Putting a new spin on an old problem
An experimental facility will help spacecraft orbit successfully after liftoff by finding a way to compensate for the motion of sloshing rocket fuel.
Southwest Research Institute (SwRI) recently commissioned a new experimental facility, the Spinning Slosh Test Facility (SSTF), that will help ensure future spacecraft can enter orbit successfully after liftoff. During this insertion maneuver, the spacecraft is still attached to the upper stage of its launch rocket, and together they rotate like a slowly spinning bullet to obtain a measure of gyroscopic stiffness that aids in keeping the spacecraft pointed to its final orbit. The upper stage control thrusters have to "fine tune" the flight of the spacecraft during the maneuver, and if, like a spinning toy top, the unavoidable wobbling (more exactly, the nutation) of the combined spacecraft and upper stage grows to an excessive amplitude, the thrusters are not able to keep the spacecraft pointed to the desired orbit. In a worst case, the spacecraft and upper stage start to tumble, and the entire mission is lost. Such disasters already have occurred and are extremely costly -- a satellite typically costs $100 million or more. Hence, NASA, the manufacturers and owners of spacecraft, and the suppliers of launch boosters have a vital interest in ensuring reliable orbit insertion.
When a spacecraft spins and nutates, the liquid propellants in the tanks slosh about in ways that cannot be predicted easily nor characterized by tests of nonspinning tanks. Sloshing produces forces and torques that accentuate the spacecraft nutation and are the main cause of excessive nutation. The SSTF, a ground-based test facility, was designed to investigate the magnitude of these liquid-induced forces and torques by subjecting a full-scale tank from the spacecraft to the same kinds of motions it would experience during the orbit insertion maneuver. NASA's Kennedy Space Center, through a contract with Boeing, sponsored construction of the SSTF. SwRI was selected because of its extensive background in spacecraft propellant dynamics, especially in analyzing liquid motions in spinning tanks. SwRI's recent Liquid Motion Experiment on Space Shuttle Flight STS-84, flown in May 1997, also demonstrated the Institute's expertise in measuring liquid motions in spinning tanks and in understanding the test measurements.
The key to conducting realistic tests of a spinning, partially full tank is to duplicate the spinning and the up-and-down, to-and-fro oscillation of the tanks in the nutating spacecraft. The magnitude of the nutation is measured by the cone angle, which is the angle between the actual direction of the spacecraft's spin axis and its desired direction (to be precise, the direction of its angular momentum vector); in effect, the spacecraft is tilted slightly with respect to its desired flight direction. The tilt appears to rotate around the angular momentum vector at a rate called the nutation frequency, which is somewhat smaller than the spin rate. After a preliminary study of various methods of achieving this kind of nutation motion with a laboratory apparatus, SwRI designed an apparatus that would give the most flexibility in testing and the most realistic simulation of the spacecraft motion.
The SSTF uses two separately controlled electric motors to produce the desired motion. The motor attached to the lower frame (anchored to the laboratory floor) rotates an upper frame through a gear reducer and belt-and-pulley arrangement. The upper frame is mounted on a large slewing ring bearing fixed to the lower frame that allows relative motion between the upper and lower frames. The upper frame is tilted at an angle equal to the desired cone angle. The pulley driven by the upper motor rotates the spin table. Just as with the connection between the lower and upper frames, the spin table is mounted on a slewing ring bearing fixed to the upper frame to allow relative rotation between the upper frame and the table.
The sum of the rotational rates of the upper and lower pulleys is equal to the spacecraft spin rate (neglecting the small effect of the upper frame tilt), and the nutation frequency (that is, the frequency of the tank's up-and-down, to-and-fro motion) is equal to the rotational rate of the upper motor pulley. By adjusting the rotational rate of the motors, both the spin rate and nutation frequency of the table can be varied over any desired range. Electrical power to the upper motor and the instrumentation on the spin table is supplied by a system of slip rings. Unique multi-axis load cells, which support the test tank, measure the forces and torques created by the motion of the liquid in the tank.
The SSTF generally uses larger spin rates than the spacecraft does to make the centrifugal acceleration of the tank much larger than Earth's gravitational acceleration. By doing so, the SSTF simulates the coasting period of the orbit insertion maneuver when the upper stage engine is not firing and the spacecraft appears to be in a low-gravity environment. To measure the liquid forces and torques, a spacecraft tank containing simulated propellant is mounted in a load cell frame composed of four force and torque sensors. The sensor signals are sent to a data acquisition system mounted on the spin table, where they are processed and transmitted, via the slip rings, to the SSTF control room and recorded for later analysis. Because the effects of more than one tank are additive, the fact that the SSTF uses only one tank is not a limitation. The SSTF is large enough to test a full-scale spacecraft tank as large as 40-inch diameter at distances up to 4 feet off the spin axis.
The SSTF tested a full-scale tank of the Genesis spacecraft, scheduled for a 2001 launch. The objective of this mission is to collect samples of the material flung out from the sun's surface into the solar wind, with the hope that these samples will help scientists determine the origin of the solar system. NASA used analytical models of the Genesis spacecraft dynamics and proved that the spacecraft should remain stable. If the test data and analytical models had proved otherwise, the control system parameters of the rocket would have to be modified, or the mass distribution of the spacecraft-rocket combination would have to be changed, or some other change would be needed to make the nutation growth acceptable -- none of these changes is easy to accomplish during the final stages of spacecraft design.
At present, the SSTF is the best experimental method available to investigate liquid motions in spinning spacecraft tanks that contain flexible bladders. These tanks are relatively common since they are able to hold the propellant over the tank outlet in the low-gravity environment of space. The lack of an applicable test facility in the past meant that the nutation characteristics of any spacecraft that used them had to be predicted very conservatively.
The most popular alternative to the SSTF is a drop tower, in which dynamically scaled models of the spacecraft are allowed to fall freely for several seconds while spinning at a high rate. A drop tower test has the advantage that it can obtain a low-gravity environment directly but it has to employ rather small models to meet the limitations inherent in drop towers. This dependence on small models and small tanks implies that tests of a bladdered tank will generally not be reliable, since making an accurate scale model of such a tank that retains all its essential dynamics has not yet been demonstrated successfully.
In addition to the SSTF's advantages of using long duration test times and full-size tanks, the ease with which the SSTF can change test conditions such as spin rate, nutation frequency and tank fill level make it ideal for fundamental investigations of liquid motions in spinning tanks. For all these reasons, many additional programs are already scheduled or planned for the SSTF in the coming year, including nutation tests for the tanks of the CONTOUR spacecraft (a spacecraft that will TOUR the Nucleus of three COmets) as well as several basic research investigations of bladdered tanks in general.
With the inauguration of the SSTF, SwRI has strengthened its recognized status as a world leader in the experimental investigation and analysis of propellant dynamics effects in spacecraft tanks. The Institute had its first program in this area in the 1950s, even before there was a National Aeronautics and Space Administration, and SwRI's research and development activities have continued unabated since then, with hundreds of programs applied to dozens of individual spacecraft and rockets, with several hundreds of millions of dollars in funding. It is clear that the SSTF will help the Institute maintain this strong program in propellant dynamics in the future.
Published in the Spring 2001 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.