SpaceX Mars Plans: Jon’s First Take

I’ve had a lot of friends ping me today about my thoughts on Elon’s Mars talk today. I was in a meeting when it happened, and literally was pinged by half a dozen people during the meeting… Now that I’ve had a time to chew and digest things a bit, here’s a bit of a stream of consciousness take on the plan.

Overall my feelings are mixed. I think the plan has a lot of good points, could probably work given enough money, and would likely be a better use of money than whatever NASA does for its so-called Journey to Mars, but also am really skeptical about a lot of the technical choices made, and the likelihood of hitting the price points SpaceX is predicting.

First, what I liked (some minor, some major):

  • In-Space Refueling: Elon’s plan is built around the idea of refueling his ITS (Interplanetary Transport Ship) in LEO prior to Mars departure. The BFR would have to be a lot bigger, and/or 3STO, to loft a fully-fueled ITS without LEO tanking. This is a point that ULA has been making for over a year now as well–“Distributed Lift” allows you to do a lot more with a given sized booster. As someone running a company trying to develop the rendezvous/capture and propellant transfer technologies needed for Distributed Lift, this was a welcome choice.
  • Mars ISRU: Bob Zubrin has been beating on this drum since I was a teenager, and while I think Elon’s handwaving the challenge of generating 1000s of tonnes of propellant on the Martian surface, not having to haul that all the way from Earth saves a lot of launch mass to LEO, and makes reuse of the lander a ton easier.
  • Lifting, High-Alpha Mars Entry and Supersonic Retropropulsion: By coming in sideways, and doing a somewhat lifting reentry, they’re able to bleed off enough energy early on so that the supersonic retropropulsion delta-V can stay modest even though ballistic coefficients suffer from the square cube law. If they had tried a base-first, low-lift entry, they probably would need a lot more landing propellant.
  • Reusability in General: I’ve always been a fan of both in-space and earth-to-orbit reuse. I think it’s a key to get costs anywhere close to what Elon wants.
  • Depots for Extension Beyond Mars: I also really liked Elon’s point about how his architecture can eventually include depots in various orbits, and that once you do that, you could theoretically use these vehicles for traveling almost anywhere else in the solar system you want. Many of those flights would require more radiation shielding, and possibly some level of artificial gravity to work out, but he’s totally right that once you have depots, the solar system is your oyster. Regular readers of this blog won’t find that too surprising.

Now for what I didn’t like as much (some minor some major):

  • Too Big: Something 3.5x the size of a Saturn V seems like overkill. Doing a BDB-sized launch vehicle but with near-SSTO mass ratios and the highest chamber pressure large propulsion systems to ever fly is a challenge, and I think SpaceX is likely underestimating the challenges with doing an RLV that size, and with that many engines. One thing I wonder about is if they’ve done the acoustics analysis on launching something that big. We’re talking about something several times bigger than Saturn V or Shuttle, and an order of magnitude bigger than the Falcon 9 powered landings they’ve already done. May be a non-issue, but I think both Bezos and Musk are making a mistake with how big of vehicles they’re going after.
  • Swiss-Army Knife ITS: Trying to make the ITS do so much doesn’t bode well to me for keeping it affordable. We’re talking a near-SSTO performance level reusable upper stage, who’s ascent and landing propulsion has to double as a rapid-response launch abort system, with a what amounts to a 100 person space station on top of it. Many SpaceX fans seem to think that slapping more requirements onto the most challenging piece of the overall architecture will somehow save costs compared to developing two or three more optimized system elements, but I’m really, really, really skeptical. This seems like repeating one of the dumbest mistakes from the Space Shuttle.
  • Vertical Mars Landing: I know SpaceX is sticking with what they know, but the crew and payload section on ITS looks like it’s ~30 meters off the ground level. That’s some really long ladders and/or elevators or cranes, which aren’t going to be light. Heck, ITS makes the LSAM look reasonable when it comes to payload accessibility. I know a horizontal powered landing approach ala DTAL/XEUS would require additional engines and potentially landing complexity (depending on what changes you made to the rest of the architecture), but they would probably require less mass than hauling everything down from what amounts to an 8-9 story tall building.
  • Not Refueling In Mars Orbit: I don’t have the exact EDL delta-V requirements for ITS, but it’ll probably take close to 1/3 of the Mars Orbit mass in propellant. Aerocapture/braking first into Mars orbit doesn’t really change the vehicle requirements much, it allows you to cut down on the TMI mass by 1/3. Launching ITS from the surface with only enough prop to get to Mars orbit, and then refueling in Mars orbit, can also dramatically cut down on the overall ITS size. I’m not sure which mission phase is the driver for the ITS propellant tanks and landing engines, but I wouldn’t be surprised if in-space refueling in Mars orbit didn’t cut down both on the overall size of ITS per passenger complement, or if it cut down on the IMLEO of the system. And really, you already have to develop ITS tankers and ISRU on the Mars surface. Just ship a tanker or two along with your original landing group. Use the prop it shipped for refueling the other landers, making sure to leave enough for it to land empty. Even without an actual depot in Mars orbit you can take advantage of that. No new tech, no new elements, but likely a decent savings right from the start. Refuel Early, Refuel Often.
  • Methane Uber Alles: I think Elon oversells his case on how awful Hydrogen is compared to Methane. Sure, for his specific architecture, Methane might make more sense, but I can think of many other architecture where LOX/LH2 could probably be quite competitive for all but maybe Mars Ascent and Landing. Sure, if you insist on having one vehicle do it all, sticking with one propellant makes sense, and sticking with one like Methane probably makes your life easier. But there are so many assumptions baked into that logic chain.
  • Expensive Work In Process Inventory: The major cost driver on the mission is the ITS, because it can only do a Mars trip once per synodic period at best. This is somewhat the nature of the beast for Mars travel–whatever you send to/from Mars is going to take a long time to get there and back, which means you’ll have to amortize its cost over a lot fewer missions. Which is why it would seem like you would want to minimize the cost of the assets that get tied up like that. Having your Swiss Army Knife vehicle be the one that can only fly 12 times in a half century seems like a poor way to optimize for the problem.
  • No Landing Gear on the BFR Stage: I know that my colleagues at Masten also really like the landing-cradle approach, but I’ve never been a fan. Is it doable? Sure. Does it save a lot of cost and time when it works? Sure. But how reliable is it really? I can’t honestly answer this question, but my gut suggests that if your vehicle has a decent chance of surviving landing on an unprepared surface, there are going to be many situations where an abort, a large last second disturbance, or some other error could be survived when a gear-less vehicle is toast. Missing the landing cradle by 10m due to a last-second engine-out scenario probably means loss of vehicle and major pad repairs, where with landing gear it’s a non-event. We could totally build jetliners today without landing gear, using landing trolleys or other things. Or fighter jets landing on sleds on carriers. But we don’t because there’s no way aircraft would be as reliable as they are without having things like landing gear that give them options when something goes off-nominal. Let me put it this way–I don’t think you’re likely to ever see an RLV design that can survive long enough to average 1000 reuses (like SpaceX has baked into their BFR economics) without including landing gear.
  • Crazy Raptor Performance: 4500psi chamber pressure with a LOX-rich preburner sounds like a recipe for fun engine development. This is probably doable, and the Russians eventually tamed RD-180 class engines which have almost as high of chamber pressure, but how reliable will they really be, and how long lived will they really be? I just worry that SpaceX is trying to have its cake and eat it to, by pushing the bleeding edge of performance always, while also trying to push down manufacturing costs and up reusability at the same time. My guess is something is going to give–either the engines will end up being more expensive and finicky, or less reliable than they’ll really want for such a system, or nowhere near as reusable. I just don’t think 4500psi staged combustion seems like a good recipe for a 1000 flight engine. And the failure modes of a 4500psi staged combustion engine when you have 42 of them on your first stage also doesn’t sound likely to be graceful. I could, and genuinely hope I’m wrong. Once again repeating some of the mistakes of the Shuttle by trying to push crazy performance out of their first attempt at a staged-combustion rocket engine.

