Abstract:
NMSET and related device uses and improvements to the forces generated. Laminar flow control systems benefit from NMSET and related devices as they simplify installation and are easier to retrofit on existing aircraft. Necessary temperature gradients can me generated by using a heated material with the sides at different energy accommodation coefficients. Surface geometries can be used to increase the force generated. Photovoltaic film can be embedded into the membrane, providing a source of energy that can offset the power required for desired thrust. Intake scoops improve the air flow through the micro thrusters and surface geometries, and airflow diffusers increase air flow interaction with the hotter surface resulting in higher thrust outputs.

Description:
TECHNICAL FIELD 
       [0001]    These inventions relate to optimizations of micro thruster propulsion systems, laminar flow control systems and optimizations of micro-scale thermal transfer systems. 
       BACKGROUND OF INVENTION 
       [0002]    Devices for the movement of gases are widely utilized. The very first aircraft engines were piston driven propellers. They worked by coupling a piston engine to a propeller. This simplicity led to widespread adoption until jet engines were invented. Turbojet engines work by the principle of coupling a turbine to a fuel combination system. Spinning of the turbine compresses a fuel-air mixture which, when burned, provides thrust and torque to rotate the turbine. The first turbojet engines derived their thrust from exhaust leaving the engines. Modern variants of the turbojet engines include turbo prop and turbofan engines, which use torque generated by the exhaust to drive a propeller or fan in addition to compressing the fuel-air mixture. Rocket engines are possibly one of the oldest mechanical propulsion systems, and have not changed much since their inception. A rocket comprises a tube or cone in which sits (or into which is fed) a fuel oxidizer mixture. Expanding gas from combustion of this mixture creates thrust. Rockets, while offering the highest fuel-thrust ratio of any existing propulsion systems, cannot easily vary the amount of thrust they generate. Even adding an ability to turn a rocket on or off significantly complicates its design. 
         [0003]    The ability of a temperature differential to drive gas flow at a surface has long been known. In 1873, Sir William Crookes developed a radiometer for measuring radiant energy of heat and light. Today, Crookes&#39;s radiometer is often sold as a novelty in museum stores. It consists of four vanes, each of which is blackened on one side and light on the other. These are attached to a rotor that can turn with very little friction. The mechanism is encased inside a clear glass bulb with most, but not all, of the air removed. When light falls on the vanes, the vanes turn with the black surfaces apparently being pushed by the light. 
         [0004]    Crookes initially explained that light radiation caused a pressure on the black sides to turn the vanes. His paper was originally supported by James Clerk Maxwell, who accepted the explanation as it seemed to agree with his theories of electromagnetism. However, light falling on the black side of the vanes is absorbed, while light falling on the silver side is reflected. This would put twice as much radiation pressure on the light side as on the black, meaning that the mill is turning the wrong way for Crooke&#39;s initial explanation to be correct. Other incorrect explanations were subsequently proposed, some of which persist today. One suggestion was that the gas in the bulb would be heated more by radiation absorbed on the black side than the light side. The pressure of the warmer gas was proposed to push the dark side of the vanes. However, after a more thorough analysis Maxwell showed that there could be no net force from this effect, just a steady flow of heat across the vanes. 
         [0005]    The correct explanation for the action of Crookes radiometer derives from work that Osborne Reynolds submitted to the Royal Society in early 1879. He described the flow of gas through porous plates caused by a temperature difference on opposing sides of the plates which he called “thermal transpiration.” Gas at uniform pressure flows through a porous plate from cold to hot. If the plates cannot move, equilibrium is reached when the ratio of pressures on either side is the square root of the ratio of absolute temperatures. 
         [0006]    Reynolds&#39; paper also discussed Crookes radiometer. Consider the edges of the radiometer vanes. The edge of the warmer side imparts a higher force to obliquely striking gas molecules than the cold edge. This effect causes gas to move across the temperature gradient at the edge surface. The vane moves away from the heated gas and towards the cooler gas, with the gas passing around the edge of the vanes in the opposite direction. Maxwell also referred to Reynolds&#39; paper, which prompted him to write his own paper, “On stresses in rarefied gases arising from inequalities of temperature.” Maxwell&#39;s paper, which both credited and criticized Reynolds, was published in the Philosophical Transactions of the Royal Society in late 1879, appearing prior to the publication of Reynolds&#39; paper. See, Philip Gibbs in “The Physics and Relativity FAQ,” 2006, at math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html. 
         [0007]    Despite the descriptions by Reynolds and Maxwell of thermally driven gas flow on a surface dating from the late 19th century, the potential for movement of gases by interaction with hot and cold surfaces has not been fully realized. Operation of a Crookes radiometer requires rarefied gas (i.e., a gas whose pressure is much less than atmospheric pressure), and the flow of gas through porous plates does not yield usable thrust, partially due to the thickness and due to the random arrangement of pores in the porous plates. 
