When they are saying the 8ghz bandwidth is restricted by the bandwidth of the antenna does this only apply to AESA and PESA radars? (photo-detectors and electro-optical modulators determine bandwidth for Photon radars)

[USER=”77174″]panzerfeist1[/USER] – They are talking about the transmission portion of the signal. It doesn’t matter whether the transmit and receive portions are set up like an AESA or a PESA. In the test setup described in the nature article they electrically generated a signal centered at 5.5GHz with a 2GHz bandwidth. So, signal pulse would be between 4-5-6.5GHz. But then the optical elements multiplied that by a factor of four, and so what is transmitted out is a 18 to 26GHz pulse. The difference is your bandwidth = 8 Ghz. If you started out with a diode signal of a higher bandwidth your final radiated signal would then also have a higher bandwidth (it is always a factor of four higher in their setup). The opposite if you used a lower bandwidth signal. The setup (PESA or AESA) is completely irrelevant.

I wouldn’t be too confident that anybody will have a working LIDAR/ROFAR system operational in the field in the early 2020s. The article accurately stresses that there are still considerable unresolved difficulties. The core problem is that everything is analyzed digitally. This is the case because of something called the Nyquist Sampling Theorem (and corresponding Nyquist Frequency). You can look into that if you want but it all comes down to you have to sample to *just over* twice the frequency of the received signal to be able to properly analyze it without incurring aliasing. They used a 500 million samples per second analog to digital converter in their setup. That is pretty good. It gets hard to make (and really expensive) ADC that samples much faster and still provides good signal fidelity.

Taking Nyquist into account, a 500Msamp/s ADC lets you correctly and with full precision observe a 249.999MHz signal. This is way less than their 5.5GHz transmitted signal (from the laser diode), but they take care of that in the end with the part labeled “PM” in the figure.

On the transmit side, they multiplied (optically) by a factor of four. They mention that a multiple of up to eighteen is now possible, but towards the end when they are talking about the problems in current ROFAR technology, they affirm that signal to noise ratio plummets just like with electrical multipliers. No current advantage. Something else to consider is that multipliers provide you with a wide bandwidth which comes with disadvantages as well as advantages. A RWR is a device that continually sweeps through its designed spectrum. The larger the bandwidth that you transmit the greater the likelihood that it will detect you. For low probability of intercept radar, you actually want a low bandwidth pulse, and you want to jump around in frequency often. Engineering is all about tradeoffs.

On the receiver side, a ROFAR system would seem to offer certain advantages in compressing the signal back into something that can actually be sampled. With a typical radar you have to down convert it electrically which takes a complicated circuit. Here, you are still working optically and so the trick is to slow the incoming wave signal down with a material with the correct refractive index. It’s now just a material property. That’s an obvious advantage. The rest of the receiver portion is just amplifiers and filters to prep the signal for the ADC. And there is signal degradation in all of this (lowered SNR).

In summary, the problems with ROFAR now is:

1) Signal degradation in upconversion and downconversion is similar for moderate multipliers.

2) Extremely high frequencies can be achieved (useful for detecting stealth aircraft) only by exceptional multiplication factors. But this causes increased degradation of the signal, and it creates enormous bandwidths. The extremely high bandwidths (really just the upper portion of them) helps to counter stealth technology, but they make your radar easier to pick up by a RWR, AND the increased degradation of SNR together with higher atmospheric attenuation at higher frequencies means aircraft equipped with a ROFAR system is actually at a disadvantage vice non-stealth aircraft with current radar systems.

3) The negative effects could be diminished while allowing higher frequencies by designing faster ADCs, but this is true with current radar systems as well.

It was a good read, but trust me, there is a lot of work to do in that field to make it practical in the field.

[USER=”5300″]Blueshark[/USER] and Blitzo – The last bit of talk about the Ws-15 was that it still had a few minor problems but they were confident that they would be resolved “soon”. Translation (if a true statement): they will fly it within the next 12-18 months. Soon means they will probably have a solution in just a few months but then you want to run it for a while on the bench to make certain that your fix didn’t create any new, and unanticipated, issues. It’s likely to still be a while.

PD-35 is a scaled up, evolved version of the core. They are upscaling the PD-14 design and adding in some new technologies such as CMCs and a carbon fiber epoxy fan. It’s not a “from the ground up” new design. PD-10 likely will not happen, at least not any time soon. The Russian aerospace engine industry has a full plate with continued PD-14 teething support (if needed), and then Izdeliye 30, NK-32 series 2, PD-12V, PD-35, etc. development.

