Abstract:
A micro-nozzle thruster comprising a micro-nozzle having an inlet at a first end perpendicularly aligned gas supply channel at a first end, and a thruster outlet at a second opposed end; said inlet in fluid communication with a gas supply channel, said gas supply channel perpendicularly aligned with a longitudinal axis of the micro-nozzle; a cathode within the gas supply channel and an anode external to the gas supply channel and proximate to the inlet, so as to create a plasma flow from said gas.

Description:
FIELD OF THE INVENTION  
       [0001]    The invention relates to micro- and nano-satellites, and in particular, thrusters for the maneuvering of said satellites. 
       BACKGROUND  
       [0002]    Satellites and spacecrafts miniaturization is a recognized trend covering the whole range of space missions from simple university missions designed by students to sophisticated military satellites and interplanetary probes. The rationale for miniaturization comes from the drastic launch and manufacturing cost reductions in combination with the growing offer and availability of new miniaturized parts, subsystems and satellite buses. 
         [0003]    Besides such scientific missions, the first obvious tasks for micro and nano-satellites are Earth observation for environmental, military and geological uses, while telecommunications will follow as soon as the other technological building blocks will be available. Last but not least, missions related to entertainment and space tourism will become common as soon as rocket plane launches will be offered on regular basis. 
         [0004]    In fact, the so called “New Space” movement will be finally able to solve the major obstacle to the diffusion of small satellites: the launch. Until now, launches are always arranged in piggyback fashion whereby the largest cost of the launch is paid by a very big satellite and the nano or microsatellite are taken in an orbit and with a schedule which are not necessarily the optimal for their needs and interests. The space tourism technologies developed by the “New Space” entrepreneurs will enable weekly or daily regular launches from Spaceports distributed all over the world. In this way the full potential of micro and nano-satellites will be finally available possibly having extremely short time missions, launched on demand and with hardware of very low cost. 
         [0005]    The usual definition of satellite sizes classifies Microsatellites between 10 kg and 100 kg, Nano-satellites between 1 kg and 10 kg and Pico-satellites below 1 kg. 
         [0006]    Among the nano-satellites the Cubesat ranges from 1 kg to 3 kg with a nominal cross section of 100 mm×100 mm and nominal length between 50 mm and 350 mm. This standard, introduced by Stanford University and California Polytechnic is extremely important because of the use of a standardized deployment system called P-Pod (5) which completely de-couples the integration of the nano-satellites from the rocket vehicle making very simple the launching of a nano-satellite even for a small university team. All rocket interface issues are taken care by the P-Pod standard deployment system. Various commercial ventures are even promoting satellites of 750 g, of cylindrical shape, called “Tubesats”, for US$ 8000 launch included on very Low Earth Orbit (LEO) with the first launch scheduled for 2011. 
         [0007]    One of the improvements long waited for will be the advent of really miniaturized and efficient propulsion systems which in combination with other subsystems miniaturization efforts will allow the use of micro and nano-satellites for missions comparable to the more conventional and larger spacecrafts ones. 
         [0008]    A challenge for achieving a small thrust is to do it efficiently in order to have the best use of the limited amount of propellant available on board. This as well as thrusters efficiencies have to cope with the mission requirements of spacecraft velocity change ΔV. 
         [0009]    Generally speaking a list of typical manoeuvres with an indication of ΔV ranges from is:
       Transfer to planetary trajectory (3600 to 4000 m/s)   Orbit transfer to GEO (˜4000 m/s)   Plane change (100˜1000 m/s)   Orbit rising, drag make-up, controlled re-entry (50˜1500 m/s)   Orbit maintenance and attitude control (10˜100 m/s per year)   Relative motion of spacecrafts (1˜100 m/s per manoeuvre)       
 
         [0016]    To this end, for a nano-satellite, the standard design performance expected may fall within the following parameters:
       Thrust between 500 μN and 500 mN;   Minimum ΔV 10 m/s;   Average ΔV 100 m/s (includes orbit injection correction, acquisition and maintenance), up to 200 m/s if including de-orbiting;   Maximum ΔV 2000 m/s (including Moon, transfers or orbit changes);   Dry mass budget 1 kg (possibly 400 g);   Propellant mass budget 3 kg;   Tank size max 200 mm diameter;   Power budget, max 10 W, average 2-3 W;   1 DoF for high ΔV, 3 DoF for low ΔV, 6 DoF for moderate ΔV, μN for attitude control.       