         [0008]    Thermal transpiration refers generally to the formation of a pressure gradient in gas inside a tube, the pressure gradient formed when there is a temperature gradient in the gas inside the tube, and when the mean free path of the molecules in the gas is a significant fraction of the tube diameter. 
         [0009]    Construction of a thermal transpiration device to operate at 1 ATM (standard atmosphere pressure) is difficult as, optimally, the hot and cold sides must be within 100 nm or less of each other. A 100 nm thick film exposed to an unfiltered, uncontrolled environment tends to be too fragile to withstand typical environmental stresses, such as, for example, impact from debris and/or handle the sheer forces produced by changes in air current. 
         [0010]    Furthermore, the only insulation that is generally efficient at that scale is a vacuum. This means that that if the Bernoulli effect is used to draw a vacuum between the two membranes, at least one of the membranes used to form the thermal transpiration device must be thinner than 50 nm. Such a thin membrane would not last long due to the typical environmental stresses placed on the device when in use. 
         [0011]    Thus there is a need for a way to optimize the thermal transpiration/radiometric effect described above for practical uses. 
       SUMMARY OF INVENTION 
       [0012]    Apparatuses and methods to optimize the thermal transpiration and radiometric effect are described herein. Several inventions address optimizations applicable to individual thrusters which may or may not be part of a larger collection. This includes novel systems and methods of maintaining the multiple volumes of gases in close proximity (&lt;0.1 Knudsen Number (Kn)) at different temperatures as well as maximizing the difference in temperatures between the multiple volumes, given a surface temperature and the surfaces&#39; corresponding energy accommodation coefficient and/or surface to gas convection coefficient. Several more inventions address optimizations applicable to a collection of thrusters. These inventions include systems and methods optimizing gas flows to the intakes, as well as optimizing the gas flows through the thrusters in a way that increases the net force. Another invention addresses a system and methods for decreasing energy requirements by integrating a photovoltaic/thermoelectric generator to convert solar energy into electrical energy for use by the aircraft. The last of the inventions are for practical applications for the NMSET technology. A Laminar flow control system and apparatus and a system and methods for characterizing the speed of heat conduction through a given material. 
         [0013]    The present inventions optimize devices that benefit from the thermal transpiration/radiometric effect. They also describe practical applications and describe a system and methods to decrease energy requirements by making use of the membrane to collect and convert solar energy. 
       Overview 
       [0014]    In preferred embodiments, the apparatus described here may be referred to as Networked Micro Scale Electric Thrusters (NMSet). The basis of operation of the NMSet makes it possible to apply an NMSet in the fields of propulsion, adhesion, and refrigeration; depending on the manner in which an NMSet is employed. In preferred embodiments, NMSets and related devices provide lightweight, compact, energy-efficient creation of a gas pressure differential with adjustable flow velocity. 
       Principles of Operation 
       [0015]    Although many different geometries of NMSet devices are possible, the principle of operation of NMSets remains the same. Operation of an NMSet uses energy to lower entropy on some device surfaces and transfer lowered entropy to a gas in contact with the surface. The device can optionally donate energy to the gas by raising the gas temperature. The function of the NMSet may be therefore divided into three areas:
       the means by which entropy on surfaces of the device is lowered;   the means by which the lowered entropy is transferred to the gas; and   the optional means by which the gas temperature is increased       
 
       Kinetic Force Inequality 
       [0019]    As shown in  FIG. 5.1 , The Momentum Flux Imbalance is the primary force component of a thermal transpiration/radiometric device. This is an area affect and is caused by a surface e 1 . 4  transferring more thermal energy to the surrounding gas e 1 . 6  and therefore resulting in a higher kinetic force on the surface, and the opposing surface e 1 . 3  transferring less thermal energy to the surrounding gas e 1 . 5  and therefore experience less kinetic force on the surface. There is no non-local pressure gradient as an open system is mostly isobaric, as any increase in pressure is quickly dissipated with distance. This is different from an isochoric system, where at a starting pressure of 1 atm, pressure would change by 0.05 psi per degree Kelvin. 
         [0020]    Kinetic force inequality can be achieved by maintaining the two surfaces at different temperatures. However in an isobaric system, with sufficient gas flows, a kinetic force inequality can also be achieved if the two surfaces have a different energy accommodation coefficient (“EAC”). EAC is a measure of the average efficiency of the energy exchange per encounter of a gas molecule with the solid at the gas-solid interface. This causes a gas impinging on the surface with a higher EAC to gain energy faster, while the gas impinging on the surface with a lower EAC gains energy slower. 
       Local Density Inequality 
       [0021]    Most literature refers to Density Imbalance as thermal creep or thermal transpiration. However, while this force is observed in the transitional/slip flow regime, this is not an exotic force or one limited to the transitional/slip flow regime; instead this is a simple and fundamental force. In an isobaric system, when the temperature of the gas changes, to preserve pressure, density decreases, when it cools, to preserve pressure, density increases. In an isobaric system you have two separate volumes of gas at the same pressure, however at different temperature and densities. If the barrier separating the two volumes is removed, the densities and temperatures will equalize. Since there is more cold/denser gas than there is hot/rarer gas, density will equalize faster than the temperature and the gas will flow from cold to hot at a rate related to the diffusion coefficient, the concentration gradient and the distance as it relates to the mean free path. 
