If I remember correctly from the presentation, Must claimed radiation was not a significant concern with the exception of things like solar flares. Can anyone comment on the accuracy of that?

Well, it's not nice out there, but hardly crippling. Cosmic radiation on the ISS is about 150 milliSieverts a year. Interplanetary space could get up to about 800 mSv a year. Not good, but if you can get there in, say, three months, then you've cut your exposure down to what an ISS crewmember would get in a year. The actual danger is solar flares, and for that you'll need some kind of shielding compartment onboard. The ISS is reasonably well-protected from those, but they'll really roast you if you're caught in interplanetary space.

I think Musk mentioned it somewhere in the presentation, but the initial plan for shielding if you're aware that there's going to be a solar flare is to angle the spaceship so that the engines are between the Sun and the crew compartment, and then have everyone packed near the water tank... or something along those lines (it might have been in response to a question in the Q&A time afterwards; I don't remember).

I don't know that they'll have this on the first trips there, but I'd guess that magnetic shields might eventually be a standard component:

http://space.stackexchange.com/questions/3772/how-much-power...

I mean authors have been using that idea for years. The Red Mars trilogy for example (which was an at the time scientifically accurate example of mars colonization).

Each of the stuff for which we read here people jumping and yelling "visionary!" "revolutionary!" "unforeseen breakthrough!" "brand new plan!" "disruptive improvement!", has been dealt with dozen of times in hundreds of SF novels in the last 80 years, which explored all the possible problems (problems are a more exciting matter for a novel than everything-goes-fine-and-as-planned).

But since for most of those people, SF and anticipation is limited to Star-Wars + a couple of other movies/series and super-hero block-busters, they are not aware, and all these stuff appear as genuinely unprecedented technical and sociological thought breakthrough.

Yes, I expect that more advanced active shielding will eventually be developed - particularly since you can detect solar flare events before the radiation arrives.

The challenge with flares is that the radiation can come in from multiple directions, so a cylindrical water container that people can pack into is probably a better defense than just the bulk of the engines.

You need radiation shielding on Mars as well. Unless you live underground, I suppose.

Perhaps that's one of the advantages of using a really big ship; you can put more shielding / water tanks between the sun and the passengers.

If it's a major problem, they might just orient the ship in a direction that gives the best radiation protection to the passengers and just leave it like that for most of the trip.

Why would it hurt the momentum?

Because it wouldn't be as sensational if he said it cost $10 million to go vs $200k.

The idea of the $200K ticket price is only once the system is mature and well-tested. The early trips will have to cost a lot more. Probably a lot more than $10 million a seat.

Where is that price structure and timeline laid out?

In the presentation: https://youtu.be/IFA6DLT1jBA?t=42m29s

Early trips would of course not have a large crew, since they'd be mostly focused on setting up a base, and be more about cargo and equipment. Saying "several tens of millions" is because that's generally what it costs right now to get someone to low Earth orbit. If we can get early trips to Mars down to what it would cost for a current orbital mission, that would be very impressive.

The graph he's showing at the point you linked to shows the max cost around $550k to Mars, which fits with some extremely simplified ballpark math for making the trip efficient:

Start with ~$30 million (based on "several tens of millions") to get a person to orbit using current tech. Current tech doesn't use reusable rockets, and the cost of building a rocket greatly exceeds the cost of the fuel for the launch, so we'll assume fuel costs are 0. If you make a rocket that you can reuse for 10 launches, your price tag is down to $3 million to get a person to orbit. The next step is to make a bigger rocket: currently the Soyuz rocket is the only rocket taking people to the ISS (assuming this is roughly what you mean by getting someone to low Earth orbit), and it can only take 3 people at a time[0]. If you make your reusable rocket big enough to take 100 people at a time, your price tag goes down ~33x to $100k to get a person to orbit. Of course, a bigger rocket will probably cost more to make than current "small" rockets -- but a big reusable rocket would have to cost 330x more to build than a modern small rocket for future seats to cost ~$30 million per ticket.

So even the first trip to Mars should be well under $1 million per person as long as they make the rockets big and reusable.

[0] http://www.nasa.gov/audience/forstudents/k-4/stories/nasa-kn...

Right, the challenge is mainly that the first missions will have to carry a lot of cargo (priced into the cost of sending the crew) and that it'll be easier to bear the cost of development and construction if the early missions have a higher price.

