In the decades-long quest to develop reusable aircraft that can reach hypersonic speeds – Mach 5 (approximately 3,300 miles per hour/5,300 kilometers per hour) and above – engineers have grappled with two intertwined, seemingly intractable challenges: The top speed of traditional jet-turbine engines maxes out at roughly Mach 2.5, while hypersonic engines such as scramjets cannot provide effective thrust at speeds much below Mach 3.5. This gap in capability means that any air-breathing hypersonic vehicles developed today would use disposable rockets for one-time boosts up to operating speed, limiting the vehicles’ usefulness.
To help remove these constraints and lay the framework for routine hypersonic flight with reusable vehicles, DARPA has launched its Advanced Full Range Engine (AFRE) program. AFRE seeks to develop and demonstrate a new aircraft propulsion system that could operate over the full range of speeds required from low-speed takeoff through hypersonic flight.
AFRE aims to explore a turbine-based combined cycle (TBCC) engine concept, which would use a turbine engine for low-speed operations and a dual-mode ramjet – which would work efficiently whether the air flowing through it is subsonic (as in a ramjet) or supersonic (as in a scramjet) – for high-speed operations.
The AFRE program will initially conduct system design, sub- and large-scale component development, and ground demonstration, and will culminate with a large-scale, integrated test series or a propulsion wind tunnel free-jet test of the integrated low- and high-speed flowpaths.
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