Next, let’s look at the one big assumption we already made—the ISRU plants. In “The Case for Mars,” Zubrin describes the ISRU prototype plant he built at Martin Marietta in the early 1990’s (pp. 153-156, and see http://www.marspapers.org/paper/Zubrin_1994.pdf). A 20 kg plant was demonstrated that could generate 400 kg of LOX/liquid methane, using hydrogen feedstock, a simulated Martian CO2 atmosphere, and 300 watts of power over a ~500 day run. Nostromo will need more—a lot more. Specifically, it will need 1,100 tons of propellant per ship, or nearly 3,000 times more than the Zubrin/Martin Marietta prototype plant. Just doing the algebra, it’s actually 2,750 times more propellant, which means the 20 kg plant becomes 55,000 kg. That’s 55 tons, without considering Zubrin’s point that scaling up the plant makes it more efficient, since instrumentation and control equipment become a smaller portion of the overall effort. The 1994 paper concludes 10 kg is a realistic goal for the pilot plant, but does not examine the mass of tank insulation, lining, circulation pumps, refrigeration, and all the sort of things that come into play when you scale up a deep-cryogenic storage tank by a factor of 3,000. Previously, we arbitrarily allocated 100 tons to the ISRU plants, so 55 tons each appears practical. We’ll assume we can shave 5 tons off the mass of each plant. We’re at 100 tons since we need two plants to fill the tanks for both Constellation and Finity’s End in time for their scheduled return. More importantly, each plant will need 825 kilowatts of power, which is why nuclear reactors are often mentioned as a prerequisite for ISRU propellant production.
What sort of solar power field is required to generate 825 kilowatts of power? Round figures, if it were in Earth orbit that would be 8 times the size of the International Space Station’s solar panels (which generate 84-120 kW total). Of course, Mars only receives 43.3% of the solar irradiance of Earth, so this suggests each solar power field is sized with an equivalent to 1.9MW at Earth, or 19 times the ISS capacity.
ISS is a handy reference, but these solar panels are on the ground. So, again the question is posed: what sort of power field is required to generate 1.9MW of power? What’s the mass, what’s the area, what’s the volume?
Visiting https://mitsubishielectricsolar.com, we find a variety of commercial grade solar panels rated down to -40 degrees Celsius. Obviously, we’ll need more than that, but this is a good starting point. The neo solar power (NSP) D6M series provides consistent voltage down to 200 W/m2 irradiance, well below Mars’ 590 W/m2, just reducing the current as the irradiance decreases. The 350W rating applies to Earth irradiance, around 1000 W/m2, which gives about 9 amps. Just to keep the math simple, we’ll use their 350W version, which tips the scales at 23 kg and has approximate dimensions of 1m x 2m x 35mm. At Earth irradiance, it’s spec’d at 38.34V and 9.13 amps, which yields 350.6 W. These values are comparable in similar products.
At Mars irradiance, the spec sheet indicates about 5.6 amps, still 38.3 V, or 214 W. Now that we’re looking at power production with Mars numbers for the equipment, we no longer need the 1.9 MW equivalent; we’re actually interested in what it will take to provide 825 kW with the gear we’ve selected (remembering we need this much for each plant).
To get 825 kW, we’ll need 4,000 of these panels (3,855 to be precise, but let’s go with the round figures and end up with 856 kW). That works out to 8,000 m2 for our solar power field, or about one and a half football fields (end zones excluded). The algebra gives a total mass of 92,000 kg, or 92 tons, for the entire field. And, we need two of these.
At a first approximation, this is alarming, because we only have a remaining budget of about 50 tons, based on 100 tons for the twin ISRU plants. But, these panels are designed for Earth gravity, Earth winds, and Earth snow. We won’t face any of that on Mars, so the design can be much lighter. Can we get it down to 50 tons? A lighter structure is obviously possible, but a large proportion is made up of the photovoltaics and glass. It seems unlikely we can reduce the mass this much.
We do have options here. First, Zubrin assumed 500 days; we’re arbitrarily assuming 550. We’ll need to be more precise about which synod and which trajectories we’re really going to use to get any closer on this approximation; besides, we’re only talking 10% difference. Second, if we don’t have enough power to get the job done, then could always just send one ship back. There is certainly some suggestion that SpaceX intends to leave quite a few ships on Mars. Lack of power for the ISRU plants could be a very practical reason.
There’s another alternative, though. The whole reason to work out a six-ship mission architecture was to replace variables with constants. We previously argued for one manned ship and one freighter per cycle, but then eliminated one freighter in favor of a refinery ship. The math now suggests we need to modify this plan, and sent a freighter along with Heart of Gold and Nostromo. That freighter’s sole mission is to carry enough solar panels to fully power Nostromo’s twin ISRU plants. With 150 tons available on the freighter, and 50 tons available on Nostromo, we’re now at 200 tons. That’s more than our 184 tons, which our initial algebra suggests, so we’re well within the engineering ball park. If we can’t get the ISRU plants and solar power field working within these budgets, we might be forced to skip one ship’s return until we can bring more solar panels. But, at a first approximation, this seems to be an achievable engineering goal. There are plenty of challenges, but a reasonable budget to solve them. There will be plenty of time later to argue over kilograms.
Based on a variety of sources, as of late 2018, this architecture is actually a little slimmer than SpaceX’s proposed plan. The press reporting is for two robotic freighters in the first synod (we’re planning for one freighter and one ship with life support) and four ships in the second—two robotic, two with life support. The architecture here requires one ship less, so we’re still very much on the conservative approach. However, these power calculations are specific to the ISRU plants, which is essentially “reserved” cargo. We still need to look at the power for the base and its equipment. That will be for a later discussion.
The change we’ve just made also leads to a new question—the name of our new freighter. To keep things simple, recognizing this is a bit of a one off, but also helping us remember this ship solves a significant problem by bringing Solar panels, let’s call this freighter Serenity.
Back on our reference architecture, Serenity will accompany Heart of Gold on its cycles back to Earth. There’s a nice symmetry here.