Combat aviation is at a crossroads, with the major western air powers set to launch new programs: the Next Generation Air Dominance in the US and the Future Combat Air Systems in France, Germany and the United Kingdom. While the first is aimed at counter-air operations, the latter is actually several separate national programs looking to define the whole air combat architecture of these countries from the 2030s onward.

Western air forces have enjoyed a relative superiority over the last decades: in the first Gulf War, the US dismantled the Soviet-style Iraqi air defense system and laid waste to the enemy the ground forces, NATO managed to assert its dominance in the skies over the Balkans in the 1990s, and the operations in Afghanistan or the second Gulf War did not see western air power being threatened.

However, it was only temporary. Russia got back on its feet after the fall of the USSR and rebuilt its forces, and China’s economic and military development now make it a peer competitor to the US. Both countries manufacture area/access denial systems aimed at keeping modern western air forces at a safe distance, and are selling them worldwide. As the chief of staff of the French Air force puts it:

“We are preoccupied by access denial, and I even more as an airman. No air operation, in fact no military operation, is possible without air dominance. Some countries develop airspace denial strategy to counter western air superiority. It is paramount we keep on working to preserve this air superiority. The trend is that efficient surface to air missile are sold to regional powers, making the situation more complicated in some theaters. Latest-generation fighters are also an issue regarding air to air combat. Cyber is also a concern. In air warfare, it is necessary to pierce through enemy defenses to get inside a theater of operations. To do that, we must know how the threats will evolve in the 2030s […] then develop technologies to maintain superiority. We have to work on stealth for instance, but also on weapons to suppress defenses. It’s what’s at stake in the next decade, because the future of combat aviation and airborne nuclear deterrent depends on it.”

Integrated Air Defense Systems

These access denial systems rely of a multitude of sensors and weapons. The main threat they pose is that of long-range surface to air missiles, like the Russian S-400, which can reach targets a few hundreds of kilometers away. Medium-range system with a range around a hundred kilometers are also a challenge, as they can have more missiles in store due to their smaller size. All these missiles often carry active sensors, so they don’t have to rely on illumination by a ground radar to home in on the target in their terminal attack phase.

Point defenses based on guns and short-range missiles can also pose a threat not only to enemy planes, but also to the munitions they fire. For instance, the Russian Pantsir S-1 carries both types of weapons and was designed to protect longer-ranger SAMs against anti-radiation missiles.

Regarding the sensors, there can be both passive sensors, or active ones. Passive sensors use the energy produced by the aircrafts: for instance, their radar emissions, which is why it is hard for a plane with its radar on to be stealthy. Another kind of emission is the infrared signature of the engines, which can be detected from a distance by InfraRed Search and Track sensors (IRST).

However, the most common way to detect planes remains radar. Its principle is simple: a radio wave is sent from the ground, hits the plane, and bounces back to the radar. Radars constitute the backbone of an air defense system from the early detection to the final weapon engagement. Consequently, all major world powers, and especially China and Russia, have developed modern flat-panel radars to ensure their safety.

These radars are often mobile, so that they can be quickly redeployed and are not sitting ducks waiting to be attacked by the enemy air force. Long-range radars can also be carried by early warning planes and heavy fighters. Such radars have an advantage: because they are at high altitudes, they can see further away and can see in valleys better, so they make terrain-hugging planes more detectable.

Radars and stealth

In practice, things are a bit more complicated than in theory: the size of a radar’s antenna plays a big role in its range and precision, and the wavelength (or the frequency) of the radio wave a radar uses also has an influence on these factors. Larger wavelengths are less accurate, so they require big antennas to have an exploitable precision. For instance, in the image above, the radar with the largest antenna operates in the VHF, with a wavelength around one meter.

One might wonder then, why Russia and China have gone to the trouble of developing metric-wavelength radars that are cumbersome to use. The answers lies in the development by the US of stealth airplanes, like the F-22 or the F-35. These planes are specially shaped so that their surface reflects short-wavelength waves away from the radar. In effect, the surface of the plane acts like a mirror for those wavelengths: when it is not perpendicular to the incoming radar wave, it reflects it at an angle.

Long-wavelength waves however have a different behaviour: as can be seen in the graph below, when the wavelength of the radar approaches the dimensions of the object to be detected, there is a big spike in the strength of the radar return. This is called Mie scattering.

This occurs not only when the wavelength matches the size of the whole plane, but also when it matches the size of the control surfaces, the weapons it carries, or anything that sticks out. That’s why all recent combat drone projects or the B-2 stealth bomber all have smooth shapes and no tails: it is necessary for long-wavelength stealth.

