This paper summarizes and compares the results of systematic research programs at two independent laboratories regarding the injection of cryogenic liquids at subcritical and supercritical pressures, with application to liquid rocket engines. Both single jets and coaxial jets have been studied. Cold flow studies provided valuable information without introducing the complexities of combustion. Initial studies utilized a single jet of cryogenic nitrogen injected into a quiescent room temperature nitrogen environment with pressures below and above the thermodynamic critical pressure of the nitrogen. Later, the work was extended to investigate the effects of a co-flowing gas. Parallel to this work, combustion studies with cryogenic propellants were introduced to understand high pressure coaxial injection phenomena with the influence of chemical reaction. Shadowgraphy and spontaneous Raman scattering were used to measure quantities such as growth rates, core lengths, turbulent length scales, fractal dimensions, and jet breakup regimes. It is found that jets injected at supercritical pressures do not atomize as they do at subcritical pressures. Rather, they behave in many respects like variable density turbulent gas jets.

The following properties have been measured for mercury clusters: (1) ionisation potentials of Hg_n by electron-impact ionisation, (2) dissociation energies of Hg⁺ _n, and (3) mass spectra for negatively charged mercury cluster ions (n 3). Cohesive energies for neutral and ionised Hg clusters have been calculated from the data. The transitions in chemical binding are discussed. For small clusters Hg_n is van der Waals bound (n⩽ 13), the binding changes to covalent for 30 ⩽n⩽ 70, and then to metallic (n 100). A sudden transition from covalent to metallic bonding is observed. It is discussed whether this can be considered as being analogous to a Mott transition for a finite system. The experimentally observed transitions in chemical bonding are much more pronounced than those calculated in a tight-binding calculation. This points to strong correlation effects in Hg clusters.

Hydrogen is a widely used rocket fuel and methane is particularly of interest in Europe as a promising substitute for H2. Experimental investigation of cryogenic reactive coaxial sprays with oxygen as an oxidizer and hydrogen and methane as fuels is conducted to prove whether concepts from LOX/H2 injector design can be transferred to LOX/CH4 injection. The liquid oxygen has been atomized in shear co-axial atomizers, the sprays and the flames have been investigated by visualization methods like shadowgraphy and imaging of the flame emission. LOX-sprays are characterized for both propellants by the intact core lengths and droplet numbers and the combustion is analyzed in terms of the flame anchoring mechanism and the flame spreading angle. The results for LOX/H2- and LOX/CH4-spray combustion are compared, also the influence of the injection conditions of the propellants on atomization and spray flame is discussed. Significant differences of the sprays and flames have been observed for the two propellant combinations at similar injection conditions as defined by Weber number and momentum flux ratio. The flame stabilization process has shown a strong influence on the atomization and flame characteristics