Would it be a good idea to put solar arrays on the torque plate on the Webb telescope

[Themohawkninja]

As I understand it, the James Webb Space Telescope will have a metal plate on it (I think it's called a torque plate, but I may be wrong) that is designed to counteract the force of the solar wind to keep the telescope in the correct alignment, and at the L2 point. What I wonder is, would it be a good idea to put solar panels on the plate to give more/redundant power?

[Nibb31]

Why would that be a good idea ? AFAIK, it doesn't have any power problems. Its biggest issue is budget overrun and technical complexity. Your proposal would only increase cost, delay, complexity, failure modes, and weight.

[Themohawkninja]

Why would that be a good idea ? AFAIK, it doesn't have any power problems. Its biggest issue is budget overrun and technical complexity. Your proposal would only increase cost, delay, complexity, failure modes, and weight.

I didn't think it had power problems, I'm just wondering if you could do it for the sake of redundancy. I'm mainly wondering if the torque plate would be less effective if the stellar wind is absorbed rather than deflected.

[TouhouTorpedo]

I'm not aware of this plate (some more info would be neat) but from what your saying it sounds like its intended to use solar radiation pressure to produce a counter torque to disturbances on the telescope (likely also from solar radiation pressure).

The problem is that extra power has to go somewhere. A spacecraft at this higher orbit has a minimal battery reserve, so the power isn't going there. The only way to get rid of the power is by shunting it back through the arrays and generating heat. Which on a spacecraft designed to be passively cold, is a very bad thing.

Another problem is simple expense on a program already spiraling in cost - solar arrays of decent output and space compatible materials aren't all that cheap, and nor is launching the extra mass.

Its an interesting thought as I had to wonder for a moment on it myself (if its gonna be in the sun, why not?), but theres big reasons for not doing it once I thought about it.

[softweir]

Looking at This Diagram, it appears the torque plate (labelled "Momentum Trim Tab" in the diagram) is significantly smaller than the solar array already attached to the telescope. Furthermore, it looks like it is designed to be moved so as to adjust solar radiation pressure, and may spend long intervals hardly facing the sun at all. If the main solar array was too small to power the telescope then it would be unable to rely on the additional array you suggest, and if the main solar array is large enough then the additional array is redundant.

Now I, personally, am fond of redundant design. Unfortunately it is very hard to create redundant design that actually reduces the risk of failure - each additional component introduces additional ways things can go wrong, and with things like power systems those failure modes can all too easily cause other components to fail:- for instance, a short in the flexible cable leading to the Momentum Trim Tab would very likely cause a system-wide short and power-down the whole craft, and flexible cables in space are very prone to mechanical wear and electrical shorts. So the designers need to add a whole additional set of components to protect the craft from such a contingency (over and above those already needed to deal with electrical failures in the primary solar array) and since they have a small chance of failing you end up with an increased risk of major failure.

Incidentally, NASA put a great many redundant systems and components in the Apollo craft. This was necessary because the engineering was considerably less reliable than it is now, and it was possible because there were so many fewer components that it was easier to isolate failures between systems. Still, they got a few things wrong: nobody in NASA or its contractors thought the Apollo crews would have to use Command Capsule CO2 scrubbers in the Lunar Module, and the incompatibility between the design of the two types of scrubber very nearly killed the APollo 13 crew when the LM scrubbers became saturated!

[TouhouTorpedo]

Yeah those trim tabs seem to be just what I expected. GEO satellties are sometimes fitted with the same thing.

Now you've mentioned flexible cables that brings up another reason - "Flexible cables" Typically isn't how transfering power from a solar array is done, least if its moveable. That usually involves a SADM, which is an integrated gear box, drive motor and slip rings for power transfer. They are a bit more expensive and heavier than their non-slip ringed counter parts. And as you say - they aren't actually that reliable either. If I've remembered the figures right, at 1-2 satellites launched a year WILL have DEPLOYABLE solar array failure (often resulting in no power from the array whatsoever) Although the hold down mechanisms can also be to blame.

Typically though, Solar arrays themselves are quite robust. They are often made up of individual strings with their own connection to the power supply unit, and some damage to an array does not shut it down entirely. They are also designed too large in the first place. Solar arrays will decay over their lifetime in space, and there will be a typical mission profile used to work out maximum power consumption in W/Hr. this is used together to size the solar array to have plenty of power up until the mission end, where it has just enough power.

In the event of damage, the array early in its life may still be just fine. Later in the missions life, the mission plan may be slightly altered to change how the power is consumed, and keep running at lower power levels. This is how missions can be extended - since they won't have the power levels they were supposed to by this point.