 
         [0026]    Since low thrust propulsion systems have become available few decades ago, we have learned how to benefit from continuous-thrust manoeuvres instead of impulsive manoeuvres. This is going to be even more important for nano and microsatellites which necessarily have extremely low power availability and small mass budget for propellant storage, requiring high specific impulse engines working at low thrust for long time. 
         [0027]    The selection of a propulsion technology for a given spacecraft and mission requires consideration of the whole system whereby the engine is accompanied by a reservoir or tank and a power supply which includes batteries and solar cells. 
         [0028]    Propulsion systems include the five main groups of elements: mass storage and supply, electric storage and supply, thermodynamic acceleration of the propellant, propellant ionization, propellant electrical acceleration. The various combination of such elements create the different propulsion system which all have the common goal to produce the highest possible momentum of the ejected propellant with the smallest possible use of propellant mass and electric power. 
         [0029]    Chemical propulsion systems derive their energy from the chemical energy content of the propellant which is endo-thermically heated or, in addition to it electro-thermally heated, leading to the achievement of propellant exit velocity which depends on the achievable propellant temperature ( 24 ). 
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         [0030]    The temperature achievable by the propellant is ultimately limited by the combustion chamber and expansion nozzles materials giving a practical limitation to the specific impulse of chemical propulsion systems. 
         [0031]    Propulsion systems will soon enable very advanced small satellite missions including constellation and formation flight with distributed sensors, communication networking, assembly of larger structures in Space, maintenance of larger spacecrafts, de-orbiting, Moon exploration and others. While many laboratories worldwide are studying and prototyping systems based on various principles, there are very few examples of micro and nano-satellites carrying a micro-propulsion unit. Furthermore, such units are generally limited to a micro-propulsion payload and do not perform a primary mission requirement. This is clearly due to the modest performances of the systems available to create a new micro-propulsion system really miniaturized in all its components providing sufficient Thrust, Specific Impulse and Efficient use of the limited available power and storage volume and mass budget. 
       SUMMARY OF INVENTION  
       [0032]    In a first aspect, the invention provides a micro-nozzle thruster comprising a micro-nozzle having an inlet at a first end perpendicularly aligned gas supply channel at a first end, and a thruster outlet at a second opposed end; said inlet in fluid communication with a gas supply channel, said gas supply channel perpendicularly aligned with a longitudinal axis of the micro-nozzle; a cathode within the gas supply channel and an anode external to the gas supply channel and proximate to the inlet, so as to create a plasma flow from said gas. 
         [0033]    In a second aspect, the invention provides a micro-nozzle thruster comprising a micro-nozzle having an inlet at a first end co-axially aligned gas supply channel at a first end, and a thruster outlet at a second opposed end; said inlet in fluid communication with a gas supply channel, said gas supply channel perpendicularly aligned with a longitudinal axis of the micro-nozzle; a cathode within the gas supply channel and an anode external to the gas supply channel and proximate to the inlet, so as to create a plasma flow from said gas. 
         [0034]    In a third aspect, the invention provides a method of propulsion comprising the steps of: supplying a gas along a channel; passing said gas across a cathode and an anode so as to create a plasma stream; directing said plasma stream along a nozzle perpendicular to the channel, and; ejecting said plasma from an outlet of said nozzle. 
         [0035]    In contrast to chemical propulsion, there is no material temperature limitation to the speed of ions leaving an electric propulsion rocket and therefore very high specific impulses can be obtained. This makes electric propulsion interesting for very high ΔV requirements thanks to the reduced amount of propellant that needs to be stored. 
