P-61 Gun-Laying Radar

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Decided not to edit, probably because he forgot what he was editing
 
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Come on Sr6, think like a pilot. If you're focused on getting a firing solution on a target in front of you, a radar that is also pointing out other targets is nothing more than a distraction and an information overload. That was beyond the technology of the time. Only recently (in the long view) have AI radars been able to lock and track while still keeping a scan going. This is a capability that tends to lean toward a two-crew interceptor, as it can lead to task-saturation of a single pilot. (Biff, you up?) He's the pro in this game.
Cheers,
Wes
 
wuzak said:
1. Ever heard of the cavity magnetron?
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Yes
Without it the SCR 720 would not have been possible. And the British gave the US one in 1940 in exchange for extra access to US industry.
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I didn't know they were 1000 times more powerful than anything we had in the US

Looking at the links, the radar covers the whole forward hemisphere ranging from 5-degrees down, 50-degrees up, and spins at 360 degrees per-second, has a maximum search-range of 100 miles, a maximum effective range of between 9-10 miles for bombers, 4-5 miles on fighters

In the article, I noticed that in addition to the details on the various radar systems developed by the USAAF, there was a brief mention for both a 10cm airborne-interception radar, and a 10cm radar-directed gun-layer. The latter seems an accurate description of what the P-61 was built for if Dana Bell is correct.

Looking at the radar range of travel: While the azimuth isn't 360-degrees, 180-degrees permits forward attacks from the 9-o'clock to the 3-o'clock position instead of the normal 12-o'clock position only; the elevation limits of -5 to +50 degrees seem to cover the turret elevation of up to 50-degrees up.

For the purpose of the turret-fighter concept the British were fond-of, I guess it made it easier to put guns on target without having to place the nose on the target.

If the target was dead ahead then why wouldn't you use the 4 x 20mm cannon?
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Personally, that makes enough sense to me. Truthfully, I don't see the turret as being needed if the plane is agile enough to get on the bomber's tail.

The maximum g-load was 7.3g x 1.5 which is actually quite good
 
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If I am reading it correctly, the radar can sweep in one of 4 selected bands.

Zero elevation - sweeps horizontally only
-5° to +5° - sweeps from just below horizontal to just above - total of 10°
+5° to +20° - sweeps a 15° band above the horizon
+20° to +50° - sweeps a 30° band.

So the maximum area the radar can cover is 180° x 30°. Not very useful as a gun laying radar for a turret that has 160° x 90° (0° to +90°) coverage.



The 10cm airborne intercept radar was the SCR-520 and SCR-720 radar sets.

The article notes that the laboratory was directed to develop a 10cm gun laying radar, not that they did develop such a device, or if they did if it was successful.


Zipper730 said:
For the purpose of the turret-fighter concept the British were fond-of, I guess it made it easier to put guns on target without having to place the nose on the target.
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The British didn't develop a turret armed night fighter, although they used Boulton Paul Defiants in the night fighter role (without radar IIRC).

The enthusiasm for turret fighters had largely dissipated after the BoB. Though there was a turret-fighter (day) prototype of the Mosquito, which went nowhere.


Zipper730 said:
Personally, that makes enough sense to me. Truthfully, I don't see the turret as being needed if the plane is agile enough to get on the bomber's tail.
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Manoeuvrability should not be an issue for a night fighter going up against an average bomber in WW2.

The Mosquito had lower limits, but it was more than up to the task of fighting German night-fighters.
 
