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Hybrid Rocket Technology Exploration

Periodic Reporting for period 1 - HyTeX (Hybrid Rocket Technology Exploration)

Reporting period: 2021-03-01 to 2023-02-28

The space industry is going through a transformation phase: in the next years a significant increase in the market of small satellites is expected and current launch capacities cannot cover needs. A dedicated service is necessary to launch small satellites to specific orbits and time windows. HyImpulse intends to bring the key technologies for a game-changing, unique, low risk, innovative small launcher to the market, with a 500 kg payload capability to LEO. The launcher will be based on a combination of novel and established technologies, using inherently safe hybrid propulsion (paraffin/LOX propellants). The distinct advantages of hybrid rocket engines are summed up as follows: inert fuel and liquid oxidizer never form an explosive mixture. The fuel regression rate does not depend on the chamber pressure, hybrid engines can be throttled and the possibility to shut-down and re-ignite a hybrid motor is a major point for precise orbit insertion. High safety and low costs: inert fuels reduce costs for safety precautions and only one feedline system is required for the engine. Green propellants: the hybrid motor that will be developed will use liquid oxygen and paraffin. System complexity: even if a turbopump is required for a small launcher, the system complexity is still reduced compared to bi-liquid motors. In that context the overall objectives of the project were to develop and exploit a new fuel regression rate measurement method, to optimize the fuel grain production process and to study and improve the hybrid motor performance (paraffin composition, stability, combustion efficiency). All these scientific objectives have been achieved during the period of the action. The regression rate measurement method has been employed intensively on the medium and large-scale motors developed at HyImpulse to determine the regression rate performance of the fuel in the early stage of the development. At the end of the action period, that measurement system is available and exploited from time to time when there is a need of detailed investigations on the fuel regression rate. The wax-based fuel composition has been extensively investigated, from material characterization (mechanical properties, viscosity, influence of temperature) and from behavior during real combustion tests at different scales. Good composition and operating conditions have been identified and demonstrated to be efficient regarding both aspects: mechanical integrity (avoiding material degradation or mechanical failure under loads and combustion environment) and regression rate (in order to match the desired oxidizer to fuel ratio). Fuel grain production process has been regularly improved and has obtained a good level of maturity. The quality and reproducibility of the manufactured grains reached satisfying levels. The engine performance has been improved over the action duration and many different pre- and post-combustion chambers configurations have been tested and evaluated at medium and large scales. The motors have demonstrated very good ignition characteristics and combustion efficiencies that make them very close to the qualification for the rocket maiden launch. The stability topic has been a major part of the scientific work performed. It is a very complex topic involving many different processes interacting together and is the biggest issue regarding a rocket motor: an unstable rocket motor cannot fly since it would risk damaging either the rocket, or the payload, or both. After deep investigations involving combustion testing, literature analysis, and CFD calculations, a good understanding of the key phenomena and parameters related to that aspect has been reached and the stability issues have been mitigated.
During the project, a dedicated method has been implemented to measure the axial distribution of the fuel regression rate along the axis of the engines. This consists in filling a motor standing vertically with water, measuring the mass flow rate and level of the water during the process, and reconstruct the internal contour before and after firing tests. Combined with a ballistic reconstruction algorithm, this system is exploited to study detailed parameters of the fuel behavior during combustion tests. This contributed to improve the fuel grain regression rate through geometry or composition changes over the action to match the targeted requirements for the motor performance. In addition, extensive analysis has been performed to understand and improve structural aspect of the fuel components. Other aspects of the motors have been studied and improved such as the combustion efficiency thanks to different post-combustion chambers geometries and mixing devices, that allowed to reach very high levels of efficiencies. The nozzle erosion has been considered with attention and several mitigation solutions have been studied such as the use of passive film cooling from insulation materials, this work led to an international conference publication and a journal article is being written at the end of the action. The last aspect of the motor development that has been investigated extensively was the stability. Unstable combustion is common in hybrid engines that employ cryogenic oxidizer. In fact, the oxidizer requires a very high amount of heat and good flow conditions in order to be vaporized properly and avoid what is called a “vaporization lag” leading to local flame extinction. Over the action, tens of medium-scales as well as more than 10 large-scale engines have been fired and the progress done regarding stability has been significant. The pressure oscillations have been reduced to a level allowing for first flight in the next months after a qualification campaign already planned later this year. To reach that level of performance, CFD calculations as well as dedicated LOx injector test bench have been performed in combination with many hot fire tests at different scales. These testing activities have been realized partially at the DLR test site of Lampoldshausen and in the Shetland Island where HyImpulse owns a test facility at Scatsta Airport. Finally, a novel and proprietary fuel production facility has been developed and improved over the action. The facility is currently exploited for all the fuel grains of the large motors and will be scaled up in the future when increasing the production cadency will become necessary. The dissemination plan of the project has been performed through different manners and supports such as a dedicated webpage on HyImpulse website, the participation to international conferences or other type of events, the publication of promotional videos, the writing of company newsletter as well as the support of the company open day.
Noticeable progress has been done regarding the modeling of nozzle erosion through the establishment of a new reconstruction algorithm that should be published in a journal in the coming months. The water measurement system as well as its coupling with ballistic reconstruction algorithms also goes beyond the state of the art and provides very valuable data for the motors development. Finally, the whole process of development and testing of various hybrid motors of 75 kN has been part of the outstanding results, achieved partially thanks to the project. This type of performance was never reached so far in Europe and is a significant step towards a more independent European access to space in the coming years.
Top view of a HyPLOX75 firing test.