A Hacker’s Mars-Ascent Vehicle

DIY_Mars_Rocket Mars One is a project that hopes to send the first colonists to Mars. It will be a one-way trip, which vastly simplifies the technology compared to what’s required for a round trip (and makes it cheaper). Cynics assert that the thousands of people who have volunteered to be colonists must be out of their minds, that the trip is some sort of suicide mission. I disagree. And in the spirit of full disclosure, let me explain that I’m on the Board of Advisors for Mars One. So, I would be expected to disagree with that view.

There are many motivations for becoming one of the first settlers on Mars, none of them insane, in my opinion. They include a noble sense of self-sacrifice, if you believe that extending human presence into the solar system will help ensure the survival of the species; a desire for the immortality that comes with fame, despite the risk to life and limb—in some sense, putting yourself at risk helps ensure your influence on the history of humanity will outlive your physical being; a desire for personal accomplishment, or overcoming a challenge, the same sort of thing that drives people to join expeditions to the summit of Mount Everest; self-interest, including the prospect of making money as a Mars entrepreneur, or helping your family and friends back on Earth do the same.

Here’s the thing. I reject on its face that this trip is a Kobayashi Maru scenario. One of the first things I would do upon landing is to begin building the tools, the equipment, and even the economic structures to one day construct an Earth-return vehicle. Such a hope would not be irrational. In fact, it’s easier to lift off from Mars and enter Mars orbit than to do the same on Earth. I’m confident that the right group of engineers could pull this off over the course of a couple of decades, at least returning to cislunar space. And remember—they’ll likely have Internet access, probably with the help of lasercomm. So, the folks back home can send them schematics, analyses, even test results, to help make that effort a success.

Let’s take a look at what a space-hardware hacker might build to enable his or her return journey. But not just one hacker. Let’s say three people will plan to make the return trip in a single vehicle.

In coming up with a design, we’ll have to keep in mind that exquisite, custom components will be unavailable or in short supply. The hacker is unlikely to find spools of carbon filament for high-tech composites, an unlimited supply of cryogenic valves, electric motors, circuit boards, or other high-precision special-purpose parts. Maybe a few items can be sent from Earth, on demand. I wouldn’t count on it.

What we can count on is a ready supply of water. As much as the hacker wants. NASA has documented the abundance of water on Mars many times over. It’s just beneath the surface. Meteor impacts expose it in the form of an ice-strewn field fairly often. So, hydrogen and oxygen are no problem. They’re available by electrolysis, which any kid can achieve on a small scale. So can graduate research assistants at Cornell University.

We’ll start by keeping it simple, with a single-stage-to-orbit design. The hacker’s goal is to return to Earth in a human-habitation module sized for three people. NASA’s Mars Design Reference Architecture 5.0 mission (circa 2009) sizes a two-stage system for six astronauts. We’ll cut that mass exactly in half: the structure, solar panels, the food, everything. We’ll take the added penalty of carrying the mass that DRM 5.0 associates with the first stage all the way to orbit. So, the vehicle we anticipate that our hacker will build is conservatively heavy at 26,025 kg (without propellant). After all, it’s to be expected that the Mars hacker isn’t able to build something as weight-optimal as NASA contractors can. Also, our design doesn’t need methane and liquid-oxygen cryocooling hardware required by DRM 5.0, which our hacker probably couldn’t build anyway, saving considerable mass, and adding even more margin for error.

It doesn’t need that cryogenic hardware because the hacker uses gaseous hydrogen and oxygen as propellant. That gaseous mixture results from electrolyzing 89,000 kg of water, which fills four spherical aluminum tanks (each about 3 m in radius). The image at the top of this post shows the vehicle, including its tanks, sized about right. These tanks are just over 2 cm thick, holding H2 and O2 gas at 10 Earth atmospheres of pressure (over 1,000 N/m2, for those of you on Mars right now taking careful notes). At that pressure, material in a spherical tank experiences about 50 MPa (megaPascals) of stress, which represents a factor of safety of 2.5 times what the welds in the aluminum can withstand (1.5 margin). Let me emphasize that this concept is not a critique of DRM 5.0. It’s what you or I might do if you didn’t have expectations of that level of sophistication and, frankly, safety.

