We report on the results of numerical-simulation investigations of ignition characteristics of hydrocarbon-fuel blends expected from thermal cracking of typical jet fuels, at conditions relevant to high-Mach-number, air-breathing propulsion. A two-point-continuation method was employed, with a detailed description of molecular transport and chemical kinetics, focusing on the effects of fuel composition, reactant temperature, additives, and imposed strain rate. It captured the entire S-curve that describes the processes of vigorous burning, extinction, and ignition. The results demonstrate that ignition of such fuel-blends is dominated by the synergistic behavior of CH4 and C2H4. A fuel temperature of Tfuel = 950K was employed throughout. At higher air temperatures (Tair = 1200K), addition of small amounts of CH4 to C2H4 moderately inhibits C2H4 ignition, while at lower Tair = 1050K, CH4 promotes ignition. Large amounts of CH4, however, inhibit C2H4 ignition at all Tair's. Ignition promotion was also investigated through the independent addition of H2 and F2 in the reactant streams. H2 addition (e.g., 2-10%) produces a two-stage ignition and sustains higher ignition strain rates. Small amounts of F2 (1%) result in F-radical production, contributing to efficient fuel consumption, enhancing ignition characteristics. Ignition strain-rates of ~4000/s, as compared to ~250/s for pure C2H4, can be attained with such additives at lower temperatures (Tair = 1050K).