19 thoughts on “Improvements On Elon’s Mars Plans”

  2. Dumb proposal. He forgot that a fair bit of the delta v to land on Mars comes from aerobraking and likewise the return to Earth.
    For a rocket engineer he makes a pretty good car writer.
    I’m sure artificial gravity could be arranged by sending two ships with tether between them. Same for on orbit refuel. The tanker and ship dock and then rotate slowly around the mutual axis to settle the propellants. Wouldn’t need much to settle them.
    That cylindrical structure in the center of the living areas could be the housing for the carbon fiber tube that extends forwards and docks with its counterpart on the other ship forming a robust tether, transfer system and redundancy for emergencies.

  3. I’ve seen the short video of the BFR but I haven’t watched Elon’s entire presentation yet. It sounds like he just threw a basic outline out there for others to crowdsource and suggest refinements and improvements. I like that.

  4. I like the idea of min/maxing passengers on launch and then docking with a transfer vehicle with habs. Under Musk’s plan, they don’t go directly to Mars anyway. The vehicles dock in a parking orbit to transfer fuel and passengers.

    Mike Borglet suggested the other day to build a scale model of the BFS. They could do this but along the lines suggested at the link, without all of the entertainment space. Just a pure passenger vehicle. This would allow SpaceX to build up off current, or near term, launch capabilities.

    The big unknown then is the space ship. Initially, maybe there wouldn’t even be a Mars lander or return vehicle. A large number of people could be placed close enough to Mars to do real time telerobotics to prospect any future landing sites and answer some of those pesky questions some want answered before people land.

    Preferably this spaceship(s) could integrate with Musk’s Mars SSTO vision.

    Perhaps Musk needs some other billionaire to work on the in-space transfer market segment.

    1. Perhaps Musk needs some other billionaire to work on the in-space transfer market segment.

      I’m starting to remember MirCorp. I wonder how long it will be until we see a commercial space station. I hope soon after Dragon V2.0 and Boeing CST-100 are available.

  6. If you’re going to have a non-aerobraking hab, you’re adding at least 2110 m/s of Δv to Mars and 3600 m/s back to Earth. If you’re assuming the same Mars-bound payload, then getting that extra 58% Δv is going to take your mass ratio from 14 to about 64, even assuming magic extra tanks that weigh nothing.

    One thing I will say for this design, though: It almost looks like it could have launch escape. With Elon’s, not so much. There’s a lot to be said for launching your 100 passengers like spam in a can if it means that you don’t occasionally blow them all up. Of course, my guess is that you occasionally broil/flambée them during aerobraking anyway, so maybe that’s not adding that much risk. A lot more cameras on Earth to watch it happen, though…

  7. No launch or in flight escape on a Boeing 777 and we sometimes lose them or throw them carelessly at the runway at SFO due to crew inattention.
    Also no escape during re-entry on any spacecraft up to now.
    For the scale model BFS just put a Dragon V2.0 on top.
    Anyone got any idea of the likely terminal velocity of the BFS near the Martian surface before rocket powered landing? Just a wild guess says maybe 500 meters/sec?

  8. Oh, the design has so many issues it’s hard to know where to start.

    Even empty, the crew section is going to have a re-entry loading of about 200 lbs/square foot because it’s very large and filled with lots of stuff, Toss in propulsive landing and it’s probably coming in around 400-600 lbs/square foot. It isn’t aerobraking so much as a crashing meteor burning through the atmosphere. It’s a cube law (mass) versus square law (area) sizing issue. For comparison, Curiosity came in at about 42 lbs/sq ft and Apollo was about 100 lbs/sq ft.

    The requirements for Earth launch and re-entry are quite different from those on Mars, so Elon’s plan makes about as much sense regarding mass budget as early lunar direct ascent proposals. Don’t send a ship capable of Earth ascent and descent all the way to Mars because it’s not optimized for Mars.

    1. Are you worried about the structural loading or the ballistic coefficient? If you assume 450 t payload, 150 t dry mass, and (rectally extracted) 20 t landing propellant, you get 620 t.

      Max diameter is 17 m, so let’s go with avg = 15, and effective length = 48ish m. For a half-cylinder (we can fill it with spherical cows!) aspect ought to be roughly A = πrL = 1131 m².

      I think the Space Shuttle had a drag coefficient of about 3 in the high-AOA hypersonic region, so I’m going to blindly use that.

      That gives us β = M/(C_d * A) = ~180.

      For comparison, they were estimating Red Dragon at about β = 300, so we’re not doing terribly.

      One of the big takeaways from the Red Dragon study was that you had to have pretty good lift-to-drag to get the reentry chord length long enough to avoid auguring in, and I suspect the strategy with this is pretty much the same.

      Again, I can’t speak to the structural issues, but it looks plausible ballistically.

      1. I think your math may be off.

        I calculate the cross sectional area at around 700 sq meters (7500 sq ft). I think you may have roughly calculated total surface area.

        I can’t find my earlier reference to dry weight, but I also doubt it could have a dry mass that’s only a third of the payload, as the Shuttle’s payload was only 40% of the dry mass, for an orbit mass about 3.5 times larger than payload mass. That would indicate an unfueled orbital mass of about 3.5 million pounds. (It has a theater inside)

        The 450T payload alone would give it a loading of about 130 lbs/sq ft. A Space Shuttle orbital weight/payload ratio would give it a loading of about 460 lbs/sq ft.

