P-61 Gun-Laying Radar

[Zipper730]

Regarding Endurance at Altitude

the original specification/thoughts for the P-61 called for about an eight hour "Loiter time." . . . Trying to stay airborne for anything close to eight hours with a pair of R-2800s is going to take a lot of fuel.

Wait a second, how much fuel did the P-61 carry on-board in terms of gallons (I've seen some weight figures, but I'm not sure how much is the ammo)? Could it have loitered 8 hours even under the fuel consumption estimates predicted, and the actual figures?

Early night fighting tactics and theory being a bit different than what was used later.

What were the early allied night-fighter tactics, and how'd they compare with later allied night-fighter tactics?


Regarding Gun-Laying Radar

Doesn't involve quotes, just one question: Why dd the USN go with gun-laying radar for night-fighters when the USAAF didn't?


Regarding Wave Propagation in General

The total feedline length is lengthened and shortened in various ways. . .

... In the previous example, 2 antennas 4 meters from the transmitter each fed with 5 meters of feedline. I have a 5 meter section of coax going to each antenna. And at one end of each feedline I have a coaxial switch that allows me to select an additional meter of coax in the path or to bypass that meter of coax. Throw a switch and the total length of cable feeding an antenna is now 6 meters, flip the switch the other way it is now 5 meters. I can now switch in and out, at will, a delay equal to the propagation time through that additional meter of coax, a delay, or phase shift, that can now be added or subtracted to each, either, or both, antennas.

Just to recap, at this point are we talking about simply lengthening an electrically conducting circuit that leads the device that generates and emits the beam, or are we talking about having an already formed electromagnetic beam go a greater or shorter distance... yes, I know this probably sounds stupid.

 

[XBe02Drvr]

are we talking about simply lengthening an electrically conducting circuit that leads the device that generates and emits the beam

Yes. Lengthening or not lengthening at will.

 

[XBe02Drvr]

What were the early allied night-fighter tactics, and how'd they compare with later allied night-fighter tactics?

Early days, lower performance fighters and shorter range radar they had to launch early and loiter at altitude until the path and targets of intruders was known. Later, with better ground radar and GCI combined with higher performance fighters (think Mosquito), they could delay takeoff and then get vectored directly to their victims and still get there in time.
Cheers,
Wes

 

[Token]

The total feedline length is lengthened and shortened in various ways.



An example might be coaxial cable and switches. The coaxial cable is the feedline.



In the previous example, 2 antennas 4 meters from the transmitter each fed with 5 meters of feedline. I have a 5 meter section of coax going to each antenna. And at one end of each feedline I have a coaxial switch that allows me to select an additional meter of coax in the path or to bypass that meter of coax. Throw a switch and the total length of cable feeding an antenna is now 6 meters, flip the switch the other way it is now 5 meters. I can now switch in and out, at will, a delay equal to the propagation time through that additional meter of coax, a delay, or phase shift, that can now be added or subtracted to each, either, or both, antennas.

Regarding Wave Propagation in General


Just to recap, at this point are we talking about simply lengthening an electrically conducting circuit that leads the device that generates and emits the beam, or are we talking about having an already formed electromagnetic beam go a greater or shorter distance... yes, I know this probably sounds stupid.

Basically, yes.


The devices that generate and then emit the beam are, in this case (and much of the time), separate devices.


The transmitter generates the energy. The antenna forms and emits the beam. The energy has to get from the transmitter to the antenna somehow, so the feedline conducts the energy from the transmitter to the antenna. The basic configuration is that the Radio Frequency energy starts in the transmitter, passes through the feedline, and is emitted by the antenna.


By changing the length of the feedline you change the time the energy takes to get from the transmitter to the antenna. Any changes in time with an RF signal (or any cyclic signal) can be expressed as changes in phase.


T!

 

[MIflyer]

"Why dd the USN go with gun-laying radar for night-fighters when the USAAF didn't?"

