karn@eagle.UUCP (Phil Karn) (07/16/83)
Well, it has FINALLY become public information. ESA has released a statement explaining the attitude problem of AMSAT-OSCAR-10 during the first few days after launch on 16 June. Telemetry from the launcher showed that all events up to and INCLUDING separation of OSCAR-10 from the third stage of the Ariane were perfectly nominal. This explains why the "event frame" stored away by the Oscar-10 computer 1 second after separation showed proper sun angles, etc. At separation time, two pyrotechnic bolt cutters fired to release our spacecraft, and three separation springs mounted on the attach fitting propelled us out from the lower SYLDA half-shell and away from the top of the Ariane at about .5 meter/second. At 53 seconds after separation and again about 1 second later, the accelerometers on the launcher registered "bumps". This was the third stage slamming back into our payload. It turns out that the onboard sequencer had been programmed to vent extra liquid oxygen, probably through the engine bell, and the resulting impulse slowly accelerated the stage to the point where it "caught up" with us. The impact velocity was estimated to be 1.5 meters/sec. If the stage had been reoriented to point in a different direction before venting, the collision would have been avoided. It is not clear at what points the impact occurred. The most likely thing to have happened was for the end of an arm to hit the lip of the SYLDA cannister; this would have tipped over our satellite, resulting in the second collision, of another arm or the kick motor on the side of the SYLDA. Quite possibly the two-meter antennas were involved in the collision, and their "springiness" are probably what absorbed much of the rotational momentum of our spacecraft and started it rotating slowly in the opposite direction. Amazingly enough, however, it doesn't look like there was any serious damage to the antennas, as they have given patterns as expected. Neither has there been any observable damage to the solar arrays. The 70cm high gain antenna was also tested and found to give predicted patterns. The only puzzle is the asymmetrical spin modulation on the 2 meter omni, but it is hard to envision a collision that would have bent the 2m omni (which is on the top center) without also damaging the other antennas. Phil Karn, KA9Q