Tech Briefs

These engines offer potential safety and cost advantages over solid-fuel engines.

"Hybrisol" denotes a proposed rocket engine that would contain hybrid and solid-propellant parts within a single combustion chamber. ["Hybrid" as used here denotes a type of rocket engine in which a solid fuel is burned by use of a liquid or gaseous oxidizer and the flow of the oxidizer can be throttled to control the engine. Unlike conventional solid rocket propellants, a solid fuel for a hybrid rocket engine can be made relatively inert in the absence of the oxidizer and therefore presents little hazard of explosion or inadvertent ignition.] Inside a hybrisol rocket engine, there would be two concentric tubular energy-storage layers: an outer layer of conventional solid rocket propellant and an inner layer of solid hybrid fuel (see figure).

In operation, the solid hybrid fuel would be consumed first. As the hybrid fuel approached burnout, the burning would ignite the outer layer of solid propellant. The hybrisol concept would thus confer the following advantages:

  • The use of the burning of the hybrid fuel to ignite the solid propellant would increase reliability. It would also enable a reduction in overall rocket mass because there would be no need to carry a device to ignite the solid propellant.
  • The rocket would operate initially in the hybrid mode, which would afford the inherent safety of that mode plus the controllability that is typically needed during the early phase of ascent. The later burning of solid propellant would provide the high thrust typically needed in the thinner upper atmosphere.
  • During ascent, mass could be reduced sharply by jettison of the empty oxidizer tank immediately after burnout of the hybrid fuel.
A Hybrisol Rocket Engine is a combination of a hybrid and a solid-propellant rocket engine. As the hybrid fuel nears burnout, its burning ignites the solid propellant.

Detailed calculations have shown that a hybrisol design could be executed at about half the cost of its nearest competitor. In addition to the advantages mentioned above, during the hybrid phase of operation, the hybrisol concept offers the advantage of lower (in comparison with other rocket-engine concepts) pollution from its exhaust and lower temperatures. The lower temperatures make it easier to solve heat-transfer and heat-related material problems.

Potential applications for hybrisol rocket engines range beyond the spacecraft launching market to such terrestrial applications as sounding rockets for science, distress markers, and rockets for triggering small avalanches to prevent larger ones. To the degree to which there is a toy and/or amateur rocket market, the hybrisol rockets could be attractive as safer alternatives to conventional solid-propellant rockets.

This work was done by Kumar Ramohalli of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Machinery/Automation category. NPO-20387

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