Even if the cost for the first few trips was $50 million per person, there'd be no shortage at all of people who would be paid for by governments or organizations with that kind of money. Might as well use it, and apply these funds to expanding the program.

Full reusability is really the essential part of this system - you can't afford to throw away any stage of a rocket this big. Fortunately, the larger the vehicle, the easier reusability tends to be (up to a point of course).

Is there a natural limit to this? If you built a reusable rocket to hold a million people, would it be essentially free?

Well, there's only so many people you can fit onto a rocket of a particular size. And eventually increasing rocket size gives you diminishing returns, so there's an upper limit. Once you've used a rocket a few hundred times, though, most of your costs become things like fuel and oxygen, pad maintenance, and rocket refurbishment.

There are practical limitations from engineering and economic perspectives. From the engineering standpoint, architectures tend to run into scale problems as you make things bigger.[0] For example, if you take an existing rocket design and decide to make the same thing except 25% taller, while keeping the other proportions the same, you'd actually increase the mass of your rocket by a factor of 2 (width and depth also increase by 25% -> 1.25^3 = ~2). From the article I linked to, breaking strength varies with cross-sectional area rather than mass, so the rocket's breaking strength would only have increased by a factor of 1.56. So your rocket design might no longer be sturdy enough, and it might warp or break or explode when you try to launch it. (Breaking strength isn't the only factor to consider, either; there are tons of factors in materials science that would affect the many components of a rocket.)

The implication is that you'll probably want to design an entirely new rocket if you want to make one that's significantly bigger than what you've made in the past, because rocket designs aren't going to be fully reusable at significantly different scales.

That's where the economic perspective comes in: rockets are expensive, and the bigger they are, the more expensive they'll be, from a raw materials perspective at a bare minimum. The hundred person rocket that Musk has proposed would already be significantly larger than any actually constructed rocket to date, so it has to be a new design, which means they're going to have to build and test prototypes. And some of the prototypes will probably blow up during testing -- hopefully not many, or they'll run out of money and won't be able to keep going.

Now if you want to build a rocket that can hold a million people, you'd probably be talking about the largest man-made structure ever. The only structure on this scale that I'm aware of (which was never built) is the X-Seed 4000, which would have housed 500,000 to 1,000,000.[1] Its proposed height was 4000 meters, which is about 33 times taller than the proposed SpaceX interplanetary rocket. If you scaled up the SpaceX rocket design to that height, the mass would increase 36000x, and the breaking strength would only increase 1000x. I think we would probably need some major advances in materials science to be able to make a rocket that big that didn't just disintegrate on launch.

Secondly, what about the economics of a million person rocket? The X-Seed 4000 would have cost roughly a trillion dollars (in 2016 USD) to construct, and it just has to sit on the ground. I'm going to assume that at the barest, most insane minimum, it would cost at least 10x as much to build a rocket that size. So that's 1/7 to 1/10 of global GDP (in 2014)[2] -- for one rocket -- that will probably disintegrate.

But! Let's assume that we make major materials science advances and can actually make it work, and we make it reusable up to 10 launches, and somehow we get the whole planet to agree to build this thing. The price tag actually isn't so great... $10 trillion / 10 million people = a million dollar price tag per ticket. So maybe my estimate is too high and we could build a rocket that big for a trillion, no more than building a building that big. Even then you're still looking at $100k per ticket.

So, in short... My guess is that there might be some technological and economic sweet spots that are a bit bigger than than the SpaceX design, but I think it's a good starting place in terms of feasibility. And yes, in practice there are some limits to how big you'll make your rocket.

PS. I don't think "breaking strength" is likely the most relevant factor for rockets (I an not a rocket scientist), but it's the principle of the scaling thing that matters. Plus a rocket has to have supporting structures to help it keep its shape, so breaking strength might be relevant after all.

[0]https://www.av8n.com/physics/scaling.htm#sec-strength (I linked to the section on how breaking strength varies with scale because it seemed the most relevant to rocketry, but the whole page is useful for explaining many of the ways in which things vary nonlinearly with scale).

[1] https://en.m.wikipedia.org/wiki/X-Seed_4000

[2] https://en.m.wikipedia.org/wiki/World_economy