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CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application of, and claims priority to and the benefit of, co-pending U.S. Provisional Application Ser. No. 62/268,752, filed Dec. 17, 2015, the full disclosure of which is hereby incorporated by reference herein for all purposes. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Disclosure 
         [0003]    The present disclosure relates to a system for use with a borehole excavating system that employs reactive materials that selectively generate impulse forces in the excavating system. 
         [0004]    2. Description of Prior Art 
         [0005]    Hydrocarbon producing wellbores extend below the Earth&#39;s surface where they intersect subterranean formations in which hydrocarbons are trapped. The wellbores generally are created by drill bits that are on the end of a drill string, where typically a drive system above the opening to the wellbore rotates the drill string and bit. Cutting elements on the drill bit scrape or otherwise impact the bottom of the wellbore as the bit is rotated and excavate material from the formation thereby deepening the wellbore. Drilling fluid is typically pumped down the drill string and discharged from the drill bit into the wellbore. The drilling fluid flows back up the wellbore in an annulus between the drill string and walls of the wellbore. Cuttings produced while excavating are carried up the wellbore with the circulating drilling fluid. 
         [0006]    During drilling, cutters or teeth formed on the cutting surfaces of the drilling bits impart forces onto the subterranean formation. The forces include shear forces generated by rotation of the drill bit with respect to the bottom of the borehole. Compressional forces are also transferred between the bit and formation, where the compressional forces are from a combination of the weight of a drill string on which the bit is attached and a column of drilling fluid flowing within an axial bore in the drill string. Except when changing bits due to wear or failure, the bit remains in contact with the formation during drilling of the wellbore. 
       SUMMARY 
       [0007]    Disclosed herein is an example of a system for excavating within a wellbore and that includes a drill string, a housing having an end that couples to the drill string, actuators in the housing that are selectively extendable and that each have an end coupled with the housing, and a ram assembly having an end coupled to a drill bit, and that couples to ends of the actuators opposite from the ends of the actuators that couple with the housing, so that when the actuators are selectively extended, the drill bit selectively extends a distance from the drill string. 
         [0008]    In an example, each of the actuators exerts a force onto the ram assembly when selectively extended, and wherein the actuators are arranged in series in the housing such that a sum of the forces is transmitted to the ram assembly. In an example when the actuators are selectively extended, the drill bit is axially displaced an amount substantially equal to the axial elongation of a one of the actuators. The members can optionally be made from an activatable material that elongates in response to applied electricity. Examples of activatable material include piezoelectric material, a magnetorestrictive material, and combinations thereof. 
         [0009]    In one embodiment, the bit is made up of an outer bit having an axial bore, and an inner bit that reciprocates within the axial bore in response to the actuators being changed into the activated state. The actuators can be axially elongated when selectively activated. Optionally, the housing can hollow with bulkheads formed in the housing at axially spaced apart locations, and wherein outer peripheries of each of the bulkheads couple with an inner surface of sidewalls of the housing. In this example, the ends of the actuators that couple with the housing are in abutting contact with the bulkheads. In an embodiment, planar radial walls are provided inside of ram assembly, and that extend in a direction transverse to an axis of the ram assembly, and wherein ends of the actuators that couple with the ram assembly abut the radial walls. In an alternative, the ram member coaxially moves within the housing when the actuators are selectively extended. 
