Elon Musk answered questions on Reddit Space yesterday about the Spacex BFR

Elon Musk answered questions about the Spacex BFR, Raptor Engine, Mars colonization and other space related topics.

Here are Elon Musks answers to questions.

* Elon on space radiation – Ambient radiation damage is not significant for our transit times. Just need a solar storm shelter, which is a small part of the ship.
Buzz Aldrin is 87.
* Our goal is get you there [to Mars] and ensure the basic infrastructure for propellant production and survival is in place. A rough analogy is that we are trying to build the equivalent of the transcontinental railway. A vast amount of industry will need to be built on Mars by many other companies and millions of people.
* Best mass ratio is achieved by not building a box in a box. The propellant tanks need to be cylindrical to be remotely mass efficient and they have to carry ascent load, so lowest mass solution is just to mount the heat shield plates directly to the tank wall.
* Wouldn’t call what BFS has a delta wing. It is quite small (and light) relative to the rest of the vehicle and is never actually used to generate lift in the way that an aircraft wing is used.
It’s true purpose is to “balance out” the ship, ensuring that it doesn’t enter engines first from orbit (that would be really bad), and provide pitch and yaw control during reentry.
* Some parts of Raptor will be printed, but most of it will be machined forgings. We developed a new metal alloy for the oxygen pump that has both high strength at temperature and won’t burn. Pretty much anything will burn in high pressure, hot, almost pure oxygen.
* The control thrusters will be closer in design to the Raptor main chamber than SuperDraco and will be pressure-fed to enable lowest possible impulse bit (no turbopump spin delay).
* The heat shield plates will be mounted directly to the primary tank wall. That’s the most mass efficient way to go. Don’t want to build a box in box.
* At first, the tanker will just be a ship with no payload. Down the road, we will build a dedicated tanker that will have an extremely high full to empty mass ratio (warning: it will look kinda weird).
* Thrust scaling is the easy part. Very simple to scale the dev Raptor to 170 tons.
The flight engine design is much lighter and tighter, and is extremely focused on reliability. The objective is to meet or exceed passenger airline levels of safety. If our engine is even close to a jet engine in reliability, has a flak shield to protect against a rapid unscheduled disassembly and we have more engines than the typical two of most airliners, then exceeding airline safety should be possible.
That will be especially important for point to point journeys on Earth. The advantage of getting somewhere in 30 mins by rocket instead of 15 hours by plane will be negatively affected if “but also, you might die” is on the ticket.
* Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don’t need the high area ratio, deep space Raptor engines.
Next step will be doing orbital velocity Ship flights, which will need all of the above. Worth noting that BFS is capable of reaching orbit by itself with low payload, but having the BF Booster increases payload by more than an order of magnitude. Earth is the wrong planet for single stage to orbit. No problemo on Mars.
* Landing will not be a hoverslam, depending on what you mean by the “slam” part. Thrust to weight of 1.3 will feel quite gentle. The tanker will only feel the 0.3 part, as gravity cancels out the 1. Launch is also around 1.3 T/W, so it will look pretty much like a launch in reverse….
* The main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.
* 3 light-minutes at closest distance. So you could Snapchat, I suppose. If that’s a thing in the future.
* But, yes, it would make sense to strip the headers out and do a UDP-style feed with extreme compression and a CRC check to confirm the packet is good, then do a batch resend of the CRC-failed packets. Something like that. Earth to Mars is over 22 light-minutes at max distance.

