Day 19 - Power without People
We’ve identified that goal #1 after landing is getting enough power to the ships to keep them healthy. We sketched out a preliminary design for unloading a tractor, trailer, and sets of solar panels. Now, let’s dig a little deeper. This is still work that SpaceX will need to do, but having a general idea of what it needs to look like will inform our designs.
The first question is, “How much power does a ship need if there’s no crew?” A NASA Spaceflight Forum study (https://www.nasaspaceflight.com/2018/08/evolution-big-falcon-rocket/6/) suggested a 100 kW figure for a ship with crew, but since power is so critical, using this figure for every ship gives us a valuable safety margin. Absent better information, we’ll use that. Recall our NSP D6M panels provide approximately 214W apiece. (See https://www.mitsubishielectricsolar.com/images/uploads/documents/specs/NSP_350W_355W_360W_lo_res.pdf) With dimensions of 1m x 2m x 35mm, they mass 23 kg each. On Earth, they would weigh about 50 pounds. On Mars, they’ll weigh 19 pounds—technically, pounds force, or lbf. That’s 86N, but I’m betting that’s a meaningless number even for those of you in metric countries. (An interesting aside—since Mars has only 0.38 Earth gravity, what units will permanent Mars residents use when they talk about how much something “weighs”? On Earth, we say something is X kilograms, but we unconsciously factor in 9.81 m/sec2, so we have a sense of what 23 kilograms “means.” It won’t mean the same thing on Mars.)
These panels will need to be placed autonomously, which will mean something between a robot arm and crane. This won’t be an ISS-style Canadarm; rather, something more like an industrial robot crossed with a backhoe. With a requirement to only move a 19 lb. panel, this can still be a fairly lightweight setup. A spreader seems to be the most practical grappling solution; for only one panel, that can be a lot lighter than our ship’s crane. The arm will need the ability to switch between the spreader and other types of tools.
Sticking with the D6M panels, we’ll need about 467 of these to provide 100 kW power. Of course, that’s just average irradiance, so we’ll be looking for margin. We’re using these panels to charge batteries, initially aboard the ships and eventually within the base. A couple of interesting article at https://www.washingtonpost.com/news/energy-environment/wp/2015/05/01/what-backing-up-your-home-with-teslas-battery-might-be-like/?noredirect=on&utm_term=.04359f447eba and https://en.wikipedia.org/wiki/Tesla_Powerwall#cite_note-wp2015-05-13 provide some useful specifications.
First, it appears we want 400-450V for charging Tesla-manufactured lithium-ion batteries. Conveniently, the D6M’s can be wired in series to obtain a variety of voltages up to about 1000V, or in parallel to increase current. Twelve in series will give us 450V at 5 amps. (We’ll round down in all these power calculations.) This “voltage segment” package will amount to 276 kg, or 100 “Mars pounds” on the trailer. We’ll put a junction box at the start of each voltage segment. For deployment, let’s assume rows with 5 voltage segments of 12 panels each, making the row 60 panels long. Nine of those rows gives 540 panels and 115 kW. Each 12 panel voltage segment is wired in parallel across the 9 rows, so we have 108 panels providing 45 amps at 450V per power segment. The five power segments feed the ship independently. This provides redundancy, and means our junction boxes, cabling, etc. only need to handle 450V at 45 amps constant power.
To store this power, a Tesla Powerpack 2 is spec’d at 200 kW. It’s only rated to -30C, but that just means it needs to be aboard the ship or in a partially heated part of the base when it comes time to offload. More importantly, it’s only 1.6 tons and 2.2m x 0.8m x 1.3m. That’s 616 Mars pounds, which shouldn’t present a serious problem for our crane, when the time comes. For redundancy, let’s initially plan on two for the ship, with each ship bringing two more for the base. (Until it is time to offload, we probably want the base Powerpacks also tied into the ship grid.)
540 panels masses 12.4 tons...which is a reminder just how much flexibility we have with the scale of these ships. Adding in four Powerpacks is another 6.4 tons. (Technically, the ship will need battery storage regardless, but it’s more conservative to count them all as cargo.)