Correct, I figured one could train in from a distance using less elevation, then get closer and adjust the elevation to keep the target in the search area. It was kind of a guess
The article notes that the laboratory was directed to develop a 10cm gun laying radar, not that they did develop such a device, or if they did if it was successful.
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I was just relying on what Dana Bell had said earlier
The British didn't develop a turret armed night fighter, although they used Boulton Paul Defiants in the night fighter role (without radar IIRC).
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I knew the Defiants were used as night fighters, but I thought they kluged a radar into it.
The enthusiasm for turret fighters had largely dissipated after the BoB. Though there was a turret-fighter (day) prototype of the Mosquito, which went nowhere.
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Frankly, I'm surprised they didn't just develop the P-61 prototypes with two different gondolas: It would have made a good hedge if they were working on a gun-laying radar.
  • Most WWII radars didn't involve a lock-on capability, so one would have to use the radar and maneuver to visual range to carry out the kill.
  • A gun-laying radar would include a wide range of elevation and azimuth, a lock-on capability, and a ballistic computation and ranging ability based not just on your speed, g-load, as well as turret position.
  • A hedge would include normal elevation and azimuth, ballistic computing and ranging, and lock-on, which would be fine if you were just using the 20mm's, which are more destructive anyway per hit. The ballistic computing and ranging would also be easier to work out if you didn't have to factor the guns being re-positioned in flight.
The hedge would have also allowed the gondola to be the same length in theory, a reduction in crew from three to two, with the associated removal of 200 pounds about, plus the weight of the turret, the ammo for the 0.50's, and each gun-barrel. This combination would provide longer range, better acceleration, possibly better climb as well. Even if the blind-shooting system didn't work at all, you'd have the normal features of any other night-fighter, and less weight and drag.
Manoeuvrability should not be an issue for a night fighter going up against an average bomber in WW2.
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It's still useful, plus if it was agile enough, it could be used in day-time as well.
The Mosquito had lower limits, but it was more than up to the task of fighting German night-fighters.
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The Mosquito could pull around 8g right?
 

Lock on capability?

What WW2 are you using that could take advantage of that? Remembering that guns were more effective at closer ranges too.

The Automatic Gun-Laying Turret did not have a lock on capability. Initially the radar operator shouted instructions to the gunner, but the later versions placed and indicator blip on the gun sight, the gunner manoeuvring the turret to get the blip in the reticle.

Making two lots of gondolas probably would have delayed the whole program. In the end a P-61B was converted to a 2 seater XP-61E (radar replaced with 4 x 0.50" hmgs in the nose), but this had little or no performance improvement over the P-61B despit the weight saving.
 
wuzak said:
Defiant Mk Is did not have radar.

Defiant Mk IIs had the AI Mk IV radar installed.
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Okay, where was the radar mounted, and did the turret guy operate the radar or the pilot?
Making two lots of gondolas probably would have delayed the whole program.
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I wouldn't have thought so for the prototype stage: Most prototypes in those days were hand built, there were two XP-61's built, and the XA-26 were built in three forms (level-bomber, night-fighter, strafer).
 
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Zipper730 said:
Okay, where was the radar mounted, and did the turret guy operate the radar or the pilot?
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The radar was AI.MkIV, which used antennae fitted to the wings.

You can just make out some of the radar installation in the port wing of the Defiant



It was copied in the US as the SCR-540 radar.

US Night Fighter Radars of WWII

I believe the pilot operated the radar - doubt there was enough room in the turret for the equipment.


Zipper730 said:
I wouldn't have thought so for the prototype stage: Most prototypes in those days were hand built, there were two XP-61's built, and the XA-26 were built in three forms (level-bomber, night-fighter, strafer).
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The A-26 just changed the nose, not the whole fuselage.
 
wuzak said:
The radar was AI.MkIV, which used antennae fitted to the wings.
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The arrowhead...
I believe the pilot operated the radar - doubt there was enough room in the turret for the equipment.
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So it would have a bit of a high workload?
The A-26 just changed the nose, not the whole fuselage.
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True, but the gondola's nose, underside, wing-body junction, and after-body would have the same shape.
 
I'm confused about something: With later versions displaying the blip on the gun-sight, I assume the position of the blip would depend on the radar operator either putting the antenna on the spot, and the turret operator adjusting with the radar operator so the blip is right in the middle of the turret?

My explanation might not be articulate...
Making two lots of gondolas probably would have delayed the whole program.
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You might be right: I was just figuring with prototypes everything is hand-built and the two could be designed in a way that's modular. The nose would be the same, the lower fuselage section would be the same with the upper section altered. The XP-61E had more fuel capacity which seems useful.
XP-61E (radar replaced with 4 x 0.50" hmgs in the nose), but this had little or no performance improvement over the P-61B despit the weight saving.
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That's quite a surprise, though the guns ports in the nose might have added some drag...
 

The radar scanned.

In the original versions the radar operator would direct the gunner as to where to aim. Later versions, when the blip was on the gunner's sight, the radar operator would still call out.

I don't believe that the operator directed the radar as such, but monitored it and adjusted the operating parameters.
 
wuzak said:
1. Ever heard of the cavity magnetron? Without it the SCR 720 would not have been possible. And the British gave the US one in 1940 in exchange for extra access to US industry.
Click to expand...