Question – Why was Raptor thrust reduced from ~300 tons-force to ~170 tons-force?
One would think that for (full-flow staged combustion…) rocket engines bigger is usually better: better surface-to-volume ratio, less friction, less heat flow to handle at boundaries, etc., which, combined with the target wet mass of the rocket defines a distinct ‘optimum size’ sweet spot where the sum of engines reaches the best thrust-to-weight ratio.
Yet Raptor’s s/l thrust was reduced from last year’s ~300 tons-force to ~170 tons-force, which change appears to be too large of a reduction to be solely dictated by optimum single engine TWR considerations.
What were the main factors that led to this change?
ElonMuskElon Musk – We chickened out
ElonMuskElon Musk
The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk. In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you’ve lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

40 thoughts on “Elon Musk answered questions on Reddit Space yesterday about the Spacex BFR”

  1. 1. “No, they aren’t. Neither in their launch points nor where their trajectories are going are they anything like an ICBM.”
    Tdperk, yes they are. A lot of ICBMs are mobile and their trajectory is identical since the slowdown of the BFR will happen at the very end of the flight.

    2. “Why would the G forces be higher than 1.3G as stated?”
    It’s been one year since Musk announced BFR, and he changed quite a lot. What makes you think that 1.3G will stay the same? Also, Musk has a very long record of blatantly lying.

    3. “Why would there be a special suit?”
    For the same reason airplanes have oxygen masks. And since the BFR will go to space, just an oxygen mask won’t do it.

    4. “…would the noise be so much higher than can be accommodated by a barge offshore?”
    Yes, it would. Have you ever witness a rocket launch? They are very loud! Even the small ones.

    5. “Why do you presume the most negative idiocies possible will obtain, instead of what is known to be true and likely?”
    Which part of my comment was an “idiocy”?! And “what is known to be true and likely” is quite the opposite of what Musk is trying to do. So the question becomes, why do YOU tdperk presume the most positive idiocies instead of what is known to be true and likely?

    6. “They can be as safe as airplanes.”
    Yeah, that’s easy to say when you haze zero understanding of the engineering behind that statement. There’s a reason why rockets aren’t used today in point to point transportation.

    7. “Claiming “physics” says otherwise is the same sort of error the people made who said the “the rocket equation” meant SpaceX could never vertically land a rocket and launch a useful payload.”

    Vertical landing of rockets have been demonstrated since the 60s so it was not a “physics” problem, it was an economics one. Existing rocket companies just didn’t (and still don’t) think it makes economic sense. Musk thinks it does, and on this, I agree with him.

    8. “Yep, because these are the same paid trolls making these arguments, I see exactl same arguments made in Russian media all the time.”
    “Yeah, there’s that too. The amount and themes of the attacks seems to follow some perverse method.”
    Igor and Tchernik, wonder what the reasons behind my attacks are…
    Is it because I’m paid by Big Oil, ULA and Ariane to make these comments? Of course this would be on top of my miserable salary as an FSB agent. This definitely would fit with the standard cult mentality, “us against the world”.
    Or is it because this plan of his makes no sense and it’s just a PR stunt?

    I strongly suggest you learn some marketing, it helps be less gullible and see through company / ceo / politician bs.

    • 1) No, the BFR used P2P no more resembles an ICBM than a jet airliner resembles a bomber. You troll to bring it up.
      2) Why presume this will change. You have no reason to do so, or then even if it did change, that the change would be problematic. Are you trolling us to pretend it will go from 1.3 to 10, or some such nonsense?
      3) As a practical matter, there is no reason to presume depressurization will not be addressed by simply building the airframe well. Catastrophic depressurization–the whole thing comes apart–is not survivable in airplanes either, and slow leaks can be better dealt with by just such oxygen masks and aborting the flight. Because the BFR cabin must withstand full vacuum without continual pressurization from air breathing engines, I have no reason to think an abort sequence will not be a built in subroutine. Neither do you.
      4) Before you make more of a fool of yourself, take note the weight of the BFR system in a P2P application will be far less than that of a fully loaded trip to LEO, and sound will be proportionately less, and can in any case be addressed easily as it is noe–by distance.
      5) A great deal of your comments are idiocy, such as claiming the BFR cannot be told from an ICBM. I presume no idiocies, only that what has been known to be ture for 4 decades can be realized now far more easily than it could be then–that access to space need cost no more than a few multiples of the fuel cost for the energy required to get there.
      6) No I do understand, you don’t. The reliability of airplanes comes from iterative improvement drastically aided by examination of success and failure. This is with re-use something now as applicable to rockets as to airplanes, something not previously true since they were discarded into the depths before.
      7) I never said that “physics” or the rocket equation meant a rocket could not land vertically. Gary Church has claimed that numerous times, and he lies to claim he has not. I never claimed it had not been done before ever. It had not been done before the F9 that a rocket carrying useful payload to orbit had been landed vertically for re-use, still less with an eye towards re-fuel to re-use operation.
      8) I have no idea if you are literally a paid Russian troll or not, but I can believe it. The alternative of course is you are actually so stupid that you take seriously what you write.

