the first embodiment of the atmospheric propulsion engine is illustrated schematically in fig1 and generally by reference numeral 1 . it is a single - cycle ( unicycle ), spark ignition engine and compressor having a cylinder 02 with cylinder heads 012 a and 012 b on each end and having a piston 03 slidably interposed therebetween , forming alternate combustion and compression chambers 010 a and 010 b . cylinder heads 012 a , 012 b contain fuel injectors 08 a , 08 b and igniters 09 a , 09 b . the engine has thrust nozzles 04 a , 04 b with associated valves 06 a , 06 b and actuators 014 a , 014 b which sense piston 03 obturation of nozzle ports 011 a , 011 b to effect appropriate valve , fuel injection , and ignition timing for sustained operation as further explained below . the engine also has common exhaust / inlet ports 07 a , 07 b which perform the dual functions of exhausting combustion gasses and admitting atmospheric air for propulsion , scavenging , and cooling . exhaust / inlet ports 07 a , 07 b are opened and closed by valves 05 a , 05 b and associated actuators 013 a , 013 b which sense obturation of exhaust / inlet ports 07 a and 07 b and enforce the appropriate valve action of valves 05 a , 05 b , 014 a , and 014 b . the valves , actuators , fuel injectors , and igniters for the first embodiment are conventional elements whose description will be omitted here for the sake of brevity . referring to fig2 piston 03 is positioned in cylinder 02 such that the air charge and fuel mixture in chamber 010 a is at the required combustion pressure . with piston 03 in this position , igniter 09 a is energized to initiate combustion in chamber 010 a and begin the cycle . as further shown in fig2 nozzle valve 6 b is open , nozzle valve 06 a is closed , and valves 05 a , 05 b are closed at this part of the cycle and piston 03 begins accelerating to the right due to the combustion pressure in chamber 010 a ( dashed line arrows indicate movement ). as piston 03 moves to the right under the impetus of the combustion pressure in chamber 010 a , the air / exhaust mixture in 010 b is compressed and expelled through nozzle port 011 b , open nozzle valve 06 b and thrust nozzle 04 b , thus generating the thrust tb . as shown in fig3 when piston 03 crosses exhaust / inlet ports 07 a , actuator 013 a senses port 07 a closure and opens nozzle valve 06 a and causes actuator 13 a to open exhaust / inlet port 07 a via the valve 05 a . fig4 shows the opened state of nozzle valve 06 a and exhaust / inlet port 07 a . at this point , piston 03 reaches its maximum velocity . the remaining unexpanded low - pressure combustion gasses are then exhausted through exhaust / inlet port 07 a and nozzle port 011 a , nozzle valve 06 a , and thrust nozzle 04 a . meanwhile , piston 03 continues to travel to the right in cylinder 02 due to its inertial energy . the continuing rightward movement of piston 03 , draws atmospheric air into chamber 010 a through exhaust / inlet port 07 a and nozzle 04 a . nozzle 04 a is open at this event time to provide scavenging and dilution of the exhaust products . the distance between nozzle port 011 b and cylinder wall 012 b is prefixed such that the mass of air charge required for subsequent combustion in chamber 010 b is attained as piston 03 crosses and obturates nozzle port 011 b as shown in fig4 . at this point , the loss of high pressure in port 011 b sensed by actuator 014 b initiates five actions , shown in fig4 : the actuator 013 a for slide valve 05 a closes off exhaust / inlet ports 07 a ; the actuator 014 b closes nozzle port 011 b ; the injector 08 b injects a metered amount of fuel into chamber 010 b ; actuator 014 a for nozzle valve 06 a opens nozzle port 011 a to nozzle 04 a ; and a delayed signal is sent to fire the igniter 09 b when piston 03 achieves maximum compression in chamber 010 b as shown in fig5 . the remaining inertial energy of piston 03 is dissipated in achieving the required combustion pressure in chamber 010 b . the atmospheric propulsion engine has completed one cycle and is in position to repeat the next cycle in the opposite direction . the sequence of this next cycle can be followed by substituting the a and b components for one another and reversing the piston &# 39 ; s direction . [ 0065 ] fig6 illustrates how the inventive engine can be