Design, Optimal Guidance and Control of a Low-cost Re-usable Electric Model Rocket Lukas Spannaglz, Elias Hamppy, Andrea Carronz, Jerome Sieberz, Carlo Alberto Pascucciy, Aldo U. Zgraggeny, Alexander Domahidiy, Melanie N. Zeilingerz Abstract— In the last decade, autonomous vertical take- off and landing (VTOL) vehicles have become increasingly important as they lower mission costs thanks to their re- usability. However, their development is complex, rendering even the basic experimental validation of the required advanced guidance and control (G & C) algorithms prohibitively time- consuming and costly. In this paper, we present the design of an inexpensive small-scale VTOL platform that can be built from off-the-shelf components for less than 1000 USD. The vehicle design mimics the first stage of a reusable launcher, making it a perfect test-bed for G & C algorithms. To control the vehicle during ascent and descent, we propose a real-time optimization-based G & C algorithm. The key features are a real-time minimum fuel and free-final-time optimal guidance combined with an offset-free tracking model predictive position controller. The vehicle hardware design and the G & C algo- rithm are experimentally validated both indoors and outdoor, showing reliable operation in a fully autonomous fashion with all computations done on-board and in real-time. Index Terms— Aerial Systems: Mechanics and Control, Mo- Fig. 1. The EmboRockETH during an outdoor flight. tion and Path Planning, Optimization and Optimal Control I. INTRODUCTION [9], these techniques also pose new challenges in terms of performance and functional validation. Sustainable and repeatable access to space requires drastic In order to address these problems, we designed an in- changes in the way space transportation systems are designed expensive small-scale vertical take-off and landing (VTOL) and operated. In this regard, novel approaches to guidance, aircraft, informally dubbed EmboRockETH and shown in navigation, and control are key enabling technologies as Figure 1, which allows the analysis and validation of G & C they allow launchers to be reused by performing pin-point algorithms before their deployment on more complex sys- landings. This significantly lowers the cost of access to tems. The EmboRockETH is designed to replicate some of space and maximizes the science return for missions on the key aspects of a reusable rocket’s first stage including other planetary bodies, where it is necessary to land close aerodynamic control fins and a gimbaled main engine. Our to relevant geological features. Concerning guidance and contribution is twofold: we detail the complete hardware control (G & C), this paper focuses on the use of numerical design of the EmboRockETH and we propose a G & C optimization to generate trajectories and actuation profiles pipeline for indoor and outdoor flights.1 arXiv:2103.04709v1 [cs.RO] 8 Mar 2021 on-board and in real-time. Similar techniques are already The remainder of the paper is organized as follows. Next, used by SpaceX [1] demonstrating their potential in the real- we discuss existing related work and introduce the notation, world. The last decade of advances in embedded computa- before detailing the hardware design of the EmboRockETH tional hardware, numerical algorithms [2], [3], [4], [5], and in Section II. Subsequently, we outline the guidance and optimization software [6], [7] has made optimization-based control architecture of the vehicle in Section III, discuss G & C algorithms feasible for space applications. Despite experimental results in Section IV, and conclude the paper the clear potential and the active research in this area [8], in Section V. This work was partly supported by the European Space Agency (ESA) A. Related Work under the Future Launchers Preparatory Programme (FLPP) Contract No. 4000124652/18/F/JLV. In recent years, several test platforms similar to the yE. Hampp, C.A. Pascucci, A. U. Zgraggen, and A. EmboRockETH have been developed or are currently un- Domahidi are with Embotech AG, 8005 Zurich, Switzerland der development. The two most notable examples being fhampp,pascucci,zgraggen,[email protected] zL. Spannagl, A. Carron, J. Sieber, and M. N. the Grasshopper [10], which was used by SpaceX for the Zeilinger are with the Institute for Dynamic Systems and Control (IDSC), ETH Zurich, 8092 Zurich, Switzerland 1A video summarizing this paper and presenting footage of an outdoor fspalukas,carrona,jsieber,[email protected] experiment can be found here: https://youtu.be/r4V7S1Rsbkg early testing of their propulsive landing maneuvers, and A Xombie from Masten Space Systems, which was used within the frame of the ADAPT project [11] to test the lossless convexification-based GFOLD algorithm [12] and B the terrain relative navigation system for the Mars 2020 mission [13]. More recently, the European Space