karn@eagle.UUCP (Phil Karn) (07/23/83)
During the past week, AMSAT-Oscar-10 has been on "autopilot". That is, the onboard computer has continued to spin up the satellite and reorient it for the second kick motor firing. The target spin rate is about 50 rpm. This figure is high to provide enough centrifugal force to ensure that the fuels in the half-empty tanks are properly picked up by the engine feed tubes. There is one worry having to do with telemetry readings of decreasing helium pressure. Several days after the first burn, a sudden drop from about 15 bar to 6 bar was seen on the output of the helium pressure regulator, followed by a slow decline to about 4 bar now. The reason for these readings is still not know for sure, but there are several plausible possibilities: 1. The readings are false and there is no leak. 2. There is an actual leak somewhere between the helium tank, the regulator output and the three-way check valves which lead to each fuel tank and the engine control valve. 3. There is a leak in the plumbing associated with one of the fuel tanks, or perhaps in the regenerative cooling jacket which passes UDMH fuel over the engine bell, possibly caused by the collision with the launcher. There are problems with each of these theories. The pressure sensor is a high-reliability unit that cost something like $350. You'd expect something that expensive to work right! If there was an actual leak, you'd expect a smoother, exponentially decaying pressure curve instead of the sudden drops we've seen. If #3 were right, we would have lost much of our fuel immediately after the collision and the first motor burn shouldn't have worked. #2 is considered the theoretically most likely possibility, considering the reliability and complexity of the various components. If this is the case then things aren't all that bad. The fuel and oxidizer tanks, which are 20 liters each, are currently half empty. Assuming that the check valves (which are safety rated) hold, the helium already in each tank should keep them pressurized at 15 bar. The engine could still be fired in a "blowdown mode", utilizing this trapped helium pressure. Emptying the tanks would only double the ullage (empty space), resulting in a tank pressure drop to only 7.5 bar, enough to keep the engine going although not at maximum efficiency. The second burn is tentatively scheduled for 01:13:50 UTC on 25 July (Sunday evening USA time). Listen to 3850 khz for real-time information. After the burn, whether it works or not, the spacecraft will be reoriented to an operational attitude and the transponder switched on for open use. The end is near! 73, Phil Karn, KA9Q