Abstract
The low regression rates for hydroxyl-terminated polybutadiene (HTPB)-based solid fuels and poor mechanical properties for the alternative paraffin-based liquefying fuels make today hybrid rocket engines far from the outstanding accomplishments of solid motors and liquid engines. In this paper, a survey is conducted of several innovative methods under test to improve solid fuel properties, which include self-disintegration fuel structure (SDFS)/paraffin fuels, paraffin fuels with better mechanical properties, high thermal conductivity fuels and porous layer combustion fuels. In particular, concerning HTPB, new results about diverse insert and low-energy polymer particles enhancing the combustion properties of HTPB are presented. Compared to pure HTPB, regression rate can be increased up to 21% by adding particles of polymers such as 5% polyethylene or 10% oleamide. Concerning paraffin, new results about self-disintegrating composite fuels incorporating Magnesium particles (MgP) point out that 15% 1 μm- or 100 μm-MgP formulations increase regression rates by 163.2% or 82.1% respectively, at 335 kg/m2·s oxygen flux, compared to pure paraffin. Overall, composite solid fuels featuring self-disintegration structure appear the most promising innovative technique, since they allow separating the matrix regression from the combustion of the filler grains. Yet, the investigated methods are at their initial stage. Substantial work of refinement in this paper is for producing solid fuels to fulfill the needs of hybrid rocket propulsion.
Highlights
Hybrid engines testing dates back to the very beginning of rocket propulsion development, both in USA and Russia
hydroxyl-terminated polybutadiene (HTPB), isophorone diisocyanate (IPDI), dioctyl adipate (DOA), dibutyltin diacetate (TIN), and four kinds of polymer particles (PE paraffin, oleamide, polydextrose, and polyethylene glycol (PEG)) are the raw materials used for sample preparation
15% 100 μm-Magnesium particles (MgP), which indicates that the regression rate of composite fuel with small size magnesium particles is significantly affected by oxidizer mass flux
Summary
Hybrid engines testing dates back to the very beginning of rocket propulsion development, both in USA and Russia. In spite of these very precocious activities, still today hybrid rocket propulsion is far from the huge success later obtained by the companion liquid and solid rocket propulsion. The hybrid engine featured an 11-port wagon wheel solid grain This system was throttleable, burned smoothly, and exhibited impressive high performance with a specific impulse efficiency of 93%, achieving a delivered vacuum specific impulse of about 380 s with a nozzle area ratio of 40. After intensive static fire testing of large hybrid engines burning LOx/HTPB (8- or 15-port wagon wheel solid grain) capable of 1.1 MN thrust in vacuum up to 80 s burning time, NASA gave up despite the potential advantages for safety, cost, and versatility of propellant selection. The intrinsic architecture of HRE forces a macroscopic diffusion flame conducive to poor combustion efficiency compared to solid and liquid rockets
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