The Maretian

by Kris Overstreet

Chapter 222: Sol 409

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There may be a problem.

We began Sirius 7 with the Whinnybago loaded as it would be, more or less, for the final trip:

ROVER 2 (empty, with extra hydrogen cells) – 3.7 tons
FRIENDSHIP TRAILER (empty) – 14.5 tons
FOOD (110 days supply) – 0.65 tons
BOOSTER SYSTEM (substitute equivalent weight of rocks) – 4.5 tons
SPARKLE DRIVE plus 7 batteries to power it – 0.5 tons

To be specific, the trailer’s total loaded mass is a little more than seventeen tons, with Rover 2 carrying the balance.

This load was propelled on ten wheels, eight of which are powered, two of which have the clutch disengaged so they rotate freely. (All the rover wheels have their own built-in electrical motors, rated at roughly fifty watt-hours per kilometer of travel on a normal load. This is more than double the normal load rating, with Rover 2 almost at maximum emergency load and the trailer miles beyond that..) The two rover batteries plus four Hab hydrogen storage cells add up to fifty-four pirate-ninjas to power all of this.

Bear with me. I’m laying all of this out so I can think.

We got a bit of a late start, so the sun was already up before we began rolling. We drove for about three hours, until the battery readouts showed 10% power remaining. (The rover computer is smart and can detect the extra, unauthorized power storage and monitor its charge level. Which is good, for reasons which will become obvious in a moment.) This got us 69.66 kilometers away from the Hab. All well and good, right?

We stopped, unloaded the fourteen extra solar cells from their stacks on top of Rover 2, and set them out for recharging the system. With a clear sky each panel provides 120 watts at peak power. The ongoing cirrus cloud coverage knocked that back a little, but with forty-two out of the Hab’s fifty-four panels with us, we figured we had power to spare. Also, we have the 100 watts provided by the RTG, which isn’t much, but it’s 24/7.

So we retired to the trailer for the rest of the day, gathering in the habitat deck when the sun went down and it began to get uncomfortable in the bridge. I set the alarm for first light, about an hour before dawn, expecting to get up to find a full power system and an easy drive back the way we came to return to the Hab.

Nice theory. Too bad it didn’t quite pan out.

We got up when my alarm went off. I suited up, went outside, picked up the solar panels in the Martian pre-dawn, and got into Rover 2 for the drive home. And that’s when I discovered that the batteries were only recharged up to 70%.

Remember, it took 90% of the batteries to drive seventy kilometers yesterday. And since 70% is less than 90%, we definitely weren’t going to get seventy kilometers today. But, since this was a test, we pushed on anyway. 90% got us seventy klicks, so 60% should get us two-thirds as far, right? Forty-six and two-thirds kilometers, no problem, yeah?

Nope. Barely forty kilometers. And that’s where we are now, as I type this; thirty kilometers from the Hab, and temporarily out of contact with Earth.

I’ve got a lot of questions I need to find answers to. Where did my recharge go? Why is my driving performance fourteen percent less efficient on the second day? And, assuming I find answers, what can I do about it?

I do know one thing: forty kilometers a day is not going to do it. That’s over eighty days- more than half our safety margin for modifying the MAV gone.

So we’re cancelling today’s read-along. No D&D. All of us are doing math and brainstorming solutions to this issue. I’m keeping this log open and using it for, well, kind of the minutes of the meeting. If we come up with good ideas, this will help us remember.

Okay, going forward.

Dragonfly asks how much power each solar cell produces. On Earth, with its almost circular orbit, sunlight adds about 1400 watts of heat energy per square meter of surface. Mars is a lot farther out, and its orbit is a lot more elliptical. Raw solar energy ranges from 500 to 700 watts per square meter. The solar cells turn that energy into electricity we can use at a 10.2 efficiency rate. That means, on a clear, day, each 2 sq. m. solar panel should have a peak power of about 120 watts.

Of course, we aren’t having clear days lately. Cirrus clouds let in most of the sunlight, but not all of it. Also, the northern hemisphere’s summer corresponds almost perfectly with the Martian apisol- that means farthest point from the sun in orbit. Mars is gradually getting closer to the sun, but this hemisphere is tilting away from the sun as we approach the equinox, so it’s kind of a wash, energy-wise.

Fireball points out that estimates aren’t the same as actual testing. Okay, so thing to do: connect one of the power meters in my tool kit to a solar panel and monitor its performance. That will give us an exact measurement.

Starlight Glimmer does some math and works out that, assuming twelve hours of good sunlight, our forty-two solar panels ought to produce a total of 60,480 watt-hours, or more than enough to fill up the batteries without the RTG. Nice idea, except that fourteen of the solar panels aren’t producing while we’re driving. We can’t start driving until there’s at least enough twilight for the ponies to see beyond the range of their suit helmet lights, so some recharge time during the day will be lost to driving.

Fireball asks: doesn’t that mean that the batteries are charging from the twenty-eight solar panels on Friendship while we drive? Good point… come to think of it, damn good point. Let’s think about that for a moment.

It takes at least half an hour after sunrise for the sun to be high enough off the horizon for the solar cells to get a decent current going. Before then the angle is too low and the panels are catching more photons reflected from the atmosphere than direct from the sun. But after that, the current is close enough to peak as makes little difference. And yesterday- and in the prior power test, come to think of it- we started driving at or after that point. That means that, in addition to burning what was in the batteries, we were also using 100 watts from the RTG and as much as 3,360 watts from the solar cells every hour. In three hours, that comes up to maybe 10,380 watt-hours.

