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The engine sat on a steel examination table in a requisitioned workshop at RAE Farnborough, Hampshire, on the morning of the 14th of July, 1944.
British engineers analyzed a captured German V-1 pulsejet engine in 1944, revealing its combustion rate defied known physics and engineering principles.
Key Takeaways
- The V-1 pulsejet engine was a simple yet highly effective propulsion system operating beyond theoretical combustion limits.
- German engineers prioritized frequency and performance over durability, reflecting the weapon's short operational life.
- The pulsejet's acoustic resonance was finely tuned, demonstrating advanced applied acoustics in wartime mass production.
- The British technical examination provided critical insights that aided in countering the V-1 threat.
- The engine's design challenged existing combustion and propulsion engineering knowledge of the time.
What the video covers
- On July 14, 1944, British engineers at RAE Farnborough examined a captured German V-1 pulsejet engine.
- The engine was simple in construction, weighing 353 lb with thin sheet steel casing and only one moving part: 18 spring steel flap valves.
- Despite its crude design, the pulsejet operated at a combustion frequency of 47 times per second, exceeding the theoretical maximum of 35.
- The engine powered a weapon responsible for killing 2,752 civilians and destroying over 75,000 buildings in just 7 weeks.
- The valve design was deliberately loose to allow high-frequency operation, optimized for a short lifespan of about 22 minutes.
- The fuel system used standard 80 octane aviation gasoline with a simple pressure-fed injection system, no pumps or cooling.
- Each combustion pulse released about 4,200 joules of energy, totaling nearly 200,000 joules per second through an uncooled steel tube.
- Engineers were initially baffled as the combustion chamber's behavior defied textbook acoustic resonance models.
- The V-1 was mass-produced with research-grade acoustic tuning features like circumferential grooves to optimize combustion.
- The acoustic signature of the engine was used by Allied forces for early warning detection of incoming V-1 attacks.
Chapters
- 00:00Introduction to the captured V-1 engine
- 00:54Impact and scale of V-1 attacks
- 01:48Comparison with modern turbojet engines
- 02:39Pulsejet operation and design overview
- 03:23Transport and secrecy of the captured weapon
- 04:10Valve assembly examination and engineering
- 05:43Fuel system analysis and combustion energy
- 07:36Engineers’ reaction and wartime context
- 08:20Combustion chamber measurements and resonance
- 09:18Acoustic tuning and production engineering
Full Transcript — Download SRT & Markdown
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It was 11 ft and 4 in long. The casing was thin-gauge sheet steel rolled and spot-welded, the kind of construction you would expect from a factory turning out agricultural equipment, not propulsion systems.
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There were no cooling fins, no intake compressor, no turbine wheel. The exhaust pipe was a straight tube.
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The whole assembly weighed 353 lb. One of the engineers who first walked around it, a propulsion specialist named Dr. William Crichton from the Power Jets Research Division, later wrote in his preliminary notes that his first impression was that someone had sent him
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a drain pipe with ambitions. He was not joking. He was making a technical observation.
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Because what was sitting on that table had in the 7 weeks before it arrived at Farnborough killed 2,752 civilians in London and Southern England, destroyed more than 75,000 buildings, and was being launched at a rate of more than 100 per day, and the
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engine that powered it, the Argus AS 109-014 pulse jet, was operating at a combustion frequency that should have been physically impossible.
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When Crichton's team finally measured it, they found it was firing 47 times per second.
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The textbook said the theoretical ceiling for a resonant combustion cycle in a duct of that diameter and length was 35.
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To understand what they were looking at, you need to understand what a pulse jet actually is, because it is the simplest form of jet propulsion that exists, and that simplicity is precisely what made it so disturbing.
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A turbojet engine, the kind that powers a modern aircraft, has thousands of moving parts.
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Compressor blades, turbine stages, fuel nozzles, variable geometry intakes. It requires precision manufacturing to tolerances measured in thousandths of an inch.
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The Rolls-Royce Merlin engine powering a Spitfire had 11,000 individual components. The Argus pulse jet had one moving part.
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One. A grid of spring steel flap valves at the inlet. 18 of them arranged in a rectangular array, each one a stamped metal shutter roughly the size and weight of a playing card. That was it.
