The Maretian

by Kris Overstreet

Chapter 226: Sol 418

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It’s been a little while since my last entry, but I’ve been busy, what with hunting down electrical leaks in the Whinnybago, helping do pre-trip physicals, and doing other prep work. So I’ve got a lot of ground to cover to tell you how I got where I am now, which is back in the Hab after another attempt at Sirius 7.

It really helps that the skies are clear again- well, except for the fact that it’s now considerably colder in the Whinnybago at night. The RTG and the insulation in the habitat compartment help with that, but this morning we woke up in a cuddle-pile, and we definitely didn’t go to sleep that way. And getting up and suiting up in the chill was no fun at all, let me tell you. But it’s not really uncomfortable yet, so we’re dealing with it.

Over the past week I’ve been monitoring the noontime power output of the solar cells. On Sol 410, with the clouds still in full effect, the panel I tested put out 108 watts at high noon. Today, Sol 418, with the sky clear except for the normal pink haze, we got 122 watts. That’s excellent news. So long as we have this kind of weather, we’ll get maximum recharge out of the system.

Over the past week we went over the electrical system of the Whinnybago twice and Rover 2 once. We found four bare spots on the wiring and one outright break (in a nonfunctional system, obviously), not counting the four entire wiring harnesses we removed because nothing they led to still functioned. We didn’t throw them away, though; they got added to the scrap and tools in the back of Rover 2. There are so many potential uses for wire that I just don’t want to part with it unless I have to.

Between that inspection and double-checking that the remaining cut ends are both switched off and insulated, we’ve secured the circuits about as well as we can do without actually dismantling the pony ship. I mean, more than it already is. The thing already looks like it spent six months at a U-Pull-It parts wrecker yard.

And, finally, we performed the two tests for Sirius 7B. Yesterday we left the Hab on a full electrical charge, one hour before dawn, with the harness for the solar panels on the roof of the trailer disconnected, so that only the RTG was still putting power into the system. Everything else, of course, was pulling power out. We ran until the power readings read 10%, which means more or less 48,600 watt-hours consumed. Distance traveled: fifty-seven and one-ninth kilometers, for a consumption rate lowered to 850 watt-hours per kilometer, probably thanks to the power leaks we patched.

We reconnected the solar panels, spread out the spares from Rover 2, and spent the day more or less as before. We pre-cooked four days of potato rations before leaving, so each round of taters only required about four minutes to bring from freezing to edible. (Quick thought; if we bring in tomorrow’s potato rations from the saddlebags to thaw each day, we can cut even that in half.) In every other respect we acted just like before- playing with the computers, talking, reading, recharging suits, whatever. And this morning, when we woke up, the battery charge was within 1% of full.

Yeah! Go team! Protect those pirate-ninjas!

This led to today’s experiment; drive back the way we came, with the solar panels disconnected again- basically, run all the same conditions as before- with the motor clutches on the rear two wheels of Rover 2 disengaged.

Here’s the logic behind this. The wheel motor systems are designed to produce a relatively low speed but outrageous levels of torque. Bear in mind, Rover 2 by itself hauled the wreck of the pony ship- a weight two and a half times its own. (Okay, it didn’t do it entirely by itself. We had a unicorn and a dragon to help over the gullies. But if the ground had been as flat as it looks from orbit, it would have. And if the ponies had used larger wheels for their landing gear, we could have done it a lot quicker than the one kilometer per hour. Seriously, the Ares rovers are fucking beasts.)

Now, the logic is that electric motors have a flat efficiency curve, i. e. that so long as the load isn’t zero or too heavy for the motor to budge, it’s at or near peak efficiency, and thus pouring all the electricity to one engine or distributing it among four or eight makes no difference. Thing is, that’s not necessarily so. In fact, once the load on an electrical engine drops below fifty percent of its rated capacity, its efficiency drops off. Below twenty percent, it becomes outright shitty.

The reason is friction. Friction constantly steals a bit of any engine’s efficiency- the bearings rub against each other, they rub against the housings, etc. When you lower the load you lower the electricity needed to move it, yes... but you also raise the percentage of the electricity that’s being eaten by that constant friction drain.

And as I said, the engines in each rover wheel are monsters. NASA wanted energy-efficient rovers, but they wanted a vehicle that would be able to climb over bad terrain and get its crew home a hell of a lot more. And the same idiots who gave us safety-glass helmet faceplates and one-use disposable CO2 filters said, “Well, there’s no kill like overkill,” so they gave us motors which could pull England across the Channel and connect it to France, nearly.

