[vannevar]

Don't fool yourself, there is a lot more to landing humans on Mars than building a big launch vehicle. The only data point we have is the Apollo project, and Mars is an order of magnitude more difficult. Apollo was around $150B in today's dollars, so if anything, the $100B estimate for Mars is low. Which at this point puts SpaceX closer to Mars One than it is to Mars itself.

[Robotbeat]

The situation is wildly different. A half century of progress has been made, and SpaceX is taking a much more affordable approach to the whole thing.

We used expendable rockets for Apollo. And there were at least 6 different stages, depending on how you count (3 stages for the Saturn V, plus the command module which had a propulsion section and a reentry module plus the lunar module which had descent and ascent stages), all of which were expended. There were at least 3 different propellant combinations with even more engine types. All of these components were basically dedicated to the Apollo program and had to be paid out of that budget. Each mission requires the expenditure of all those stages.

The BFR for SpaceX, on the other hand, is just two stages. The two stages are very similar, using the same propellant type and basically the same engines. BFR will be used for satellite launches initially, so its development can be paid for separately. The advantage of BFR is that it is fully reusable, so you literally can use the same rocket to send stuff to Mars as is used for satellites. The biggest part, the booster, never needs to leave a few hundred kilometers from the launch site, and can be reused for satellite launches. SpaceX is able to get by with just two stages because they simply refuel the landable upper stage again and again instead of dropping stages. They're leveraging reuse not just to lower launch costs, but to dramatically simplify their mission architecture (and thus reduce development costs).

The reusable upper stage also serves as the lander, once refueled. And it is designed to be refueled by an essentially identical vehicle. And the lander itself can also be reused for the next Mars window. So in a single Mars mission, NOTHING needs to be expended, and at most you're just tying up a single stage during the duration of the mission; the rest of the architecture elements can earn their keep launching satellites while astronauts are on Mars.

Before SpaceX, launch costs for the US were around $20,000 per kilogram to LEO. SpaceX's F9 and Falcon Heavy have brought the cost down to around $1000-2000/kg. BFR, since it's fully reusable, could bring the costs down to $100/kg or even $10/kg. SpaceX designed an architecture that could reduce the cost to Mars by orders of magnitude and whose primary components can be developed and paid for the same way Falcon 9, Falcon Heavy, and Dragon are paid for (i.e. by delivering cargo for paying customers).

[vannevar]

There are many good reasons to rationalize away the possible costs. But there are equally good reasons to believe we're underestimating the cost by not accounting for the unknown. Experience leads me to believe that the latter case is much more likely to be true, given the singular nature of a manned Mars mission.

Full disclosure, I love SpaceX and absolutely want to see them succeed. But we should be realistic about the challenges that Mars presents.

[Robotbeat]

This isn't just rationalizing away possible costs.

I'd argue that we are more fully prepared for a human Mars mission now than we were for Apollo when Kennedy made his speech. We have decades of operational experience operating robotics on the planet and decades of on-orbit human spaceflight experience with durations of the same order of magnitude as a Mars mission. In contrast, the US had just shot someone on a suborbital flight into space by the time JFK gave his speech, and we had next to no understanding of the Moon's surface or long-duration human spaceflight.

I've thought intensely about Mars. I think a human Mars surface mission is easily within reach if the right architectural approach is used. SpaceX has a particularly good architectural approach. Using a reusable upper stage also as a tanker and a lander is a game-changer.

Scaling that up to a sustainable mass settlement initiative, on the other hand, is absolutely full of unknowns, as you say. That remains a crazy proposition without guarantee of success.

The first human mission is primarily about launch, entry/reentry, and logistics. SpaceX has a good handle on those and has retired a lot of risk by demonstrating supersonic retropropulsion (in a portion of the atmosphere approximating Martian conditions) and landing almost two dozen times, and I think SpaceX has a good plan to mitigate the rest of the risk.

EDIT:Another thing: SpaceX is building the lander and the rocket anyway and plan to pay for it just by switching over all their services from Falcon 9, Heavy, and Dragon. BFR is also big enough to serve as the transit hab and surface hab. So by just consolidating F9, FH, and Dragon to the BFR vehicle, they'll have 95% of the Mars architecture already built and paid for. The lone additional thing they need is ISRU: the mining equipment, the power production (primarily solar as it'll be cheapest), and the chemical plant. Chemical plant and solar are well-understood as both have been tested at small scale either on Mars or on ISS or similar, but the mining equipment only has some not-terribly-representative testing by scientific sample handling on the various landers and rovers of Mars. That is a question the first uncrewed mission will need to address.

[greglindahl]

I don't recall saying that all there was to landing humans on Mars was a big launch vehicle. Perhaps you're arguing with a straw man instead of the people you're replying to?

[vannevar]

And that target's a lot less than $10B; most of the BFR engineering is not about Mars but about a fully-reusable launcher, which is useful for launching anything, not just going to Mars.

Sorry, I read your comment to read that you thought the cost would be much less (as little as their target of $10B), since they had already developed the launch vehicle.

If your point was just that if they can't do it for $10B, they won't, that may be valid but it's not the issue under discussion. We're talking about how likely it is that SpaceX will go to Mars, not whether SpaceX continues as a going concern.