Agency has joined these efforts with the Future Launchers Preparatory Programme (FLPP) [14]. However, all the aforementioned C vehicles are complex systems that require an appropriate bud- get, a crew and stringent safety measures for their operation. Hence, the use of quadrotors has been predominant in aca- demic research, since these vehicles enable rapid prototyping and real-world testing of advanced G & C methods [15], [16] in an agile setting. Nevertheless, quadrotors share only a D few basic principles with reusable first stages. Therefore, the vehicle presented in this paper is aimed at bridging this gap by providing an agile research platform for advanced G & C methods with an improved dynamical representation of reusable launchers. Fig. 2. Structural overview of the EmboRockETH: (A) electronics & battery, (B) 3D-printed fins, (C) carbon fiber central spine, (D) gimbaling mechanism & propeller cage. B. Notation In this paper, we denote vectors with bold letters, i.e., a 2 Rn, matrices with bold capital letters, i.e., A 2 Rn×m, and A. Structural Design the time derivative of a vector or matrix with the dot notation, The EmboRockETH’s core structure is a carbon fiber i.e. a_ . The symbol H denotes the set of unit quaternions and square tube mounted on four flexible non-retractable landing ei denotes a unit vector along the i-th axis. We denote with legs made from carbon fiber. The housing of the electronics the symbols p 2 R3 and v 2 R3 the position and velocity and the battery are mounted opposite of each other at the top of the EmboRockETH in the world frame, respectively. The of the central square tube, while the gimbaling mechanism quaternion q 2 H denotes the aircraft’s attitude, while ! 2 is mounted between the landing legs at the bottom of the R3 is the associated angular velocity. The symbol T 2 R3 vehicle, see Figure 2 (A, C, D). The gimbal consists of an represents the thrust vector applied to the aircraft and g 2 R3 universal joint connecting the central spine to a propeller denotes the gravitational force. Given a quaternion q 2 H, cage. Additionally, the vehicle is equipped with four 3D- the rotation matrix R(q) 2 R3×3 is defined as printed fins, which enable aerodynamic control of the vehi- 2 2 2 3 cle. The four fins are mounted on a ring below the electronics 1 − 2(qy + qz ) 2(qxqy − qzqw) 2(qxqz + qyqw) 2 2 R(q) = 42(qxqy + qzqw) 1 − 2(qx + qz ) 2(qyqz − qxqw)5; and are symmetrically arranged around the core structure as 2 2 2(qxqz − qyqw) 2(qyqz + qxqw) 1 − 2(qx + qy) shown in Figure 2 (B). where qx=y=z=w are the components of the quaternion q. B. Actuators Finally, given a quaternion q 2 , we define the quaternion H The EmboRockETH’s main propulsion system consists propagation matrix as of two coaxially mounted propellers, each actuated by a 2 3 0 −!x −!y −!z brushless DC motor. To enable thrust vectoring, the two 6!x 0 !z −!y7 rotors are mounted in a propeller cage, which is gimbaled Q(!) = 6 7 ; 4!y −!z 0 !x 5 by two servo-motors. For aerodynamic steering, the four 3D- !z !y −!x 0 printed fins are independently actuated by one servo-motor each. where !x=y=z=w are the components of the quaternion !. C. Electronics & Sensors II. VEHICLE DESIGN The EmboRockETH uses a Raspberry Pi 4 [17] as its main In this section, we detail the design of the EmboRockETH, computational platform, while a PixHawk 4 autopilot [18] which is an electric small-scale VTOL vehicle and has provides the sensing capabilities and the low-level flight the capability to fly both indoors and outdoors. The control. The Raspberry Pi hosts the high-level decision EmboRockETH is mainly built from off-the-shelf compo- algorithms, i.e., guidance and position control, running on nents with a total cost of less than 1000 USD. The fully the robot operating system (ROS). The Pixhawk autopilot assembled vehicle weighs 1.16 kilograms, stands 1.05 meters interfaces the Raspberry Pi through MAVROS [19] and hosts tall, and can continuously hover for about 5 minutes. In the low-level attitude control and state estimator. A more the following, we discuss the overall structural design, the detailed overview of the algorithms and their respective actuators, and the electronics of the vehicle. execution platform is shown in Figure 3. Additionally, the Raspberry Pi 4 Pixhawk Autopilot Tx Tx Optimal Position fpf g psp Guidance v Controller MavROS Attitude Att. Rate sp qsp qsp !sp τ Control S MPC 25 Hz Controller Controller Allocation w^ P 250 Hz PID 1 kHz CT-DT q^ !^ Kalman Filter x^ x^ y MavROS EKF Fig. 3. The above schematic represents the guidance and control architecture of the EmboRockETH, including the computational execution platform of the individual algorithms. The Raspberry Pi hosts the optimal guidance (1) and the high-level MPC position controller (7), while the Pixhawk hosts the low-level PID controllers and the control allocation.