Reconfigurable On-Board Guidance for (Micro-) Launchers
Authors:   Astos Solutions, Stuttgart, Germany, Gkoutzos Dimitrios , Neuss Malte , Milhano Tiago , Mohamad Said 
Published: January 2024

The rapidly growing and highly competitive nature of the commercial New Space market encourages its companies to cut costs and speed up development. However, emerging micro-launcher companies usually lack the expertise acquired over decades of experience or existing technology that can be utilized to accelerate the development of a new launch vehicle. Therefrom arises the need for industry tools that can compensate for these impediments. The design of the GNC flight software presents one of the most expensive and time-consuming tasks in the development of a launch vehicle. An ‘Off-The-Shelf’ GNC software that is be applicable to different stages in the design process, ranging from preliminary analyses and actuator sizing up to detailed design, could accelerate the development tremendously. The primary requirement for such a GNC software is to be easily and rapidly reconfigurable to any launch vehicle and mission.
Within the ESA-funded activity "Off-The-Shelf Guidance & Navigation for Microlauncher" (MLGN), Astos develops highly modular and reconfigurable guidance and navigation (G&N) algorithms up to TRL 6 in response to the needs of (micro-) launcher companies. This paper focuses on the guidance and outlines some of the developments that facilitate its rapid and simple reconfiguration. These developments can be separated into architectural changes within the guidance framework and design choices that shaped the development of the guidance algorithms. These will be discussed in turn once the central components of the guidance algorithm have been outlined.
The central components of the guidance algorithm include a set of guidance modes that are designed to meet the requirements unique to different types of mission phases, such as coasting, deployment or deorbit phases. They further encompass a mode manager that controls the transition from one guidance mode to the next. This system provides the flexibility that enables a user to quickly add or reorder certain phases in the mission timeline. The endo-atmospheric ascent mode uses an open-loop table that is generated from an offline trajectory optimization performed with the ASTOS software. For the exo-atmospheric ascent, a closed-loop trajectory optimization is performed using the PEG algorithm developed by NASA. Its extensive flight heritage which includes the Space Shuttle Program, the Space Launch System (SLS) and Orion spacecraft, demonstrates its adaptability to different missions and launch vehicles. In combination with its modular architecture and low computational load, it is a sound choice as an on-board trajectory optimizer.
The guidance framework was developed to simplify and accelerate the reconfiguration of the guidance algorithm. It contains a file structure that minimizes the number of manual inputs the user must supply, while providing the option to tune guidance performance through configurable parameters. This is achieved by extracting and processing all relevant information of the mission and launch vehicle from a file that can be automatically exported from ASTOS. This allows for a rapid and simple reconfiguration as any changes within the launch scenario can be easily transferred to the guidance algorithm by re-executing the guidance export and initialization.
The design of the guidance algorithm is focused on maximising the variety in missions and launch vehicles, while ensuring that the desired performance is met. A large variety in applicable (micro-) launcher missions can be achieved through a modular architecture of configurable guidance modes and a flexible guidance mode manager. However, variations in the launch vehicle or maneuver type, such as circularization or de-orbit maneuvers, require the closed-loop trajectory optimization to work with a large range of thrust to weight ratios. Even though PEG has a large range in viable thrust to weight ratios, its performance is known to deteriorate for low thrust to weight ratios and long thrust arcs. To extend this range, modifications to the PEG algorithm, as well as the guidance as a whole, were performed. These alterations include enhancements to the PEG algorithm that were proposed by NASA as part of SLS, the use of different orbit propagators, and an online optimization of coasting durations to name a few. The specific modifications, their motivation and the results of their examination will be discussed in further detail in the subsequent paper.
Furthermore, the paper will demonstrate the reconfigurability of guidance by applying it to two representative micro-launchers. Despite differing in their launch vehicle, mission and target orbit, they successfully reach their target orbit with minimal alterations to the guidance.