Ticket Price Economics Thoughts:

  • Per-Vehicle Costs: BFR costs seem to be assuming a per kg cost less than half that of F9 FT. Which seems optimistic to me given the higher performance, more complex engines, and the use of composites for the propellant tanks, and the general scale of the thing. Once again, trying to do a Big Dumb Booster with bleeding-edge performance. But it’s the ITS that I’m really skeptical about. You’re really going to make a spacecraft that has to have all the life support capabilities of ISS, but for 16x the crew, and cram it into a high performance upper stage, a reentry/landing vehicle, and all of that for less than half the cost of a 747-8? Especially given that you’re likely only making a tiny fraction of the number per year. Dragon currently costs probably $30-40M each to produce, and we’re saying that a Dragon designed for 14x the number of people, and 30-50x the duration, with a nearly Saturn V first stage class propulsion system built in is going to only cost 5x as much? Color me extremely skeptical. I think that they’d be lucky to have the production cost of an ITS with all of its subsystems necessary to get 100 people safely to Mars and return reliably down below $1B each anytime in the foreseeable future.
  • BFR Reuse Numbers: I’m also really skeptical they’ll get BFR reliability or engine life high enough to get anywhere near the 1000 reuses they’re claiming. I think they’d be lucky to average 100 flights each once again through the foreseeable future, based on the technology choices they’ve made.

The upshot is that if I’m right on those three items, you’re still talking less than $500M per Mars mission, and a ticket price in the ~$5M per person price range. That’s still three orders of magnitude better than what NASA could realistically do with its architecture. So while I think Elon doesn’t have an architecture that really gets down into the “cost of a median US house” range, he is getting into a range that a lot of people could afford. Having a Mars architecture this affordable would still be absolutely amazing, even if I think it could be done better.

How would I do things differently? Honestly I haven’t put as much thought into it as Elon has, but I have a few high-level thoughts:

  1. Split Things Up More: I’d separate TMI propulsion, the actual transfer habitat, and the Mars to surface and back into three separate elements. If you combine with the next point, the TMI stage can literally just be a normal upper stage like the Falcon 9 upper stage or ACES. The transfer hab would want to aerocapture at Mars, but could do so with a much more modest propulsion, and the Mars landing system can be smaller and higher flight rate. Honestly I think developing three systems that are more optimally split like this will not only cost less to develop than the swiss army knife approach, but will also be lower cost to operate, and open things up more for technology advancements over time.
  2. Go a Bit Smaller: Unless Induced Torpor works out, 100 people in a single vehicle seems really big for the transport stage. Breaking things up into convoys of smaller say 10-20 person vehicles might make more sense. This would mean transfer habitats would be small enough that you could use a TMI stage that is more reasonably sized for use in Cislunar space, so you don’t need a dedicated TMI stage. The transfer habs could be small enough that you have a range of options for aerocapture (inflatable, deployable, in-space assembled, or if it pans out magnetoshell aerocapture). These transfer habs will also have a lot more in common with LEO and cislunar orbital habitats, and possibly early mars surface habitats.
  3. Post TMI or TEI Boostback: I think Dave Masten and I have discussed this in the past, and Robert Zubrin hit on it today in his comments–It probably makes sense to have a separate TMI propulsion system that does the equivalence of a boostback maneuver after the TMI burn is done, to decelerate back into a highly-elliptical Earth orbit, where it can then aerobrake back to LEO. By not sending that along with the transfer hab, you enable it to be reused a lot more, since it’s not tied up for four years now. While not doing transfers to Mars, it can be sending payloads to/from Cislunar space, or to/from GEO. On a similar note, you could send one or two TMI stages along with the transfer habs to serve as TEI stages on the Mars side of things, using a similar post-TEI boostback maneuver and aerobraking to return back to Mars orbit for reuse. The more of your architecture is in the “can get 100+ flights in its lifetime” category vs “synodics mean I can only fly 12x in my lifetime” category, the cheaper things will be overall.
  4. Reusable Horizontal Mars Landers: Having separate landers, possibly smaller than the transit habs makes a lot of sense. The same landers can be used both for hauling people/cargo to/from Mars during arrival season, but also can be used for prepositioning propellant in Mars orbit, and Martian suborbital point-to-point transportation when not being used for Mars arrival landing. Horizontal landing on Mars is trickier than on the Moon, and I’m not totally wedded to the concept, but it seems like a much better way of getting people and heavy cargo onto/off of the surface.
  5. Using LH2 for More of the In-Space Elements: Once you’ve split-up Mars landing/ascent from the TMI/TEI burns, it makes sense to start looking again at LOX/LH2 for those segments. Those are two of the highest delta-V portions of your mission, so the higher performance could help. And LOX/LH2 can be made from Martian, Lunar, NEO, and possibly Phobos/Deimos ISRU sources. I know SpaceX is allergic to LH2, but most of the people I know who’ve worked with it have said “sure it’s a pain, but it’s not as evil as people make it sound”.
  6. Find Ways to Use Transfer Habs for Other Destinations: Say you can only realistically send a reusable Mars transfer hab on every other Mars window. That leaves a decent amount of down-time in-between. If the hab could be used for say taking tourists to/form the Moon or Venus when waiting for the planets to re-align from Mars, you can get much higher utilization out of the transfer hab elements. If you can take the one element in your system that currently can only be amortized over 12 flights (~50 year lifetime), and add in cislunar trips say during the “off-season”, you’re now amortizing it over 100+ flights instead of just 12. If you look at Elon’s architecture, 2/3 of the cost of a Mars ticket is due to the transfer hab’s low number of flights (the same if you make my more pessimistic hardware costs). If you could divide that over 100 flights instead of 12, that would make more of a difference for Mars ticket prices than almost anything else. Could you theoretically do this with the ITS as is? Sure, but without a source of CH4 on the Moon, you’d need to fly a lot more lunar tankers to make that work. Not impossible, but the economics aren’t as good as it would be if ITS could run on LOX/LH2.
  7. Leverage the Moon and NEOs for ISRU More: I still think there are ways that lunar ISRU can eventually beat earth-launched RLV prices for propellants in orbit. Especially if you stage out of a highly-elliptical earth orbit or EML-1 or 2. Investigating if there’s a way to tap into that wouldn’t be a bad idea, and would be a lot easier with the other suggestions I’ve given above.

Anyhow, that’s kind of off-the-cuff, but those are some of how I’d do things differently. As I said above, Elon has a lot of great architecture ideas, but I really don’t think he’s found the “One True Way” to get people to Mars as inexpensively as possible. Worlds better than NASA’s Journey to Mars? Definitely. Technically feasible? Probably. Cheap enough to be interesting? Sure. The best path forward from where we are today? That’s what I’d quibble with.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
This entry was posted in Commercial Space, ISRU, Launch Vehicles, Lunar Commerce, Mars, NASA, Propellant Depots, Space Development, Space Exploration, Space Settlement, Space Transportation, SpaceX, ULA, Venus. Bookmark the permalink.

