P-61 alternatives

The RO in the F-82 did kind of have his hands full with that radar. But he did have a few basic flight instruments. Also attached is one of my favorite Scale Modeler covers. Exactly what the F-82's shot down that day seems a bit uncertain. They reported the enemy aircraft had an observer to some have asserted it was a Yak-11, as shown, But the Yak-7V fighter also was a two-seater and it probably is more likely it was one of those.

I don't think it would have been possible to move the P-82 up significantly in time. The fuselage of the production P-82 was "based" (and I use that word loosely because I've heard different things about how much actual commonality there was) on that of the P-51H that itself didn't make it into combat in WWII. The best alternative for the USAAF to the P-61 would have been either the F6F-5N or the F7F-2N. In my mind, if it was given enough priority, the F7F could have been moved to another facility to give it more attention. The Navy put the F7F on the slow track to avoid delaying F6F production.

Here's the special test P-38 with the extended center nacelle. It still would have not been able to mount the SCR-720.

As for moving up the P-82, I think that if they job was given to Fisher instead of that XP-75 monstrosity it could have been moved up. Basing it on the P-51D instead of the P-51H should have been entirely feasible.

MIflyer said:

As for moving up the P-82, I think that if they job was given to Fisher instead of that XP-75 monstrosity it could have been moved up. Basing it on the P-51D instead of the P-51H should have been entirely feasible.

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Preach on, brother

MIflyer said:

. . . that XP-75 monstrosity . . .

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I will say ten .50 cal machine guns as standard armament was an impressive number. Though I wonder what the actual firing rate would have been for the four nose-mounted guns which had to be synchronized to fire between two (counter-rotating) propellers.

33k in the air said:

I will say ten .50 cal machine guns as standard armament was an impressive number. Though I wonder what the actual firing rate would have been for the four nose-mounted guns which had to be synchronized to fire between two (counter-rotating) propellers.

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Better install an ejection seat.

Shortround6 said:

Please note that the dish could and did turn 90 degrees from the fuselage axis and is nearly there in the photo.

The radar used on the F4U, F6F and P-38M used a 19in parabolic reflector.

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When discussing the 520 and 720 it should be kept in mind that they were basically the same radar, with the same basic modes and performance characteristics, with the 720 being a newer, more compact, and lighter version that addressed some shortcomings of the 520. It was improved, but the displays and basic capabilities were very similar with only minor differences.

The dish of the SCR-720 (and SCR-520 also) turned rather further than 90 degrees off axis. The only scan that the -720/520 had was a full 360 degrees in azimuth.

The antenna used what is called a helical scan or a unidirectional sector scan. It spun completely around 360 degrees, stepped up, spun 360 again, stepped up, etc. When it reached its upper scan limit it started back at the lowest currently set elevation. The radar only transmitted while the radar was pointed in the forward hemisphere, and was blanked when pointed aft. This made the mechanics of the radar more simple, and simple means light. Basically it consisted of an azimuth drive motor that spun the antenna around, the same direction all the time, and a more restricted elevation drive motor that controlled elevation. To construct an antenna, and more importantly drive system, that sweeps an antenna rapidly side to side is more complex. And for reference such a scan, back and forth in a sector, is called a bidirectional sector scan.

Working from memory the basic scan rate of the radar was about 5.7 scans per second, meaning the antenna spun on its azimuth axis about 350 times per minute. Certain models included other operator selectable scan rates. The SCR-720 actually ended up being used in many different applications, including as a short range, high angle, air search radar mounted on ships. The last one I saw, and used, was mounted in the back of a truck for instrumentation purposes on a test range.

It has been a long time, and I may have this part wrong, but I seem to remember that in the 100 mile mode of operation the scan rate was slower. The 100 mile mode was the beacon mode, and it makes sense that you might sweep a bit slower for that.

The APG-1 and 2, which were in development for the P-61, would have been a totally different animal, with multiple and very different modes of operation, including auto track. Unfortunately they never became operational on any platform, and were only tested briefly on the P-61.

The AIA and AN/APS-6 radars used in such things as the F6F, F4U, P-38M, etc, uses a totally different scan from the -720. It has a couple of scan modes, depending on what mode the radar is in, but the basic search mode is a spiral scan, inner to outer. The shooting or targeting mode is a conical scan centered on straight ahead of the aircraft.

