P-61 Gun-Laying Radar

[basil]

Thank you for the reply. Yes, average output among the radars you mentioned may be similar but it still makes no sense to me why for example the
peak power of a Hughes E-5 (F-94c) or of an AN/APS-21 (F3D-2) is described with 200 kW while the latest Irbis E (Su-35) has a peak of 20 kW, an AN/APG-81 (F-35) of about 17 kW, or the Raven of the newest Gripen "more than 10 kW". Is it really just a matter of pulse repitition?

 

[XBe02Drvr]

peak power of a Hughes E-5 (F-94c) or of an AN/APS-21 (F3D-2) is described with 200 kW while the latest Irbis E (Su-35) has a peak of 20 kW, an AN/APG-81 (F-35) of about 17 kW, or the Raven of the newest Gripen "more than 10 kW".

Among other reasons, those old tube sets were nowhere as efficient as today's solid state miniaturized circuitry. The transmission lines and antennas were more lossy and less efficient at converting generated power to effective radiated power. Internal timing was less precise, as was the test equipment to calibrate and monitor it. Antennas were less effective in focussing the radiated power, and had less inherent gain to catch the return, and thanks to compact and efficient phase lock loop circuits, modern radar receivers are able to achieve usable S/N with much weaker returns. And then there's digital signal processing.
Today's GCI is far more sophisticated, thus requiring less detection range in the interceptors. The APQ72 in the F4B had a peak power near 1,000 KW with detection ranges in the hundreds of miles, to operate beyond carrier borne GCI and the puny E-1 AWACS.
I'm sure others here will enlarge upon these oversimplified points.
Cheers,
Wes

 

[basil]

No question that today's fighter radar circuits are by magnitudes more sensible in addition to all their processing power. However, this still does not explain the huge gap in pulse power output to me, which is described between the 20- to 100 fold as much for the old radars compared to modern ones.
BTW, I do not agree that for modern fighter aircraft detection range is less required because of GCI than in earlier decades. The big picture of the airspace is important as ever and there is not always an AWACS at hand

 

[XBe02Drvr]

BTW, I do not agree that for modern fighter aircraft detection range is less required because of GCI than in earlier decades. The big picture of the airspace is important as ever and there is not always an AWACS at hand

In the half century plus since the radars you cited were designed the largely un-radared world has become covered with a network of ATC radars, the raw data of which is available to the relevant military commands that defend regional airspace. This network is by no means all encompassing, but the big picture of the airspace is much less the mystery it once was.
What today's fighters need more than range is ECM sophistication, target analysis, and effective look-down capability to track targets in the airspace below radar coverage, especially in this world of terrain hugging cruise missiles UAVs, and manned aircraft.
Cheers,
Wes

 

[basil]

Returning to the original topic of this thread: William Wolf describes on page 417 of his excellent book "U.S. Aerial Armarment in World War 2" following P-61 installation:
"There were seven P-61B-25s built which were test beds for the Western Electric APG-1 Gun Laying Radar that was linked to the GE remote controlled dorsal turrets. The SCR-720 radar fed data into the analogue computer which directed the turret guns onto the target. One P-61B-15 and the first six P-61B-20s were also modified to this configuration. These aircraft were tested by the Air Proving Command at Eglin Field, FL and at the night fighter training unit at Hammer Field, CA."

 

[Token]

Returning to the original topic of this thread: William Wolf describes on page 417 of his excellent book "U.S. Aerial Armarment in World War 2" following P-61 installation:
"There were seven P-61B-25s built which were test beds for the Western Electric APG-1 Gun Laying Radar that was linked to the GE remote controlled dorsal turrets. The SCR-720 radar fed data into the analogue computer which directed the turret guns onto the target. One P-61B-15 and the first six P-61B-20s were also modified to this configuration. These aircraft were tested by the Air Proving Command at Eglin Field, FL and at the night fighter training unit at Hammer Field, CA."