        The hypersonic drag ratio of an infinite cylinder is 1.33, while a sphere is 1.0, so I’ll guess a Cd of 1.2.

        That gives a ballistic coefficient of β = 530 to β = 4,100 kg/m^2

        If we can firm the numbers up, perhaps someone will crunch the numbers on re-entry heat load and max heating rate, given some reasonable guess at re-entry angle.

        1. I think you’re pretty close to right–certainly closer than I was.

          Yes, I did forget to multiply by sin α, which looks like about 60° from eyeballing the pretty reentry picture in the deck, so that makes A = 980 m²

          Seems like a finite cylinder ought to have Cd greater than an infinite one (more flow separation), and remember that we’ve got the little stubby wings at high α as well. I’m willing to back off to Cd = 1.5. (We’ve now reached the “Middle Eastern bazaar” phase of aerodynamics…)

          My prop estimate is way too low. Red Dragon used a prop mass fraction of 32%. If you scaled the same (which is probably ballpark, because there’s a pretty narrow window to fool with), then prop mass would be 282 t, and total entry mass would be 882 t.

          That gives the new β = 882 t / 980 / 1.5 = 600.

          On the other hand, Red Dragon assumed L/D = 0.27. A real-live lifting body’s going to do substantially better than that, so maybe we’re still in the ballpark.

          On the dry mass: I’d expect Elon to be more hyperbolic on the amenities than on the dry mass and payload, so I’m taking those as givens. He’s clearly making some extrapolations about the state of the art in terms of carbon composites five years from now, but he tends to be pretty good at that sort of thing.

          Bottom line: I don’t think his numbers are crazy, but they’re certainly… aspirational.

  9. George, if it is that obvious how come Elon has this plan? He has a bunch of smart people working for him and he said this was from their CAD drawings. Surely someone did his/her homework?

  10. Maybe they did, maybe they didn’t. SpaceX hires a lot of people straight out of college who are going to be good at math but not necessarily knowledgeable about all the old design studies that have gone before.

    My loading numbers are from the cross-sectional area (given the proposed length and diameter and a guess at the shape) and the stated dry weight.

    Given it’s shape and loading, I’d say a successful re-entry and vertical landing is rather improbable. They were probably based on studies they did on re-using a Falcon upper stage. But the F9 2nd stage dry mass divided by its cross-sectional area is only 16 lbs/sq ft, not 200.

  11. I’m not an aerospace engineer, but it seems to me going with this BFS plan is leaving lots of hardware that could work and probably for a lot less, behind on the shop yard. What is so wrong with devising a colonization plan around the Red Dragon? How about figuring out a way to land Dragon trunks on the ground with tankage and a Raptor engine or two capable of putting a Red Dragon back on an Earth return trajectory assuming you have already landed the heavy equipment needed to stack a Red Dragon atop it and have already landed the equipment needed to re-fuel using ISRU?

    As was previously mentioned you could tether some Dragon derivatives and spin along a co-axis to provide artificial gravity during the transit phase. I was just so surprised by the BFR/BFS approach. I had always assumed the BFR would be to get much much more of Red Dragon derivative & Red Dragon derivative support hardware (like tankers) into orbit an onto its way to Mars. Not a total do over. Kinda of contrary to the tradition that SpaceX itself started with, build a little, test a little, operate a little and refine as you go….

    BTW this link reminds me much of the Nautilus-X approach. One I like very much. Gotta say I’m skeptical of the current Musk plan. I suppose however it does good to get the discussion going. After all the original Apollo proposal had everything landing on the moon and lifting off from there as well. One of the reasons the Service Module engine was so oversized for its eventual role.

  12. You know, the Dr. Evil in me says an opportunity could be had to launch a LUNAR colony by an NGO or even NASA, using spare F9H and Dragon hardware contracted to be built by SpaceX while its R&D is off in this wild pursuit of BFR/BFS to Mars…

    If NASA could just get the US Senate off it’s back…

  13. Space cadets always think their own chosen architecture is best. But what if this things flies as described, except it can’t be made reliable enough? Maybe it’s only 95% and every twentieth flight blows up.

    It’s still a fully reusable LEO space shuttle with a cargo bay of 300 ton payload capacity that can operate unmanned. Even losing five percent of them over time, it’s still cheaper to operate per kg than anything to have come before, $100mln or so per 300 tons, charging the lost flights in the price, assuming they cost $2bln apiece.

    Want to launch comsats with it? Put a big hypergolic tug in the cargo bay that can deliver your satellite to GTO and then come back to the payload bay for return to Earth for reuse. Want to fly manned? Put an abort to orbit ejection cabin in place of the payload bay, and fly. With refueling, go to the Moon, go to Mars if you want, just with a less passengers and cargo.

    The only issue I see with this is, as described each booster needs its own LC, but Musk did briefly mention offshore platforms in his presentation. I wouldn’t worry about the winds of Mars, even if it got there, but I might want to worry about the hurricanes of Earth. There will need to be weather enclosures, I think.

    Build this, or something like it, and we might go to the stars one day. Or we can just take Sean Penn’s advice and get it over with.

  14. What constraints did Musk put on the design?

    Fully reusable. 100 mt/crew to the surface of mars in 3 months. Get the ticket price below $200k.

    Given those constraints there are probably better ways.

    But first someone has to prove it’s even possible. Including proof that we can live on mars. Otherwise no competition will develop and the talking is endless.

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