Comparative analyses of European horizontal-landing reusable first stage concepts

Reusable launch systems have the potential to significantly impact the space launch service market if both a high reliability and low refurbishment costs can be achieved. This study delves into the Vertical Takeoff and Vertical Landing (VTVL) methodology, as currently employed by SpaceX, and forms a segment of the ENTRAIN study by DLR. This broader study encompasses an examination of both Vertical Takeoff Horizontal Landing (VTHL) and VTVL reusable first stages, exploring their performance across high-level design parameters. This manuscript’s primary objective is to assess the quantitative impact of high-level design factors on launch vehicle performance, particularly in relation to the development of a future European reusable launch system featuring a VTVL first stage. For a two-stage vehicle with a payload performance of 7.5t into GTO, the effect of varying propellant combinations, staging velocities and engine cycles are assessed. The study encompasses an iterative, multidisciplinary analysis and sizing process for ten different configurations. Each design iteration not only entails a structural design analysis but also includes optimization of the ascent and descent trajectories. Finally, the developed vehicle concepts are compared to derive quantitative insights into the trade-offs associated with key design choices.

Comparative analysis of European vertical landing reusable first stage concepts

Wilken,

Stappert

2024

Overview of system study on recovery methods for reusable first stages of future European launchers

Dietlein,

Bussler,

Stappert

et al. 2024

The design of a reusable launch vehicle implies the need to provide for a means to safely retrieve the component to be re-used. Following economic considerations, reusable launch vehicle concepts tend to be designed such that large parts, like entire stages, are to be recovered. These are usually significant in size and weight and have acquired a considerable amount of energy during their primary mission. This poses the challenge of how to recover them in a way that makes it available for further re-uses. In the past and present, different methods were and are used. Depending on the selected recovery method, the system design is very different necessitating different technologies and competencies to be acquired for a successful design. Two major classes of recovery methods can be distinguished: those recovery methods ending with a vertical landing of the reusable stage and those ending with a horizontal landing. Both have their own benefits and drawbacks. In 2016, The German Aerospace Centre DLR has initiated a large in-house study with the aim of investigating, in a comparative manner on system level, both classes of recovery methods on a system level for two-stage-to-orbit launch vehicles with a reusable first stage and an expendable upper stage to be operated within a European context. Fuel choice and engine cycle were major design parameters that were considered during the study. The present paper presents the framework of this study describing the adopted study logic, providing an overview of the major findings obtained at the end of the first study phase and gives an outlook to the work of the second study phase. It ends with providing a view of a possible demonstrator and technology roadmap toward the realization of an operational two-stage-to-orbit launch system with a reusable first stage.

Selection of propulsion characteristics for systematic assessment of future European RLV-options

Sippel,

Wilken

2024

The propulsion system as the key-element of any space transportation concept is systematically investigated supporting a study on potential future European RLV. Different liquid propellant combinations are compared and evaluated. Subsequently, main stage rocket engines are defined for the two cycle options: open gas-generator and closed staged-combustion. Four different propellant combinations are considered all based on liquid oxygen (LOX) as oxidizer and with the fuel options liquid hydrogen (LH2), liquid methane (LCH4), liquid propane (LC3H8) and kerosene (RP1). These combinations result in eight different generic engines with sub-variants using different nozzle expansion ratios in first and upper stage application. Engine characteristics are similar, as far as conceivable, to be well-suited for the system level comparison of pre-defined RLV-launchers. However, characteristics are not necessarily identical as different engine architectures and propellants might make individual choices necessary. Engine performance characteristics are compared with similar existing engines when available. It is shown that closed-cycle staged combustion engines bring significant performance gains, particularly in sea-level operations. On the other hand, hydrocarbon- and open-cycle gas-generator engines offer a better thrust-to-weight-ratio than hydrogen and staged combustion cycle.