The DISCOS Drag-Free SatelliteDISCOS was the first Drag-Free Satellite ever flown. The DISCOS module was built at Stanford University in the late 1960's with the help of the Applied Physics Laboratory at Johns Hopkins University who constructed the other two modules of the TRIAD I satellite. It was built as a test vehicle to attempt to improve the accuracy of the U. S. Navy's TRANSIT navigation-satellite system. The Stanford team was led by Prof. D. B. DeBra and the APL effort by Dr. Robert E. Fischell. From a modern point of view (particularly for LISA), DISCOS demonstrated two things: 1) No field gradient at the proof mass exceeded about 10-7 / sec2 and 2) the disturbance-force calculation techniques (and in particular the method of determining the gravity field and its gradient at the proof mass by calculating the contribution from every mass element in the satellite) are viable. This means that a LISA specific force disturbance of about 10-16 meters / sec2 / Hz1/2 from internal disturbance field gradients has been demonstrated in flight assuming a modern Drag-Free controller performance of 10-9 meters / Hz1/2 (See the discussion below). DISCOS was launched on September 2, 1972 and flew in a polar orbit at 750 km for a little more than one year. The name TRIAD I refers to the entire satellite. DISCOS is the Disturbance Compensation System built at Stanford University. APL Technical Digest Special Issues1) APL (Applied Physics Lab, John Hopkins) Technical Digest, Vol. 12, No. 2, April-June, 1973. The entire issue is devoted to DISCOS and its flight results. It consists of three articles:
2) APL Technical Digest, Vol. 12, No. 4, October-November, 1973. This issue contains two articles detailing some of the equipment on the satellite:
TRIAD I AIAA PaperIn 1974 a paper summarizing the results of the DISCOS flight was published in the AIAA Journal of Spacecraft and Rockets: Staff of the Space Department of the Johns-Hopkins-University Applied Physics Laboratory and Staff of the Guidance and Control Laboratory at Stanford University, A Satellite Freed of All but Gravitational Forces: "TRIAD I", AIAA J. Spacecraft, Vol. 11, No. 9, September 1974, pp.637-644. Advantages and Disadvantages of Locally-Level Attitude Control and a Large Limit CycleThe DISCOS / TRIAD I satellite's attitude was gravity-gradient stabilized. This attitude-control system was used because it is simple and reliable, but it is the worst system if it is desired to have an orbit which is not disturbed by the internal forces in the outer satellite. (Also see the discussion of the original AIAA DFS paper.) Gravity-gradient stabilization is the worse method because a component of the disturbing forces is always parallel to the satellite velocity vector and thus changes the orbit energy and, most importantly, the period. For example, a disturbance fdx,y of 10-11 ge causes a deviation from a pure gravity orbit of 1.5 fdx,y t2 = 1.5 * 10-10 meters/sec2 * (3 x 107 sec)2 = 1.5 x 105 meters = 150 km in one year. The Navy was interested in predicting for periods much shorter than one year, for example, two weeks = 106 seconds. For this time, the in-track error was 1.5 * 10-10 meters/sec2 * (106 sec)2 = 1.5 x 102 meters, or about 150 meters in two weeks. Prior to DISCOS, air drag and solar radiation pressure of about 10-8 ge had restricted the orbit prediction time to about 12 hours. Thus gravity-gradient stabilization was adequate for the Navy's purposes.But every cloud has a silver lining. While locally-level attitude control is the worst for Drag-Free performance, it is the best for measuring the size of the disturbing forces by means of satellite tracking. Furthermore, the limit cycle amplitude of about 1 mm while totally unacceptable for a modern high-performance mission such as LISA amplified the measurement of the disturbance-field gradients at the proof mass. The performance showed that no field gradient exceeded about 3 x 10-8 / seconds2. If one had deliberately set out to design a mission to measure the disturbances at the proof mass, these are probably the choices that would have been made, i.e. gravity-gradient (locally-level) stabilization and a large (1 mm) limit cycle. Because of this, it was possible to accurately measure the level of the disturbing forces in DISCOS. This has been useful in confirming the expected gyro drift in a Relativity-Gyro Experiment (PDF), calculating how well the cancellation could be done in a DC-Cancellation High-Accuracy Equivalence-Principle Experiment (PDF), and estimating the proof-mass electric charge and discharge rate in DISCOS' 750-km polar orbit. TRIAD II / TIP IIOn October 12, 1975, a second version of the TRIAD spacecraft also known as TIP II (Transit Improvement Program II) was launched by APL. TRIAD II (Go to the year 1975.). TRIAD II was similar to TRIAD I with the very important exception that it was a single-axis DFS with the Drag-Free axis parallel to the orbital velocity vector. Since the atmospheric drag is also parallel to the velocity vector, a single-axis DFS can cancel most of the disturbing forces acting on the satellite. In addition, Pulsed-Plasma thrusters were tested in space for the first time. The single-axis Drag-Free proof mass was mechanized as a Gold/Platinum cylinder about 5 mm in diameter and about 25 mm long. It had a hole in the center parallel to its long axis through which a straight copper wire acting as the support was strung. An AC current flowed through the copper wire, and the eddy currents in the cylinder provided a repulsive force which caused the cylinder to float off of the support wire. The cylinder was free, however, to slide along the wire in the axis parallel to the orbit velocity vector. Its position along the wire was measured optically with two light beams so that it provided the single axis sensor for the DFS. TRIAD II demonstrated the single-axis technology but had problems because the cylinder kept hitting the wire. It turned out that this was caused by an undamped mode where the CM of the cylinder orbited (not spun) about the suspension wire. The problem was fixed by making the copper wire hollow and inserting a magnet wire with a linear dipole moment which damped the unwanted mode. This fix was implemented in TIP III and Novas I, II, and III. TIP IIITIP III was the third and last in the Transit Improvement Program series. It was a single-axis DFS with the fix described above. A problem which showed up with TIP III was caused by an attempt to avoid disturbances from the "Radiometer Effect". The Radiometer Effect causes disturbing forces on the cylindrical proof mass due to a temperature gradient in the proof mass. Temperature differences at different points on the proof-mass surface cause molecules from the residual vacuum to rebound with different momenta. To solve this problem it was decided to vent the cavity to space to improve the ambient vacuum. The vent hole, however, was placed on only one side of the cavity. Outgassing currents caused a variable bias on the proof mass which lasted for about 3 months. This problem was solved in the next three satellites by putting a vent hole on both sides of the cavity. Drag-Free Satellites Become OperationalAfter TIP III, the U. S. Navy launched three more single-axis Drag-Free satellites for the TRANSIT Satellite-Navigation program. They were the NOVA I, II, and III series. The NOVA satellites were very similar to TIP III but were constructed by RCA. |
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