Fuel gas generator for airbreathing propulsion systems

A gas generator propellant formulation suitable for use in an air turbo ret (ATR) which employs an air breathing system that uses fuel gases produced by a gas generator propellant to operate the engine's turbine is provided which can also be used with other airbreathing propulsion systems that require a high gravimetric heating value (GHV). The basic fuel gas generator propellant formulation comprises in weight percent a tetraalkylammonium borohydride 50-100; lithium nitrate 0-50; and optional additives of hydroxy proply cellulose 0-20 and silica or silicon 0-20. The basic fuel gas propellant formulation can also employ an encapsulated tetraalkylammonium borohydride which employs an encapsulation polymer selected from the group consisting of polyethylene, polypropylene, and ethyl cellulose. When employing an encapsulated tetraalkylammonium borohydride a binder is employed selected from the group consisting of polybutadiene and polyether cured with 0.50% selected from hexamethylene diisocyanate and isophorene diisocyanate. The encapsulated fuel gas generator propellant formulation comprises an encapsulated tetraalkylammonium borohydride 50%; binder 0-45%; and lithium nitrate 5%. Lithium nitrate in the fuel gas generator propellant formulation enables the composition to be ignited with a hot wire. The tetraalkylammonium borohydride is selected from the group of tetraalkylammonium borohydrides consisting of tetramethylammonium borohydride, tetraethylammonium borohydride tetrapropylammonium borohydride, and tetrabutylammonium borohydride.

BACKGROUND OF THE INVENTION 
An air turbo rocket (ATR) is an air breathing propulsion system that uses 
fuel gases produced by a gas generator propellant to operate the engine's 
turbine. The turbine expands these hot gases and provides energy to the 
compressor. The compressor then compresses the air from the air inlet and 
the air flows from the compressor to the combustion chamber. The fuel 
gases flow from the turbine to the combustion chamber where they react 
with the compressed air. The combustion gases from the combustion chamber 
are expanded through a nozzle that produces the rocket's thrust. 
A turbojet is an air breathing propulsion system that uses the gases 
produced in the combustion chamber to operate the engine's turbine. The 
turbine expands the gases produced in the combustion chamber and provides 
energy to the compressor. The compressor then compresses the air and the 
air flows to the combustion chamber. Fuel is injected into the combustion 
chamber where it reacts with the compressed air. The combustion gases go 
through the turbine and to a nozzle where the gases are expanded to 
provide thrust for the rocket. 
The ATR has several advantages over the turbojet propulsion system. Since 
there is no turbomachinery located downstream of the combustion chamber, 
the ATR can operate at higher combustion temperatures. The ATR can operate 
at higher speeds because the turbine temperature is independent of air 
inlet conditions. At subsonic conditions, where the air is not compressed 
much because of the lower rocket velocity, the ATR operates with better 
performance since the energy supplied by the turbine is independent of air 
inlet conditions. 
The use of an ATR in a tactical weapon system has not been considered 
because of limitations in present gas generator formulations. The ATR has 
not been able to compete with the turbojet propulsion system because the 
turbojet uses a liquid fuel, JP-10, which has a gravimetric heating value 
(GHV) of approximately 18,000 btu/lb whereas gas generator propellants for 
the ATR have GHVs between 5,000 to 9,000 btu/lb. 
The object of this invention is to provide a gas generator formulation that 
can provide enough energy for an ATR turbine but also have a GHV of 18,600 
btu/lb. 
Another object of this invention is to provide a gas generator formulation 
that can be used with other airbreathing propulsion systems that require 
fuel gases with a high GHV. 
Further exploitation of this invention is the use of a gas generator 
propellant formulation in a pulse detonation engine (PDE). A PDE is 
essentially a shock tube into which both a fuel gas and air is introduced 
before an ignition device detonates the explosive mixture of gases. The 
PDE engine is throttled by varying the gaseous flow rates. This engine has 
a significant weight advantage over the ATR or turbojet because is has no 
turbomachinery. This engine can also be made of lower cost materials that 
do not have the high temperature requirements of a turbojet turbine. 
