CFS Engineering participated in several ESA Projects through past years. CFSE work in these projects concerns mainly space re-entry and all problematics link to it, including:
- Shock waves
- Heat transfer
- Particles ejection
- Stability (6 DoF)
The successful development of launch vehicles depends on finding solutions in the critical and challenging areas of propulsion and aero-thermodynamics, which are the key elements of any launch vehicle. In the past aerodynamic and aero-thermodynamic development work was almost exclusively based on the use of engineering and empirical methods. Today the use of Computational Fluid Dynamics (CFD) has matured to the point where it can provide valuable physics based input where in the past empirical or engineering methods required to strongly simplifying the physics involved. Although CFD based methods are considered as the best solution to accelerate and refine the design process of launch vehicles, CFD methods have their own shortcomings and limitations. First, CFD simulations are time consuming, in particular for unsteady flow simulations. Second CFD solvers employ physical models, as for example for turbulence and combustion. Although much progress has been made in turbulence modelling for unsteady flows over the last 15 years, uncertainties in computed results remain important. Combustion modelling, and in particular the influence of turbulence on the combustion process, is an area where still much research is needed. When using a solid propellant additional complexities arise due to the presence of particles in the exhaust gases, and the importance of radiation phenomena that influence the combustion process.
The hot plume project is mainly concerned with the modelling of the after body flow of launch vehicles (also called the base flow region). This flow is characterized by large regions of unsteady separated flow induced by the abrupt changes in geometry of the vehicle. In this region hot gases from the nozzle exit mix with the cold flow coming around the launch vehicle leading to very complex aerodynamic phenomena which today are only poorly understood. As a result there exists a large area of uncertainty in launch vehicle design.
A well-known example is the base flow buffeting experienced on the Ariane5 launcher during flight: unsteady pressure fluctuations in the base region generated unexpected high side loads on the nozzle. As a result the nozzle deforms and the launch vehicle cannot be used at full capacity.
Over the past 15 years a large number of experimental and numerical studies were performed to better understand the complex flow phenomena in the base region. However, wind tunnel tests in Europe use cold plumes for the flow from the nozzle exit (jet-on conditions), but in-flight measurements on the Ariane5 and on the inaugural flight of VEGA have shown that this approach underestimates the base pressure and hence over estimates the vehicle drag. Furthermore, measurements during the first flight of VEGA showed that the predictions performed in the design phase largely over-predict heat loads observed in flight. This implies an influence of the plume temperature on the complex flow field surrounding the base region, which in turn implies an uncertainty in the heat loads and fluctuating loads on the nozzle itself.
In the Hot Plume project (financed by the European Space Agency) the NSMB CFD code will be extended to permit the numerical simulation of hot gases with particles. In particular the following models will be implemented:
• chemistry models (frozen, equilibrium and non-equilibrium) for the simulation of hot plumes
• a Lagrangian particle tracking model to track solid particles coming out of the nozzle exit from VEGA type of launch vehicles
• a simple radiation model for solid particles
The Hot Plume project focuses on a VEGA type of launcher and of particular interest is the first stage engine firing in the transonic regime (Mach = 0.8). However, the extensions made to the CFD tool are applicable to other launch vehicles including retro-jet firing for stage separation with multiple plumes.
The IXV Post-Flight Analysis carried out by CFSE is made under the FLPP program and started in December 2018. The objectives of this project are to better understand different phenomena observed during the IXV flight, to exploit the data measured during flight (aerodynamic forces, surface pressures and temperatures) in order to validate and improve design tools (including Computational Fluid Dynamics (CFD) tools). The overall goal is to improve the design of future re-entry vehicles, and in particular the Space Rider in the short term.
The planet Mars always attracted a large interest since humans were capable of sending capsules into space. Already in the 1960’s both the Russians and Americans launched several vehicles to the planet Mars, some of them with the objective to make photographs, others with the objective to land on the planet. Many of these missions failed, but some of them were rather successful. This continued in the 1970’s with the first successful landing in 1971 by a Russian lander, whoever stopped transmission after 15 seconds. Mars exploration was quiet in the 1980’s but started again in the 1990’s and continued since then. Today discussions are underway to launch human exploration missions to Mars in the 2030- 2040 time frame.