         [0036]    It is the low thrust and high specific impulse that makes the electrical propulsion interesting for long duration non impulsive manoeuvres. 
         [0037]    In one embodiment of the invention, the thruster may include a resistor to pre-heat the gas reaching the nozzle or to vaporize a liquid before it reaches the nozzle (resistojet preheating). Further, a resistor may be used to pre-heat the cathode thereby enhancing the release of electrons by thermionic emission (cathode emitter). 
         [0038]    In a further embodiment, heat lost through the nozzle walls may be regenerated to pre-heat the gas flowing to the nozzle (regenerative nozzle). 
         [0039]    In a further embodiment, a magnetic field inside the nozzle may be produced by permanent magnets or coils placed outside the nozzle, to create a magnetic confinement of the plasma in the central portion of the nozzle thereby reducing wall heat losses. In the case of the nozzle having a prismatic structure, the nozzle may be inserted in the gap of a magnetic circuit. Such an arrangement may have minimal losses of magnetic flux (magneto hydrodynamic confinement). 
         [0040]    In a further embodiment, a combination of magnetic fields and electric fields transversally to the nozzle may create a force that confines and accelerates the plasma through the nozzle. Again, in the case of a prismatic structure of the micro-fabricated nozzle, such electric fields may be achieved by deposition of electrodes on faces of the nozzle (magneto hydrodynamic thruster). 
         [0041]    In a further embodiment, magnetic fields may be added inside the nozzle to separate different ionized species and drive them to different exits areas or to multiple nozzles by exploiting the different ratio between mass and electric charge resulting in different radius of the path produced by:the magnetic fields (magneto hydrodynamic distillation). 
         [0042]    In a further embodiment, a mix of gases or gases and liquids or gases and solid particles may be injected to facilitate ionization, to increase the momentum of the ejected material or to produce a thermal or deposition coating effect to materials placed in the ejected stream (plasma torch). 
         [0043]    In a further embodiment, the thruster according to the present invention may be a basic cold gas micro-thruster. Alternatively, it may be a Resistojet/Arcjet. Further still, it may be a Resistojet-Mixer/Bipropellant 
         [0044]    The efficiency of propellant use in thrusters may depend on the temperature of the gas supplied before the expansion nozzle. Such temperature corresponds to the energy content of the gas per unit of mass and such energy is converted in speed during the gas expansion along the convergent-divergent shape, of the nozzle. Higher temperature means higher energy and therefore higher speed or momentum transferred from the gas leaving the nozzle to the nozzle itself thereby creating the aimed propulsive effect. The gas is stored or provided at a certain temperature compatible with the storage materials and technologies and there may be additional energy available in other forms, usually electricity that can be transferred to the gas just before the nozzle to raise the gas temperature and therefore the thruster efficiency. 
         [0045]    The more intimate such transfer can be achieved inside the gas, the less energy will be lost to the nozzle walls. One of the best ways to provide such energy is by creating a current through the gas in form of a spark, a discharge arc or plasma. The core of the arc can be sufficiently far from the nozzle walls to minimize the thermal losses and maximize the transfer to the gas reaching temperatures between 5000K and 10000K or more without damage to the nozzle walls. 
         [0046]    This is usually achieved in so-called Arcjet thrusters where a cathode is inserted in proximity of the nozzle throat that is electrically connected as anode. The high voltage difference between cathode and anode produces the arc discharge in the gas and therefore heats up the gas flowing through it. 
     
    
     
       BASIC DESCRIPTION OF DRAWINGS   
         [0047]    It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. 
