Prologue
In the Research & Development Gallery at the National Museum of the USAF near Dayton, Ohio stands an aircraft towering over all the other aircraft in the gallery, the centre piece, in the shadow of its wings stand several other aircraft appearing to shelter there.
This is the XB-70 Valkyrie, arguably one of the most influential aircraft of all time.
The Nuclear Deterrence
Before we dive into the XB-70 Valkyrie, it’s important to understand the backdrop which led to her development.
The 1945 nuclear events of Hiroshima and Nagasaki clearly established the importance of nuclear deterrence. The cold war was ramping up and the modified B-29s used to deliver the nuclear ordnance were inadequate.
By 1941 Britain was at the risk of falling to Nazi Germany. America was looking for a new bomber that had at least a 5,700 mile range ( Gander – Berlin roundtrip), the ability to deliver a 10,000 pound ordnence load and return. Furthermore the bomber had to have a service ceiling of 40,000 feet and a cruising speed of approx 275 mph. The events of Pearl Harbor ensured the B-36 would only enter service post WW2.
Convair won the contract and the aircraft was originally designated the B-35, later switched to B-36 to avoid overlap and confusion with the Northrup YB-35 flying wing. The aircraft first flown in 1948 was huge with a wingspan of 230 feet and a length of 162 feet, was propelled by six pusher props. Later models had four turbojets on the outboard wings making a total of ten power plants ‘ Six Turning & Four Burning’(the maximum on any production bomber aircraft ever), could carry over 80,000 pounds of ordnance.
The B-36 was relegated to obsolescence with the advent of the MIG 15 over North Korea by 1950. It was too slow for the faster interceptors Russia was producing. America needed an all jet powered bomber that was quicker.
The B-47 Stratojet entered operational service in 1951. While the requirement goes back to 1943 for a jet powered reconnaissance bomber, the original model 424 was essentially a version of the B-29 . Following the 1945 inspections of captured top secret German documents on swept wings the jet powered game pivoted on its head. With a 35 degree sweep and a wingspan of 116 feet with wings mounted on the fuselage shoulder, the aircraft was powered by six turbojets. The nuclear capable bomber had a max payload of 25,000 pounds and a range of 2500 miles. With a cruise speed of approx 500 mph the B-47 was the backbone of the Strategic Air Command’s ( SAC) bomber fleet through the 1950s. (Note: The B-45 operated from 1947-59 however had many shortcomings that severely curtailed its usefulness)
While the B-47 operated in tandem with the B-36 there was a clear gap in the Range / Payload / Speed doctrine and most importantly reliability, enter the B-52.
The eight jet B-52 is a venerable veteran among bombers globally, first entering service in 1955 and still in active service to this day. With a wingspan of 185 feet and a length of 159 feet, the aircraft cruises at 525 mph, has a range of 8,800 miles and service ceiling of 50,000 feet. The aircraft can carry 70,000 pounds of ordnance and is nuclear capable.
Through the 1950s aircraft got faster and the push for air superiority quickly moved aircraft into the supersonic era. Starting with Gen. Chuck Yeager’s famous 1947 first in the Glamorous Glennis. Aircraft such as the F-86 Sabre and the F-100 Supersabre made sure that supersonic was here to stay. The Russians were making supersonic strides themselves with their MIG 19 ‘Farmer ‘ . Bombers needed to go supersonic.
The B-58 was designed with nuclear strike capability and was the very first operational Mach 2 bomber. While the B-58 was a clear statement of intent the aircraft had a limited range of 4,000 miles and payload capacity of approx 20,000 pounds. The delta wing (a recent innovation) made low speed handling very difficult and the aircraft had a high incident rate. SAC issued a fresh directive for new aircraft.
WS-110A
In 1955 the SAC issued ‘ General Operational Requirement No. 38 ‘ the foundation for an operational bomber that had the capabilities of both the B-52 and the B-58. The conventional fuel powered jet version of this requirement was called ‘ Weapons System 110A ‘ or WS-110A.