60 Responses to SpaceX Mars Plans: Jon’s First Take

  1. commentdenied says:

    And i read all of this as a praise. Someone on twitter quipped , “dude, how are you going to take 20 TRL-2 technologies to TRL-7 in 10 years and 4 departure windows” ?

    IMO he is fishing for some sort of an arrangement for funding a smaller scale version of all this, while doing back-burner investments to keep the story semi plausible. The investments he is doing are not small, comparatively to the rest of the aerospace industry, and he is moving the needle in significant areas. But tip of the iceberg of the whole challenge, and he knows it.

  2. Would it be feasible to refuel in Mars orbit? It seems like you’d need lots of propellant tanks on the ground separate from all the vehicle tankage flying around for that to be feasible so I’m not sure it would be anything you could do until after the first 5+ rounds of colony trips.

    And I expect that the ITS delta-v number is set by it having to contribute most of the delta-v from getting into Earth orbit. Plus it should be somewhat lighter after having dropped it’s cargo off on Mars even if it’s returning with people.

    Not to say that other parts of the plan don’t sound dangerous or crazy ambitious, just that I think that part makes sense.

  3. Jonathan Goff Jonathan Goff says:

    Actually, I don’t think you need extra tanks to enable refueling in Mars orbit on the way in. There are a few ways to make this work:

    1- Fly a tanker along with a group of passenger ITS vehicles. Use the tanker to transfer prop into the landers after TMI but before landing. After that, you use the surface ISRU to refill the tanker first, and send it to Mars Orbit, then refuel the passenger ITS vehicles in Mars Orbit on the way home. Possibly with tanker landings and relaunchings in the middle somewhere. Once the return vehicles are outbound, refill the tanker and wait in mars orbit for the next wave of arriving vehicles. You’d have to runt he numbers on how many tankers to transports you need, but if the TMI + landing propellant ends up being the driver for ITS size this would help a lot.

    2- Pre-fly a robotic tanker along with the ISRU plant. Or if possible have a way of converting the ISRU plant hauler into a tanker after it has dropped off the ISRU plant (harder). Have it launch fill-up and then launch to Mars orbit before the passenger fleet arrives.

    Either way you don’t need extra hardware you wouldn’t need anyway.

    The bigger question is which mission segments drive which requirements. The problem is that when you have so many requirements, it’s often the case that they can feed on each other. Maybe the peak thrust is driven by launch abort, while the prop tank size is driven by launch to orbit (or maybe TMI + EDL). Without more details it is hard to know. But even if you can’t shrink the ITS by using Mars orbital refueling, you can probably reduce the number of LEO flights required per Mars mission, and the overall mass involved. I’d need to run more numbers, but I’m pretty confident that stopping and refueling is almost always better than not refueling. Especially if it doesn’t require any new infrastructure you don’t already need.


  4. G. says:


    Good suggestions, especially on breaking up the missions more so you don’t have a whopper of a structure that you can only use once every cycle.

  5. Jonathan Goff Jonathan Goff says:

    It is possible to reuse the ITS for other destinations while it’s waiting for the next Mars launch season, but it may be harder due to their selection of CH4 for the fuel. It would be funny if the only way to make the business case close for Mars involved lots of Lunar Tourism flights allowing the Mars transporter to amortize itself over more flights…


  6. Dave Salt says:

    Hi Jon,

    That’s a very thoughtful post and I tend to agree with all your points, especially those with respect to size and number of elements. I think Elon’s monomania with Mars has blinded him to the advantages of a flexible transportation infrastructure that could serve many more missions/destinations and so help make us a truly space-faring civilization.

    I’d also note that a launch system with the performance (payload mass/volume and flight rate) and, especially, cost attributes he’s aiming for would also enable large scale space industrialization and support the building of solar power satellites… and even O’Neill colonies!

  7. in situ says:

    Epic. That’s the only word to describe Elon’s system.

    A space capsule the size of an apartment building. A payload 4x that of the next biggest rocket ever, the Saturn. Landing a rocket the height of a tall office building on a cradle. Taking 100 Saturn loads to orbit with one booster + tanker over a couple week period, and so on.

    But the system’s epicness reaches its maximum in the three word metric: Part Number Count.

    A system to take people to and from Mars, with a part number count of 2.5, something no one has ever imagined.

    There are a lot of benefits to simplicity. Each of Jon’s alternatives, with possible exception of size, increases part number count and complexity.

    Regarding the massive size, while it increases vehicle fabrication and ground system costs and reduces launch ops overhead (per unit payload), it has an less obvious effect as well. It makes the whole endeavor more heroic, and expensive. These last two aspects may actually make it easier to get the funds needed to build it, for various reasons.

    There is one key missing piece, however: Elon did not mention a feature length movie about traveling to Mars on his system. This is my only suggestion.

    Perhaps the story of the daughter of an initial colonist who supported her father’s desire to go even though she wasn’t quite an adult yet, communicating with her father from afar, eventually going herself, being the first marriage on Mars, and so on.

    Possibilities for plots are pretty limitless, but the point is to allow people to visualize the whole endeavor in a way they couldn’t otherwise. Apollo was preceded by a movie about space travel.

  8. Dave Huntsman says:

    Good thoughts, Jon; I much more agree with you than I would quibble on any points.


  9. Andrew Swallow says:

    I suspect the design is being driven by the 150 ton dry mass of the passenger section. That needs reducing. If there are no passengers on board then launch abort is not needed reducing the number of engines.

    Passengers, consumables and cargo can be loaded into the passenger section in LEO. They could reach the spacestation using several Falcon 9s and Dragons. The spacestation could be the prototype passenger section used for the multi-year reliability testing of the life support systems.

  10. Jonathan Goff Jonathan Goff says:

    In situ,
    Parts (or elements) count isn’t the only driver of costs/complexity. The amount of requirements on each element also drives complexity. Look at the F-35. It’s also a jack of all trades. Stealth fighter, scarred for enabling VSTOL operations for the Marines, carrier landing for the Navy, etc, etc. Sure it reduces the number of fighters types needed by the various services, but stacking all of those requirements on top of each other creates a faster than linear growth in complexity.

    But epic? Sure. Especially if he somehow pulls it off. If not, it’ll be the Disney Mars mission concept that never flew of the 21st century.

  11. Jonathan Goff Jonathan Goff says:

    Thanks Dave! To be honest, I’d probably quibble with myself on several of these points. 🙂

  12. Jonathan Goff Jonathan Goff says:

    Dave Salt,
    This is why I’m keeping fingers and toes crossed for Masten’s XS-1 Phase 2 bid (and if they win it, on their execution of said bid). Somebody needs to work on a small/medium RLV to counterbalance the billionare ego-sized rockets… 🙂


  13. john hare says:

    I can visualize an engine architecture that would justify that crazy raptor performance without breaking the bank. Of course when that visualization is uncontaminated with such trivialities as experience or formal training…….

  14. in situ says:


    I’m not trying to claim that the epically low part number count of the proposed Mars transport system automatically makes it the best, I’m just pointing out that it is a totally epic aspect that isn’t getting the attention it deserves.