         [0010]    Also disclosed herein is a method of excavating within a wellbore and that includes rotating a drill string in the wellbore that includes drill pipe, a drill bit coupled to the drill pipe, and actuators disposed between the drill pipe and drill bit, generating actuating forces with the actuators by selectively elongating each of the actuators a designated distance, and imparting a summation of the actuating forces against the drill bit to urge at least a portion of the drill bit away from the drill pipe an urged distance that is substantially the same as the designated distance. 
         [0011]    The actuators can be elongated at a resonant frequency, such as a resonant frequency of the drill string, or a resonant frequency of a formation that surrounds the wellbore. Selectively elongating each of the actuators a designated distance can involve directing electricity to a magnetorestrictive member disposed in the actuator that axially expands and generates a one of the axial forces. The portion of the drill bit urged away from the drill pipe can be an inner bit that is proximate an axis of the drill bit. In one embodiment, at least a portion of the drill bit is all of the drill bit, and when at least a portion of the drill bit is urged away from the drill pipe the urged distance, the drill bit is urged into excavating contact with a bottom of the wellbore. 
         [0012]    Another example of a system for excavating within a wellbore is described herein and that includes a bottom hole assembly that selectively couples to a drill string, actuators in the bottom hole assembly that are selectively extendable a designated distance and that each exert a force when extended, a drill bit coupled with the bottom hole assembly, and a means for transferring the combined forces exerted by the actuators to the drill bit, and urging the drill bit a distance away from the drill string that is substantially the same as the designated distance. The actuators can include members made up of material that is responsive to an application of electricity. The bottom hole assembly can further include a housing that is coupled with the drill string, and wherein members are arranged in series in the housing, and ends of each of the members are coupled with the housing. In one alternate embodiment, the bottom hole assembly includes a ram assembly that couples to the drill bit, and wherein ends of the members opposite from the ends that couple with the housing couple to the ram assembly, so that when the members expand, the ram assembly is urged axially a distance that is substantially the same. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]    Some of the features and benefits of the present disclosure having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a sectional view of an example of a drilling system having actuators for delivering an axial force to a drill bit. 
           [0015]      FIG. 2  is a sectional view of an alternate example of the drilling system of  FIG. 1 . 
           [0016]      FIG. 3  is an axial view of an example of the drilling system taken along lines  3 - 3  of  FIG. 1 . 
           [0017]      FIGS. 4A and 4B  show an example of the drilling system of  FIG. 1  respectively in a retracted and an extended configuration. 
           [0018]      FIGS. 5A and 5B  show an example of the drilling system of  FIG. 2  respectively in a retracted and an extended configuration. 
       
