P-61 Gun-Laying Radar

fubar57 · Oct 29, 2017

Zipper730 said:
When did the AN/APS-6 come out?

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Westinghouse APS-6 History

Shortround6 · Oct 29, 2017

Zipper730 said:
The AWACS idea isn't so crazy: The US Navy did in fact modify a B-17 derivative with a HUGE antenna. I figure if you have that, you have the hunter element; with the P-61's with the APG-2 as the killer side...

The problem in WW II was for the AWACS plane to try to tell the "killers" where to look. Were the "displays" in the aircraft of a type that the operator/s could easily see the relationship between the target and the "Killer" so that the controller could vector the killer onto the target?

Modern radars use a lot of computing power to modify the display into a more user friendly form.

British MK VIII radar display

Horizontal line is ground clutter, target is the arc on the left, about 20 degrees left of the nose and and at two miles range.

Radar displays were constantly evolving but trying to control an intercept from a airborne, moving radar platform was going to need an awful lot of practice. the radar operator having to estimate the course and speed of both of the other aircraft and try to work out intercept angles/courses either in his head or with a paper and pencil (unless radar plane was so big they could put a plotting board in it)

all done with computers now and displayed in high definition on a bright screen in multi colors

Token · Oct 29, 2017

wuzak said:
So doesn't provide gun trajectory information?

As near as I can tell the APS-6 was not connected to any kind of computer to predict trajectory.

That radar had 5 skin track range positions, 65 miles, 25 miles, 5 miles, 1 mile, and "Gun Aim" which is 1000 yards.

In the 65 and 25 mile ranges there was no way to tell elevation at all, you had a B scope which showed you azimuth and range in a top down view. In the 5 and 1 mile ranges you had a modified B scope, which showed each target as 2 pips, the relationship of the 2 pips on the scope indicated if the target was above or below you.

In the 1000 yard Gun Range setting you had a C Scope, a 2 dimensional scope (elevation and azimuth) that indicates the relationship of the target to the forward / roll axis of your aircraft. If the target was dead ahead of you it was in the center of the scope.

T!

Token · Oct 29, 2017

Zipper730 said:
How short? Because if that range is around 400 yards, that puts you within an effective gun-range firing solution...

The "Gun Aim" setting was a 1000 yard max range. (edit) Removed reference to range pips, I was looking at a C model, post war, manual. That radar included a second display with a top down range view which included range markers. The WW II model had only the B and C Scopes I described earlier.)

Zipper730 said:
The tighter the arc, the more gain right?

The tighter the beam the more gain. Any beam can be swung in any sized arc that the mechanical system is designed to support.

T!

Dana Bell · Oct 30, 2017

Last week I found another note on the AGL for the P-61. The following is from a 15 Dec 1943 memo, with the Office of Assit Chief of Air Staff (MM&D) writing the Materiel Command at Wright Field. The last paragraph reads:

5 The status of development, test, and production of the SCR-702 gun laying equipment, which may replace the SCR-720 AI equipment is such that a definite statement can not be made at this time regarding the use of this equipment in future night fighters. Therefore, no attempt should be made at present to install SCR-702 in production line aircraft. It is requested, however, that mock-ups and tests of the SCR-702 be expedited by Materiel Command in order that a decision may be reached as soon as possible regarding the availability [sic] of substituting the SCR-702 for the SCR-720..

Replacing the AI radar with the AGL suggests that the mount would have been in the nose, not the turret, and that the AGL would have had no use aft of the wing. Again, there's still more of the story to discover...

Cheers,

Dana

Koopernic · Oct 30, 2017

Dana Bell said:
Last week I found another note on the AGL for the P-61. The following is from a 15 Dec 1943 memo, with the Office of Assit Chief of Air Staff (MM&D) writing the Materiel Command at Wright Field. The last paragraph reads:

5 The status of development, test, and production of the SCR-702 gun laying equipment, which may replace the SCR-720 AI equipment is such that a definite statement can not be made at this time regarding the use of this equipment in future night fighters. Therefore, no attempt should be made at present to install SCR-702 in production line aircraft. It is requested, however, that mock-ups and tests of the SCR-702 be expedited by Materiel Command in order that a decision may be reached as soon as possible regarding the availability [sic] of substituting the SCR-702 for the SCR-720..