I think we just found that fourteen percent efficiency loss. We didn’t lose any efficiency. We were just burning more juice than we thought we were.

This morning we got rolling long before dawn and ran out of juice in a bit less than two hours of driving. The solar cells were putting out negligible amounts of current for about two-thirds of our drive time today, so we didn’t get the benefit of their juice.

Doing the math again. With a normal load, the rover wheel motors are rated for fifty watts per kilometer per wheel, or (with eight drive wheels running) four hundred watts per kilometer, total. But more weight requires more juice. Yesterday’s performance was (I thought) sixty-nine kilometers on 49.6 pirate-ninjas, or in round numbers about 720 watt-hours per kilometer. But it turns out we were probably closer to 60 pirate-ninjas, or roughly 870 watt-hours per kilometer.

More than twice as much energy consumption, for more than twice the rated load. There are a lot of reasons why this could have been different- lack of air resistance, rolling load, Mars gravity, all sorts of other shit- so I never bothered to run this calculation before. But…

… eight hundred seventy watt-hours per kilometer, at seventy kilometers, requires 60,900 watt-hours- call it sixty-one pirate-ninjas. We can only store fifty-four pirate-ninjas at a time, and an all-day recharge cycle gets us not more than 60.5 pirate-ninjas per sol. That’s not sustainable. Either we find a way to use less power in motion, or else we accept a maximum theoretical range of (fifty pirate-ninjas divided by 0.87 pirate-ninjas per klick) fifty-seven kilometers per day.

The obvious answer is to lighten the load. The problem is, that’s impossible. The ship is stripped down to the absolute minimum systems. We need all the magic batteries for emergencies, for magic rations to keep Dragonfly from crawling back into a cocoon, to top off the jumbo batteries for launch day, etc. We’ll have to think of something else.

Anyway, next time we do a dress rehearsal run, we drive without the solar cells plugged into the electrical system until we stop driving for the day. That’ll give a more accurate idea of what kind of daily driving range we can sustain without making changes.

But anyway, yesterday we had nine good hours of sunlight recharge with all solar panels. Let’s say the clouds knock ten percent off their peak performance- it shouldn’t be that much, but let’s say. That should have got us almost 41 pirate-ninjas back in the tank, plus the 5 pirate-ninjas in reserve. And during our sleep cycle, when even the computers are powered down, the RTG should add close to another pirate-ninja. We should have had 46,500 watt-hours in the system when I got up this morning. Instead we had 37,500.

There is an electricity thief somewhere on Mars, and it’s stolen nine pirate-ninjas from us already.

Cherry Berry beats me to my suggestion: make a list of everything, absolutely everything, that draws electricity.

Okay. First are the rover computer and the five Hab laptops, all of which we’re taking along for reasons of morale. If all six are running at the same time, they draw a total of ninety watts, more or less.

The rover life support, with air circulation fans and air sampler, draws an average of twenty watts.

Two Hab light strips for illumination in the ship; twenty watts each.

Rover radio system, about ten watts. It’s on to let us track the Hab beacon when we get within twenty-five kilometers of either the Hab or (at the end of our drive) the MAV. This was NASA’s idea, and it’s an excellent one. Schiaparelli is one of the largest craters on Mars, and the MAV isn’t all that large in geological terms.

Microwave. Or, as I shall henceforth call it, Slayer of Pirate-Ninjas. Twelve HUNDRED watts. We’ve been cooking our potatoes in it, and only three or four potatoes fit in it at a time. And microwaving a raw potato into what we laughingly call an edible condition takes a fucking long time compared to just heating up a pre-cooked meal pack. The three ponies and I ate a combined thirty potatoes yesterday cooked by the Slayer. I’m willing to guess that it ate two entire pirate-ninjas yesterday by itself.

Can anyone else think of anything? No. That’s it.

So tonight we’ll shut everything down except the life support and the rover computer. That’s roughly thirty-five watts, which means the RTG should be recharging the battery system at a rate of sixty-five watts per hour even in pitch black. If it’s anything different, we’ll know we missed something.

So, recap:

Tonight- turn things off, check for electricity thief.

Tomorrow- get back to Hab, reactivate Hab. (Everything’s shut down except for Pathfinder, because the Hab currently has only six solar panels left.) Tell NASA our results and plans.

Two sols after tomorrow- second dress rehearsal run. Drive without solar panels.

After that: fuck if I know.

Meeting adjourned.

Author's Notes:

Yesterday I got up at 6 AM, left home at 6:45, and got home at 12:45 AM this morning. I drove about six hundred miles, drove a load of boxes of things to my friend's new lodgings, and did what I could to otherwise lighten her load. (No details about the load: not my story to tell.)

I paid for it today. Despite seven solid hours of sleep, I more or less sleepwalked through today. And the planned chapter for today was always going to be an infodump/problem solving one, so I didn't get to make up the missed chapter today.

The electricity problem, two days ago, looked even worse than it was at one point, until I went through the page of math I did in my notes and realized that I was calculating the power output of the solar panels as if they were one meter square instead of two meters square. Basically, I was halving the power in the system due to a math error. There's still a power shortfall, but it's no longer catastrophic.

Next Chapter: Sol 410 Estimated time remaining: 7 Hours, 43 Minutes
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