    
    
       [0019]    Embodiments described here are not intended to limit the present disclosure to those embodiments. On the contrary, the present disclosure is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of what is described. 
       DETAILED DESCRIPTION 
       [0020]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term “about” includes +/−5% of the cited magnitude. In an embodiment, usage of the term “substantially” includes +/−5% of the cited magnitude. 
         [0021]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
         [0022]    Shown in a side sectional view in  FIG. 1  is one example of a drilling system  10  for use in forming a wellbore  12 . In this example wellbore  12  intersects formation  14 , and a wellbore wall  15  is defined at the intersection of wellbore  12  and formation  14 . A drill string  16  is shown projecting into wellbore  12  and which is rotated by a rotary table  18  on surface. Sections of drill pipe  20  may be added on top of drill string  16  with use of a derrick  22  shown mounted over an opening of wellbore  12 . Optionally, a top drive (not shown) may be mounted to derrick  22  and used for rotating drill string  16  in lieu of rotary table  18 . A bottom hole assembly (“BHA”)  24  is shown coupled to drill string  16 . BHA  24  is made up of an elongated housing  26  that is hollow and whose outer periphery is made up of sidewalls  27  that extend along a length of the housing  26  and curve around an axis A X  of BHA  24 . The outer surface of sidewalls  27  resembles a cylindrical shape. Inside of housing  26  are elongate compartments  28   1-n  that are formed in series. The compartments  28   1-n  are defined between planar bulkheads  30   1-n  that project radially between the sidewalls  27  of housing  26  at axially spaced apart locations. A ram assembly  32  is shown coaxially disposed within housing  26 , and which has sidewalls  33  that define an outer lateral periphery of the ram assembly  32 . Sidewalls  33  of the ram assembly  32  are curved around the axis A X  of the bottom hole assembly  24  and extend generally parallel with sidewalls  27  of housing  26 . Similar to the bulkheads  30   1-n  in housing  26 , are planar radial walls  34   1 ,  34   2  that extend radially between the sidewalls of the ram assembly  32  at axially spaced apart locations to form compartments  36   1-n  within ram assembly  32 . 
         [0023]    BHA  24  further includes actuators  37   1-n  that selectively apply a cumulative force against the housing  26 , and an opposing force against ram assembly  32 . More specifically, actuators  37   1-n  of  FIG. 1  are made up of reactive members  38   1-n , that in the illustrated embodiment are disposed in housing  26 . Further illustrated is that each of the reactive members  38   1-n  have an end that is coupled with the housing  26  via contact with an associated bulkhead  30   1-n . Examples of the reactive members  38   1-n  include things that change in size or shape. Embodiments exist where the change in size or shape is in response to applied energy, such as electricity or magnetism; or introducing a fluid to the actuators  37   1-n  such as hydraulic or pneumatic. Changes in size include becoming longer, shorter, wider, thinner, or combinations thereof. Example constituents of the reactive members  38   1-n  include electro-active materials, magnetostrictive materials, magneto-active materials, lead-zirconate-titanate, lead-magnesium-niobate, terfenol-D, galfenol, and combinations thereof. An opposing end of each of the reactive members  38   1-n  couples with the ram assembly  32  via resilient members  40   1-n  where each of the resilient members  40   1-n  are in contact with the ram assembly  32 . In the example of  FIG. 1 , resilient member  40   1  abuts a drill chuck  42  shown formed on a lower end of ram assembly  32 . As will be described in more detail below, ram assembly  32  and drill chuck  42  are recriprocatable with respect to the housing  26  and drill pipe  20  portion of the drill string  16 . In the illustrated example, resilient member  40   2  mounts on radial wall  34   1 , resilient member  40   3  mounts on radial wall  34   2 , and resilient member  40  mounts on radial wall  34   n . Examples of the resilient members  40   1-n  include springs, Belleville washers, elastomeric members, combinations thereof, and the like. In an alternate embodiment, resilient members  40   1-n  are not included so that the ends of the reactive members  38   1-n  directly contact the ram assembly  32 . 
