US Air Force Research will develop Skylon pre-cooler heat exchanger suitable for flight for two stage to orbit hypersonic space plane

In 2015, the United States’ Air Force Research Laboratory (‘AFRL’) confirmed the feasibility of the UK Synergetic Air-Breathing Rocket Engine (‘SABRE’) engine cycle concept.

The investigations examined the thermodynamic cycle of the SABRE concept and found no significant barrier to its theoretical viability provided the engine component and integration challenges are met.

In late 2015 / early 2016, BAE Systems invested £20.6 million in Reaction Engines to acquire 20 per cent of its share capital and also enter into a working partner relationship.

BAE Systems will collaborate to accelerate Reaction Engines’ development of SABRE – a new aerospace engine class that combines both jet and rocket technologies with the potential to revolutionize hypersonic flight and the economics of space access.

SABRE is an advanced combined cycle air-breathing rocket engine. This new class of aerospace engine is designed to enable aircraft to operate from standstill on the runway to speeds of over five times the speed of sound in the atmosphere. SABRE can then transition to a rocket mode of operation, allowing spaceflight at speeds up to orbital velocity, equivalent to twenty five times the speed of sound.

The USA AFRL is bullish on the technology. The lab will reveal two-stage-to-orbit SABRE-based concepts either this September, at the American Institute of Aeronautics and Astronautics’ (AIAA) SPACE 2016 conference in Long Beach, California, or in March 2017, at the 21st AIAA International Space Planes and Hypersonic Systems and Technologies Conference in China, said AFRL Aerospace Systems Directorate Aerospace Engineer Barry Hellman.

The key SABRE technologies that AFRL, based in Ohio, will start work on later this year, and possibly fly in the future, are related to the engine’s precooler. This device precools the air entering the engine at speeds greater than four times the speed of sound (Mach 4). SABRE’s precooler will cool such air from more than 1,832 degrees Fahrenheit (1,000 degrees Celsius) down to minus 238 F (minus 150 C) in one one-hundredth of a second. The oxygen in the chilled air will become liquid in the process.

Heat Exchanger


The AFRL precooler test program, which is called Durable Pre-cooling Heat Exchangers for High Mach Flight, consists of three phases, the last of which could involve test flights, according to an AFRL description.

Reaction Engines has raised more than 30 million pounds sterling ($41.9 million at current exchange rates, not including the BEA investment) in private funding.

SABRE and the Skylon space plane were invented by Alan Bond and his team of engineers at Reaction Engines. Bond now works three days a week for the company in a chief scientist/chief engineer role.

Two SABREs will power Skylon — a privately funded, single-stage-to-orbit concept vehicle that is 276 feet (84 meters) long. At takeoff, the plane will weigh about 303 tons (275,000 kilograms). AFRL officials views a single-stage-to-orbit Skylon space plane as “technically very risky as a first application [of SABRE],” and this is why the lab is developing two-stage-to-orbit concepts.

SBIR for the AFRL precooler project

Durable Pre-cooling Heat Exchangers for High Mach Flight SBIR

High Mach flight will be growing in importance for the Air Force to execute its five core missions and that precooled propulsion could be an enabler for new platform capabilities. The objective of this topic is to mature the technology for a lightweight and compact pre-cooler heat exchanger for high Mach propulsion that uses turbomachinery.

No pre-cooling heat exchanger has ever been flown. The biggest difficulty is getting the heat exchanger light weight and compact enough to be practical for flight. In many industries, modern manufacturing has allowed for lighter weight components and unique geometries to be built. This topic will leverage modern manufacturing techniques (e.g., additive manufacturing, friction stir welding, C&C milling, etc.) to develop a pre-cooler heat exchanger that is practical to be used in a propulsion system on a high Mach flight system. At the end of the Phase II, it is expected to fabricate a scaled prototype of the heat exchanger and conduct initial evaluation testing. Throughout this topic, it is important to address thermal integration for the necessary systems involving the heat exchanger.

Important attributes of pre-cooler heat exchangers (of roughly equal importance) that need to be addressed include durability, affordability, ability to integrate with propulsion and flight systems, scalability, manufacturability, impact on ground operations, material and manufacturing maturity, amount of pressure drop across the heat exchanger, and maintainability. The pre-cooler should be able to cool incoming freestream air to about 500 degrees F or cooler for flight conditions at altitudes above 55,000 feet. It is also expected the heat exchanger to be developed will have a specific power of at least 15kW/lbm.

Both Phase I and Phase II will consist of an appropriate level of design and systems engineering efforts to understand what it will take to fully develop the proposed solution. These efforts should address all issues but focus on the demonstrations that will be performed in Phase II. Modeling of the heat exchanger’s performance and its integration is needed throughout both phases to understand its potential. Recommend developing one or more reference vehicle platform designs for one or more Air Force core missions to show how the heat exchanger could enable that capability.

A letter of endorsement from a Versatile Affordable Advanced Turbine Engines (VAATE) participant is highly encouraged.

Commercialization of the pre-cooler heat exchanger involves integration of the pre-cooler into high-speed propulsion systems for DoD and/or commercial needs such as point to point cargo and access to space. Commercialization of the heat exchanger can also be used for propulsion thermal management and terrestrial applications.

Location of the pre-cooler in the Skylon engine

Remote access to the DoD Supercomputing Resource Center (DSRC) to cleared personnel will be made available if needed.

PHASE I: Conduct initial design of the pre-cooler heat exchanger with an emphasis on its integration and manufacturing. Based on higher level platform requirements, derive requirements for the heat exchanger components that have early verification and validation. Develop plans for the Phase II fabrication and testing.

PHASE II: Fabricate a scaled prototype of the heat exchanger utilizing the proposed manufacturing approach. Conduct testing in a relevant laboratory environment. Develop and validate performance and lifting models based on the testing. Utilize this information to increase the understanding of how the heat exchanger integrates into a platform or platforms.

PHASE III DUAL USE APPLICATIONS: Phase III will focus on maturing the heat exchanger and beginning to integrate it into a full propulsion system and a vehicle platform. Additional Phase III activities can consist of applying the heat exchanger and its manufacturing to other defense and commercial domains.

SOURCES- Air Force research, Reaction Engines,, Parabolic Arc