Boeing’s B-52, Keeper Of The Peace Or Flying Test Lab!
By Lt. Colonel William D. Sluru, Jr., February 1980 issue of Wings
Little was it realized when the first YB-52 thundered down Boeing Field, Seattle, on its maiden flight in 1952 that the U.S. was launching one of the most venerable and versatile military aircraft ever produced. Not only has the B-52 been the mainstay of our strategic bomber force for the past two decades, but it has played an important part in the development of many new aerospace concepts . For example, the X-15 and X-24 both began their careers from beneath the wings of BUFF, Big Ugly Fat Fellow.
The 8-52 has also been used for flight testing new jet engines and rockets. Today, the B-52 is still in the research and development business, and is being used to launch a new research tool, the Remotely Piloted Research Vehicle (RPRV). The idea of launching a small plane from a larger one is by no means new. In the early 1930s, the huge Navy Airships Akron and Macon could carry, launch, and retrieve five Sparrowhawk biplanes. Later in that same decade, the British experimented with carrying a small monoplane, the Mercury atop one of their giant Empire flying boats.
There was even a proposal to have a Hurricane fighter carried by a Liberator bomber for convoy escort duty during World War II . Then there was the midget Goblin fighter that was designed to ride as a parasite beneath the B-36. Of course. all these ideas were attempts at increasing range by allowing the “passenger” aircraft to start out at altitude with a full fuel load. The practice of air launching research craft from a bomber began after World War II . The Bell X-1 , the first aircraft to fly supersonically , was launched from a B-29, as were other pioneer aircraft like the X-1 A, X-1 B, X-1 E and the Navy’s 0-558 II Skyrocket. The X-2. which was designed to fly at three times the speed of sound was launched from a B-50.
In the mid-1950s. the U.S. started thinking seriously about the space age and manned spaceflight. There were legitimate questions as to whether manned spaceflight was even possible. Could such a craft be piloted above the fringes of the atmosphere? Could man function under weightlessness and the high “g’s” of reentry. and was he even needed in space? These questions demanded answers before the U.S. could embark on more ambitious space ventures. Thus the X-15. probably the most successful research aircraft ever. was born as a joint NASAAir Force-Navy program. (See Airpower May 1978) It is important to note that while the X-15 was originally conceived as a means to obtain technical data on hypersonic aeronautics. the immediate benefactor of the X-15 flights was the manned space program.
It was obvious that in order to fly the X-15 flight envelope of speeds up to 4000 mph at altitudes of over 50 miles a fuel gulping rocket engine was the only way to go. In fact. to get about two minutes of powered flight from the X-15’s 60.000 pound thrust engine. the X-15 had to carry nearly 20.000 pounds of fuel and oxidizer. Thus the X-15 had to be air launched and the B-52. The USAF’s newest bomber was the craft to do the job.
The B-52 was extensively modified by USAF and North American technicians to become the mothership for the X -15. A six by eight foot section was cut out of the bomber’s right wing flap to make room for the X-15’s wedge tail. A pylon to mate the X-15 to the B-52 was installed between the bomber’s in-board engines and the fuselage. The lines and wires that were the X-15’s umbilical cord during the captive flight below the B-52 passed through this pylon. Large liquid _oxygen tanks were placed in the B-52’s bomb bays for topping off the X-15’s LOX system prior to separation. A closed circuit TV system was added so that the B-52 crew could carefully watch the X-15 and its pilot prior to launch. There was an elaborate launch control system to insure that the X-15 was released at precisely the right instant and with the right attitude. Actually, two B-52s were modified as motherships. No. 003 and 008. As their tail numbers indicate, they were among the earliest B-52s to roll off the Boeing production lines.
March 10, 1959 and a typical sunny day at Edwards AFB. A giant B-52 thunders down the runway, its tall orange fin towering above the shimmering pavement. Tucked below its right wing is the black, 8 ton X-15 almost invisible by comparison to the 150 ton mass of metal that is the “Stratafortress”. Air Force Captain Charles Bock is at the controls of the B-52, while North American test pilot, Scott Crossfield, sits in his silver pressure suit within the X-15 cockpit. However, on this first flight the X-15 would not be launched. lt would return and land while still attached to the B-52. This was only the first of several captive flights made to check out the X-15 and the X-15/B-52 combination.
Even so, this was not a routine flight. On the descent before landing, the X-15’s cockpit suddenly filled with smoke. Crossfield and the B-52 crew feared the worst. A fire in the X-15 was a very serious situation, for the X-15 sat directly below the B-52s huge fuel tanks. A flame could have caused an explosion that could have blown both aircraft out of the sky. Should the X-15 be jettisoned or should they try to land with a potential bombshell? After a quick check by the B-52 crew and the pilots of the chase planes that were monitoring the flight, the decision was made to land. It turned out to be the right one, the smoke had been caused by a failed X-15 generator and it had found its way into the rocket~plane’s cockpit through the ventilation system.