         [0024]    A drill bit  44  is shown mounted to drill chuck  42  on an end of drill chuck  42  that is opposite from its connection to ram assembly  32 . Drill bit  44  is equipped with cutters  46  on its cutting face for excavating wellbore  12 . Further shown in  FIG. 1 , is a controller  48  which connects to a communication means  49  for communicating signals and/or electrical power to the reactive members  38   1-n . In one example of operation, reactive members  38   1-n  respond to applied electrical energy (such as that provided from controller  48  via communication means  49 ) by elongating, which imparts a force against the housing  26 , and another force against ram assembly  32  that is in a direction opposite to the force applied to the housing  26 . Embodiments exist where controller  48  includes a power supply (not shown) from which electricity is selectively provided to reactive members  38   1-n . In an alternate embodiment, a dedicated power supply  50  is shown with an output line connecting to communication means  49  and through which electricity is routed downhole. An interface  51  between the controller  48  and power supply  50  provides communication from controller  48  to power supply  50  for providing electricity to communication means  49 . It should be pointed out that ram assembly  32  is axially movable with respect to housing  26 , so that the oppositely directed forces applied by the reactive members  38   1-n  to the housing  26  and ram assembly  32  causes ram assembly  32  to move axially with respect to housing  26 . In one example, the applied forces of the reactive members  38   1-n  axially urges the ram assembly  32 , thereby axially moving drill chuck  42  and drill bit  44  in a direction away from drill string  16  and towards the bottom of the wellbore  12 . Further, the axial movement of the drill bit  44  is with respect to the rest of the drill string  16 , increases the force exerted by the drill bit  44  against the bottom of wellbore  12  to above that of the weight on bit. 
         [0025]    Thus selectively generating forces against ram assembly  32  with reactive members  38   1-n  can generate a reciprocating motion of bit  44  against the bottom of wellbore  12 , wherein the resultant force is greater than the standard weight on bit that takes place during a normal drilling operation. An advantage of the strategic combination of the reactive members  38   1-n  within housing  26  and ram assembly  32  creates a resultant force on the ram assembly  32 , and thus drill bit  44 , which is cumulative of the forces generated by each of the reactive members  38   1-n . Moreover, the axial displacement of the ram assembly  32  with respect to the rest of the drill string  16  is about that of an axial extension of a single one of the reactive members  38   1-n  rather than a sum of all of their elongations. In one example, controller  48  energizes actuators  37   1-n  at designated intervals of time, and at designated durations of time, so that the frequency at which the bit  44  strikes the bottom of the wellbore  12  is at a designated frequency. Examples of designated frequencies are a resonant frequency of the drilling system  10 , a resonant frequency of the rock making up the formation  14 , or a combination thereof. Resonance is a phenomenon seen by some cyclical systems, whereby energy from one cycle is stored by the system and used in the next cycle. In one example of the drilling system  10  described herein, recycling of energy between cycles allows for a greater impact force of the percussive elements than could be achieved for a non-resonant percussive system using the same energy input. It is well within the capabilities of one skilled in the art to operate controller  48  so that the actuators  37   1-n  are energized at the designated time intervals and durations so the bit  44  strikes the bottom of the wellbore  12  at the designated frequency. 
         [0026]    The high frequency vibration imparted against the formation  14  creates a series of impacts that cause compressive failure of the formation  14  under load, which is in addition to the shear failure caused by rotating the bit  44  while in contact with the formation  14 . Tuning the frequency of vibration of the drilling system  10  to a resonance mode increases drilling efficiency above that of operating at a range of different frequencies, or by rotating the drill string  16  alone. An advantage of the arrangement shown is that although the actuators  37   1-n  are arranged in series, the resulting force is as though the actuators  37   1-n  were in parallel, that is, the resulting force is substantially equal to the sum of force exerted by each of the actuators  37   1-n . Moreover, in an example the axial displacement of the bit  44 , due to the cumulative axial displacement of the actuators  37   1-n  is substantially the same as if the actuators  37   1-n  are in parallel. In an embodiment, the Young&#39;s modulus of the rock making up the formation  14  can be inferred from the frequency of vibration of the BHA  24 , as the stiffness of the rock will have an effect on the resonant frequency of the system  10 . 
         [0027]    The velocity of the mass m of the bottom hole assembly  24  changes by Δv during impacts of the oscillator of period τ, due to the contact harmonic force F=P d  sin(πt/τ) which is governed by Equation 1, for the changing momentum of the system. 
         [0000]    
       
