Helios was the second engine developed by SRT in two years. There are two major versions of this design distinguished by the oxidizer flow control and injector design. The oxidizer flow control is directly interchangeable with the NP-915 Icarus engine system. The engine features a light-weight composite tank (Luxfer), custom ball-valve actuation system (Vers. II), removable coin injector system, and a cubic graphite nozzle. All fittings are 304/316 Stainless Steel, while the injector and combustion chamber are made from 6061-T6 Aluminum.