utilized to generate reverse thrust . essentially , the thrust assembly including nozzle port , valve , actuator and thrust nozzle is duplicated . specifically , nozzle port 011 c is disposed opposite to nozzle port 011 a and has attached thereto a valve 06 c , actuator 014 c and reverse thrust nozzle 04 c . a corresponding nozzle port 011 d is disposed opposite to nozzle port 011 b and has attached thereto a valve 06 d , actuator 014 d and reverse thrust nozzle 04 d . to generate reverse thrust , valves 06 c and 06 d would be activated instead of valves 06 a and 06 b , but with the same timing relationship as for valves 06 a and 06 b described above . the result is the generation of reverse thrust tc and td . [ 0066 ] fig7 illustrates a tandem engine design in which two engines 1 are mounted back to back as shown . a tandem configuration joining structure 015 is utilized to affix the two engines 1 to each other in the tandem configuration . in the tandem configuration fuel injection and ignition are synchronized to eliminate reactive engine movements . this synchronization may be accomplished via conventional rack and pinions , linkages , gears , or other mechanical means . of course , the tandem design may also include a reverse thrust arrangement like the one shown in fig6 . [ 0067 ] fig8 shows a spring centered , slidably mounted engine to allow reactive movements . in other words , the atmospheric propulsion engine can be slidably mounted to a vehicle structure 018 via slidable engine mounts 016 a and 016 b and a centering spring mechanism 017 as shown in fig8 . in this way , the engine 1 can traverse a small distance back and forth with slidable engine mounts 016 a , 016 b and centering spring mechanism 017 compensating for reactive forces generated by the engine 1 . [ 0068 ] fig9 is a schematic showing how the unicycle engine indicated by reference 1 can be integrated into an operating system containing adjunct fuel and electrical systems . gas lines 019 with check valves 028 a and 028 b are picked off of cylinder 02 to pressurize high pressure gas reservoir 020 and feed the pressurized fuel tank 021 and turbine generator 023 . the fuel lines 022 feed fuel to fuel injectors 08 a and 08 b . turbine generator 023 charges battery and electronics package 024 which transmits a timed firing signal to igniters 09 a and 09 b at the predetermined event time through electrical lines 025 . appropriate sensors may be utilized to sense the position of the piston via obturation of ports 07 a , 07 b , 011 a , and 011 b so that the actuators 013 a , 013 b , 014 a , 014 b as well as the fuel injectors 08 a , 08 b and igniters 09 a , 09 b can be activated at the correct timing relationship that is described above . these sensors and activators may be , for example , electrical or pneumatic . referring to fig1 , the primary elements of a second embodiment of the invention which is essentially a free piston intermittent pulse rocket engine includes two combustion cylinders , 2 a and 2 b , coaxially located within , and separated by , a thrust chamber cylinder 7 . the combustion pistons 3 a , 3 b and thrust piston 4 are connected and slidably inserted into cylinders 2 a , 2 b , and 7 respectively , forming combustion chambers 5 a , 5 b and thrust chambers 6 a , 6 b . intake check valve assemblies 20 a , 20 b provide a valved inlet for air into thrust chambers 6 a , 6 b via thrust intake ports 19 a , 19 b . the opposing ends of combustion cylinders 2 a , 2 b are closed by cylinder heads 21 a , 21 b that contain fuel injectors 18 a , 18 b , respectively . fuel injectors 18 a and 18 b are fed by the pressurized fuel supply line 28 . the opposing ends of thrust chambers 6 a , 6 b are closed by thrust chamber flanges 8 a , 8 b . injector control gas ports 16 a , 16 b are provided in cylinders 2 a , 2 b and are connected to injector gas control lines 17 a , 17 b , respectively . the other ends of injector gas control lines 17 a , 17 b are , in turn , connected to fuel injectors 18 a , 18 b . as further described below , injector control gas ports 16 a , 16 b activate fuel injectors 18 a , 18 b as combustion pistons 3 a , 3 b cross respective injector control gas port 16 a , 16 b while moving on the compression stroke . exhaust ports 9 a , 9 b formed in combustion chambers 2 a , 2 b allow for expulsion