I can’t say if the XB-70 Valkyrie was the coolest airplane ever built, but I figure it has to be in the top 5 at least.
In case you were wondering what a “Valkyrie” is: in Norse mythology they were the “choosers of the slain”. They looked something like this:
The specification for the XB-70 goes back to 1955, when the mantra was “higher and faster”. The requirements were for a new bomber that could carry a 50,000 pound payload 7,500 miles at Mach 3 at an altitude of 70,000 feet. Oh, and it can’t be any bigger than a B-52 because we want to use the facilities we’ve already built.
This was a tall order indeed at a time when we were just building our first Mach 2 fighter jets. Now we were going to build a bomber that could not only achieve Mach 3 but also cruise at that speed?
I’m always amazed at how fast things were advancing at this time. In 1955 there were still B-17s in the inventory and now we were talking about going Mach 3.
The problem was three-fold. Well I’m sure it was much more than that but I’m simplifying here.
1. It needs to be aerodynamically capable of Mach 3.
2. It needs thrust. Lots and lots of thrust.
3. It needs to not melt. When you go that fast things start to get hot. Really really hot.
On top of all that, this thing was going to be a bomber, not a one-off experimental job.
That means it had to do all of the above plus:
It had to operate from existing bases. Fortunately SAC bases already had very long runways.
It had to operate in all weather.
The average Air Force pilot had to be able to fly it.
The average Air Force crew chief had to be able to maintain it.
It had to be mass produced so it couldn’t use too much in the way of exotic materials.
It couldn’t run on some exotic fuel that can’t be made in large quantities.
That’s a pretty tall order but North American and Boeing both went to work on it. I don’t think Boeing’s design ever got past the drawing board. North American, however, actually made it work.
The aerodynamic problem was solved by a unique concept called “compression lift”. How does it work? Hey, I just drive ‘em, I don’t know what makes them go.
The short answer is: at supersonic speeds the XB-70 would “ride its own shock wave”. At least that’s what every story I’ve ever read about it says. Works for me.
The end result was a large, delta-winged aircraft with a canard up front for pitch stability.
Note that “canard” is the French word for a duck. A canard airplane is kind of shaped like a duck, with the wings in back and a long neck up front. Mmmmmmmm………...duck…….
Sorry, shouldn’t write before dinner. Back to the XB-70.
The most unique feature of this very unique aircraft was the movable outer wing sections. These had three positions:
Horizontal for takeoff, landing and subsonic flight.
Drooped 25 degrees for speeds up to Mach 1.4
Drooped 65 degrees for speeds above Mach 1.4
Lowering the wingtips helped trap the supersonic shockwave under the aircraft and helped it make use of the compression lift concept. It also improved lateral (side to side) stability and effectively reduced the wing area (better for going fast). One other benefit was this kept the center-of-lift from moving rearwards and causing a big trim change as speeds increased. All in all a pretty elegant solution.
Okay, we’ve got a nice pointy delta-wing airplane that can theoretically go Mach 3. Now we need some big honkin’ engines to actually push it that fast.
The XB-70 used six General Electric YJ93 turbojets. These were surprisingly conventional engines for such an exotic aircraft. It was basically a scaled up version of the J79 that powered multiple fighter jets of the era. That will prove important later.
Each of the engines produced 19,000 pounds of thrust “dry” and 28,800 pounds with afterburner. That’s about 10,000 more pounds of thrust than the J79 it was based on. Quite a lot for the day. Plus it had a “six pack” of them.
Now the final hurdle. Spend enough time at Mach 3 and things start to heat up. Parts like the wing leading edges will exceed 600 degrees (fahrenheit) and the rest of the plane will still be an oven-hot 450 degrees. Hot enough to roast a……….duck.
Titanium can handle that kind of heat but building the whole thing out of titanium would have been much too expensive. We did that with the SR-71 but that was a much smaller aircraft as well as a very limited production run. I doubt we even had enough titanium in the early 1960s to build a fleet of B-70s out of the stuff.
Stainless steel can handle a lot of heat but it’s relatively heavy. The solution was to use a “sandwich” of stainless steel panels with a thin honeycomb between them. This made for a lightweight, heat-resistant material. The stainless steel honeycomb construction allowed the engineers to save the rare and expensive titanium for the places that needed it most.
Even as early as 1959 the Air Force was already having second thoughts about the B-70 program. The development of effective surface-to-air missiles was making high altitude penetration an iffy proposition — even at Mach 3.
Not that the early Russian SAMs were all that great. Still, the Air Force realized that you’re not going to win a “higher and faster” game against missiles. They can always build a missile that can go higher than your bomber. Even the incredible SR-71 never penetrated Soviet airspace, and it actually used an early version of stealth technology. There was no way a huge, slab-sided B-70 could ever be made stealthy.
Likewise advances in ballistic missiles were putting the entire concept of manned nuclear bombers into question. A missile would reach the target much faster than even a Mach 3 bomber.
Converting the B-70 to the low-level role was considered, but it just didn’t bring that much extra to the table. At low altitude it could only go .95 Mach and the airframe really wasn’t stressed for the rough ride at low level.
Finally, as always, politics came into play.
When JFK ran for President in 1960 he actually accused the Republicans of being weak on defense. Not something you hear much today. Once he was elected, he quickly realized that the “bomber gap” and “missile gap” with the Soviets were a myth. Oh the gap was real alright, it was just massively in our favor.
The Kennedy administration decided that the B-70 wasn’t really needed, but a Mach 3 research aircraft would be pretty useful for developing a Supersonic Transport.
Two XB-70 prototypes were built, AV-1 and AV-2. The NASA test pilots referred to them as “Ship 1” and “Ship 2”.
