Another redesign of BFS
Elon Musk has done it again. Just a few tweets were needed to throw space enthusiasts into speculation frenzy of the upcoming changes of BFR. It took ten days to announce new Falcon 9 upper stage based on mini-BFS concept, and to retire it again. Instead, another great non-intuitive design change is coming.
So what could be such a radical simplification? Dropping vacuum version of Raptor definitely shortens the schedule. Horizontal landing? Not really. Real wings? Well, BFS does not need to fly in the air, it just needs to slow down in time. Rearrange cargo and tank positions? Good luck with that.
The very reason Elon Musk stated for mini/BFS atop of Falcon 9 is really a test vehicle that would be used to explore EDL profile from orbital to subsonic speeds. To earn some money while testing, it could also be used to fly some LEO payloads. You know, the ones that are not that expensive to make. Underpants and cookies for ISS astronauts do not fall into that category. But since Starlink satellites will be made by the thousands, there is no big deal if one or two launches fail. Yet that new mini-BFS still needs to be developed. And most of the integration work on real BFS would probably need to wait until test results are available.
We can even put a price estimate on mini-BFS - 500 million USD. That is the difference from announced loan amount that SpaceX was looking for just before mini-BFS announcement (750 million USD) and reduced amount of 250 million USD reported by Bank of America. In SpaceX terms this is a serious money. That much was spent for development of Falcon Heavy.
So SpaceX is obviously in a cash crunch for development of BFR and Starlink. It does not participate in any NASA lunar development programs. It is not part of US Air Force Launch System Prototype development program. And it would not be wise for Elon Musk to start selling Tesla shares just yet.
So what on Earth could they do to reduce costs and shorten schedule?
They already obtained 9m tooling. They are building the composite tanks and test sea level Raptors as we speak. So obviously that will not change. BFS is scheduled to start Grasshopper style tests in a year. So propulsion, tanks and landing gear are probably "frozen" at this time.
Whole idea of mini-BFS was brought by requirement to launch BFS to suborbital or orbital speeds. In other words they need BFR (the lower stage) monster with elongated tanks, 31 Raptors, and its own landing gear. It would be really great if they could postpone building BFR. But what could lift such a heavy payload as BFS to at least 50km height?
Except SLS, they could try to mount it on top of Falcon Heavy as not fully fueled payload. But 9m of width as twice as much current fairing. And four time longer. It would not fit into the hangar. It could not pass through existing hangar doors. And T/E would not fit. And GSE would require upgrades. And empty BFS weight of 85 tonnes is still much higher that 60 tonnes that Falcon Heavy supports. So...it seems a bad idea.
But wait, what about using Falcon Heavy boosters as BFS boosters? They are designed to be reusable, and can help BFS to reach orbit without payload (or very small payload, such as, I don't know, 20 Starlink satellites). And in the future, they could be replaced with three BFS stacked together in parallel staging. Maybe even crossfeed? That would be something. Side BFS boosters would actually be tanker versions of BFS, full of fuel. Two of then could come close to BFR booster capacity. But SpaceX was already there. Remember 2011 and Falcon Heavy announcement? How hard can it be to stick three boosters together? It turns out it is quite hard. And they had to redesign central stage from the start. Elon Musk does not repeat the same mistakes. Ok, except when using Twitter. But nobody is perfect.
So what other options do they have? They are building BFS as we speak. With propulsion, tanks, landing gear. Why not stack one on top the other? It could easily reach orbit that way. But it could not lift off the ground. Total mass of 2600 tonnes at lift off would require at least 13 Raptor Engines.
Luckily, latest BFS design shown during Dear Moon announcement enables addition of outer ring of Raptor engines.
Notice the aft cargo pods? They could be replaced with additional Raptor engines. Thrust structure should be changed so that accommodate this addition, as well as fueling lines. What about whole structure strength? Since stacked BFSs could reach at most 3g acceleration during booster stage, it would not require radical redesign of the lower stage.
There is just one missing piece - new interstage connecting 1st and 2nd stage BFS. It would be essentially the same composite used to build outer tank wall. It would probably be the only non-reusable piece of the whole architecture. Possibly even that could be reused if booster BFS would not used belly for aerobraking but instead use Falcon 9 style tail first renetry profile.
By designing BFS to handle varying number of sea level Raptor engines (between one, seven, thirteen or nineteen engines) with the same tanks and thrust structure essentially any BFS could be used as lower or upper stage. The change would also make BFS sturdier (at the expense of dry mass weight) and more reusable. For example, each booster BFS could be retroffited at the end of usable life to deep space missions.
