It's seventy years to the day since No. 617 Squadron of the Royal Air Force returned from Operation Chastise, in which specially designed bouncing bombs were dropped in an attack on the Möhne, Sorpe and Eder Dams in Germany during World War II. Though the bouncing bomb is without doubt the invention for which Barnes Wallis is most renowned (thanks in no small part to its depiction in the film Dambusters) Wallis' other work before, during, and after World War II was of great importance, and in some cases, far ahead of its time. Gizmag spoke to Dr. Andrew Nahum, Principal Curator of Technology at the Science Museum where many of Wallis' papers are archived, about swing-wing aircraft, earthquake bombs, improbable mathematics lessons, and the geodetic Wellington Bomber.

Gizmag: How troublesome was development of the bouncing bomb?

Dr. Andrew Nahum: I think the concept was okay. The problem was making it hold together. The conception of the rotation, the skipping, was pretty sound. I think it was more of a mechanical problem, of engineering, to withstand the drop. It was to do with having a defined height, and a very accurate low height, but also strengthening the casing. It was quite an ingenious thing, in that the backspin imparted to it was there to give it a lift force. It made the projectile roll down the dam wall once it reached it, and he'd marked the drawing, as you see, lateral force, and that's a fluid dynamic effect called the Magnus effect. It's kind of more sophisticated than it looks as a great big spinning dustbin.

GM: Do you think the historic view of Operation Chastise altered over the years; for instance, in terms of recognizing German casualties, and a realistic assessment of its military impact?

AN: Are you referring to people implying that strategic bombing was ineffective in general? You're getting into a whole new area, really. That's a question you ought to be asking about the whole strategic bombing campaign, and not just the dams raid. That is one element in it. It's a matter of record that there was a lot of opinion expressed after the war that the bombing had been relatively ineffective, but that's still open to debate. One of the more economic histories of the war by Adam Tooze, Wages of Destruction, had started to suggest that it was more effective than has generally been thought. I'm talking about strategic bombing as a whole, not the dams raid alone. I'm not saying that's my opinion. I'm simply saying the question has been re-opened at an academic level. I think another thing that's worth saying is that the raids were never repeated, and I think it was concluded that, having had surprise once, and having had very heavy casualties on the attacking force, they could never go back and do the same thing, especially with the long slow approach which the bomb required.

Wallis arrived at the idea of a bouncing bomb when thinking about how he, as an engineer, might help shorten World War II. His idea was that a bomb dropped from an aircraft could be made to bounce along the surface of a river before striking a dam where it would sink and detonate. The bouncing would allow evasion of anti-torpedo nets placed in the river to protect the dams, and allow the bomb to explode right next to the dam, where it would be most effective, and potentially breach. Numerous tests were carried out before arriving at the dustbin-shaped Upkeep bomb design, and determining optimal backspin and drop height. On May 16, 1943, 19 modified Lancaster Bombers left on their mission to destroy the Möhne, Eder and Sorpe Dams in an effort to disrupt Nazi supply lines. After several attempts, the Möhne and Eder Dams were breached, though the Sorpe Dam obtained less damage. Eight aircraft were lost, with 53 of the 56 crew aboard them killed. The mission is thought to have resulted in more than 1,300 causalities on the ground, many of whom were overseas workers. The mission did strike the Nazi war machine, however, destroying 25 bridges, 11 factories, and various other mines and infrastructure, and damaging many more.

GM: Are there any items in the Science Museum's archive that shed light on Operation Chastise?

AN: There's a lot of documentation on it. There's a very early sketch which I think is very intriguing, because it shows that the concept was formed very early in his mind. To me, one of the most intriguing things about it is that he had no standing as a tactician, if you like. He simply thought up this plan, how can we strike an economically effectively blow, and then started lobbying people. So the film has some elements of truth, [such as] him being rebuffed by officialdom but at the same time there was quite an appetite in Britain for, what should I say … what Churchill's people called funnies, odd weapons that might work in a new way. You could include in that some of the flail tanks that landed at D-Day, the Mulberry harbours, the pipeline under the ocean that brought oil across The Channel, and the floating ice aircraft carrier that was never built, or never deployed, made of a mixture of woodchip or sawdust and water, intended to be assembled in Canada and towed off the coast for battle.

