Sunday, May 31, 2015

Thoughts on LightSail

This is the first of what is likely going to be multiple posts on solar sails, and in particular my thoughts on using solar sails on CubeSats. Although there are many more basic starting points than what I'm going to talk about in this post, I'd like to dig in immediately and talk about the recent trouble with The Planetary Society's CubeSat LightSail. For those who haven't heard, LightSail is a CubeSat mission with the goal of showing solar sails can be deployed from a CubeSat in space with a secondary goal of making sure the extra inertia can be controlled by the ADCS (attitude determination and control system). Although I'm an unrepentant partisan for all things small sats and a big fan of Bill Nye, especially after his heavy promotion of science education during the past few years, I am simply unsure about this mission.

First, it seems only marginally interesting at best. In the Low Earth Orbit (LEO) they have selected for the CubeSat, atmospheric drag will far outweigh solar radiation pressure by many orders of magnitude. This means even if they manage to deploy the solar sail it will be useless as a propulsion system, instead doing the opposite and pulling the CubeSat down even faster than normal by increasing the drag force (proportional to area) on the CubeSat and sail. This isn't necessarily a useless thing however, as there's actually a certain niche for deorbiting devices! This is because those of us in the small sat world tend to love our LEO orbits and not want to clutter them with debris, so we've all agreed that every CubeSat has to burn up in the atmosphere at the end of its life. Having a deorbiter allows you to maintain a higher altitude LEO while still meeting this requirement so it's actually useful! But that's not what the purpose of the sail on LightSail is, nor what it is continually billed as in the media. It's instead billed to be a solar sail, a la Clarke's concept, so why not simply select an orbit which will allow the sail to actually function as a propulsion system? Although there could be many reasons why TPS didn't do this (e.g. what if the sail doesn't deploy and LightSail itself becomes space debris in a higher altitude orbit?), it doesn't make sense to actually go through the entire design process just to do a marginally interesting technology demonstration. Now that I think of it, this could just as easily been done on a series of suborbital parabolic flights. Despite these qualms however, this thing was selected, designed, constructed, and presumably passed some sort of reviews, although recent issues make me think these must have been minimal.

Now let's get to LightSail's current situation: which is that it's just recovered after experiencing a software failure. Basically because LightSail beacons telemetry data every 15 seconds for some unknown reason and because the flight software stores these beacons for some even more unknown reason, the on-board computer (OBC) experienced a system-wide software failure and became frozen. The excuse for this was that the flight software was unpatched by some commercial distributor, an excuse I don't believe I've ever heard before, doubtless because it is so weak. It is unbelievable that you would pass Flight Readiness Review without having convincing validation of your software and without running it for at least a few days in simulation! Most spacecraft have specially written flight software for exactly this reason and suffering a failure because of not having a patch (which will now be a recurring issue if you cannot uplink the required changes) sounds more like an excuse for a computer game not working than a spacecraft failing in flight. Ridiculous.

To make matters worse, a second critical design flaw--one that can't be fixed by telemetry--prevented TPS from power cycling to overcome the issue: they have no analog reset! Weird stuff happens to flight software all the time and frequently you just have to power cycle and resume where you left off. But how do you know to do this? The standard solution is to have an element in the Electrical Power System (EPS) which constantly pings the OBC and, if it fails to receive a response to the ping for a certain amount of time (typically hours or days), initiates an analog reset into the deployment state by pulling bus power and firing up a timer circuit which in turn initiates the boot phase for the OBC (alternatively the timer can be left out or the pinger can be internal to the OBC board and simply reboot as soon as a freeze is detected). Evidently TPS didn't include this basic component and so they tried, unsuccessfully, to telemeter a commanded reset knowing their OBC was frozen! This is beyond words for me. Of course they failed and simply had to shrug and say they hoped a single-event-upset would reboot them. It is beyond fortunate that this occurred in a short amount of time since their entire mission could have easily failed otherwise.

The now inexplicably long wait time of 28 days to deploy the sail, itself bizarre and simply asking for something like this to interfere with their primary objective, has been thoughtfully revised to be as soon as they are sure they've regained control of the spacecraft. All this drama and they've not even attempted their rather trivial threshold objective. Moreover, if the sail deployment mechanism design is anything like their OBC or EPS I am not so sure they will achieve even the small token of having deployed a sail in space. I'm sad to say LightSail is increasingly looking to me like a total waste of time and money. Moreover the publicity it has garnered due to Nye's celebrity, publicity which could have been used to help bring small sats more into the mainstream if they had only selected a good mission and diligently designed it, will doubtlessly arm those of my colleagues who still clutch the dying belief that CubeSats can only ever be toys with a high failure rate with yet more ammunition in reviews, publications, and formal and informal discussions informing (or rather misinforming) the attitude of our field in general.

So far LightSail has been a huge disappointment for me and I wish Bill Nye of all people could have given a little more thought to exactly what he was doing. I hope they are successful. I hope they can successfully deploy the sail and even swing the spacecraft around a bit with the sail deployed, but no matter the outcome I ultimately think LightSail will go down as a rather marginal and unremarkable piece of small sat history.

Edit: As of June 7 2015, the CubeSat is once again failing to communicate with the earth with the batteries in a suspected safe mode. The current cause of the error is unknown.

Edit 2: As of June 8 2015 the CubeSat has recovered from the second failure and the boom deployment motor shows a record record of it moving to deploy the sail. Media and LightSail personnel have somewhat disingenuously reported this as "the sail is deploying" when in truth we have no images to make sure the deployment is proceeding correctly. Hopefully they are correct in assuming that just because the motor is turning the sail is correctly deploying.

Edit 3: It seems as if they have an image of the sail and it deployed successfully, a major relief to all of us. I still find their attitude laughable: "LightSail Test Mission Declared Success; First Image Complete"--a heading which has to be the most arrogant way I've yet seen of announcing a spacecraft has achieved its rather tawdry threshold objectives. It is unclear whether or not this means this spacecraft will now be left to rot until it reenters since most good missions typically have more than one objective, but given that the CubeSat now has a giant reentry device attached to it I can't see much else they can do before it reenters. 

No comments:

Post a Comment