Saturday, September 15, 2018

A Technical Appendix to June/July 1948: Berlin By Night

So here's a technical appendix that looks at radar developments, and a particularly significant aircraft development delay due to problems with the "avionics," as we say now.

It will also not escape attention that it is a "Zombie Day" post, as I've already mentioned. It's because of overtime. On the bright side, my boss has decided to burn three of my accumulated paid days off leading into my holiday at the end of next week, so expect a bit more activity around here in the near future. 

Three years ago, the RAF spent its nights over Berlin in a perverse attempt to hasten an age of peace and love with a wild spasm of violence. This year, seventy years past, they are bombing it with candy. Okay, their ally is: I wanted to work the Candy Bomber and the night offensive into the same paragraph, and it was hard. Also, radar.

In our long, rear-view mirror interpretation of the beginning of the Cold War, stories tend to be shaped to fit. In the final version of the story of the English Electric Canberra

the main stories are Britain's "exhaustion" after WWII, and the Cold War alliance of the NATO powers, which obviously started with the Cold War, which began . . . when it did. Iron Curtain Speech? Berlin? Korea? Let's just say, given that the Cold War is a metaphor, that it can begin when I say it does, and this month, I say it starts with the Berlin Airlift.

Neatly folded into the story is the production triumph of the Canberra. Nine hundred were built in the United Kingdom for the RAF and the RAAF, and an additional 400 at the Martin works in Baltimore, Maryland as the B-57, which is quite an extraordinary thing to happen. The military-industrial complex frowns on producing foreign weapon systems, and for fairly good reasons. 

Since the Canberra had a somewhat late and troubled service entry, a model narrative has Britain "broke," and the United States paying for it, because the Cold War might get hot, in which case the RAF would have to do its part in stopping the Soviet steamroller from reaching the Channel in 48 hours, or whatever. 
Given that Britain did receive Marshall Plan aid, it's hard to argue this to the last ditch. Even given that this is a massive misunderstanding of the nature of Marshall Plan aid for Britain, the license payments for the production of 400 American Canberras really were a solid stream of dollar earnings that helped offset the "Boeing Washington" purchase. That all of this has to do with the Korean War, which hasn't happened yet, can be swept under the rug in the interest of proving the truth of the main story, which is that a massive war effort funded by borrowing must bring nemesis in the form of austerity and cancelled programmes. There is no room in this story for a budget surplus,
. . and the idea that the national debt is a smaller charge on the budget in 1948 than in 1938 is not to be born! 
The reality, of course, is different, but the Canberra was delayed, and we might, in the end, get to something like the "austerity" story, if you'll bear with me. That plexiglas bubble at the front was not in the original design, which had a shorter, solid nose. This was because the Canberra was conceived around an automatic radar bombsight. The window had to be added at a late date, along with a bombardier/navigator, and an  ejection seat to accommodate. The ejection seat, a first for service jets, is an interesting aspect of the story in its own right, but the key point is the visual bombing, accomplished by the bombardier laying prone in the window and aiming a good old T.14 sight. Thae practice was still far better than WWII operations, because the Canberra was equipped for Gee-H control. Unfortunately, the range of Gee-H is quite limited, leaving the Canberra somewhat ineffective for the "main event" of a strategic air offensive against Russia.

That being said, no-one cared very much, as it was a very nice plane, aerodynamically speaking, and was a perfectly fine replacement for the WWII-vintage medium bombers in use in Korea.

Also, the Martin plant was just down the road in Baltimore, the Space Race hadn't even started yet, and the Flight 421 accident might have been the most egregious of all the postwar civil aviation accidents, which is a pretty high bar when you're competing with British South American Airways. 

All I'm saying is that Martin needed the work. 

So the issue with the nose and the delay is that original, as Wkipedia calls it, "H2S automatic radar bombsight." It did show up eventually, in the V-bombers, while late-model Canberras did get a simplified Green Satin, the Blue Silk side-looking radar, good enough for navigation, if not bombing. This augmented the Ground Position Indicator. (Old Canberra drivers reminisce over at the Prune Forums.)

