Abstract:
A hybrid rocket motor includes a supply of oxidizer, a first solid fuel element positioned around the supply of oxidizer, a second solid fuel element positioned concentrically around the first solid fuel element, and a combustion port positioned between the first and second solid fuel elements. The oxidizer interacts with the first and second solid fuel elements within the combustion port to produce a combustion product. A nozzle is in communication with the combustion port for combustion discharge of the combustion product.

Description:
REFERENCE TO PRIORITY DOCUMENT 
       [0001]    This application claims priority of co-pending U.S. Provisional Patent Application Ser. No. 60/818,001 filed Jun. 29, 2006. Priority of the aforementioned filing date is hereby claimed and the disclosure of the Provisional Patent Application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to propulsion systems, and more particularly to a hybrid propulsion system. 
         [0003]    Liquid systems and solid systems are the two basic types of rocket propulsion systems that are most widely used in the rocket industry. In a solid propellant system, solid rocket fuel and an oxidizer are mixed together and allowed to cure inside a rocket case to form a solid propellant material, which is then ignited in the rocket case. Upon ignition, pressure forms within the rocket case and gases are released through a nozzle to produce thrust. In a solid propellant system, the solid propellant burns uninterrupted until all the propellant is exhausted, which can be undesirable in certain circumstances. 
         [0004]    In a liquid system, a liquid oxidizer is fed into a combustion chamber in combination with a liquid fuel. The oxidizer and liquid fuel are mixed in the combustion chamber, where they react to produce gases under high temperature and high pressure. The gases exhaust through a nozzle from the combustion chamber to thereby produce thrust. Although widely used, there are certain drawbacks associated with liquid propulsion systems. 
         [0005]    Another type of rocket propulsion system is the hybrid system. A hybrid system combines aspects of both liquid systems and solid systems in that one propellant is stored as a solid and another propellant is stored as a liquid. In a typical system, the solid material is used as the fuel and the liquid material is used as the oxidizer. A variety of materials can be used as the solid fuel, including Plexiglas (polymethyl methacrylate (PMMA)), high density polyethylene (HDPE), hydroxyl terminated polybutadiene (HTPB), etc. Nitrous Oxide is a commonly used as the oxidizer, although other oxidizers can be used. 
       SUMMARY 
       [0006]    There is currently a need for improved hybrid rocket systems. Disclosed is an improved hybrid rocket system. In one aspect, there is disclosed a hybrid rocket motor comprising a supply of oxidizer, a first solid fuel element positioned around the supply of oxidizer, a second solid fuel element positioned concentrically around the first solid fuel element, and a combustion port positioned between the first and second solid fuel elements. The oxidizer interacts with the first and second solid fuel elements within the combustion port to produce a combustion product. A nozzle is in communication with the combustion port for combustion discharge of the combustion product. 
         [0007]    In another aspect, there is disclosed a hybrid rocket motor, comprising a oxidizer tank containing an oxidizer, a main casing surrounding the oxidizer tank, and at least one injector adapted to inject oxidizer from the oxidizer tank into the main casing. The main casing includes a first, annular solid fuel grain; a second, annular solid fuel grain positioned concentrically around the first, annular solid fuel grain; a combustion port positioned between the first and second annular solid fuel grains wherein the oxidizer interacts with the first and second solid fuel elements within the combustion port to produce a combustion product; and a nozzle in communication with the combustion port for combustion discharge of the combustion product. 
         [0008]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a schematic view of a single port hybrid rocket motor. 
           [0010]      FIG. 2  is a cross-sectional view of the motor of  FIG. 1  along line  2 - 2  of  FIG. 1 . 
           [0011]      FIG. 3  is a schematic side view of a hybrid rocket motor having concentric fuel grains. 
           [0012]      FIG. 4  is a cross-sectional view of the motor of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows a schematic of the configuration of a conventional, single port hybrid rocket motor  100 . The motor  100  generally includes a liquid fuel tank  101  coupled to a main tank or casing  102 . The casing  102  encloses a combustion chamber  110 , a mixing chamber  115 , and an elongate combustion port  120  extending therebetween. A solid fuel or “grain”  130  is located within the casing  102 . As shown in the cross-sectional view of  FIG. 2 , the solid fuel grain  130  may have a “wagon wheel” cross-sectional shape such that the solid fuel grain is divided into wedge-shaped portions  136  that are arranged around a central port. It should be appreciated that the portions  136  do not have to be wedge-shaped and that any quantity of portions can be used. 
         [0014]    The wedge-shaped portions  136  have spaces therebetween that form the combustion port  120 , which extends along the axial length of the grain  130 . The combustion port  120  permit combustion gas to flow across the length of the solid fuel grain, as described below. With reference to  FIG. 1 , the solid fuel grain has a length L and a diameter D. It is generally desirable to maximize the ratio of the length L to the diameter D in order to improve performance of the rocket motor  100 . 
         [0015]    With reference still to  FIG. 1 , an injector  122  communicates with the combustion chamber  110  for injecting a liquid phase oxidizer from the liquid fuel tank  101  into the combustion chamber  110 . In use, the oxidizer is injected into the combustion chamber  110  via the injector  122 . The injected oxidizer is gasified and flows axially along the combustion port  120 . This forms a boundary layer edge  125  over the exposed surfaces of the solid fuel grain  130 . The boundary layer edge  125  is usually turbulent in nature over a large portion of the length of the combustion port  120 . A diffusion flame zone  135  exists within the boundary layer edge  125 , which diffusion flame zone  135  extends over the entire length of the solid fuel  130 . 
