DISRUPTIVE & ATTRITABLE TURBOJET ENGINES FOR FUTURE UAV-BASED C-UAS PLATFORMS

Take a more thorough look into the challenges of UAS threats we are increasingly facing today, and find out more about how the use of smart gas turbine engines can save lives tomorrow.

1. COUNTER-UAS TECHNOLOGY CLASSIFICATION

Counter-drone technology, also categorized as C-UAS (Counter-Unmanned Aerial System), industry has surged rapidly in the past 5 years. In 2019 there were 537 dedicated C-UAS systems in use globally. The C-UAS industry is very extensive and needs to address countering solutions for UAVs with various size, speed and technology. [1]

To put things in perspective – out of the operatable and dedicated 537 C-UAS systems, only 6,33 % are integrated on drones. The vast majority of the identified systems is ground based. These 6,33 % represent UAV-based C-UAS systems of various drone sizes which are intended to deny growing threats within the unmanned aviation sector. [2]

Unmanned aerial systems can be classified into 5 groups according to U.S. Department of Defense. Depending on the size, altitude and speed of the UAS, diverse C-UAS technology is needed for detection and interdiction methods. [3]

 According to the U.S. DoD classification 3 we can further classify the UAV-based C-UAS solutions likewise. To address the threats possessed by the various UAS we can assume that the countering-UAV architecture of the Group 1 and the Group 2 UAS targets will be based on small platforms (e.g., quadcopters) which integrate an electric engine powering its rotors.

Aside from the VTOL UAV-based C-UAS systems (like the quadcopter) the Group 1 and Group 2 UAS can also be countered with non-VTOL platforms which integrate an electric engine powering a propeller.

Countering the Group 3 UAS which have service ceiling of up to 18 000 ft. and MTOW of almost 600 kg. requires different approaches than the ones used for the Group 1 and Group 2. Most UAV-based solutions in this category will feature a more powerful energy generator – e.g., a turbojet engine which can be used to leverage the UAV-based C-UAS system in multiple ways described later in the arctile more thoroughly.

C-UAS Counter-Swarm Technology Combats Threat Platform | Lockheed Martin MORFIUS [7]

2. UAV-BASED C-UAS PLATFORM INTEGRATING A SMALL-SCALE GAS TURBINE ENGINE

Using a small-scale highly efficient turbojet engine over a electric or small-scale rocket engine leverages the platform’s performance in multiple aspects. The following is considered to be the principal advantages while countering Group 3 UAS threats with the use of a turbojet equipped platform:

• Immense energy density.
• Fire & forget missions’ availability.
• Time frame reduction for threat acquisition, detection & interdiction.

The specific energy density of gas turbine engine fuel stands at 43.15 MJ/kg. The specific energy of today’s battery accumulators (Lithium-ion) stands at 0.875 MJ/kg making the power to weight ratio of small-scale gas turbine engine superior when compared to batteries powering an electric motor. Immense energy density is thus achieved for both propulsion and very light weight power generation system for the on-board systems, respectively for the non-kinetic effector. [5]

Fire & forget UAV ability is today’s standard within other non-C-UAS turbojet equipped platforms. These platforms include missile guidance kits, cruise missiles, loitering munitions and other smart UAVs and have been proven to be very successful lately in Europe, US and other nations like Turkey or the UAE. The platform types listed above share the benefits of small-scale gas turbines such as greater manouverability (e.g., when compared to solid fuel rocket engines) or extended range. The same principle of leveraging the benefits is expected to be seen also within the C-UAS market.

Vastly adjustable/controllable flight true air speed range of gas turbine engines (0 – 0,9 M) ensures the platforms maximum versatility. The ability to control the speeds enables the C-UAS platform to both loiter or impact directly the threatening UAS. Increased manoeuvrability and final flight stage increased thrust/velocity result in reduced time for the final interdiction of the threat.

