The Microgravity Research Experiments (MICREX) Database

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When spacecraft attitude control motors are fired, both (1) the vehicle and (2) liquid propellant held within the fuel storage tank, accelerate. The dynamic response of the fluid in the tanks coupled with the tank design determines the distribution of the fluid within the container. The storage tanks are often designed to use the surface tension of the stored liquid to move or maintain the liquid at the appropriate position within the tank. Propellant must be adequately positioned within the tank for several reasons including (1) motor fuel intake and (2) vehicle stabilization.

The objectives of this STS Get Away Special (GAS) canister experiment included (1) determining the low-gravity, dynamic behavior of a partially filled hemispherical tank via experimental methods, (2) correlating the experimentally-obtained data to mathematical models designed to describe similar dynamic behavior and, (3) evaluating the outflow of the liquid in the test tank through a tube and solenoid-valve and into an evacuated aluminum chamber, (particularly investigating "...the orientation and stabilization of the fluid (propellant) under the influence of a propellant management device (PMD) especially with respect to outflow cases.") (4, p. 166)

"Specific aims [of the experiment] were:

o definition of all significant natural frequencies and damping of the fluid in the low frequency range of 6 Hz;

o determination of the generalized propellant mass, to facilitate the necessary analysis assumptions;

o determination of sloshing forces and the pressure distribution in the fluid and tank;

o observation of the dynamic behavior of the propellant under quasi zero-gravity conditions;

o observation of the fluid orientation stability effect, including the shape of flow during critical operation phases." (4, p. 167-168)

The experimental apparatus was designed to simulate fluid sloshing within a partially-filled satellite liquid propellant tank. A transparent, hemispherical tank contained the test liquid. Details of the tank design were not presented in any document published after the experiment was actually performed. Two documents published prior to the shuttle flight (Reference (2) and Reference (9)) detailed the expected experiment setup. It is clear, however, that the overall experiment design was changed somewhat from the preliminary description provided in the references. For example, the earlier system (1) was to be excited up to 10 Hz (whereas the actual experiment was excited up to 6 Hz) and (2) was to perform four different experiment phases (whereas the actual experiment performed five different phases). Because there is no other source available which describes the setup, the preliminary design concept is summarized here.

"A hemispherical plexiglas tank is suspended in a rocking mechanism such that purely linear, nearly frictionless oscillation can be induced within a limited displacement range. A stepping motor is connected to the tank rocking mechanism by means of a variable stiffness coupling (with a spring). Pressure sensors, force sensors, temperature sensors and accelerometers are located at selected positions on the experiment assembly: certain phases of the experiment will be recorded with a high speed camera.

"The mechanical assembly is locked down during launch, landing and during non-operational periods in orbit. During operation the stepping motor is driven according to a pre-determined program and generates a variety of known excitation profiles comprising shock waves of different forms and sinusoidal sweeps from 0,2 to 10 Hz. The coupling stiffness will be switched in accordance with the experiment run program. In addition, the amount of liquid in the tank can be varied by transferring a portion of it to an evacuated container. The system decay dynamics will be investigated over a range of different natural frequencies." (2, p. 41)

Additional details concerning the drive train, transmission coupling system, and tank support assembly can be reviewed in Reference (2) on pages 42-43.

"Besides the hemispherical test tank, initially 50% filled with fluid, a second evacuated aluminum container is provided, connected to the test tank by a tube and solenoid valve. Outflow of propellant from the test tank will be simulated by opening the valve at specific times." (2, p. 43)

"The major portion of the active operations is assigned to data production by transducers and sensors. The time allowed for filming is very limited, because the high speed used rapidly consumes the film magazine; hence only critical fluid phases are filmed where sensor data cannot provide an adequate picture of the fluids behavior." (2, p. 48)

Reportedly, one of the critical fluid phases to be filmed was "...the influence of the PMD in assuring correct fluid orientation and stability, and the response of the fluid to simulated thruster propellant demand. A source of diffuse light is provided for illumination of the fluid." (2, p. 44)

A discussion of the four expected phases of the experiment was provided: "In phase 1 of the experiment...the mechanical hold-down locks will be released, thus freeing the moving assemblies. The propellant tank receives an impulse to settle (pre-orientate) the liquid propellant. A short time will be allowed for the system to return to test [probably should be rest] and then an additional period allowed for the liquid to stabilize itself. The liquid behavior during the impulse and decay period will be recorded on film...

"Phase 2

In this phase the propellant tank is subjected to a variety of sinusoidal oscillations. Overall, the vibrations will sweep the frequency range 0.2 Hz to 10 Hz in three intervals: 0.2 Hz to 1 Hz in 20 incremental steps; 1 Hz to 5 Hz in 40 steps; 5 Hz to 10 Hz in 50 steps. The amplitude of the oscillations will be small. This procedure will be repeated six times with different amplitude settings and different spring tension settings on the rocker assemblies...