    Obviously, it is possible other architectures could be competitive. But if you recall, the whole motivation for big dumb booster and MCD were cost studies showing that, historically, each stage of a rocket, regardless of size, cost about the same. SpaceX takes this lesson to the extreme: a part number count of 2.5 for an entire Mars transport system, incredible. And using basically one new rocket engine.

    My feel is that this architecture is the result of a long and rigorous analysis by SpaceX. Variations that appear to have advantages, such as staging at earth escape, may result in difficulties such as getting the proper ballistic coef for aero capture at Mars, for example.

    Obviously, more modestly scaled variations appears easier, but could also be more challenging to aerocapture; hard to say without doing a lot of math. I’m guessing the short transit time/aerocapture requirements drives a lot in this design.

    Still, they need a feature length film, so everyone from potential passengers to potential billionaire investors to average filmgoers can really visualize and appreciate and understand it. Should be able to get one off the ground with $4M in seed funding. The setting is fixed, but the story is completely flexible, so it shouldn’t be hard to make it awesome, and there are no shortage of potential challenges and excitement given the setting.

  15. Nathan says:

    Jon, excellent post!

    A few thoughts:

    I think we now have the answer to how Spacex is going to recover the second stage of F9R (as we once called it). That’s something the internet has been arguing over for over a decade! I prefer that approach to the base first one as I could never see how to protect the engines from reentry or solve the c.o.g issues, especially if returning a payload.

    I wonder how much the size of the vehicle is influenced by the necessity of having a large number of raptors on the upper stage for engine out purposes? This would be needed on both ascent to LEO and landing on Mars.

    What power source is used for making propellant on Mars? They need one 20X that used in Mars direct, right?

    Biggest risk or challenge is the heatshield. Need’s to work twice once on entry to Mars and then again on Earth reentry plus have to protect during journey to Mars and 18 month stay on Mars.

    One problem with your strategy of using a dedicated Mars lander is you’d need to inspect and refurbish your lander’s heatshield on Mars. I suspect that is one of the reasons Musk didn’t go down that route. Think how much effort goes into the Shuttle heat shield inspection and then note that the MCT’s heatshield is far larger.

    One question I have is how hot does the heatshield get on Mars entry? Most architechtures for Mars missions have employed a separate heatshield – which is then jettisoned – for the Mars lander/habitat, partly because the heatshield gets very hot.

  16. Bob Steinke says:

    @in situ

    I’m curious why you say the part number count is 2.5 and not 3? Is the .5 because the passenger module and the tanker are similar, or is it because the booster doesn’t reach orbit?

  17. Nathan,

    Regarding the heat shield inspection, a lot depends on which technology base is used, and on how high of a ballistic coefficient you have. There are some options that should enable very low maintenance/inspection heat shields. As one data point, NASA Langley looked at the idea of reusable Mars landers, and didn’t think the heat shields were a show-stopper.


  18. in situ says:


    Yes, the first, because of all the upper stage commonalities.

  19. Dante80 says:

    This architecture is derided by some due to an apparent focus on it being a Mars transport system.

    Here is some food for thought. What is Elon going to do with his huge re-usable boosters and tankers between every Mars-Earth synod?

    Or, to put it in another way, what would he enable others to pay him for 20 months out of every 26?

    Think about that a little (especially if you are in the Moon, Asteroid or O’Neil Cylinder camp of HSF fanboy-ism). 😉

  20. Jonathan Goff Jonathan Goff says:

    While I”m more interested personally in the Moon, I have no problem with Mars as a destination. It’s interesting and a worthy destination too. As for being excited about BFR for lunar missions? Maybe. I care a lot more about getting regular low-cost transportation, and am not particularly picky how it is done. BFR is one possible option, but there are a lot of other options for getting to low transportation costs. We’ll see.

    As for reusing BFR for lunar missions, it’s also worth re-mentioning the point I made in the blog post that ITS is the item that needs help finding more flight opportunities between synods. Even if BFR was only used for launching ITS vehicles, it is going to fly 6x more frequently than the ITS already. And I think the ITS passenger vehicle is likely going to cost a lot more than the BFR first stage.


  21. xoviat says:


    I believe SpaceX has used this approach because, according to Musk, the tanks on the spaceship are dry once it enters LEO. Therefore, at 100 people and cargo, nothing can be downsized.

    Then, as I believe you did, you ask whether the LEO ship can be downsized. What I believe is happening is that SpaceX has realized that launching to LEO is expensive, period. Consider their customer, NASA. What is the cost per-person to LEO using their dragon architecture? Probably somewhere around $25 million.

    SpaceX may have come to the conclusion that this cost cannot be reduced significantly without adding more people, because there exists a minimum for the per-launch cost. If this is correct, it may have driven their entire design.

  22. Chris says:

    Breaking TMI burn into it’s own stage using hydrolox seems like it has a lot of hidden, or maybe no so hidden, costs. For one their carbon fiber tank solution may not be up to the task of cryogenic hydrogen even if it passes for deep-cryo methalox applications. If that’s the case it’s back to lithium-aluminum with all separate tooling structure along with insulation techniques. I know NASA was looking at CF ET at one point but not sure how that testing every worked out. Additionally, development of a separate low production (relatively) hydrogen LPRE sounds like a way to balloon the cost of the whole system, but for sure the development costs. So wouldn’t it be likely a hydralox stage would be built completely different than the booster or spaceship stages?

  23. Jonathan Goff Jonathan Goff says:

    I wasn’t suggesting SpaceX had to do a LOX/LH2 stage. I was saying how I would do it, not how I would do it if I were SpaceX. Subtle but important difference.

    There’s at least one other company already working on a pretty capable LOX/LH2 stage. Not big enough for a full ITS sized vehicle, but it could probably handle sending a more reasonable sized transit hab, especially if you used a reusable Mars-based lander instead of shipping the landing hardware through every single high delta-V maneuver.


  24. Rok Adamlje says:

    Im guessing that actual life support equipment and crew accomodations arent included into the stated dry mass.

    If TWR of the engines is anywhere near Merlin, the engine section may weigh about 15t. If so, added engines may only serve as redundancy , not nesesity for high thrust maneuvers.

    Requirement for the cargo to be palletised is a nuisance. So would be on orbit cargo transfer and even more so maintenance of the lander on mars before nesesarry infrastructure.

    That said, im not sure they can just land on any unprepared surface.

  25. Paul451 says:

    Re: All-singing, all-dancing ITS.

    My impression is that Musk’s whole original concept is based around the commonalities between the various tasks required to get from Earth to Mars and back.

    The MTI delta-v and Earth direct-return delta-v mean that the same vehicle can serve as its own upper-stage during Earth launch. To be reusable, it has to re-enter and land on Earth, and that allows it to perform EDL on Mars. The be powerful enough to do Mars ascent SSTO, it is also capable of being its own escape vehicle during Earth launch. To be able to carry sufficient payload mass to Mars for a colony, it is going to be large enough to carry the passengers.

    It’s not that they’ve shoe-horned all the different requirements onto one vehicle, it’s that the requirements feed into each other. If you’ve got a reusable upper-stage for a HLV, then you have 95% of ITS. If you’ve got a Mars-fuelled ERV, you’ve got 95% of an upper-stage. If you’ve got a LEO-refuelable HLV upper-stage, you have… well, not 95%… but certainly the propulsion system for an interplanetary transporter.

    Each task provides almost the entire solution for one of the other tasks.