         [0022]    The limit of a force on a heated plate in an isochoric system that started at ambient pressure and temperature is equal to the pressure produced by the temperature difference between ambient and that of the heated plate. Then the limit of the force generated by a radiometric device is equal to half the pressure produced by the temperature gradient. A 1 m 2  membrane operating at 1 atm with ideal materials, aperture size, packing and optimizations is limited to 172.8 N per degree K. This is further relaxed by ratio of aperture area to surface area. Therefore if is 10 μm 2  of aperture area per 40 μm 2  of surface area (apertures account for 25% of the membrane surface), the limit of force will be reduced by 25%. 
       Applications 
     Propulsion 
       [0023]    In some embodiments, NMSet can offer one or more of the following improvements in the field of propulsion:
       1. Improved Resiliency: Damage to any area in a conventional propulsion system can lead to system-wide failure. NMSet provides enhanced redundancy and robustness.   2. Lightweight: NMSet does not need a particular fuel, and itself can be microns thick. With the right setup, fuel load vanishes and membrane weight is immaterial.   3. Scalability: Conventional propulsion systems cannot scale easily; optimal turbojets for small aircrafts are not scale reductions of optimal turbojets for large aircrafts. However, such scalability issues are not present with NMSet.   4. Response Time: Thrust from NMSet can be easily and quickly adjusted in response to changes of need.   5. Power Independence: Conventional propulsion systems require a specific type or class of fuels in order to operate, whereas NMSet only requires a source of temperature differential, which can established by electricity.   6. Green Propulsion: Because NMSet does not have to rely on fossil fuels to operate, it can be setup to not produce polluting exhaust (e.g., carbon monoxide, nitrogen oxide) during ordinary operation.       
 
       Adhesion 
       [0030]    In some embodiments, an NMSet device may be used as a lightweight mechanical adhesive. The process can be reversible as the only step required to reverse the adhesion is to cut power to the NMSet. Using NMSet can provide further benefit over electrostatic adhesion in that NMSet does not require a material to be adhered to be flat or conductive surface. Compared to other mechanical adhesion processes, using NMSet may not require a surface being adhered to be pretreated. 
       Gas Compression 
       [0031]    Because an NMSet device can be arranged to drive gas flow through a surface, all or part of a pressurized vessel may function to provide gas compression. Thus, in some arrangements, separated pumping and pressurized containment may not be required. Moreover, because, NMSet&#39;s action generally occurs over a short distance, it is possible, in some embodiments, to use NMSet as a highly compact compressor by stacking multiple stages of NMSets. Conventional propulsion systems generally operate over length scales of centimeters and sometimes meters. Thus, stacking conventional propulsion systems tends to be a complex and expensive proposition. By contrast, an NMSet can operate over micrometers. Furthermore, the versatility of an NMSet means that an NMSet can be readily adapted to function as a high-pressure pump, a standard atmospheric pump, or with a sufficient number of stages, as a high vacuum pump. 
       Laminar Flow Control System 
       [0032]    As shown in  FIG. 4.1 , traditional aircraft use engines to produce forward thrust and then utilize the wings d 1 . 1  to produce vertical lift. Air flows over the wings d 1 . 2  remain laminar until boundary layer separation occurs and turbulent flows result in section d 1 . 3 . As illustrated in  FIG. 4.2 , it is well known in the art that a wing d 2 . 1  with suction intake will prevent boundary layer separation from occurring until the air flow is toward the end of the wing at d 2 . 3 . Installing NMSET or related device d 2 . 4  so that air flows through housing in the wing d 2 . 1 , and through an exhaust d 2 . 5  which is located on the body of the aircraft, preferably the bottom or end of the wing d 2 . 1 . Current laminar flow solutions are bulky, often require major redesign of the aircraft&#39;s wings and sometimes the body. NMSET or related devices are easier to integrate into a wing surface, while providing the necessary suction to maintain a laminar flow for longer distances d 2 . 2 . 
       Temperature Gradients 
       [0033]    Temperature Gradients are generally required for NMSET or related devices to operate. Temperature increase of a hot side of a device is desired as long as the structures do not negatively affect the isobaric dynamics of the system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The present invention will now be described with reference to the accompanying drawings, in which: 
           [0035]      FIG. 1  shows a cross section of a thermal transpiration/radiometric device with multiple apertures. 
           [0036]      FIG. 2  shows a cross section of a thermal transpiration/radiometric device with a photovoltaic/thermoelectric component. 
           [0037]      FIG. 2   a  shows a cross section of another thermal transpiration/radiometric device with a photovoltaic/thermoelectric component. 
           [0038]      FIG. 2   b  shows a cross section of a further thermal transpiration/radiometric device with a photovoltaic/thermoelectric component. 