      Potential problems != Showstoppers, or even grounds for skepticism…especially when so many plausible fixes are self-evident.

  2. About SpaceX being a rocket manufacturer and spaceliner at the same time…

    Wasn´t Douglas both an aircraft manufacturer and airline operator at the same time?

    • No, that was Boeing. Roosevelt’s FAA split them off from one another, and Mr. Boeing was so pissed that he left for China, while the airline became United Airlines.

  3. If Spacex delivers on the BFR and corners the market in orbital, cislunar and inner-solar system transport, it’s enemies and competitors will attempt to break it up via anti-trust laws. That’s what happened to Jim Boeing’s company in 1934.
    SpaceX will not be allowed to be a rocket manufacturer and a spaceliner operator.

    From Wikipedia:

    After the Air Mail scandal of 1934, the U.S. government concluded that such large holding companies as United Aircraft and Transport were anti-competitive, and new antitrust laws were passed forbidding airframe or engine manufacturers from having interests in airlines, and United Aircraft and Transport broke into three separate companies. Its manufacturing interests east of the Mississippi River (Pratt & Whitney, Sikorsky, Vought, and Hamilton Standard Propeller Company) were organized as the United Aircraft Corporation (now United Technologies Corporation), headquartered in Hartford with Rentschler as president. The western manufacturing interests (including Northrop Aviation Corporation, formerly Avion Corporation), were folded into Boeing, headquartered in Seattle. United Airlines became a separate company.

    • SpaceX will not be allowed to be a rocket manufacturer and a spaceliner operator.

      Good point.

      That’s a likely outcome. And a desirable one in the longer term, if SpaceX manages to transform rockets into reusable space liners.

      Currently the manufacturing and launch businesses are one and only, because there are not that many lunches per year for any single company, and nobody can make the rockets and sell them to strangers to launch and relaunch as a standardized product, given the complexity and very customized processes of single launch rockets.

      For achieving their goals, rockets must become heavily productized, and that means producing and following new standards and maintenance procedures, coming as part of the product’s envelope. Very much as airplanes did.

      But before that happens, they have to show their rockets really are mature, reusable and worthy of becoming a standardized product. So far that has not happened.

  4. “How do you get this place to show comments as a thread tree?”

    By persuading Brian to switch to a different commenting system?

    Seriously, I think there’s a browser extension for that.

  5. Suppose the PTP earth transport cost only $10M per launch.
    Suppose it can haul 500 customers (probably too high).
    That’s still $20,000 per ticket, one-way – and with overhead and other costs probably $30,000.
    Could they get 500 customers per launch to one destination at that price?

    • “Could they get 500 customers per launch to one destination at that price?”

      Tom, the issue is not the price. There will always be people who would gladly pay $20k to save a few hours. The issues with this kind of travel are:

      – You could die. Rockets just can’t be safer than airplanes. It’s just physics. They operate in extreme environments (speed, heat) and if something goes wrong, there’s no time to do anything about it, even if there was anything to do.
      – No city on earth would allow rocket launch facilities close to them (or rockets fly over them). The noise would be insane. And if the rocket explodes, there’s going to be a lot of damage to the surrounding area.
      – No country would allow these kind of rockets to come their way. These are IDENTICAL to ICBMs, you can’t tell the difference. Sure you can put a transponder on them, but there’s no way to make sure it’s not a surprise attack.
      – It will be extremely uncomfortable (g-forces, special suit) and I don’t think there will be much saving on time since you’ll have to travel to the launch site (which will be a lot farther away than the airport); go through security, go through the special prep for rocket travel (you can’t just board it) and repeat the process after the landing (if you survive. lol).

