|
|
Post by VALIS on Mar 16, 2010 17:41:13 GMT -4
I always thought it was interesting that Babylon 5 had its long axis perpendicular to the planet it orbited. Although the station appeared to be too high to be affected by atmospheric drag, there would still be utility in minimizing the tidal effects within the station. Of course, it was probably just done for aesthetic reasons, but I've always had the Marvel "No-Prize" mindset of "don't tell me that it's wrong - Think of a reason why it is right." I didn't watch the show but if you have the long axis in that orientation, you can use the tide to your advantage: you would feel gravity at both ends. Probably not much gravity (unless the station is really long) but still. |
|
|
|
Post by randombloke on Mar 16, 2010 18:10:01 GMT -4
How long is really long?
B5 was canonically five miles long. As space vessels go that's pretty big.
Of course on most useful astronomical scales it's somewhere between 'tiny rock' and 'small rock' and therefore totally insignificant...
|
|
|
|
Post by ka9q on Mar 17, 2010 18:30:12 GMT -4
Since velocity on a spacecraft is never constant (with gravity a] slowing you down and b] pulling your trajectory into a curve) you are constantly accelerating on a trip to the Moon...  True, and it's my fault that I wasn't more specific about the kind of acceleration I was talking about. One kind is when you feel a force pushing you against a wall of your spaceship cabin. Another kind is when your velocity vector, measured in an inertial frame, changes with time. Because of gravity, one can happen without the other. Without gravity, the two would always occur together. |
|
|
|
Post by ka9q on Mar 17, 2010 18:41:57 GMT -4
Because the performance of the launch vehicle can be inferred from the mass properties of the payload, and the performance of the Saturn V was not something we wanted the Soviets to know about in much detail. That's what I thought. The classification notices on some Apollo documents also talk about specific performance and anomaly information. But it seems a little silly given all the information that was routinely published about Apollo. You can produce some pretty good estimates about Saturn V performance from easily attainable figures like its external dimensions and the burn time of each stage. The public affairs officers were always telling us the exact perigee and apogee of every orbit and the details of every midcourse correction, and those certainly said something about the accuracy of our guidance and navigation systems. So it's hard to see exactly what they were trying to deny the Soviets and what the Soviets could have done with what we were trying to deny them. As I recall, they routinely stationed "fishing trawlers" in international waters off the Florida coast in those days. Each Saturn stage transmitted telemetry that, as far as I know, wasn't encrypted. And then you have the constant propaganda from our side about how completely open our program was compared to the Soviets'. And it was open. Something I keep trying to tell the hoaxheads who insist that "compartmentalization" allowed NASA to keep nearly 400,000 workers fooled into thinking that Apollo was real. As I recall, the Russians knew very well why their N1 didn't perform (even had it flown) as well as the Saturn V: it burned LOX and RP-1 (or their equivalent) in all stages while we burned LH2 in the upper stages. |
|
|
|
Post by VALIS on Mar 17, 2010 19:35:46 GMT -4
How long is really long? B5 was canonically five miles long. As space vessels go that's pretty big. Of course on most useful astronomical scales it's somewhere between 'tiny rock' and 'small rock' and therefore totally insignificant... I computed the gravity gradient that would be applied on a 5 mile spacecraft in equatorial LEO (300km) and got only .02 m/s 2. That's way less than I thought. I expected more since some satellites control their attitude this way. Disappointing.. Apparently it works better on integral trees. The way I computed this is that I subtracted the gravity at 296km from the gravity at 304km, figuring that this would be what would put tension on the spacecraft |
|
|
|
Post by ka9q on Mar 17, 2010 20:19:13 GMT -4
I computed the gravity gradient that would be applied on a 5 mile spacecraft in equatorial LEO (300km) and got only .02 m/s 2. That's way less than I thought. I expected more since some satellites control their attitude this way. Disappointing.. Yup, that's about right. But it's enough considering that this force acts continuously (in a circular orbit) without anything to oppose it. Actually, there are other forces acting on spacecraft. In earth orbit the two most significant ones are solar radiation pressure and the effects of the earth's magnetic field. At 1 AU (earth's distance from the sun) solar radiation pressure is 4.6 micropascals when the photons are absorbed and twice that when they're perfectly reflected. Since this is a pressure, the force depends on the area presented to the sun. At earth distance the solar wind (the stream of charged particles) is orders of magnitude weaker than the radiation pressure and can usually be ignored. The effects of the earth's magnetic field are more complex. If the structure is conducting, as most spacecraft are, then the main effect is the generation of eddy currents when the spacecraft rotates within the field. These currents, like all eddy currents, produce torques that oppose the rotation, eventually causing it to stop. Many small amateur radio satellites designed for LEO are passively spin-stabilized through the use of permanent magnets and selective painting of spacecraft surfaces. You start by ensuring that the desired spacecraft spin axis has a greater moment of inertia than the other axes; this is essential for stability. Permanent bar magnets aligned with the spin axis provide a torque to align it with the earth's magnetic field. VHF and UHF "turnstile" antennas constructed from a metal tape measure are painted white on one side and black on the other. Since the solar radiation pressure is greater for reflecting surfaces, the differential solar photon pressure creates a torque around the spin axis. But the spin rate won't increase forever, as it is counteracted by a magnetic torque created by the eddy currents induced in the spacecraft frame by its rotation in the earth's magnetic field. It's a simple and highly effective system that's been used dozens of times. More sophisticated spacecraft use electromagnets and gravity gradient booms to provide a more active form of attitude control without having to expend propellant in thrusters. |
|
