Test Activities on Hybrid Rocket Engines: Combustion Analyses and Green Storable Oxidizers - A Short Review

Hybrid rocket engines (HREs) offer a low-cost, reliable, and environmentally friendly solution for both launch and in-space applications. Hybrid propellants have been identified as green thanks to their use of non-toxic, non-carcinogenic oxidizers. Of particular relevance are storable oxidizers, namely high-concentration (≥90 wt.%) hydrogen peroxide (HP, H2O2) and nitrous oxide (N2O). This work provides a survey of experimental activities based on H2O2 and N2O for hybrid rocket propulsion applications. Open literature data are completed with original thermochemical calculations to support the discussion

Role of Pressure and Aluminum Size in Solid Propellant CCP Generation

Aluminum combustion in solid propellants generates condensed products leaving the burning surface. The population of this particles is quite wide, spanning from smoke-oxide to molten metal drops. Their properties depend upon both intrinsic propellant features and combustion conditions (e.g. composition, microstructure, combustion pressure, and propellant burning rate). In propellants, aluminum is typically used in the shape of a micrometric powder. This class of energetic materials produces spherical agglomerates having the size between some tens to few hundreds of micrometers. When the metal fuel turns to nanometric, flake-kind aggregates emerge from the burning surface. Some macroscopic properties, such as the burning rate, are affected. This paper presents some results obtained from a set of aluminized propellants based on inert binder (hydroxylterminated polybutadiene) and ammonium perchlorate. The effect of both powder size and pressure is explored in terms of ballistics and condensed combustion residues. A nonstraightforward trend with pressure emerges when the condensed combustion products of propellants containing micro-aluminum and nano-aluminum are compared

Thermite-for-Demise (T4D): Preliminary assessment on the effects of a thermite charge in arc-heated wind tunnel experiments

The use of thermite to aid spacecraft demise during atmospheric re-entry is investigated in the ESA-TRP SPADEXO project. The Thermite-for-Demise concept was tested in L2K arc-heated wind tunnel facility. In this paper, the design of the samples for the experimental campaign is presented. The SCARAB software was used to tune the heat load on the samples and to predict the test results. A new extension for the representation of exothermic reaction was implemented. A set of numerical and experimental data were compared, confirming good agreement in terms of thermal behaviour of the samples, extra heat provided, and ignition time

A Critical Analysis of Paraffin-based Fuel Formulations for Hybrid Rocket Propulsion

A wide chemical, thermal, mechanical and ballistic investigation of paraffin-based solid fuels for hybrid propulsion is discussed in the present paper. Different formulations are investigated and characterized, ranging from pure solid paraffin waxes to compositions containing strengthening additives. Thermal properties are studied using a DSC technique to obtain data about the transitions typical of paraffin waxes (solid/solid and solid/liquid) and about the thermal decomposition of the added energetic fillers. Rheological and mechanical properties of the fuels were investigated as well, with a focus on the viscosity of the melted fuels. Firing tests were performed in a lab-scale burner enabling time-resolved regression rate measurement

Activated Aluminum Powders for Space Propulsion

Aluminum powders are commonly used in solid propellants to enhance the performance of space propulsion systems. During combustion, a fraction of the fuel metal particles, which emerge from the bulk, tends to merge into aggregates. These structures eventually leave the combustion surface in the shape of partially molten agglomerates which can reach the size of hundreds of microns. These condensed combustion products partake in nozzle expansion and hinder the delivered specific impulse of the rocket unit. The enhancement of original particle reactivity improves combustion quality and may reduce sensibly agglomerate size and relevant losses. More reactive aluminum fuel can be obtained by activation of micron-sized powders, without resorting to the use of nano-metals. One of the methods consists of a chemical treatment with a processing solution which alters the standard oxide layer at the surface of the particles. Such modifications grant lower ignition temperature and faster propellant burning rates but deplete a fraction of the active metal content, as a result of the chemical reaction. The present paper compares the features of three batches of aluminum particles which were treated with fluorine-based activating solutions of different concentrations. The batches were supplied in the frame of HISP FP7 European Project. The characterization focused on physical, chemical and thermal properties, looking at the reactivity of the samples and at the alterations introduced by the chemical processing. Finally, activated aluminum batches were tested in lab-scale propellants, monitoring the variation of ballistic properties with respect to a reference formulation