Life support, landing sites, water mining, ISRU propellant production…there are dozens, if not hundreds of things that would benefit from prototyping in precursor missions. Of all those, three items stand out as “must do” if SpaceX’s plan is to become real: 1) on-orbit, large-volume refilling, 2) long-term storage of cryogenic propellant in carbon-fiber tanks, 3) reliable operation of large, reusable spacecraft. Everything else is simply expanding what we already know how to do, and tailoring it to Mars.
Let’s take each in turn.
There are some obvious possibilities with regards to on-orbit propellant transfer. ISS routinely transfers fuel from Progress supply ships, but that’s on a small scale in comparison with what SpaceX is proposing. A BFR needs 1,100 tons of LOX and liquid methane. The international standard docking port, which includes fuel transfer lines, simply isn’t going to cut it for this purpose. There are also a lot of questions about keeping the liquid settled over the pumps, etc.
In Musk’s 2017 IAC presentation, the presentation showed a much better solution than the one shown in 2016. In 2016, the ships were side-by-side, requiring all sorts of additional docking mechanisms, pumps, etc. In 2017, they dock back-to-back, and now with thrusters, we can simply use “gravity feed” for propellant transfer, augmenting with pumps if needed. Thrusters let us maintain head pressure at the turbo-pump inlets; pumps let us significantly speed propellant transfer.
We can test gravity feed versus pumps using derivative Falcon 9 gear—two second stages configured with pumps, extra fuel, thrusters, and docking ports. They transfer propellant back and forth under varying conditions of thrust. If we make them reusable, then we can run more excursions. Musk has hinted he finds the idea of reusable Falcon 9 second stages tempting, but doesn’t want to be distracted. Well, here’s the excuse. Once we have a good sense as to how much force we need to apply, we’ll test that on the full-scale tanker and ship, and iterate until we get it right.
A much harder question than on-orbit refilling is how the carbon fiber tanks of the ship will stand up to years storing LOX and liquid methane. This is hard because ultimately, the only way to be sure is to expose the tank material to the cryogenic liquids—and temperature cycles—for a long time. I wouldn’t be surprised to learn SpaceX already has subscale experiments in progress. Ultimately, we need to be sure the tanks will handle the cryogenic liquids indefinitely. It isn’t enough that they last an entire mission, or even two or three. For the ships to be reusable for 30 years, their tanks must handle LOX/liquid methane for 30 years, because the only time they won’t be full of propellant is during their six-month transits. The rest of the time, some tank will be slowly filling with liquid oxygen or liquid methane—whether that tank is on Nostromo or one of the cyclers.
While on Mars, the propellant needs to be liquid, but not necessarily deeply cryogenic. Additional refrigeration can occur shortly before launch, if necessary. (It’s unclear whether SpaceX’s figures show deep-cryo methalox is absolutely necessary for Mars-Earth return, but that features prominently in the discussion.) Ultimately, this is a materials problem, driven by the physics of cryogenic propellant storage.
Oxygen’s boiling point is -183C, but SpaceX’s desire is to use LOX just above the melting point, which is -219C. Mars rarely gets below -150C, so additional refrigeration will be required, either way. Methane’s boiling point is -161C and its melting point is -183C. So, deep cryogenic LOX could be used in a heat exchanger to drop methane to just above its melting/freezing point, but we’ll still need liquefaction equipment. (Check “Hampson-Linde” cycle on Wikipedia; also “liquid air.”)
On Earth, liquid natural gas is kept cold by allowing a small amount to boil off. The energy of evaporation transfers heat out of the liquid. The same technique can be used here, with active refrigeration then used to re-cool and liquefy the gas, and then return it to the tanks. I leave it to cryogenic fluid experts to recommend the best technique for returning the re-cooled liquid, and whether it’s best to keep the liquids static or mixed. Of note, it appears the Amos-6 mission was lost because the liquid oxygen actually froze, so we should spend extra time examining the implications of this. In particular, what long-term temperature do we want to use, and what do we do when the time comes for extra cooling prior to launch? Another obvious question is what temperature the propellant fluids should be at when they are transferred from Nostromo to the return ship. Given the scale of this industry on Earth, it seems a safe bet these are easy questions to answer. Only the tank material interaction is in question.
The third major problem almost sounds a bit absurd…operating giant reusable spaceships? That’s all? But in all seriousness, this isn’t just “make the science fiction work.” This is a very specific problem: SpaceX is going to build very big ships. They will have lots of new systems. Establishing what works, what breaks, and what the malfunction scenarios are will be much like the early days of trans-oceanic flight. Early on, designers wondered if long-range airliners should be designed to float if they crashed. The answer was that a better solution was to ensure they didn’t crash. Early on, overwater flights required four engines, in case one failed; even today, there’s a special name for long-range twin engine operations overwater—ETOPS (Extended range, Twin-engine Operations). Rocket engines have a tendency to explode when they fail; jet engines only fling their fan blades into the wing and fuselage. By comparison, getting hit by a shredded engine blade is minor. But jet engines are still designed to contain their debris, and they are tested for containment using deliberately damaged blades. We need the same concept for rocket engines.
Disasters happen. Airliners crash. We won’t have to worry about the most common problems—hitting another aircraft in the vicinity of an airport, controlled flight into terrain by a distracted crew, even spatial disorientation. But, we will need to establish what fails, how often, and how catastrophically. Multiple engines shutting down can be handled with software. One engine exploding may well destroy the vehicle. We need to figure out how and why that happens, and take ruthless steps to ensure it doesn’t or protect the rest of the ship if it does. So, heavily instrumented equipment with very conservative shutdown protocols seems the right approach—fail safe, in this case, is also fail-op. And the only way to be confident we’ve thought of the likely failure scenarios is to fly those big ships over and over, off Earth and back to Earth, preferably unmanned. Losing a $200 million ship is terrible; losing a $200 million ship and a 4 person crew could be mission-ending. Of course, once you have that part figured out, then a 4 person crew saving a $200 million ship gives you heroes with the Right Stuff. That won’t be a landing scenario, but could well occur on orbit or in deep space.
There are other issues, and we’ll go through them in turn, but these are the deal breakers.