      This was all just a PR stunt by Musk. And the fact that almost all the tech media is talking about this, shows Elon’s genius at public relations. Most people don’t like being played for a fool though, so when they get tired of his charlatanry, his cherished reputation will be ruined. And even his genuine accomplishments (like falcon 9 launch / landing) would be tarnished.

      • ” Rockets just can’t be safer than airplanes. It’s just physics. ”

        They can be as safe as airplanes. Claiming “physics” says otherwise is the same sort of error the people made who said the “the rocket equation” meant SpaceX could never vertically land a rocket and launch a useful payload.

        • Yep, because these are the same paid trolls making these arguments, I see exactl same arguments made in Russian media all the time.

          • Yep, because these are the same paid trolls making these arguments

            Yeah, there’s that too. The amount and themes of the attacks seems to follow some perverse method.

            • I am glad you noticed. It is mostly two main arguments Russian trolls will try to make 1. Musk is a charlatan and is lying and 2. His business live off the government handouts.

              • “It is mostly two main arguments Russian trolls will try to make 1. Musk is a charlatan and is lying and 2. His business live off the government handouts.”

                It’s not called trolling when you state facts Igor. But I guess for Musk / Trump clan everything that doesn’t praise the fearless leader is “fake news”.

                Anyways… I don’t know why I bother replying, the cult around Musk is worse than Scientology. You guys are no different than climate change deniers, moon landing truthers, vaccine deniers, etc. Facts to you guys are not important, faith and tweets are all that counts.
                And people used to complain about Apple fanboys. lol I guess there’s always worse.

                • @ tdperk: “But you usually don’t state relevant facts. You usually troll instead.”

                  Td just because you don’t like what you hear, doesn’t make it trolling. Don’t be like Trump. You should face criticism, not hide from them.

                  As for relevant fact, what I posted you are relevant facts. And by the way, it’s not me who needs to prove a negative, it’s Musk who needs to prove that earth to earth rocket travel makes sense, and he has not done that and never will because it was just a PR stunt, so from his perspective, the mission was accomplished. The cult members on the other hand don’t understand this and they keep religiously defending stupid ideas without knowing that they supposed to be stupid.

                  Ask yourself this, why am I not “attacking” the other part of his plan?
                  It’s because it’s doable and it’s a logical thing to do. Why waste time on three systems (F9, FH and BFR) and you can focus only on one.
                  As usual more time than he stated to make it happen, but he’ll probably deliver the BFR.

                  Stop religiously defending something just to protect the honor of the fearless leader. It makes no sense. And read a marketing book, it will help you understand the actions of politicians and business people. None of them are stupid, everything they do is calculated.

                • I am amused by you Kudoz. Your trolling is an entertainment.

                  You should provide relevant, factual criticism if you have any. You don’t.

                  ” he has not done that and never will because it was just a PR stunt ” <– A statement demonstrating your bad faith, that you are here to troll. Equally your nonsense claiming the BFR in P2P use will be problematically seen as an ICBM.

                  " Why waste time on three systems (F9, FH and BFR) and you can focus only on one. " <– F9 because the F1 could not hope to raise the revenue he needed, and the F3 and F5 sized concepts had the same problem. When re-use was fully considered, he saw he needed to build the largest rocket he could which was road transportable because fuel cost was trivial–hardware is expensive–and sea transport too slow for the quick turnarounds he was needing to bring costs down. FH was originally conceived of because the original builds of F9 were lower in capacity than many payloads. It was given lower priority as the degree and rate at which the F9 could be uprated was understood. FH happens at all now only because there are customers whose payloads require it who he does not want to leave waiting until the BFR is finished. F9 development is now largely completed with the BLock 5 build, and unless problems are seen in the FH, it is largely completed too. What time do you pretend was wasted? Wasted how? The BFR is their R&D focus as soon as FH is settled, why do you doubt that?