I exaggerate a bit, but the key point is that the rover motors are overpowered. That’s a good thing for getting a twenty-six ton load started, but once it’s moving it only takes a little juice to keep it moving. The apparent load drops off a cliff, and friction- aggravated by the excess weight of the Whinnybago- starts going all om nom nom on the efficiency. And telling the computer to cut all power to those motors doesn’t help, because if you do that the motors immediately become dynamos, producing a massive drag on the other engines that more than eats up any power they produce.

Now, of course deactivating two wheels out of eight is not going to give us a twenty-five percent efficiency boost. First, when we’re getting up to speed, all that torque is welcome. As beefy as these engines are, twenty-six tons from a dead start on six motors is a bit above one hundred percent of rated load, so the efficiency takes a hit until we’re up to speed. Also, every time we brake the connected motors regain a bit of the electricity we’ve lost, but the wheels with the clutches disconnected don’t do that. Free-wheeling wheels don’t turn dynamos. So with the six-wheel configuration we lose efficiency both starting and stopping.

And then there’s up-slopes. The six-wheel configuration does not like anything above a one in four upgrade. I actually had to get out four times today and re-engage the two wheel clutches long enough to get us out of gullies we had to cross, because we couldn’t find any banks less steep than a thirty degree angle. When we make the trip for real, that represents lost time, which means lost recharge, which means shorter legs of the trip. It also means wasted energy stopping and then accelerating again.

NASA tried the experiment on the streets of JSC (and that must have been a thing for the tourists to see, though I feel sorry for the engineers who had to move their cars out of their on-street parking). They got an efficiency gain of twelve percent in Earth gravity on perfectly flat streets with no obstacles and little braking or accelerating.

So what did we get? Well, yesterday we got 57.11 kilometers on 48,600 watt-hours. Today we got… drum roll… 60.53. That’s a 5.5% efficiency improvement, 805 or so watt-hours per kilometer instead of 850. After that we recharged for a couple hours and drove the short distance back to the Hab, which we had to drive past before. And here we are.

Five point five percent helps, but not one hell of a lot, especially when you consider there’s going to be a lot of terrain where we won’t be able to move without those two extra wheels. And critically, we drove more or less in our own tracks going back to the Hab in that second test, so at least a bit of that efficiency improvement is down to not having to slow down to pick a way around obstacles. (And there’s going to be a lot more of those where we’re going than there are in Acidalia.)

In short, we can only disconnect two motors if we can count on a really long, mostly level stretch where we can just barrel on through. Otherwise it’s not worth the hassle.

Now, to be fair, the issue isn’t really power consumption so much as power generation. You can put up with shitty efficiency so long as you have fuel to throw at the problem. And we do have an advantage in that right now Mars is getting rapidly closer to the sun, and will continue to get closer during the trip. To make things better, Schiaparelli is almost on the equator- 3 degrees south latitude. That means, if anything, we’ll get a slight gain in power from the solar panels as the trip progresses.

But that’s not enough. We don’t know what Mars will throw at us next. We might break down for days for some reason. We might find an obstacle NASA hasn’t spotted from space that makes us detour. We might have more dust storms- autumn is the beginning of the main dust storm season, as the southern hemisphere warms up and gets really active. We really need that seventy kilometers a day.

So let’s look at current ideal, best-case recharge rates. With eleven good hours of recharge time in a sol, if we use them all we’re guaranteed of a full battery. But my math says there’s very little margin. If we lose more than half an hour of prime recharge time, we don’t start the next day on a full battery.

I say eleven hours, but that’s not precisely true. There’s almost twelve hours of good charging daylight each day. The problem is, I use an hour of it each day for driving, in addition to the pre-dawn drive. If I reconnect the solar cells and drive with constant recharging, I get about 3.6 pirate-ninjas in that hour, or about enough power for three and a half kilometers more. The less efficient charging right at dawn would probably stretch that to four, which requires maybe an extra nine minutes of driving. Push it any farther, and it becomes unsustainable.

I can’t throw more solar panels at this. We only have six spares, and anyway with the saddlebags and roof storage already accounted for there’s no place to put them.

Maybe Starlight Glimmer could stick those solar power catchers she made for the cave farm on top of the panels. Not the same ones, of course. The solar panels are lightweight and can’t stand to have a big fucking slab of quartz sitting on top. But maybe a thin layer of glass…

Maybe I see a way out of this. Yeah. Time to talk to the man with the plan… or the unicorn with the horn… or something.

Author's Notes:

The exact efficiency curve of electric motors depends much on the motor and its power rating. It's not far off a horizontal line, but that's only after the first twenty percent or so. An electric motor moving less than twenty percent of its rated load, according to my reading, really is shit for efficiency.

Next Chapter: Sol 419 Estimated time remaining: 7 Hours, 14 Minutes
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