Combustion Processes in Hybrid Rockets

Abstract

This paper presents the latest results achieved at the Space Propulsion Laboratory (SPLab) of Politecnico di Milano in the area of hybrid propulsion. Focus is put on four specific research topics, currently under investigation, and strongly linked: 1. solid fuel formulations development; 2. investigation of liquefying fuel formulations responsible for the entrainment phenomenon; 3. development of a vortex flow pancake (VFP) designed for in-space missions; 4. numerical simulation approaches. A wide chemical, thermal, rheological, mechanical and ballistic investigation of traditional polymeric formulations and paraffin-based solid fuels has been performed in the last years and is shortly summarized here. Firing tests are performed in a radial lab-scale burner enabling timeresolved regression rate measurements. The results of this activity pave the way to the challenging horizon of liquefying fuel formulations. The entrainment of melted fuels is investigated by a dedicated setup designed for the study of the oxidizer stream/melt surface interaction under cold-flow conditions, to understand the droplet formation mechanism and to measure their size distribution. The effects of liquid layer entrainment on the combustion processes seem attractive for the development of unusual geometries, such as the VFP. The VFP hybrid rocket configuration offers a compact implementation with motor length-to-diameter ratio lower than 1, giving a breakthrough opportunity for inspace missions that could strongly benefit from the system affordability, with low recurring costs joined to high operating flexibility. The VFP development requires a strong support of numerical simulation activities, developed through OpenFOAM, and described in the last part of the paper.

A new strategy for the reinforcement of paraffin-based fuels based on cellular structures: The armored grain — Mechanical characterization

“…In these analyses, the reinforcing materials are used to create a binding matrix hosting wax pockets, or blending the paraffin with strengthening agents. The key point of these reinforcing methods is the remediation of the brittle and frail behavior of wax with limited alterations of its original low viscosity [16,30,31].…”

“…Conventional (i.e., non-liquefying) fuels as hydroxyl-terminated polybutadiene (HTPB) feature high mechanical properties, though the r f is limited. Their use as reinforcing binders for fast-burning paraffin wax was one of the first methods proposed for the creation of a fuel formulation with suitable mechanical and ballistic properties [13][14][15][16][17][31][32][33][34][35][36][37][38]. Boronowsky [32] tested the combustion behavior of paraffin, HTPB and two different heterogeneous HTPB-based fuels consisting of 15 wt% and 30 wt% granulated paraffin chunks (with size in the range 0.3-0.7 mm).…”

Nano-Sized and Mechanically Activated Composites: Perspectives for Enhanced Mass Burning Rate in Aluminized Solid Fuels for Hybrid Rocket Propulsion

“…Current liquefying fuels offer low mechanical properties. Typical mechanical properties reinforcement techniques yield a reduction of the entrainment effect due to the augmented viscosity of the melted fuel layer [10][11][12][13][14]19]. Recent efforts proposed innovative reinforcing strategies exerting minor effects on the liquid phase viscosity such as self-disintegrating structures [64], as well as the use of three-dimensional (3D) printed cellular structures [65,66].…”

“…Data sampling extended until the end of the combustion (depending on visualization quality). Diameter sampling was performed with the maximum possible frequency (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) Hz under the investigated conditions, with higher frequencies in the earlier phases of the burning process). During the combustion, central port diameter was sampled along different radial directions (depending on combustion uniformity).…”

“…Different studies are currently ongoing to obtain reinforced fuel grains combining ballistic and mechanical properties. Paraffin wax blending with reinforcing polymers is a strategy to achieve this purpose [9][10][11][12][13][14]. The improved mechanical properties of the blended formulations are accompanied by increased viscosity of the melted phase; this lessens the entrainment mass transfer, thus decreasing r f and .…”