gas turbine engine systems involving tip fans are provided , several exemplary embodiments of which will be described in detail . in this regard , some embodiments of a gas turbine engine system incorporate the use of a fan that can adapt to a variety of operating conditions , such as supersonic and sub - sonic loiter conditions . in some embodiments , the fan is a multi - stage fan that incorporates a tip fan and is driven by a differential gear assembly . notably , the differential gear assembly enables stages of the multi - stage fan to exhibit different rotational speeds . in this regard , reference is made to fig1 , which schematically depicts an exemplary embodiment of a gas turbine engine system . as shown in fig1 , system 100 incorporates a multi - stage fan 102 that includes a forward fan stage 104 and a rear fan stage 106 . notably , the rear fan stage incorporates an inner fan 108 and a tip fan 109 . specifically , each of the blades of the rear fan stage includes distal end portions that form the tip fan . each of the fan stages includes a corresponding set of rotatable blades , with each of the sets of blades being powered by a differential gear assembly 110 . differential gear assembly 110 is coupled to a low - pressure turbine 112 via shaft 114 . in addition to providing torque for rotating the multi - stage fan , low - pressure turbine 112 powers a low - pressure compressor 116 . low - pressure turbine 112 is located downstream of a high - pressure turbine 118 that is connected through shaft 120 to a high - pressure compressor 122 . a combustor 130 is located downstream of the high - pressure compressor and upstream of the high - pressure turbine . low - pressure compressor 116 , high - pressure compressor 122 , combustor 130 , high - pressure turbine 118 and low - pressure turbine 112 are located along an annular gas flow path 140 . gas flow path 140 also receives a flow of gas from multi - stage fan 102 . however , gas from multi - stage fan 102 also is directed along an annular gas flow path 142 , which is located radially outboard of gas flow path 140 , and along an annular gas flow path 144 , which is located radially outboard of gas flow path 142 . specifically , tip fan 109 is positioned along gas flow path 144 . in operation , the differential gear assembly enables rotational speeds of the fan stages of the multi - stage fan to accommodate various operational requirements . by way of example , for high - speed flight operations , the forward fan stage can be set to a moderate rotational speed while the rotational speed of the rear fan stage is set to a higher rotational speed . notably , achieving a desired rotational speed can be accomplished by altering the pitch and / or camber of the blades of one or more of the fan stages . for instance , by increasing the pitch and / or camber of the blades of the forward fan stage , fan stage work and fan pressure ratio of the forward fan stage is increased , which causes a corresponding decrease in rotational speed of the forward fan stage . responsive to this speed decrease , the differential gear assembly causes the rotational speed of the rear fan stage to increase . with respect to low - speed operations , the forward fan stage can be controlled via pitch and / or camber change to exhibit a higher rotational speed , whereas the rear fan stage can exhibit a higher fan pressure ratio and a corresponding lower rotational speed . in transitioning to high - speed operations , the pitch and / or camber of the blades of the forward fan stage can be increased , which causes a corresponding decrease in rotational speed of the forward fan stage and an increase in rotational speed of the rear fan stage . additionally or alternatively , the tip fan 109 can be used to influence high - speed and low - speed operations . in addition , the flow characteristics of the secondary bypass stream 144 can be used separately , or in concert with the primary bypass stream 142 to affect exhaust system cooling and / or engine or vehicle thermal management . in this regard , moderate rotational speed typically is exhibited by the forward fan stage during high - speed operations . in this mode of operation , airflow to the tip fan can be restricted . as such , the tip fan is not able to perform a high degree of work and , therefore , the tip fan does not significantly reduce the rotational speed of the rear fan stage , which rotates at a relatively high speed . in contrast , for low - speed operations in which slower rotational speed of the rear fan stage typically is exhibited , airflow to the tip fan can be increased . this tends to slow the rear fan stage and reduces the pressure ratio across the rear fan stage . correspondingly , the rotational speed of the forward fan stage increases . it should be noted that the embodiment of fig1 includes two fan stages that are configured to exhibit different rotational speeds . in other embodiments , various other numbers of stages can be used . in some of these embodiments , two or more of the stages can be controlled to exhibit the same rotational speed . fig2 is a schematic diagram depicting another embodiment of a gas turbine engine system . as shown in fig2 , system 200 includes a multi - stage fan that incorporates a forward fan stage 202 and a rear fan stage 204 . notably , the rear fan stage incorporates an inner fan 203 and a tip fan 205 . each of the fan stages includes a corresponding set of rotatable blades , with first and second sets of blades ( 206 , 208 ) of a low - pressure compressor 210 being located between the fan stages . each of the blades of the rear fan stage includes an inner portion , an intermediate portion and a distal end portion . the inner portions are located along an annular inner gas flow path 212 , the intermediate portions are located along an annular outer gas flow path 214 ( located radially outboard of gas flow path 212 ), and the distal end portions are located along an annular gas flow path 216 ( located radially outboard of gas flow path 214 ). notably , the distal end portions form the tip fan . for instance , blade 213 includes an inner portion 215 located along gas flow path 212 , and an intermediate