Replacing the AI radar with the AGL suggests that the mount would have been in the nose, not the turret, and that the AGL would have had no use aft of the wing. Again, there's still more of the story to discover...

Cheers,

Dana

From the point of view of the P-61's B-29 like GE gunnery system all that was needed was a range, bearing, elevation and maybe closure rate figure for the mechanical computer to provide offsets to aim the turrets guns. These values were transmitted by Synchro transmitters from the optical sights of the B29 or P61 sighting station. The signals could easily be switched in from Synchros mounted on the SCR 702 tracking aerial rather than the optical sighting stations.

Once you have that data you can do anything with it including use it to deflect a reflector gyro sights mirror or even inject it into the auto pilot.

Zipper730 · Oct 30, 2017

Shortround6 said:
The problem in WW II was for the AWACS plane to try to tell the "killers" where to look.

The US Navy did seem to use TBF's as AEW aircraft. I thought the PB-1's were used during the war, but it wasn't until after.

Regardless: Did the technology exist to create such a radar and aircraft earlier? I'm curious because sometimes developments occur because of technological limits, other times because nobody thought of the idea.

Token said:
Thew "Gun Range" setting was a 1000 yard max display with 250 yard range pips.

Pips?

The tighter the beam the more gain.

Gotcha

Dana Bell said:
Last week I found another note on the AGL for the P-61. The following is from a 15 Dec 1943 memo, with the Office of Assit Chief of Air Staff (MM&D) writing the Materiel Command at Wright Field. The last paragraph reads:

5 The status of development, test, and production of the SCR-702 gun laying equipment, which may replace the SCR-720 AI equipment is such that a definite statement can not be made at this time regarding the use of this equipment in future night fighters. Therefore, no attempt should be made at present to install SCR-702 in production line aircraft. It is requested, however, that mock-ups and tests of the SCR-702 be expedited by Materiel Command in order that a decision may be reached as soon as possible regarding the availability [sic] of substituting the SCR-702 for the SCR-720..

The SCR-702 was that the APG-2?

Replacing the AI radar with the AGL suggests that the mount would have been in the nose, not the turret

That adds up

the AGL would have had no use aft of the wing.

Makes enough sense

Again, there's still more of the story to discover...

You should write a book on this -- you helped unearth a whole lot of stuff!

Token · Oct 30, 2017

Dana Bell said:
Replacing the AI radar with the AGL suggests that the mount would have been in the nose, not the turret, and that the AGL would have had no use aft of the wing. Again, there's still more of the story to discover..

The SCR-702 / 580 was the APG-1 / 2, and fit in the nose in the same location as the SCR-720. It did mount in the nose. And like the SCR-720 the APG-1/2 was limited to an area in the forward hemisphere.

T!

Token · Oct 30, 2017

Zipper730 said:
Pips?

(Edit) The WW II version of this radar as used did not have the range markers I was describing, that is from a manual of a post war version for a different type of aircraft. I have removed the incorrect information.

Zipper730 said:
The SCR-702 was that the APG-2?

That is debatable.

One very good period source states that the SCR-702 and the SCR-580 were the same radar from different vendors. This was a common practice, to change designations with different suppliers. If the SCR-702 and the SCR-580 are the same radar that might explain the differing APG-1 and APG-2 designations. Some sources call the SCR-702 the APG-1, and other sources call the SCR-702 the APG-2. If the -580 and -702 are the same radar from different sources then obviously the SCR-580 might be the "other" radar in the APG set, be it APG-1 or APG-2.

