A ‘Dark Knight’ and contrail suppression – will stealth and visibility collide? An intellectual property perspective.

Article | April 28, 2023

A ‘Dark Knight’ and contrail suppression – will stealth and visibility collide? An intellectual property perspective.

A ‘Dark Knight’ and contrail suppression – will stealth and visibility collide? An intellectual property perspective.


Like most people dealing routinely in technology, I have spent my share of time delving into the world of aerial drones. Many years ago I purchased a new-to-me DJI F550 flame wheel hexacopter. It wasn’t tuned right, however, so my initial foray into this world involved a reassembly exercise that journeyed me through the core components and gave me an understanding as to how these things operate.

In doing this, it doesn’t take long to get a real feel for the power that the (increasingly) democratisation of this technology possesses – not only for the fun that can be had flying the various platforms around the local park, but for ‘other’ purposes.

It’s actually quite scary once you start to consider how simple these things can be to build so as to be ‘mission’ ready.

My F550 was remote controlled using (now redundant) DJI proprietary componentry. As expected, proprietary componentry places inherent constraints on the operational features available. However, I soon discovered the open-source world of aerial drone components, firmware, and mission planning software. Suddenly, everything took on a whole new level of scary – any perceived limits of operation dissolved completely, leaving a scope of possibility that was limited only by one’s ethics and moral firmware.

I hadn’t really considered the possibility of a propulsion system for a drone being anything other than propellers. This changed one day at work years ago when I came across jet engine technology being developed by an Australian company for use in autonomous target drones.

As I discovered, this isn’t very new – it’s just now become far more advanced.

Fast paced development

From looking into recent patent filing activity (which is relatively straight forward to interrogate due to the very well-structured nature of patent databases these days), it is clear that this technology is developing at an extremely fast rate, expanding the limits of what is possible.

Research conducted by Finnegan.com reports that the issuance of drone related patents in the US rose around 660% from 2015 to 2022. Interestingly, this time period aligns with the regulatory framework implemented by the Federal Aviation Administration (FAA) in 2015. Prior, the FAA issued flying clearances on an application basis resulting in back logs of flying approvals being awarded – in a way, choking operational activity.

It therefore seems that with clearer regulation in place a more stable operating environment has been provided giving commercial industry more comfort in investing in IP protection for drone technology.

A pinch of stealth

Of course, drivers of the commercial drone industry often differ from those steering military development. For intelligence gathering purposes, being invisible presents significant opportunity – this is, of course, a huge understatement.

When you inject a ‘stealth’ flavour into this eclectic mix of ‘autonomous’, ‘unmanned’, ‘aerial’ ingredients, you begin to procure the ability to reshape modern warfare. For example, there is now no need to risk troops in reconnaissance missions – send in a (sacrificial) drone (very quickly), get what you need (eg. target identification, intelligence gathering), and get out before being seen.

The underlying advantage of ‘stealth technology’ as applied to aircraft is the reduction in detection by air defense systems – this is due to the reduction of radar and infrared signatures of the aircraft. This is achieved by reducing radar reflection back toward the radar emitter. This has been shown to be achievable in a number of ways: by designing the airframes using orthogonally aligned metal panels forming corner reflectors, avoidance of any external bumps or protrusions in the frame, parallel alignment of edges or even surfaces, burying the engines within the wing or fuselage, specially developed radar deflecting coatings, use of internal construction techniques that trap and confine penetrating radar waves within the airframe structure.

Introducing Australia’s ‘Ghost Bat’

On 27 February 2021, Boeing Australia and the Royal Australian Air Force successfully completed the first test flight of the ‘Loyal Wingman’ unmanned aircraft – 2 years after only revealing the program to the public at the Australian Airshow outside of Melbourne. Considering the feature rich capability promised, this development time is very impressive.

On 21 March 2022, the official designator for this aerial vehicle was released to the public as the ‘MQ-28 Ghost Bat’.

I think it looks pretty cool – but that’s just me.

In what I think is a fantastic achievement for the Australian aerospace industry, the Ghost Bat represents the first military aircraft to be designed, manufactured and flown in Australia in over 50 years – designed from a ‘blank canvas’ by a team of multi-disciplinary specialists.