           [0048]      FIG. 1  is a detail view of the micro-nozzle thruster according to one embodiment of the present invention; 
           [0049]      FIG. 2  is a plan view of the micro-nozzle thruster according to a further embodiment of the present invention; 
           [0050]      FIGS. 3A to 3D  are sequential views of the micro-nozzle thruster according to  FIG. 2 ; 
           [0051]      FIGS. 4A and 4B  are elevation views of the micro-nozzle thruster according to a further embodiment of the present invention; 
           [0052]      FIGS. 5A and 5B  are isometric views of a micro-propulsion system according to a further embodiment of the present invention; 
           [0053]      FIG. 6  is an isometric view of a micro-nozzle thruster assembly according to a further embodiment of the present invention; 
           [0054]      FIG. 7  is an isometric view of a micro-nozzle thruster according to a further embodiment of the present invention; 
           [0055]      FIG. 8  is an isometric view of the inlet area of a micro-nozzle thruster assembly according to a further embodiment of the present invention; 
           [0056]      FIG. 9  is a sectional elevation view of the micro-nozzle thruster according to a further embodiment of the present invention; 
           [0057]      FIG. 10  is an isometric view of micro-nozzle thruster assembly according to a further embodiment of the present invention; 
           [0058]      FIG. 11  is a sectional isometric view of the micro-nozzle thruster according to a further embodiment of the present invention; 
           [0059]      FIG. 12  is an isometric view of the micro-nozzle thruster according to a further embodiment of the present invention, and; 
           [0060]      FIG. 13  is a sectional elevation view of a co-axial micro-nozzle assembly according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]    In describing the invention, an embodiment is shown in  FIG. 1 . The invention is directed to miniaturized arcjets  5  which uses the following basic elements:
       1. a generally prismatic micro-nozzle  35  which may be micro-machined using techniques such as micro-lithography, Deep Reactive Ion Etching, Anodic Bonding and Dicing using substrates  40 ,  45 , such as silicon and glass with inlet  25  in the central area of the substrate and outlet, or exit, on one of the sides.   2. A gas supply channel  10  made of substrates of insulating material such as ceramic or glass reaching the nozzle inlet perpendicularly to the substrate in correspondence of a through hole micro-machined in one or both the substrate(s).   3. An anode element  20  placed externally to the gas supply channel, but internally to the nozzle inlet  25 , possibly in the form of an electrically conducting ring and oriented between the channel and the nozzle throat  30 ;   4. A cathode element  15  placed internally to the gas supply channel  10 , possibly in the form of an electrically conductive tip.       
 
         [0066]    Alternatively, the whole nozzle can be made of conductive substrate such, as doped silicon and therefore used as electrode, possibly as anode. 
         [0067]    The configuration of cathode internal to anode may be reversed by switching electrical polarity between the two elements. Multiple anodes and cathodes may also be used. 
         [0068]      FIG. 2  shows a plan view of an actual micro-nozzle  55  having the gas supply channel  75  having a cathode  60  and anode  65  located proximate to the inlet  70 , which defines the arc discharge area. The subsequently formed plasma then passes through the throat  80  of the micro-nozzle and through the convergent-divergent micro-nozzle directed to the outlet, through which the gas is supplied to the inlet. Without limiting and independently from the functionality of the invention, the gas flowing through the arc between cathode  60  and anode  65  is further ionized and becomes plasma with mainly positive charge. 
         [0069]    Furthermore, by placing a negative electrode along the nozzle or a negatively polarized grid outside the nozzle it is possible to further accelerate such plasma through the nozzle to obtain even higher exit speed and thruster efficiency. 
         [0070]    One embodiment of this arrangement is shown in  FIGS. 4A and 4B . Here the micro-nozzle assembly  120  includes a gas supply channel  130  for feeding gas to a nozzle  140 . The micro-nozzle assembly includes an extension  125  having an extended nozzle channel  150  so as to pass through a magnetic field source  135  and electric field source  145 . Such an arrangement applies a force to the plasma stream so as to maintain the stream within the bore of the nozzle  150  and so avoid heat loss and other frictional loss through contact with the nozzle faces (walls). By adding a bias voltage to the MEMS nozzle, which may be formed of silicon or by adding metallized layers, a repulsive effect to enhance ion acceleration already inside the divergent part of the supersonic nozzle may be achieved. Furthermore, by adding a screen and acceleration grids outside the nozzle we can create an “electro-dynamic-afterburner” where the plasma already accelerated in the nozzle can be further electro-statically accelerated to reach very high specific impulses. 
