The present study focuses on increasing the performance of a hydrocarbon-boron gel fuel ramjet. The research examines the option of condensing the combustion products, thus releasing the latent heat stored in the gaseous phase before they exit the engine and increasing the combustion efficiency. A two-step combustion process, using bypass air, was suggested to increase condensation. Three different models for the condensation of boron oxide were examined. The effect of bypass air injection on the condensation process was investigated, and the time required for complete condensation was estimated. The concentration of solid nuclei particles in the stream was found to have a major effect on the required time for the completion of boron oxide condensation; the more particles are present in the stream, the faster the condensation occurs.air mass ratio f st = stoichiometric fuel-to-air ratio g 0 = gravitational acceleration m∕s 2 H = altitude, km I sp = specific impulse, s J = particles flux through area per time unit, mol∕m 2 · s Kn = Knudsen number K B = Boltzmann constant, m 2 · kg∕s 2 · K L = latent heat of vaporization, J∕kg l = large eddies length scale, m M = Mach number M w = molar mass, kg∕kmol m = mass, kg _ m a = air mass flow rate, kg∕s _ m f = fuel mass flow rate, kg∕s N A = Avogadro number O∕F = air-to-fuel ratio p = partial pressure of B 2 O 3g in the mixture, Pa p c = stagnation pressure at nozzle entrance, Pa p e = nozzle exit pressure, Pa p v = vapor pressure of B 2 O 3 , Pa R = droplet radius, m R = universal gas constant, J∕kmol · K Sc = Schmidt number T = temperature, K T a = ambient temperature, K T 05= stagnation temperature at the nozzle entrance, K t = time, s u a = ambient velocity, m∕s u e = nozzle exit velocity, m∕s u g = gaseous phase velocity, m∕s u s = condensed phase particle velocity, m∕s V = volume, m 3 v t = velocity fluctuations, m∕s Y i = mass fraction of species i Y s = mass fraction of condensed phase α = condensation coefficient γ = specific heat ratio δ = angle between two chin bypass air injections, deg λ vap = effective mean free path, m θ = bypass air injection angle, deg ν t = turbulent kinematic viscosity, m 2 ∕s ρ = fluid density, kg∕m 3 ρ g = gaseous phase density, kg∕m 3 ρ s = condensed phase density, kg∕m 3 ϕ = molar concentration, mol∕m 3