           [0039]      FIG. 3.1  shows a cross section of a thermal transpiration/radiometric device made with a heated film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. 
           [0040]      FIG. 3.1   a  shows a cross section of another thermal transpiration/radiometric device made with a heated film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. 
           [0041]      FIG. 3.2  shows a cross section of a thermal transpiration/radiometric device made with a thermoelectric/peltier or other dual temperature film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. This design is best suited for films where the cooler surface is of a higher temperature than the ambient gas. 
           [0042]      FIGS. 3.2   a  and  3 . 2   b  show cross sections of another thermal transpiration/radiometric devices made with a thermoelectric/peltier or other dual temperature film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. 
           [0043]      FIG. 3.3  shows a cross section of a thermal transpiration/radiometric device made with a thermoelectric/peltier or other dual temperature film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. This design is best suited for films where the cooler surface is of a lower temperature than the ambient gas. 
           [0044]      FIG. 3.3   a  shows a cross section of a thermal transpiration/radiometric device made with a thermoelectric/peltier or other dual temperature film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. This design is best suited for films where the cooler surface is of a lower temperature than the ambient gas. 
           [0045]      FIG. 3.3   b  shows a cross section of a thermal transpiration/radiometric device made with a thermoelectric/pettier or other dual temperature film whose surfaces have different energy accommodation coefficients/surface to gas convection coefficients. This design is best suited for films where the cooler surface is of a lower temperature than the ambient gas. 
           [0046]      FIG. 4.1  shows a cross section of a wing with boundary layer separation and turbulent flows over the top of the wing. 
           [0047]      FIG. 4.2  shows a cross section of a wing with the top skin incorporating microthrusters such as NMSET to maintain the boundary layer further toward the end of the wing. 
           [0048]      FIG. 5.1  shows a cross section of a single thermal transpiration/radiometric device without geometric enhancements. 
           [0049]      FIG. 5.2  shows a cross section of another single thermal transpiration/radiometric device with geometric enhancements. 
           [0050]      FIG. 5.3  shows a cross section of yet another single thermal transpiration/radiometric device with geometric enhancements and energy accommodation coefficients/surface to gas convection coefficients enhancements. 
           [0051]      FIG. 6.1  shows a cross section of a device with a microthruster system perpendicular to the direction of travel, and the resultant air flows. 
           [0052]      FIG. 6.2  shows a cross section of a device with a microthruster system parallel to the direction of travel, and the resultant air flows. 
           [0053]      FIG. 6.3  shows a cross section of a device with a microthruster system at an angle to the direction of travel, and the resultant air flows. 
           [0054]      FIG. 6.4  shows a cross section of a device with a scoop in over the microthruster system that is parallel to the direction of travel, and the resultant air flows. 
           [0055]      FIG. 6.5   a  shows a side view of a movable flap system designed to channel air flow through the microthruster assembly. 
           [0056]      FIG. 6.5   b  shows a front view of the movable flap system of  FIG. 6.5   a.    
           [0057]      FIG. 6.5   c  shows a top view of the moveable flap system of  FIG. 6.5   a.    
           [0058]      FIG. 6.6  shows a front view of an actuated movable flap system designed to channel air flow through the microthruster assembly. 
           [0059]      FIG. 6.7   a  shows a side view of a fixed flap system designed to channel air flow through the microthruster assembly. 
           [0060]      FIG. 6.7   b  shows a front view of the fixed flap system of  FIG. 6.7   a.    
           [0061]      FIG. 6.7   c  shows a top view of the fixed flap system of  FIG. 6.7   a.    
           [0062]      FIG. 6.8   a  shows a side and front view of a fixed flap system designed to channel air flow through the microthruster assembly. 
           [0063]      FIG. 6.8   b  shows a front view of the fixed flap system of  FIG. 6.8   a.    
           [0064]      FIG. 7.1  shows a cross section of multiple thermal transpiration/radiometric thrusters in operation and the resultant airflows. 
           [0065]      FIG. 7.2  shows a cross section of multiple thermal transpiration/radiometric thrusters in operation and the resultant airflows being shaped by a secondary layer. 
           [0066]      FIG. 7.3  shows a cross section of multiple thermal transpiration/radiometric thrusters, at an angle to each other, in operation and the resultant airflows. 
       
    
    
     DETAILED DESCRIPTION 
       [0067]      FIG. 1  illustrates a simple NMSET membrane, microthruster, or thrust generating membrane. The membrane is made of two materials shown by a. 1  and a. 2 . These materials form two surfaces, a cooler surface a. 3  and a warmer surface a. 4 . Apertures a. 7  are located in the membrane to connect the cooler gas a. 5  with the warmer gas a. 6 . For NMSET to function, a. 5  must be a different temperature than a. 6 . In preferred embodiments described herein, a temperature differential can be established in a solid-state electrodynamic mechanism. However, the devices and methods described here are not limited to electronic or purely solid state devices. For example, a temperature differential may be established by conduction of heat from combustion using a fluid coolant, exothermic chemical reaction, or other chemical source. A temperature differential may be established by simple resistive heating, by the Peltier effect, by the thermo-tunneling enhanced Peltier effect, thermionic emission or by any other suitable means, such as explained below. The effectiveness of NMSET and related devices is depended on establishing the largest possible temperature gradient between the two volumes of gas a. 5  and a. 6 . 