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Fuel injected, valves slammed shut by combustion pressure, gas expanded rearward, pressure dropped, valves snapped open again. Repeat.
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The acoustic resonance of the tube itself maintained the cycle. No compressor, no turbine, no lubrication system, no cooling.
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In theory, it was a dead end, a curiosity that generated thrust through brute rhythmic combustion, but could never be efficient enough to matter.
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The Germans had built a weapon around it anyway, and they had made it work in ways that Crichton's team was not prepared for.
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The V1 that arrived at Farnborough had been recovered largely intact from a forced landing in a field outside Robertsbridge in East Sussex on the 9th of July, 1944.
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The airframe had sustained undercarriage damage, but the engine and fuel system were essentially undamaged.
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It had been transported under armed guard and covered with tarpaulin because the government did not yet want photographs of the complete weapon in circulation.
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The examination team was led by Crichton and included two aeronautical engineers from the Royal Aircraft Establishment, a fuel chemist named Dr. Harold Roxbee Cox, who would later become chief scientific adviser to the Ministry of Fuel and Power, and a senior engineer
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from Bristol Aeroplane Company named Arthur Rubbra, who had spent most of the war working on the Hercules radial engine.
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These were not theorists. They were men who had spent years with their hands inside running machinery.
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And if you are finding this examination as compelling as we think you are, this is exactly the kind of very technical history this channel covers every week.
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If you want more of it, subscribe now and hit the bell. Every video goes this deep.
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They began with the inlet valve grid. Rubbra removed the forward fairing first, exposing the rectangular intake face of the engine.
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The valve assembly came out as a single unit, a welded steel frame holding 18 individual flap valves, each one formed from spring steel strip 0.018 inches thick.
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Rubbra measured the spring tension on each valve using a small gauge he had brought from Bristol.
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The valves were set to open at an inlet pressure differential of approximately 0.3 pounds per square inch and to close under a back pressure of roughly 1.1 pounds per square inch.
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The numbers were written down and then stared at for a long moment. Because those tolerances were loose.
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Not by accident, by design. The Germans had made the valves deliberately compliant, deliberately easy to open and easy to close because they needed the cycle to run fast and tight tolerances would have damped the resonance.
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Rubbra noted in his report that the valve engineering was, in his phrase, "brutally pragmatic." He meant it as engineering admiration.
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A tighter valve might have lasted longer. This valve was optimized for one thing only: frequency.
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Lifespan was not the objective. The V1 flew for 22 minutes on average. The engine only needed to survive 22 minutes. The fuel system came next.
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Roxbee Cox took the fuel lines apart while Crichton examined the combustion chamber dimensions. The V1 burned 80 octane aviation gasoline, not a specialist fuel, not a secret compound, standard Luftwaffe aviation spirit of the same grade used in the Focke-Wulf 190.
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The injection system was a simple pressure-fed arrangement, a main fuel tank holding 150 L pressurized by ram air from a small inlet on the nose, feeding through a single main jet and a set of eight secondary spray nozzles
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arranged in a ring around the forward section of the combustion chamber. No fuel pump, pressure-fed, gravity and ram air.
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Roxbee Cox calculated the fuel flow rate from the tank capacity, the known range of the weapon, and the measured nozzle diameters.
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It came to approximately 57 L per hour at cruise. He then calculated the thermal energy being released per combustion cycle.
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Each firing pulse, each individual detonation in that 11-ft tube, was releasing roughly 4,200 J of energy, 47 times per second.
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That was 197,400 J per second flowing through a steel tube with no cooling system, no heat sink, no thermal management of any kind.
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Roxbee Cox wrote one line in the margin of his notes at this point. It said, "This should not be intact." Between the fuel analysis and the combustion chamber measurement, the team broke for a working lunch. Sandwiches brought in from the canteen, eaten
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standing at the bench because none of them wanted to leave the room. Crichton later described this in a post-war interview.
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He said the feeling in the workshop was similar to the feeling of reading a letter that you know contains bad news and keeps delaying the moment it tells you what that news is.
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They knew something was wrong with their understanding of this engine. They did not yet know what.
