6.T. NORGREN COLLOID PROPULSION METHOD AND APPARATUS FLled March 12, 1963 FIG. I FIG. 2 ~NV~NTOR~ CARL T. NORGREN 1 are charged at the nozzle exhaust by any convenient 3,173,246 means, such as a corona discharge. The resulting padi- COLLOID PROPULSION METHOD AND APPARATUS cles are then accelerafed by suitable means such as an Carl T. Norgren, North Olmsted, Ohio, electrostatic accelerator. It is likewise contemplated that United States of America as repre [i the colloidal particles can be charged in an electron bom- Administrator of the National Aeronautics and Space bardment engine and then accelerated. In an alternate Administration embodiment of the invention the colloidal particles are Filed Mar. 12,1963, Ser. No. 264,735 accelerated in the nozzle alone to supply small corrections 10 Claims. (C1.60-35.3) to planned orbits. (Granted under Title 35, Code (19521, sec. 266) US. 10 It is, therefore, an object of the present invention to The invention described herein may be manufactured provide an improved method of propulsion in which sub- and used by or for the Government of the United States micron sized particles are utilized which enables realistic of America for governmental purposes without the pay- accelerating potentials to be used. ment of any royalties thereon or therefor. Another object to the invention is provide an improved This invention is concerned with a method and appara- 15 thrustor which utilizes a colloidal particle generator tus for propelling space vehicles, and more particularly having a supersonic nozzle. with an onboard colloidal particle generator for an elec- Other objects of the invention will be apparent from tro static engine. The invention is especially concerned the specification which follows and from the drawings with an improved thrustor which utilizes a colloidal par- in which like numerals are used throughout to identify ticle generator. 20 likeparts. Colloidal particle electrostatic rocket engines have been FIG. 1 is a schematic view of an electrostatic engine proposed as propulsion systems for space missions where having a thrustor constructed in accordance with the in- specific impulses from 1,000 to 10,000 seconds are re- vention, and FIG 2 is a perspective view of a colloid quired. High power efficiency, high propellant utiliza- particle generator of the invention. tion, reasonable field strengths, physical and electrical 25 Referring now to the drawings there is shown in FIG. 1 reliability, light weight, and long-duration operation are an electrostatic engine 10 for rockets and the like that all required of these engines. may be used with electric spacecraft. The electrostatic Various methods have been used to form colloidal engine 10 comprises a power supply 12 and a thrustor 14. particles such as expanding material into a cooling or The power supply 12 may be any of a number of different calming chamber through a high presscre nozzle. Chem- 30 types such as a nuclear reactor that is used to drive a ical reactions such as the action of ammonia vapor in turbine or a nuclear battery. hydrogen chloride have likewise been utilized along with The thrustor 14 comprises a propellent vaporizer 16. the oxidation of a particle forming material. Colloidal a supersonic nozzle 18, charging means 20, and accel- particles have also been produced by physical grinding erators 22 and 24. An electron trap 26 is provided where such as mechanical disintegration in a colloidal mill. 35 the vaporizer 16 and the nozzle 18 are operated at ground These techniques have disadvantages when applied to potential to prevent the back streaming of electrons from propulsion systems. For example, the atomization of a the charging means 20. material, such as a liquid, 'tends to produce a very poor The vaporizer 16 and the nozzle 18 compnse a colloidal size distribution which is completely unsuitable for an particle generator which is indirectly heated in a con- electrostatic engine. Other devices which provide good 40 ventional manner for supplying sub-micron sized particles size distribution of the particles do not produce a suffi- to be charged and accelerated. By way of example, an cient quantity of particles to be useful. Also methods A.C.' Nichrome heating unit 28 wound around the va- which produce relatively uniform prepared particles porizer 16 and a similar heating unit 30 wound around often require a carrier, and it can be quite difficult to [he nozzle 18 have been successfully used. Heat shields separate the particles from the carrier. Moreover, even 4.j 32 and 34 are provided to reduce radiation heat losses. when the particles are separated from the carrier they The heating units 28 and 30 are connected to an auxiliary tend to form aggregates which are very difficult to sepa- power supply 36 and the entire colloidal particle genera- rate. tor