Hybrid rockets have very interesting characteristics like simplicity, reliability, safety, thrust modulation, environmental friendliness and lower costs, which make them very attractive for several applications like sounding rockets, small launch vehicles, upper stages, hypersonic test-beds and planetary landers. In recent years, advancements have been made to increase hybrid motor performance, and two of the most promising solutions are vortex injection and paraffin-based fuels. Moreover, both technologies can be also used to tailor the fuel regression rate, in the first case varying the swirl intensity, and in the second case with the amount and type of additives. In this way, it is possible not only to design high-performing hybrid motors, but also to adjust their grain and chamber geometries to different mission requirements, particularly regarding thrust and burning time. In this paper, the knowledge about these two technical solutions and their coupling is extended. Three sets of experimental campaigns were performed in the frame of the Italian Space Agency-sponsored PHAEDRA program. The first one investigated a reference paraffin fuel with axial and standard vortex injection. The second campaign tested vortex injection with low values of swirl numbers down to 0.5 with a conventional plastic fuel, namely polyethylene. Finally, the last campaign tested another, lower regressing, paraffin-based fuel with the same low swirl numbers as the second campaign.

The use of thermite to assist satellite demise is a novel methodology under exploration in the frame of the CleanSpace Initiative of the European Space Agency. The idea consists in inducing an additional heat source during the reentry of a spacecraft, supporting the destruction of those massive components which may not demise completely. Embedding thermites into some components is under investigation within SPADEXO, an ESA-TRP project. One of the key aspects of the energetic component design is the understanding of the best heat transfer mechanism. This paper presents a preliminary on-ground experimental campaign aiming at the quantification of the heat exchanged between a thermite charge and a surrounding metallic structure. This activity aims at demonstrating the capability of spontaneously igniting a thermite charge embedded in a metal vessel subject to external convective flow as well as evaluating the efficiency of the heat transfer process.

In this work the regression rate performance and flow physics of a lab-scale hybrid rocket engine burning gaseous oxygen and paraffin-based fuels are experimentally and numerically investigated. Regression rates are obtained by thickness-over-time averaging procedures and through a non-intrusive optical method enabling fuel grain port diameter tracking. A numerical rebuilding of the experimental data is performed with axisymmetric Reynolds-averaged Navier-Stokes simulations, using sub-models accounting for the effects of turbulence, chemistry, radiation, and fluid-surface interaction. Simulations are performed with different computational setups, also considering the fuel grain shape variation over time, obtaining a fairly good agreement between the numerical and experimental data. A parametric analysis is also performed to assess the variation of the fuel regression rate with swirl intensity.

Hybrid rockets have many advantages over pure solid or liquid propellant rockets, but low solid fuel regression rates and correspondingly low thrust have hindered their application to operational systems. Paraffin-based fuels regress significantly faster than traditional polymeric formulations, such as HTPB, and paraffin inclusion in HTPB represents a potential tool for performance augmentation in hybrid rockets. A survey of the available literature indicated disparities regarding the utility of this approach which are resolved herein. Fuel specimen consisting of plain HTPB; plain paraffin; and HTPB loaded with molten macrocrystalline paraffin wax (10-75%) or solid microcrystalline paraffin particles (10-60%) were manufactured and evaluated for their thermal decomposition and ballistic properties. Fuel samples were heated (10 IC/min) in an argon atmosphere in simultaneous TGA/DTA experiments. The inclusion of macrocrystalline paraffin enhanced the low-temperature decomposition of HTPB, while the inclusion of microcrystalline paraffin had the opposite effect. The prepared fuel grains were burned in gaseous oxygen on one of two lab-scale hybrid rockets over a range of oxidizer mass fluxes (5-430 kg/m(2)-s) and pressures (0.5- 1.0 MPa). The plain macrocrystalline paraffin fuel exhibited a 300% increase in regression rate over plain HTPB. However, none of the mixed-fuel formulations exhibited notable, if any, regression rate enhancement at the evaluated operating conditions. First principles modeling was completed for the combustion of plain HTPB, plain paraffin, and mixed-fuel systems comprised of HTPB containing molten liquid paraffin or solid paraffin particles. The combustion of mixed-fuel systems is dominated by the pyrolysis of HTPB which does not allow for the formation of a melt layer at the fuel surface, such that any enhancement is due to an increase in the vaporization rate of the fuel and not entrainment effects. This study was the first to concurrently evaluate the inclusion of both molten liquid paraffin and solid paraffin particles in HTPB and demonstrated a lack of performance augmentation with either strategy in two separate laboratories. The results presented herein resolve the disparities in the literature and indicate that paraffin inclusion in HTPB is not a viable means for tailoring the combustion behavior of hybrid rocket systems. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Conventional propulsion systems are difficult to change between propelling and non-propelling modes. Throttleable propulsion that results from the control of the input energy is a significant further step toward application in various space missions. We present our work on a novel propulsion mode producing throttleable thrust under the control of our low power CW laser. This photosensitive propellant is fully capable of repeated ignition and interruption, while generating gases that are more environmentally friendly with decreased solid residues. Laser ignition and combustion performance of such modes are characterized. Laser-controlled combustion behavior examples are shown and discussed. These results show potential applications in many aspects of space missions, such as maneuvers in space, attitude control, orbit raising and microsatellite deorbiting.

The 3D printing is changing the paradigm for manufacturing, thanks to the ability to rapidly produce customized and complex structures. The Space Propulsion Laboratory is exploiting the benefits offered by the 3D printing to create a new family of paraffin-based fuels featuring both mechanical and ballistic performance: the armored grain. In the present study, the armored grains feature a macro-crystalline wax embedding 3D printed structures made of polylactic acid. Four different reinforcing structures are considered: gyroid, Schwarz P, straight honeycomb, and twisted honeycomb with 1/4 turns per inch. The investigation of the armored grain concept involves: the structural assessment of the mechanical behaviors of the 3D printed reinforcements, and the mechanical and ballistic characterization of the armored grains. The results prove that the armored grain approach is successful in increasing both the yield stress and the strain energy of the pristine paraffin, mitigating the typical brittle behavior of the alkane fuel. The firing tests indicate that the presence of the 3D printed reinforcements does not lessen the ballistic performance of the pure paraffin wax.