                  " Stop religiously defending something just to protect the honor of the fearless leader. " <– That has nothing to do with it.

                  " It makes no sense. " <– Your concern trolling FUD makes no sense, and fighting that is what it is about. At best you are pathologically skeptic, at worst you are a paid concern troll.

        • No one claimed not to be able to upright-land a first stage based on the rocket equation(s). Instead, it was said that to do so would exact a payload mass penalty. Which is exactly what it did.

          • No, Gary you’ve said that repetitively. Don’t bother denying it now.

            Or deleting your comment, I’ve screenshotted it.

        • The physics argument also says that aeroplanes will always have much higher energies involved than say… cars. And hence air travel will always be less safe than car travel.
          Whereas we know that air travel is in fact significantly safer than catching the taxi to the airport.

      • ” These are IDENTICAL to ICBMs ”
        No, they aren’t. Neither in their launch points nor where their trajectories are going are they anything like an ICBM.

        Why would the G forces be higher than 1.3G as stated? Why would there be a special suit? Why when a P2P launch will have nothing like an orbital launch weight, would the noise be so much higher than can be accommodated by a barge offshore?

        Why do you presume the most negative idiocies possible will obtain, instead of what is known to be true and likely?

        • Russia does not have radar coverage over Florida or central CONUS, so when a flight to Beijing or Tokyo enters Russian radar cup near Alaska, they won’t know or care that it came from Florida and not S. Dakota. I don’t think it is a deal breaker, but there will be challenges, it isn’t trivial. To sophisticated radar, the BFR will look different enough from a Minuteman-3, but on older radar… some effort will obviously need to go into networking with foreign powers to get over the regulatory hurdles of routine suborbital reentry happening all over the globe all day long.

      • Yes a P2P rocket looks identical to an IRBM/ICBM. But a P2P jet airliner looks identical to a jet bomber.
        The issue of Airliner or Bomber was sorted out to the point where there is only about 1 error per decade, and everyone seems prepared to live with it.

    • I suspect Elon/SpaceX et al are thinking the PTP launches will be far less than $10M a pop. Probably cost of fuel (SpaceX keeps saying 200,000 per launch, which sounds low to me) plus amortization of the vehicle which depends on a lot of things. He also tweeted after the IAC talk that a PTP ticket would cost the same as full fare economy today so, what’s that? A few thousand dollars between LA and Singapore?

  6. A lot of criticism in bulletin boards and social networks are centering around Elon Musk’s mental health. And his stuttering on IAC 2017 didn’t help at all.

    I don’t know about the stuttering. The guy probably had jet-lag and/or was emotional. But the criticism itself isn’t that surprising, given we have lived more than 43 years in a world without Apollo missions. A world devoid of any space ambitions, where clearly nothing was affordable or even possible.

    Thus someone contradicting those mantras obviously must be crazy.

    • But at the same time. We have had examples where genius’ where very close to be mentally unstable. Musk is a very good business man, excellent engineer and ambitiously visionary but not a certifiable genuine genius. So I dont worry about that.

    • Some people just get nervous when doing public speaking, it is a form of mild and common social fobia, it has nothing to do with being mentally unstable. I am like that as well, no reason to worry about Musk’s mental health.

      • You’ve got that right. I once had to speak in public, and my teeth ended up chattering so hard I chipped a tooth. And by most accounts I’m sane.

        Well, at least never been diagnosed with anything…

  7. I was surprised and encouraged that Elon is seriously planning to develop PTP earth transport. I think if they added a couple extra engines to the BFS they could do most of the routes without the booster which could make it simpler and cheaper.

