SSTO Reusable Launchers: a Critical Comparison of Propulsion Concepts
Creators: name:  S. SALVI 
name:   C. PARAVAN 
name:   L. GALFETTI 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
AIR BREATHING
RAMJET
SCRAMJET

description: The growing interest in reusable Single-Stage-To-Orbit (SSTO) launchers establishes the necessity to develop a suitable propulsion system, able to provide an effective alternative to conventional thermochemical rocket launchers. Among all the technologies proposed over the years, Rocket Based Combined Cycles (RBCC) appear as the most attractive choices for this purpose. A specific, promising application is the deeply precooled Synergetic Air-Breathing Rocket Engine (SABRE), which uses sub-cooled liquid hydrogen as its fuel and sub-cooled liquid oxygen as the oxidiser in rocket mode. In rocket mode the engine operates as a closed cycle; in air-breathing mode the liquid oxygen flow is replaced by atmospheric air. The airflow is compressed by an axisymmetric intake and is cooled by a pre-cooler heat exchanger to cryogenic temperatures. The pre-cooler heat exchanger is an element of a closed cycle helium loop which uses the hydrogen fuel as the heat sink before it enters the combustion chamber. In the combustion chamber fuel mixes with air, which has been compressed after cooling. This engine is proposed to propel the spaceplane SKYLON to over Mach 5 at an altitude of 25 km. The SABRE/SKYLON system is capable of placing into an equatorial low Earth orbit a 12 tonnes payload, at a gross takeoff mass of 275 tonnes. The proposed air-breathing solution significantly reduces the subsequent less efficient rocket phase of the ascent trajectory to low earth orbit. The aim of this paper is to perform a preliminary comparison between a RBCC engine, the SABRE solution and a conventional rocket baseline for a Low Earth Orbit mission. Due to the lack of data available, performance computation is based on simplified models. RBCC engine numerical model is developed on the basis of the approach presented in 1, providing an analysis tool that gives thrust, specific impulse and specific fuel consumption for given flight conditions and geometry of the engine, which is here selected to be the one of the propulsion system designed for Hyperion SSTO 2. A hypothetical constant dynamic pressure trajectory is selected to compute performance parameters according to flight altitude and Mach number during airbreathing propulsion phase. RBCC engine works in three different airbreathing modes: ejector up to Mach 2, ramjet between Mach 2 and 5 and scramjet between Mach 5 and 10. The SABRE engine is modelled according to data provided in 3 to examine the same physical quantities as RBCC engine analysis, according to the same trans-atmospheric trajectory, working in airbreathing mode from sea level steady conditions up to Mach 5. Final acceleration is provided in both cases by pure rocket working mode of engines, allowing to reach the target velocity for orbit insertion. Eventually, mass ratios are computed with Tsiolkovsky equation for each engine, and results of numerical simulations are compared with a hypothetical rocket launcher, based on properties of the space shuttle main engine, to demonstrate the unsatisfactory behavior of a non-airbreathing SSTO vehicle. Simulations results illustrate that despite its complexity SABRE engine is the most suitable solution to develop a SSTO launch vehicle, because of its high thrust and high specific impulse during the entire airbreathing propulsive phase. The design of single stage to orbit launch systems, using combined cycle engines that work both in airbreathing and pure rocket modes, is discussed in this paper analysing the characteristics of the different solutions proposed for the SSTO project. Precooled hybrid airbreathing rocket engines precool the airstream and use a high pressure rocket combustion chamber for both airbreathing and rocket operating modes. This solution allows mass saving and installation simplifications, supporting the potential of the airbreathing mode advantages.