         
           
             
               
                 
                   
                     m 
                      
                     
                         
                     
                      
                     Δ 
                      
                     
                         
                     
                      
                     v 
                   
                   = 
                   
                     
                       
                         ∫ 
                         0 
                         τ 
                       
                        
                       
                         
                           P 
                           d 
                         
                          
                         
                           sin 
                            
                           
                             ( 
                             
                               πt 
                               τ 
                             
                             ) 
                           
                         
                          
                         dt 
                       
                     
                     = 
                     
                       
                         
                           P 
                           d 
                         
                         ( 
                         
                           
                             2 
                              
                             τ 
                           
                           π 
                         
                         ) 
                       
                       . 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0028]    In one example, the uniaxial compressive strength of a rock is defined as the value of the peak stress sustained by a rock specimen subjected to failure by uniaxial compression. It is the maximum load supported by the specimen during the test divided by the effective contact area subjected to the compression. Thus the compressive strength of the rock; 
         [0000]        U   S   =P   d   /A   e ,  Equation 2;
 
         [0000]    where A e  is the effective area, which in an example is assumed to be about 5% of the area of the hole drilled. 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                           ( 
                           
                             
                               2 
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                         0.05 
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                           D 
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                       ( 
                       
                         by 
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                         substituting 
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                           1 
                         
                       
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                    
                   
                       
                   
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                   3 
                 
               
             
           
         
       
     
         [0029]    Assuming that the drill bit  44  performs a harmonic motion between impacts, in this example the maximum velocity of the drill bit is V m =Aω, where A is the amplitude of the vibration and ω=2πf is its oscillation frequency in rad/s. Assuming further that the impact occurs when the drill bit  44  has maximum velocity V m  and that the drill bit  44  stops during the impact, then Δv=V m =2Aπf. Accordingly in this example, the vibrating mass is expressed as: 
         [0000]    
       
         
           
             
               
                 
                   m 
                   = 
                   
                     
                       
                         0.05 
                          
                         
                           D 
                           2 
                         
                          
                         
                           U 
                           s 
                         
                          
                         τ 
                       
                       
                         
                           4 
                            
                           π 
                            
                           
                               
                           
                            
                           Af 
                         
                          
                         
                             
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
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                   4 
                 
               
             
           
         
       
     
         [0030]    The period of the impact, τ, in the above expression can be determined by many factors including the material properties of the formation  14  and the bottom hole assembly  24 , other factors include the frequency of impacts. In one example of operation, τ is estimated to be about 1.0 percent of the period of oscillation, that is, τ=0.01/f. By substituting τ into Equation 4 a lower bound estimation of the resonant frequency that can provide enough impulse for the impacts is given by Equation 5 as follows. 
         [0000]    
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       
                         
                           
                             D 
                             2 
                           
                            
                           
                             U 
                             s 
                           
                         
                         
                           8000 
                            
                           
                               
                           
                            
                           π 
                            
                           
                               
                           
                            
                           Am 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   5 
                 
               
             
           
         
       
     
         [0031]    In an example, Equation 5 provides a lower bound estimate for the stable frequency of the oscillator. The use of a frequency too much greater than this lower bound frequency can generate a crack propagation zone in the formation  14  that is in front of the drill bit  44  during operation, which could lead to compromise borehole stability and reduced borehole quality. Moreover, if the oscillation frequency is too large then accelerated tool wear and failure may occur. A scaling/safety factor, S f , with appropriate value less than 1.0 can be applied to the frequency as a precautionary measure. 
         [0032]    The dynamic force, P d , applied to the oscillation system can be calculated by rearranging Equation 2 and can be expressed as follows: 
         [0000]        P   d   =A   e   U   S =π/4( D   e   2   U   S )  Equation 6;
 
         [0000]    where in this example D e  is an effective diameter associated with effective area (A e ) of the rotary drill bit  44  which is the diameter, D, of the drill bit  44  scaled according to the fraction of the drill bit  44  which contacts the material being drilled. Thus in this example, the effective diameter, D e , can be defined as: 
         [0000]        D   e =√{square root over ( S   C )} D   Equation 7;
 
         [0000]    where S C  is a scaling factor corresponding to the fraction of the drill bit  44  which contacts the material being drilled. For example, estimating that only 5% of the drill bit surface is in contact with the material being drilled, D e =√{square root over (0.05)}D. An appropriate value of scaling/safety factor can be introduced to the dynamic force, P d , according to the material being drilled so as to ensure that the crack propagation zone does not extend too far from the drill bit  44 , and consequently compromising borehole stability and reducing the borehole quality. 
         [0033]    Another factor to consider is that the resonant frequency changes when drilling through different rock types. The compressive strength can be related to an optimal frequency range. It was therefore considered that the lower frequency range can be in relation to changing rock properties, looking at the right hand side of Equation 5 and introducing a factor, S f . 
         [0000]      √{square root over (( D   2   U   S /8000π Am )))}≦ f≦S   f √{square root over (( D   2   U   S /8000π Am )))}  Equation 8.
 