and scavenging of burnt combustion gases via exhaust ducts 10 a , 10 b and exhaust thruster nozzles 11 a , 11 b . scavenge purge lines 14 a , 14 b allow high pressure air from thrust chambers 6 b , 6 a to scavenge combustion chambers 5 a , 5 b through scavenge ports 12 a , 12 b and scavenge inlet ports 15 a , 15 b , when pistons 3 a , 3 b opens combustion chambers 5 a , 5 b to exhaust . scavenge port check valves 13 a , 13 b prohibit counter - flow during the combustion , expansion and compression cycles of each combustion cylinder as further described below . thrust chamber exhaust separators 24 a , 24 b ensure separation of exhaust from combustion chambers 5 a , 5 b to thrust chambers 6 a , 6 b . main thruster check valves 22 a , 22 b interconnect main thruster nozzles 23 a , 23 b with thruster ports 25 a , 25 b and thrust chambers 6 a , 6 b , respectively . pneumatic starter valves 26 a , 26 b allow compressed air from a compressed air source ( not shown ) to enter combustion chambers 5 a , 5 b and permit engine starting . operation of the engine will be described here , while construction and operation of the preferred fuel injector 18 will be described in following paragraphs . assume that piston 3 a is in its compression position to the left of cylinder 2 a as shown in fig1 . when piston 3 a is in its compression position , the volume of chamber 5 a is at its minimum , and compression pressure therein is at a maximum . fuel injection has been accomplished and combustion is underway . piston 3 b has opened chamber 5 b to exhaust through ports 9 b , exhaust duct 10 b and exhaust nozzle 11 b . thrust chamber 6 a has completed expulsion of its thrust gas and its pressure is approaching atmospheric . thrust chamber 6 b has completed its air intake stroke and is near atmospheric pressure . injector gas control port 16 a is at atmospheric pressure through exhaust port 9 a . check valve 13 a is closed since thrust chamber 6 b is at low intake pressure . as the combined piston ( 3 a - 4 - 3 b ) begins moving to the right under the impetus of compression pressure and fuel combustion , the following actions occur : thrust chamber 6 a begins intake of air through check valve 20 a , while check valve 25 a prevents entry of air through nozzle 23 a . pressure builds up in thrust chamber 6 b with the subsequent expulsion of air and generation of thrust through thruster port 25 b , check valve 22 b and nozzle 23 b . check valve 20 b prevents loss of air through the inlet port 19 b . piston 3 b begins closure of cylinder 2 b exhaust ports 9 b . as shown in fig1 , when the combined piston ( 3 a - 4 - 3 b ) has moved right to the point where piston 3 b has closed cylinder 2 b exhaust ports 9 b , the following actions have taken place or now occur : piston 3 b begins compression of the combustion air in chamber 5 b . piston 3 a has uncovered scavenge port 12 a , but check valve 13 a prevents any flow . piston 3 a has uncovered injector gas control port 16 a and reset of the injector 18 a for the next cycle has begun . this will be explained in a following paragraph describing injector operation . expansion of combustion gas in chamber 5 a is increasing the velocity of piston 3 a - 4 - 3 b to the right . under the impetus of piston 4 , pressure is increasing in chamber 6 b , with the resultant increase of mass flow and thrust out of nozzle 23 b . chamber 6 a is ingesting atmospheric air through valve 20 a . as shown in fig1 , when piston 3 a is around mid point of its stroke in cylinder 2 a , its maximum velocity is attained , and it begins to decelerate due to the pressure degradation in chamber 5 a and the opposing forces generated by the increase in pressures in chambers 6 b and 5 b . as shown in fig1 , when piston 3 a crosses exhaust ports 9 a , the following events have taken place or now occur : chamber 5 a is vented to atmosphere through ports 9 a and exhaust nozzle 11 a , with some thrust generation . the pressure in chamber 5 a drops below the pressure in chamber 6 b , thus allowing fresh air from chamber 6 b to enter chamber 5 a through port 15 b , line 14 a , check valve 13 a , and port 12 a . this air then scavenges chamber 5 a through exhaust ports 9 a and exhaust nozzle 11 a . note that the scavenged air is not wasted , but used to generate thrust through exhaust nozzle 11 a . piston 3 b is approaching the point of maximum compression in chamber 5 b . piston 3 b has crossed injector gas control port 17 b and communicated it with exhaust ports 9 b and nozzle 11 b . this begins activation of fuel injector 18 b . this function will be explained in a paragraph . describing injector operation . chamber 6 b is reaching maximum pressure , mass flow through check valve 25 b and nozzle 23 b , and is generating maximum engine thrust output . as shown in fig1 , the mass inertia of piston 3 a - 4 - 3 b then carries it to the point of maximum compression pressure in chamber 5 b , and its velocity reaches zero . at this time , the following conditions exist and the engine repeats the foregoing cycle in the opposite direction as follows : fuel injector 18 b is injecting fuel into combustion chamber 5 b and combustion has begun . the pressure in thrust chamber 6 b has decayed to atmospheric and scavenging of chamber 5 a is complete , while chamber 5 a remains open to exhaust and check valve 13 a ceases interflow between 6 b and 5 a . thrust chamber 6 a has ingested its maximum volume of air and is at near atmospheric pressure . starting of the engine may be accomplished via pneumatic starter valves 26 a , 26 b . specifically , a source of compressed air may be connected to at least one of the pneumatic starter valves 26 a or 26 b . for example , compressed air may be passed through pneumatic starter valve 26 a and enter combustion chamber 5 a thereby moving the piston ( 3 a - 4 - 3 b ) to the right until the operational state shown in fig1 is achieved . at this point , the fuel injector 18 b injects fuel into combustion chamber 5 b , combustion begins , and the engine starts . alternatively , a conventional igniter can be added to at least one of the cylinder heads 21 a , 21 b and utilized as a starting means with appropriate utilization of the pneumatic starter valves to inject compressed air to move the combined piston 3 a - 4 - 3 b to a desired position , actuate a fuel injector 18 and thereby start the engine . furthermore , pneumatic starter valves could also be added to the engine 1 of the first embodiment as an alternative method of starting that engine . furthermore , the system shown in fig9 can be utilized with the engine of the second embodiment as indicated by the common usage of pressurized fuel line 022 . the engine of the second embodiment is preferably equipped with the fuel injector shown in fig1 . for ease of reference , fuel injectors 18 a and 18 b will be collectively referred to as fuel injector 18 it being understood that the same fuel injector 18 design is used for both 18 a and 18 b . as shown in fig1 , the self - actuated , uniaxial fuel injector 18 consists of an injector body 50 into which is slidably mounted an intensifier piston 53 containing a slidably mounted fuel pintle 52 , closure spring 56 , and pintle stop 55 . all of these elements are coaxially located . an annular intensifier piston cylinder stop 54 is attached on the combustion chamber side of the injector body 50 to constrain motion of the intensifier piston 53 . on the opposite end , a fuel quantity plug and stop 51 with seal 64 is centrally located and threadably inserted into the injector body 50 . the fuel quantity plug and stop 51 contains the pressurized fuel inlet connection 36 , fuel inlet passage 62 , and check valve 63 . the check valve 63 allows fuel to flow into the fuel cavity 65 when the cylinder pressure pi is less than the inlet fuel pressure p 4 , enabling the fuel cavity 65 to refill and reset the intensifier piston 53 when the combustion cylinder enters the exhaust phase . the threaded insertion of the fuel quantity and plug 51 into the injector body 50 allows for simple adjustment of the amount of fuel metered for each injection cycle . when installing the fuel injector 18 , the pressurized fuel inlet connection is connected to pressurized fuel line 022 . the fuel injector 18 accomplishes the following functions : meter the amount of fuel required for a single combustion action ; contain that fuel until injection is required ; multiply the fuel injection pressure by the ratio of a 1 to a 2 above the cylinder compression pressure ; inject the fuel into the combustion chamber when the engine piston crosses the gas control port ; reset the pintle and