Electric J79-GE-3 turbojet, the NF-104A was powered by a Rocketdyne LR121-NA-1 rocket motor. The J79 generated 15,000 pounds of thrust in afterburner and burned JP-4. The LR-121 produced 6,000 pounds of thrust and burned a combination of JP-4 and 90% hydrogen peroxide. Rocket motor burn time was on the order of 90 seconds.

The NF-104A was kinematically capable of zooming to altitudes approaching 125,000 feet. As such, it was a combined aircraft, rocket, and spacecraft. The pilot had to blend aerodynamic and reaction controls in the low dynamic pressure environment near the zoom apex. He was also required to fly in a full pressure suit for survival at altitudes beyond the Armstrong Line.

On Friday, 06 December 1963, Bob Smith took-off from Edwards and headed west for the Pacific Ocean. Out over the sea, he changed heading by 180 degrees in preparation for the zoom run-in. At a point roughly 100 miles out, Smith then accelerated the NF-104A (S/N 56-0760) along a line that would take him just north of the base. Arriving at Mach 2.4 and 37,000 feet, Smith then initiated a 3.75-g pull to a 70-degree aircraft pitch angle. Turbojet and rocket were at full throttle.

Things happened very quickly at this point. Smith brought the J79 turbojet out of afterburner at 65,000 feet and then moved the throttle to the idle detent at 80,000 feet. The rocket motor burned-out around 90,000 feet. Smith controlled the aircraft (now spacecraft) over the top of the zoom using 3-axis reaction controls. The NF-104A’s arcing parabolic trajectory subjected him to 73 seconds of weightlessness. Peak altitude achieved was 120,800 feet above mean sea level.

On the back side of the zoom profile, Bob Smith restarted the windmilling J79 turbojet and set-up for landing at Edwards. He touched down on the main runway and rolled out uneventfully. Total mission time from brake-release to wheels-stop was approximately 25 minutes.

Much more could be said about the NF-104A and its unique mission. Suffice it to say here that two of the aircraft ultimately went on to serve in the ARPS from 1968 to 1971. The only remaining aircraft today is 56-0760 which sits on a pole in front of the USAF Test Pilot School (TPS) at Edwards.

Bob Smith went on to make many other noteworthy contributions to aviation and his nation. Having flown the F-86 Sabre in Korea, he volunteered to fly combat in Viet Nam in his 40th year. Stationed at Korat AFB in Thailand, he commanded the 34th Fighter Squadron of the 388th Tactical Fighter Wing. Smith flew 100 combat missions in the fabled F-105D Thunderchief; many of which involved the infamous Pack VI route in North Viet Nam.

Bob Smith was a true American hero. Like so many of the airmen of his day, Smith was a man whose dedication, service, and courage went largely unnoticed and underappreciated by his fellow countrymen. Bob Smith’s final flight came just 3 months shy of his 82nd birthday on Thursday, 19 August 2010.

Posted in Aerospace, History