CEAS Space Journal https://doi.org/10.1007/s12567-020-00313-9 ORIGINAL PAPER Preliminary aerodynamic design of a reusable booster fight experiment C. Merrem1 · V. Wartemann1 · Th. Eggers1 Received: 27 August 2019 / Revised: 31 March 2020 / Accepted: 5 April 2020 © CEAS 2020 Abstract The Reusable Flight Experiment (ReFEx) is an experimental vehicle currently in development by the German Aerospace Center (DLR), which simulates the reentry of a winged reusable booster stage. The topic covered in this paper is the aero- dynamic design through the Mach number range from 5.0 to 0.8, including the transonic regime, by CFD simulations using the DLR TAU-Code. For steering and lift generation, the vehicle is equipped with multiple aerodynamic surfaces: canards, wings and a vertical fn. A VSB30 was chosen as the carrier rocket, leading to numerous limiting conditions that had to be fulflled. The resulting tradeofs are discussed. Also the process to achieve a confguration that meets all needs regarding stability during ascent and descent, controllability and energy management to autonomously follow a desired trajectory is explained. Since this vehicle is under development, the presented geometry is not necessarily the fnal shape. However, the completed design iterations strongly indicate that only details will difer between the presented state to the fnal and fyable ReFEx, scheduled to fy in 2022. Keywords Reusable fight experiment (ReFEx) · CFD · Aerodynamic design · Reusable booster Abbreviations p Pressure x y z x y Latin , , Body-fxed -, -, z-axes C Aerodynamic coefcient Abbreviations Ma Mach number CFD Computational fuid dynamics d Diameter DLR German Aerospace Center x, y, z Force in x-, y-, z-axes direction MRP Moment reference point Greek ReFEx Reusable fight experiment SM Static margin α Angle of attack TAU​ Triangular adaptive upwind β Sideslip angle Subscripts CG Center of gravity 1 Introduction NP Neutral point l, m, n Moment around x-, y-, z-axes Access to space is no longer a privilege to few industrialized countries, but is part of the globalized world’s technologies at a growing number of countries’ disposal. This techno- * C. Merrem logical advancement comes with an increasing number of [email protected] competing governmental agencies and also companies in V. Wartemann the private sector. Therefore, it has become important to not [email protected] only build reliable but also cost-efcient launch vehicles. Th. Eggers One attempt to reduce the cost of launching payload into [email protected] orbit is to use parts of the carrier rocket multiple times, dem- 1 onstrated by SpaceX’s Falcon rocket. They reuse the rock- Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR), Lilienthalplatz 7, et’s booster stage after landing it vertically. An alternative 38108 Braunschweig, Germany take on the landing procedure is a horizontal, airplane-like, Vol.:(0123456789)1 3 C. Merrem et al. approach. This is the basic idea of ReFEx, a fight experi- gear, the current plan is to perform a fare to touch ground ment to prove this concept. with least possible damage to the vehicle. Reusable spacecraft which land horizontally include the The main focus of the aerodynamic investigations lies Russian Buran and the American Space Shuttle. However, on the experimental phase. It starts outside of any notable they are designed to withstand a high-speed reentry from an atmospheric infuence. A cold gas system will orientate orbital trajectory, whereas ReFEx is only exposed to Mach ReFEx into an aerodynamically stable state to enter the aero- numbers up to 5.0 for a relatively short time period. Also, dynamically dominated part of the reentry without any dif- the objective of a reusable booster stage is to transport fuel, fculty. The initial deceleration will be done at high angles of not astronauts or satellites. A vehicle with a rather similar attack to keep the structural and aerothermal loads within an purpose to ReFEx is the Russian Baikal Booster stage, which acceptable range. Later, the angle of attack will be decreased is under development. It is a reusable booster stage, so most to fy at angles close to a state of the maximum lift-to-drag of its requirements are the same as ReFEx. Aerodynamically, ratio, which is around α = 10°. During the entire experimen- the main diference is the Baikal Booster’s rotatable wing, tal phase, the energy management is of great importance to which, in conjunction with a jet engine, allows the Baikal reach the destination at the desired speed. To achieve this, a Booster to cover a large distance over ground during the corridor of fyable pitch angles and bank angles around the return fight. This way it can return to its launch point even one preset to maintain the trajectory has to be made availa- if the trajectory is rather fat-angled. ble. A more detailed description of the mission can be found Other projects in this feld include the