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Two-stage-to-orbit (TSTO) conceptual-level vehicle designs were evolved by the Lockheed-California Company in the mid-1960s. The purpose was to provide a vehicle-systems-level basis for assessing the payload performance potential of a new class of Stage 1 propulsion systems: combined-cycle airbreathing/rocket engines. TSTO configurations were also established as conventional all-rocket and all-airbreathing engine comparison cases. These vehicle designs and their operating characteristics, along with their orbital payload-delivery capabilities, are presented for consideration by today's space transportation systems planning community.

Transportation aspects of commercial spaceflight in the opening decades of the Twenty-first Century are envisioned. A specific Spaceliner concept is described, one predicated on combined-cycle airbreathing/rocket (“Synerjet”) propulsion. Its technical literature recorded heritage is reviewed. NASA's long-term goals, and its ongoing program relevant to this class of transportation concepts are discussed. A key observation, one of potentially distant, but great significance to the future of aviation and the air carrier industry, is that Spaceliner class systems, once available, can -- in addition to providing orbital payload service -- fly transglobally, conducting point-to-point terrestrial transportation services, all at ‘highest speed.” Providing an aviation/air-carrier perspective on all of this, in an annex paper attached, the candid views of an esteemed former airlines executive, the late Willis Player, are highlighted.

A landmark study of Synerjet propulsion for fully-reusable Earth/orbit transport missions was conducted for NASA in 1965-67 by a study team of Marquardt, Rocketdyne, and Lockheed (the present author led this effort). Synerjet propulsion systems are fully integrated aerospace vehicle power-plants, comprising both airbreathing and rocket hardware subsystems and technologies. They are thereby capable of multimode operation using airbreathing, rocket, and certain mixed airbreathing/rocket thermodynamic operating cycles. The final report (9 volumes) deriving from this 20-year ago study could have applicability to post-Shuttle systems assessments currently underway. However, for reasons of passage-of-time and the (then) classified nature of the study, its documentation seems not well known, and to be in scarce availability today. This paper ameliorates this limited information-availability situation by providing a readily accessible, study overview for the interested reader.

Under Department of Energy sponsorship, the Los Alamos Scientific Laboratory is presently road testing a U.S. intermediate sized automobile adapted to liquid-hydrogen fuel. The overall project objective is to document a general experience base as a point of departure for future development. A semiautomatic fueling station developed by the Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt (DFVLR) of the Federal Republic of Germany is being used to service (refuel) the experimental vehicle. The DFVLR also provided the 150-ℓ (39.6-gal.) cryogenic liquid-hydrogen storage tank mounted in the trunk compartment and the dashboard fuel gauging system. The V-6 turbocharged engine was converted to hydrogen operation by the Billings Energy Corporation. Both the container and the engine modifications represented state-of-the-art technology when the program began.

The objective of the project reflected in this summary paper is to document an experimental design data-base on hydrogen-fueled automotive engines. The effort, sponsored by the U.S. Department of Energy, is directed toward possible future designs of airbreathing piston engines using hydrogen. To this end, pertinent performance and emissions (NOx only) characteristics of 16 engine configurations are presented graphically. Configurational variations of a 1600 cc automotive test engine included: throttled and unthrottled operation (i.e., quantity and quality power level control); central manifold, port, direct cylinder injection [not included]; single and divided combustion chambers; exhaust gas recirculation (EGR), water induction and air injection; and certain other features.

Developments in the energy and environmental sectors have awakened interest in hydrogen-fueled internal combustion engines. Based on a 50-engine survey of recent U.S. research and demonstration activities, the technical status of hydrogen engines is summarized. A comparative hydrogen/conventional-fuel orientation is maintained in assessing basic fuel property, engine power and efficiency, emissions and general operating characteristics. Key theoretical and experimental results, and correlations where evident, are noted. Engine problem areas and their solutions are discussed, accompanied by some thoughts toward future development possibilities and supporting research needs.