intensifier piston , and refill the injector for the next cycle . refer to fig1 and 18 and assume that injector 18 is filled with fuel ( fuel cavity 65 and passages 62 and 66 ) and ready to perform the injection function . as pressure ( p 1 ) rises in the cylinder chamber 5 during compression , the control gas inlet 28 ( p 3 ) and chambers 58 , 60 and 61 track this pressure through gas control port 16 and injector gas control line 17 until piston 3 crosses gas control port 16 . during this period , the annular volume and area 60 ( p 3 ) is at the same pressure as the compression chamber 5 ( p 1 ), thus , that portion of a 2 is counterbalanced and the effective area under p 1 is equal to a 1 . since the top area of the slidable intensifier piston 53 in contact with the incompressible fuel in fuel cavity 65 is also equal to a 1 , the pressure in fuel cavity 65 ( p 2 ) is equal to p 1 . also , since the gas control pressure p 3 is communicated to control gas pintle cavity 61 , the areas and pressures on top and bottom of the pintle 52 being equal , this allows the pintle closing spring 56 to maintain the pintle 52 in the closed position , thus preventing fuel flow into the cylinder chamber 5 . when the piston 3 has crossed gas control port 16 , control gas inlet 28 ( p 3 ), passage 57 and chambers 58 , 60 , and 61 are vented to atmosphere through injector gas control line 17 , gas control port 16 , exhaust port 9 , and exhaust nozzle 11 . when this occurs , the effective area of the intensifier piston 53 exposed to the compression pressure p 1 is now equal to a 2 , while the effective area on the opposite end in contact with the fuel in cavity 65 ( p 2 ) is still equal to a 1 . thus , the fuel injection pressure p 2 increases in the ratio of a 2 to a 1 ( p 1 × a 2 = p 2 × a 1 ) . this pressure increase is consistent with the operation of a conventional intensifier piston . typically , a cylinder compression pressure p 1 of 1000 psi , might yield a fuel injection pressure p 2 of 4000 psi , but this can be tailored for any specific design by appropriately adjusting , for example , a 2 and a 1 . at the same time , the release of pressure in pintle cavity 61 allows compression pressure p 1 and the increased fuel injection pressure p 2 to act on the pintle 52 nose at injection nozzle 67 , overcoming the force of the pintle closing spring 56 and causing the pintle to snap open . fuel is now injected into combustion chamber 5 at pressure p 2 until the fuel cavity 65 is depleted and the intensifier piston 53 contacts fuel quantity stop and plug 51 as shown in fig1 . pressure p 2 then drops to the more benign pressurized fuel inlet value p 4 , and any further fuel flow through the fuel delivery passage 66 is prevented by its inlet being in contact with the stop 51 . this state and mechanical condition of fuel injector 18 remains constant until there is a change in the gas control pressure p 3 . as piston 3 in cylinder 2 reverses direction for its power stroke under the impetus of compression pressure and combustion , piston 3 recrosses injector gas control port 16 , again communicating control gas inlet 28 ( p 3 ) with combustion pressure p 1 through injector gas control port 16 and gas control line 17 . gas control chambers 58 , 60 , and 61 then rise pressures equal to p 1 . pintle 52 now has equal pressures on both ends , therefore , the pintle closing spring 56 causes the pintle 52 to return to its closed position as shown in fig2 , expelling any residual fuel in its cavity . intensifier piston 53 , however , has an effective area of a 1 exposed to pressure p 1 , and since p 1 is much higher than the fuel supply pressure p 4 , intensifier piston 53 will remain against stop 51 until piston 3 uncovers exhaust ports 9 , and p 1 in combustion chamber 5 decays to near atmospheric pressure . at this point , the fuel supply pressure p 4 is greater than the chamber 5 pressure p 1 , fuel cavity 65 refills until intensifier piston 53 reaches piston cylinder stop 54 . the fuel injector 18 is now reset , primed and ready for the next injection cycle . the invention being thus described , it will be obvious that the same may be varied in many ways . such variations are not to be regarded as departure from the spirit and scope of the invention , and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims .