A Deep Dive into the Revolutionary Hypersonic Propulsion Technologies: Ramjet and Scramjet

The Dawn of Hypersonic Era

As we stand on the precipice of an unprecedented era in the aviation industry, the quest for hypersonic speeds is leading the way towards unprecedented technological innovation. Among the trailblazers in this field, Hermeus Corporation has made a notable decision to rely on the mature technology of Ramjets for its hypersonic propulsion needs, instead of the latest scramjets. This strategic choice could potentially restrict Hermeus’s aircraft to speeds of Mach 6 or below, yet the manoeuvrability at such speeds would make Hermeus’s aircraft as elusive as the most advanced hypersonic missile — all within a reusable package.

Ramjet and Scramjet: A Comparative Study

While on the topic of engines, it’s imperative to explore further the core of Hermeus’s ongoing hypersonic efforts: the turbine-based combined cycle engine known as the Chimera. A product of intricate design and engineering, the Chimera integrates two distinct types of air-breathing jet engines that perform optimally at different speeds.

The conventional turbofan and turbojet engines, which power most of today’s aircraft, employ a compressor and fan at the front. These components suck air in, compress it, mix it with fuel, and ignite it to create propulsion at the back. Crucially, these compressors allow these engines to function even when the aircraft is stationary. However, their effectiveness begins to wane beyond Mach 3.

Contrarily, Ramjets, devoid of compressors or any moving parts, depend on the pressure of air flowing into the intake at high speed to achieve compression. Consequently, they can’t function at a standstill, requiring a minimum speed to initiate their operation. Ramjets use an internal structure mounted in the intake to manipulate airflow, slowing it to subsonic speeds before it enters the combustion chamber. While they may be dormant at a dead stop, once the aircraft reaches Mach 3, Ramjets awaken and can potentially accelerate aircraft beyond Mach 5, reaching as high as Mach 6.

The latest and most advanced hypersonic propulsion technology, scramjets, extends the Ramjet concept by eliminating the internal structure and allowing the air to flow through the engine at supersonic speeds. In theory, scramjets can maintain much higher speeds, possibly surpassing Mach 10. However, they present significant engineering challenges. Sustaining ignition in an engine with supersonic airflow is a daunting task, akin to keeping a match lit in a hurricane. Consequently, while scramjets have existed for years, they have only been used for single-use technology demonstrators due to the technical difficulties associated with them.

Ramjets in Action

The Ramjet technology, despite its limitations, has been successfully incorporated into various platforms. One such example is Europe’s Meteor air-to-air missile. However, all platforms powered by Ramjets or scramjets have been single-use until now. Given these engines’ inability to function at low speeds, they are typically lofted into the sky with a conventional rocket booster. After reaching suitable altitude and speed, they are detached and allowed to operate independently. However, because they can’t function at lower speeds, they can’t slow down for landing and instead always crash. Presently, this feature is mainly exploited in missiles where the crash serves a purpose.

The Future of Combined Cycle Engines

A combined cycle engine, integrating a turbofan and a Ramjet or scramjet, could theoretically take off under its own power, accelerate to around Mach 2.8 or 3, transition to the Ramjet or scramjet for hypersonic flight, and then switch back to the turbofan for deceleration and landing. However, despite its enticing promise, no organization has managed to accomplish this feat yet.

Lockheed Martin, with its SR-72 concept exemplified by the Dark Star aircraft featured in Top Gun Maverick, has long aimed to integrate a combined cycle turbofan scramjet system. Similarly, the Air Force Research Laboratories’ (AFRL) Mayhem program aims to achieve something analogous. If successful, the resulting aircraft might be capable of reaching speeds exceeding Mach 10. However, the timeline and budget for these ambitious programs remain speculative.

Lockheed Martin has been collaborating with Aerojet Rocketdyne on their combined cycle turbofan scramjet concept since at least 2018. The scale of their investment remains undisclosed. Similarly, the AFRL’s Mayhem program granted Leonidos a $334 million contract in 2022 to continue developing their concept.

Conclusion

The race towards hypersonic speeds is not just a quest for prestige; it’s a pursuit of next-generation technology that could redefine aviation. As scramjets and combined cycle engines continue to evolve, their development challenges and costs must be balanced against their prospective benefits. Despite the roadblocks, companies like Hermeus and government programs like AFRL’s Mayhem continue to forge ahead, backed by their belief in the game-changing potential of these technologies. The dawn of hypersonic travel might be closer than we think, and when it arrives, it will undoubtedly herald a new era of aerospace innovation.