       Non Uniform Thermal Conductivity 
       [0068]    As the NMSet device is made thinner, in many cases it becomes increasing difficult to maintain desired temperature gradients. An improved method of establishing a temperature gradient between two volumes of gas is illustrated in  FIG. 3.1  and described herein. When heated, a single membrane c 1 . 8  would transfer thermal energy to the gas c 1 . 5  on one side of the device c 1 . 13  and the gas c 1 . 6  on the other side of the device c 1 . 13  equally, without a temperature or density gradient. 
         [0069]    However if the thermal energy imparted per collision with surface c 1 . 3  is different from surface c 1 . 4 , the volumes of gas c 1 . 5  and c 1 . 6  will heat at different rates. While the heating rates are dependent on flow rates through aperture c 1 . 7 , if sufficiently high temperatures are generated by the membrane c 1 . 8 , and sufficient difference in EACs between surface c 1 . 3  and c 1 . 4  exists, a temperature/density gradient will appear between gas volume c 1 . 5  and c 1 . 6 . 
         [0070]    The temperature gradient is due to an imbalance in energy transferred from surface to gas between the two materials c 1 . 3  and c 1 . 4 . This energy imbalance significantly relaxes design and development constraints when manufacturing NMSets for higher pressures. 
         [0071]    In  FIG. 3.1  the membrane c 1 . 8  is covered with another material c 1 . 1  such as molybdenum or other material with a low EAC. This material can be further optimized if smooth or polished, as this decreases the available surface area for energy transfer to the gas. Material c 1 . 2  covers the other side of membrane c 1 . 8 . This material c 1 . 2  can be copper, oxygen implanted tungsten or other material with a higher EAC. This material can be further optimized if the surface is rough or a geometry is chosen to increase the overall surface area of c 1 . 4 . 
         [0072]    In another embodiment shown in  FIG. 3.1   a , if the EAC of the heated membrane c 1 . 10  of the device c 1 . 14  is sufficiently high, a material c 1 . 9  with lower EAC is what is required. Similarly, if the EAC of the heated membrane c 1 . 10  is sufficiently low, a material c 1 . 9  with higher EAC is required. 
         [0073]    Additionally, as shown in  FIG. 3.2 , a heated membrane c 2 . 16  with a hot side c 2 . 14  and a hotter side c 2 . 8  will benefit if a material with a lower EAC c 2 . 1  covers the hot side c 2 . 14 , so that the surface c 2 . 3  transfers less thermal energy to the ambient gas c 2 . 5 . A material with a higher EAC c 2 . 2  covers the hotter side c 2 . 8 , so that the surface c 2 . 4  transfers more thermal energy to the ambient gas c 2 . 6 . This will increase the flow rate through aperture c 2 . 7 . 
         [0074]    As another example shown in  FIG. 3.2   a , when the EAC of the hotter surface c 2 . 10  is sufficiently high, and only a material c 2 . 9  with lower EAC is necessary to cover the hot side c 2 . 13  to lower the thermal energy transferred to the ambient air. Further, as can be seen in  FIG. 3.2   b , when the EAC of the hot side c 2 . 15  is sufficiently low, only a material with a higher EAC c 2 . 12  is necessary to cover the hotter side c 2 . 11 . 
         [0075]    Additional benefit can be achieved by a membrane where the temperature gradient is achieved by peltier, thermionic emission or other active heating/cooling method. For example, as shown in  FIG. 3.3 , where the hot material c 3 . 8  is hotter than the ambient gas and the cool material c 3 . 14  is cooler than the ambient gas it is more advantageous to cover the cool material c 3 . 14  with a material of a higher EAC c 3 . 1  so that the cool surface c 3 . 3  will more efficient in cooling the ambient gas c 3 . 5 . It is also advantageous to cover the hot material c 3 . 8  with another material with a higher EAC c 3 . 2 , so that the hot surface c 3 . 4  will more efficient transfer thermal energy to the ambient gas c 3 . 6 . This will increase the flow rate through apertures c 3 . 7 . 
         [0076]    As another example shown in  FIG. 3.3   a , if the EAC of the hotter surface c 3 . 10  is sufficiently high, and only a material c 3 . 9  with higher EAC is necessary to cover the cool side c 3 . 13  so that the cool surface will be more efficient at cooling the ambient air. Additionally, as shown in  FIG. 3.3   b , when the EAC of the cool side c 3 . 15  is sufficiently high, and only a material with a higher energy coefficient c 3 . 12  is necessary to cover the hotter side c 3 . 11 . 
