Air Force

Fast ignition rocket fuels

A potential replacement for bipropellants used in orbiting spacecraft propulsion or boost and divert maneuvers


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Illustration of selected frames from the high-speed video of a hypergolic reaction.

A conventional, storable bipropulsion system uses hydrazine as the fuel component. This fuel affords useful performance characteristics and has a fast ignition with the oxidizer. This fast ignition provides system reliability for an on-demand action of the propulsion system. However, this system is limited by its inherent energy density that can be traced, in no small measure, to the density of the fuel. Also, there are significant costs and operational constraints associated with handling the fuel that derives from the fuel’s very toxic vapor.

The challenge of making a fuel and oxidizer combination that is hypergolic is difficult. Hypergolicity is self-ignition that occurs within milliseconds after contact of fuel with an oxidizer. Hypergolic ignition is valuable because it offers high reliability, eliminates the inert mass of a separate ignition system, and provides an ability to restart for missions that require multipulse operation.

Recently, Air Force researchers developed ionic hypergolic bipropellant fuels incorporating a salt or ionic liquid fuel, containing a dicyanamide anion and a heterocyclic based cation and b) an oxidizer. While salt molecules include highly energetic high-nitrogen anions, the dicyanamide-based molecule solely displays fast ignition. The preferred oxidizer to fuel ratios ranges from 1:1 to 7:1. These advanced fuels have been synthesized and demonstrated.

These fast igniting, ionic liquid fuels provide a means to overcome significant limitations of the state-of-the-art, storable bipropulsion system. Such ionic liquid fuels can provide greater than 40 percent improvement in density over hydrazine fuels, which confers greater energy density to the bipropulsion system. Also, the negligible vapor pressure of ionic liquid fuel provides a means of significantly reducing costs and operational constraints associated with handling the fuel.

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