         [0071]    While miniaturization is a clear trend in satellites and spacecraft design, the use of micro-propulsion systems has not been very diffused both for the difficulty to find suitable systems and the practical way to provide basic attitude control by other methods such as reaction wheels and magnetorquers. 
         [0072]      FIGS. 7 and 8  show detailed views of a further embodiment of the present invention. Here a micro-nozzle assembly  195  is mounted to a thruster body  240 . The micro-nozzle, having an inlet, throat  225  and outlet  230  is formed from a series of substrates  200 ,  205 ,  210 . The micro-nozzle is positioned along an axis at right angles to a gas supply channel, with a cathode  215  within the gas supply channel and an anode  220  external to the channel but intermediate the channel and the micro-nozzle. In this particular configuration the anode  220  is realized from a conductive rod  235  extended from the side of the assembly. 
         [0073]    The relationship between the components can be better illustrated from the sectional elevation view shown in  FIG. 9 . Here the micro-nozzle assembly  245  includes the gas supply channel  260  terminating at the inlet  255  of the micro-nozzle  27 . A cathode  250  is located within the channel, with a tip of the cathode proximate to the inlet  255 . An anode  265  is further positioned proximate to the inlet  255  so as to create a plasma flow from the gas within an arc discharge area at the inlet. 
         [0074]    Different orientations of the micro-nozzle assembly  285 ,  320  are shown in  FIGS. 11 and 12 . In  FIG. 11 , the micro-nozzle is formed from substrates  305 ,  310  into which the gas supply channel  315  directs gas. A cathode  311  within the channel is positioned with a cathode tip  290  proximate the anode  295  for converting the gas flow to plasma, for final ejection through outlet  300 . 
         [0075]      FIG. 12  shows a micro-nozzle assembly  325  mounted to a thruster  320 , with the outlet  330  providing the ejection of plasma for the requisite propulsion where the upper element  335  is encapsulating the micro-nozzle for thermal insulation purposes. 
         [0076]      FIG. 10  shows an further embodiment of the micro-nozzle assembly  270  which includes a micro-valve and a pressure sensor and has the nozzle  280  sandwiched between the thruster body  275  and a lid for easy assembly and replacement. The embodiment using a Cold-Gas configuration may be enhanced by adding a pre-heater section used as warm-gas or vaporizing unit depending on the choice of propellant. The same section may be further improved to become a micro-arcjet module to further increase the gas temperature before nozzle expansion and possibly initiate the ionization. In a further embodiment, a double flow unit that can pre-mix two species before entering the heating section is possible. This is used as colloid thruster or to achieve heating by the reaction of the two species such as in bipropellant thrusters. In the Cold-Gas version and for low temperature Resistojet the nozzle is sealed to the thruster body with a Viton O-ring, while for the high temperature versions it is sealed with ZrO 2  ceramic bond that can withstand over 3000° C. 
         [0077]    Embodiments of the Cold-Gas and Resistojet may have attended values of nominal thrust (i.e. 1 mN+/−20 μN) and the expected values of specific impulse reaching more than 50 s for the Cold-Gas and up to 120 s for the Resistojet. The resistojet has been successfully tested as vaporizing thruster with various liquids thereby confirming the potential of very high Δv. 
         [0078]    Gas ionization may be achieved by means of a sustained spark in the nozzle area. Given the small size of the electrode gap in the arc jet augmentation module, reasonably low voltages are expected to be necessary. In fact sustaining glow discharges can be achieved with as low as 200V to 400V and 0.1 mA to 8 mA for Argon at atmospheric pressure on micro-hollow cathode arrangements with holes varying from 200 μm to 1000 μm. 