       Surface Geometry Optimizations 
       [0077]    A simple NMSet is illustrated in  FIG. 5.1 . This figure provides an illustration of the edge effects that take places with an NMSet. Other apertures and a planar sheet are not shown, but the effects illustrated herein will take place at all NMSets in a group, which may make a set of microthrusters. 
         [0078]    The cooler side e 1 . 1  is stacked on the hotter side e 1 . 2 . As in previously discussed, an NMSet operates by transferring more heat from the hotter surface e 1 . 4  to the ambient gas e 1 . 6 , than the cooler surface e 1 . 3  transfers to the ambient gas e 1 . 5 . Because the device operates as an isobaric system, the gas near the hotter surface e 1 . 4  is less dense than the gas near the cooler surface e 1 . 3 . In the aperture, or around the edge of the membrane e 1 . 14 , less dense gas e 1 . 6  diffuses into higher density gas e 1 . 5 . As the gases diffuse into each other, the hotter gas will gain density and the cooler gas will lose density. This process creates the flow of gas particles from cold to hot. 
         [0079]    Density imbalances are greatest at the boundary layer e 1 . 15 , and decrease with distance, illustrated as rings e 1 . 10 , e 1 . 11 , e 1 . 12 , and e 1 . 13 . Diffusive flux decreases with the concentration gradient and distance as it relates to the mean free path. Therefore such a system will have a maximum effective radius at e 1 . 13 . In a large structure, only part of the hotter e 1 . 8  and cooler e 1 . 7  surface is effective. Furthermore, due to mass flow resultant from diffusion, these gas particle interactions near the wall e 1 . 9  generate a parasitic force in the direction of cold to hot. 
         [0080]      FIGS. 5.2  and  5 . 3  illustrate ways to improve the force generated by an NMSet shown in  FIG. 5.1 . The membrane e 2 . 10  in  FIG. 5.2  has a surface e 2 . 2  made of a material A that is hotter than surface e 2 . 1 , which is made of material B. At the edge, the maximum effective area is shown by radius e 2 . 7 . Effective surface area for the cooler section e 2 . 3  is shown without modifications. If the cooler section e 2 . 3  is warmer than ambient air, the cooler section should be as smooth/polished as possible to minimize heat transfer to the gas. 
         [0081]    It is preferable for the hotter section e 2 . 4  to transfer as much heat energy as possible. A sloped geometry helps maximize the surface area near the boundary layer e 2 . 9 , where the rate of diffusion [of gas particles] is the highest. The geometry [of the hotter section] can also be curved as illustrated by e 2 . 6 , and/or rough, to further maximize surface area to exchange thermal energy with the ambient gas. 
         [0082]    Furthermore, when the temperature gradients are driven by active heating/cooling and the cooler side is cooler than ambient gas, it is preferable for the cooler side to exhibit the same characteristics as the hotter side. A minimal sidewall e 2 . 5  is preferable to minimize resistance with high density gas as it flows from cold to hot. An optimal sidewall e 2 . 8  is only limited by structural integrity of the material. 
         [0083]      FIG. 5.3  further illustrates another embodiment of the invention, similar to that shown in  FIG. 5.2 , with a single resistive membrane e 3 . 6  and a low energy accommodation film e 3 . 5  over the cooler side of the membrane. Similar principles apply as with  FIG. 5.2 . If the temperature of the section e 3 . 1  is higher than ambient gas, and it is cooler than the opposite surface, low energy accommodation film and/or smooth/polished surfaces are preferred as they minimize heat transfer to the ambient gas. If the temperature of the surface is lower than ambient gas, or the temperature of the surface is the hotter surface e 3 . 2 , e 3 . 3 , then surface area of e 3 . 2  and/or e 3 . 3  should be maximized and higher energy accommodations films should be used. As with  FIG. 5.2 , a minimal section e 3 . 4  is preferable so that resistance with a high density gas is minimized as it flows from cold to hot. 
       Energy Utilization 
       [0084]    Some implementations of NMSets will require a power source to drive temperature gradients. Depending on the pressure they are operating in, the payload carried, current velocity, and other factors, the power load changes. Furthermore, in some applications a large portion of NMSET may be exposed to atmosphere and sunlight. 
         [0085]      FIG. 2  illustrates an NMSet with a photovoltaic membrane designed as a supplementary power source, which may also be referred to as a power generating membrane. In this diagram, a cooler layer b. 1  and a hotter layer b. 2  are in a stack. The surface of the cooler layer b. 3  is cooling the ambient gas b. 5 , while the surface of the hotter layer b. 4  is heating the ambient gas b. 6 . During normal operation, the cool side is exposed to the sun and the photovoltaic membrane b. 8  would be placed on top of cooler surface to collect solar energy. The solar energy is fed back into the system for use or storage. Placement on the top surface may be undesirable for multiple reasons, including little to no damage resistance, undesirable energy accommodation profile and others. 
