I think some things can be seen from the compressor stages of the GCAP engine model and the fuselage model modifications, but I am not a professional and completely welcome challenges to my views.
1
I saw some people on Twitter using the first picture to prove there's a variable cycle, because the engine in the first picture seems to have three ducts above but only two below.
However, from the reflection in the glass in the second picture where Shigeru Ishiba is, it can be seen that the so-called "third duct" above the engine in the first picture is actually an illusion caused by shadows or a pole.
Both pictures are from "DESI JAPAN" in May this year (although to my knowledge, this engine model first appeared at last year's Vandenberg Air Show). This corroborates the previous statements by engineers that the engine will not adopt a variable cycle, at least not at present.
2
The engine in both pictures has an integrated coaxial generator, which should be their confidence to increase the power generation of a single engine to 1MW.
3
Most strikingly, this monster engine has an 8-stage fan (accurately, low-pressure compressor), and a 10-stage high-pressure compressor, with a seemingly low bypass ratio.
This indicates that this engine is designed for a high-altitude, high-speed air superiority interceptor. Only when considering very high-altitude operating environments would so many compressor stages be designed. Air superiority interceptors are also widely recognized as the core requirements for both the UK and Japan.
At the same time, increasing so many compressor stages greatly improves the overall pressure ratio and adiabatic efficiency, leading to significant improvements in fuel efficiency at both high and low altitudes. However, considering they abandoned the variable cycle, and the surge challenge brought by low altitude, and most importantly, traditional engines are inherently good at high-altitude, high-speed performance, there is reason to believe that they will focus on optimizing high-altitude performance, while compromising on low-altitude and low-speed performance.
The CG animation and model of XA100 show it as an engine with a 2-stage low-pressure compressor and a 7-stage high-pressure compressor. We really have reason to believe that the thrust of the GCAP engine will be the world's highest at that time.
4
Momosir mentioned above that the IHI and RR models are two shafts. I don't know which RR engine he was referring to, but for now, it seems IHI's engine is temporarily using the XF9 model. It's clear that the monster engine is definitively the GCAP engine (at least the current placeholder version).
I personally cannot tell at all whether this engine is twin-spool or triple-spool, but so many compressor stages will bring huge surge challenges. I speculate that a triple-spool design is likely a way to solve this problem.
5
Although the appearance of the second version of the model is somewhat controversial on this forum (well, to be honest—ugly), there should be significant progress in all-aspect stealth. Although it is often said not to estimate RCS area with the naked eye, I believe physical reflection area still has reference value.
The first version had separate V-tails from the wings, with the V-tails connected outside the protruding engine at the rear. This meant that radar waves from ground radar and early warning aircraft with limited flight altitude could illuminate the tail, especially from the side view.
The second version integrated the wing and fuselage, enlarged the wing area, and moved the V-tails above the wings. Additionally, these V-tails are clearly much smaller than those of the F-35 and F-22, which means that radar waves from below will be blocked by the large wings, whether from the front or side. This significantly improves anti-long-wave radar effectiveness. As for radars on enemy sixth-generation fighters at the same altitude, a V-tail is not a major issue, as sixth-generation fighters primarily guard against long-wave radar, not to mention that V-tails can also bring advantages like maneuverability.
It's worth noting that there is a sixth-generation fighter, despite being touted as "tailless," has a huge third air intake that doesn't even offer maneuverability advantages. Yet, miraculously, in the internet military circles, being tailless means victory, and no one cares if it has something even stranger than a V-tail.
Considering the news published by bobbymike, BAE director Jonny Moreton stated that the GCAP second version model is definitely not the final shape, so we won't discuss this aspect too much further.
6
Variable cycle technology is not flawless; it also has its "troubles." The third duct would make the engine diameter larger.
This means that under the same technological conditions, if we want an engine with the diameter of an F119, we would get a variable cycle engine with less thrust than an F119; if we want an engine with F119-level thrust, we would get a variable cycle engine with a larger diameter than an F119. This directly relates to the frontal area and high-speed drag of the aircraft, and engineers must make trade-offs.
At the same time, the third duct will be completely closed in high-speed mode to reduce the bypass ratio, but the valves will still cause drag, and various variable structures will bring extra weight. These mechanical complexities are the price variable cycle pays for achieving "versatility."
Therefore, my idea is that the increased diameter and frontal area brought by the third duct of the variable cycle engine hinder its high-speed performance, possibly not performing better than the GCAP engine.
While traditional low-bypass ratio engines excel in high-speed conditions, they must anchor a compromise point, so their performance in any specific condition cannot be optimal.
The GCAP engine, however, has an exaggerated number of compressor stages, as well as lower drag and lighter weight compared to a variable cycle engine of the same class. This gives it excellent fuel efficiency in high-speed conditions, while also increasing fuel efficiency in low-speed conditions. So, while not as good as a variable cycle in low-speed environments, it is still better than a traditional fixed cycle.
That is, variable cycle can achieve a grade of B in all conditions.
A traditional low-bypass ratio fixed cycle engine, due to needing to anchor a compromise point in various conditions, can only achieve a C+ in its home ground of high-altitude, high-speed conditions, a C in low-altitude, high-speed conditions, and only a D in low-altitude, low-speed and high-altitude, low-speed conditions.
The GCAP engine, relying on its exaggerated compressor stages and optimization for high-speed conditions, can achieve an A in high-altitude, high-speed conditions, a B in low-altitude, high-speed conditions (facing surge challenges, but if engineers solve them well, it can still perform impressively), and the fuel efficiency improvement brought by the compressor stages, while not as good as variable cycle, can still achieve a C++ in low-altitude, low-speed and low-altitude, high-speed environments.
I guess these are what RR engineers meant by "the trade-off between the cost and the capability is a pretty fine one." Considering that the GCAP engine is already quite heavy due to its excessive compressor stages, perhaps they cannot accept adding variable cycle technology to build a larger and heavier engine.
Therefore, they focus more on thrust and frontal area, fuel efficiency in high-speed conditions, and believe that increasing compressor stages, adopting variable stator vanes, triple-spool design, and other technologies can optimize fuel efficiency to a certain extent. After all, they might prefer an 'air overlord' rather than an 'all-rounder'.