I am not sure what you mean by Gun Laying Radar.. Some USN PB4Y-2's and some USAAF B-29's used radar directed guns in the tail turret. While the USN did have single seat night fighters with the F4U and F6F, while the USAAF used two seat night fighters with the P-70, P-38, and P-61, both the USAAF and USN used similar techniques to guide the fighters to their target. The SCR-720 radar used in the P-61 and some Mossies had a lot greater range than the other airborne radars.

 

[Zipper730]

I am not sure what you mean by Gun Laying Radar..

A radar that could auto-track a target

 

[Token]

A radar that could auto-track a target

I think it would be more correct to say that a gun laying radar was/is a radar that can provide azimuth, elevation, and range to a target in such a way, and with adequate accuracy, that that information can be used to derive a firing solution for guns. Such a radar would not necessarily have to be capable of automatic track, as such information can be derived from radars with man-in-the-loop to maintain precise track.


So it would have to be a radar capable of tracking a target, be that track automatic, man-in-the-loop, or some combination of the two.


Some documents specify an AGL (Automatic Gun Laying radar) from an GL. One might assume that an AGL would have to be auto-tracking.


T!

 

[MIflyer]

Interestingly enough there was a widely used radar altimeter in WWII, the APN-1. But there was a "gun laying" version of it, the APG-1, that was used to help figure out the altitude to launch torpedoes., Same avionics, just a different purpose. One was for "navigation" and one was for "gun laying." I have an APN-1; I used to crank it up and watch it put a black bar across the TV on CH 14.

The APG-30, which I have an example of out in my garage, was used on the F-86 and other jet fighters of that vintage in order to feed range information to the gyro gunsight. On the earlier gunsights you had to dial in the target aircraft's wingspan and then adjust the edge of the sight window to match it, in order to provide range information needed by the gunsight. The APG-30 had an X-band radar and a computer that figured out what the range was and fed it to the gunsight.

 

[Token]

"Why dd the USN go with gun-laying radar for night-fighters when the USAAF didn't?"


I am not sure what you mean by Gun Laying Radar.. Some USN PB4Y-2's and some USAAF B-29's used radar directed guns in the tail turret. While the USN did have single seat night fighters with the F4U and F6F, while the USAAF used two seat night fighters with the P-70, P-38, and P-61, both the USAAF and USN used similar techniques to guide the fighters to their target. The SCR-720 radar used in the P-61 and some Mossies had a lot greater range than the other airborne radars.

There is a difference between an AI (Air Intercept) radar and a gun directing, or gun laying, radar. Eventually, such gun-laying radar became called an FCR, (Fire Control Radar).


In the case of the PB4Y-2's and B-29 the radar was the APG-15 (and other models, just going to mention the late war -15 for now). This was a radar capable of automatically tracking the target and automatically aiming the guns at the target. Earlier versions had either manual radar track or semi-automatic (man in the loop) radar track to guide the guns.


On a fighter there typically is not a turret with guns that can be guided by a gun laying radar. Yes, the P-61 was one such equipped, more on that in a bit. So, at least during WW II, self radar guided night fighters (as opposed to purely GCI, Ground Controlled Intercept, night fighters) typically used an AI radar to get close to the target and then the gunner or pilot fired the guns manually using some kind of sighting system. Some night fighters (none of the ones mentioned in your post) used other radar based techniques to trigger the guns.


The F4U, F6F, and P-38 used the AN/APS-4 AI radar, the P-61 and P-70 used the SCR-520 or SCR-720 AI radar, and (mid to late war) Mossie used the British Mk VIII or Mk X AI radar (themselves derived from the SCR-520/720).


These AI radars had the ability to guide the aircraft to a target, but it was up to the human to aim the guns. On page 3 of this thread I lay out the basics of how that was done on the P-61.


Some of the following I have little documentation to support, but seems to be strongly indicated. Dana Bell may have documentation to support some of it, but I am not sure.


