Last month, SpaceX CEO Elon Musk made a presentation before the International Astronautical Congress in Adelaide, Australia, a followup to his 2016 talk where he unveiled SpaceX’s Interplanetary Transport System and his ambitions to colonize Mars. This year’s presentation showcased some updates and design changes to his plans and the rocket itself — he wants to put all of SpaceX’s resources into the BFR, and use it to provide long-distance travel around planet Earth.

On Saturday evening, Elon Musk hosted an Ask Me Anything session in r/space, where he answered questions about his the ITS, the engines that power it, and more.

BFR AMA on r/space in 2 hours — Elon Musk (@elonmusk) October 14, 2017

Interplanetary Transport System

The key focus of the AMA was on the ITS vehicles that Musk hopes to use to travel between Earth and Mars — otherwise known as the Big Fucking Rocket. SpaceX made some changes to the design between the 2016 and 2017 presentations, and one user asked about the change to a more cylindrical shape. Musk noted that he wanted to get the “best mass ratio” and that “the propellant tanks need to be cylindrical to be remotely mass efficient and they have to carry ascent load, so lowest mass solution is just to mount the heat shield plates directly to the tank wall.” Some users also asked about the changes to the design of the propellant tanks, going from a series of spherical tanks to something more elongated. Musk replied that it was to “avoid/minimize plumbing hell, but we don't super love the current header tank/plumbing design,” but that “further refinement [are] likely.”

Another user observed that the 2016 design appears to have its wings and heat shield integrated into the airframe, while in this year’s presentation, they seemed to be modular, prompting them to ask if could be detached. Musk pushed back on the characterization that they were delta wings, and said that they’re not designed to generate lift, but to ensure that the ship “doesn't enter engines first from orbit,” and to “provide pitch and yaw control during reentry.” However, he didn’t really speak to whether or not they could be detached. In another question about the wings, someone noted that while there were two on the ship, there wasn’t a tail (noting that the space shuttle had one), and asked how vertical stabilization worked for the rocket. Another user jumped in, saying that the shuttle didn’t really use it for reentry, and that it’s likely that the BFR doesn’t need it either, to which Musk affirmed, and followed up by saying that “tails are lame.”

Another user asked about the heat shield, asking if it was mounted on the ship or embedded into the skin of the rocket, to which Musk said that they would be mounted to the primary tank wall.

Other questions concerned the cargo that the ship would carry between Earth and Mars, with a user asking about what the BFR tankers might carry: will they go over empty, or will they include extra propellant? Musk replied that the first tanker will “just be a ship with no payload,” but that he plans to build a rocket with an “extremely high full to empty mass ratio” that will be used as a tanker, adding that it “will look kinda weird.”

Keeping the ship’s propellant cool during the trip to Mars was the source of another question: one user pointed out that a rise in temperature in the tanks would eventually boil off the fuel, and asked insulating the tanks will be enough, or if they would require liquid methane and oxygen to keep cool. Musk explained the tanks will be insulated, but that they might add a cryocooler.”

When it came to landing the rocket, someone crunched some numbers of the available thrust and weight of the rocket, and asked if the BFR would perform a “hoverslam” landing when being used on Earth. Musk noted that the “landing will not be a hoverslam, depending on what you mean by the ‘slam’ part. Thrust to weight of 1.3 will feel quite gentle. The tanker will only feel the 0.3 part, as gravity cancels out the 1. Launch is also around 1.3 T/W, so it will look pretty much like a launch in reverse....”

Another minor change between the 2016 and 2017 renders was the number of landing legs, going from three to four. When asked about the change, Musk simply replied, “because 4,” and followed up with “Improves stability in rough terrain.”

Finally, someone asked one of the big questions left unanswered by Musk’s 2016 talk: what about the threat of radiation for astronauts? Musk said that “Ambient radiation damage is not significant for our transit times,” and that the BFR would just “need a solar storm shelter, which is a small part of the ship.” He followed up by saying that Buzz Aldrin is 87, seemingly implying that his short trip to the moon and back left no lingering damage.