FCAS requirements

Air combat systems of the future will have to penetrate modern Integrated Air Defense Systems (IADS), with mobile long-wavelength radars networked to long-range missiles through highly directional links that can hardly be jammed or even detected. As the recent strikes in Syria showed, stealthy cruise missiles remain an option to penetrate such a system and destroy fixed objectives known well in advance.

However, cruise missiles are expensive and for most countries their numbers are very limited. Besides, they require extensive mission planning: a 3D model of the target has to be built and loaded into the missile, and the weapon’s trajectory has to be defined and optimized to avoid enemy radars using terrain masking. So if a large volume of fire is to be delivered in a theatre, it means cheaper, lighter munitions have to be used. It also means the enemy IADS has to be dealt with to be able to operate with acceptable risks. Unfortunately this IADS will probably be mobile and maneuver often enough to defeat long targeting loops.

Consequently, any combat system aiming at reducing the IADS will have to find its components among a wide theater of operations, and engage them quickly after detection lest they move.

FCAS recon & strike drone design

This requires bringing stealthy, high-performance sensors deep inside hostile territory, and doing so while flying high so that the sensors have a wide field of view, making nap-of-the-Earth flight and terrain masking a non-starter. So the only remaining option is to have a platform with broadband stealth, wide-area search sensors and carrying weapons able to penetrate the point defense found around the major IADS assets. While the Future Combat Air System proper will comprise manned fighters, tankers, cruise missiles and the command and control architecture linking them all, we will use the designation FCAS for this stealthy penetrating platform, which in all likelihood will be unmanned. This is supported for instance by statements from the French Defense procurement agency DGA:

“[The FCAS] will be a system of systems, with various networked platforms. There will not be one plane, but a mixed wing of piloted planes, reconnaissance drones or strike drones, hypersonic missiles and somewhere an AWACS or the successor of the AWACS.”

Sensors

The drone’s description sounds a lot like the flying wings shown above: they all have smooth shapes that minimize Mie scattering, they have a good planform for long-endurance flight, and some carry the right sensors for the job. For instance, the B-2 carries a Ku-band AN/APQ-181 radar with large antennas, which gives the radar a very narrow beam. This, plus other techniques to make the radar Low Probability of Intercept (LPI) make it very hard to detect, especially if it is not pointed right at the detector, in which case it might already be too late.

It is not a coincidence the B-2 is a good fit for the FCAS design: it was probably designed to hunt down mobile ICBM launchers located deep in the USSR’s IADS, so it had almost the same requirements.

LPI radar is one way to perform a wide-area search, and has the advantage of going through weather and being able to easily detect mobile vehicles. However, being LPI does not ensure the probability of detection is 0: for that, a fully passive system is required. Like the passive ground systems described above, it can have two components:

an emitter detection and location system, able to work over a wide range of wavelength so it can find low-frequency enemy search radars and high-frequency fire control radars. With luck, it might also come across the directional communication links networking the IADS. Being able to link several drone though tight, stealthy communication links would make it possible to efficiently triangulate emitters, giving a quicker location fix.

an optical search system, probably working in infrared to be effective at night and see better through haze, clouds and smoke. Since the goal is wide-area search, the system must have either a wide field of view or be able to scan along the aircraft’s path over a wide swath. This is the solution chosen for the French-British FCAS concept:

Like the radars, the optical sensors are duplicated so that they are not front-facing (which would reduce stealth) and can search both sides.

In order to minimize the communication bandwidth requirements, automatic target detection must be performed onboard the drone. It requires image processing hardware and efficient algorithms. The recent advances in object detection thanks to deep learning should enable an almost fully automatic search, with only a confirmation by a human operator on the ground or flying as far as possible from the threat area. The French optronics manufacturer Thales is already proposing to add such algorithms and hardware to its AEROS recon pod to quickly perform wide-area search.

The radars could also do double-duty as high-frequency jammers against fire control radars, but jamming has to be used in a smart way, or it will make the drones more detectable and more vulnerable.

Datalinks

As outlined before, the drones will have to carry a directional, low probability of intercept and jam-resistant datalink to share information among a patrol and help triangulate enemy positions using passive sensors. This is close to the MADL datalink carried by the F-35 for instance, which relies on phased array antennas spread over the surface of the plane.

Such a datalink can also be used to communicate with other platforms that stay further away from enemy forces because they are more vulnerable, such as tankers, AWACS and manned fighters. The radar horizon at 30 000ft being more than 600km away, if the drones and the supporting assets operate at this altitude they can maintain contact even if the drones are deep within hostile territory. Such links can be used to issue high-level orders to the drones, confirm target detection and authorize weapon use, thus keeping a human in the loop.

For longer-range communications, satellite communications are required. They allow direct control from the force’s operation center, but are expensive if high bandwidth is required, although laser communication might change this. Communication satellites are also highly vulnerable to counter-space operations.

