Part 35: Interlude: De Havilland Comet (Spaced God)

Instead of doing classwork, I’m writing dumb stuff for a thread on a forum. Let’s chat about an aircraft that both revolutionized civil aviation, and is a great engineering case study.

Squares need not apply: The De Havilland Comet

Back in 1943, the Brits were optimistically already looking forward towards the post-war aerospace industry. They established the Brabazon Committee to figure and flesh out the five type of aircraft England needed postwar. This Committee is super fascinating, and a lot of really cool planes (and monstrosities) came from this committee and I could probably write a post just on this committee alone but you guys have lives. Also think the Comet is the only one in the game anyways.

Anyways, the fourth type outlined in the report was one that brought skepticism: a jet powered aircraft capable of carrying 100 people. Moreover, it was suggested it could carry an imperial ton of payload at 350 knots (~640 kph) and have a pressurized cabin. Jets at the time were fairly new, and were known to guzzle fuel, waste money, and catch explode if you looked at it wrong -- AKA great features for an aircraft designed to fly transatlantic carrying very rich people.Sir Geoffrey de Havilland liked the idea (in fact he used his spot on the committee to ensure a jet aircraft was included on the list), and his company was granted a contract to develop a Type IV aircraft in February 1945, except carrying only 24 passengers instead of 100. Appointed to lead the project was chief designer Ronald Bishop of Mosquito fame, who was known for his creative designs. Moreover, despite the untested technology and high cost with little proof of a return on investment (and despite the unusual designs Bishop had), the state-owned airline BOAC ordered 10 Type IV aircraft from de Havilland in December of the same year.


Designs for the Comet. The first two were what made BOAC say “Yes, I want this.”:

Over the next year, designs became more and more like the “standard jet aircraft” we know today. 20 degree wingsweep for high speed flights, extra seats added to bring total count to 36, and powered flight controls all make their appearance in their final design. If you look at the above image, though, you’ll notice there are no engine nacelles similar to normal aircraft. The jet engines instead were built into the wing roots, with large intakes at the leading edge. For some reason, British aircraft do this for quite some time and though it looks cool as hell it leads to some strange consequences. Had an engine had one of those explodey moments, you’d lose the wing rather than just the nacelle. Moreover, it required anyone maintenance crews to dig around in the wing rather than simply detach the engine from the nacelle. Anyways, things were tested, pieces were built, and the first Comet prototype made its public appearance at the 1949 Farnborough Airshow (one of the largest airshows in England). The first flight was made in 1950, and a second prototype was built and put through over 500 flight hours for training. More tests, more construction, and finally the first fare flight of a jetliner occurred on from London to Johannesburg on 2 May 1952, carrying three dozen passengers and equal bags of mail. The 5-stop trip took only 21-and-a-half hours.

From the start, the aircraft was beloved. Comet flights cut travel time in half compared to the fastest competitors. The jets were quieter than pistons and produced less vibrations, which increased the passenger experience by orders of magnitude. In the first year, 30,000 passengers were carried to various destinations in style, including the first British Royalty to travel by jet when the Queen Mother, Queen Elizabeth, and Princess Margaret traveled via chartered Comet. By the end of 1953, 8 airliners had put orders in for Comets, including three American airlines (Pan-Am, National, and Capital).

… And theeeen shit went wrong.

In January 1954 a BOAC Comet crashed shortly after takeoff from the Italian airport of Ciampino. It had broken up in-flight over the Met with a loss of all souls aboard. There had been some hull losses in the past year, sure, but most were pilot error or easily explainable; every time the aircraft was not at fault. This was different though: there was nothing to indicate a fault. Quoth a de Havilland test pilot: “It was a perfect airplane as far as we were concerned. We were absolutely puzzled by the problems.” Some assumed sabotage, others assumed an engine had failed, taking the rest of the wing with it. All were hoping a cause would be found, and that it was a one-off tragedy.

And then it happened again.

