Vacuum Test Chamber Research Articles

Analysis and Testing of a Froude Scaled Helicopter Rotor with Piezoelectric Bender Actuated Trailing Edge Flaps

This paper presents an analytical model and validation tests of a Froude scaled rotor featuring piezoelectric bender (bimorph) actuated trailing-edge flaps for active vibration suppression. The analytical model for the coupled bimorph actuator trailing-edge flap dynamic response in the rotating environment takes into account the aerodynamic, centrifugal, inertial and frictional loads acting on the actuator-flap system. The linkage arm length associated with the mechanical amplification mechanism is selected to maximize flap performance in the rotating environment. The bimorph clamping is improved to prevent actuator slippage under high centrifugal loads. The analytical model is validated by carrying out a series of bench tests, vacuum chamber tests and hover tests. In hover, flap deflections of ±6 degrees at 4/rev flap excitation are achieved at 900 RPM, thus demonstrating the potential of the piezoceramic bender as a lightweight and compact actuation system for individual blade control purposes. This paper also includes a feasibility study for piezo-bimorph actuation of a trailing-edge flap for a Mach scaled rotor model.

Spark conditioning procedures for vacuum interrupters in circuit breakers

The breakdown voltage between the open contacts of vacuum circuit breakers (VCBs) immediately after manufacturing is low and subject to high scatter. To attain a sufficiently high breakdown voltage, these unknown and undefined surface conditions must be improved by conditioning. Several usual conditioning procedures applied to a large number of 12-kV circuit breakers of a standard design are compared. A large number of conditioning experiments have been made and statistically evaluated. Additional studies have been performed in a vacuum test chamber in which the contacts could be interchanged easily. The aim was to find the most important conditioning parameters and to explain the underlying physical processes. The studies show that the conditioning procedure can be optimized. >

Energetic ion emission for active spacecraft control

Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. In the near-Earth space these spacecraft are usually solar-cell powered; consequently, parts of their surface are most of the time exposed to solar photons. Outside the plasmasphere, a positive surface potential due the dominance of surface-emitted photoelectrons over ambient plasma electrons is to be expected. Photo- and ambient electrons largely determine the potential and positive values between a few Volts up to 100 V have been observed. Active ion emission is the obvious solution of this problem. A liquid metal ion emitter and a saddle field ion emitter are nearing the stage of flight unit fabrication. We will attempt to clamp the spacecraft potential to values close to the plasma potential. We present first results from vacuum chamber tests and describe the emission behaviour and characteristics of emitters producing, respectively, In + and N 2 + beams with an energy of ≥ 5 keV.

New technique for oil backstreaming contamination measurements

Due to the large size and the number of diffusion pumps, space simulation chambers cannot be easily calibrated by the usual test dome method for measuring backstreaming from oil diffusion pumps. In addition, location dependent contamination may be an important parameter of the test. We have measured the backstreaming contamination in the Space Power Facility (SPF) near Sandusky, Ohio, the largest space simulation vacuum test chamber in the U.S.A. We used small size clean silicon wafers as contamination sensors placed at all desired measurement sites. The facility used diffusion pumps with DC 705 oil. The thickness of the contamination oil film was measured using ellipsometry. Since the oil did not wet uniformly the silicon substrate, two analysis models were developed to measure the oil film: (1) continuous, homogeneous film and (2) islands of oil with the islands varying in coverage fraction and height. In both cases, the contamination film refractive index was assumed to be that of DC 705. The second model improved the ellipsometric analysis quality parameter by up to two orders of magnitude, especially for the low coverage cases. Comparison of the two models for our case shows that the continuous film model overestimates the oil volume by <50%. Absolute numbers for backstreaming are in good agreement with published results for diffusion pumps. Good agreement was also found between the ellipsometric results and measurements done by x-ray photoelectron spectroscopy (XPS) and by scanning electron microscopy (SEM) on samples exposed to the same vacuum runs.

Compact radio-frequency glow-discharge atomic hydrogen beam source

An atomic hydrogen beam source, compatible with ultrahigh vacuum and clean surface conditions, has been developed. Hydrogen atoms (H0) of thermal energy distribution are produced by dissociation of hydrogen molecules in a radio-frequency (rf) glow discharge. The resulting mixture of atoms and molecules is allowed to flow from the discharge vessel to the vacuum test chamber through a capillary, resulting in the formation of a beam. Dissociation fractions [ 1/2 H0/(H2+ 1/2 H0)] of >0.5 and H0 fluxes of >2×1017 H0/s were achieved. Locating the rf power source adjacent to the discharge vessel resulted in a compact device which is easy to maintain and operate.

