Schlieren imaging of microrocket jets

Abstract: In this report, microrockets from the company NanoSpace were studied using schlieren imaging techniques. The rocket chips are manufactured using MEMS technology, which requires compromises regarding the shape of the nozzle. The rocket chips are 22x22x0.85 mm, manufactured from laminated silicon. The nozzles are approximately 20 µm wide at the throat, and 350 µm wide at the exit. A semi in-line schlieren apparatus was designed, set up, and aligned. A small vacuum chamber was constructed, and a series of tests was conducted in order to qualitatively evaluate the consequences of these compromises, and other performance issues. It was found that the existing 1 kW quartz-tungsten-halogen lamp was sufficient as a light source, standard photographic equipment served well as an imaging device, and a 400 mm, f/7.9 achromatic doublet as schlieren lens, resolved enough detail in the exhaust gas to perform the studies. At maximum magnification, the viewing area was 7 by 4.5 mm, captured at 14 Mpixel, or about 1.5 µm/pixel. Several different rocket chips were studied, with helium, nitrogen and xenon as propellant gases. Feed pressure ranged from 0.5 bar to 3.5 bar, and the rockets were studied at atmospheric pressure and in vacuum, and with and without heaters activated. Through these studies, verification and visualization of the basic functionality of the rockets were possible. At atmospheric pressure, slipping of the exhaust was observed, due to the severe overexpansion of the nozzle. In vacuum, the nozzle was underexpanded, and the flow was seen to be supersonic. There was a measurable change in the exhaust with the heaters activated. It was also shown that the method can be used to detect leaks, which makes it a valuable aid in quality control of the components.