It's ALIVE: Juno back online after reboot NASA's Juno space probe has successfully rebooted after a recent Jupiter fly-by was disrupted by an unexpected “safe-mode” event. The data-gathering on the October 19 flyby didn't happen, because a software performance monitor rebooted its computer just beforehand. The probe then entered safe mode while NASA worked through its …
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According to your calculations, Symon, the thrusters used for this maneuver have a specific impulse of 147 seconds. Before analyzing that, let's compare it to my homework.
Mass of Juno postburn: 1700.0kg
Mass of expended fuel: 3.6kg
deltaV: 2.59 m/s
The ideal rocket equation:
2.59m/s = (Isp)(9.8m/s^2)(ln(1703.6/1700)) gives an Isp of 125 seconds, differing from your estimate because I estimate Juno is 300kg closer to empty than you do. Still, our numbers match fairly closely.
What does this mean? Juno's RCS thrusters use hydrazine monopropellant, which has a theoretical maximum Isp of ~230 seconds. So this burn was in fact fairly inefficient in a thermal sense. Since hydrazine decomposition is exothermic, this inefficiency may be attributed largely to the small size of the engine, and the desire to run 'cold' as to avoid damage to the motors and payload. The engineers who figured this stuff out when planning the mission probably realized the savings on engine mass, thermal shielding, and radiators, would outweigh the fuel mass efficiency of larger, hotter RCS engines of the same reliability.
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