Noise generation by instabilities in low Reynolds number supersonic jets

An experimental investigation of noise generation by instabilities in low Reynolds number supersonic air jets has been performed. Sound pressure levels, spectra and acoustic phase fronts were measured with a traversing condenser microphone in the acoustic field of axisymmetric, perfectly expanded, cold jets of Mach numbers 1·4, 2·1 and 2·5. Low Reynolds numbers in the range from Re = 3700 to Re = 8700 were obtained by exhausting the jets into an anechoic vacuum chamber test facility. This contrasts with Reynolds numbers of over 10 6 for similar jets exhausting into atmospheric pressure. The flow fluctuations of the instability in all three jets have been measured with a hot-wire and the results are documented in a previous paper by Morrison and McLaughlin. Acoustic measurements show that the major portion of the sound radiated by all three jets is produced by the instability's rapid growth and decay that occurs near the end of the potential core. This takes place over a relatively short distance (less than two wavelengths of the instability) in the jet. In the lower two Mach number jets the instability has a phase velocity less than the ambient acoustic velocity. In the Mach number 2·5 jet the instability phase speed is 1·11 times the ambient acoustic velocity. In this case the acoustic phase fronts indicate the possibility of a Mach wave component. It was also determined that low level excitation at the dominant frequency of the instability actually decreased the radiated noise by suppressing the broad band component.

Pulsed Gas Shutter

To reduce the heavy gas load of the high-intensity duoplasmatron ion source, a rotating disk pulsed gas shutter has been constructed and tested on a test vacuum chamber. The gas shutter is to be installed at the nozzle of the duoplasmatron. The timing of periodic opening is synchronized to an external 20 Hz master pulse. Since the average Q-value has been reduced from 2.8×10-2 Torr·1/sec to 2.8×10-3 Torr·1/sec, it is possible to replace a large pumping by a one-order smaller one.

Pulsed Gas Shutter

To reduce the gas flow from the ion source to accelerating tube, rotating disk pulsed gas shutter mounted on test vacuum chamber has been constructed and tested. The timig of periodic opening is synchronized to an external 20 Hz master pulse. Gas flow into the ion source is continuous and the outflow from the source to accelerating tube is pulsed. Average flow has been reduced from 2.7×10-2Torr.l/sec to 2.7×10-3Torr.. l/sec for operation, it is possible to eliminate the use of a conventional high speed, large capacity pump.

The design and performance of a solar simulation vacuum test chamber: G M Garett, Inst Environmental Sciences Annual Tech Meeting, San Diego, California, April 1966

The design and performance of a solar simulation vacuum test chamber: G M Garett, Inst Environmental Sciences Annual Tech Meeting, San Diego, California, April 1966

Space molecular sink simulator facility design.

Space molecular sink design to create vacuum environment, emphasizing pumping and test chamber instrumentation

Factors in Life Testing Ion Engines

The goal of contemporary ion rocket research and development is the attainment of reliable, long-life ( 10,000 hr) operation. In addition to vacuum test chamber environment, the interaction of electrode thermal characteristics and neutral efflux from the ionizer are dominant in determining surf ace-con tact ion engine lifetime. Several key physical processes are discussed in the light of recent experimental results. Charge exchange erosion is the fundamental limitation to engine lifetime. Although experiment and analysis indicate attainable lifetimes of 5000 to 20,000 hr at 10 ma/cm using the best present-day ionizers, shorter lifetimes result when interrelated secondary physical phenomena within the acceleration region of the engine emerge. Cesium ionization on the focus electrodes is considered in detail, since it is a lifetime barrier in those designs which utilize such beam-forming elements. Tolerable charge exchange erosion of accel electrodes, to assure a given lifetime, is used to provide boundary conditions for ionization efficiency, focus temperature, and focus ion current. The effect of focus electrode roughness on accel erosion patterns is studied in detail. The influence of the ground-based test chamber environment vs a true space vacuum environment is also considered.

Vapor jet control of space vehicles

This paper describes a reaction jet attitude control technique which affords significant advantages in terms of accuracy, reliability, fuel economy and operational flexibility. These advantages are realized by the use, in combination, of low-thrust vapor jets and time-dependent on-off switching circuits. An accuracy potential comparable to inertia wheel control is thus provided, while the proverbial wheel problems of speed saturation, bearing life, threshold nonlinearities, gyroscopic coupling and vibration excitation are avoided. Very-low thrust magnitudes are attained by simply opening a small orifice to allow fuel to vaporize into the surrounding vacuum. Fuel storage, pressurization, circulation and mixing requirements are thus minimized. By augmenting conventional on-off valve switching circuitry with electronic networks that generate thrust pulses of small but constant time duration, vehicle angular rate can be controlled to a very-low threshold. This minimizes fuel consumption and valve cycling frequency. The capabilities and limitations of this design approach were substantiated by an analog computer program incorporating breadboard switching circuits, and by vacuum chamber testing of critical components. These technique and component developments are applicable to such space missions as astronomical observation, earth reconnaissance and stellar navigation. Design guides are presented for synthesizing a reaction jet system to meet any particular set of performance specifications.