Therefore a further object of this invention is to provide a family of gas 
generators that can be used with airbreathing engines such as the ATR and 
PDE. The ATR goal is to provide enough energy for an ATR turbine but also 
have a GHV of 18,600 btu/lb. The PDE goal is to provide an effluent with a 
GHV of 18,600 btu/lb. but also have good detonation properties. 
SUMMARY OF THE INVENTION 
A gas generator propellant formulation that is comprised of 95% of a 
tetraalkylammonium borohydride and 5% lithium nitrate can be ignited with 
a hot wire igniter. This will cause the formulation to decompose and 
liberate gases and form a solid klinker. Experiments have been performed 
to determine the gas yield from the tetraalkylammonium borohydride, 
tetramethylammonium borohydride, which produces equal amounts of hydrogen 
and nitrogen gases with the balance of gas produced being methane. Seventy 
percent of the weight of the original propellant formulation produced 
gases while 30% of the original propellant formulation remained as a 
klinker. Chemical analysis decomposition. Seventy percent of the weight of 
the original propellant will be the liberated gases while 30% of the 
original propellant will be the weight of the klinker. Chemical analysis 
of the gases indicates that the gases are 65% methane, 17.5% hydrogen, and 
17.5% nitrogen by weight. Measurements of the gas temperature indicates a 
temperature of 567.degree. F. The GHV of the gases is 18,600 btu/lb based 
on the chemical analysis of the effluent indicates that the gases are 65% 
methane, 17.5% hydrogen, and 17.5% nitrogen by weight. The temperature of 
the effluent was 567.degree. F. The GHV of the effluent is 18,600 btu/lb. 
The propellant grain is formed by pressing the powdered ingredients. When 
the grain is pressed into a pellet form and is ignited, the klinker 
retains the shape of that the unreacted pellet. Since this propellant 
would be used with a filter system, a propellant that produces a klinker 
in the shape of the original pellet would require less filtration. This 
propellant formulation is not chemically compatible with urethane cure 
methods. When a urethane cure was attempted with this formulation the 
binder did not harden thereby resulting in a grain having very weak 
physical properties. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
This fuel gas generator propellant formulation can be utilized in an ATR, 
ducted rocket or pulse detonation engine propulsion system. It can also be 
used in any airbreathing propulsion system that requires a fuel that has a 
high GHV. A minor amount of other oxidizers (e.g., potassium nitrate, 
sodium nitrate, ammonium nitrate, or ammonium perchlorate) is required to 
maintain the decomposition. The basic formulation is comprised of 0-50% 
lithium nitrate and 50-100% tetraalkylammonium borohydride. The amount of 
lithium nitrate can be higher than 5% if gas temperatures above 
600.degree. F. were required. The tetraalkylammonium borohydride can be 
selected from the group of tetraalkylammonium borohydrides consisting of 
tetramethylammonium borohydride, tetraethylammonium borohydride, 
tetrapropylammonium borohydride, and tetrabutylammonium borohydride. 
Experimental results indicates a decrease in the amount of hydrogen and 
nitrogen and a corresponding increase in the amount of the other gaseous 
component i.e., ethane, propane, and butane, increases as the molecular 
weight of the tetraalkylammonium borohydride increases. Based on the 
experimental results obtained, tetramethylammonium borohydride is 
preferred for maximum decomposition rate which is achieved in proportion 
to the amount of nitrogen gas effluent. If the propellant is in the form 
of pressed pellets, 0-20% hydroxy propyl cellulose is added to improve the 
physical properties of the pellet. Also 0-20% of additives selected from 
silica, or silicon to increase the strength of the klinker can be added to 
the formulation. The formulation could have a binder such as hydroxy 
terminated polybutadiene if the tetramethylammonium borohydride were 
encapsulated in another thermoplastic polymer such as polyethylene, 
polypropylene or ethyl cellulose. The binder could be 0-50% of the 
formulation and could be either polybutadiene or polyether cured with 
hexamethylene diisocyanate or isophorone diisocyanate could be 0.50% of 
the formulation and could be either polybutadiene or polyether cured with 
hexamethylene diisocyanate or isophorone diisocyanate.