Of course, much is secret and still in development, but we are told that the Ghost Bat is primarily designed to fly autonomously alongside, and in controllable association with, crewed aircraft (eg. fighter jets) as would (consistent with its initial name tag) a ‘Loyal Wingman’.

Even with this brief role description, it doesn’t take too much imagination to appreciate what could be possible with what we know of this high-tech platform.

We understand that the Ghost Bat incorporates low-observable stealth characteristics, high fighter like maneuverability, advanced avionics, first-of-its-kind autonomous and artificial intelligence systems allowing it to perform a range of mission types, a digital open-architecture ‘back-bone’ which enables integration of new sensor packages allowing this ‘Dark Knight’ to undertake intelligence, surveillance and reconnaissance (ISR) and air combat operations through to tactical early warning missions.

With political tensions becoming more prominent in the Asia-Pacific region, its easy to understand the benefits of the perceived capability of the Ghost Bat to Australia. The ‘wingman’ tag could be simple misdirection – could the stealth flavoured, autonomous flight capable, AI loaded Ghost Bat provide the invisible intelligence gathering platform that extends Australia’s reconnaissance reach (ie. spy missions) without anyone becoming the wiser?

The short answer based on what we (are allowed to) know: it could very well be the case.

But it still remains to be seen what steps have been taken to mitigate one of covert aviation’s oldest inconveniences – the formation of contrails, the suppression of which continues to be the focus of development efforts from a number of high-profile aerospace OEMs. We know this through what can be revealed through patent literature.

The problem of contrail formation

The issue of contrail visibility extends back to World War II, and has revealed the presence of mystery ‘bat-winged’ aircraft as recent as 2020.

During WWII, contrails produced by approaching US bombers were greatly exploited by German fighter pilots – the contrails could be easily seen long before the aircraft were actually visible, allowing German pilots to refine their attack runs with deadly precision.

As we’ve all experienced during winter times, condensation is formed by warm, moisture-laden air mixing with cold, dry air. For the specific case of contrails, the condensation forms as tiny ice crystals – which form around very tiny particles output from engine exhaust systems.

Interestingly, as aircraft propulsions systems moved from piston engines to jet turbines, contrail development became more distinct.

Could contrail visibility be the Ghost Bat’s Achilles heel in revealing Australia’s intelligence gathering endeavours to its neighbours?

The bottom line seems to be that contrail suppression is still an ongoing problem, but various approaches have been used to manage the issue.

Early fighter pilots discovered that changes in altitude (to regions of air having different characteristics) could eliminate contrail development. In those days pilots had limited visibility behind them making it difficult to monitor contrail formation before it was too late – this led to the inclusion of rear-view mirrors being used for spy missions. A more technologically advanced form of the rear-view mirror was later developed using a form of light detection and ranging (LiDAR) system which emitted a laser beam into the gas exhaust to measure the scattering of light from ice particles (if present) thereby enabling the pilot to be warned of imminent contrail development.

If changes in altitude were not palatable, other solutions were needed. In terms of military development now known, attempts at modifying the combustion output using various sulphur fuel mixes were trialed but with inconsistent success. Injections of acid compositions into the exhaust mix provided some success – this concept didn’t eliminate contrail formation but made them much less visible by reducing the particle size of the ice crystals. The problem with this solution was in the amount of equipment needed to be carried (hardware and acid solution) and the corrosive nature of the acid composition – not to mention the high level of toxicity dispersible into the atmosphere (not too much of a concern for spy missions). Sophisticated software models which predict where contrails are likely to form have also been used, enabling stealth missions to be rerouted as needed to reduce the risk of contrail management.

Of course, any successful developments in contrail development by the military are likely to be closely guarded as trade secrets – so there could be other technologies in play dealing with contrail development that are not (yet) public.

Response from the commercial aviation sector appears to be significant – largely due to global pressures to be seen to be environmentally responsible. Research suggests that a warming effect from contrails (which can disperse significantly from formation to form high-altitude cirrus clouds) can be greater than CO2 emission from burnt aviation fuel. Therefore, other than stealthy motivations, there is a very green reason to get on top of contrail development.