The specifications of the bomber was a cruising speed of Mach 0.9, 50,000 pound payload and a combat radius of 4,000 miles. Boeing & North American Aviation both were included in round one of the development along with other leading companies.
By the mid 1950s USSR in addition to its supersonic fighters such as the MIG-19 had SAMs (Surface to Air Missile). The missiles were a threat to a Mach 0.9 aircraft. The rules of engagement changed to a Mach 3 heavy strategic bomber and a cruising altitude of 70,000 feet.
The initial designs from both companies had take-off weights in excess of 750,000 pounds and both the proposals were dismissed ‘ being too large ‘. Gen Curtis LeMay, the commander in chief of the SAC is said to have commented on seeing one of the proposals “ this is not a bomber, it is a three ship formation!”
Both companies were told to refine designs.
NACA Supersonic Studies
In 1951 Richard Whitcomb put forward the ‘ Area Rule’. His discovery stated that ‘ Total cross sectional area ‘ of the aircraft was responsible for drag in the transonic ( Mach 0.8 – 1.2) regime and not just the wing cross section. This finding resulted in the ‘coke bottle fuselage’ , a narrowing of the fuselage where the wing cross section came into play.
In 1956 A J Eggers & Clarence A Syverton published ‘ Aircraft configurations developing high lift-drag ratios at high supersonic speeds’. The principle investigated the design concepts of aircraft at high supersonic speeds. The long title would come to be known as compression lift or wave riding.
The 1951 ‘Area Rule’ was first tested on the redesigned F-102A Delta Dagger. The rule which required the original F-102 to be lengthened by 11 feet , with narrowed coke bottle design in the middle, a new canopy along with redesigned wings and a pushed back tail, resulted in a much faster , more stable aircraft that comfortably sustained Supersonic speeds.
The 1956 internal memorandum was studied in detail by NAA and they figured compression lift had to be central to the WS-110A design philosophy along with area rule.
By early 1958 the WS-110A would be officially designated the XB-70. The Air Force had transitioned the project from a concept ( Weapons System or WS) to an experimental program (XB). The name Valkyrie was the winning name submitted by Sgt. Francis Seller in a naming contest held by the USAF. Valkyrie the Norse Goddess is the ‘chooser of the slain’, guides souls lost in battle to Valhalla(the hall of heroes). Valkyrie was chosen from over 20,000 suggestions.
The Canards & Forebody
The XB-70 experienced significant ‘ Mach Tuck’ at high Mach speeds. This was caused by the centre of pressure moving aft as the aircraft accelerated through the speed regime.
The automatic canards managed by the FACS (Flight Control Augmentation System) adjusted continuously to manage the tuck. With a span of 28 feet they were significant in trimming out pitch shifts and helped smooth shock transitions.
The canards worked in conjunction with the elevons on the wing’s trailing edges.
The forebody of the XB-70 like most supersonic aircraft today was sharply tapered through to the canards. The underside as were the sides were not only flat and shallow, but also contoured to create the primary shockwave.
Behind the nose the contour widens and transitions towards the engine nascelles. It is here the coke bottle design is clearly visible.
Please be sure to read an about the evolution of the Flying & Blended Wings in the two part series here. http://theaviationevangelist.com/2025/09/13/the-evolution-of-the-flying-wing-part-one/
The XB-70 used a retractable windshield ( the first of its kind). The windshield serviced multiple purposes. The first was to create a clear aerodynamic line. The second was heat insulation for the cockpit at 600 degrees F (it did heavily restrict forward visibility). To augment visibility, the canopy had flat, heat shielded windows on the sides. Aircraft such as the Concorde and TU-144 followed a similar concept with their droop noses.
The Wings
The large & thin wing area with a high aspect ratio ( the wingspan divided by the mean distance between the leading & trailing edges of the wing a.k.a average chord) managed sub / transonic lift (aerodynamic lift).
The sculpted leading edges of the wing helped control vortices the delta wings generated. Vortex lift is important during high angles of attack (specific to delta wings) during take off and landing. Concorde is a famous example of using vortex lift.
