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Cryogenic Production for Airborne Thermal Management

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Develop and demonstrate a practical lightweight system for an on wing cryogenic liquid production system applicable to airborne thermal loads, and suitable for implementation on future US Army rotorcraft. 

DESCRIPTION: Emerging aircraft concepts are calling for increased electrical power and in many cases all electric aircraft designs. While accommodating the electric power requirement is a challenge in and of itself, thermal management of the components for the system becomes a priority engineering challenge as well. Cooling technology needs for these aircraft architectures are driven by the power and cooling demands of aircraft components such as superconducting motors/generators, Directed Energy Weapons (DEWs), and high powered jammers. From a military perspective high pulse power loads devices, such as DEWs, offer significant opportunities, as well as challenges, compared to traditional kinetic weapons. Airborne DEWs could perform a variety of Army missions including missile defense, counter-UAV operations, suppression of enemy air defenses, precision strike, etc. Although not limited to high energy lasers (HELs), the challenge of integrating such a device into a mission equipment package (MEP) illustrates the technical challenges ahead. HELs do not consume ammunition in the traditional sense, though they do consume significant amounts of electrical power. Lasers are intrinsically inefficient, and the “wall plug” efficiency can be considerably less than 50%. This means that to fire a 50 kW laser, 100 kW of electrical power would be needed. In addition, the net 50 kW of waste heat that is rejected to fire the laser must also be cooled which requires a cooling device that adds more size, weight, and power (SWaP). Typically the laser itself has tight thermal constraints which limit temperature changes to +/- 1° Celsius. Additionally, operating temperatures of the device are close to the operating temperatures of the external ambient air. The low quality heat is extremely difficult to move around. Therefore, significant incentive exists to maximize the efficiency of airborne DEWs to minimize both power and thermal management requirements. One way to enhance the efficiency of a laser is to cool it and operate the laser at cryogenic temperatures. Research has shown that wall-plug efficiency in excess of 70% is possible for cryogenically cooled solid-state lasers using liquid nitrogen (LN2). An airborne DEW operating at this efficiency would have significantly reduced power and thermal management requirements, but these benefits are only possible if the system is cryogenically cooled. For a cryogenic cooling system to be attractive, the benefits of increased efficiency must outweigh the cost, complexity and SWaP penalties associated with its implementation. If successfully developed, a low weight cryogenic liquid generator could be applied to a cooling system for superconducting motor/generators, high powered jammers, and/or HEL type devices. Metric goals and characteristics for the on-wing cryogenic liquid generator are as follows. The ratio of the amount of liquid cryogen produced per day to the device weight ((liters/day)/kg) should be greater than 1; with higher ratios being consequentially more desirable. For close looped systems assume the system has already been chilled and/or the reservoir is available. Production (cooling capability) goal is greater than or equal to 100 liters/day. Weight of the production equipment should not exceed 250 kg; weight is highly valued on a rotorcraft, hence any that can be removed should be. Operational ceiling is up to 5500 meters; performance must be characterized over the operational range. Temperatures up to 50 Celsius should be considered. If bleed air from an aircraft engine or secondary power unit (SPU) is required in the design, limit the bleed flow to a maximum .45 kg/sec (1 lb. /sec). Electrical power to drive equipment can assume 270VDC and an allotment of up to 40 kW of electrical load. Note, this system is not limited to LN2, innovative concepts which use any low temp fluid will be considered, such as, but not limited to liquefied natural gas (LNG), Ammonia, liquid ethylene, etc. 

PHASE I: Develop a design for a cryogenic liquid generator which meets or exceeds the aforementioned specifications. Utilization of models, and a systems engineering perspective is encouraged. 

PHASE II: Develop and demonstrate the critical components of the cryogenic liquid generator through prototyping and laboratory component/system testing. 

PHASE III: Phase III options would include development of a fully-functional prototype that could be used for cryogenic DEW ground and flight testing. For dual us applications the same technology could also be applied for other ground, naval, and airborne thermal management applications such as cooling superconducting systems including high-efficiency, compact motors and generators and lightweight power distribution systems. 

REFERENCES: 

1: J. P. Perin, et al., "Cryogenic Cooling For High Power Laser Amplifiers", IFSA 2011 - Seventh International Conference on Inertial Fusion Sciences and Applications, Bordeaux, France.

2:  R. Paschotta, article on 'cryogenic lasers' in the Encyclopedia of Laser Physics and Technology, 1. Edition October 2008, Wiley-VCH, ISBN 978-3-527-40828-3

3:  Rodger W. Dyson, "Novel Thermal Energy Conversion Technologies for Advanced Electric Air Vehicles"

4:  Scheidler, Justin J. PhD., "Preliminary Design of the Superconducting Rotor for NASA’s High-Efficiency Megawatt Motor. AIAA Propulsion and Energy Forum. July 9-11, 2018.

5:  "U.S. Army Weapons-Related Directed Energy (DE) Programs: Background and Potential Issues for Congress", Congressional research Service Report R45098 (2018).

6:  Vretenar, N., et al., "Cryogenic Yb:YAG Thin Disk Laser", AFRL Technical Paper AFRL-RD-PS-TP-2016-0004 (2016).

KEYWORDS: Directed Energy Weapons, Auxiliary Power Unit, Thermal Management, Cryogenic Cooling 

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