         [0034]    Referring now to  FIG. 2 , shown in a side sectional view is an alternate example of a drilling system  10 A used in forming a wellbore  12 A in a formation  14 A. In this example, the drilling system  10 A includes many of the same elements of the drilling system  10  of  FIG. 1 , that is, a drill string  16 A in the wellbore  12 A, a rotary table  18 A, drill pipe  20 A, a derrick  22 A, a BHA  24 A having a housing  26 A, and sidewalls  27 A on the housing  26 A. Further making up the BHA  24 A are compartments  28 A 1-n  the housing  26 A, and bulkheads  30 A 1-n  at opposing axial ends of the compartments  28 A 1-n  A generally cylindrically shaped ram assembly  32 A is coaxially disposed in the housing  26 A having axial sidewalls  33 A and radial walls  34 A 1-n  that are transversely mounted within sidewalls  33 A. Axially between the radial walls  34 A 1-n  are compartments  36 A 1-n  which actuators  37 A 1-n  are provided and that include reactive members  38 A 1-n  Resilient members  40 A 1-n  provided in the compartments  36 A 1-n  exert a biasing force against reactive members  38 A 1-n . 
         [0035]    A difference between the embodiments of  FIGS. 1 and 2  concerns the bit  44 A. As shown, bit  44 A is made up of a main bit  52 A having an axial bore  54 A extending therethrough. An inner bit  56 A is included with the main bit  52 A that reciprocates within bore  54 A. Here, the inner bit  56 A has an upstream end that attaches to a lower end of ram assembly  32 A via a connecting rod  58 A. Thus, in this example, actuating the reactive members  38 A 1 ,  38 A 2 , . . . ,  38 A n  generates a resultant force in ram assembly  32 A which transfers only to inner bit  56 A to reciprocate it within the main bit  52 A. Further, main bit  52 A is shown mounted to a lower end of housing  26 A. 
         [0036]    Because housing  26 A is not axially motivated by actuators  37 A 1-n , main bit  52 A does not axially reciprocate in response to operation of actuators  37 A 1-n  and thus generally maintains its axial distance from the lower end of drill string  16 A. Instead, main bit  52 A is limited to rotation within wellbore  12 A, much like a standard drill bit. Further, cutters  60 A,  62 A are shown respectively formed on the downhole ends of inner bit of  56 A and outer or main bit  52 A. In bits that rotate about their axes, the radial speed of the bit, and thus the cutters on the bit, becomes lower with proximity to the bit axis. Meaning the region of a bit proximate its axis is less effective for rotational drilling that regions of the bit distal from the bit axis. An advantage of focusing the axial vibration of the effective bit area towards its inner radius is that when the cutters  60 A on the inner bit  56 A are out of contact with the formation  14  (due to reciprocation of the inner bit  56 A), the amount of cutting force per bit surface area lost is less than that if an outer portion of the bit  44 A is moved away from the formation  14 . As such, adding the axial vibration and forces on the ensuing rock enhances the operational functionality of the bit  44 A of  FIG. 2 . Examples exist where cutters  60 A,  62 A are formed from composites, such as poly-crystalline diamond. 
         [0037]      FIG. 3  is an axial sectional view of an example of the BHA  24  taken along lines  3 - 3  of  FIG. 1 . In this example, a coil  64  is shown between ram assembly  32  and reactive member  38   1 . As is known, selectively energizing the coil  64  with electricity generates an electrical field that as explained above axially elongates the reactive member  38   1 . Electricity for energizing the coil  64  can be from surface, such as from controller  48  or power supply  50  ( FIG. 1 ), from a battery (not shown) included with the bottom hole assembly  24 , or from a downhole generator (not shown) that converts fluid flow to electricity. As shown reactive member  38   1  coaxially inserts into a sleeve  66  that can provide protection/isolation for the reactive member  38   1 . Further illustrated are supports  68  that extend radially between the ram assembly  32  and housing  26 . Annular spaces  70  are defined in the circumferential spaces between adjacent supports  68  and the radial spaces between the ram assembly  32  and housing  26 . In an example of operation, drilling fluid flows downhole within the annular spaces  70 , and back uphole within an annulus  72  between the outer surface of the housing  26  and walls of the wellbore  12 . 
