Nuclear Friday: Thrust Issues

I’ve always been a fan of clumsy yet effective workarounds to address technological limitations. One of the clumsiest was JATO, Jet-Assisted Take-Off. That name confused me when I first learned it. It’s not really a jet at all, it’s just little rocket motors stuck onto an aircraft to help it take off. But there wasn’t really much of a distinction made between air-breathing jet engines and rockets in the US in the very early forties. In Britain, where jet engine research was much more advanced, they settled on the much less confusing RATOG, Rocket Assisted Take-Off Gear for similar systems. JATO, RATOG, and the German Starthilfe (literally “start-help”) systems would see only limited use in WWII. This would change in early Cold War aviation, when one US bomber would be both the most advanced aircraft ever built, and at the same time almost non-functional. The B-47 Stratojet had serious thrust issues, and JATO would solve them.

But before we get to the B-47 here is a picture gallery with captions showing WWII systems:

Both the UK and Germany fielded combat capable jet aircraft during World War II, and the UK and US both commissioned several design programs for jet bombers during the war. A few of these craft would go into production after the war, and in general their performance and capabilities were unimpressive, only slightly better than their piston driven predecessors. The one exception was the English Electric Canberra, which would set altitude and Atlantic crossing records in the early fifties. But planners in the US wanted something truly futuristic, and with the help of German wind tunnel data and engineers captured during the war, Boeing would attempt to leap into the future.

They would almost get there. Boeing’s design team had been working on a jet bomber during the war. In 1945 they started applying their newly-acquired German know-how to their project and would deliver their B-47 prototype in 1947, almost two years before the Canberra entered service. And look at it:

jatob47

That’s a completely modern jet aircraft. Swept wings, engines in pylons rather than wing roots, and it really is the progenitor of all modern jet airliners.

There was just one problem. It could just barely get off the ground. Test pilots loved it once it was flying, but there was no way this beautiful aircraft was going to take off with a full load of fuel and a nuclear bomb payload.

The problem was that jet engines were a very new technology, and the swept wings and high wing-loading (wing area divided by total weight) that made it a joy to fly at high speeds and altitudes gave it poor efficiency at low altitude. In general, jet engines perform better at high altitude, and since the sixties, we have designed them to be able to perform well at low altitude as well, but in the late 40’s to early 50’s these problems had not been solved. High wing loading and the innate problems of early jets were why the Arado bomber pictured above needed Starthilfe. Other postwar jet bombers had broad wings to lower loading. This curtailed performance at high altitude and only the Canberra performed adequately. The B-47 would be better, if it could take off with a full load.

In spite of these problems, SAC would get its medium range bomber. It took until 1953 and many solutions were attempted, but eventually the B-47 entered operational service with JATO. At first the B-47 carried JATO rockets inside the rear of the fuselage. The nozzles stuck out of little holes. The rockets burned for less than seven seconds and accelerated the Stratojet to takeoff speed without need for a double length runway nor facing the dangers of a long run on the ground at high speed in ground turbulence.

Obviously, carrying JATO rockets internally is a bad idea. They are explosive. They are also heavy, and the little JATO ports caused aerodynamic drag. Later B-47’s carried their JATO on an external mount called a “Horseshoe”. The horseshoe would be dropped shortly after takeoff. This reduces danger, and saves weight.

Here are what the two systems looked like:

So there it is; A clumsy low-tech solution to technological overreach. The most advanced airplane in the world can’t get off the ground without help from solid fuel rockets that William Congreve would have recognized 150 years in the past.

The B-47 entered service in 1953, and began withdrawal in 1963. All were taken out of combat roles in 1966. The B-52 had replaced the deep attack role. ICBM’s had replaced the quick response role, and new compact designs of nuclear weapons meant that light tactical aircraft could take over the short range role.

I’m surprised the B-47 lasted as long as it did.

Video research link:

So, you wanna see this thing take off, here you go. Video shows two takeoffs with internal JATO, but is mostly about external JATO. Actor portraying a psychologist talks about guilt and death a lot. Very weird.

Second Video Link:

Is JATO used today? Rarely, and only when a heavily loaded cargo plane has to take off on an inadequate runway. But sometimes folks use JATO just for fun. The Blue Angels flight team used to travel with a cargo plane called Fat Albert that carried all their gear. At some of the Angels airshows Fat Albert would close the show by performing a very short JATO takeoff. It is really cool.

 

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