T!

The 720 had a 10 mile normal range while the APS-6 had a 5 mile range under good conditions, especially at higher altitude. The APS-6 was fine if you were under tight control of ground radar and were being vectored onto nearby targets at higher altitudes. But if you you want to get out and look around, see what you can find, do a nighttime fighter sweep, the SCR-720 was much better.

MIflyer said:

But if you you want to get out and look around, see what you can find, do a nighttime fighter sweep, the SCR-720 was much better.

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That is, if you can lug it into the air. Was that "shape" in the P82 photograph actually crammed full of electronics, or partially a low drag fairing for the antenna? After reading Token's description of the 360° scanning antenna, I'm wondering if that "shape" isn't an attempt to get the antenna down and forward for a wider azimuth usable scan?
The earliest AN/APQ72s in the F4 were all tube, no solid state (I'm told) and many times the power and range of the SCR720, but they weren't THAT big.

MIflyer said:

Basing it on the P-51D instead of the P-51H should have been entirely feasible.

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The P-82 was based on the XP-51F, which was a lightened version of the P-51 (and the basis of the P-51H).

XBe02Drvr said:

That is, if you can lug it into the air. Was that "shape" in the P82 photograph actually crammed full of electronics, or partially a low drag fairing for the antenna? After reading Token's description of the 360° scanning antenna, I'm wondering if that "shape" isn't an attempt to get the antenna down and forward for a wider azimuth usable scan?
The earliest AN/APQ72s in the F4 were all tube, no solid state (I'm told) and many times the power and range of the SCR720, but they weren't THAT big.

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The APQ-72 antenna was only 2 inches larger than the SCR-720 antenna. However, since the -72 is X band (just under 10 GHz) and the -720 was S band (about 3 GHz) the gain of the APQ-72 antenna was much higher. The 30 inch SCR-720 dish had a gain of just under 25 dBi, while the 32 inch APQ-72 dish had a gain of about 36 dBi. This 11 dB delta enhanced separately both the transmit and receive sides of the equation.

So not only did the -72 have higher transmitter peak power than the -720 (on the order of 4 to 10 times as much), it also had 11 dB more transmitter gain, this makes the ERP at least 17 dB (and maybe 21 dB) higher, or the transmitter power into the air at least 50 times greater and maybe as much as 115 times greater. And on the receiver side the more modern -72 probably has at least 10 dB better sensitivity, and quit possibly 20 dB better MDS. Combined with the 11 dB greater antenna gain and the -72 receiver might be as much as 1000 times more sensitive.

As for the F-82's, various models carried the SCR-720, the APS-4, and the APG-28. I have no idea what model is in the picture mentioned.

Working from memory here, but, want to tie it all back to the P-61? I believe that the APG-28 that the later models of the P-82 carried was a modified version of the APG-1 that was developed for, and never operationally used in, the P-61.

T!

Token said:

As for the F-82's, various models carried the SCR-720, the APS-4, and the APG-28. I have no idea what model is in the picture mentioned.

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The picture is in post #247 in this thread and is alleged to be an SCR720 installation. Just eyeballing the photos, it looks as if that 360° dish could get close to 270° unobstructed usable azimuth at zero elevation angle and below, and maybe 220 or so above that. That pod looks awfully big for an AI radar. It's hard to imagine that entire volume is filled with electronics.

One reason the P-82 pod is so big is that they needed to get the radar out front so the props were not in the way. The APS-4 used on the P-82D used the same pod as the P-82C.

MIflyer said:

One reason the P-82 pod is so big is that tgey needed to get the radar out front so the props were not in the way. The APS-4 used on the P-82D used the same pod as the P-82C.

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So if you integrated that into an enlarged P38 schnoz it might work after all, and perhaps counterbalance the aft RO position? It would be already out in front of the props.

The P-38 had the guns up there, too, so there was a lot less room to work with. Initially on the P-38 night fighter they tried putting the radar pod below the aft center fuselage. When that proved to be a not very good location they moved it out under the Right wing outside of the prop.