    For example,

    Re: VTOL vs VTOHL.

    The VTO from Mars pretty much requires VL. How else are you going to launch the refuelled ITS back to Earth? You’re worried about unloading cargo, how do you propose to raise the ITS back to vertical for relaunch, so that the main engines are pointed in the right direction.

    VTOL ends up being the most parsimonious solution.

    Re: Shuttle-like performance demands.

    If the SSMEs or SRBs didn’t perform as advertised, the Shuttle orbiter didn’t make orbit. 100+ tonne orbiter with around 20 tonnes payload. Not much margin for performance loss.

    If Raptor isn’t quite as powerful or efficient, then maybe you lose half the payload capacity. 200 tonnes to LEO. Still a hell of a launcher for the first generation while they get the kinks out of it. And still large enough for the initial, small-crew exploratory missions. Hence, unlike the STS, you don’t need to tick every box just to reach minimum usefulness.

    Re: Size.

    Yeah. Really can’t see why you wouldn’t at least start with a Raptorised launcher on a par with FH (50-70 tonnes to LEO) that replaces the Falcon/Merlin fleet. Lets you cut your teeth on the larger system, while standardising commercial ops on a single new vehicle. Which lets other people pay you to work out the inevitable kinks in Raptor and methalox, and while you are practising upper-stage reusability. Once the first version of the BFR first-stage is running, it starts out with that reusable upper-stage, letting it be used for paying customers while ITS is in development. Then ITS starts life as a larger reusable upper-stage, then gradually develops the extra capacity for Mars.

    And on that subject…

    Re: Vehicle costs / low numbers.

    Musk seems to imply that F9 and FH will be operating alongside of BFR. That seems bizarre to me. If BFR can lower launch costs to the point where it makes Mars colonisation affordable, then LEO/GEO must be downright cheap. Sure you’re way over-capacity, but who cares as long as it’s fully reusable and doesn’t require extraordinary costs to operate. You can load GEOsats like a nanosat deployer on F9.

    I just can’t see the point in continuing with a semi-reusable launcher production like that requires different tanks, different engines, different launch systems, etc, once you have a giant, low cost, fully reusable system available.

    Re: Landing cradle.

    Scares the crap out of me.

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  27. Andrew Swallow says:

    Re: Vehicle costs / low numbers.

    Musk seems to imply that F9 and FH will be operating alongside of BFR. That seems bizarre to me. If BFR can lower launch costs to the point where it makes Mars colonisation affordable, then LEO/GEO must be downright cheap. Sure you’re way over-capacity, but who cares as long as it’s fully reusable and doesn’t require extraordinary costs to operate. You can load GEOsats like a nanosat deployer on F9.

    I just can’t see the point in continuing with a semi-reusable launcher production like that requires different tanks, different engines, different launch systems, etc, once you have a giant, low cost, fully reusable system available.

    Lorries come in different sizes from enormous juggernauts down to 1 ton payload vans. So launch vehicles are likely to follow the same patten.

    SpaceX could create a replacement for the Falcon 9 that uses Raptor engines.

  28. Ken Brown says:

    I should have thought of the acoustic aspects given my background in audio system engineering. 42 engines of that size are going to be exceptionally loud and it might be necessary to build an entirely new launch facility to handle the loads. I have many of the original papers and notes from the lead engineers that created an acoustical test apparatus to certify missile structures. The fidelity sucked, but it did get loud.

    It would be irrational to think that any hardware is going to be useful for 50 years. Ten years might be a maximum which would be roughly two Earth-Mars-Earth cycles. The Space Shuttles were designed around 50 years ago and looking at their systems now is a good indicator of how technology has changed. There will also be a lot of refurbishment of the transports. I bet it will get smelly with 100 people living in a confined space with almost no access to cleaning supplies that we take for granted on Earth. It’s not like it can air out on the way back either.

    The rocket technology is engineering and money. I don’t see any theoretical roadblocks for the hardware. I do see issues with the physical and mental well being of the colonists. It’s one thing to have a group simulate living through the trip crammed together in a mocked up module on Earth where they have a little reminder in the back of their mind that in X number of days they will get out and be able to run free again in the open air. Those going to Mars won’t have that to look forward to. I spent an afternoon talking with Ann Sutton and we discussed a wide range of health concerns for a trip to Mars. One major conclusion was that we (humans) don’t have any data on living in low G environments. None of the astronauts that walked on the Moon were there long enough to make any difference. The only thing that is know is that human bodies in 0G for extended periods of time have problems. The most likely part of the journey for problems to happen will be at Mars and the crew will need to be in good physical condition to be able to tolerate G and stressful work. Brittle bones or an abuse endocrine system will be a liability. How well does the body do in 1/6G? It would be very useful to find out. A facility could be built on the Moon to simulate Mars gravity for even more data. It could be done in space too, but the ISS operators do not want to deal with the gyroscopic properties of a spinning section and the vibrations would disturb other experiments.

    While I can think of many commercial opportunities that could financially justify establishing a colony on the Moon, I can’t think of any that would make selling a Mars trip to investors an easy task, closing in on impossible. At least not broadly enough to raise the billions of dollars it will take. A colony on the Moon would raise the TRL on a Mars venture considerably when it’s time to go to Mars.

    Having humans living on Mars tickles the imagination, but dreams don’t have an accounts payable department.

  29. Herp McDerp says:

    The thing that surprises me is that Musk doesn’t use Aldrin cyclers in addition to the crew module. They’d make life support for the voyage much simpler. Since the cyclers don’t have to land — or even slow down — at Mars or Earth, they can be large and ungainly … and you can make them spin for artificial gravity.

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  31. One of the things that jumped out at me is that there’s no way to do pad or boost-phase abort with the ITS ship. You’ve got 9 pump-fed, spark-ignited engines, enclosed by an interstage, trying to lift a 2250 t ship out of the way of 6700 t of exploding propellant? And note that 6 of the 9 engines are vacuum variants, which will survive at sea level for only seconds, if that.

    You can build an abortable capsule to ferry passengers up 100 at a time and use the BFR to do it, but then it’s essentially an SSTO, and you’re unlikely to be able to reuse it.

  32. Robert Clark says:

    In his presentation at about the 54 minute mark Musk discusses that the second stage in its tanker form or in its spaceship form will be able to reach orbit when used as a single stage. He states though the tanker will not be able to land, presumably because of insufficient reserve fuel. Then it could be an expendable SSTO.

    However, he states it could be used as cargo ship for fast intercontinental deliveries. In this case it would need to land so presumably he means this would be at speeds just below orbital.

    Also, notable is that the upper stage at about a third the size of the booster could instead be used as the booster for a smaller launch system. You would then develop also a smaller upper stage. This results in a system about 1/3rd the size so could transport 1/3rd the number of people, about 35 or so.

    This is interesting because Elon said they may have a development upper stage within 4 years, which could then be used instead as the booster. It may even be possible to use an existing upper stage on an existing rocket for the smaller ITS upper stage we now need, such as the Delta IV upper stage or even the Ariane 5 core used as upper stage. This would clearly reduce the development cost if we could use an existing upper stage.

    A quite high payload to LEO could be done using the Ariane 5 core as the upper stage in this scenario:

    3820ln(1 + 2500 ÷ (90 + 170 +240)) + 4650ln(1 + 158 ÷ (12 +240)) = 9,100 m/s

    So we could get 240 metric tons to LEO with this much smaller system.