The best source I know of calls the SCR-702 the APG-1, however I personally saw a radar marked SCR-702A / APG-2.

I think it is very safe to say the SCR-702 was one or the other APG-1 or APG-2, if not both. The question, to me, comes in with how does the SCR-580 fit? Was the -580 the early experimental version and the -702 the production version? Why then the APG-1 and APG-2? Could the APG-1 have been the SCR-702 and the APG-2 the SCR-702A?

T!

Zipper730 · Oct 30, 2017

Token said:
The SCR-702 / 580 was the APG-1 / 2

Oh, okay

Range markings along the range axis of the display. The total displayed range of the display in this mode was 0 to 1000 yards, with markings along the range every 250 yards so that you could visually estimate the range. These kinds of markings are called range pips.

Do you mean like minute of angle or mil-dots?

One very good period source states that the SCR-702 and the SCR-580 were the same radar from different vendors. This was a common practice, to change designations with different suppliers.

Like how the TBF/TBM were basically the same plane?

The best source I know of calls the SCR-702 the APG-1, however I personally saw a radar marked SCR-702A / APG-2.

That might be a possibility...

Shortround6 · Oct 30, 2017

Zipper730 said:
The US Navy did seem to use TBF's as AEW aircraft. I thought the PB-1's were used during the war, but it wasn't until after.

See: Lockheed EC-121 Warning Star - Wikipedia

Even over Vietnam the EC-121 could not actually control an intercept.

There is a huge difference between an early warning aircraft and an Airborne early warning and control aircraft.

It was tried in a basic form during WW II by the British.

But apparently only over water?

pbehn · Oct 30, 2017

Shortround6 said:
See: Lockheed EC-121 Warning Star - Wikipedia

Even over Vietnam the EC-121 could not actually control an intercept.

There is a huge difference between an early warning aircraft and an Airborne early warning and control aircraft.

It was tried in a basic form during WW II by the British.

View attachment 470255

But apparently only over water?

I think that was used against air launched V1s late in the war.

Koopernic · Oct 30, 2017

Shortround6 said:
See: Lockheed EC-121 Warning Star - Wikipedia

Even over Vietnam the EC-121 could not actually control an intercept.

There is a huge difference between an early warning aircraft and an Airborne early warning and control aircraft.

It was tried in a basic form during WW II by the British.

View attachment 470255

But apparently only over water?

Note in the above photograph the censor has blended out the antenna dipoles and reflectors to hide the operating frequency.

wuzak · Oct 31, 2017

Shortround6 said:
See: Lockheed EC-121 Warning Star - Wikipedia

Even over Vietnam the EC-121 could not actually control an intercept.

There is a huge difference between an early warning aircraft and an Airborne early warning and control aircraft.

It was tried in a basic form during WW II by the British.

View attachment 470255

But apparently only over water?

pbehn said:
I think that was used against air launched V1s late in the war.

Yes, it was called Operation Vapour.

Trials were conducted in November and December of 1944. The aim was to detect low flying aircraft, specifically He 111s with air-launched V1s.

The trials aircraft was the Wellington XIV, which was fitted with ASV Mk VI.

One or Two night fighter types would hold station behind the Wellington, one being further behind and higher than the other. The controller would send one of the fighters to the interception guiding it until an AI contact had been made. They would fly in a straight line at low altitude (2000ft) until the end of their patrol line, then return.

In trials 23 of 26 intercepts were successful.

Operational controls began in January 1945. The patrol area was off the coast of the Netherlands. Contacts were successfully intercepted, but, as far as I can tell, they all consisted of allied aircraft which did not respond to IFF interrogation. There was one where the controller sent a fighter to intercept a small boat, recalling it quickly after realising his mistake.

The equipment used was not suitable for use at high altitudes.

Token · Oct 31, 2017

Zipper730 said:
Do you mean like minute of angle or mil-dots?