         [0016]    The heat generated in the flame, which is located approximately 20-30% of the boundary layer thickness above the fuel surface, is transferred to the wall mainly by convection. The wall heat flux evaporates the solid fuel and the resultant fuel vapor is transported to the flame where it reacts with the oxidizer, which is transported from the free stream by turbulent diffusion mechanisms. The unburned fuel that travels beneath the flame, the unburned oxidizer in the free stream, and the flame combustion products mix and further react in the mixing chamber  115 . The hot gases expand through a nozzle  140  to deliver the required thrust. 
         [0017]    The “wagon wheel” solid fuel grain configuration shown in  FIG. 2  can provide a beneficial ratio of exposed surface area to cross sectional area for the solid fuel grain. However, the wagon wheel design has disadvantages. For example, due to the slow burning rate of the fuel, the fuel grain webs become very thin during the last portion of the burn and the motor has to be shut down. This undesirably results in a high residual. It has been attempted to reinforce the wagon wheel fuel grain by incorporating solid stiffening sheets in the spoke or web portions of the grain. This too has not proven satisfactory since the fuel grain tends to separate from the solid sheets during burning. 
         [0018]    The disadvantages of such a configuration can be overcome using the hybrid rocket motor of the present invention.  FIG. 3  shows a side, cross-sectional view of an exemplary embodiment of the hybrid rocket motor  300  that is configured in accordance with the present invention.  FIG. 4  shows a downward cross-sectional view of the motor  300  along lines  4 - 4  of  FIG. 3 . The motor  300  includes a central, liquid fuel tank  305  that is surrounded by an annular, outer casing  310 . The liquid fuel tank  305  contains a liquid fuel or oxidizer. The outer casing  310  contains a pair of solid fuel grains or members (referred to individually using reference numerals  320   a,    320   b  and collectively using reference numeral  320 ). The solid fuel members  320  are annular and are arranged in a concentric manner around one another and the liquid fuel tank  305 , as described in more detail below. An annular combustion port  325  is formed between the two solid fuel members  320 . The combustion port  325  extends along the axial length of the solid fuel members  320  in the space between the solid fuel members  320 . The combustion port  325  provides a communicative pathway between a combustion chamber  328  and a mixing chamber  329 . 
         [0019]    As best shown in the cross-sectional view of  FIG. 4 , the solid fuel members  320  are ring-like and substantially circular in cross-sectional shape with the combustion port  325  being formed therebetween. The solid fuel members  320  need not be circular, but can have other shapes. In addition, there can be additional solid fuel members that are concentrically arranged around the solid fuel members  320   a  and  320   b.    
         [0020]    With reference still to  FIG. 4 , the dimensions of the liquid fuel tank  305 , the solid fuel elements  320 , and the combustion port  325  can be at least partially defined by radii R 1 -R 4 . The liquid fuel tank  305  has an outer radius R 1 , the solid fuel element  320   a  has an outer radius R 2 , the combustion port  325  has an outer radius R 3 , and the solid fuel element  320   b  has an outer radius R 4 , with the radii being relative to a center point C. 
         [0021]    With reference to  FIG. 3 , a liquid fuel port  330  is disposed in communication with the liquid fuel tank  305 , such as at an upper region of the liquid fuel tank  305 . The liquid fuel port  330  is adapted to facilitate the transfer liquid fuel from the liquid fuel tank  305  into the combustion chamber  315 . In this regard, the liquid fuel port  330  includes an opening that communicates with the interior of the liquid fuel tank  305  such that the liquid fuel can exit the liquid fuel tank  305  via the port  330 . The liquid fuel port  330  communicates with one or more conduits  335  through which the liquid fuel can flow toward the combustion chamber  315 . Each of the conduits terminates in an injector  340  that communicates with the combustion chamber  315 . It should be appreciated that the motor  300  can include any quantity of conduits and injectors. In one embodiment, a plurality of conduits communicate with a plurality of injectors with the injectors being spaced evenly or sporadically around the circumference of the upper region of the combustion chamber  315 . 
         [0022]    With reference now to  FIG. 3 , the outer casing  310  tapers moving downward along the motor  300 . In this manner, the outer casing  310  forms into a nozzle region  345  at the bottom end of the casing  310 . The nozzle-region  345  includes an internal throat  350  formed within the bottom region of the casing  310 . At its upper end, the throat  350  communicates with the combustion port  325  and the mixing chamber  329 . The throat  350  also communicates with the outside environment at an opening  352  at a lower end. In one embodiment, the nozzle region  345  comprises an aerospike. 
         [0023]    A space  337  is positioned below the liquid fuel tank  305  and below the fuel element  320   a.  The space  337  can be used for various purposes. In one embodiment, the space  337  communicates with the throat  350  via one or more holes or passageways therebetween. The passageways can be used to facilitate liquid injection vector thrust control (LITVC) of the nozzle  345 . Other uses of the space  337  are also possible. 
         [0024]    As discussed, the liquid fuel tank  325  contains a liquid fuel or oxidizer. The type of liquid fuel can vary. In one embodiment, the liquid fuel comprises liquid oxygen (O2). The type of solid fuel can also vary. In one embodiment, the solid fuel is plexiglass. 
         [0025]    In use, the oxidizer flows out of the liquid fuel tank  325  via the port  330 . The oxidizer flows through the conduits  335  toward the combustion port  325  and is injected therein using the injector(s)  340 . The injected oxidizer is gasified and flows axially along a combustion port  325  between the solid fuel members  320 . Ignition causes combustion of the fuel-oxidizer mixture at the exposed surfaces of the fuel grain, resulting in the generation of thrust as the high pressure combustion products are discharged through the rocket nozzle  345 . 
         [0026]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the claims. Accordingly, other embodiments are within the scope of the following claims.