         [0086]    Further, if the cooler layer is optically transparent, the photovoltaic membrane b. 9  can be sandwiched between the cooler side b. 1  and the hotter side b. 2  as shown in  FIG. 2   a . It is also possible to place the photovoltaic film on top of the hotter side, as shown in  FIG. 2   b , if both the cooler and hotter side are transparent, and/or the device is designed for forward flight away from the sun. 
       Intake Optimizations 
       [0087]      FIG. 6.1  illustrates an aircraft surface f 1 . 1  moving forward through a gas f 1 . 4 . An NMSet is being utilized to move ambient gas. Gas flows f 1 . 3  are shown flowing through the membrane, perpendicular to the aircraft surface. 
         [0088]      FIG. 6.2  illustrates an aircraft surface f 2 . 1  moving through a gas f 2 . 6 . NMSet f 2 . 2  is being utilized to move ambient gas. Gas flows f 2 . 4  are shown flowing through the membrane perpendicular to the aircraft surface and the ambient flow of gas f 2 . 3 . The desired action is to maintain an upward force f 2 . 5  on the aircraft surface f 2 . 1  to maintain the aircraft&#39;s vertical position. As forward momentum f 2 . 6  increases, flows across the surface f 2 . 3  become more laminar and air flow f 2 . 4  through the microthrusters f 2 . 2  decreases, which decreases the vertical thrust f 2 . 5  placing an upper bound on the forward velocity of the aircraft. 
         [0089]      FIG. 6.3  illustrates an aircraft surface f 3 . 0  compensating for the loss of vertical thrust by positioning part of the aircraft surface f 3 . 1  at an angle to the direction of travel f 3 . 7  to increase the airflow f 3 . 4  through the microthrusters f 3 . 2  in an effort to increase the vertical thrust component f 3 . 6 . While this increases airflow through the microthrusters f 3 . 4  and therefore the vertical thrust generated, the aircraft surface at an angle f 3 . 1  as well as the microthrusters f 3 . 2  contribute to downward drag f 3 . 5  produced by gas flows f 3 . 3  at an angle or perpendicular to the direction of travel f 3 . 6 . This results in the expenditure of more energy to overcome the downward drag f 3 . 5  component. 
         [0090]    Intake Scope 
         [0091]    A more efficient design is illustrated in  FIG. 6.4 . In  FIG. 6.4  an aircraft surface f 4 . 0  positions a scoop f 4 . 1  over the microthrusters f 4 . 2  and into the direction of travel f 4 . 7 , which guides airflow f 4 . 4  through the microthrusters f 4 . 2  and recombines the airflow with airflow under the surface f 4 . 3 . This increases airflow to the microthrusters f 4 . 2  and increases the vertical thrust component f 4 . 6 . The positioning of the scoop f 4 . 1  creates upward drag f 4 . 5  and drag opposite of the direction of travel f 4 . 8 . An intake scoop over a microthrusters array parallel to the surface is novel, and enables the microthrusters to operate in forward velocities faster than the velocities the microthrusters are capable of creating, this allows them to supply the upward thrust while other thrust producing apparatus supply the forward momentum. Further illustrations will explore further optimizations to the microthrusters intake scope invention. 
         [0092]    Adjustable Scoops 
         [0093]    As the forward velocity increases, drag against the direction of travel f 4 . 8  and the pressure underneath the scoop f 4 . 1  increases. Microthrusters are typically designed to operate inside of a range of pressures. To support a range of forward velocities, desired microthrusters pressures need to be maintained. The microthruster sets shown in  FIGS. 6.5   a ,  6 . 5   b ,  6 . 5   c , and  6 . 6  maintain desired pressure through the use of adjustable scoops. 
         [0094]      FIGS. 6.5   a ,  6 . 5   b , and  6 . 5   c  illustrate an aircraft surface f 5 . 0  with one or more groups of microthrusters f 5 . 1 . An adjustable scoop assembly is shown as a structure f 5 . 4  that elevates one side of the flap f 5 . 6  over the other that covers a predetermined group of thrusters. It is preferable that the flap f 5 . 6  does not extend to the aircraft surface f 5 . 0 . This leaves room for airflow to pass through without building up excessive pressure. The adjustable scoop assembly further f 5 . 4  contains actuators, pressure sensors, control circuitry, and power circuitry, which are not shown here but are known to one skilled in the art. 
         [0095]    Adjustable scoops can vary in size, height, placement and orientation dependent on the desired operation. Illustrated is an adjustable scoop toward the back of the aircraft structure f 5 . 0  is made of a taller support structure f 5 . 5  for the adjustable flap f 5 . 7 , to maintain higher pressures due to lower available gas pressure as some of the gas has been directed through the microthrusters f 5 . 1 . 
         [0096]    Further, as shown in  FIG. 6.6 , an adjustable scoop assembly can operate in pairs. When gas pressure near microthrusters f 6 . 1  is low, flaps f 6 . 8  and f 6 . 9  connected to support structures f 6 . 5  installed on an aircraft surface f 6 . 0  can be lowered to increase the pressure to desirable levels. 