The flat undersurface of not just the wings but also engines ‘6 pack’ was critical to the XB-70’s most important design feature, ‘ Compression Lift’. The wings outer panels ( last twenty of the trailing edge on each side ) drooped by up to 65 degrees. The droop was important to trap the shock waves created off the sculpted engine intake splitter & the intakes themselves.
While most of us think of shockwaves coming off a supersonic aircraft horizontally, the splitter was responsible for generating shockwaves vertically, these waves being trapped by the folded wingtips creating a wave cushion. The XB-70 generated up to 30% of required supersonic lift through compression lift. Shock waves would bounce into the engine inlets too. The folded wingtips improved yaw handling a great deal and the XB-70 needed much smaller vertical stabilizers as a result.
The XB-70 is the first aircraft to use three different kinds of lift across the speed regime. The swept back wings at 65 degrees reduced transonic drag and improved handling.
The wings flexed and bent considerably through the speed regime. To help keep the wing flexible the engineers at NAA intuitively integrated six elevons (combination flaps & airelons) on each wing and avoided binding the wing. Furthermore by doing so they managed extreme hinge and actuator loads inflight (hinge moments).
The six elevon setup gave the FACS more flexibility as it managed pitch / trim (inboard elevons) and roll (outboard elevon). As the wingtips drooped (25 – 65 degrees) the two outboard elevons were faired to zero and became part of the folding wingtip. Lastly, having six elevons helped with redundancy. All hydraulics on the aircraft were at 4,000 psi.
AV1 had a flat wing with zero degree dihedral, while AV2 had a five degree dihedral as a design refinement. This gave AV2 better directional and roll stability over AV1 and also gave AV2 better compression lift efficiency.
AV2 was unfortunately lost on June 8 , 1966 during a formation photo flight. General Electric had a photo session using the XB-70, F-104 Starfighter, F-4 Phantom II & a T-38 Talon. All of them used GE engines.
Test pilot Joe Walker (the most experienced supersonic pilot then) in his F-104 was sucked into the starboard wingtip turbulence of the XB-70, flipped over the vertical stabilizers of the XB-70 and crashed in a fireball. The doomed XB-70 flew level for a few seconds before going into a steep spiral and crashing, taking with it co-pilot Carl Cross. Pilot Al white ejected using the crew escape capsule engineed for high altitude ejection or depressurisation while retaining control of the aircraft (in event of depressurization).
The Engine Nascelles & Intakes
The engine nacelles not only fed the engines with air but also were an integral part of the compression lift generated by the XB-70.
The entry was split by a vertical splitter fins. The engines were split three on each side. They also projected the airflow towards the drooped wing tips to trap shockwaves. The nacelles created oblique shock waves at the inlet lips as they began slowing air to about 400 mph from supersonic speeds as stable air was directed to the engines. This kept engine compressor pressure within a constant bandwidth. The trailing edges of the three moveable ramps behind the engine inlets hinged inwards or outwards (between 10 – 30 degrees or upto one foot) as per the Mach number and compressor requirement. The entire system including the ramp angles & bleed doors (for excess air) was continuously adjusted by the inlet control system.
The engine nacelle had a 2D rectangular configuration and had a maximum height of 4 feet. The length of the intake from the nacelle to the engines was approx 30 feet.
The underside of the entire intake ramp was flat as it aided in compression lift.
The Engines a.k.a ‘The Six Pack’
The XB-70 had six General Electric ( GE) YJ93-GE-3 turbojet engines.
Each axial flow engine generated 19,900 pounds of dry thrust and 28,800 pounds with afterburners. The engines had no thrust reversers and used drogue chutes as a stopping device. With eleven compressor stages and of which six were low pressure and five high pressure.
The engines were made of Nickel based alloys and stainless steel. Advanced blade cooling allowed the engine to survive high exhaust gas temperatures (EGT). The engines used high flash point JP-6 fuel.