is maintained at a zero potential with respect to These problems have been solved by the present in- ground. vention which utilizes the condensation of a homogenous 50 In operation, a supply of propellent is enclosed in the vapor in a supersonic nozzle to obtain sub-micron sized vaporizer 16 and heated by the heating unit 28 to form colloidal particles. In this manner vaporization, nuclea- a homogenous vapor which is then expanded in the nozzle tion, condensation, aggregation and possible revaporiza- 18 to form sub-micron sized colloidal particles. The tion occur at an extremely rapid rate, on the order of heating unit 30 prevents condensation of these particles one millisecond. According to the present invention 5.j on the nozzle surface. a material is vaporized and then passed through a con- The particle formation on a nozzle condensation prw- vergent-divergent nozzle wherein adiabatic expansion ess is highly complex, and is dependent primarily on of the vapor produces the required supersaturation for reaching a critical nucleation rate in the vapor flow. particle growth. The collision rate of the particles is This rate is a function of the supersaturation, tempera- controlled by the physical properties of the vapor as well GO ture, and physical properties of the propellent in the as the dimensions of the apparatus, and in this manner the vapor stream. particle growth rate and size is controlled so that the Supersaturation is controlled by the flow rate in the particles have atomic mass units (a.m.u.'s) that are nar- nozzle 18. Because the nozzle throat functions as a critical flow orifice, the vapor pressure of the propellent rowly distributed in a predetermined mass range. The system is operated at a maximum pressure determined 6.j in the vaporizer 16 must be high enough so that adiabatic expansion can occur in the nozzle 18. This expansion by the vapor pressure of the material; consequently the creates the necessary conditions for the supersaturation exhaust pressure can vary from a hard vacuum to a rela- required in nuclei formation. The vapor pressure of tively high pressure and still produce colloidal particles. the propellent in the vaporizer 16 is controlled by regulat- The method and apparatus for generating colloidal io ing the power input to the heating unit 28. Particles according to the present invention can be used A predetermined supersaturation is required for nu- in numerous types of spacecraft. The colloidal particles cleation, and for some materials this state is reached =r /. 3,173,246 3 4 ahead of the nozzle throat. In this situation subsequent from metal foil that is supported on a relatively light expansion is not required for nucleation; however, the frame. expansion does cool these materials. This cooling is While one example of the invention has been dis- necessary to reject heat from the vapor stream to insuure closed and described it will be appreciated that various efficient formation of sub-micron colloidal particles from 5 modifications may be made to the structure as well as nuclei. The process is stopped with the formation of the method without departing from the spirit of the nuclei if low mass particles are desired. The formation invention or the scope of the subjoined claims. For ex- of the stable nuclei is quite complex and can vary from ample, it is contemplated that water as well as carbon 100 a.m.u.’s to over 600,000 a.m.u.3 for different ma- dioxide may be used as the propellent in the colloidal terials. 10 engine. While an important function of the nozzle 18 is to What is claimed is provide for supersaturation so that nuclei can be formed 1. A method of propulsion comprising the steps of from a given material, it further insures an adequate vaporizing a material, mass flow rate at a given supersaturation to provide a expanding said material in a supersonic node, con- sufficient quantity of nuclei suitable for use in an electro- 15 densing said material to form a plurality of sub- static engine. Also the length of the nozzle 18 is selected micron sized colloidal particles, to provide the required time necessary for nuclei forma- charging said particles, and tion and/or particle growth. The nozzle has an expan- accelerating said particles. sion ratio which not only provides the supersaturation 2. A method of propulsion comprising the steps of required for nuclei formation but also insures cooling so 20 heating a coloidal particle forming material to vapor- an efficient conversion of vapor to particles can occur ize the same, from a thermo-dynamlc viewpoint. expanding said vaporized material in a supersonic Particles in the range of 103 to 105 a.m.u.3 are pre- nozzle, ferred because they can be accelerated to a desired im- controlling the flow rate of said vaporized material in pulse by reasonable accelerating voltages.