Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!kcarroll From: kcarroll@utzoo.UUCP (Kieran A. Carroll) Newsgroups: net.columbia Subject: Re: Launch Windows Message-ID: <5188@utzoo.UUCP> Date: Thu, 7-Mar-85 19:32:57 EST Article-I.D.: utzoo.5188 Posted: Thu Mar 7 19:32:57 1985 Date-Received: Thu, 7-Mar-85 19:32:57 EST References: <225@tellab2.UUCP>, <5179@utzoo.UUCP> Organization: U of Toronto Zoology Lines: 45 (a virtual line) Henry Spencer mentions that most spinning satellites are not free to orient themselves in the best possible position for receiving solar power through their panels of solar cells while in transfer orbit; this leads to a desire to keep low-earth-to-geosynchronous transfer times as short as possible, so as not to run down the batteries on the spacecraft before geosynchronous orbit is achieved (at which time, all the solar panels can be deployed, and power ceases to be a constraint). Quite true. In case anybody's interested, here's one of the reasons that spacecraft orientation is constrained during the transfer orbit: During transfer, the vehicle must be able to sense it's attitude (ie. its orientation with respect to some inertial reference frame), so that, when it reaches apogee, it'll know in which direction to point its apogee kick motor, to circularize its orbit. The most common way to perform this attitude sensing, for a rotating spacecraft, is with sun-sensors and earth-sensors. These sensors have a limited field of view, and so must be pointed roughly in the direction of the earth or the sun (whichever is applicable) to start with; after that, provided nothing disastrous happens, they'll stay locked on their target. Also, one has to be careful not to allow an earth sensor to face the sun (or the moon, for that matter), or else it might get confused, and the satellite might lose its attitude reference. These requirements translate into a set of constraints as to which direction the satellite's spin axis may point; generally, pointing perpendicularly to the line joining the satellite with the sun is not the best orientation. Thus, the solar cells on the sides of the spacecraft don't face directly towards the sun, and power-collection is restricted. For some missions, the spinning solar cells don't even collect enough power to make up for the housekeeping power drain during transfer orbit; the extra power is drained from batteries on board the satellite (fully charged at launch). The satellite must reach geosynchronous orbit, stop spinning and deploy the rest of its solar cells before the batteries run down. This places a constraint on the amount of time the satellite can spend in transfer orbit. Another problem is maintaining an acceptable temperature range in the satellite during transfer orbit, but I won't go into that right now... There! My spacecraft design class taught me something, after all! -- Kieran A. Carroll @ U of Toronto Aerospace Institute {allegra,ihnp4,linus,decvax}!utzoo!kcarroll