Finally, on June 8, 1959, the X-15 was released. Crossfield had it all to himself. On this true first flight the rocket was not lit and the X-15 was flown entirely as a glider. The first rocket powered flight would come in September of 1959. Incidentally, the first two dozen powered X-15 flights would not use the big XLR-99 rocket engine, but would use two smaller XLR-11 engines, 8000 pound thrust power sources that were essentially the same engines used in the earlier X-1 aircraft. Before the X-15 program was over in 1968, 199 flights would be made by the three X-15s. NB-52 No. 008 carried the X-15 aloft for 140 of these flights, while 003 was the.mothership for the rest.
On a typical flight, the B-52 would release the X-15 after attaining an altitude of about 45,000 feet, and a speed of 500 mph. First, the X-15’s LOX tanks would be topped off. The rocket engine would be lit for 80 to 120. seconds, with external tanks attached to the X-15 for the longer burns. The remainder of the 10 to 11 minute flight would be as a glider. Generally, one of two types of flight profiles was used; a steep climb to maximum altitude or a level flight to attain maximum speed. During the X-15 program some twelve military and civilian test pilots would fly the bird, including future astronauts Joe Engle and Neil Armstrong.
Several Air Force pilots earned their astronaut wings for flights above 50 miles. Several unofficial records would be set; fastest speed at 4,520 mph (Mach 6.7) and highest altitude of 354,200 feet (67 miles). But more important that mere records was the vital technical data gained during the more than 30 hours of flight time.
Some 700 technical documents were needed to report on the results of the X-15 flights. This was important data for future projects like Apollo, the Space Shuttle, and for high speed aircraft of all types. The X-15 flights showed that man could function under the stress of spaceflight and, indeed, had an important place here. In the X-15, means of controlling craft both inside and outside the earth’s atmosphere were demonstrated and pilots learned to cope with this new flight regime. The X-15 showed the value of the flight stimulator for preparing pilots for the unknown. The list of X-15 contributions could go on and on, but without the B-52, the program and its progress would have been severely limited.
The Lifting Bodies
The B-52 motherships would see action next with lifting body research aircraft that both the USAF and NASA started testing in the mid-1960s. As their name implies. these craft obtain their lift from the unique aerodynamic shape of their bodies rather than from wings. Thus wings can be essentially eliminated from spacecraft that must survive the heat of re-entry through the earth’s atmosphere. Without wings, such craft can better survive the friction of reentry and can be made far stronger structurally. NASA and the USAF were interested in lifting bodies because they meant manned spacecraft could return from routine space missions without the need for huge recovery forces such as required by the Apollo program.
Lifting body research was really done in two parts. There was the hypersonic flight regime where an unmanned test vehicle was boosted to extremely high speeds and altitudes via rocket. Then there were the manned lifting bodies to investigate the problems of flight after re-entry, as well as control during landings. It was during this latter regime that the B-52 again had a chance to play a significant role. NASA used its M2-F1 /F2/F3 and HL-10 craft, while the joint NASA/USAF X-24A and X-248 lifting bodies were introduced later. Incidentally, tests were conducted with the M2-F1 as early as 1963. However. this tubular steel and plywood craft was not launched from a B-52, but was towed behind a C-47.
NASA and USAF research craft were all quite similar in size, weight, and general appearance. However, there were differences in body shape and in the location of the cockpit and control surfaces. All of the vehicles would be flown as gliders. as well as being equipped with the veteran XLR-11 rocket engine that could boost them to higher speeds and altitudes than the B-52 could.
The lifting bodies were carried aloft using the same pylon as created for the X-15, with a special adapter to mate the craft to the pylon. In the forward end of this adapter were four 3000 psi air tanks to supply air conditioning and cabin pressurization for the parasite craft while it was still attached to the B-52. The B-52’s liquid oxygen tanks were used to top off the tanks in the lifting body prior to release on rocket power flights.
Again the craft were released at an altitude of about 45,000 feet and with speeds approaching 450 mph in order to give maximum tree-flight times. For example, on initial flights there was sufficient time to make a practice landing flare at altitude before the pilot had to make a real one on the lakebed. But even before the lifting bodies were released, several captive flights were made to check out the structural integrity of the lifting body vis-a-vis the B-52 mating system, subsystems within the lifting body and even, in some cases. the operation of the landing gear. Some of the lifting bodies had escape system that allowed ejection even at zero altitude.or while still attached to the B-52. Almost all the 144 lifting flights were dropped from B-52, No. 008. These included both powered and unpowered flights.
