Methalox Propellant for Future Launch Vehicles

Abstract

Contrasting experience with Reusable Launch vehicles—Space Shuttle and Falcon 9— has established that potential of reusable launch vehicles to achieve low launch costs is driven by the design choices made. Methalox propellant is one such design choice that has been touted to power future missions and potentially replace traditional propellants — Hydrolox and Kerolox. The high density of methalox compared to hydrolox and improved specific impulse compared to kerolox, potentially make methalox an ideal propellant choice. To justify any new design choice, cost-based analysis is essential, especially given the persisting issue of high launch cost. Additionally, it is essential to benchmark the performance of any new design choice with existing practices, especially when they are expected to replace current practices. For this, a cost based comparative analysis of methalox based launchers with hydrolox and kerolox launchers is performed using a tool capable of launch vehicle design and cost analysis. Rather than designing a tool from scratch, existing First Stage Recovery Tool (FRT), which was developed by M. Rozemeijer to modify and cost existing expendable launchers to include reusability, was extended to include a launch vehicle design module, which was previously lacking. A Multidisciplinary Design Analysis and Optimization (MDAO) methodology was applied for the design module. The design module was developed by verified and validated models implemented from literature for hydrolox, kerolox and methalox propellants and an optimization scheme to minimize for Gross Lift-Off Mass (GLOM). This design module in conjunction with the FRT enables design and costing of expendable and reusable launcher configurations. For the current study, two missions were considered—15600 kg payload to Low Earth Orbit (LEO) and a 5000 kg payload to Geostationary Transfer Orbit (GTO)—for different propellant combination-based launchers. Additionally, different launcher configurations— expendable, reusable via non-propulsive recovery and reusable via propulsive recovery—were considered, enabling cost comparison of propellants for different scenarios. Results indicate that methalox based launchers are cost-effective solution when compared to hydrolox, regardless of mission type or launcher configuration considered in the current study. Compared to kerolox, only a marginal cost benefit can be achieved, for the case of expendable configuration and in combination with kerolox. For reusable configurations, purely methalox shows potential to achieve costs within 10% of kerolox. Sensitivity analysis showed the potential to reduce this gap by including the lower refurbishment requirement of methalox, given low soot formation possibility. Furthermore, it showed the need for a better engine model for methalox, to refine comparison between methalox and kerolox. The tool, however, is not complete and should be extended to include reliability assessment, especially for methalox systems, which are not flight proven unlike hydrolox and kerolox. There also remains issues with the accuracy and uncertainty in certain models, which make the current version suitable only for comparative studies.