Where would the hacker get these tanks? All four total just over 20,000 kg. I would suggest sand-casting many identical equilateral triangles and welding them together to form a geodesic approximation of a sphere. We’ll still need to ensure that enough aluminum is available. It would have to come from recycling bits of the lander, or other equipment. Alternatively, combining available aluminum powder with water produces H2 gas and alumina (aluminum oxide, or Al2O3). That’s a ceramic that has very nice properties, and it’s strong enough for this application. Its main advantage is that because Al2O3 is half aluminum (by weight), using alumina stretches the aluminum available to the hacker. A downside is that the tanks must be assembled by sintering, a kind of 3D printing technique, and that’s more time-consuming that melting down, casting, and welding aluminum triangles.

Yet another solution would be to create watertight inflatable sacks. Cotton fiber has a very high strength-to-weight ratio. So, worn-out t-shirts, lint from the clothes dryer in the Mars hab, and some sort of rubberized liner might also do the trick. The result might even be lighter than metal tanks.

But for this exercise, we’ll stick with these aluminum geodesic spheres. In any case, testing will be essential. The test campaign includes launching smaller tanks of water into Mars orbit. You’ll see why in a moment.

Fueled up, with three passengers and 1,325 kg of food, the hacker’s Mars ascent vehicle weighs 133,000 kg. The water-electrolysis propulsion system in development at Cornell is simple, requiring no solenoid valves, cryogenic plumbing, or other subtleties. Its specific impulse approaches 371 sec. That efficiency is lower than cryogenic H2/O2, but the simplicity can’t be beat. Furthermore, the gaseous mixture is stoichiometrically ideal (i.e. there’s just enough of each molecule to burn completely). As a gas, the combination is extremely well mixed, which eliminates the need for the combustion chamber to include an injector (which is complicated and expensive to fabricate).

The propulsion subsystem provides about 4,000 m/s delta-V. That’s enough to reach a 300 km altitude Mars orbit. The vehicle won’t stay there long. It needs to refuel, and then it heads to Earth. It refuels by docking with tanks of water that have been launched to orbit over the past two years. Those tanks refill the now-empty aluminum tanks on the ascent vehicle. The water is much denser than the gaseous H2/O2 mixture. About 493,000 kg of water can fit in those tanks. So, the resulting vehicle’s delta-V capability is far greater. The electrolyzers that the hacker has been using on the surface of Mars are installed in each tank before liftoff. After the water is on board, these same electrolyzers slowly bubble the water into combustible fuel.

Water propellant can impart over 10,000 m/s delta-V to this spacecraft. It does so much more slowly than the gaseous mixture did at liftoff, but that’s OK. Unlike lifting off from the surface, the return trip does not require a lot of thrust at once. Low thrust delivered over many months would do it. According to Bobby Braun and his colleagues, that 10,000 m/s is well above what’s necessary to return to Earth, more than twice what’s needed (depending on the timing). So, this design forgives a little imprecision in hacker navigation, and a little underperformance in other areas.

The spacecraft spins. The resulting centripetal acceleration separates the water from the gas. The plumbing pulls the H2/O2 gas from the inboard-most point on each spherical tank, leaving the water to electrolyze. The spin need not be fast, and indeed too fast a spin speed would have bad results. These large water tanks offer a final benefit: they shield the astronauts from radiation. Water is among the best substances to keep humans safe from the space environment. And if the drinking water, even the oxygen, runs out in the habitat, our three hackers know where to find more.

It will be essential for all involved in a one-way trip to Mars to proceed with a clear understanding of the risks: that is, the probability of success (or failure) and the consequences. To me, that’s the key ethical concern: transparency. If people choose to undertake this risky activity with full knowledge of the risks, I see no ethical issues. In fact, I think we may be morally obligated to permit people the freedom to do so, and not impede their desire to realize their dreams by imposing our own fears or superstitions based on uninformed perspectives. But the real key to success in living on Mars will be the hacker ethos: make due with what you have. Be creative. Your instinct for innovation may even be enough to bring you home again.

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One thought on “A Hacker’s Mars-Ascent Vehicle

  1. Matt

    This is a very interesting post. I like the “Hacker” thinking which seems to be uncommon in the space flight business.

    You mention the Mars Design Reference Architecture 5.0 mission and latter that your idea has a lower safety margin. I wonder what would be possible with a design reference architecture done with a lower safety margin from the beginning? Would it be viable? Would informed people be willing to take the risk? I imagine it would be a lot of work to re-do the whole design. I find the whole idea intriguing.

    Reply

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