    Bob Clark

  33. gbaikie says:

    –Ken Brown says:
    September 30, 2016 at 9:56 pm

    I should have thought of the acoustic aspects given my background in audio system engineering. 42 engines of that size are going to be exceptionally loud and it might be necessary to build an entirely new launch facility to handle the loads. —

    What about using ocean launch?
    If engines started when 50 meters above the ocean surface, how would that compare
    sound wise to launching above pit on land with it being doused with water?

  34. gbaikie says:

    “In-Space Refueling: Elon’s plan is built around the idea of refueling his ITS (Interplanetary Transport Ship) in LEO prior to Mars departure. The BFR would have to be a lot bigger, and/or 3STO, to loft a fully-fueled ITS without LEO tanking. This is a point that ULA has been making for over a year now as well–“Distributed Lift” allows you to do a lot more with a given sized booster. As someone running a company trying to develop the rendezvous/capture and propellant transfer technologies needed for Distributed Lift, this was a welcome choice.”

    It would be helpful if NASA developed an operational depot in LEO, and if NASA did this, then I think Musk should depart from Earth starting at EML-1.
    Or I think good idea to make first depot in LEO- in terms of developing it, but once there is an operational depot in LEO and want to go to Mars, one should make another one in EML-1.
    Or from EML-1, one can get to Mars in less than 3 months, and you can’t really do this very easily from LEO. So maybe from EML-1 you get to Mars in 4 to 6 months- because too expense to lift the extra amount rocket fuel to get there faster. But even if going to Mars 6 months or more there are advantage of leaving from EML-1.
    One advantage is in station keeping. Another is less risk from space debris- which also related to station keeping in terms of avoidance. Third with LEO you have to pick an inclination and leave from that inclination. Forth can get more Oberth effect starting from high earth orbit with perigee at low orbital distance. Fifth one can reuse boosters which give Earth to mars trajectories more easily [booster returns to Earth earth orbit]. Sixth could choose to reuse stages from LEO to Earth orbit. And environment at EML-1 is most like the environment of your Earth to Mars trajectory.
    It also seems that if returning to Earth, it easier to stop at EML-1 rather direct earth re-entry or Earth low earth orbit capture.
    It also seems that cheapest spot near Mars to get to is Mars high orbits [L-1 or 2] and high orbits have more solar energy available.

  35. gbaikie says:

    “…But it’s the ITS that I’m really skeptical about. You’re really going to make a spacecraft that has to have all the life support capabilities of ISS, but for 16x the crew, and cram it into a high performance upper stage, a reentry/landing vehicle, and all of that for less than half the cost of a 747-8? Especially given that you’re likely only making a tiny fraction of the number per year. Dragon currently costs probably $30-40M each to produce, and we’re saying that a Dragon designed for 14x the number of people, and 30-50x the duration, with a nearly Saturn V first stage class propulsion system built in is going to only cost 5x as much? Color me extremely skeptical. I think that they’d be lucky to have the production cost of an ITS with all of its subsystems necessary to get 100 people safely to Mars and return reliably down below $1B each anytime in the foreseeable future.”

    It seems only advantage of 100 seat spacecraft is to fudge the cost per seat. And it seems the only way to for 100 seat to work is from Earth and Mars high orbit AND
    get to Mars in less than 3 month- or rocket fuel is cheap in high earth orbit.
    If don’t go fast, have +1000 seat and go high orbit to high orbit- and give it artificial gravity [and basically cruise ship type accommodations-the trip is as important as the getting somewhere].

  36. Paul451 says:

    Me: (Operating Falcon alongside ITS/BDB is dumb.)

    Andrew Swallow says: “Lorries come in different sizes from enormous juggernauts down to 1 ton payload vans. So launch vehicles are likely to follow the same patten.”

    However, global freight is measured in the million-tonne-kms (or million-ton-miles). There’s enough market to sustain every possible scale of transport, from bicycle couriers up to Triple-E cargo ships.

    The total number of payloads in the world for a decade wouldn’t fill a single small Panamax for a single run. The total number of payloads each year wouldn’t be enough to keep a single semi-trailer owner-driver in business, unless he picked up outside work.

    [Now that I think about it, I’m curious whether NY bicycle couriers carry more tonnage of freight each year than the global launch industry. Can’t find any good estimates for the former. But certainly any single vehicle that qualifies as a “lorry” would.]

    However, my issue with Falcon is that it’s semi-reusable. It can’t compete with it’s own giant sibling on price. You could waste two-thirds of the payload capacity on ITS/BDB and still launch payloads cheaper than Falcon. So why would you keep an entirely separate production line open? With specialised tooling, handling and transport, for every component. No overlap between the two systems.

    And by using it commercially during its testing phase, you get other people to pay for your development. Exactly the strategy for F9-recovery testing.

    “SpaceX could create a replacement for the Falcon 9 that uses Raptor engines.”

    More a replacement for FH. A single core less-than-100t-to-LEO launcher, with a fully reusable upper-stage.

    It would be a stepping stone to BDB. Limited to say 9 Raptor engines on the first stage, with a single vac-Raptor upper-stage. Lets the engineers cut their teeth on a larger system, before trying the full 42-engine monstrosity. Develop and test the 12m composite tanks (shorter than BDB/ITS, obviously, but same diameter, same tooling, same materials.) Test Raptor under launch conditions. Experiment with that insane RTLS cradle capture, with a slightly smaller rocket. And once ITS is flying, only the length of the stages differs, allowing parallel production without having very different production lines for tanks, engines, handling and transport, etc.

    And as Bob says, the tanks and engine-numbers for any upper-stage could serve as the first stage for the previous generation. Letting you ratchet up your development in increments.

    However, Musk’s 2022 timeline rules all of this out, even allowing for Musk-time.

  37. Paul451 says:

    Ken Brown,
    “A facility could be built on the Moon to simulate Mars gravity for even more data.”

    That’s just silly.

    “A colony on the Moon would raise the TRL on a Mars venture considerably when it’s time to go to Mars.”

    There’s almost no overlap between moon and Mars that matters, in which working on the moon wouldn’t make more difficult and expensive anyway. Even basics like habitat ECLSS, there’s nothing that the moon adds to Earth-side testing that isn’t made billions of dollars more expensive by redesigning it to handle the unique conditions of operating on the moon.

    Re: Moon colony economics.

    There’s nothing to prevent ITS landing on the moon. If there’s a market for it, as you claim, then those investors can buy a flight or ten from SpaceX. Musk is focused on Mars because of his personal obsession, and I strongly disagree with it, but his method of operation is to develop the most general-purpose systems that can be used anywhere for any mission.

    Herp McDerp,
    “The thing that surprises me is that Musk doesn’t use Aldrin cyclers in addition to the crew module.”

    AIU Musk’s reasoning, the system he’d need to develop to built an Aldrin cycler (especially enough cyclers to allow flights every synod for the number of passengers he’s imagining) would be capable of sending people to Mars directly… so why not…

    Once there’s a large colony (or colonies) on Mars, you might have enough traffic going both ways to justify a network of cyclers.

    Aldrin’s idea was really intended to get around the horrible expense of Earth launch. (Build the big thing once, minimise its ongoing fuel use, then only launch crew/cargo on small short-range vehicles.)

    Musk’s concept is based around reducing the cost of Earth launch, that inherently solves all the problems that Aldrin’s cycler is intended to solve.