Raptor Engines

Another major topic that users focused on was on the ITS’s Raptor engines. During this year’s presentation, Musk unveiled a scaled-down version of the ITS, one that utilizes 31 Raptor engines, rather than 42, as initially planned, along with some other changes. One user asked why the Raptor engine was reduced from 300 tons to 170 tons of thrust. Musk first said that they chickened out, but explained that “The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk.” He went on to explain some of the thinking behind it:

In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

Another user asked for an update on scaling up the prototype Raptor engines to their final size. Musk replied that scaling up the engines is the easy part, and went on to explain that their objective “is to meet or exceed passenger airline levels of safety.”

The flight engine design is much lighter and tighter, and is extremely focused on reliability. The objective If our engine is even close to a jet engine in reliability, has a flak shield to protect against a rapid unscheduled disassembly and we have more engines than the typical two of most airliners, then exceeding airline safety should be possible.

This is, he noted, critical for SpaceX’s ambitions to use the BFR to travel around Earth. “The advantage of getting somewhere in 30 mins by rocket instead of 15 hours by plane will be negatively affected if ‘but also, you might die’ is on the ticket.”

Another question was about the engine’s autogenous pressurization system, in which the propellent is pressurized in the tanks, rather than using a system using helium. The user asked if the engines would use a heat exchanger system, where the propellent is heated in the engines, with hot gas routed back into the tanks to help pressurize the propellent. If that was the case, would all of the raptor engines would use them? Musk was a bit snarky about this, saying that they’ll be using the Incendio spell from Harry Potter, but followed up with a “yes and probably.” Another user asked about the control thrusters, and if they’ll be “derived from Raptor or from SuperDraco engines,” to which Musk noted that they’ll be “closer in design to the Raptor main chamber than SuperDraco and will be pressure-fed to enable lowest possible impulse bit (no turbopump spin delay).”

One detail Musk unveiled during his talk was that the BFR has two different types of Raptor engines: four that can operate in a vacuum, while two others are designed to operate at sea level. When asked if those vacuum engines could work at sea level in the case of an emergency, Musk replied that they could but that it wasn’t recommended.

Finally, someone asked Musk whether the rocket engines would be 3D printed, given that the company already has experience 3D printing with its SuperDraco engines. Musk replied that some components would be printed, but that “most of it will be machined forgings.” He went on to note that the company has “developed a new metal alloy for the oxygen pump that has both high strength at temperature and won't burn.”

Mars

Another line of questions was about SpaceX’s ambitions on Mars, and how the company would set up infrastructure for its colonists. The first question was about whether or not SpaceX would put more satellites around the planet, or if the satellites in place would be sufficient. Musk answered yes, but it’s not clear what question he was answering. He also didn’t quite answer a question about specific landing sites for SpaceX’s base, saying only that the site needs to be in a “low altitude to maximize aero braking, be close to ice for propellant production and not have giant boulders.” He also noted that somewhere near the equator will be good for solar panels.

Musk did say that SpaceX will design the ISRU (in-situ-resource utilization) system designed to collect materials for propellant on Mars and that the design is pretty far along. Someone also asked if they’ll transport one of the boring machines that Musk has been using with his Boring Company, to use to mine ice or build tunnels. Musk only replied “more boring!,” so take that as you will.

Another big question about SpaceX’s Mars plans have been how the company would keep a colony supplied: one user asked “what companies are you working with to provide the technology that SpaceX isn’t focused on?” Musk didn’t really provide any details on the first two points, but did lay out that their “goal is get you there and ensure the basic infrastructure for propellant production and survival is in place,” comparing it to the development of the transcontinental railway. Importantly, he noted that industry on Mars would “need to be built on Mars by many other companies and millions of people.” It looks as though Musk will have to get help from other companies to realize his vision of a self-sustaining colony.

When asked about the illustrations of the Martian city in his 2017 presentation, where some rockets are depicted as permanently anchored, Musk said that we shouldn’t “read too much into that illustration.”

Preparations for 2022

Finally, Musk answered a questions about the preparations to get to Mars. One was about what to expect between now and 2022: would SpaceX perform hop tests with the BFR, build new facilities, and test out propellant? “Yes, yes, and yes,” was his reply.

He elaborated by saying that SpaceX will start off with short, hops of a “few hundred kilometers altitude and lateral distance,” and then will begin “orbital velocity Ship flights, which will need all of the above.”