karn@eagle.UUCP (07/26/83)
At 00:32:00 UTC today, 26 July, an attempt was made to fire the motor on Oscar-10 for the second time. This burn was planned to deplete our fuel, raising inclination to 50 degrees and lowering perigee to about 2100 km. At the proper time the electrically operated auxiliary valve was opened, and this was confirmed by telemetry. However, no changes in helium pressure or bottom surface temperature were seen, and the computer closed the valve one minute later as it was programmed to do in the event this happened. It appears that the helium pressure readings we had been seeing in the past weeks since the first motor burn were valid and that there was insufficient helium to open the main engine valve, preventing us from firing the engine. The main valve is specified as requiring 5 bar of helium pressure to open, while the regulator output pressure reading at the time was about 3.6 bar. The most likely cause according to MBB, the manufacturer of the engine, is damage to the welds on the engine's regenerative cooling jacket caused by the collision with the Ariane at separation. Either the leak did not form until after the first burn, or the small size of the leak combined with the relatively low vapor pressure of UDMH (the fuel) kept enough fuel in the tank to allow us to do the first motor burn. Once the tank had been pressurized to 15 bar, however, the leak rate increased to the point where either the fuel was expelled or helium escaped, depleting the helium tank. My personal theory, although not widely held, is that the slow leak existed from the time of the collision. The nice thing about this theory is that it also explains why the delta-v (change in velocity) in the first burn was higher than even the longer burn duration should have given. If fuel had been leaked, this would result in a smaller spacecraft mass and therefore a larger velocity change for a given thrust and duration. The regulator pressure readings during the first burn, which were normal, would be explained by no gas being lost from the leak until most or all of the liquid had been expelled. All is certainly not lost, however. The first burn put us in a very stable orbit in which atmospheric drag is negligible. The rest of the satellite appears perfectly functional, and it will now be reoriented to operating attitude. It will take perhaps a week to point the high gain antennas at earth, followed by another few days for transponder checkout, and the satellite will become available for general use. There will be some side effects of this orbit that aren't particularly severe, but will have to be lived with: 1. The inclination (approx 26 deg) is low enough that at certain times of the year the high gain antennas will have to be off-pointed from the earth in order to maintain acceptable sun angles on the solar panels. This is likely to be more of a nuisance than a real problem, however, and would have had to be done for the first year even if we had reached the desired orbit until the apogee point had precessed far enough north to make this unnecessary. 2. There will be slightly more radiation in this orbit. It is not known exactly how long the satellite will survive, but it will still most likely be measured in years. 3. The argument of perigee will precess much faster, but this is mitigated by the fact that apogee will never move further north than 26N or further south than 26S. The rate of precession is about .272 deg/day, which means that the first northernmost apogee will occur in a little less than a year. After another year apogee will be back at the equator, a year later it will be at 26S, etc. 4. The long DX possibilities for high latitude stations will decrease. In the ideal "Molniya" orbit, long mutual 3-way visibility windows would exist between, say, Tokyo, London and New York. These windows will be shorter and less frequent in this orbit. 5. The southern hemisphere gets a much better deal. The low inclination will not penalize stations in the southern hemisphere as much as the apogee moves north. 6. There is still probably half a tank of corrosive nitrogen tetroxide on board. While the tanks were designed with considerable margins of safety, it is not known how well the N2O4 tank will hold up over a period of years. In any event, the satellite should finally be operational soon, albeit in an unplanned orbit! Phil Karn, KA9Q