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Opinion | Reviving The Aerospace Plane Program

Skylon with SABRE engines. Credit: Reaction Engines

Accessing space for security, scientific discovery, and economic interests is now commonplace. But success of Earth-oriented-and-beyond space missions and the inevitability of a human presence in space require assured, affordable, and safe access to low Earth orbit (LEO). Assured access means robust, responsive, and resilient transportation. Moreover, the affordability of space access for these purposes will determine the degree to which near-Earth space can be commercialized. And, as launch service providers appreciate, safety of humans and critical cargo is paramount for customers and stakeholders alike.  

A fully reusable Earth-to-LEO transportation vehicle, using a revolutionary propulsion system and operated with commercial airline industry practices, could greatly reduce transportation costs to LEO for medium-weight (5–15 MT) payloads (humans and cargo). For this, the Aerospace Plane Program should be revived.

Historically, transportation progress has been contingent on revolutionary changes in propulsion ‘modes’. For example, aviation transformed when jet-powered aircraft replaced propeller-driven aircraft. Such revolutionary advances in propulsion transcend non-propulsive advances, such as swept wings, stressed skin metal structures, and low-drag engine cowlings. We know that because air-breathing propulsion during atmospheric flight eliminates the need for onboard oxidizer, it is a better mode of propulsion than chemical rocket engines. Moreover, vehicles with air-breathing engines are more resilient and responsive than rocket-engine-powered systems.

To fully develop human presence in near-Earth space—commute stations, tourist destinations, laboratories, and manufacturing and construction facilities—we need safe routine transport and safe emergency rescue vehicles with wide-entry windows, wide-ranging flight paths, and low rates of entry deceleration. A rescue vehicle with high maximum hypersonic lift-to-drag ratio, preferably around 3.5, could have a large enough cross range to provide a wide daily launch window and a range of flight paths to reach a target landing site. To be acceptable to disabled humans during entry and descent, the space ambulance must also restrict g-forces. For example, deceleration should be limited to about 1.1 g for people with cardiovascular issues.  

The aerospace plane program began to develop a single-stage-to-orbit (SSTO) aerospace plane for transportation to LEO in the late 1950s. When the feasibility of the SSTO concept became uncertain, two-stage-to-orbit (TSTO) system concepts were considered. The National Aero-Space Plane (NASP) Program, HOTOL, Sänger II, MAKS-M, X33/VentureStar, and Skylon were other substantial efforts. Almost all efforts were terminated. Only the Skylon endeavor, which involved from the HOTOL concept, is ongoing principally because the conceptual design of Skylon incorporates the conceptual Synergistic Air-Breathing Rocket Engine (SABRE), having potential breakthrough propulsion advances.

 Current commercial efforts focus on affordability of rocket-powered launch systems via improved and innovative designs, innovative management and development practices, advanced technologies, and partial reusability. Public-private partnerships augment private investment in evolved launch systems and result in competitive launch services.

The US Strategic Defense Initiative Organization demonstrated the concept of vertical takeoff and landing with Delta Clipper Experimental (DC-X). Subsequently, this concept was demonstrated with the first stage of Falcon 9 and with New Shepard. The Soviet Union conducted at least twelve suborbital and orbital flight tests of the BOR-4, a subscale reentry winged vehicle. The NASA HL-20 concept evolved from the BOR-4. The Dream Chaser was developed from the HL-20. And, the Dragon Spacecraft, the CST-100 Starliner, and the Orion Crew Module evolved from the Apollo Command Module.

To advance space transportation beyond what commercial efforts can achieve, a few governments conduct, support, and create an environment for high-risk research, breakthrough technology development, and proof-of-game-changing-concepts. Successful efforts can result in revolutionary technologies, game-changing applications, and innovative products—and failures can provide very good lessons. A government can undertake such efforts at much higher degree of risk tolerance than entrepreneurs and launch service providers.

While the launch industry is taking steps to demonstrate economic advantages of partially reusable orbital launch systems, we need a national strategy to develop fully reusable systems with revolutionary and innovative technologies. If a government were to lead the way for development of a fully reusable launch vehicle with a revolutionary propulsion system, commercial aerospace planes would follow.

The strategic objectives are to fully commercialize and economically develop low Earth space and to provide responsive and resilient space transportation. The former would be an appreciable force multiplier for Earth-to-LEO transportation industry and for space exploration and security. The latter is crucial for security and for either civil or commercial emergency.

The design of an aerospace plane strongly depends on the propulsion system. With successful ground and flight tests of the SABRE, it could be available for use in an aerospace plane in the 2020s, decades before the availability of appropriate turbine-based combined cycle (TBCC) engines [which are designed either with a turbine engine plus a dual-mode ramjet (or dual-mode scramjet) engine] for an accelerating atmospheric flight reaching Mach ~11. The USAF hypersonic roadmap projects technology readiness in the 2040s for a hypersonic cruise aircraft using a TBCC engine. Consequently, the first-generation operational aerospace planes would use SABRE and the second-generation planes would use TBCC engines.

The economic imperative to significantly reduce the Earth-to-LEO transportation cost, recent breakthrough advances−the partially demonstrated heat exchanger technology needed for the SABRE and independent partial verification of the feasibility and potential performance of the SABRE concept−and significant relevant innovations since the beginning of the NASP program in 1986 suggest that this is the time to revive the Aerospace Plane Program conceivably as a joint US-UK program. The revival would further mature technologies and develop and test a fully reusable aerospace plane—an Xplane.

The speed goal for this plane should range from 6.8 to 7.8 km/s. Its design, the technology suite used, design sensitivities, operations, and so on would determine the achievable maximum speed. The outcome of flight trials would assist in the development of the first operational aerospace plane. This outcome would also determine whether this plane would be designed for a TSTO or SSTO mission.

We can learn from previous reusable launch vehicle programs and from recent innovative management and development practices. Visionary and responsible leadership and the will to make a revolutionary change in the Earth-to-LEO transportation paradigm can make the revived Aerospace Plane Program a success.


Unmeel Mehta is an associate fellow at American Institute of Aeronautics and Astronautics and a member at Sigma Xi, the Scientific Research Society.

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Opinion | Reviving The Aerospace Plane Program

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