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That same week, the week of the 14th of July 1944, Allied forces in Normandy were fighting their way through the bocage south of Caen.
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Operation Goodwood was being planned. The ground campaign that would eventually break German lines in France was still 2 weeks away from launch.
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And in London, every night the V1s were coming in. Not one or two, waves of them.
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The examination at Farnborough was not academic. The people in that workshop knew that what they found in that engine would shape how Britain tried to stop the weapons that were destroying the city 35 miles to the northeast.
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Crichton measured the combustion chamber himself. The internal diameter of the tube at the widest point was 535 mm, just over 21 in.
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The length from the valve face to the exhaust exit was 3,390 mm, approximately 133 in.
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He calculated the acoustic resonant frequency of the tube using the standard formula, the speed of sound in hot combustion gas divided by twice the effective length of the resonant cavity.
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Using an assumed gas temperature of 1,200° C ins
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He put the pencil down. The measured operating frequency, which had been recorded from acoustic analysis of the weapon in flight by a team at the National Physical Laboratory in Teddington, was 47 cycles per second.
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The engine was running five cycles per second faster than the acoustic resonance of its own tube should have permitted.
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This was not a marginal discrepancy. This was the engine telling them that their model of how it worked was wrong.
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Robrow was the one who found the answer, and he found it by doing something none of them had thought to do yet.
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He measured the temperature of the outer skin of the combustion chamber at multiple points along its length using a contact thermometer.
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The results were plotted on a strip of paper pinned to the wall of the workshop.
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What the temperature distribution showed was that the combustion was not occurring in the zone where Crichton's model assumed it was.
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The main combustion event, the peak energy release, was happening approximately 18 in further rearward in the tube than the injection point.
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The fuel was not igniting immediately at the nozzles. It was being swept rearward by the intake airflow during the valve open phase, mixing with residual hot gas from the previous cycle, and igniting as a distributed charge across a longer
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section of the tube. The effective acoustic length of the resonant cavity was not the physical length of the tube.
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It was shorter. Significantly shorter. The Germans had designed the injection geometry, the angle and velocity of the fuel spray from those eight nozzles, to deliberately shift the combustion centroid rearward, shortening the effective resonant length, raising the natural frequency of the
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system. They had tuned the combustion location the way a musician tunes a pipe organ.
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By changing where energy entered the resonant column, not by changing the column itself. And by doing so, they had pushed the cycle frequency five beats per second above what the tube's dimensions should have allowed. Crichton sat down when
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Rubra showed him the temperature plot. Not because he was tired. He sat down because the implication of what he was looking at took a moment to reach him fully. The Germans had not stumbled onto this frequency.
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They had engineered it. The 47-cycle operating speed was not a byproduct of the design.
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It was the target. And the reason it was the target was noise. At 47 cycles per second, the Argus pulse jet produced a sound at approximately 47 hertz.
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A tone in the low bass range, a frequency that propagated further through structures and through the human body than higher frequencies did.
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It was harder to locate directionally. It was harder to filter out. It induced a specific kind of physiological stress, the same low frequency resonance that makes people feel unease in certain large buildings, in the presence of certain industrial
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machinery. The V1 was not just a delivery system. The acoustic signature had been refined.
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The population of London was not just being bombed. They were being subjected to an engineered sonic environment at a frequency calculated to be maximally disturbing. Roxbee Cox pulled the exhaust section apart last. The tailpipe of the pulse jet was a simple conical
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exhaust cone with no internal structure. But on the inner surface of the cone, beginning approximately 8 inches from the exit plane, Rubbra found something that had not been mentioned in any intelligence report about the V1 engine.
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A series of shallow circumferential grooves, rings machined into the inner surface of the steel. Each groove approximately 2 mm deep and 4 mm wide, spaced at irregular intervals that varied between 11 and 17 mm. Rubbra ran his finger along them and initially
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assumed they were manufacturing artifacts, tool marks from the forming process. Then he measured the spacing.
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It was not random. The groove spacing decreased in a specific sequence from the throat of the cone toward the exit.