Bypass ratio is not as closely correlated with fuel efficiency as you suppose. You have three main sections in a turbofan – the fan, the compressor, and the turbine. (You got to take the different characteristics of the HP and LP sections of the turbine and compressor too.) Each of these have their individual “ideal” point for rotational speed to yield maximum efficiency. Trouble is, you got only one main shaft that they are all coupled to. You can divide this into two or three spools to provide for a little bit of optimization between them, but not much. (Unless if you put a hefty gear mechanism between the fan and the compressor, because the fan has the largest difference in optimum rotational speed).

So, to make use of a larger diameter fan – a greater bypass ratio – you need to design a compressor and turbine that can operate with a slower rotational speed. In practice, you might lose a little bit of efficiency in the turbine and compressor. Taken together, you generally still see a slight gain. Try to compare all factors (if you can find them) – fuel consumption, noise emissions, NOX emissions, and electrical power provided – and you will likely see the LEAP getting the leg up by just a little everywhere. But both designs are so efficient that you are going to have to look at everything to see much difference.

Pretty cool stuff, Dr. Snufflebug. It looks like they are using optical time domain reflectometry. They basically emit pulses of light through the fiber optic network and measure the signal that gets reflected back. When the skin material is stressed it changes its refractive index – leading to a change in the signal that is reflected back. So you can measure any and all stresses. The hard part (what will require considerable characterization and verification) is distinguishing between normal stresses (such as incurred under a high g maneuver) and abnormal stresses (caused by damage). They put a hefty computer in the plane; they will be glad they did! Once they’ve determined the normal stress patterns then it is just a matter of computational power.

I’m not surprised that they are continuing to tinker and improve a little. Izdeliye 30’s testing is likely to go well into 2020, and so other concurrent work is almost free. But when are they going to stop? They might be able to verify the wing by that point (or up to 12 months later) but are they going to do the vertical and horizontal tails too? That will take more time. At some point they have to declare the aircraft to be done enough. I am concerned that it appears that the project is still open ended.

Yeah, but can the Su-57 itself got to 20.000 meter in the first place ?

If the publicized performance figures that float around are accurate then it should. It’s service ceiling should be around 65k feet.

I mean typical fighter combat ceiling would be 12000 (39000 ft)

That is for a legacy fighter which carries all weapons externally. Service ceiling is defined for clean wing condition. Since legacy fighters carry all ordinance on external pylons (adding both weight and drag) their actual useful ceiling under combat conditions drops significantly from their designed service ceiling. But a 5th gen fighter can carry weapons and still keep its wings clean. You may, in some cases, still have to factor in for the extra weight of weapons. But the Su-57 is believed to have fuel capacity of more than 20,000lbs. A weapon of a similar size to the Kh-58 would weigh around 1,500lbs (maybe just a bit more). So you can still reach service ceiling by reducing the fuel load to compensate.

The Su-57 should be able to deploy such weapons from much higher altitudes…unless if it has all of its pylons loaded in beast mode. Or if this really does turn out to be just a slightly smaller, external weapon. And I’m with St. John on that. Such a weapon doesn’t seem useful.

[USER=”70376″]stealthflanker[/USER] – Is that taking into account the fact that the Su-57 can deploy the weapon from 65,000ft? With most weapons shots the Su-57 will actually be able to get a little more range (and peak velocity) vice the Su-30/34/35 just by virtue of deploying at altitudes where the air is thinner. It won’t be a day vs. night difference but even a few percent of increase will make the weapon more difficult for defenses to keep up with.

I agree that it appears to be a mock-up. The J-20 has a smooth transition from a flat surfaced top to the contoured rear part housing the engines. Plus, that doesn’t even seem to be housing engines. There aren’t any exhaust petals and the characteristic space between the exhaust nozzle and the rear fuselage isn’t there on this mockup.

But the pics don’t match with the satellite photos. The pictures show a flat topped roof on the building. That satellite image shows a slanted roof. Interesting story.

Fantastic news for the F-35 program, but it likely means the domestic program for a 5G fighter is dead. Maybe they will join with a nascent 6th gen program instead?

[USER=”77048″]St. John[/USER] – 40 of the B variant seems excessive for just the two ships. The standard aviation compliment for the Izumo class is just nine helis. I’m sure you can do more, and you can perhaps pack the F-35’s a little tighter. But they aren’t going to be able to handle more than 14-16 combined fighters and helis while conducting ops. There just isn’t enough deck space. I think 25 to 30 would be a more reasonable estimate. That would still leave some spares for training and such.