Weapons

Once the IADS elements, or other high-priority targets, have been located, they have to be quickly engaged with weapons carried by the FCAS, so that the target does not have time to move. Since such important targets will often be protected with point defenses optimized to shoot down anti-radiation missiles and bombs, the weapons have to be able to penetrate this protection bubble. The cheapest way is probably to saturate the defenses with simple glide bombs: they carry no propulsion so can be lightweight and affordable, but still have ranges in the tens of kilometers. An optical or radar sensor, or laser guidance, might be necessary since GPS will probably be jammed around the targets. A weapons such as the GBU-53 has all these guidance modes plus a datalink, making it a good fit for this task. At only 113kg, a dozen can be carried for less than 1.5t, giving a good chance to overwhelm point defenses. The Chinese and Europeans are developing equivalent weapons, and the US has bought an even lighter version, the 30-kg Small Glide Munition

A rack of MDBA Smartglider light bombs (SDB II equivalents) A 113-kg GBU-53 SDB II A 30-kg GBU-69 Small Glide Munition

However, depending on how stealthy the drone is, the range of glide bombs might not be enough to close the distance to powerful search radars without being detected. Besides, their slow speed and low maneuverability makes them easy targets. Another option is then to keep using small munitions, but to give them propulsion. A design such as the 50-kg British Brimstone 2 missile, with a range of 60km and a supersonic speed thanks to its rocket engine, can be used. It also has a 3-mode seeker.

The French have developed a family of rocket-boosted glide bombs called AASM or HAMMER, with a variety of guidance modes: purely GPS+inertial, or with an infrared seeker added for metric precision, or with a laser seeker added. They range from 60km for the 250-kg version to 80+km for the 125kg version (which is not in production). The former appears in renders of the FR-UK FCAS, with 2 being carried in each weapon bay. However, one can wonder if such a small number of glide bombs can penetrate point defenses, although the canards at the front of the munition and the energy given by the booster make it very maneuverable in the terminal phase.

Another solution to give more range and speed to glide bombs is to give them a small turbojet, like MBDA proposes to do on the SPEAR III, which is basically a Brimstone with such an engine.

However while the range is claimed to be in the hundred of kilometers, the miniaturized engine has a higher complexity than a rocket, so its price must be higher. The weapon is also unlikely to be supersonic given its wing shape, making its survivability questionable. It can nevertheless graze the terrain to some degree, , which makes it harder to detect, and then gain altitude afterwards for the attack phase thanks to its sustained propulsion.

Instead of relying on lightweight munitions for saturation, the US Navy has opted for a different approach: it is procuring a new version of its AARGM anti-radar missile, with a bigger rocket booster for more range, and a lifting shape that should further increase the range and make evasive maneuvers possible. The missile will however not be small, filling a whole F-35 weapon bay, but it carries a heavy warhead, should have a range around 200km and has an dual active/passive radar seeker.

Using speed and maneuverability to defeat defenses is also the approach used by air-breathing hypersonic weapons currently in development, such as the Russian Zircon missile or the French ASN4G project.

Finally, while classical cruise missiles may not be a good fit for anti-IADS work, their anti-ship counterparts could. Indeed, they have to penetrate one of the worst kinds of IADS, a modern warship’s interlinked radars, missiles, point defenses and countermeasures, and do so without knowing exactly the position of the target. To do so, modern Western anti-ship missiles are stealthy, fly low, and perform automatic target recognition through infrared sensors. They also perform strong terminal evasive maneuvers to avoid gunfire. One such missile is the Norwegian JSM, derived from the Naval Strike Missile and repackaged to fit inside a F-35’s weapon bay. It has all the characteristics above, can also perform automatic target recognition of land targets and the Australian version will have a passive radar sensor.

The recent US LRASM is a comparable missile, with passive infrared and radar sensors, a small signature and a target recognition algorithm able to find the right target in a fleet, with a range around 500km.

While an anti-ship missile’s warhead might be oversized for anti-vehicle work, all their other features make them suited for anti-IADS tasks, so some air forces might find a way to exploit the commonality.

Conclusion

In order to maintain their superiority in the air, western powers will have to develop new platforms to hunt down and destroy enemy air defenses. Projects such as the B-21 and RQ-180 in the USA, the French and British combat drone prototypes and concepts are examples of broadband stealthy reconnaissance and strike aircraft that fill that need. The French-German Future Combat Air System that was recently announced in Berlin will also likely include a drone with similar design.

Further reading

On stealth

Bill Sweetman on the Taranis drone and broadband stealth

A series of article by Aviation Week on stealth:

stealth part 1

stealth part 2

stealth part 3

part 4

part 5

An aviation week podcast on stealth

Works on radar signature theory and modeling

Naval Post-graduate School course

Rough modeling of F-35 radar signature

On the Russia IADS