The engine control system synchronised with automatic inlet control management to prevent compressor stalls and upstarts (happens when airflow to engines is unstable due rapid speed changes).
With so many different systems working in tandem on such a precision piece of engineering the YJ93 was a high maintenance product.
The Landing Gear
The XB-70 had the conventional hydraulic tricycle gear.
The rearward folding nosewheel had two wheels.
The main gear had two bogies with four wheels each. The main gear had a complex mechanism of folding the bogie in, then a twist and then folding into the wheel wells. The wells had a flap that closed and aerodynamically sealed the wheels inside.
The main gear each had one small wheel between the outer pair of wheels. This small wheel acted as a braking sensor was an early ABS mechanism. During rejected takeoffs the brakes could heat up to 1,000 degree F.
The landing gear struts were made of forged chromium-molybdenum steel for its exceptional strength and fatigue resistance. The struts were more than capable of handling the 500,000 pound gross weight during heavy landings at over 200 knots.
The tyres were made by Goodyear and had aluminium woven into them to withstand the high landing temperatures of over 300 degrees F. Each tire was Nitrogen inflated to over 250 psi.
The XB-70s brakes had a multiple disc setup. Each disc is made of forged steel. They were heat treated to resist warping and cracking under extreme thermal loads.
The Fuel System
The aircraft carried approximately 43 – 46,000 gallons of JP-6 Fuel. Everything about the system was about managing heat, aircraft stability & structural integrity in addition to feeding the engines optimally across the speed range.
Fuel was stored across eleven fuel tanks distributed across the fuselage and wings of the aircraft. The tanks themselves were constructed using the same honeycomb sandwich panels used for the fuselage skin. The honeycombing did throw up sealing issues which was resolved using advanced epoxy compounds. Although some tanks never properly sealed and hence were never used (ex: the tail tank).
Using the JP-6 fuel as a coolant was a first ! The fuel was circulated through ten heat exchangers throughout the aircraft to absorb and dissipate heat. The heat exchangers were part of the engines fuel pumping system enroute to ignition.
The tanks themselves had heatsinks within each of them to draw excess heat. Furthermore to prevent vapor ignition the tanks were inerted using 700 pounds of liquid nitrogen held in dedicated tanks. As fuel was consumed nitrogen filled the empty tanks to maintain pressure, displace oxygen and reduce fire risk at elevated temperatures.
The fuel management system was integral to the Centre of Gravity Management system. The system actively transferred fuel between tanks as Mach numbers increased. As speed increases the aerodynamic centre of the aircraft moves rearward. The centre of gravity needs to coincide with this to avoid a Mach Tuck. By drooping the outer wingtips in conjunction with its canards, the aircraft effectively moved the centre of pressure forward. The fuel management system worked in conjunction with the compression lift mechanism by moving fuel forward to balance the rearward move of the aerodynamic centre. The wing tanks were typically burned off first.
We observe here that all systems were dependent on each other to maintain stability.
Lastly the JP-6 fuel was specifically developed for the XB-70 program and its extreme speed regime. It addressed the issues of aerodynamic heating, high speed engine performance and safety & high altitude operation. The fuel performance exceeded all the XB-70 operating parameters and was developed as an alternative to zip fuels (high calorific value boron based fuels). Zip fuels had caustic byproducts that caused engine wear and posed toxicity risks.
Kerosene based JP-6 was the safe alternative that provided for all requirements without the byproducts.
The Materials of the XB-70
Over ninety percent of the external structure of the XB-70 ( fuselage skin, nacelles ) was made of type 321 stainless steel built as a honeycomb structure. The material and construction had high thermal resistance of up to 600 degrees F with minimal distortion at Mach 3. The structure itself was rigid, lightweight and thermally stable.
The hot areas such as engine bays & aft of bays & internal structure was made of a titanium alloy called Ti-6Al-4V also known as Grade 5 titanium. The alloy was 90% titanium, 6% aluminium, 4% vanadium and had excellent thermal resistance of over 1000 degrees F with an excellent strength to weight ratio.
