Abstract:
An ignitor for use with the MC-1 rocket engine has a cartridge bounded by two end caps with rupture disc assemblies connected thereto. A piston assembly within the cartridge moves from one end of the cartridge during the ignition process. The inlet of the ignitor communicates with a supply taken from the discharge of the fuel pump. When the pump is initially started, the pressure differential bursts the first rupture disc to begin the movement of the piston assembly toward the discharge end. The pressurization of the cartridge causes the second rupture disc to rupture and hypergolic fluid contained within the cartridge is discharged out the ignitor outlet.

Description:
ORIGIN OF THE INVENTION 
   This application is a continuation-in-part of U.S. patent application Ser. No. 09/877,800 filed Jun. 6, 2001 now U.S. Pat. No. 6,497,091. 

   This invention was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or thereof. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an ignitor for a rocket engine, and more specifically, to a hypergolic liquid ignitor for use with a rocket engine, such as the MC-1 engine. 
   2. Prior Art 
   Various hypergolic ignitor designs have been developed in the past. These ignitors are utilized to commence the burning of the rocket engine propellants in the combustion chamber. Hypergolic fluid is designed to ignite spontaneously upon contact with an oxidizer. The prior art ignitors are mounted off the main injector of the rocket engine and are typically incorporated into a fuel bypass line feeding the injector. These ignitors dispense the hypergolic fluid through the injector into the combustion chamber where the fuel was ignited. Fuel pressure from the feed or fuel system forces the ignitor fluid into the combustion chamber where it ignites the rocket engine propellants. 
   The traditional hypergolic ignitor designs suffer from a plurality of disadvantages. First, they are typically non-reusable and expensive to construct since they are not constructed with off-the-shelf components. Secondly, the filling of the prior art ignitors requires a high degree of complexity. Furthermore, the prior art ignitors deliver the ignitor fluid through the combustion chamber injector instead of directly into the combustion chamber. This introduces a plurality of additional design considerations for both the ignitor and the injector. 
   Thus, a need exists for an efficient, cost effective ignitor which may communicate directly with the combustion chamber rather than requiring the complexity of additional valves or flow control devices to deliver ignitor fluid through the injector. 
   Another need exists for a modular design for an ignitor allowing faster assembly and interchangeability of parts. 
   A further need exists for a method of joining two structural members while providing for disassembly at a later date. 
   SUMMARY OF THE INVENTION 
   Consequently, it is a primary object of the present invention to provide a cost effective ignitor for use with rocket engines, including the MC-1 engine. 
   It is another object of the present invention to provide an ignitor with purge grooves providing side chamber injection of hypergolic fluid to reduce the complexity of the combustion chamber injector. 
   Accordingly, the present invention provides an ignitor having a cartridge contained within end caps. Each of the end caps contains rupture disc assemblies. A piston is located within the cartridge and is moveable from one end of the cartridge to the other, a discharge end. The ignitor is designed to provide a low pressure, hypergolic liquid to produce a sustainable ignition source for a rocket engine chamber. 
   The cartridge is filled with a mixture of hypergolic fluid, Triethylaluminum and Triethylborane (TEA/TEB). A first rupture disc is ruptured to provide the motive force to drive the piston. As the piston moves toward the discharge end, the second rupture disc ruptures to deliver the hypergolic fluid into the combustion chamber. The rupture discs are preferably interchangeable and located so that any leakage past the rupture discs is retained within the ignitor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawing in which: 
     The FIGURE is a partial cutaway elevational view of an ignitor for use with a rocket engine in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the FIGURE, an ignitor  10  for use with a rocket engine is illustrated. The ignitor  10  of the preferred embodiment is adapted to work with an MC-1 rocket engine currently produced by SUMMA Technology, Inc. of Huntsville, Ala. for NASA. 
   Structurally, the ignitor  10  is comprised of a cartridge  12  with end caps  14 ,  16  on either end of the cartridge  12 . The cartridge  12  is preferably substantially cylindrical with a cavity therein which initially contains hypergolic fluid which is utilized to ignite the propellant, i.e., rocket fuel in the combustion chamber (not shown). The end caps  14 ,  16  each contain openings  18 ,  20  where rupture disc assemblies  22 ,  24  connect to the end caps  14 , 16 . In the design illustrated, the first rupture disc assembly  24  has been located within the cartridge  12  and connected at the opening  20  of the first end cap  16 . Both of the rupture disc assemblies  22 ,  24  are located internally within the ignitor  10 . 
   The rupture disc assemblies  22 , 24  preferably include threads  26 , 28  which cooperate with threads  30 , 32  at the openings  18 , 20  within the end caps  14 , 16  to secure the rupture disc assemblies  22 , 24  to the end caps  14 , 16 . The openings  18 ,  20  represent an outlet and an inlet, respectively of the cartridge  12 . The first rupture disc assembly  24  is located within the cartridge  12  and downstream of opening  20 . The end caps  14 , 16  have shrouds  34 ,  36  which extend away from the cylinder  12  as illustrated. The shrouds  34 ,  36  prevent any fluid that would otherwise leak past threads  26 ,  30  or  28 ,  32  from being exposed to oxygen which could be a severe problem. Within the shrouds  34 ,  36  and plugs  35 ,  37  are chambers  31 ,  33 . The cartridge  12  is located intermediate the chambers  31 ,  33 . 
   The rupture disc assemblies  22 , 24  contain rupture discs  38 , 40  which are typically metal domes scored in a pattern such that the discs  38 , 40  break, or rupture, at a specific pressure differential across the discs  38 , 40 . The rupture disc assemblies  22 , 24  are preferably created within female housings  45 , 47  which allow for quick assembly from inexpensive parts. Furthermore, the discs  38 , 40  themselves are off the shelf items which provide reliable, precise and repeatable performance since they can be easily replaced in this design. By using a single housing type, one cannot inadvertently switch the rupture disc assemblies  22 ,  24  during installation. 