    • I wasn’t suprised. Elon, just like other narcissistic megalomaniacs (Trump), craves media attention.
      And if the “announcement” helps divert attention from Tesla’s problems, It’s a big bonus.

    • I remember that Robert Zubrin as part of his critique of Elon’s 2016 ITS presentation. said that Musk should consider intercontinental travel on Earth as a market to subsidize his Mars spacecraft ambitions. So maybe that helped shift Musk’s thinking? Or else, great minds just think alike.

  8. The most interesting thing for me was that in spite of many who have been disparaging the seriousness of plans to use BFS as a P-P Earth Transport, they are changing the design to make this more possible. Even since the Adelaide IAC presentation, they have added another mid-range expansion ratio engine to BFS, to make it safer to land it on Earth, as safe as an airliner, as Musk wants it.

    I hope this means they soon will begin to address legal issues like MTCR and ITAR restrictions on who they can sell these vehicles to. Iniside the US there will be no problem, but the State Department has shown itself glacial in letting US companies talk to foreign buyers, even when it’s people like Richard Branson of Virgin Galactic. The markets inside the US may be worthwhile for the BFS alone, without its booster.

    This is because Musk said that the first flight tests will be done by the BFS over distances of a “few hundred kilometers, vertically and horizontally. He also said that the BFS alone could carry about 1/10th of its normal payload to LEO. That means it could carry some larger fraction to shorter distances. One wonders where the ranges for longest and shortest distance for the BFS alone would be in terms of economic payoff. At the shortest, the ranges in those first ballistic flight tests may show what can be accomplished. The range from an offshore LA launch site to Las Vegas is about 400 kilometers. A 10 minute trip to Las Vegas, in that heavily “impulse buy” dominated market, would bring in more high-rollers than the current 4 hour drive, or the 65 minute airline trip. Then, some longer trips, like from Seattle to San Diego, might be viable for the BFS alone as well. Hard to say if you could reach Hawaii from LA with an economical payload, but it wouldn’t hurt the islands to have that available, …nor Alaska, for that matter.

    If those more humble markets than NY to Shanghai are opened first, the experience in handling BFS and lots of revenue could be acquired, even as the State Department grinds slowly through its ITAR process for exporting the whole system as a transport vehicle from one continent to the next. That, in turn, could make it that much easier to finance the full system to make flights elsewhere in the Solar System with it possible.

    • I have my doubts about P-t-P ever seeing the light of day. The rocket would be astoundingly loud and not very trustworthy at the beginning, requiring expensive sea platforms miles away from the coast to deploy them. Plus the strong reticence from the government to allow such kind of vehicle that behaves so much as an ICBM. Space launches can be monitored much more easily than hundreds of intercontinental flights per day.

      Also is the P-t-P BFR/BFS any different from the orbital version?

      That is, can a P-t-P be used for launching military or hostile cargo into space?

      If so, convincing the gubmint to allow its sale and use by foreign powers for ferrying people and cargo over Earth would be very hard.

      • Why would SpaceX have to sell its vehicle to foreign powers to achieve the outlined vision? It can be entirely US owned and operated. No, the problem with the “gubmint” is in terms of safety regulation. Permitting rocket launches in NY Harbor or San Francisco Bay will give lots of regulators stomachaches.

    • I’m missing something. Why would SpaceX have to sell its vehicle to the Chinese to permit a, say, NY to Shanghai PTP launch? China would have to license/permit the landing/launch in their territory but that doesnt mean SpaceX has to sell them the vehicle (although the Chinese may want that for obvious espionage reasons). Does United Airlines sell its aircraft to China to operate intercontinental flights to Shanghai? No, United buys the plane from Boeing and then works with the local authorities to plan routes.

      • Think of SpaceX as Boeing or Airbus…sell the vehicle (or lease) to various “spaceliner” companies operating the individual routes. China, et al, would then have to have the ability to service/repair as needed the vehicle(s).

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