mdf@ziebmef.uucp (Matthew Francey) (04/21/89)
First, I would like to thank Mr. Yee for posting the "press kit". Real information is always like a breath of fresh air... But there seems to be a problem. The IUS described in the kit does not seem capable putting Magellan into an Earth-escape trajectory. Here is the IUS as described: mass at ignition 15160kg (excludes Magellan) total mass at ignition 20750kg (includes Magellan) 1st stage: burn time 150s thrust 187000N fuel 9700kg burn rate 65kg/s exhaust velocity 2.88km/s 2nd stage: burn time 108s thrust 80000N fuel 2700kg burn rate 25kg/s exhaust velocity 3.20km/s Here is how it is used: initial orbit is 297x297km (6675x6675km from geocentre) orbital speed is 7.73km/s. first stage burn the delta-V comes to 2.88*ln(20750/(20750-9700)) = 1.81km/s 1st stage separate an unknown mass is jettisoned. See below... wait 150s from the 1st stage burn, the resulting orbit has a semi-major axis of 14000km, and an eccentricity of 0.523. Assuming that the IUS/Magellan is at the perigee of this orbit then at the end of the 150s wait, IUS/Magellan is now 6730km from the geocentre, and its speed is 9.49km/s. second stage burn the escape velocity at this point (6730km) is 10.88km/s. So, the IUS must now supply a velocity increment of at least 1.39km/s. From this, we can calculate the >minimum< mass of the jettisoned 1st stage. Sparing the details, it comes to 3390kg. And this is the problem. The inert (non-fuel) mass of the IUS at launch is 15160 (total IUS) - 9700 (total 1st stage fuel) - 2700 (total 2nd stage fuel) = 2760kg. It seems that a piece of Magellan will have to be disposed of as well. Can someone explain this away? Perhaps the engines on the IUS have a strange thrust curve? Maybe the data in the press-kit was incorrectly/badly rounded? NASA has made a horrible mistake? I can't use a calculator? -- Name: Matthew Francey Address: N43o34'13.5" W79o34'33.3" 86m mdf@ziebmef.UUCP uunet!utgpu!{ontmoh!moore,ncrcan}!ziebmef!mdf