The scenario was very similar. The aircraft, operating as a South African Airways flight, was examined and preflighted by the same team as the previous mysterious BOAC, and took off on 7 April from London bound for Johannesburg. The stop in Rome was nominal, barring some minor faults. But, after fixing them, the aircraft left en route to Cairo for another refueling stop. The aircraft broke up at 35,000 feet over the Met, and all aboard were killed. Like the BOAC flight, no immediate cause was found. The entire fleet was grounded, and engineers from across the globe threw themselves at the problem to find the root cause of these tragedies. Churchill even famously said “The cost of solving the Comet mystery must be reckoned in neither money nor manpower.” The aerospace industry of England was at stake, and a solution needed to be found fast.

HOVERCRAFT INTERLUDE

Hovercraft are vehicles that can travel over most any terrain using fans or turbines to create a cushion of air upon which the vehicle rides. It’s hard to distinguish when hovercraft first were thought of, because a lot of early thinkers thought of craft more as hydrofoils than hovercraft proper. The difference is that a hovercraft can, as the name implies, hover, whereas a surface effect vehicle requires motion to get ground effect. But, with that being said, in 1929 a Ford engineer named Andrew Kucher started blowing air through a hole in a metal disc. This “Levapad” is considered an ancestor to the modern hovercraft, but it never reached any major use other than a cool demonstration piece.

During World War Two Charles Fletcher, an engineer for Bell Laboratories in New Jersey, built a hovercraft the size of a small car named the Glidemobile. Many (including myself) consider it the first hovercraft. It wasn’t patented, however, because the US Government was working on a similar design through a classified project. A man in black’s visit to Fletcher’s door later, and the Glidemobile was locked in a shed for a while. Years later, when a Brit by the name of Cockerell was trying to claim he invented the first hovercraft, Fletcher dusted off his craft to strike down the Brit’s patent.

How do I know all about this? Well what I just wrote was more-or-less what I told people as I did tours at the museum I worked at, where Fletcher’s Glidemobile currently resides.


The Glidemobile at the Aviation Hall of Fame and Museum of New Jersey. Since this photo was taken, restoration improved the condition of the vehicle. Also, I worked here!

ANYWAYS, where was I?

After a few weeks, investigators on the Comet Mysteries discovered something about the metal in the fuselage: It showed signs of severe metal fatigue. That didn’t make sense, though. Pre-production testing had shown airframes could last tens of thousands of hours before fatigue would begin to show. The airframes involved only had a few thousand or so hours on the airframe. But then the investigators remembered something important about the jetliner, the fact that it was pressurized. In Italy, autopsies on the passengers had shown injuries consistent with that of an explosive decompression. The investigators began to piece the clues together. No other aircraft had gone through pressurizing cycles as fast and as often as the Comet. That cycle incurred stress on the airframe which weakened it over time. Testing the effects pressurizing cycles had on the airframe concluded that fatigue occurred forty times faster than expected in airframes. A prime suspect was discovered.

To double check their findings, investigators put a pressurized Comet fuselage into a tank full of water to simulate spending time at cruising altitude and pressure. 9,000 simulated hours in, a loud bang was heard and the pressures inside the fuselage fell; their theory was confirmed. The tank was drained, and what they saw horrified them. Starting from the window frame, a crack extended to the top of the radio antennas and back along the other side of the fuselage, nearly bisecting the craft with results looking more like a bomb than anything else. Upon further research and calculations, engineers discovered that the strain put against the square windows at the corners were astronomically high, leading to the formation of a crack. From that point on it was clear: Square windows on pressure vessels would not work.


Fuselage fragment from the South African incident. Note the square windows and point of fragmentation

The Comets were immediately rebuilt with rounded windows to fix the issue. The first variant of Comet never flew a passenger flight again. Meanwhile across the Pond, Boeing had completed development of its own jetliner, the 707 -- this time with rounded windows. De Havilland had started a jet race, and everyone was pumping out their own jetliners. They competed for a while, with the last Comet flight taking place in March of 1997. They continued to make small jets like the Trident and the Heron, but nothing as innovative as the Comet. For de Havilland, they were gobbled up by Hawker Siddeley in 1960 as a subsidiary until HS went under in 1992.

So that’s the story of the de Havilland Comet. It changed the entire landscape of commercial aviation, and brought the world into the jet age. We’re getting into the modern era in game now, so I’m not sure what other aircraft are left to unlock, if any. If y’all have any questions or requests for things I should cover in the remaining time of the LP, just give me a PM or post in the thread.