The P-61 was apparently originally envisioned to have a combined AI / gun laying radar that could both guide the aircraft to the target and then assist the pilot or gunner in firing the fixed guns or directing the turret mounted guns. I am assuming this was what was to become the AN/APG-1 and 2, although I am not sure a model was specified in the early P-61 requirement. I assume, based on confirmable timelines of development, that the AN/APG-1/2 was not ready as early as the P-61 was, and so an SCR-520, and later 720, was fitted.


The SCR-720 became the standard radar for the P-61. A small block of 6 or 7 aircraft (P-61B-25's) did receive the APG-1/2, and they were tested. As near as I can tell the APG-1/2 was removed from all of those airplanes that survived and the SCR-720 was installed in its place.


Of interest, I see you mentioned an APG-1 in your next post as a UHF gun layer for altitude. Could that be a typo? I have seen documentation and pictures that indicate the AN/APG-1 and -2 were 10 cm gun layers with conical scan and auto track capability. For example, "The Signal Corps: the outcome (mid 1943 through 1945)" specifically lists the AN/APG-1 as an AGL (Automatic Gun Layer) and described it as a "10 centimeter S band set". More detail can be found in the MIT Rad Lab series, but basically saying the same thing. Also, I have seen pictures of a Block 25 P-61B with radome removed, annotated "P-61 with APG-1 radar" and a dish similar in size, but with different feed (conical scanning), to what is seen with the SCR-720.

T!

 

[MIflyer]

We actually found pictures of what clearly was an APN-1 with some slight difference in connectors on the front, marked as an APG-1. I figured it was a typo but actual research found that there was an APG-1 torpedo launching version. The APN-1 initially was known as the ARN-1, so they clearly were trying to figure out what designations to use when they started on the AN designation system. The ARR-1 was not even a receiver, although that probably was more deliberate subterfuge than it was confusion.

 

[Zipper730]

The F4U, F6F, and P-38 used the AN/APS-4 AI radar

Wait, I thought some used the AN/APG-1...

The P-61 was apparently originally envisioned to have a combined AI / gun laying radar that could both guide the aircraft to the target and then assist the pilot or gunner in firing the fixed guns or directing the turret mounted guns. I am assuming this was what was to become the AN/APG-1 and 2, although I am not sure a model was specified in the early P-61 requirement.

It's possible that either the AN/APG-1/2 wasn't as ready as the plane was, or that the original requirement simply dictated the SCR-520/720.

 

[Token]

Wait, I thought some used the AN/APG-1...

I suppose it is possible, however I am not aware of any of those aircraft, F4U, F6F, or P-38, being equipped with the AN/APG-1. And since the APG-1 was a gun layer/tracking radar I can think of no particular reason for it to be fitted to those aircraft, since their armament was mounted parallel to the central axis of the aircraft and could not be individually directed.

It's possible that either the AN/APG-1/2 wasn't as ready as the plane was, or that the original requirement simply dictated the SCR-520/720.

Since the P-61 had a turret as original specified, and the SCR-520/720 could not direct a turret, I doubt the SCR-720 was the final goal as originally conceived. It makes sense that the end goal was something like the APG-1/2, however tactics change, and possibly by the time the APG-1/2 was ready the methods of use for the P-61 had evolved from the original concept.


T!

 

[Zipper730]

I suppose it is possible, however I am not aware of any of those aircraft, F4U, F6F, or P-38, being equipped with the AN/APG-1. And since the APG-1 was a gun layer/tracking radar I can think of no particular reason for it to be fitted to those aircraft

I can -- normally you'd have to acquire on radar and then maneuver into visual range. With gun-laying you just lock onto them and spray them from 400 to 1000 yards depending on accuracy -- no need for a visual.

Since the P-61 had a turret as original specified, and the SCR-520/720 could not direct a turret, I doubt the SCR-720 was the final goal as originally conceived.

Probably, but many night fighters did basically acquire their targets, then maneuver into visual range and open up. Once in visual range, the turret would be fine.

As I grasp it, the RAF's interest in turreted fighters was sort of a "Schrage Muzik with Options" as you could vary the inclination of the gun.