"Phase 3

The experiment run continues with the generation of shock waves (pulses). Following the input of a pulse to the propellant tank, the drive train will be disconnected and the moving mass allowed to decay naturally. A stabilization period enables the liquid in the tank to take up its natural configuration.

"This procedure is carried out nine times with various settings for pulse duration, amplitude and spring stiffness settings, following which all the steps are executed once again. A single pulse input is followed by a decay period of 10 seconds, during which data is generated and a stabilization period of 5 minutes.

"Phase 4

Phase 4 begins with a propellant expulsion operation, in which the tank is emptied to the 30% fill level. The liquid is transferred to an evacuated container, the flow being limited by a solenoid valve. For this operation the camera is switched on to provide visual data of propellant management characteristics under fuel demand conditions. This phase continues with a repetition of the pulsed excitation program carried out in phase 3, following which the propellant tank is emptied completely. This operation will also be filmed. The dynamic characteristics of the empty tank system are then further investigated with a set of shock inputs... (2, pp. 48-49)

Reference (9) (which was written prior to Reference (2)) presented a more detailed description of the expected design of the test tank. The reader is again reminded that the final experiment configuration may have differed considerably from this description.

The test tank "...is built out of a circular glass cylinder of 10 cm diameter and a length of 10 cm. The glass cylinder is closed on [the] bottom and top sides by aluminum plate...." (9, p. 566) A detailed description of the tank surface coatings and the role of these coatings was also presented in the reference.

It was reported that the test fluid was to be Methylenbromid. Its "good surface properties" were detailed in Reference (9). Please refer to Reference (9) for a detailed description of the expected measurement system.

During the low-gravity shuttle mission, the tank was "...subjected to oscillatory disturbances corresponding to inputs used for theoretical simulation and analysis." (4, p. 166)

Each of the five experimental phases had a specific operating mode and objective. It appears (from Reference (4)) that prior to the start of any of these phases the tank had to be in a state of stationary equilibrium. Therefore, time slots (time that the experiment was most likely operationally inactive) were programmed into the experiment sequence to allow for the fluid stabilization. During the over 3 hours the experiment ran, 40% of this time was dedicated to this fluid stabilization. Transducers and sensors documented the majority of the fluid behavior. Critical phases of the fluid dynamics were captured photographically. (The amount of photographic data was limited because "...the high speed employed rapidly used up the film." (4, p. 173)

At the time Reference (4) was written, extensive post-flight analysis of the transducer, sensor, and photographic data was not complete. However, preliminary analysis indicated that the "experiment functioned perfectly." Further, "In principal, the underlying mathematical models developed by MBB/ERNO for the design of surface tension tanks have been supported by the experiment results, and the validity of current product design features has been confirmed. The results will provide additional insight into the effect of fluid sloshing on spacecraft attitude control systems." (4, p. 173)

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References/Applicable Publications:

(2) Gilbert, C. R., Netter, G., and Vits, P.: Study of Liquid Sloshing Behavior in Microgravity. In Goddard Space Flight Center's 1984 Get Away Special Experimenter's Symposium, Post Symposium Proceedings (Addendum), 1984, pp. 38-54. (preflight)

(3) Kolcum, E. H.: Fuel Contaminant Threatens Delay in Shuttle Launch, AW&ST, June 17, 1985. (preflight).

(4) Gilbert, C. R.: GAS Payload No. G-025: Study of Liquid Sloshing Behavior in Microgravity. In NASA Goddard Space Flight Center's 1985 Get Away Special Experimenter's Symposium, October 8-9, 1985, pp. 165-176, NASA CP-2401. (post-flight)

(5) STS 51-G Press Kit, NASA Press Release 85-83, June 1985, p. 19. (Preflight)

(6) Get Away Special... the first ten years. Published by Goddard Space Flight Center, Special Payloads Division, The NASA Gas Team, 1989, p. 28. (post-flight, very brief description)

(7) Netter, G. and Beig, H. G.: Preliminary Results From the EMTE Fuel Slosh Flight Experiment-STS-16. In ESA Proceedings of an International Symposium on Fluid Dynamics and Space, pp. 145-157.

(8) Netter, G. and Eckhardt, K.: Fluid Dynamic Experiment in a Surface Tension Tank: Phase 1/Phase 2A. Final Report, June 1981. MBB/ERNO, Report #BMFT-FB-W-83-002. (in German, English summary)

(9) Eckhardt, K. and Netter, G.: Experiment for Investigation of the Dynamic Behavior of Fluid in a Surface Tension Tank Under Microgravity Condition. Acta Astronautica, 1982, pp. 565-571. (preflight)

(10) Ridenoure, R.: GAS Mission Summary and Technical Reference Data Base. Ecliptic Astronautics Co., Technical Report #EAC-TR-TWT 87-11, October 2, 1987. (Get Away Special Canister mission history)

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