         [0097]    Fixed Scoops with Pressure Bleed Off 
         [0098]    When the aircraft travels at a known speed, fixed intake scoops can be constructed due to their simplistic nature. As illustrated in  FIGS. 6.7   a ,  6 . 7   b , and  6 . 7   c  an airframe f 7 . 0  with groups of microthrusters f 7 . 1  is out fitted with fixed intake scoops f 7 . 2 . Flaps can also be designed to maintain a particular pressure difference, above which they will bleed pressure off. In  FIGS. 6.7   a ,  6 . 7   b , and  6 . 7   c  a flap f 7 . 4  is attached to a column f 7 . 3 . As pressure builds up underneath flap f 7 . 4  it will rise to bleed the pressure off. Further, the flap f 7 . 4  may be weighted to provide the desired pressure difference. 
         [0099]    Scoops on a Parallel Surface 
         [0100]    Air intake system can be further separated from the propulsion system.  FIGS. 6.8   a  and  6 . 8   b  illustrate one such example. An aircraft surface f 8 . 0  contains groups of microthrusters f 8 . 1  on a surface f 8 . 2  above the microthrusters. Further, intake scoops f 8 . 4  and f 8 . 5  and through holes f 8 . 3  are installed. As the aircraft moves forward, gas enters intake scoops f 8 . 4 , f 8 . 5  and is forced toward the microthrusters f 8 . 1 . The maximum pressure can be controlled by varying the height of the surface f 8 . 2  with the intake scoops f 8 . 4 , f 8 . 5 . The geometry of an intake scoop f 8 . 5  can further be modified to provide with desired gas flow profiles through the intake f 8 . 5  as well as around it. 
       Exhaust Optimizations 
       [0101]    NMSET and other thermal gradient driven propulsion systems that operate in the slip/transitional flow regime require effective energy transfer to the incoming gas g 1 . 3  flowing from the cooler side g 1 . 1 , to the hot side g 1 . 2  through the apertures g 1 . 4 . The heat exchanged when the gas flow g 1 . 3  reaches the hot side g 1 . 2  is not optimized. This greatly reduces effectiveness and is one of the main reasons behind ineffective thermal transpiration devices, and hence, force per area. 
       Surface Geometry &amp; Surface Characteristics 
       [0102]    Geometry considerations can be important when considering gas flowing through the membrane. An increase in active surface area as shown in  FIGS. 5.1  and  5 . 2  provides additional area to transfer heat energy to the gas flowing through apertures. Furthermore, high EAC increases the amount of energy transferred per collision and surface roughness further increases total surface area. 
         [0103]      FIG. 7.2  illustrates another method for improving gas flow from the cooler surface g 2 . 1  to the hotter surface g 2 . 2  through apertures g 2 . 3 . Sections of the hot/cold vane g 2 . 0  are set at an angle to increase the exposed hotter surface g 2 . 2  to the flow of gas from the cooler side g 2 . 7 . Additionally, a section of the hot surface is covered with a cooler surface g 2 . 6 . This minimizes heating of the cooler gas, thereby increasing flow rates. The cooler side can further be set at an angle as shown by g 2 . 5  to increase aperture size and gas flow volumes. 
       Exhaust Diffusers 
       [0104]    While geometry and surface characteristics are helpful in increasing energy transfer to the gas flowing through the apertures, more aggressive means may be considered when dealing with a range of pressures. At lower Knudsen numbers, as shown in  FIG. 7.1 , less gas in the center of the aperture g 1 . 5  is able to reach the hotter surfaces g 1 . 2 . 
         [0105]    To allow for better results,  FIG. 7.3  illustrates the use of a parallel surface with gas diffusers g 3 . 6  to spread the gas flows g 3 . 5  flow the cooler surface g 3 . 1  through the apertures g 3 . 5  to the hotter surface g 3 . 2 . As the gas flow g 3 . 5  encounters gas diffusers g 3 . 6 , the gas flow g 3 . 5  spreads out and increases the surface area of the hotter surface g 3 . 2  that is contacted. The section of the gas diffuser g 3 . 3  facing the hotter surface g 3 . 2  should be covered in a material with a low EAC. Preferably g 3 . 3  is made of a material that is actively cooled. The opposite section of the diffuser g 3 . 4  can be optionally covered in a material with a high EAC and/or made from a material that is actively heated to help increase flow rates and velocities. 
         [0106]    Using the provided figures and descriptions, one of ordinary skill in the art will readily understand that the inventions can be combined to increase efficiency. As has been described, embodiments of the present invention have many applications. In particular, though not limited thereto, the uses and improvements can be in the form of micro-thrusters, and even more particularly NMSet micro-thrusters of many forms and variations disclosed elsewhere herein. 
         [0107]    While the foregoing specification teaches the principles of the present invention, with examples provided for the purpose of illustration only, it will be appreciated by one skilled in the art from reading this disclosure that various changes and modifications in form and detail can be made, and equivalents employed, without departing from scope of the appended claims, which are to be given their full breadth.