The SCR-720 did not feed data to a computer to direct a gun turret. The -720 was an AI radar, and provided an air picture for the pilot or gunner to direct the guns. The APG-1/2 did feed such data, but the APG-1/2 and the SCR-720 are different radars. The SCR-720 (and its predecessor, the -520) is the radar used by most other P-61 aircraft, not the 7 used for this testing. These 7 aircraft are, as near as I can tell, the only ones outfitted with the APG-1/2, and never saw operational service in such a configuration. There is some indication that at least some of these aircraft were retrofitted to the standard SCR-720 radar after testing was complete.

The information quoted, including the block numbers of the aircraft and the testing at Hammer, has been stated a couple times in this thread.

T!

 

[Token]

Thank you for the reply. Yes, average output among the radars you mentioned may be similar but it still makes no sense to me why for example the


peak power of a Hughes E-5 (F-94c) or of an AN/APS-21 (F3D-2) is described with 200 kW while the latest Irbis E (Su-35) has a peak of 20 kW, an AN/APG-81 (F-35) of about 17 kW, or the Raven of the newest Gripen "more than 10 kW". Is it really just a matter of pulse repitition?

PStickney, in post 1464241, gave you the most complete single answer. However, that is only a partial answer, as there really is no single answer that gives you what you are looking for. There are multiple aspects to consider and which weigh in to drive the modern state of the art in airborne intercept radars. Any attempt to tie this to one factor will make an error of omission, and be potentially flat out wrong.

Yes, radar receivers today are more sensitive than in the past. For example, the SCR-720 radar may have had an MDS of around -110 or so dBm, but a modern radar in the same frequency range would have an MDS more along the lines of -126+ dBm, maybe up to the -134 dBm range. This means the receiver is literally ~100 times as sensitive, and this is raw sensitivity before we start talking about improved signal processing. Now put improved processing combined with pulse compression techniques on top of the raw receiver sensitivity. And so, today, a lower power radar, all other things being equal, will detect similar sized targets further away than a similar radar of the 1940's to mid 1960's.


At a very basic level, probability of detection can be tied to average power returned from the target. Old radars tended to send a big pulse less often, while modern radars send a less big pulse very often.

Realistic numbers here.

An old, magnetron based, radar, may have a peak power of on the order of 250 kW (or higher, I have worked with magnetrons over 2 MW, but just using the 250 kW as an example), but it might have a maximum duty cycle of 0.1% (high power radar magnetrons typically have a DC limitation of well under 0.2%). The PRF might be on the order of 1200 Hz (a pulse is sent every 0.00083 seconds), and the PW on the order of 0.5 microseconds. This makes the duty cycle about 0.06%. So while the peak power is 250 kW, the average power transmitted is 150 Watts.

A modern TWT / klystron / Solid State pulse Doppler radar may only have a peak power of 20 kW, but such systems are capable of duty cycles in excess of 10%. If the PRF is 100 kHz (a pulse sent every 0.00001 seconds), and the PW is on the order of 0.5 microsecond, then the duty cycle is 5%, and the average power is 1000 Watts.

And so, despite the fact the modern transmitter is less than 1/10 the peak power of the old transmitter (20 kW new vs 250 kW old), the average power transmitted today, and thus the average power received in the return, is many times more energy.

Other factors to consider:

Technology of the RF devices. For multiple reasons old radars were much more likely to use magnetrons than modern radars. Magnetrons typically have low duty cycle limitations meaning to get a desired average power you must push up the peak power.

Lower peak power transmitters typically operate at lower voltages. Why does this matter? Lower voltages are easier to mechanically keep isolated, resulting in smaller and more compact, along with lighter weight, packaging. Always desirable in aircraft applications.

Lower peak power transmitters can use simpler transmissions lines. The arcing potential (voltage breakdown) in waveguide is a factor of the voltage potentials inside the waveguide and the physical dimensions of the waveguide, along with the dielectric of the gas inside the waveguide. To get high peak powers you either need to use larger wavegudie dimensions (not possible while maintaining the mode of operation of the waveguide) or you need to use exotic gasses / complex drying methods to get the voltage hold-off within the waveguide. Lower peak powers are more forgiving, although higher average power typically requires more cooling to control heating. So feedline systems can be simplified, instead of exotic gasses or pressurization/dry air systems simple forced air to drive away heat.

