Supersonic Liquid Droplet Launcher

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Aerospace & Defense
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An idea to provide a repetitive stream of liquid drops at supersonic velocity.

A Need Exists To Launch Mach>3 Droplets At Optical Components For Erosion Studies On High Mach Number Vehicles. This proposed technique replaces other difficult techniques such as launch droplets cradled on the nose of a vertical gun-fired sabot where the sabot is slowed and deflected from line of fire (by a curved bi-track) before droplets impact on the target. The idea is to create wave velocity amplification and droplet launch in a tapered subsonic duct. The principle is based on conservation-of-momentum similar to that of 'cracking a whip'.

A whip is a long flexible transmission line tapering in area (and mass per unit length) from the handle to the tassel. A momentum wave launched from the handle propagates along the braided tapering body toward the tassel. Momentum in the wave is conserved since very little can escape to the ambient air environment; as whip mass/length in the wave decreases, conservation of momentum forces the wave velocity to increase. With proper design the tassels go supersonic as the momentum wave exits the whip 'lash'. A whip is basically a mechanical impedance transformer.

This droplet launcher similarly employs an impedance transformation scheme:
A hydraulic impedance transformer takes the form of a gradually (conical) tapered rigid pressure vessel (likely mounted vertical in a reduced pressure test cabin). The lower large end of the launcher is fitted with a piezo quartz crystal that can launch a fraction of a mm thick plane-wave into the liquid the fills the tapered wave guide. Conservation of momentum will cause the wave to accelerate to high velocity as the waves diameter is reduced by the wave guide. The launched water droplet speed will still be subsonic relative to sound speed in the water medium but supersonic in the air outside the nozzle. The area ratio of the piezo crystal face to the launch nozzle area can be varied (in addition to the crystal amplitude) to achieve the desired droplet launch velocity.

The conical shaped wave guide bore can be fabricated by rough step drill & ream followed by wire-EDM. The area ratio of the input (crystal end) to output (nozzle end) will determine the wave amplification. A thin bolt-on nozzle plate will seamlessly continue the waveguide duct into a slightly divergent shaped nozzle to facilitate supersonic droplet formation.

The tapered waveguide and shaped launch nozzle would be coated to inhibit wetting (adhesion).

The piezo pulse width is made short enough to assure the momentum wave thickness is small compared to the taper rate of the wave guide which will help prevent edge distortion of the wave at the wave perimeter. The entire apparatus could be operated in a reduced pressure somewhat above the vapor pressure of the launch water. A fairly high rate of fire like 100 droplets per second or much more should be achievable with settling time between shots.


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  • Name:
    David Carpenter
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