Variable cycle engine: technology advances smoothly, application still controversial

The road to the development of aviation engines is difficult and long, and it also requires huge investment when put into use, so it is understandable to be cautious.

On September 7th, GE announced that it has started testing the second XA100 adaptive cycle engine on its high-altitude test bench. The first engine of XA100 began testing in December 2020, and now the testing of the second engine will enable GE to continue collecting more comprehensive test data, further maturing the design of the engine.

These two XA100s are also the only two prototype engines that GE had previously planned in the AETP program. GE stated that XA100 integrates three key innovations: adaptive engine cycle, three channel architecture, and extensive use of advanced component technology. The use of these innovative technologies has increased the thrust of XA100 by 10% and fuel efficiency by 25% compared to engines of the same size.

After completing the first phase of testing at GE, the second XA100 will continue to be tested at the Arnold Engineering Development Complex (AEDC) of the United States Air Force to meet the testing requirements of the Air Force's Adaptive Engine Conversion Program (AETP). XA100 is a product of GE's AETP program, designed to be directly mounted on and integrated with the F-35 fighter jet according to AETP requirements.

However, although GE's XA100 testing is progressing smoothly, the idea of the AETP program requiring its products to be directly replaceable for the F-35 has recently encountered some controversy.

Is the replacement of the current F-35 encountering variables?

The US Air Force has recently expressed the view that this new type of "adaptive engine" is significantly different from the engines currently used on the F-35 fighter jet, and may not be directly retrofitted as a power unit for existing models. The person who put forward this viewpoint is General Dale White, the head of the relevant departments of the US Air Force. He expressed his views to reporters at a seminar held at the US Air Force Lifecycle Management Center on August 12th.

General White stated that the current focus of the US Air Force is on how to fully utilize existing equipment to maximize combat effectiveness, rather than looking for new possibilities for the future. Compared to current engines, adaptive variable cycle engines are very different in terms of technology and usage, and replacing and upgrading engines for current aircraft is a complex matter that must consider how much return on investment such an investment can bring. Therefore, this variable cycle engine technology is more likely to be applicable to the next generation of fighter jets and other future projects.

However, in subsequent speeches, some people stated that General White, although responsible for most of the aircraft's engine projects, is not specifically in charge of the F-35, and his remarks are not feedback or implication on the F-35's plan to replace it with a variable cycle engine. In response, the F-35 Joint Program Office stated that the current requirement for the F-35 is not to replace it with an adaptive variable cycle engine, but they are working with relevant manufacturers to evaluate this new technology for future use on the F-35.

In fact, statements similar to those made by General White have not appeared for the first time. For example, in the spring of this year, a senior leader of the US Air Force stated that it was not possible to afford a comprehensive replacement of existing engines within the expected budget range. The newly appointed Secretary of the Air Force, Frank Kendall, also stated in an interview that future versions of the F-35 fighter jet are more likely to use new engines developed with adaptive cycle change technology rather than replacing the existing F-35. （He added: “There are also some technologies that could go into future upgrades that could reduce some of the operational costs, such as fuel, significantly, but we’re not ready to commit to those at this point.” He nodded when asked if he was referring to a “next-generation engine.”）

For such debates, engine manufacturers naturally will not ignore them, as it is related to whether their huge investment can be rewarded. For example, the latest statement from Pratt&Whitney, the manufacturer of the current F-35 engine, is that improvements can be made to the F135 engine to increase thrust and efficiency, and the cost of the improvements is much lower than replacing the adaptive variable cycle engine.

Jennifer Ratka, Vice President in charge of the F135 engine project at Pratt&Whitney, stated that the current AETP technology is only applicable to the F-35A and F-35C models, and cannot be applied to the F-35B with vertical/short takeoff and landing capabilities; If another engine besides AETP is to be developed for the F-35B, the budget for the F-35 replacement program over its 50 year lifecycle may increase to $40 billion. Therefore, although General White may not have calculated the "$40 billion" figure in detail before, his concerns about the cost of swapping are not unfounded.

The improvement plan for Pratt&Whitney's F135 is based on the upgrade of the F-35 fighter jet, and the improved engine can use the same production and maintenance facilities, so the cost is relatively low. Regarding the upgrade project itself, Pratt&Whitney stated that it can increase the thrust and fuel efficiency of the F135 by 10% each, increase the thrust of the F-35B by 5% during vertical takeoff and landing, and enhance its thermal management capability by 50%.

In the Block 4 upgrade program of the F-35, the aircraft is required to carry heavier payloads and stronger electronic capabilities, which requires more cooling. Without better engines, the upgrade of Block 4 may not achieve the expected goals. Of course, Ratka believes that the current F135 "has no defects", but the upgrade of the F-35 has exceeded the performance requirements of the engine at the beginning, so the upgrade is necessary, and it is hoped that it can be achieved once throughout the entire lifecycle of the F-35 project.

Regarding the AETP project, Ratka did not comment on whether Pratt&Whitney's XA101 is suitable for the replacement of the F-35. He only said that the engine has always been intended to be the power unit of the sixth generation aircraft, and also believes that the new technology carries a lot of risks and requires a lot of validation work, so it is not suitable for the current F-35.

Advancing technological research and development is the key

It is not difficult to see from the views of different individuals above that the plan to replace the F-35 with a new engine within the US military seems to be changing, but the US military still holds a supportive attitude towards the technological development of the new engine itself.

General Charles Brown, the Chief of Staff of the United States Air Force, stated on September 8th that funding for the development of the new F-35 engine should continue, even if it ultimately turns out to be a pile of scrap metal.

Previously, the US House Armed Services Committee passed a 2022 defense policy bill that requires the US Department of Defense to explain the necessity of installing better engines on the F-35. Therefore, Brown expressed the above viewpoint and believed that the development of new technologies is necessary, so that there will be opportunities for choice in the future; If we stop research and development, we won't even have the opportunity to choose in the future.

Source: International Airlines