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A geometric progression. Crichton recognized it immediately. It was an acoustic liner, a passive device for managing the internal acoustic field in the exhaust section, absorbing energy at specific frequencies, reducing standing waves that would otherwise interfere destructively with the combustion cycle. The Germans had
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lined the inside of the exhaust cone with a tuned acoustic absorber to stabilize the combustion at 47 hertz.
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This was not production engineering. This was research-grade acoustics applied to a mass-produced weapon. The V-1 was being built at a rate of 3,000 per month in the Mittelwerk underground factory, and every single one had those grooves.
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The discovery, the finding that made Crichton put his pen down a second time and not pick it up for several minutes, was the combination of what the temperature plot and the exhaust grooves taken together proved about the German development
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program. The combustion centroid positioning and the acoustic liner were not independent solutions to separate problems.
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They were paired. They were designed together, tuned against each other to achieve a specific operating point. The combustion geometry pushed the frequency up.
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The acoustic liner held it stable at that elevated frequency by suppressing the harmonic interference that would otherwise cause the cycle to break down.
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This required someone to have modeled the acoustic behavior of a resonant combustion system in enough detail to design a tuned absorber for it.
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In 1944, without computers, the mathematics alone. The calculation of groove spacing to achieve a targeted absorption profile in a conical duct carrying hot combustion gas represented months of theoretical work.
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Someone at Argus Motoren or at the Luftfahrtforschungsanstalt in Braunschweig where the engine had been developed had done fluid acoustic modeling at a level that British and American engineers were not doing yet.
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Not for jet propulsion. Not for anything. The engine on the table was not a crude weapon.
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It was the output of a sophisticated acoustic engineering program that the allies had not known existed. Britain changed two things because of what Crichton's team found at Farnborough.
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The first was immediate. The acoustic signature data from the examination was passed to the teams developing early warning detection systems for V1s. The characteristic 47 hertz tone propagating ahead of the weapon at low altitude became one of the signatures used to
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queue anti-aircraft gun laying radar in the gun belt along the North Downs. The improved detection contributed to a dramatic rise in the interception rate.
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In the first week of July 1944 before the gun belt reorganization informed by the Farnborough examination anti-aircraft command was destroying approximately 24% of V1s. By the end of August the rate was 74%.
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The second change was longer in coming. The acoustic line of findings from the Farnborough report was circulated within the British Aeronautical Research Establishment and eventually fed into post-war research on combustion instability in jet engines.
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A problem that would plague early turbojet development through the late 1940s and that the grooves in that exhaust cone pointed toward solving. Arthur Rubbra's written description of the liner geometry was cited in a 1951 paper by the National Gas Turbine
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Establishment on resonant combustion damping. The footnote acknowledged that the initial observation had been made during a wartime examination of a captured German weapon.
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It did not name the weapon. By then, almost no one would have remembered to ask. The Argus AS 109-014, examined at Farnborough in July 1944, or more precisely, the one most likely to be the same engine, or one from the
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same batch, is held in storage at the RAF Museum Cosford in Shropshire. It is not currently on public display in the main galleries. It sits in a climate-controlled storage area accessible to researchers by appointment.
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The casing is still the same thin-gauge spot-welded steel. The valve grid is intact. If you were to look into the exhaust cone with a torch and run your thumb along the inner surface near the exit, you would feel those grooves, the
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spacing shifting under your fingertip in a sequence that is not quite regular, not quite random, but tuned. Most people who have handled it over the decades have assumed the grooves are manufacturing marks. The label on the storage tag describes the object as a
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German pulse jet motor, V1 flying bomb, recovered 1944. It does not mention the acoustic program.
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It does not mention Crichton or Rubbra, or Roxbee Cox. It does not mention that the men who examined it sat in a workshop in Hampshire and discovered that the weapon killing London had been designed in part as an instrument of sound.
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That someone in Germany had done the mathematics to make a steel pipe resonate at exactly the frequency most difficult for a human being to endure.
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And had then machined the interior of the exhaust to make sure it held that note. The V1 was not a blunt instrument.
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And the 353 pounds of steel on that Farnborough table proved it.
Topics:V-1 pulsejetWW2 technologyBritish engineeringpulsejet combustionArgus AS 109-014RAE FarnboroughGerman flying bombacoustic resonancewartime propulsiontechnical history