portion 217 located along gas flow path 214 , and a distal end portion 219 located along gas flow path 216 . the first and second sets of blades ( 206 , 208 ) of the low - pressure compressor also are located along inner gas flow path 212 . each of the sets of blades of the multi - stage fan and of the low - pressure compressor is powered by an epicyclic differential gear assembly 220 . the differential gear assembly is coupled to a low - pressure turbine 222 via shaft 224 . low - pressure turbine 222 is located downstream of a high - pressure turbine 228 that is connected through shaft 230 to a high - pressure compressor 232 . a combustor 234 is located downstream of the high - pressure compressor and upstream of the high - pressure turbine . in the embodiment of fig2 , differential gear assembly 220 incorporates a forward epicyclic gear 240 and a rear epicyclic gear 250 . the forward epicyclic gear includes a carrier 242 , planet gears ( e . g ., planet gear 244 ) held by the carrier , a ring gear 246 surrounding the planet gears , and a sun gear 248 about which the planet gears rotate . the rear epicyclic gear includes a carrier 252 , planet gears ( e . g ., planet gear 254 ) held by the carrier and a ring gear 256 surrounding the planet gears . notably , the rear epicyclic gear and the forward epicyclic gear share sun gear 248 . in operation , the first and second sets of blades ( 206 , 208 ) of the low - pressure compressor rotate with corresponding sets of blades of the fan stages . specifically , the forward fan stage 202 and first set of compressor blades 206 rotate with carrier 242 of the forward epicyclic gear . in contrast , the rear fan stage 204 ( i . e ., the inner fan and the tip fan ) and second set of compressor blades 208 rotate with ring gear 246 of the forward epicyclic gear . note that the fan stages , and thus the first and second set of compressor blades , are counter - rotating . the counter - rotating configuration embodied provides high relative velocities between adjacent low pressure compressor blades resulting in relatively high levels of pressure ratio . this counter - rotating arrangement allows for a preservation of core supercharging and thermodynamic efficiency as fan speeds are modulated through the epicyclic differential gearbox . in operation , the differential gear assembly enables rotational speeds of the multi - stage fan and the low - pressure compressor to accommodate various operational requirements . by way of example , for high - speed flight operations , the forward fan stage and first set of compressor blades can be set to moderate rotational speeds , while the rotational speeds of the rear fan stage and second set of compressor blades can be higher . achieving a desired rotational speed can be accomplished by altering the flow of air to the tip fan . for instance , by increasing the flow of air to the tip fan , fan pressure ratio of the rear fan stage is increased , which causes a corresponding decrease in rotational speeds of the rear fan stage and the second set of compressor blades . responsive to this speed decrease , the differential gear assembly causes the rotational speeds of the forward fan stage and the first set of compressor blades to increase . with respect to low - speed operations , the forward fan stage can be controlled to exhibit a lower fan pressure ratio , which results in corresponding increased rotational speeds of the forward fan stage and the first set of compressor blades . responsive to these increased speeds , the rear fan stage fan can exhibit a lower rotational speed ( which also is exhibited by the second set of compressor blades ) and a corresponding increased fan pressure ratio . in transitioning to high - speed operations , the flow of air to the tip fan can be decreased , which causes a corresponding increase in rotational speeds of the rear fan stage and the second set of compressor blades . this can be accomplished by selectively closing one or more valves ( e . g ., valve 262 ) of an inlet valve assembly 260 . in this embodiment , the inlet valve assembly includes an annular arrangement of valves that can be controlled to alter airflow to the tip fan . it should be noted that , in transitioning to slower speeds , spillage drag oftentimes is experienced by gas turbine engines as intake air required by the engine for reduced thrust reduces quicker , and to a level ultimately lower , than the aircraft inlet &# 39 ; s ability to deliver flow to the engine . during such a transition , inlet valve assembly 260 can be adjusted to an open position . in the open position , excess air , which could otherwise cause spillage drag , could be diverted from gas flow path 216 to gas flow path 214 . with respect to low - speed operations , one or more valves of inlet valve assembly 260 can be maintained in the open position . as such , an increased flow of air is provided to the tip fan , which causes the work of the rear fan stage to increase . responsive to the increase in work and pressure ratio across the rear fan stage , rotational speed of the rear fan stage slows , which causes a corresponding increase in the rotational speed of the forward fan stage as described above . the embodiment of fig2 also incorporate an inter - stage valve assembly 270 , which includes an annular arrangement of inter - stage valves ( e . g ., inter - stage valve 272 ) that can be controlled to alter airflow to the tip fan . the valves of the inter - stage valve assembly are located between the forward and rear fan stages . the valves of the inter - stage valve assembly can be selectively adjusted to cause air to be diverted from gas flow path 216 to gas flow path 214 . in some embodiment , this can be done to reduce the effects of spillage drag . it should be emphasized that the above - described embodiments are merely possible examples of implementations set forth for a clear understanding of the principles of this disclosure . many variations and modifications may be made to the above - described embodiments without departing substantially from the spirit and principles of the disclosure . all such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the accompanying claims .