Lower peak power transmitters can be combined with low duty cycle waveforms to produce LPI (Low Probability of Intercept) radars. In certain applications you want a radar that is not likely to be detected. You can achieve this many ways, but combinations of low average power, low peak power, and low revisit rates, goes long way in this direction.

And a lot more factors, but you get the general idea. There ain't no one reason.

T!

 

[basil]

Token, thank you for this great and detailed explanaition. So does this mean that the microwave source of modern (airborne) radars (TWTs) is generally not capable to deliver the same high peak pulse performance (I am not talking about average output) like old magnetron based radars? I have read the Irbis E of the Su-35 is built up with two TWTs just to get peak power up to 20 kW. On the other hand the AN/ASG-18 of the YF-12 is said to have had a peak output of 600 kW or more; similar numbers are available for the original interceptor version of the MIG-25). And this radar of course had a TWT and not a magnetron as microwave source.

 

[basil]

Ad P-61: I also think that William Wolf has mixed things up a little bit. I am aware that the SCR-720 with its helical scanning is not able to feed data into a computer (in contrary to the APG-1/2). BTW it was already well explained in earlier posts

 

[Token]

Token, thank you for this great and detailed explanaition. So does this mean that the microwave source of modern (airborne) radars (TWTs) is generally not capable to deliver the same high peak pulse performance (I am not talking about average output) like old magnetron based radars? I have read the Irbis E of the Su-35 is built up with two TWTs just to get peak power up to 20 kW. On the other hand the AN/ASG-18 of the YF-12 is said to have had a peak output of 600 kW or more; similar numbers are available for the original interceptor version of the MIG-25). And this radar of course had a TWT and not a magnetron as microwave source.

In general, TWTs will not deliver the same high peak power that can be gotten in Magnetrons, however that is not the issue here. TWTs can deliver power levels north of 100 kW, it is just less common for them to do so, I have seen TWTs in the 1+ MW range.

Don't confuse the current design trends with an inability to make power. There are reasons, other than technical ability, that lower powers are used today.

They could easily build a TWT based airborne system in the 100+ kW range, there just is no need to on fighter type targets, and lots of reasons not to.

Klystrons are even better suited to high power than TWTs. But still most airborne Klystron based fighter systems will not be at the "old" power levels.

If you want specific examples, you can look at something like the CPI (the old Varian folks) VTX-5681 coupled cavity TWT. It is capable of 120 kW peak power and 35% duty cycle, for average powers of over 40 kW. But this would not be a good fit in an airborne application. Big, heavy, separate focusing magnet (with its added complexity), must be liquid cooled, higher operating voltage, etc.

Are you sure about that 600 kW level on the AN/ASG-18? That seems wrong to me for the time of development and technology that surrounded this. Lets be clear, I have zero first hand experience with the ASG-18, however it was a PD radar with fledgling TWS, and it evolved into the AWG-9. I do know a bit about the AWG-9, and it was nowhere near that kind of power level. The AWG-9 was a TWT based system, with peak power levels well below 50 kW. The AWG-9 is sometimes quoted as a "very high power transmitter", however this is a basic misunderstanding of the difference between peak and average power. It was a very high (for the day) duty cycle, high average power, moderate-to-low peak power, design. The PRF and duty cycle was so high, especially when combined with the TWS capability, that some ESM receivers called it, inaccurately, a CW transmission.

Ad P-61: I also think that William Wolf has mixed things up a little bit. I am aware that the SCR-720 with its helical scanning is not able to feed data into a computer (in contrary to the APG-1/2). BTW it was already well explained in earlier posts

One of the problems may be in the AN/APG-1/2 designation. I have seen indications that either the APG-1 or APG-2 (not sure which, but some sources say the -1) was the SCR-702 (the other of these two APGs was the SCR-580). If the SCR-702 and the (much more numerous) SCR-720 were both in different versions of the P-61, you can see how that might occasionally get transcribed or misunderstood.

T!