The intellectual property perspective

As I’m sure all readers are aware, technology developers are incentivised by patent systems to secure exclusive rights for their technologies so that they can pursue commercial advantages – for the case of OEMs, allowing them opportunity to secure and/or increase market share. Exclusive rights conferred by patents are rewarded in return for a full disclosure of the technology to the public.

While publication of a new technology is something defence departments (who largely opt for a trade secret approach) will necessarily consider very carefully, it is an accepted necessity for the commercial sector.

Patent databases are very well structured enabling many different approaches for searching as efficiently as possible through a diverse range of technologies.

So, we can learn a lot about what patent applicants are up to by interrogating patent databases – in my view, these are extremely valuable resources for helping one keep a stealthy eye on what their competitors are up to, but also providing what could be valuable inspiration in helping solve a problem of (commercial) concern. Many new patented technologies have been derived from inspiration gained from patent literature.

Back to contrail suppression

A good example of what can be learned by ‘spying’ through patent databases is by looking at some technology developed by Rolls-Royce that provides a new type of gas turbine engine as disclosed in US 7,971,438 (US’438). US’438 was granted on 5 July 2011 and is currently in force in the US, and Rolls-Royce has also secured rights to the technology in Europe.

Without getting too technical, US’438 concerns a gas turbine engine having a condensation stage in a heat exchanger arrangement configured operable so that the level of condensation within a final exhaust gas flow from an engine is reduced. Broadly, this technology seeks to remove water content within the exhaust gas flow to reduce condensation (contrail) formation.

Its interesting stuff. If the powerplant selected by the Ghost Bat derives from Rolls-Royce, then it is possible that the technology of US’438 is included.

For the case of multiple engines, Rolls-Royce also provides a solution as disclosed in US patent 9,399,521 (US’521). US’521 was granted on 26 July 2016 and is currently in force in the US.

Again, keeping things simple, US’521 concerns an aircraft propulsion control system involving multiple gas turbine engines spaced about an airframe. Sensors are arranged to sense ‘a condition indicative of vapor trail formation’ by an exhaust flow from any of the engines. A controller is arranged to be responsive to a thrust demand (eg. by adjusting the fuel composition supplied to each engine) for the aircraft and to selectively control any of the engines so as to modify their respective exhaust flows relative to any of the other engines according to their positioning on the airframe. The proximity of an engine to a vortical wake flow created by the airframe may be used to determine suitable exhaust flow modifications.

US’521 explains that the technology is beneficial because different portions of the airframe will induce different contributions to the wake caused by the aircraft. Rolls-Royce have determined that the location of the engine exhaust flow relative to certain portions or flow phenomena of the aircraft wake can reduce vapour trail characteristics compared to other locations of the airframe. In effect, the advantage of the technology seems to be in the reduction of the net/collective ‘optical depth’ of any vapour trail formed by the engine exhaust. Optical depth is the attenuation of energy emitted for passing through a formed vapour trail – US’521 explains that the source of energy emitted could be light or sound waves.

Again, very interesting stuff. Below is a figure provided with US’521 showing one example of how the technology can be implemented.

I note that gas turbine exhaust management seems to be a popular focus for Rolls-Royce, as well as other key engine OEMs in the aerospace sector.

The bottom line

The Ghost Bat represents some cool new drone technology. And it’s a fantastic achievement for the Australian aerospace industry in terms of the technology package and the development time.

But given the basis of its existence, how will the Ghost Bat deal with the age old problem of contrail formation.

While there are a number of technical solutions that the Ghost Bat could be ladened with to mitigate the risk of contrail development, nothing seems to be completely perfect, … that we currently know about.

Based on my very basic research, while there is a significant green reason driving development of contrail management in the commercial sector, mitigation still seems to be the best approach possible.

If the Ghost Bat is ultimately equipped with an off-the-shelf power plant for in-service operation, there is a good chance we can learn more about what contrail management strategy the design team have opted for as exhaust management seems to be a strong development thread being explored by key aerospace OEMs.

If contrail elimination has been successfully achieved, then it is unlikely that we (the public) will learn about it anytime soon. If not, surely the best mitigation strategy currently available will be used until superseded.


Dan Wadsworth

Patent Attorney | Special Counsel