  38. gbaikie says:

    –Ken Brown,
    “A facility could be built on the Moon to simulate Mars gravity for even more data.”

    That’s just silly.

    “A colony on the Moon would raise the TRL on a Mars venture considerably when it’s time to go to Mars.”

    There’s almost no overlap between moon and Mars that matters, in which working on the moon wouldn’t make more difficult and expensive anyway. Even basics like habitat ECLSS, there’s nothing that the moon adds to Earth-side testing that isn’t made billions of dollars more expensive by redesigning it to handle the unique conditions of operating on the moon.–

    I think there is overlap between Moon and Mars which matters.
    I think the Moon should explored using a depot a LEO and I think a large
    portion of Lunar exploration should be spent on robotic exploration.
    And that robotic operations should support crew missions.
    Crewed lunar exploration can be quicker and do more specific types of
    exploration. I don’t think we need a lunar robotic sample return, but it would silly
    not to have lunar sample return on crew mission, as part of crewed mission is returning to Earth. So I think we should get lunar sample returns and part of what the crew do is get lunar sample return. What robotic programs can do is find what area should have a lunar sample return. And such robotic programs can do is help make crew landings safer- and cheaper.
    The depot in LEO would be robotic. And robotic and crew mission could use the depot
    to refuel LOX [the most massive component of rocket fuel which can used to get to lunar orbit and/or lunar surface.
    With the Moon one could have one robotic mission go somewhere on lunar surface, in which no robotic mission or crewed mission are going revisit this spot. But perhaps as much robotic mission will go to location in which other robotic or crew missions come afterward to this location. One could choose to call this a base- but people many wild ideas of what should be a base. One reason one could call it a base is because it’s landing area. Or it’s location in which spacecraft can land near other spacecraft- doing this is necessary ability related to future Mars bases, and is not something which ever done. Or one knows where these other spacecrafts are and one could land safely with 50 feet of them- and not damage them. There could be many ways this can be done, and perhaps a few should be tried- or at least one way.
    So, with crewed landings, crew will know exactly where and how they going to land- it’s not going to a place which has never been visited before. And one wants [and needs] to do this with Mars. Or when crew first land on Mars they need a way to leave Mars before crew show up. They should also have supplies already there- food, water, energy. Though once crew are on Mars surface they will need to land more supplies and need to some kinds of assembly in terms of establishing a base.
    If cost are low enough one might also have crew on lunar surface and land stuff after crew are already on the lunar surface [again, never done before]. But main idea is to pre-land stuff on lunar surface and it’s stuff the crew can use. An example, is one could land rocket fuel and a lunar hopper [instead of rover]. One could think of this as cheaper way for the crew to explore more of the Moon- or need less crews landing to different parts of polar region. Or maybe instead of having 4 or more lunar crew landing- one at one pole, and 3 at another pole. One does 2 crew landing, and at one pole, hop to other sites. And hopper could allow not landing in dark crater, instead one hops into a dark lunar crater.
    Such hoppers could also be used on Mars. One also have hoppers which are unmanned- robotic. And having relate able to Mars exploration, such unmanned hopper could controled by crew at lunar surface.
    And I see Mars exploration as again largely robotic- crew could remain at base and run
    unmanned hopper- or rovers, whatever.
    So commonalty is depot use, and robotic use, and establishing a landing area.
    I see depots as significant way to lower costs of Mars exploration program- and as the only vaguely rational way to do Mars exploration- you can not explore Mars without depots- though you don’t have call something which functions as depot- a depot.
    With the Moon, the Apollo crew landed with fuel tanker truck- but it was not called this, though the dominate payload going to lunar surface was rocket fuel.
    Also can’t have a real Mars settlement without rocket fuel made in space environment- whether it’s the Moon or asteroids- or Mars moons [which are essentially asteroids orbiting Mars].
    So if purpose of Mars exploration is to find alien life, and thereby have another excuse to not go to Mars, you don’t need rocket fuel made in space. But purpose is to determine if and how Mars is a viable location of human [or any kind of] settlements
    then first exploring the Moon would be the fastest pathway to Mars settlements.

  39. Chris Stelter says:

    I totally agree with in situ about the epicness of such a relatively simple Mars architecture. There are also a minimum number of vehicle configuration changes, which actually DOES reduce costs since it reduces the number of interfaces and the amount of analysis that needs to be done.

    I mean consider MSL:
    “6 vehicle configurations”
    “76 pyro devices”
    Discards cruise stage and balancing weights
    pops drogue chute
    then main supersonic parachute (a huge challenge by itself, FWIW)
    Then, in order for radar to work, drops heatshield.
    Then, the backshell and parachute are cut loose and the descent stage and MSL fall
    Rockets on descent stage fire, with
    Enters Mars as a lifting capsule
    Then discards weights and deploys drogues
    Then supersonic chute (a huge challenge by itself)
    Then heatshield is dropped so the radar can see the ground
    Then backshell and chute is cut loose, the rest falls down.
    The descent stage fires up to do a huge divert maneuver and slow down and kill horizontal velocity
    Meanwhile, the rover starts to descend on a freaking cable system (as the rockets are firing)
    Wheels extend
    vehicle touches down
    bridle is cut
    skycrane flies off.

    Retract solar arrays
    Enter sideways
    Then swandive maneuver as engines fire, slowing the thing down
    Meanwhile, legs extend

    Compare ITS to what we currently do, and it seems actually sane.

    And even Mars Direct, which is fairly simple, looks monstrously complex in comparison.

    I think the biggest strength is the architecture’s simplicity. Yes, there are ways you could improve efficiency by splitting things up.

    But I don’t think the “swiss army knife” appeal is correct, here. All the spaceship does is the same thing you’d need in every other SSTO VTVL crewed vehicle or reusable Mars lander with the exception that it’s super big (so can also be used for transit) and has solar arrays. Everything else is already a requirement.

    And I’d probably do it with an architecture 1/10th as big, but the size IS commensurate with the actual goal (unlike SLS). And a benefit to the large size is they can dial performance of any piece WAY back and still do the early missions just fine.

    And methane actually is a good fuel to choose. It’s significantly cheaper (per unit mass and per unit energy) than hydrogen on Earth (most relevant to the booster, and it is actually important for helping to achieve the per-launch costs they want), and requires less water to produce on Mars. It also makes SSTO-like performance significantly easier to achieve, giving the most bang for a given dry mass buck. They can also avoid some of the embrittlement issues and the deep cryo problems. And of course, it helps a lot with keeping the landing propellant from all boiling off.

    It’s true that hydrogen is better if you have more in-space architecture elements, but SpaceX’s architecture doesn’t.

  40. Chris Stelter says:

    In the future, you could imagine the Spaceship actually has more variants.

    For instance, a cargo-only variant.

    And a short-term passenger variant. This could even be SSTO, without the booster.

    And you could also imagine a variant of the Spaceship that doesn’t actually land. You could greatly enlarge the hab section to fit, say, 500-1000 people. The vehicle would have to refuel in high orbit instead of just LEO, but would still need the basic design since it’d need to have propulsion to leave Earth orbit and still need a heatshield to aerocapture at Mars (and on the way back). That would get you much of the benefit of separating propulsion from the hab without requiring the hab to fly without propulsion and without requiring a quick turnaround-burn by the propulsion stage. And it’d maintain some commonality with the rest of the stack.

