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:
- 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.
- 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.
- 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.
- 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.
- 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”.
- 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.
- 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.
Latest posts by Jonathan Goff (see all)
- An Updated Propellant Depot Taxonomy Part VI: Roving Depots - February 22, 2021
- An Updated Propellant Depot Taxonomy Part V: Human Spaceflight Fixed Depots (Low-Orbit) - February 16, 2021
- An Updated Propellant Depot Taxonomy Part IV: Smallsat Launcher Refueling Depots - November 14, 2020