Barnes Wallis' sketch outlining his bouncing bomb concept (Photo: BAE Systems/SSPL)

GM: Wow…

AN: This is not Barnes Wallis. But partly because Churchill had a very inventive mind and was fascinated by invention, there was a lot of time given to unconventional, novel weapons. He did have an effect in the First World War on the adoption of the tank; not his sole idea, but he was a spokesperson for it.

GM: What were Barnes Wallis' most notable achievements before or after World War II?

AN: He had quite a varied career, and he started off designing airships.

GM: …which is where we get into his geodetic airframe?

AN: Yes. The challenge for the airship is, you have to make a very large structure that's stable and rigid. It relates to exactly what you said, a geodetic structure, subsequently used for the Wellington Bomber.

GM: What was the strength of the geodetic airframe?

AN: In a sense you could say it is a kind of basketwork structure, which distributes the loads fairly completely about the fuselage, meaning, to some extent, that it is fairly resistant. There are other load paths if part of it was shot away. You'd get it if you stretched a string over a courgette [zucchini] in several places.

The geodetic airframe devised by Wallis in the 1930s is a basketwork structure which describes the aerodynamic form of the aircraft, as opposed to being an aerodynamic skin supported by a beam. By crossing geodesic members on a curved surface, the torsional loads on each are cancelled out, accounting for its great strength. The geodetic principle was used in the gasbag of the Vickers R100 airship, the design of which Wallis led. Geodetic design was subsequently applied to the Wellington Bomber. Though this construction method took longer than monocoque techniques, they resulted in robust aircraft. There are several accounts of Wellingtons returning safely, though rather less than intact, following raids on Germany, such as that pictured above, heavily damaged by anti-aircraft fire during a raid on Duisburg in 1943.

AN: A thing to observe about the Wellingtons is that, in a way, it's kind of intermediate technology. It's not stressed skin which was becoming the conventional way to build aeroplanes, what some people call monocoque. All the American bombers are like that, they're built like airliners. So the skin is carrying part of the load. But again, that's possibly a more safety-critical way of designing [aircraft]. You've got a stressed can, as it were. You can probably afford to lose less than you can with the Wellington. The stressed skin aircraft is also dependent on internal beams. There's no such thing as an aircraft where the skin is the whole structural member. It's simply a question of trying to exploit the skin so that it contributes and is not just a parasite.

If you look inside a Boeing or an Airbus, there's stiffness everywhere, and along longitudinal members, and particular beams in the wing, deep internal spars. The trick was to make the skin contribute through its tension. It can't contribute in any other way because it buckles in compression. It can contribute when it's stiffened, and part of the technique of aircraft design is to add, or machine in, channels that help it resist compression to some extent. However, its principal contribution is in tension, so you can say that when an aircraft is flying stably, there's a tension field in the under surface of the wing. The good thing about that, whether it's an airliner or a bomber, is that it makes the thing structurally more efficient.

So, for example, the Hurricane, actually built on a kind of framework structure (it's not like a Wellington because it doesn't have these diagonally-crossing members, but it's still a system of frames, which is ultimately what a Wellington is), weighed more than a Spitfire, had the same power, same engine, so typically, Spitfires were faster, faster to height, and had a higher altitude capability. The stressed skin made it structurally more efficient, lighter for its size and power, meaning its performance was better. If it was a bomber it would have meant it carried more payload, but that wasn't really the case with the Spitfire.

So not to discredit or disparage the Hurricane, but that was the difference between the previous technology and the new one, the stressed skin. In the Wellington, I say it's an intermediate technology because the basketwork is holding the aeroplane together, and then it's covered in doped Irish linen, so that's what keeps the air out, rather like, well… most biplanes were built that way. It was an intermediate step between previous framed aircraft and stressed skin. You could say it was appropriate for its day, but its day passed.

GM: What about after the war?

AN: The effect of his influence is very interesting. And after the war he became an ahead-of-his-time technological thinker with these designs of tailless variable-geometry aircraft, swing-wing aircraft, Wild Goose and Swallow hypersonic or supersonic aircraft. And he also proposed a fleet of nuclear-powered cargo submarines, which would help Britain stay at the centre of world trade.

GM: What was his goal with supersonic aircraft?

AN: Very high Mach-number flight, shrinking the world… By this stage his perspective is that Britain needed to cultivate high technology in the new world to stay afloat, really. He gave a lecture called The Strength of England in various places, dwelling on this, and arguing that Britain ought to be innovating and building or it would decline. In the case of swing-wing aircraft, he didn't persuade anyone to follow. So in away he was a bit of a prophet crying in the wilderness in the postwar period.