    Also, as Jon hinted, you could fly a tanker to Mars for refueling in Mars orbit. Really, this would require zero changes in SpaceX’s architecture. It might also be required for single-synod flights with any kind of significant payload.

    I like the landing cradle idea for uncrewed flights. In fact, you might imagine it being done on Mars, too, with the tanker spaceship instead of the booster. I had a similar idea though I used a MUCH smaller tanker vehicle for Mars than SpaceX is proposing, and I used a hook/cable for the concept.

    But I really like this architecture. I just hope SpaceX stays solvent.

  41. DougSpace says:


    I think that there needs to be a lunar development / settlement company that could have a Falcon Heavy-based architecture. In other words, I think that there needs to be the Moon company equivalent to the SpaceX Mars company. The difference is that SpaceX has to develop all of this technology to get people to Mars whereas the lunar company would be starting with an existing and fairly proven launcher. Another difference is that the lunar company wouldn’t need to develop a cis-lunar craft but just use launcher with cargo or crew module and then have it dock with a lunar ferry refueled from lunar ice. The business case would be similar to SpaceX in that it would be the intersection between those who could afford to go to the Moon and those who wanted to go to the Moon. Any idea who could head up such a company?

  42. Roderick Reilly says:

    Excellent post, Jon. I think you covered the issues well.

    As a long-time enthusiast of SpaceX, and one who acknoweledges what a remarkable man Elon Musk is, I think he jumped the Space Shark here.

    Musk’s motivations for doing this Mars thing is out side of the general technical scope of this thread, but I’ll venture there briefly, in order to explain why I think this will fail.

    Musk’s agenda is to “save the human race” by giving it an alternate home. By making that his principal priority, he lends an inflated sense of urgency to the project which, in turn, enlarges the size and scope of the mission. This is folly, plain and simple.

    He puts the cart before the horse by thinking this way. What struck me is that there is no “interim” set of missions of a scale in between SpaceX’s current capabilities and what he is proposing for his Big Mars Thing. That makes no sense to me.

    Having said all that, it’s his plan and his money with his motivations. Let him proceed, and, at least, the fallout will be major advances in launch and spacecraft technologies that can be put to more sober uses.

  43. Roderick,
    I think you summed up a lot of my thoughts better than I can. His biggest mistake is trying to jump from robotic exploration straight to colonization, without any real pioneering phase in between. This forces him to build his colonization architecture off of today’s technologies, because he thinks he’ll have it flying in 10yrs and thus can’t afford to wait for things like Induced Torpor, artificial gravity studies, or Magnetoshell Aerocapture to be tested out. If he instead offered ITS as the current concept for a longer-term plan, but focused his efforts on leveraging the heck out of Red Dragon and other nearer-term capabilities to prepare the way, I would’ve given him a higher probability of pulling this off successfully.


  44. Roderick Reilly says:

    Thank you Jon, for your quick reply and additional thoughts.

    Good points again.

    Another observation: in an age when robotics is surging, and AI is making real headway, I find his whole “people intensive” approach anachronistic. But again, Musk is guided by a sense of urgency based on a wordlview I do not share, but that is quite popular.

    A largely robotic built, modest-sized Mars base makes the most sense to me. One built over time, and ready to receive human “guests” when they finally arrive. All the habitat-building and propellant and oxygen production, and even food growing can be done with no human presence, in my view. Space is turning out to be a very different paradigm than most of us have usually imagined, and “Mars ain’t Jamestown.” Much has to be worked out yet before humans can thrive off-planet in large numbers as permanent residents. And much of the solution(s) to human habitabilty will probably come from bio-medicine and genetic manipulation advances.

    Again, just my thoughts, from my perspective.

  45. Roderick,

    Two thoughts:
    1- As someone who works with robotics a lot, I’m skeptical about how far we’re really going to be able to push things without at least some humans on site. For the Moon, where teleoperations are more feasible, I *might* be able to believe robots could set some stuff up before people arrive. For Mars, that seems a lot more optimistic. The approach I like best is to send a few people (McGyver types) semi-permanent, with a ton of robots, and a decent machineshop/lab, probably with 3d printers and lots of spare materials.
    2- The other reason why Musk is looking at human-intensive approaches, is that he’s interested in people being there partially as an end in itself. This isn’t just a research base, or a mining base, where you really want to use the minimum number of people to safely do the job. This is a settlement, where part of the point is to have large numbers of people not living on earth. I agree with you that the urgency on that seems exaggerated at the moment, but if your point is settlement, then having lots of people there is a feature, not a bug.


  46. Chris Stelter says:

    “He puts the cart before the horse by thinking this way. What struck me is that there is no “interim” set of missions of a scale in between SpaceX’s current capabilities and what he is proposing for his Big Mars Thing. That makes no sense to me.”

    Red Dragon and crew Dragon are both interim missions. And the spaceship will be developed, built, and flown first, nearly to orbit (and probably with a trajectory that’s just as harsh or worse than returning from orbit). That counts as interim to me. Additionally, there’s no reason the ITS couldn’t do a round-trip trip to the Moon and back for initial testing. I think Musk may even have hinted about it once.

    So I don’t find that part of your critique meaningful.

  47. Chris Stelter says:

    I think Musk is limiting the whole architecture to what he thinks SpaceX can do, not to all possible solutions. I think he is perfectly open to new ideas, but you can’t just assume that things like torpor or magnetoshell will work and plan around those things before they’ve even been demonstrated. He is trying to make a public case that Mars settlement can happen and that it can be accessible and not terrible without requiring magic technology. Magnetoshell & torpor don’t really help Musk in making that case.

    Musk knows the timeline is already aggressive and that it’s almost certainly going to slip. He also knows that getting the whole thing moving will take decades. Human lifespan is limited. He may lose control over SpaceX or go bankrupt. A fascist might get elected and plunge the country into ruin. And SpaceX can’t be experts in every possible new technology. So under those constraints, the plan looks quite good.

    Those things you mention would help. But they’re not strictly necessary. And if you’re making a public case, where people LOVE to hitch on to any perceived technological flaw, probably makes sense not to rely on them.

  48. Jonathan Goff Jonathan Goff says:


    You could save a little time by just replying “Nuh, uh guys, everything Elon says is true!”



  49. George Turner says:

    I have some concerns about the square/cube law loading. Mars Curiosity came in at about 42 lbs/sq ft, Apollo was 100 lbs/sq foot, the Shuttle was 130 lbs/sq ft, and the max for Dragon (returning a payload) is about 145 lbs/sq ft. I estimate the ITR to be about 7,500 sq feet in cross section, and I’ve seen references to 550 tonnes in LEO, including payload. For propulsive landing from Mach 2 I’m guessing it will need about 100 tonnes of propellant. That would give it a loading of close to 200 lbs/sq ft, and its coefficient of drag isn’t going to be very large due to its cylindrical shape. And it’s re-entry velocity for Earth is going to be above 14 km/sec instead of 7.8 km/sec, so the heat load will be about 3 times higher than it would for re-entry from LEO.

    If its thermal protection is an ablator, there’s going to be serious ablation that argues against re-use. If it’s using thermal tiles, it’ll need five Space Shuttle’s worth.

  50. Bob Steinke says:

    I do think Chris hit on an important point. One critical requirement placed on this plan is “within Elon’s lifetime”. A couple extra decades of technology demonstration and robotic precursor missions are a real problem for that.

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