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SpaceX founder Elon Musk discusses his plan to build a “Big Falcon Spaceship” at the International Astronautical Congress in Australia. Musk followed up with an “Ask Me Anything” chat on Reddit. (SpaceX via YouTube)

SpaceX CEO says he “chickened out” and made the design for the monster spaceship he’s planning to send to Mars a little less monstrous — in order to make the concept a lot more realistic.

He also confirmed that testing for the BFR, euphemistically known as the “Big Falcon Rocket,” would begin with suborbital short-hop tests on Earth.

Musk’s frank talk came today during an “Ask Me Anything” chat session conducted on Reddit’s space forum. More than 10,000 comments streamed in during the session, which Musk had promised to hold in the wake of September’s unveiling of the latest BFR concept at the International Astronautical Congress in Australia.

Musk says the BFR would be used not only to send settlers to Mars starting in the 2020s, but also to go on trips to the moon and other interplanetary destinations, deploy and retrieve satellites in Earth orbit, and take passengers on suborbital space trips anywhere on Earth in an hour or less. In short, for any space mission that SpaceX has in mind.

Redditors were anxious to get into the technical points of Musk’s BFR design, and Musk was only too happy to oblige. Here are some of the high points:

  • Musk joked that the maximum thrust of the BFR’s methane-fueled Raptor rocket engines was downsized because “we chickened out.” Actually, the downsizing was roughly in line with how much the BFR’s overall size was reduced since the first version of the design was unveiled a year ago, Musk said. He noted that more engines with lower thrust provided more versatility, plus a bigger margin of safety in case one of the engines failed.
  • The initial test flights of the BFR will begin with the combination upper stage and passenger compartment, known as the BF Ship or BFS. “Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance,” Musk said. Those tests would be followed by orbital-scale BFS flights, and then tests of the full BFR, including the BF Booster.
  • Some parts of the BFR’s Raptor engines would be 3-D printed from metal, but most of the parts will be machined forgings. “We developed a new metal alloy for the oxygen pump that has both high strength at temperature and won’t burn,” Musk said.
  • So far, only scaled-down prototypes have been test-fired, but Musk said it should be “very simple” to scale up the engine firings to full power. “The objective is to meet or exceed passenger airline levels of safety,” he said. “That will be especially important for point to point journeys on 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.”
  • Musk made clear that the BFR would not glide through the air the way the space shuttle did, and thus it’s won’t be necessary to have large wings or a tail. “Tails are lame,” he wrote.
  • One Redditor asked how much radiation shielding the BFR would have. “Ambient radiation damage is not significant for our transit times,” Musk replied. “Just need a solar storm shelter, which is a small part of the ship.” Then he added a quip that appeared to downplay the health effects of space travel: “Buzz Aldrin is 87.”
  • Another asked how much food, water and infrastructure would be sent along with Mars settlers. “Our goal is to get you there and ensure the basic infrastructure for propellant production and survival is in place,” Musk said. “A rough analogy is that we are trying to build the equivalent of the transcontinental railway. A vast amount of industry will need to be built on Mars by many other companies and millions of people.”
  • Musk said SpaceX was already “pretty far along” on the design of a processing plant for making oxygen and methane fuel on Mars. “It’s a key part of the whole system,” he wrote.
  • Musk would love to have a high-bandwidth communication link and planetary internet at Mars. When one of the Redditors geeked out on the subject, Musk called that person a “nerd” but provided additional musings: “It would make sense to strip the headers out and do a UDP-style feed with extreme compression and a CRC check to confirm that the packet is good, then do a batch resend of the CRC-failed packets. Something like that. Earth to Mars is over 22 light-minutes at max distance. Three light-minutes at closest distance. So you could Snapchat, I suppose. If that’s a thing in the future.”
  • He had a two-word answer for a question about whether he’d put a tunnel boring machine on the first BFR flight heading to Mars, perhaps to mine water ice and minerals there. “More boring!” Musk said.

In addition to geeking out over his BFR vision, Musk slipped in a couple of wry side comments: For example, he joked that he planned to use Harry Potter’s Incendio spell to heat up the BFR’s propellants and auto-pressurize the tanks. In another exchange, he acknowledged that he was “actually drinking whiskey right now.”

“Well, that explains your answers,” one of his fans on Reddit wisecracked.

For more, check out this table of Redditors’ questions and Musk’s answers, or scan the entire AMA transcript.

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