GM: Is that a theme throughout his career?

AN: No, I don't think so. He had almost complete control of the design of the R100 [airship], and it flew to American and returned successfully, around the time the R101 crashed at Beauvais. There were two airships, a national one [the R101] and a Vickers commercial one [the R100]. I think he was taken very seriously. He was in charge of it, and the same with the Wellington. In fact he became, as Vickers' chief designer in 1936 … he was supposed to have responsibility for Supermarine, which developed the Spitfire, but I don't think that worked, really. I don't think he had that much influence on Supermarine work. However, that was an incidental because he was designing the Vickers aircraft, and his authority to do that was unchallenged.

After the war, Wallis turned to the question of supersonic flight, and developed concepts for a number of such aircraft, which he called aerodynes. His Wild Goose concept was notable for the position of the wings to the rear of the aircraft. This was to counteract the rotational forces experience be a solid body moving in air at an angle – an issue he was alerted to thanks to his experience designing airships. The wings would pivot to steer the craft, and sweep back to attain greater speeds. Wild Goose proved successful as a model, but was cancelled in the early 1950s before a piloted version was completed. His subsequent Swallow concept, pictured above, also employed a swing-wing, but in a delta configuration. After promising early test flights, the loss of a test model followed by military cuts did for the development of the aircraft. His work was built on in the United States, though without the involvement of Wallis or Vickers. (Image:

GM: Did he continue to design bombs after the war?

AN: I don't think so. After the bouncing bomb attack he worked on free-fall penetration bombs [during the war]: Tallboy is one. They were used on concrete structures like the V-2 launching site that was being built in a forest near Calais. The site was Watten, and a huge concrete structure was being erected for making liquid oxygen and launching the V-2s. I'm not sure that its function was entirely understood by Britain, except that they knew it didn't mean any good. It was bombed, I think, at certain stages of its pouring on the advice of the McAlpine family. This is just hearsay, you might want to check it. I don't know which McAlpine it would have been, but one of the big contractors said [to the question of] when is the best time to bomb it, and I think the answer was when it was nearly set, but not when it was still too soft, for maximum disruption. And you can still see it. It's a huge huge blockhouse – more than a blockhouse – it was a fort – a fortified factory, I think we could call it, and launching site. And it's all slightly slumped to one angle as a result of the raid. That was Tallboy bombs.

Here, Nahum is referring to Blockhaus d'Éperlecques (pictured under construction in 1944), sometimes called Watten after a nearby town, which was subjected to numerous raids during the war, including by Tallboy bombs. The Tallboy raid was reportedly influenced by Sir Malcolm McAlpine, who advised that the attack should be made before the concrete had set. However, Gizmag is as yet unable to verify this for certain.

AN: They were also used on a gun emplacement which was a fixed gun [the V-3 cannon], supposed to be firing at London, at Mimoyecques. And they were used on U-boat pens which were heavily reinforced with concrete arches.

GM: In dry dock, you mean?

AN: I mean their operational hub installation. The idea is when they got back to France or Germany they went into these hardened pens. They were really floating docks. But the thought was, if they were just floating alongside a quay, they would be very easily broken up by a conventional raid, which is true. I think the Barnes Wallis bombs were also used in attacking viaducts, railway viaducts, as well. They call them earthquake bombs, and the idea was that they would achieve a supersonic speed under free-fall and penetrate a long way into the earth or into the structure, so they weren't a fragmenting-in-the-air kind of blast. It proved remarkably effective, and it's surprising that the bomb could actually withstand it without flying apart.

GM: Are there any other stand-out items in the Science Museum's Barnes Wallis collection?

AN: One of the things that people find intriguing and quite charming is that he courted his wife, who was called Molly Bloxam, when he was in his 30s and she was 17, and about to go to University College London. And he asked her father's permission, and he said he wouldn't oppose it, or he would agree to it, if Barnes Wallis taught her mathematics. Their daughter, Mary Stopes-Roe, published an account of this in a book called Mathematics with Love, and they were kind of personal letters with mathematics tutorials in them.

Update 20.05.2013: This article was amended to reflect that Upkeep, not Highball, was the nickname of the final bouncing bomb design. Thanks to the readers who pointed this out.