 

[basil]

Token, thx again. This is the first reasonable explanation I have read regarding the question about peak output.

The power of 600 kW for the Mig-25 radar is mentioned in "Modern Air Combat" by Bill Gunston, a book of the 1980s. Not sure if this is correct. I do not remember the source where I read about the peak power of the AN/ASG-18. However it used two TWTs in tandem to obtain the highest possible detection range with its 40 inch dish.

Of course stealth considerations are an important driver to limit power output but maximum detection range still seems to be a primary requirement for at least a certain class of "strategic" fighter aircraft like Mig-31, Su-27 follow-on series, F-22, etc. It would generally be interesting to know if modern high end fighter aircraft radar range performance is really the maximum possible or if it is more a balance between stealth and detection range.

 

[Token]

The power of 600 kW for the Mig-25 radar is mentioned in "Modern Air Combat" by Bill Gunston, a book of the 1980s. Not sure if this is correct. I do not remember the source where I read about the peak power of the AN/ASG-18. However it used two TWTs in tandem to obtain the highest possible detection range with its 40 inch dish.

There was more than one version of the MiG-25, and more than one radar used. I think you are probably talking about the Smerch-A, NATO reporting name Foxfire. The Foxfire radar of the -25A was one of the most powerful radars installed on any fighter any time.

But lets keep in mind the intended target of that airframe and radar, the B-70. The Foxfire radar was intended to track the Valkyrie at very long distances and in the presence of heavy countermeasures activity. It would have to "burn through" heavy jamming. And that takes heavy power.

Plus, the Russians often adopted a brute force approach were the west would have relied more on finesse.

And yes, several sources, including US mil sources (example "The MiG-25, A Very High Speed Interceptor and Reconnaissance Aircraft", Foreign Technology Division, Air Force Systems Command) list the Foxfire as 600 kW peak power.

But the Russians and the US would not have done things the same. Different thought processes arrive at different answers to similar problems. And it would really surprise me if the ASG-18 was in the same power range as the Foxfire, that just doesn't fit well with US trends of the time, and it certainly does not fit with the system that the ASG-18 eventually morphed into.

Of course stealth considerations are an important driver to limit power output but maximum detection range still seems to be a primary requirement for at least a certain class of "strategic" fighter aircraft like Mig-31, Su-27 follow-on series, F-22, etc. It would generally be interesting to know if modern high end fighter aircraft radar range performance is really the maximum possible or if it is more a balance between stealth and detection range.

If you are a stealthy platform (and all modern designs give some nod to detectability) you are probably going to limit your use of radar, relying more heavily on other sensors. If you do use radar it is going to be something like a cognitive LPI system, if possible. Raw power level is going to be less a factor than other waveform parameters. It is better to have the power available and turn the volume down, than to need it and not have it.

This is one of many reason magnetrons (and similar sources) have fallen out of favor. You tend to run a magnetron at max peak power all the time. But Klystrons and TWTs can be turned down in the linear region and you can control the output power better, turning it up when you need it and down when you don't.

Regardless, your radar, any radar, can be detected far beyond its useful, to you, range. Radars always tell other people as much about you as your radar tells you about them.

As for maximum ranges, this is more than a simple power problem. Your design parameters will determine the maximum range of your radar, and transmitter power is only one factor. You start with a desired probability of detection on a specific RCS sized target, the physical space and power you have available on the platform, the technologies and price points that are acceptable, and you work backwards from there.

Is there really any need to track a -10 dBsm target at 200 miles, with all the associated transmitter power and antenna size (this means size and weight on the aircraft, taking away from other systems or even basic fuel load), if you can only engage it at 40 miles? All that really does is tell the enemy where you are, long before you can do anything about them. It would be far better to use another information source, maybe a data link from a battle space management system, until a target is in range that you can do something about it.

However, lets think bigger picture. Why use that size and weight for only a radar transmitter? Would it be possible to combine functions, so that your radar transmitter is also part of your active ECM suite? With modern phased array you can form beams with your antenna, pointing in multiple directions in quick succession. Do they all have to be radar beams?