         [0038]      FIGS. 4A and 4B  provide in a side sectional view an example of how the drill bit  44  of the drilling system  10  reciprocatingly contacts the bottom  74  of the wellbore  12 , thereby creating fractures in the formation  14 . Referring specifically to  FIG. 4A , here the drill string  16  of the drilling system  10  is disposed in the wellbore  12  in a retracted mode so that the bit  44  is spaced away from a bottom  74  of the wellbore  12 . In the retracted mode, the members  38   1-n  are in an unelongated state. In an example where members  38   1-n  are magnetostrictive material, the members  38   1-n  are not energized and electricity from controller  48  or power supply  50  is not being transmitted to the members  38   1-n . Referring now to  FIG. 4B , the members  38   1-n  are depicted in an elongated state. In an embodiment where the members  38   1-n  are made from magnetostrictive material, the elongation can be due to applied electricity, such as from controller  48 A or power source  50 . In the elongated state of  FIG. 4B , the members  38   1 ,  38   2 ,  38   3 , and  38   n , have elongated over their lengths shown in  FIG. 4A  by the respective distances D 1 , D 2 , D 3 , and D n . 
         [0039]    Further illustrated is that the bit  44  has moved a distance D BIT  in the wellbore  12 . As described above, the movement of the bit  44  is in response to movement of the members  38   1-n  via the coupling between the members  38   1-n  and ram assembly  32  ( FIG. 1 ). Additionally, in one example, the distances D 1 , D 2 , D 3 , and D n  (that can be referred to as designated distances) all have substantially the same value. Further in this example, distance D BIT  has a value that is substantially the same as the value of any one of distances D 1 , D 2 , D 3 , and D n . Accordingly, in this example, the novel configuration of the housing  26  and ram assembly  32  results in the distance D BIT  not being a sum of the individual distances D 1 , D 2 , D 3 , and D n . 
         [0040]    Further illustrated in  FIG. 4B  are arrows that respectively represent forces F 38   1 , F 38   2 , F 38   3 , and F 38   4  generated by the members  38   1-n  when being actuated/elongated. Another arrow represents force FBIT which is the force being transmitted to drill bit  44  from elongation of the members  38   1-n , and which is substantially equal to a summation of forces F 38   1 , F 38   2 , F 38   3 , and F 38   4 . As indicated above, ends of the members  38   1-n  couple with the housing  26 , and opposing ends of the members  38   1-n  couple with the ram assembly  32 . Thus the ram assembly  32 , the attached drill chuck  42 , and drill bit  44 , are moved away from the housing  26  and drill pipe  20  by elongating the members  38   1-n . Strategically coupling the members  38   1-n  with the ram assembly  32  via the radial walls  34   1-n  and housing  26  via the bulkheads  30   1-n  allows for reciprocation of the drill bit  44  a distance substantially the same as the elongation of individual members  38   1-n  while also exerting a cumulative force onto drill bit  44  so that its reciprocating force F BIT  is substantially the same as the sum of forces F 38   1 , F 38   2 , F 38   3 , and F 38   4 . An advantage of reciprocating the drill bit  44 , while also rotating the drill bit  44 , is that when the drill bit  44  is reciprocatingly thrust against the bottom  74  of the wellbore  12 , fractures  76  are formed in the formation  14  adjacent the bottom  74  of the wellbore  12 . The fractures  76  can reduce inherent stresses in the formation  14 , which increases the amount of rock removed with each rotation of the drill bit  44 , that in turn increases rate of penetration of the drilling operation. 