Hopper project of in the overview paper [3]. the ESA which resulted in the 7-m-long Phoenix vehicle. A successfully completed experimental phase as It has short double delta wings and airplane-like landing described above will fulfll the following mission objectives: gear. But the project was stopped after one successful drop test from a helicopter from around 2 km height in 2004 [1]. • Precise orientation of ReFEx with an reaction control This experiment covered the landing phase. Another fown system (RCS). experimental vehicle is the Hypersonic Flight Experiment • Steering throughout a the range of Mach numbers from (HEX) from the Indian Reusable Launch Vehicle Technol- 0.8 to 5. ogy Demonstrator Programme (RLV-TD) that was propelled • Energy management system to reach the target at the to a height of 65 km at Ma = 5 in 2016. Reportedly, this desired speed. 1-m diameter vehicle few controlled for 770 s and ended its fight with a planned splashdown and was not recovered [2]. Additionally, if the landing segment is carried out suc- The midterm goal of the HEX vehicle, however, is to serve cessfully, several other capabilities of ReFEx will be dem- as a scramjet testing platform and therefore to withstand onstrated. These include stability and maneuverability in the higher heat loads. Nevertheless, criteria such as navigation low Mach number regime as well as an accurate altitude and control are quite comparable to ReFEx. measurement. Last but not least, collecting the vehicle, ReFEx is the frst fight experiment in the current DLR retrieving the stored data and examining the landing damage roadmap towards hypersonic transport vehicles. With this are important steps to gain additional knowledge for other fight experiment, a good portion of the necessary experi- upcoming fight experiments. ence and basic knowledge will be gained to build a reus- able booster stage. The long-term goal is hypersonic travel 1.2 Confguration overview around the world. The confguration originates from an older study [4] with the shared goal of designing a reusable booster stage but 1.1 Mission objectives containing diferent constraints. This low-wing confguration was laid out with two canards and two angled fns. It also has The fight of ReFEx consists of three phases: the ascending, a body fap, which was not used for ReFEx. Only longitudi- the experimental and the landing phase. First, the ascent nal investigations concerning stability were conducted (see takes place. ReFEx is propelled by a VSB30 carrier sys- Fig. 1, left). Stability investigations of the old confguration tem to a height of about 100 km and a Mach number of revealed that it was not statically directionally stable. Still, approx. 5.0. After separating all carrier-related parts, the this was a good starting point to design ReFEx. experimental phase begins. In this phase, ReFEx is tasked The design process was dominated by several side con- to autonomously follow a predefned trajectory to reach the ditions, most prominently by the launch vehicle being a target destination, an ellipsoid about 10 km above ground, VSB30 system. It constrained dimensions, mass and aero- at a subsonic speed. Reaching this target marks the start of dynamic properties of the fight experiment for the ascent the landing phase. Since ReFEx is not equipped with landing phase which means: 1 3 Preliminary aerodynamic design of a reusable booster fight experiment Fig. 1 Overview of ReFEx’s confguration. Left: geometry from the previous study. Right: reference geometry for aero- dynamic design from the frst design iteration • ReFEx had to be symmetrical because the VS30 is a bal- information is transported via second-order AUSM upwind listic missile. scheme. Validation of TAU over a wide range of geometries • Mass and dimensions needed to ft a certain window. and fow conditions has been conducted in several studies • Side wind forces during ascent had to be sufciently [6–8]. small. The discretization was done with Centaur [9]. For the Euler simulations, unstructured tetrahedron meshes were To avoid or fx most of the ascent difculties, a fairing used. The accepted convergent cell count is around 4.1 mil- will be used. It will cover the asymmetrical wings and thus lion. The Navier–Stokes simulation meshes consist of an lower the side forces. Since the wings were too large to ft additional structured boundary layer, leading to a hybrid under a fairing, foldable wings were chosen to fx this prob- mesh. The cell count for these meshes is about 11 million. lem. This allowed the fairing to be sufciently slender, as the The current aerodynamic database for ReFEx con- stability analysis in Chapter 3 shows. The space inside the sists of two types of simulations. Simulations using the fairing that was required by the folded wings led to switch- Navier–Stokes equations with a Spalart–Allmaras one-equa- ing from two angled fns to a single vertical fn.