C-UAS Platform Integrating a Small Scale Turbojet Engine | Raytheon COYOTE [8]

3. DISRUPTIVE & ATTRITABLE TURBOJET ENGINES DEVELOPMENT APPROACHES

Smart UAV-based C-UAS platforms demand engine solutions with the following features:

• Rapid start sequence
• Excellent thrust/weight ratio
• Increased longitudinal, vertical & lateral flight loads
• Governable thrust control
• Small diameter
• Sufficient electrical power availability for on-board systems & effectors

Attritable Highly Powerful Small Scale Turbojet Engine | PBS TJ-40 [6]

The features highlighted above can be looked upon in the terms of performance and costs. Lately highly emerging markets within the single-use platforms brought pressure and innovation drive to the engine makers. The performance level has been continuously increased and the costs of the engines needed to be reduced in order to meet the attritable (single-use) engine concept.

Specific small scale gas turbine engines (50-250 lbf thrust level) innovations towards attritable concepts can be demonstrated on the PBS Velka Bites engine development program in the past several years. The specific R&D results are described in the following paragraphs. [6]

Rapid start sequence within engine program R&D activities is possible to be achieved with the integration of a pyro ignition system to the attritable turbojet engine core. The time to initiate the start of the engine is then reliant only on the time frame in which the air-fuel mixture is ignited using the pyrotechnic effect.

Innovation within the Thrust-to-Weight ratio (valid for standard conditions t0 = 15 °C, p0 = 101.325 kPa, v0 = 0 km/h) can be demonstrated on confidential last year research which resulted into an increase of the value from 11,03 to 11,87 as shown in calculation below:

Turbojet Engine Thrust-to-Weight Calculation | PBS TJ-40 [6]

Increased longitudinal, vertical & lateral flight loads within the manoeuvring limitations have been innovated to the following levels for the TJ40 product line and demonstrate the ability to start the engine in any position which allows UAV-based C-UAS platforms advantageous integration. [6]

Final enhancements towards very small engine diameter and increased power output have also been achieved within the PBS engine product line in the past years. Demonstration of this innovation can be seen at the power output in which the standard DC electrical power output stands at 200 W (with output voltage of 28,2 V). Engine integrated generator equipped with an external power converter (PCTJ) is as of today available to provide 1100W (depending on flight mode and corresponding engine real speed (RPM) and flight true air speed). [6]

4. CONCLUSION

Innovated and attritable small-scale gas turbine engines in the terms of faster start sequence, design innovations (e.g., increased thrust-to-weight ratio, small diameter), increased flight loads absorption, governable thrust control and the ability to support the mission with sufficient electricity generation have proven to be continuously successful in various UAV applications.

In the last 2 years we have witnessed the very first C-UAS UAV-based platform (US made) to take advantage of the benefits described above and to integrate an attritable small-scale turbojet engine. Based on the rising number of Group 2 and Group 3 UAS systems being developed globally it is expected that there will be rising demand for integration of such propulsion solutions also to C-UAS systems.

As proven in the past 2 years this can be achieved by converting an existing UAV into a C-UAS solution (as seen in the US) or developing a clean sheet UAV with direct C-UAS mission capabilities.

Based on the key facts described above, PBS Velka Bites has in the last 3 years proven to be part of the leading small-scale gas turbine engine innovation initiative.

https://www.pbs.cz/en/Aerospace/Aircraft-Engines

5. BIBLIOGRAPHY

1. COUNTER DRONE SYSTEMS, ARTHUR HOLLAND MICHAEL, 2018
2. COUNTER DRONE SYSTEMS – the 2nd Edition, ARTHUR HOLLAND MICHAEL, 2019
3. Unmanned Aircraft System Airspace Integration Plan, 2018
4.  U.S. Department of Defense Counter-Small Unmanned Aircraft Systems Strategy, 2021
5. Energy density, Neutrinum, 2014
6. PBS Design & Engineering Bureau Archive (Security classification of report: Unclassified), 2022
7. Lockheed Martin, Archive, 2022
8. Raytheon Missiles and Defense, Archive, 2022