The talk here is all about radar, not the "computer" in the Navigation and Bombing System 

From Ridenour, Radar System Engineering. [pdf]
Not a lot has been written about the NBS, not because it was Top Secret, but because it was a boringly efficient mechanical-analogue-kludge. Out of the mainstream, it was really just an extension of the original  Air Position Indicator 
inputting data to the computing bombsight, and receiving it from a 1.5cm ( magnetron centimetric radar, hence a continuation of the "H2S" nomenclature), via a magnetically-scanned cathode ray tube, an improvement over the original, electrostatic deflection-controlled CRTs. 

Not to get all tied up in the user interface, the issue here is the radar data. This new H2S radar is to use pulse changes to measure the Doppler shift due to self-movement over ground. This is where we go from an Air Position Indicator to a Ground Position Indicator. The radar gives movement over ground, making bombing far more accurate.

This sounds like a pulse doppler radar, but isn't quite, yet. The idea is there, although not the operational requirement (look down/shoot down), and neither, more importantly, is the technology. At this early date, and we are talking about the late-war period, doppler effects are more curse than blessing. The prototypical early air-detection radar, Chain Home, used doppler effects to detect moving objects, but this was only possible with continuous wave propagation. The more common kind of early radars used pulsed emissions, comparing emitted to received, and the phase shift due to doppler effects makes this comparison very difficult. Pulse doppler turns this around by filtering returns for doppler-induced phase shifts. (It also uses antenna arrays that are not available in 1948, never mind 1944.) This is an amazing achievement in electronics signal processing for the 1950s. 

Siegbert and Purcell, "Complicated Targets," in Ridenour, ed., 90.
This is what 3cm mapping radar shows in this period. 

1.5cm radar looks pretty promising in comparison, 

Ibid, 111. 
but you will notice that the picture is taken at less than half the height, and the hatching in the centre, probably from building return at varying slant angles, is going to be a problem. The 1.5cm radar first proposed for the Canberra is just too discriminating (and also short ranged). The final system chosen is the more practical 3cm radar, codenamed Green Satin, and it is not ready in 1948, either. The team working on the mapping+bomb aiming system, probably at Ferranti, will have it ready for 1955, instead.

From the perspective of the late Computing Age, Ferranti's nonchalance over the computing component of the Navigation and Bombing System is amazing. As far as I can tell, the main technical issue is with the data feed, and, even there, more from the gyrocompass than the radar. Air Commodore Norman Bonnor wrote a very brief piece about it in the late 90s, currently hosted by Tony Blackman (pdf), and there makes it as clear as anyone who talks to engineering physicist can make things clear, that the issue was the "chopper" amplifiers. "Choppers," it turns out, are used to cut up DC signals representing various parameters into 'pseudo AC' for amplification." I honestly don't know what to say about this little technology, about which I knew nothing about a moment ago

Speaking of, Ferranti was also having enormous, and enormously quotidian continuing difficulties with the optical map projection system. The problem here is that the screen illumination was hard to balance, so you either couldn't read it at night, or on sunny days. 

Bowden, et al., "Display of Radar Information," in Ridenour, ed., 215.

I'm putting this in because I am amazed that the relationship between the movie industry and the high electronics/avionics complex, which just last month I was tentatively relating to Elliot Brothers move into the studio ghetto in North London, actually started in the war years. Also, I owe an apology to G. H. Parkes for making fun of his projected-video-stereoscope-image method of blind landing. I hadn't the foggiest idea what he was on about, but it turns out that he's describing the map-projection system of the Ground Position Indicator. 

It's neat that this relationship is so old, and fun to discover that protecting your cultural industries can have spin-offs that go beyond cheap horror films and the Carry On Gang. Finally, recognition of the need for pulse coherence led to the magnetron being replaced with a klystron.

I also want to re-emphasise 

I can draw a diagram in my head! Why is this so hard? Ibid, 217.
that the Ground Position Indicator seems to have been invented before the end of the war. All that was needed to make it a reality was the actual technology! So is this an "austerity" story? Was the GPI actually a short-term possibility in 1944,with implementation delayed until 1955 by postwar cuts in the gigantic late war research and development apparatus? Or were they being over-ambitious? Damned if I know. 

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