Test & measurement


theRmal vacuum chambeR testing foR mechanical chaRacteRisations


T


elescope performance depends on stability in the nanometer range, so the stability of the fully instrumented


backplanes is critical. They need to be tested in thermal vacuum chambers suitable for testing under cryogenic conditions, and designed to ensure unique thermal stability at temperatures below –250°C. Testing in the vacuum chamber requires accelerometers and force sensors with ultra-low temperature capabilities. Backplanes can carry the primary mirror as


well as other telescope optics and the entire module of scientific instruments. Testing allows modification of the system so that the backplanes – and ultimately the telescope – can be isolated in the chamber. Some testing environments include a new, layered helium and nitrogen cooling system: this allows the backplanes to reach the low temperatures that simulate


operating temperatures in space. They allow for cryogenic optical alignment and testing of multiple primary mirror segments in a process known as "phasing". Testing of this sort calls for accelerometers and force sensors with ultra-low temperature capabilities. Accelerometers from Kistler detect


background vibrations in the micro-vibration range. The company’s PiezoStar-based IEPE (voltage mode) accelerometers are the ideal sensors for precision vibration testing because of their low sensitivity to temperature variations. Charge output sensors or cryogenic, voltage


mode IEPE accelerometers from Kistler feature an outstanding temperature range from below the typical -54°C down to -196°C, so they can safely survive liquid helium temperatures in the thermal vacuum chamber. Exposure to the high vacuum level of a


space environment induces material outgassing that releases entrapped gas. This can condense on surfaces such as camera lenses, rendering them inoperative for the intended application. Hermetically sealed sensors and low-outgassing cables from Kistler are designed to meet all needs.


Rocket engine testing: thRust, pRessuRe and vibRation chaRacteRisation


R


ockets are exposed to very high stresses, especially during launch. To prevent malfunctioning during operation, rocket


components – and especially rocket engines – undergo extensive testing and inspection. For example, new or modified rocket engines require testing to ensure that no combustion instabilities will occur. When using liquid propellant, the supply mechanism needs to be characterised and optimised. Measuring the dynamic ignition pressure is essential for a safe rocket launch


Thrust, dynamic pressure and vibration characterisation Fuel efficiency in solid propellant for solid rockets, or fuel mixture in the case of liquid rockets, is a major concern for rocket engine designers. Characterising the thrust of the engine itself provides a clear understanding of how much thrust can be produced with a given nozzle design. This allows engineers to compute the specific impulse of the combustion material and study the different phases during the functioning of a rocket engine, such as ignition, burn-in and switch-off. Customer-specific six-component dynamometers based on piezoelectric technology are often used for such investigations. This approach also provides an in-depth


understanding of the injection and mix of fuel components, ignition time, and combustion: essential knowledge to verify the reliable performance of a rocket engine and drive the development of propulsion technologies. Piezoelectric pressure and acceleration sensors


24


from Kistler span the extreme range of ultra-high temperature stability and dynamics required to tackle the challenges encountered in extreme thrust chamber environments.


Important technologies for dynamic characterisation High natural frequency: Depending on type of rocket engine, high frequency dynamic measurements are of interest during thrust characterization. Force solutions must exhibit at least 1,500 to 3,000 Hz natural frequency. Rangeability: Piezoelectric force measurement technology allows both quasi-static and dynamic measurement with high resolution. The piezoelectric measuring chain allows to focus on the lower dynamic signals. It is through this capability that the high fidelity measurement can be achieved of the low level signals originating from thrust instabilities. Adaptability: Single and multi-component force sensors from Kistler can be configured into dynamometers to satisfy specific application requirements and allow for the


flexibility to adapt other dynamometer designs as requirements evolve.


Static pressure monitoring and characterisation Last but not least, static pressure monitoring is another important measurement in rocket engine testing. This process, performed on a rocket engine test bench, includes monitoring and controlling of propellant flow as well as measuring the static pressure in the combustion chamber. Monitoring and control of propellant flow for liquid propellant rocket engines requires static pressure sensors. Piezoresistive pressure sensors from Kistler utilise a cavity-etched, micro-machined, silicon sensing element and are suitable for applications with media that are compatible with silicone oil filled capsules.


Important technologies for static monitoring and characterisation Frequency response: Long-term static pressure measurement requires piezoresistive technology which has inherent operation from 0 Hz up to 2 kHz unlike piezoelectric sensors, which allows only for quasistatic operation. Intrinsic safety: Depending on the use and installation of the pressure sensor, inherent protection against igniting explosive environments may be required. Long-term stability: Piezoresistive pressure sensors utilise an oil-filled and cavity etched, micro- machined, silicon-sensing element which provides inherent 0.1 per cent/year long-term stability.


August 2019 Instrumentation Monthly


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82