This bit about the Brits, the invention of the cavity magnetron, and radar is a bit overstated by many. This is actually a very complex issue. To be sure, the Brits got to an applicable cavity magnetron first, no argument, I don't want to down play that. And them getting there when they did may have shortened the war, that is certainly a possibility. However the time was right and the ground work had been laid, much like for radar in general. It was a technology whose time had come, and the drive of the war spurred the research. If the Brits had not shared the cavity magnetron the US were probably less than 2 years behind on their own high power microwave transmitter source, although it is not sure the US would have gone that route instead of the cavity klystron. The Brits were in the war already, and the US was not, so there was less pressure in the US.


Why do I say the time was right and the ground work had been laid?


Without A. W. Hull (an American) developing the Magnetron, if other researchers had not found that a Magnetron, when subjected to certain types of intense magnetic fields, produced frequencies at very short wavelengths, without the work of other researchers in France, Netherlands, and the UK, the groundwork of the split anode magnetron would not have been in place, and Boot and Randall would not have been able to develop the concept of the Cavity Magnetron. If the Cavity Klystron had not been developed by Hansen in 1936, with Russ and Sig Varian further advancing the tech in 1937, Boot and Randall probably may not have applied the principle to the Magnetron, they had to have read the papers, and also the German efforts in cavity magnetrons had been published about the same time (1935 - 36).


In 1939 the Japanese, Germans, and Russians were ahead of the British in the development of the cavity magnetron specifically. Wilkins biography of Randall is pretty clear on that, and the timing of when the concept came to the team. But Boot and Randall, once they decided to go that route, set out with a specific radar oriented goal, to produce 1 kW of peak power at 10 cm wavelength. And they got there first. Further, with the team of the US and the UK, the Allies accelerated much faster than the Axis powers in this technology.


By the end of the war the Japanese, completely independent of any knowledge of British or US efforts, had a working short wavelength cavity magnetron. The Russians also had an indigenously developed version. The Germans, actually ahead of everyone in cavity magnetron development in 1935 or 1936, decided it was too unstable for their applications (and it really was, that was a separate issue to overcome on the receiver side of the radar), so they basically shelved it. And as I said before, the US possibly would have developed it on our own (there were already experiments leaning in that direction underway, but not as mature as the other nations mentioned, the US was maybe 5th in line with the technology), but the Brits gave us a leg up, so there was no need to. All we had to do was turn this working concept the Brits had into a more refined technology. The 15 kW peak power of the E-1189-b the British brought to the US in the Tizard Mission was very good, but could be much better.


The war actually slowed the development of the high power cavity klystron in the US, as the magnetron was easier to improve (no complex optics) and Allied klystron efforts concentrated more on reflex klystrons (used at lower power levels, such as local oscillators for the receivers of magnetron based radars). What the magnetron did for the transmit side of microwave radars the reflex klystron did for the receive side. But shortly after the war, by 1948, the cavity klystron far exceeded cavity magnetron capability in power, to the tune of 30 MW at S band. Today there is no question, for radar systems klystrons are better transmitter devices. They develop average power levels the magnetron cannot come close to, and they are much easier to design a coherent system around. For example very few designs have used a magnetron in an aircraft radar for 40 years or more, except when cost, and not performance, is the driving factor.


But to the SCR-720. Without the Cavity Magnetron the SCR-720, or something doing the same tasking, would still have been possible, it just would have been different. Not what we know it as today, but still possible. It may have operated at a lower frequency, with all that that entails (larger antenna for equivalent beamwidths, heavier equipment for the same power level, etc), but the basic ability of the -720 could have been accomplished without the cavity magnetron. If it operated in the same frequency range the transmitter would probably have been cavity klystron based and until quite a while later that meant bigger and heavier.


And without the klystron work of people like Sperry and the Varians (and in British labs also), the SCR-720 as we know it in history would also not have been possible, despite the existence of the cavity magnetron. Despite the existence of the transmitter source you still needed a receiver. And all that back trail I laid for the cavity magnetron above? Similar back trails exist for klystrons, Sperry, Raytheon, and the Varians could not have gotten where they got without all that proceeding work by others.


As I am prone to say, no one person, one technology, or one invention, made such radars possible. Several elements are key, the cavity magnetron is only one of them, probably the biggest factor, but not standing alone. Had the magnetron come along earlier the rest of the tech would not have been available, had it come along later some other tech would have likely taken its place in the radar transmitter chain. The right tech, at the right time, with the right reason behind it.


T!
 
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