A Review of Aerospike Nozzles: Current Trends in Aerospace Applications

Summary

In the combustion chamber, the chemical reaction between fuel and oxidizer generates gases at high pressure (on the order of hundreds of atmospheres) and high temperature (around 3000 K or more). The composition of these gases depends on the propellants used. As they expand through the nozzle, the gases accelerate to supersonic speeds and experience reductions in pressure and temperature, as well as possible changes in their chemical composition. 2.1. Relevance of Thermodynamic Properties for Nozzle DesignThe performance of a rocket nozzle critically depends on the thermodynamic properties of combustion gases, which determine fundamental parameters, such as exhaust velocity ( v e ), mass flow rate ( m ˙ ), and, ultimately, specific impulse ( I s p ). These properties are derived from energy balance and isentropic relations and are integrated with the nozzle’s geometric design to maximize engine performance.The exhaust velocity is expressed as v e = 2 γ R u T c ( γ − 1 ) m 1 − P e P c γ − 1 γ , (2) Similarly, the mass flow rate, which remains constant in steady-state conditions, is determined by m ˙ = P c A t c * = P c A t R u T c γ m 2 γ + 1 γ + 1 γ − 1 , (3) where c * is the characteristic velocity of the propellant.Thrust is calculated by combining the change in momentum flow with the effect of the pressure difference between the nozzle exit and the ambient pressure: F = m ˙ v e + ( P e − P a ) A e , (4) where A e is the nozzle exit area, and P a is the ambient pressure.From the thrust equation, the specific impulse is defined as I s p = F m ˙ g 0 = v e g 0 + ( P e − P a ) A e m ˙ g 0 , (5) where g 0 is the acceleration due to gravity at sea level.For optimal engine performance, it is essential to achieve the following:Maximize T c : A higher combustion chamber temperature increases v e and, consequently, I s p ;Minimize m: Reducing the molecular weight improves efficiency by increasing exhaust velocity;Expand gases until P e = P a : Designing the nozzle so that t...