It is normal for satellites to have solar panels on both sides of the body.
But Landsat satellites, from Landsat 4, seems to have the solar panel on only one side. Are there any specific advantages of having such a configuration?
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asked Feb 11, 2019 at 10:43
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$\begingroup$ Definitely a different question, but similar theme ;-) Why do the solar panels on the seven Sentinel configurations look so completely different from each other? $\endgroup$– uhohFeb 11, 2019 at 11:07
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$\begingroup$ see also the BCP300, BCP600, LM400 and LM1000 buses, and the JPSS $\endgroup$– user20636Feb 12, 2019 at 8:28
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2 Answers
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Solar panel technology seems to have caught up with power requirements on the satellite. Since price of components is really no object when building a system like this, super expensive panels with efficiency ratings of up to 40% can be used.
The trick to engineering something properly is using just the right amount of materials, as the old maxim goes "Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands". If the engineers were meeting the power requirements for the satellite with just one set of solar panels, why include the other? It's possible to predict the power requirements very exactly and each gram that the engineers save can be used on something else which is more mission critical such as a better imagery system.
As to why build it asymmetrically? There are basically no benefits to building it symmetrically. Modern control software can easily compensate for the shifted center of mass and you also save on motors and unfolding systems in mass.
answered Feb 11, 2019 at 11:32
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$\begingroup$ Still the answer is pending a solid argument for an asymmetric build. $\endgroup$ Feb 11, 2019 at 12:07
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10$\begingroup$ There's no reason for a satellite to be symmetrical. Symmetrical designs are often used for aesthetics, aerodynamics, or balance, none of which a satellite needs to have. In the satellite's case, form follows function so they design the functionality and this causes the shape, they don't start out with a 'satellite shape' and then fill in components. In this case, building the solar panels symmetrically would have just increased the weight as you need more motors to unfold the panels. $\endgroup$ Feb 11, 2019 at 12:36
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$\begingroup$ I anticipated some form of payload packaging efficiency given the satellite in discussion is earth observation satellite, but your argument for more motor is also a good one. Didn’t strike me nor have I come across. Thanks. $\endgroup$ Feb 11, 2019 at 12:51
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3$\begingroup$ @Dragongeek isn't there asymmetry in solar radiation pressure causing an unbalanced torque? or does it just integrate out over an orbit? $\endgroup$– costromFeb 11, 2019 at 17:40
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Landsat 4 (and its sister, 5) was a LEO satellite, and was intended to be movable - it had around 500 kg of fuel on board for orbital manoeuvres and maintenance.
That fuel is needed for a number of reasons:
- A calculable drag from the atmosphere in LEO, as its density falls off exponentially as you increase in altitude (albeit low at 700 km).
- Solar pressure
- Orbital perturbation from the non-uniform gravitational field
- The need to both correct the initial orbit and decommission the satellite into a disposal orbit.
- Anticipated mission objectives to observe a particular area or observe it sooner requiring a different orbital path
All of this adds up in this case to a significant capacity to perform orbital manoeuvres and corrections, and of all of those the solar pressure due to asymmetry in its panels is likely the smallest component. Note that in a sun-synchronous orbit and with necessarily the instruments always pointing towards Earth, the solar array would rotate all the way around the satellite over each 99 minute orbit, averaging out the asymmetry in the solar pressure.
The benefits of only having one solar array include:
- Half the amount of unfolding apparatus - as Dragongeek's answer explains
- Leaves one side of the craft free of panels: the propulsion on Landsats 4&5 is located away from the solar array, as are the sensors.
- Keeps more of the hot panels further away from the body of the craft, so they can be isolated more easily and radiate more effectively from the back
These benefits are relatively marginal, but since the costs of asymmetry are so small here, it must have proved worthwhile on balance.
Note that Landsat 4 operated for 11 years, and Landsat 5 (same design) for 29 years, both with the same initial fuel load, well beyond their original 5 year designed life expectancy.