It makes sense that the end goal was something like the APG-1/2

True enough

 

[Shortround6]

I can -- normally you'd have to acquire on radar and then maneuver into visual range. With gun-laying you just lock onto them and spray them from 400 to 1000 yards depending on accuracy -- no need for a visual.

Only if you tied the radar into an auto-pilot to steer the plane onto a collision course with the target (and even then you are lacking range and/or lead computation.)

 

[Zipper730]

Only if you tied the radar into an auto-pilot to steer the plane onto a collision course with the target (and even then you are lacking range and/or lead computation.)

First of all, this is not the 1950's where we were doing collision-course intercepts -- this was WWII where the typical interception was done from the 6 o'clock position.

I'm not sure if the APG-1/2 had any lead-computation technology in them, but I do remember something about the B-29's fire-control system involving lead-computing

 

[wuzak]

I can -- normally you'd have to acquire on radar and then maneuver into visual range. With gun-laying you just lock onto them and spray them from 400 to 1000 yards depending on accuracy -- no need for a visual.

You still need to identify the aircraft, so you can be certain that it is friend or foe.

The British (and I guess the Americans) had an IFF system, but that wasn't 100% reliable.

I believe in some instances, particularly over Britain, RAF night fighter pilots would close in range behind the target aircraft and then send an instruction over the radio, like "waggle your wings", and shoot it down if there is no response.

 

[Shortround6]

Uh, any interception or firing situation that involved deflection shooting was collision course.

Gyro sights could calculate lead but they needed a good range estimate or input or it was pretty much useless.

"Locking" radar on a target means that the radar will keep the selected target in the center of the display (or actually keep the selected target in the middle of the radar's sweep range although that can be narrowed down). turning that into a viable firing solution is a bit harder.

They started in early 1941 with the concept. Getting hardware that worked in operational aircraft with regular crews took longer than the war lasted.

see; AI Mk. VIII radar - Wikipedia

 

[Zipper730]

You still need to identify the aircraft, so you can be certain that it is friend or foe. The British (and I guess the Americans) had an IFF system, but that wasn't 100% reliable.

That's a whole different variable and would apply with or without a turret

I believe in some instances, particularly over Britain, RAF night fighter pilots would close in range behind the target aircraft and then send an instruction over the radio, like "waggle your wings", and shoot it down if there is no response.

Makes enough sense

Uh, any interception or firing situation that involved deflection shooting was collision course.

I thought collision course was either a up-the-front, front-beam, or abeam attack? Otherwise it seems you'd just come at him, merge, and swing in behind him if you could and blast the hell out of him?

Gyro sights could calculate lead but they needed a good range estimate or input or it was pretty much useless.

And the radar provides the ranging...

"Locking" radar on a target means that the radar will keep the selected target in the center of the display

But if you can do that you could keep maneuvering until you have the target dead ahead...

 

[XBe02Drvr]

And the radar provides the ranging...
But if you can do that you could keep maneuvering until you have the target dead ahead

You're right, the radar can give you the range, azimuth, and elevation of your target, but not precisely enough to land your rounds on target with the technology of the time. You clearly haven't done much shooting in the real world.
Here's an experiment. Make a 1/4 size cardboard silhouette of a BF-110 as seen from its 6 o'clock and mount it on a 200 yard target stand at your local range. Now take the rifle and ammunition of your choice calculate it's 200 yard trajectory and mount your rifle on a shooting stand elevated at the proper angle to compensate for bullet drop. Now measure the magnetic azimuth angle, line of sight, shooting stand to target. Now WITHOUT looking through the rifle's sight at the target, align its barrel to the magnetic azimuth you have determined. Now fire a box or two of ammunition through it and go count your hits. You'll see what I mean.
Another issue with a tail chase situation on an unseen target within gun range is closing velocity, Vc. I've worked with AI radars much more sophisticated than what you're talking about, and they can't display Vc changes in gun range quickly enough to "fly formation" in trail of a target without risking falling out of range or ramming your target. The lag is too great.
Cheers,
Wes

 

[fubar57]

That's why I shot trap: one of them BBs hit the bird 91% of the time