We previously outlined generally how we would deploy these panels. With a few more details worked out regarding number of panels and layout, we can return to the deployment scheme. First, for reasons we’ll get into in a later post, I’m going to recommend sizing the trailer at 2m x 3m. Second, it seems that we’ll significantly simplify the deployment process if each voltage segment is pre-wired in series. A stack of twelve 1m x 2m panels, the junction box, and associated cabling is a reasonably sized bundle that should easily fit on our trailer. Stop #1 for the tractor trailer is the ship’s fixed fin, which seems the perfect location for the power connections. The tractor unhooks, rolls around back, grabs the power cable, and plugs it into the ship. After reconnecting to the trailer, it heads out to the designated location for the field, unrolling the main power cable as it goes.
An open question is how far the power field should be from the ship. We may learn enough for planning purposes from the Starship hopper tests, but regardless, my sense is that the fields should initially be located close, to minimize autonomous, battery-sucking driving. Once additional ships arrive with crew, we have years to relocate them to a safe distance from launch exhaust.
So, the tractor unrolls the main cable to a distance of, say, 50m. That’s far enough to let equipment pass easily between the ship and power field, but close enough to minimize driving distance. It swings right, stops, and unloads the junction box using the robot arm. (This might require another unhook, drive around, rehook.) It then unloads the first panel. Viewed from the rear, the panel is set down on the left side of the trailer, away from the ship. If we use a tripod support, the left side of the panel has small feet to grip the soil. As the panel lifts away from the one underneath it, the support leg on the panel’s right side pops open under spring pressure. (It will probably require some type of protection at the end so the leg won’t scratch the underlying panel.) To the panel’s rear, it’s connected to the junction box via power cable. From the forward end of the panel, a power cable connects it to the rear of the next panel down.
From this description, the distance between panels will largely be driven by the slack needed in the series-connecting power cables. We’ll need creativity in routing the cables so they come off the trailer cleanly, and we may need the trailer’s left rear twistlock post to be removable so it doesn’t foul the power lines.
From here, it’s lather, rinse, repeat. With creativity, we might be able to carry two 12-packs of panels and two junction boxes on each load. Regardless, when the trailer is empty, it heads back to the ship to receive the next load. With this design, the only cable connections that need to be made are between each row’s junction box.
We briefly dove to this very detailed level for a reason—even this level, which took me a couple weeks iterating to work out, is actually just scratching the surface of our design. Just to identify a single detail we still need to work out, those power cables are going to need insulation that won’t deteriorate under high ultraviolet light and will stay flexible at -60C. Such materials are available, but they aren’t exactly off the shelf. http://www.appstate.edu/~clementsjs/polymerproperties/plastics_low_temp.pdf gives a short introduction.
I’ve used Mitsubishi solar panels in these calculations because their specs are readily available online. That’s certainly a possibility, but it seems a lot more likely that Musk will want to contract with Tesla, which now owns Solar City. “Likely,” but not certain. One obvious concern is that Tesla is a public company, and SpaceX may be looking for a deal. I could imagine a third party wanting the cachet of providing a Mars (or Moon) solar power field, and being willing to buy in so low that Musk could be worried about the reaction of his Tesla stockholders and the SEC if he countered. Ultimately, though, none of that matters. Someone will build the panels, junction boxes, cables, etc. They’ll need to operate at lower temperatures than Earth, and be suitable for autonomous deployment. But the specs we’ve seen pretty much tell us what the art of the possible looks like, so the mass and volume constraints should only get better. Where we’re at seems reasonable as a first cut.
Similarly, this assumes SpaceX buys Tesla battery technology, which again has a certain logic, but it’s certainly not the only option. For starters, liquid metal batteries scale extremely well, are low-cost, very safe, and the “waste” heat they generate is anything but waste on Mars. Check for an easy introduction to this technology. For now, we’ll continue with PowerPacks only because we have readily available specs to use.
We do have one major, unexamined issue: how much power reserve do we really need in the event of a months-long dust storm? Short answer, “a lot more than days.” We’ll come back to this problem, but as a sneak peek, our answer will revolve around utilizing Nostromo’s immense power field and storage banks.