         [0041]      FIGS. 5A and 5B  show in a side sectional view an example of reciprocating motion of the drill bit  44 A of  FIG. 2 . In the example of  FIG. 5A  the drill string  16 A is in the retracted configuration with the members  38 A 1-n  in an unelongated state. Further, the inner bit  56 A is spaced upward from the bottom  74 A of the wellbore  12 A with its cutters  60 A out of contact with the bottom  74 A, while the main bit  52 A is at the bottom  74 A of the wellbore  12 A and its cutters  62 A in rotating contact with the bottom  74 A. In an example where members  38 A 1-n  include magnetostrictive material, the members  38 A 1-n  are not energized and electricity from controller  48 A or power supply  50 A is not being transmitted to the members  38 A 1-n . 
         [0042]    In the example of  FIG. 5B , the members  38 A 1-n  are depicted in an elongated state. In an embodiment where the members  38 A 1-n  are made from magnetostrictive material, the elongation can be due to applied electricity, such as from controller  48 A or power supply  50 A. In the elongated state the members  38 A 1 ,  38 A 2 ,  38 A 3 , and  38 A n , have lengthened over that of their lengths in  FIG. 5A  by the respective distances D 1A , D 2A , D 3A , and D nA . Further illustrated is that the inner bit  56 A has moved a distance D BITA  with respect to the main bit  52 A. In this example the main bit  52 A is coupled with the housing  26 A by a threaded connection  78 A, and unlike the inner bit  56 A, the main bit  52 A does not reciprocate with movement of the ram assembly  32 A. As described above, the movement of the inner bit  56 A is in response to movement of the members  38 A 1-n  via the coupling between the members  38 A 1-n  and ram assembly  32 A ( FIG. 2 ). 
         [0043]    Additionally, in one example, the distances D 1A , D 2A , D 3A , and D A  (that can be referred to as designated distances) all have substantially the same value. Further in this example, distance D BITA  has a value that is substantially the same as the value of any one of distances D 1A , D 2A , D 3A , and D nA . An advantage to reciprocating a portion of the cutting surface of the bit  44 A proximate the axis A X  is that the portions of the cutting surface proximate the axis A X  have a reduced excavating effectiveness than those portions of the cutting surface distal from the axis A X . The bit  44 A therefore can remain substantially effective in excavating even when the inner bit  56 A is spaced away from the bottom  74 A ( FIG. 5A ). Moreover, the main bit  52 A is shown creating fractures  76 A in the formation  14 A adjacent the bottom  74 A, which can improve the excavating efficiency of the bit  44 A as a whole. 
         [0044]    In embodiments where the actuators  37   1-n ,  37 A 1-n , do not include the members  38   1-n ,  38 A 1-n  the distances D BIT , D BITA  will be substantially the same as elongation of one of the individual actuators  37   1-n ,  38 A 1-n  rather than a sum of their distances. Similarly, the corresponding forces F BIT , F BITA  on the bits  44 ,  44 A will be substantially the same as the sum of forces from the extended actuators  37   1-n ,  37 A 1-n  when the actuators  37   1-n ,  37 A 1-n  do not include the members  38   1 ,  38 A 1-n . 
         [0045]    The embodiments described above are well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent. While a presently preferred embodiment has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the embodiments disclosed herein and the scope of the appended claims.

Summary:
A force stacking assembly for use with an earth boring system that includes a series of actuators that each generate a force, and that are arranged to create a combined force that is cumulative of all of the actuators. The actuators include members that react in response to an applied stimulus, such as from an electrical current or magnetic field. The members are arranged in series in a hollow housing, planar bulkheads are transversely mounted in the housing. Each of the members have an end axially abutting a corresponding bulkhead. Ends of each member distal from it corresponding bulkhead couple to a ram member, that in turn couples to a drill bit. Energizing the members causes each to exert a force against the ram member, which is transferred to the bit.