AV-1, being the first built, had a few flaws that were corrected on the second prototype. They were still learning how to build the steel honeycomb panels, and at high Mach numbers they could be torn off. After a couple nearly catastrophic incidents, Ship 1 was limited to Mach 2.5.
By the time they built Ship 2 they had solved the problems with the panels and improved the intake system as well.
The intakes are critical for high speed flight because they make sure the engines don’t ingest supersonic air (bad). If the supersonic shock wave reaches the engine it can cause a violent “unstart” condition. This was a very big deal for the SR-71 and was catastrophic on at least one occasion.
An unstart on the XB-70 wasn’t nearly as violent because the engines were much closer to the aircraft center-line. Note that on Ship 1 the intake system was manually controlled. Ship 2 had an Automatic Intake Control System or AICS to keep the engines happy.
It cost roughly a million dollars per sortie to fly the XB-70 so it’s not like they could just take one up for training.To prepare for flying the XB-70, test pilots first checked out in the B-58 to get experience flying a delta wing aircraft. They then checked out in the B-52 to experience flying something as large as the XB-70.
Like any large aircraft the XB-70 was reportedly difficult to taxi. The cockpit sat very high up and very far ahead of the landing gear.
Takeoff speeds were typical of delta wing aircraft. The nose was rotated at 175 knots and held there until liftoff occurred around 215 knots.
The plane reportedly flew quite well. I suspect that any quirks could have been resolved had it gone to production. A testament to the engineers at North American.
It was reportedly quite pitch sensitive at high Mach numbers. Not surprising really. At three times the speed of sound the slightest movement of the nose is going to translate to a big altitude deviation. The problem was easily solved by using pitch dampers for “stability augmentation”.
Approach and landing speeds were high as you would expect from an aircraft without flaps. Delta wing aircraft like the F-106 and B-58 landed fairly nose-high, and the XB-70 was no exception. The cockpit was so high up on landing that pilots had a tough time judging their height above the runway. It took three large drag chutes to help the Valkyrie get stopped.
If the XB-70 had one significant problem I’d say it was the landing gear. The main gear was fairly complex and had to move in three directions to extend or retract. First the struts were extended. Then the gear had to pivot to line up with the aircraft centerline. Finally the main trucks would pivot horizontally to the level position.
That’s a lot of chances for something to go wrong and of course they did. On one occasion Ship 1 had to be landed on a dry lake bed with one of the main gear only partially extended, nearly resulting in the loss of the aircraft.
On another occasion both the main and backup systems for the nose gear failed on Ship 2. A landing without the nose gear was considered impossible and the crew would have had no other option but to bail out. Finally the copilot bypassed a blown circuit breaker with a paperclip of all things and the nose gear came down. Not my first choice, but it beats jumping out.
The XB-70 ultimately met all of its design specifications. It exceeded Mach 3 and 70,000 feet. Most importantly it actually held Mach 3 for 30 minutes on one occasion, proving that the airframe heating issues had been solved.
Tragedy struck the XB-70 program in 1966. General Electric wanted a photo shoot of the XB-70 leading a formation of GE powered aircraft: F-4, F-5, T-38 and F-104. The F-104, piloted by test pilot Joe Walker was caught in the XB-70s wingtip vortex and flipped over the back of the large bomber.
Walker was killed instantly. The Starfighter damaged the XB-70’s right wing and tore both vertical stabilizers off. Ship 2 continued on for 16 seconds before entering a flat spin. Test pilot Al White barely manged to eject in time. His copilot, Carl Cross, never made it out.
The XB-70 had escape capsules for the pilots. Escape capsules were one of those “seemed like a good idea at the time” concepts. For the XB-70’s capsule to work, the pilot’s seat would actually be retracted backwards and then the capsule would close around him like a clamshell.
The copilot’s seat, probably due to the high g forces encountered in the spin, never retracted into the capsule and thus prevented him from ejecting.
White’s seat did manage to retract, but his arm was initially caught in the doors as the capsule closed around him. If you look at the picture there wasn’t a lot of clearance. Once he got his arm loose the capsule ejected properly, except for one thing.
There was an air bag on the bottom of the capsule that was supposed to cushion the landing. White’s air bag never inflated and he hit the ground with a force of roughly 44 G’s. This would probably be like being sealed in a trash can and dropped off a two-story building. Amazingly he survived but with major injuries.
After the loss of Ship 2, the program was left with only the inferior first prototype. Ship 1 made a total of 83 test flights, its last one taking it to the Air Force Museum at Wright Patterson AFB. You should go there and see what it looked like back when we used to build cool stuff. Hey you kids! Get off my lawn!
One of the main purposes of the XB-70 program was to research sustained high speed flight for the proposed supersonic transport. Those of us who were around back then can remember how we were all going to be blasting from New York to LA in 90 minutes at three times the speed of sound. Instead it’s 2016 and we plod along at slower speeds than the 707.
Sigh. The future was much cooler in the past.
The American version of Concorde was never built, of course. All that was left over from that program was the name of Seattle’s basketball team. Except I don’t think they’re either “Seattle” or “SuperSonics” any more.
The XB-70 program taught us that big airplanes going Mach 3 leave big sonic booms along their path of flight. I once thought that the sonic boom only occurred at the moment a plane broke through the “sound barrier”. In reality the shockwave trails behind the plane the entire time it’s supersonic. An SST flying from New York to LA would be rattling windows all the way across the country.
So what happened to North American? They merged with Rockwell in the late 60s. Then they got spun off to Boeing in the mid 90s. Boeing sold them to UTC (Pratt & Whitney) in 2005, who sold them to GenCorp. Today what’s left of North American goes by the rather retro sounding Aerojet Rocketdyne. That’s how it goes in the aerospace industry.