Metallic Nanopowders: Rocket Propulsion Applications
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Active metal powders are extensively used as fuels in most solid rocket propellants because of the high energy produced during their combustion. The specific impulse (Isp) of the rocket engine is proportional to (Tc/M)1/2, where Tc is chamber temperature and M is molecular weight of combustion products. Thus the best propellants are those that produce the highest combustion temperature and the smallest possible molecular weight of the combustion products. Therefore the best oxidizers are fluorine and oxygen and the best fuels are lithium, beryllium, boron, aluminum, and magnesium. Lithium is extremely reactive and beryllium is extremely toxic so these are impractical in rocket applications. That leaves boron, aluminum, and magnesium powders as primary candidates.
Aluminum is a major ingredient in solid rocket fuels, often combined in a rubbery binder along with particles of oxidizer. When burning aluminum in solid propellants, the energy utilized can be diminished because the droplets agglomerate, producing larger droplets and slower combustion that can occur too late (after the nozzle) to be effective. The agglomerates, although partially oxidized, often slag up on the internal surfaces of the engine, reducing combustion efficiency and weighing down the vehicle.
As with solid propellants, adding aluminum to liquid fuels would also provide a theoretical advantage in higher volumetric energy density, but the metal must be uniformly dispersed and remain so in the hydrocarbon. As with solid propellants, aluminum combustion must be rapid enough so that it is consumed within the rocket engine. The most effective means of achieving complete combustion is to use powders with particle sizes at least an order of magnitude or two smaller than the metal powder ordinarily used in solid propellants. This article focuses on Alex® nanosize aluminum particles manufactured by the electroexplosion of metal wire (“EEW”) and its use in liquid and solid rocket propellants.