Satgen307 Satellite Hindsight No6 by GM4IHJ 11 Feb 95 BID of this msg is SGEN307 Please use this BID if you retransmit this msg One golden rule which should be engraved on the wall of every satellite builders workshop is " If it works. Do not change it". This instruction has been defied at some cost in several Amsats , where old designs have been replaced by untried new units such as amplifiers , with sad results. But perhaps the most expensive switch in recent space history concerned the Mars Observer space probe. Prior to the Observer flight , it had been common practise to charge up course correction motors at launch, accepting that a certain amount of fuel ( usually hydrogen ) would be lost to leakage before the motor was used. Mars Observer was to change all this, and it was allowed to proceed all the way to the point where course correction for Mars capture was imminent, before the motor systems were charged up. Suitable precautions were taken before charging, noting that hydrogen is a very explosive fuel. So electronic communications equipment on the craft was shut down before fuel charging commenced , to avoid the risk of leaking gas meeting a transmission induced spark. Having shut down the electronics the craft was then order to commence charging, and, then when charging was complete , switch the electronics back on to resume the communications command link with Earth. Something happened. No one is quite sure what. Because the communications never came back on. Indeed what ever did happen was certainly dramatic, because attempts to raise the space crafts quite separate surface probe communications also failed completely. Although, these 70 cm links in the amateur bands were tested using Moonbounce before trying to contact the probe, nothing was heard on them, suggesting that the whole space craft had blown up. It is not clear whether this "charge in space mode" will ever be tried again. With the committee convened to investigate the problem having so little evidence to go on, it was generally assumed that the launch and passage to Mars with its consequent low temperature had distorted or ruptured fuel lines, such that when charging commenced the space craft was soon surrounded in a cloud of hydrogen, which exploded during charging or was detonated when the craft switched communications back on. A similar expensive problem arose some time earlier with a Direct Broadcast TV geosat. The manufacturers had to obtain quite a lot of the electronic equipment from other countries, but where possible they decided to fit locally made units even though this meant some slight redesign. One area affected by this change was the foreign Travelling Wave tube amplifiers which were coupled to locally made high voltage supplies although a standard set of TWTs and supplies was available from the foreign supplier who had tested it out on several space missions. The satellite was launched to much local acclaim and successfully located in its geostationary slot where it worked perfectly for 1 month. Then without any warning first one and then a second of the TWT power supplies failed, reducing the satellite to a single channel only . Once again proving that, if possible, the human urge present in any new team "to build something entirely new" , should be avoided at all costs unless you have money to burn. By all means space test new equipment , but you should only do so in situations where a suitable fail safe old fashioned back up is available. 73 de John GM4IHJ @ GB7SAN