Sort of, except down both sides of the top down range view. However, it does not matter, I was describing the wrong radar. The manual I was referencing was for a different version of the APS-6 used several years after WW II ended. The WW II version of the APS-6 had no such top view range display, it only had the B and C Scope displays I previously described. Range was displayed as a function of target width on the C scope.

T!

Zipper730 · Dec 5, 2017

I'm curious about something: Could the F3D-2 use the gun-laying radar and conitnue to scan?

Token · Dec 7, 2017

Zipper730 said:
I'm curious about something: Could the F3D-2 use the gun-laying radar and conitnue to scan?

I have zero firsthand information on the F3D-2 or its radar.

However from what I can find this aircraft used the AN/APQ-35 radar, which is actually 3 separate radars in an integrated suite (some had the AN/APQ-36, an improved version of the APQ-35). The three radars in this suite were the AN/APS-21, the AN/APS-28, and the AN/APG-26. The -21 is a search radar, the -28 is a tail warning radar, and the -26 is an autotracking radar, what I assume you are calling the gun laying radar.

The APS-21 radar and the APG-26 radar appear to have used two separate antennas, both mounted in the nose. The larger APS-21 antenna is readily visible in pictures of the aircraft with the radome removed. But a few pictures also show the smaller, separate, APG-26 antenna.

The written descriptions of the radar operation that I can find appear to indicate there was a separate search display and a C scope used for the track/intercept display.

Since there are separate track and search antennas, and since there appears to have been separate search and track displays, I see no reason the system could not both track and search simultaneously.

T!

Zipper730 · Dec 9, 2017

Token said:
I have zero firsthand information on the F3D-2 or its radar.

However from what I can find this aircraft used the AN/APQ-35 radar, which is actually 3 separate radars in an integrated suite (some had the AN/APQ-36, an improved version of the APQ-35). The three radars in this suite were the AN/APS-21, the AN/APS-28, and the AN/APG-26. The -21 is a search radar, the -28 is a tail warning radar, and the -26 is an autotracking radar, what I assume you are calling the gun laying radar.

So, basically the AN/APS-21 does the forward hemispherical searching under all conditions, while the AN/APG-26 does close in shooting once locked-on, and the AN/APS-28 covers the night-fighter's six?

Seems to have been designed almost specifically to address the P-61's issues and learn some lessons the RAF found out

If you can't search & track with the same radar: Put one radar to search; another to track
The RAF fitted some rear-mounted arrow-head antennae on some Mosquitoes to avoid getting jumped

Token · Dec 10, 2017

Zipper730 said:
So, basically the AN/APS-21 does the forward hemispherical searching under all conditions, while the AN/APG-26 does close in shooting once locked-on, and the AN/APS-28 covers the night-fighter's six?

Seems to have been designed almost specifically to address the P-61's issues and learn some lessons the RAF found out

If you can't search & track with the same radar: Put one radar to search; another to track

The RAF fitted some rear-mounted arrow-head antennae on some Mosquitoes to avoid getting jumped

I don't know about specifically addressing the P-61's issues, rather this was a logical progression from the limitations of all of the earlier night fighters.

The general technology of the day just was not up to doing simultaneous search and track with a single radar. Today, using phased array, it is a given. So in the WW II and immediate post WW II period that meant two radars to do these two tasks. The APQ-35 (and -36) used multiple radars to do these multiple tasks.

As for tail warning radars, these were relatively common on mid to late war US fighters. I think the APS-13 was a very common model, indeed fitted to the P-61 (and P-38, P-47, P-51, P-63, etc) for tail warning.

T!

Zipper730 · Dec 13, 2017

Token said:
I don't know about specifically addressing the P-61's issues, rather this was a logical progression from the limitations of all of the earlier night fighters.

That's what I was kind of getting at.

As for tail warning radars, these were relatively common on mid to late war US fighters. I think the APS-13 was a very common model, indeed fitted to the P-61 (and P-38, P-47, P-51, P-63, etc) for tail warning.

I never knew any day fighter had a tail-warning active radar!

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