High temperature adhesives used to bond the honeycomb structure were made of redux and epoxy adhesives. The honeycomb structure could not be riveted as it would weaken the structure.
Non heat zones such as avionics bays, hydraulic lines & non load bearing fuselage sections were made of aluminium alloys as they were light weight, easy to machine and cooler.
The engine and exhaust area materials were made of Inconel & Rene 41. These alloys can resist very high EGTs in the range of 1,800 degrees F.
All coatings and sealants had heat resistant coatings to prevent oxidation and surface degradation due high temperatures. The sealants protected the honeycomb edges from moisture intrusion & thermal cycling damage.
Strategic Bomber to Experimental Research Platform
By the late 1950s the US & Soviet SAMs were getting bigger, faster and more powerful. President Eisenhower was a proponent of the ICBMs (Inter Continental Ballistic Missiles). His take on the XB-70 program was that “ building the XB-70 was like fighting with bows & arrows in the era of gunpowder and guns” The XB-70 just could not cope with the banks of Soviet SAM systems coming online across the entire USSR.
Gary Powers was shot down in a U2 over the USSR at 70,400 feet and this would prove President Eisenhower’s prophecy.
The program was cancelled in 1959, however to salvage the considerable expenditure already incurred (over $300 mn) the Pentagon authorized the production of a single vehicle. AV1 was almost completely handbuilt.
The XB-70 program is a great example of how politics directs expenditure. As the political tug of war continued NAA was caught in the middle of a fierce battle. The Air Force continued to support the program and even attempted to reinstate it as a combat test vehicle.
The 1960 election of President Kennedy brought fresh impetus to a failing program, the President switched the program from a manned bomber to an experimental aircraft. A total of three were to be constructed, however only two were ever completed, the third was incomplete (the avionics and other systems were actually ready).
NAA should be commended for sticking through the program at each step. Finally there was consensus across all stakeholders including the Air Force, Politicians, NASA & of course NAA.
The XB-70 in the Air
Total flights – 129
AV1 – total flights83
Total flight time – 160hrs 16min
Mach 3 flights – 1.
AV1 had several design issues that restricted speed to Mach 2.5
AV2 – total flights 46
Total flight time – 92hrs 22min
Mach 3 flights – 9
On May 19, 1966 AV2 flew at Mach 3 for 32 consecutive minutes.
Combined, the XB-70 Valkyrie accumulated a total of 1hr 48min at Mach 3+.
Each flight of the XB-70 was an adventure and there were several incidents.
The Legacy of the XB-70 Valkyrie
The XB-70 was an aircraft of many firsts, later adopted for use by the Aviation / Aerospace Industry. Below are listed a few of them!
- Variable geometry wings later adopted by aircraft such as the B-1A/B Lancer. Compression lift later used by the SR-71. The overall aerodynamic stability of XB-70 influenced several other projects.
- Material and thermal management solutions advanced the development of heat-resistant structures and cooling systems, impacting aerospace exploration technologies.
- Fuel and propulsion innovations directly contributed to the SR-71 and indirectly to modern jet engines and fuel systems, particularly for high-speed and high-altitude operations.
- Avionics and automation laid groundwork for modern flight control and safety systems, enhancing reliability and reducing pilot workload in complex aircraft .
- The XB-70’s strategic obsolescence redirected military aviation toward low-altitude and stealth technologies, while its test data shaped research and development for decades
Epilogue
Over 50 years after her last flight in 1969 the XB-70 at the National Museum of the United States Airforce, looks ready to take off and fly away to the clouds where she belongs. Makes you wonder what she would have been like in the air? A combination of size, speed, sound ,smoke & incredible power all coming together creating a show like none other.
Perhaps the Valkyrie’s greatest message to future generations is ‘ Always be innovating, it’s the only path forward’.
Credit for all pictures to the respective owners.
Please be sure to read about the Flying & Blended Wings, a two part series here. http://theaviationevangelist.com/2025/09/13/the-evolution-of-the-flying-wing-part-one/
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