The tests began in 1966 with flights of the M2-F2 and ended in 1975 with the needle nosed X-24B. A maximum altitude of 89,000 feet and a top speed of Mach 1.9 was achieved, both with the HL-10. All the craft supplied important information for the design of today’s Space Shuttle. For example. while the X-24s like the other lifting bodies landed on the dry lakebed at Edwards, test pilots John Manke and Mike Love were able to land the X-24B on the regular Edwards’ runway. This accomplishment, as well as the general success of the lifting body concept, were major factors in convincing NASA to design the Space Shuttle as an unpowered glider.
Rocket Boosters for the Space Shuttle
B-52 008 got into the space business again during the testing of the solid rocket boosters for the Space Shuttle. During launch, two giant solid rocket motors will help boost the Shuttle into orbit. At 28 miles altitude, the motors will be separated from the rest of the Shuttle. A cluster of three 115 foot diameter main chutes will lower each 80 ton booster gently into the ocean. The boosters will then be towed to shore to be refurbished and reused.
In order to test the booster recovery technique and equipment, six test drops were made using No. 008. Because of limitations in the control capabilities of the B-52, the test boosters weighed only 24 tons. In the tests, the B-52 carried the booster to a 20,000 toot altitude using the same X-15 wing pylon as on previous aircraft flights and then released it for a fall to earth.
Even though the B-52 is a quarter century old and qualities to be registered as an antique airplane, it remains part of an important new concept in flight testing, the Remotely Piloted Research Vehicle or RPRV. Instead of using manned aircraft to test new concepts, aircraft flown remotely from the ground are used. These RPRVs are more economical to build, because they do not have to be man-rated, do not have to carry life support equipment, and need not be re-qualified for manned flight each time modifications are made. In addition, less expensive ground based electronics= can be substituted tor flight qualified avionics and computers. Not only are RPRVs more economical, but they also allow more dangerous and riskier ideas to be tested without endangering the life of a test pilot. in case of an accident, expensive aircraft are not lost, not to mention pilots.
While RPRVs can be and have been built to takeoff and land entirely by remote control, the simplest approach is to carry the RPRV to a suitable altitude and drop it. This is where the B-52 fits into the picture. NASA’s Dryden Flight Research Center at Edwards AFB has been the leader in developing the RPRV technology. After the flight is over, the RPRV can be recovered by a parachute that is snatched by a helicopter or it can be landed using radio control. Stall and spin testing is one of the most dangerous tests. but nevertheless a necessary part of the verification of most new aircraft designs. When it came time to test the stall and spin characteristics of the USAFs new F-15 Eagle fighter, a 3/8 scale model of this aircraft was used by the Dryden Flight Research Center. Naturally, the RPRV was dropped from 008.
Fighters of the future will have to be much more maneuverable if they are to survive the dogfights of the 21st century. In order to try out advanced materials, flight controls, and aerodynamic concepts, two new RPRVs are being tested at Dryden FRC in a joint Air Force-NASA program. These are HiMat aircraft for Highly Maneuverable Aircraft Technology. The HiMAT is powered by a jet engine and while dropped from a B-52, it will land on skid type landing gear after its remotely controlled flight. Finally, NASA is using a modified Firebee II, BWM-34F, target drone to test such new aerodynamic concepts as extremely thin wings and flutter suppression devices. These RPRVs will be recovered using a helicopter to snatch a parachute deployed from the RPRV at mission completion.
But More Than Just a Mothership
The B-52 has contributed more to aeronautical technology than just being. a mothership. Two of the largest turbofan engines ever built were flight tested beneath the wings of a B-52. These were the TF-39 engine used on the gargantuan Lockheed C-5A transport and the JT9D engine found on the Boeing 747 airliner. For these tests, a two engine nacelle was replaced by a single turbofan test engine.
In order to see if a craft as large as the Space Shuttle could be landed as an unpowered glider. landing tests were made with a NB-52 with six of its engines at idle and two allowed to run at 75 percent power to operate its accessories. The B-52, which is approximately the same size and weight as the Shuttle orbiter. showed the Shuttle concept to be quite feasible and provided important design information. The B-52 has also been used to test forward canards as a means to obtain a super-stable aircraft and to investigate wake vortex phenomena. During the Control Configuration Vehicle (CCV) program. the B-52 was employed to demonstrate that the speed of future aircraft need not be limited to avoid flutter and structural bending. The B-52 played an important role during the development of the escape capsule for the crew of the 8-1 bomber. Prototype escape modules were carried inverted under a modified wing pylon up to altitudes of 47,000 feet. Then they were released to be recovered using the 8-1 escape system parachutes. Other drop tests preceded the module drops to test the chutes themselves under different types of emergency conditions. A total of fourteen capsule drop tests were made between 1972 and 1975.
Recently, drop tests have been made of the two-man F-111 crew module, again using the B-52. The B-52 has been an invaluable asset in the development of new military, commercial, and space aviation concepts. B-52, tail number 003 has been retired, but old 008 is still going strong. It still has many years of testing before it retires. And if things go right this ageless wonder may very well be testing space vehicles and concepts that haven’t even made their appearance on the drawing board.