Impact of module design on the performance of clustered aerospike nozzles for upper stage applications

Annular aerospike nozzles offer a promising solution for upper-stage propulsion, combining compact design with the ability to achieve high expansion ratios within constrained envelope dimensions. Although the ring-shaped configurations deliver excellent theoretical performance, they also present significant challenges, such as narrow throat gaps and severe thermal loads. These issues can be mitigated by adopting clustered modular designs. However, using conventional bell-shaped modules may result in poor clustering efficiency. This is especially true for upper stage applications, where the high targeted expansion ratio may require an impractically large number of nozzles, compromising the feasibility of the system. Alternative module geometries are therefore explored in an effort to approximate the ring-shaped exit cross-sections of annular aerospikes, with the aim of improving the trade-off between single-module performance, clustering efficiency, and the total number of modules required. CFD simulations are performed for each geometry, considering both gapless arrangements and configurations with a fixed number of modules. Results indicate that a tailored geometry, characterized by a ring sector with rounded edges as the module exit cross-section, provides an interesting compromise, achieving performance comparable to the annular reference design. A comparison with conventional bell nozzles across a range of expansion ratios representative of upper-stage conditions confirms the potential gain in specific impulse, quantifying the advantages of the proposed configurations.