   The end caps  14 , 16  preferably connect to the cartridge  12  with bolts  42  which extend through bores  44  in the end caps  14 ,  16  and into threaded bores  46  in the cartridge  12 . The bolted design has been found desirable to reduce working torque and allow specific convenient surface orientations. The rupture disc assemblies  22 ,  24  are now internal parts which simplifies leak check procedures after installation on an engine. A polytetrafluoroethylene (PTFE) O-ring  50  is preferably utilized to form an air-tight seal between the end caps  14 , 16  and the cartridge  12  after connecting the components together. 
   Inside the cartridge  12  is a piston assembly  48 . The piston assembly  48  is shown in a first position in the FIGURE at the first end  52  of the cartridge  12 . The piston assembly  48  includes a piston face  56  preferably including a plurality of nubs  58 . The piston assembly is slightly smaller than the interior of the cylinder so that the piston assembly can move from the first end  52  to the second, or discharge, end  54  of the cartridge  12 . Seals  60 ,  62  are preferably elastomeric to form a pressure barrier while allowing the piston assembly  48  to move through a portion of the length of the cartridge  12 . The interior volume, or cavity, of the cartridge  12  is initially filled with a hypergolic fluid  64 , such as Triethylaluminum and Triethylborane (TEA/TEB). As the piston assembly  48  moves from the first end  52  to the second end  54 , the fluid  64  is discharged out ignitor outlet  68 . In this embodiment, the ignitor outlet  68  is oriented perpendicular to the axis of travel of the piston  48  through the length of the cavity of the cartridge  12 . 
   The ignitor has an ignitor inlet  66  which receives discharge from the fuel pump (not shown). The ignitor inlet  66  is also oriented perpendicular to the axis of travel of the piston assembly  48  through the length of the cavity of the cartridge  12 . During the start sequence, helium is initially supplied to a turbine which begins to spin the fuel pump and provides pressure at the fuel pump discharge. A supply line (not shown) connects the ignitor inlet  66  to the discharge of the fuel pump. An ignition valve (not shown) opens in the supply line allowing the pressure at the inlet to be the discharge pressure of the fuel pump. The inside of the cartridge  12  is at about ambient pressure. This difference in pressure results in the first rupture disc  40  bursting to provide about 200 psig of pressure differential across the piston face  56 . 
   The piston assembly  48  then moves toward the second end  54  of the cartridge  12  which pressurizes the interior of the cartridge  12  creating a pressure differential across the second rupture disc  38  causing it to burst. The hypergolic fluid  64  is then directed out the ignitor outlet  68  into the combustion chamber. Liquid oxygen, or other appropriate oxidizer, is provided into the combustion chamber through the injector. When the oxidizer and hypergolic fluid  64  mix, a spontaneous combustion occurs. This will light the propellant, or rocket fuel also provided to the chamber through the injector. 
   The piston assembly  48  continues to move toward the second end  54  until it stops at the end cap  14 . In the preferred embodiment, nubs  58  come to rest against the interior surface  70  of the end cap  14 . The nubs  58  have been found effective in ensuring that a channel remains in front of the piston face  56  when the nubs  58  contact the interior surface  70  of the end cap  14 . This assists in preventing the piston face  56  from sealing against the end cap  14 . Of course, channels could also be formed in the piston face  56 . 
   In the preferred embodiment, the TEA/TEB mixture is delivered at a rate of about 1.0 lbm/sec to the combustion chamber to mix with liquid oxygen propellant supplied from the combustion chamber injector. About 35 cubic inches of hypergolic fluid are delivered in the preferred embodiment. This hypergolic mixture produces about 18,000 BTU/sec at about 3,000 degrees Fahrenheit for approximately 0.9 seconds, long enough to ignite the propellant provided from the fuel pump. Obviously, the ignitor volume, piston area and operating pressures can be tailored for desired ignition time or energy requirements. 
   Purge grooves  72  may be located in the interior surface of the cartridge  12  proximate to the second end  54  of the cartridge  12 . In the preferred embodiment, two grooves at one hundred eighty degrees apart have been found adequate. The purge grooves  72  provide a flow path for fuel to be directed past the sides of the piston assembly  48  and through the channel and out the ignitor outlet  68  when the piston assembly  48  has traversed the length of the cartridge  12  to a discharged position. 
   After the start sequence, the rocket fuel, or propellant is pumped into the combustion chamber by the fuel pump. A portion of the discharge of the fuel pump continues to be supplied through the ignitor inlet  66  into the ignitor  10 . This fuel is used to purge the remaining hypergolic fluid from the ignitor  10  which remained after the piston assembly  48  moved over the purge grooves  72  located on the interior surface of the cartridge  12  and out the channel formed due to the piston face  56  not sealing against the end cap  14  to the ignitor outlet  68 . The purge grooves  72  can be machined into the interior surface of the cartridge  12 . The fuel, and remaining hypergolic fluid, if any, continue out the ignitor outlet  68  to be consumed in the combustion chamber. It is desirable to remove any hypergolic fluid from the ignitor  10  so that when the engine is recovered, the ignitor  10  can be disassembled in relative safety. If any hypergolic fluid remains in the cartridge  12 , opening of the end caps  14 , 16  would expose the remaining fluid to oxygen thereby instigating spontaneous combustion of the remaining fluid. 
   Due to the modular, and relatively simple construction, the ignitor  10  can be reused after relatively simple refurbishment and cleaning. 
   Numerous alterations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to the preferred embodiment of the invention which is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.