Sending vehicles to Mars presents several challenges. The Mars atmosphere will generate substantial aerodynamic heating, but does not permit to reduce the terminal descent velocity sufficiently to allow for a safe landing. The Mars atmosphere density is approximately about 1% of the earths atmosphere, and re-entry vehicles reach subsonic conditions at much lower altitudes compared to Earth re-entry. This puts severe constraints not only on the re-entry system but also on the regions that can be reached on the planet. The second challenge, in particular for landing missions, concerns the surface of the planet. The Mars surface is a complex terrain covered by large rocks and craters, and no detailed information is available. In addition the several times yearly large dust storm blow over the planet, making landing hazardous, reducing visibility and the possibility to re-charge batteries using solar panels.
In the frame of the ESA Mars Precision Lander project an aero-thermodynamic assessment is made of a spike shape entry system for a ground penetrator mission. CFS Engineering performed a bunch of CFD simulations using NSMB on the Entry Descent and Landing (EDL) configuration proposed in this project. Steady CFD simulations for the generation of the Aerodynamic Data Base (AEDB) and Aero-Thermodynamic Data base (ATDB) for the EDL system, steady CFD simulations for the AEDB generation of the penetrator alone, as well as unsteady CFD simulations for the penetrator system and the penetrator alone to determine the stability derivatives have been performed during this project.
CFD repository generation for the Space Rider vehicle, with unsteady simulations for dynamic derivatives assessment (pitching and plunging forced oscillations) in transonic regime, and with steady simulation for INCAS Wind Tunnel Tests rebuilding in subsonic to supersonic regimes.
The objective of the computational campaign within the framework of the MarcoPolo-R Earth Re-Entry Capsule Dynamic Stability Characterization project, is to apply high-fidelity Computational Fluid Dynamics (CFD) methods to determine the dynamic derivatives of the Earth Re-entry Capsule (ERC) in the subsonic-transonic flow regime.
In a first step, the selected computational approach has been validated against subsonic free flight tests performed in the VMK experimental facility (vertical wind tunnel) at DLR.
The second step of the CFD activities has been to perform dedicated computations for extrapolation-to-flight conditions, and then for flight predictions of selected subsonic conditions in order to populate the aerodynamic database (AEDB) in the trajectory portion that is not covered by the ground tests (0.5<M<0.9).
ARV project (ATV evolution: Advanced Reentry Vehicle)
The very successful Jules Verne ATV mission has highlighted many new technologies and capabilities that can be used and adapted in the future for developing new spacecraft, making use of additional European know-how, such as atmospheric reentry technologies. Such development could be of great strategic importance for Europe’s role in human spaceflight endeavours in low-Earth orbit and for future exploration missions, leading to an autonomous launch and return capabilities to and from orbit.
The ultimate verification of a re-entry vehicle design is in-flight simulation with the help of experimental vehicles or a prototype. Such a flight will permit the validation of computational simulation tools and the verification and improvement of ground-to-flight extrapolation methods. The ESA Future Launchers Preparatory Program (FLPP) was conceived to provide a framework for, among other technology challenges, the development of the Intermediate eXperimental Vehicle (IXV). The IXV project was initiated in 2005 by ESA after analysis and comparison of different ESA and national concepts a slender lifting body configuration was selected as IXV geometry to permit Europe to gain experience with aerodynamic controlled re-entry.
Other objectives of the IXV project were technology experimentation (verifying the performance of advanced TPS systems and hot structures during real flight), technology validation (gathering data to investigate aero-thermodynamic phenomena in the hypersonic flight regime).
CFS carried out a large number of numerical simulations for different flight data points with and without chemistry, Wind Tunnel Rebuilding, Aerodynamic Data Base and Aero-Thermal Data Base generation. Building of the HAEDB. Study of aeroshape evolution effects. Several new chemistry models were implemented to assess the uncertainty of calculated results with respect to the chemistry models used.
During the IXV project (from 2007 until 2013) over 400 CFD simulations were made by CFSE.
The IXV Vehicle was successfully launched on February 11, 2015 using a VEGA-launcher. The vehicle reached an altitude of 412 km after which it started to descend, entering the earth atmosphere at 120 km with a speed of about 7.5 km/s. The IXV glided over the Pacific Ocean before the landing parachutes were opened to slow down the vehicle before splash down in the Ocean of the vehicle.
The Atmospheric Re-entry Demonstrator (ARD) was launched by the third flight of Ariane 5 in 1998. CFS Engineering participated in the aero-thermodynamic studies for the analysis of the flight results. Particular attention was focused on the analysis of differences in pre-flight prediction and actual flight data
This study was financed by the European Space Agency (ESA), and was managed by Aerospatiale-Matra Launchers.
The project was successfully completed in January 2001, but led to additional projects financed by ESA. For more the history and on-going interest in ARD, please see ESA-Bulletin 109.