Thinking further afield, if you are stealthy platform, why give yourself away with a high power transmitter at all unless you have to? Why not use someone else's high power transmitter, say an AWACs, in a bistatic application? Their huge transmitter lights up the sky, your receiver uses that reflected energy to track targets.

No fighter is built primarily for 1 V 1 these days. Many V Many and integrated battlespaces are the way forward. Fighter weapons systems think as much about how to support / receive support from others as how to kill the enemy.

T!

 

[Token]

The power of 600 kW for the Mig-25 radar is mentioned in "Modern Air Combat" by Bill Gunston, a book of the 1980s. Not sure if this is correct. I do not remember the source where I read about the peak power of the AN/ASG-18. However it used two TWTs in tandem to obtain the highest possible detection range with its 40 inch dish.

OK, I again have to say I know next to nothing about the ASG-19, however I know a bit about the AWG-9 that the ASG-19 eventually became, and a good bit more about radar in general. So lets talk about TWTs, peak power, and "it used two TWTs in tandem to obtain the highest possible detection range with its 40 inch dish".

At the most basic level, a TWT is an amplifier. You put a low level (power) signal in and a higher level (power) signal comes out. As such a TWT has gain, gain that can be expressed as either a ratio or in dB, most often it is expressed in dB. Most TWTs have gain of greater than 30 dB (amplification of 1000 times), with gains up to 70 dB (amplification of 1000000 times) being pretty common. I have never worked with a transmitter side TWT (TWTs can also be used on the receiver side for front end amplifiers of low level signals) with less than 24 dB, and even that low is pretty unusual.

And a TWT also has a maximum peak power level. The gun, slow wave structure (or helix, depending on design specifics) and the collector are all designed for a specific maximum power level. TWTs are not highly efficient devices, with average efficiencies of under 50% (I have worked with TWTs as low as ~25% efficient). What this means is that if a TWT is 50% efficient, has a maximum peak power of 25 kW, and a maximum duty cycle of 20%, then the "system" of the TWT is designed for an average RF power out of about 5 kW, and about 10 kW, or more, of DC input power. In other words, to get that 5 kW of output RF power you have to put more than 10 kW of energy into the tube. The wasted 5+ kW must be dealt with as mostly heat energy, energy dumped into (primarily) the collector and that must be taken away so that the collector does not suffer thermal damage. I have seen failed TWTs reduce 50 lbs of copper collector to a puddle of molten copper.

But don't confuse the input power with the RF gain and power of the TWT. A TWT designed for a maximum RF peak power of 100 kW, a maximum RF average power of 10 kW (a 10% duty cycle), 50 dB of gain, and 50% efficiency, will require certain things to operate. First of all it will take an input RF signal of about 1 Watt peak power to drive the tube to saturation. That 1 Watt (+30 dBm) peak power will be amplified to 100 kW (+80 dBm) RF peak power. And if the tube is making its maximum average power (100 kW peak power at 10% duty cycle, for 10 kW average power) it will require more than 20 kW of average input DC power. This means that more than 10 kW of wasted energy, almost all of it heat, will have to be exhausted from the system in some way.

So TWTs have relatively high gain, and have maximum operating levels that are determined by their design.

This means that if you have two high power TWTs and put them in tandem, that is series, you do not add the power of the two tubes, you end up limited by the design power of the second tube. You don't just stack two 25 kW tubes in series and get 50 kW. The way to add the power of multiple TWTs is by putting them in parallel, not series, and combining the outputs in phase with a power combiner of some type. Two 25 kW tubes in parallel, properly combined, do end up with slightly under 50 kW of total output power. I have worked with transmitters with up to eight 100 kW peak TWTs in parallel, for a total peak power out on the order of 750 kW.

So, the AWG-9 did have two TWTs. However, they were not in series (tandem), they were in parallel. But in this case it was not done that way to add the peak power of the system, it was done to raise the average power by increasing the duty cycle. Each TWT performed different tasks. For example, one made the mid-course and CW illuminator for the AIM-54 Pheonix and AIM-120 missiles, while the other performed the tracking waveforms. Remember the AWG-9 could track up to 24 targets simultaneously and fire up to 6 Pheonix near simultaneously, the combination of these two waveforms (tracking and missile guidance) exceeded the duty cycle capability of a single TWT. The two TWTs shared the load.