The first P-38 night fighter, built for use locally in the Solomans, put the MK IV VHF radar in the nose, with that arrow antenna sticking out in front. They rearranged the guns to accomodate that. They put the RO behind the pilot and relocated the radio equipment to a drop tank under one wing. Very cramped.

Naturally they based the P-82 on the latest production P-51, although there were virtually no interchangable parts. But I can see no reason why you could not have built a P-82 based on the P-51D, with the earlier Merlin. It does involve extending the fuselage quite a bit - try doing it with a model kit as I did back in the 80's and you'll find out just how much. The performance would be lower than the P-82 as it was actually built but it would still be a world beater.

Token said:

The antenna used what is called a helical scan or a unidirectional sector scan. It spun completely around 360 degrees, stepped up, spun 360 again, stepped up, etc. When it reached its upper scan limit it started back at the lowest currently set elevation. The radar only transmitted while the radar was pointed in the forward hemisphere, and was blanked when pointed aft. This made the mechanics of the radar more simple, and simple means light. Basically it consisted of an azimuth drive motor that spun the antenna around, the same direction all the time, and a more restricted elevation drive motor that controlled elevation. To construct an antenna, and more importantly drive system, that sweeps an antenna rapidly side to side is more complex. And for reference such a scan, back and forth in a sector, is called a bidirectional sector scan.

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I had heard that before, but didn't understand it until I thought about it more. The antenna would scan at the same rate and in the same direction across every scan, which is not only mechanically simpler, it would simplify the back-end electronics, making them smaller and lighter. Simplicity is a big deal when you're dealing with low-noise circuits based on tubes and transformers. An airplane is an electrically noisy environment (start with every spark plug being a broad-spectrum air-gap transmitter). A smaller circuit would need less physical shielding, so it's a win that way, too.

Regarding the P-82 as a night-fighter:

The additional weight of the radar pod created very few performance problems with the F-82F having a top speed of 460 mph. The G's SCR-720 radar weighed slightly less than the APG-28 and thus the aircraft's performance was slightly better.

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However, the aircraft was not without issues:

The F, G, and H Twin Mustang was not a pilot's ideal night fighter due to the cockpit's limited field of view, and poor landing characteristics especially at night. Moreover, during nocturnal operations, the pilot and radar operator found it difficult to maintain night vision due to engine exhaust flame, instrument glare and the bright flashes from the aircraft's machine guns.

From an operational standpoint, some pilots felt a psychological discomfort of impending doom of a mid-air collision when they caught sight of the co-pilot/radar operator's fuselage out of the corner of their eyes. Another problem was with the J-8 Altitude Gyro used during instrument flying. Pilots either loved it or hated it because it read exactly opposite to the presentation of conventional gyroscopic instruments. When the miniature aircraft on the gyro appeared 'below' the reference line, instead of descending the F-82 was actually climbing and so, during an instrument landing approach, the pilot had to remain cognizant of what the aircraft was actually doing.

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Quotations from Twin Mustang — The North American F-82 at War by Alan C. Carey, pgs. 53, 55, 59, and 61.

I'm not quite understanding why we would be looking for a sinlge-place alternative night fighter in the first place. The P-61 was VERY good at what it did and was purpose-designed as a night fighter. It was also the first warplane designed for rader from the outset. Since it DID perform well, why would we be looking for alternatives? Perhaps as interim fighters uuntil the P-61 was available? The A-20 was used before the P-61 and did a credible job ... but the P-61 was far and away a better night fighter.

The P-61 (706 built) was introduced in 1944 and the P-82 (272 built) was introduced in 1946. USe the P-61. There were more of them and it was a vastly better night fighter in 1944 since it was actually flying at that time. Actualy airframe performance would favor the P-82, but it didn't fly until after the war was over.

Maybe the Germans should have proceeded with the Bf 109Z:

I guess the question was really : "What else could we have done that would have been better than the P-61 and available at least as soon?" For defending ships, bases, and cities the F6F-3N was better than the P-61 and was available at least 9 months earlier. For long range interception and night intruding the P-82 was better than the P-61 and could have been available just as fast.

The P-70 was deemed to be too low in performance to be used in the ETO.