         [0079]    The sustained arc discharge will produce the heating of the propellant gas with beneficial effect on the specific impulse and the ionization of the gas which will be then exploited by the acceleration potential to further increase the specific impulse. Typical spark temperature can be between 3000K and 5000K. 
         [0080]    The invention may be illustrated in  FIGS. 3A to 3D , which show a sequential view of arc ignition inside a micro-nozzle  90  assembled according to the ArcJet configuration. The chosen micro-nozzle  90  has 50 m×100 m of throat cross section for 10 mN of nominal thrust at 6 bar supply of Nitrogen in Cold-Gas configuration. While N 2  was supplied  95  at 3 bar with exhaust in atmosphere (1 bar), ignition  100 ,  105 ,  115  of the arc has happened at about 1000V with stainless steel electrodes with 200 m gap; current was limited at 30 mA with the power supply available for the experiment and it stabilized around 15 mA during stable sustained arc. Given the high intensity of the arc, we expect to be able to keep it stable at current in the order of 5 mA to 10 mA and with a more precise and optimised electrode configuration we expect to be able to work at about 500V therefore keeping the arc power in the order of 5 W and the total power requirements below 10 W with a minimum power supply efficiency of 50%. 
         [0081]    The degree of ionization  4  dependence on the temperature is described by the Saha-Eggert law: 
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         [0082]    Which, if n 0  is calculated from pressure and temperature with the ideal gas law, can be plot in the practical range of interest for micro-propulsion devices (p&lt;10 bar, T&lt;20000K). 
         [0083]    A modest, but sufficient degree of ionization can, be expected, in the order of 1E-6 to 1E-5, which corresponds to a plasma density n i =6E17 to 6E18 ions per cubic meter, comparable or in excess of what obtained in miniaturized ion thrusters at JPL ( 48 ). Further acceleration and focusing, of the ionized flow is achieved by means of voltage applied to the shield and accelerator grid placed outside the supersonic nozzle. For the ion acceleration preliminary design, the voltage variation along the nozzle will be obtained by the mono-dimensional integration of the equation of energy conservation and second Maxwell equation: 
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         [0084]    Which rearranged will give the ideal performance of the electrostatic thruster ( 24 ): 
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         [0085]    One further aspect to be considered will be the neutralization of the ionized beam ejected by the thruster. This will be achieved by a second cathode emitter placed in proximity of the nozzle exit. 
         [0086]    The micro-nozzles may be manufactured by Deep-Reactive Ion Etching (D-RIE) on 4 or 8 inch silicon wafers, patterned by UV-lithography, anodically bonded on borosilicate glass and precisely diced with diamond saw. Such process guarantees profile and depth tolerances as low as 1 μm for optimal reproducibility of nozzle performances and efficiency. 
         [0087]      FIGS. 5A ,  5 B and  6 , show the micro-fabricated components such as nozzles  160 ,  180 , sensors, valves, etc. assembled in the precisely machined subsystems modules  155 ,  165  with larger machined components such as the propellant tank, mechanical interface frames, electronic board supports to obtain the complete micro-propulsion system which is easily integrated in the spacecraft. 
         [0088]      FIG. 13  shows an alternative micro-nozzle assembly  340  having a gas supply channel co-axially arranged with the micro-nozzle, so as to provide a greater gas supply, and consequently a greater plasma flow. As with the perpendicularly arranged micro-nozzle assembly, a cathode  345  is provided within the gas supply channel, having a cathode tip  360  proximate to an anode  350 , which together define an arc discharge zone at the inlet  355  of the micro-nozzle. The formed plasma  370  is then ejected from the micro-nozzle through the outlet  375 . 
         [0089]    The micro-nozzle assembly  340  of  FIG. 13  may therefore be used cooperatively with the assembly  325  of  FIG. 12  through coarse adjustment from the co-axial assembly and fine adjustment from the perpendicularly arranged assembly of  FIG. 12 .