I assume the AN/ASG-18 was similar to this, since that radar ended up becoming the AWG-9.

T!

 

[basil]

Understood and thanks for the clarification. It is a pleasure to have first hand experience persons as contributors here.
Some time ago I have read that current TWT development looks at microtip emitters (field emission array cathodes) to replace the electron gun although it has become a little bit quiet about it.
BTW is it correct that AESA radars (in contrary to PESA) do not have TWTs or klystrons as microwave source but instead generate the microwaves via semiconductors?

And another question which brings us back to the time frame of the original topic: Bill Gunston mentions in his book "Night Fighters" that a single F6F-5N was built up with two radar scanners (one on each wing). Do you have any idea what the reason could have been? It would be interesting to know if the scanners were used in parallel and how they were synchronized.

 

[basil]

... in addition to the last question above - the radar was an AN/APS-6.

 

[XBe02Drvr]

a single F6F-5N was built up with two radar scanners (one on each wing). Do you have any idea what the reason could have been?

A crusty old Chief at Avionics A School told us about a rig like that which he had seen when he was a young Airman. He said the old radar sets had a fairly narrow scan pattern (45° left and right), and they wanted wider coverage, so they put one on each wing. They were both aimed at 45° off straight ahead and linked to one scope and coordinated so the first scanned from 9 o'clock to 12, where the other took over and continued on to 3, then started back. On the scope it looked like a single sweep with 180° coverage.
He said it was some sort of experiment which was overtaken by the arrival of more capable and advanced equipment.
Cheers,
Wes

 

[basil]

Wes,
never thought that anybody had an answer to this because of the many decades back. Great, thank you.
So one crt showed the combined picture of both radar sets?

 

[XBe02Drvr]

Wes,
never thought that anybody had an answer to this because of the many decades back. Great, thank you.
So one crt showed the combined picture of both radar sets?

Well this was all word of mouth and long ago (1970), but as I remember it, he described a display that looked like half a PPI with the beam sweeping back and forth rather than rotating and with a small cross hair that slid up and down the centerline to display antenna elevation angle relative to horizon. (Gyro stabilized) A -o- icon displayed the interceptor's flight attitude. Apparently the goal was for the two sets to be so coordinated that the shared display would hand off seamlessly from one set to the other at the 12 o'clock position and all look like one. Theoretically, anyway. The fact that it was sitting off in the weeds neglected suggests it maybe wasn't one of the better ideas after all. I think the F3D was the hot new thing at the time.
Cheers,
Wes

 

[Token]

Some time ago I have read that current TWT development looks at microtip emitters (field emission array cathodes) to replace the electron gun although it has become a little bit quiet about it.

Spindt type FEAs have been a thing since the late 1960's. But lots of more traditional guns are in use.

BTW is it correct that AESA radars (in contrary to PESA) do not have TWTs or klystrons as microwave source but instead generate the microwaves via semiconductors?

There is no reason an AESA could not be built using TWTs or other vacuum tube based amplifiers, however yes, you pretty much always see AEAS with solid state T/R devices.

There is no real reason to use tube tech in an AESA. The advantage tubes bring is high power in a single device. Since AESA does not use a single high power RF source, it instead uses many low level sources working together, there simply is no reason to use tubes. Solid state is (and has been for quite a while now) capable of the multiple individual moderate power sources needed. AESA has many advantages, one of the really important ones is there is not, for either the transmit side or receive side, a single point of RF failure. If a T/R module fails, or even many of them up to several percent of the total number, the system still performs well, and it will degrade gracefully instead of going from working to completely dead with a single tube failure.

So, tubes are just not a thing with AESA, but yes, for a PESA you would benefit from the advantages in tube transmitters.

T!

 

[basil]

XBe02Drvr,

thx. Amazing to get these details after such a long time.