Such a stack could (barely) reach the orbit using 100tonnes of cargo). Adding 50mT of landing reserve would easily launch all envisioned LEO payloads in the near future. But another advantage is that booster BFS would naturally evolve into tanker version of BFS by having a second set of tanks in the upper (cargo/living) space. It is approximately 1000m3 of volume that can be used for additional 800mT of propellant, bringing the whole stack performance above Falcon Heavy in fully reusable configuration. Plus it could do GTO missions with current satellite payloads (below 7000kg). And with a few more LEO refueling missions, it could easily perform Dear Moon mission with propellant to spare.
So standard BFR booster would only be needed for extremely heavy payloads, such as frequent LEO refueling flights. It this scenario it will not be needed for at least a decade since first BFS flight. There is simply no need for it right now. While significantly lowering initial BFR launch capacity to essentially Falcon Heavy class it acts as immediate, fully reusable replacement of Falcon rocket family for most payloads. It further shortens time to first LEO flight by using the same hardware twice. Same landing gear. Same ground processing. And in this architecture, nothing prevents BFR evolution to full stack capacity as initially described in IAC 2017 presentation.
So what could be such a radical simplification? Dropping vacuum version of Raptor definitely shortens the schedule. Horizontal landing? Not really. Real wings? Well, BFS does not need to fly in the air, it just needs to slow down in time. Rearrange cargo and tank positions? Good luck with that.
The very reason Elon Musk stated for mini/BFS atop of Falcon 9 is really a test vehicle that would be used to explore EDL profile from orbital to subsonic speeds. To earn some money while testing, it could also be used to fly some LEO payloads. You know, the ones that are not that expensive to make. Underpants and cookies for ISS astronauts do not fall into that category. But since Starlink satellites will be made by the thousands, there is no big deal if one or two launches fail. Yet that new mini-BFS still needs to be developed. And most of the integration work on real BFS would probably need to wait until test results are available.
We can even put a price estimate on mini-BFS - 500 million USD. That is the difference from announced loan amount that SpaceX was looking for just before mini-BFS announcement (750 million USD) and reduced amount of 250 million USD reported by Bank of America. In SpaceX terms this is a serious money. That much was spent for development of Falcon Heavy.
So SpaceX is obviously in a cash crunch for development of BFR and Starlink. It does not participate in any NASA lunar development programs. It is not part of US Air Force Launch System Prototype development program. And it would not be wise for Elon Musk to start selling Tesla shares just yet.
So what on Earth could they do to reduce costs and shorten schedule?
They already obtained 9m tooling. They are building the composite tanks and test sea level Raptors as we speak. So obviously that will not change. BFS is scheduled to start Grasshopper style tests in a year. So propulsion, tanks and landing gear are probably "frozen" at this time.
Whole idea of mini-BFS was brought by requirement to launch BFS to suborbital or orbital speeds. In other words they need BFR (the lower stage) monster with elongated tanks, 31 Raptors, and its own landing gear. It would be really great if they could postpone building BFR. But what could lift such a heavy payload as BFS to at least 50km height?
Except SLS, they could try to mount it on top of Falcon Heavy as not fully fueled payload. But 9m of width as twice as much current fairing. And four time longer. It would not fit into the hangar. It could not pass through existing hangar doors. And T/E would not fit. And GSE would require upgrades. And empty BFS weight of 85 tonnes is still much higher that 60 tonnes that Falcon Heavy supports. So...it seems a bad idea.
But wait, what about using Falcon Heavy boosters as BFS boosters? They are designed to be reusable, and can help BFS to reach orbit without payload (or very small payload, such as, I don't know, 20 Starlink satellites). And in the future, they could be replaced with three BFS stacked together in parallel staging. Maybe even crossfeed? That would be something. Side BFS boosters would actually be tanker versions of BFS, full of fuel. Two of then could come close to BFR booster capacity. But SpaceX was already there. Remember 2011 and Falcon Heavy announcement? How hard can it be to stick three boosters together? It turns out it is quite hard. And they had to redesign central stage from the start. Elon Musk does not repeat the same mistakes. Ok, except when using Twitter. But nobody is perfect.
So what other options do they have? They are building BFS as we speak. With propulsion, tanks, landing gear. Why not stack one on top the other? It could easily reach orbit that way. But it could not lift off the ground. Total mass of 2600 tonnes at lift off would require at least 13 Raptor Engines.
Luckily, latest BFS design shown during Dear Moon announcement enables addition of outer ring of Raptor engines.
Notice the aft cargo pods? They could be replaced with additional Raptor engines. Thrust structure should be changed so that accommodate this addition, as well as fueling lines. What about whole structure strength? Since stacked BFSs could reach at most 3g acceleration during booster stage, it would not require radical redesign of the lower stage.
There is just one missing piece - new interstage connecting 1st and 2nd stage BFS. It would be essentially the same composite used to build outer tank wall. It would probably be the only non-reusable piece of the whole architecture. Possibly even that could be reused if booster BFS would not used belly for aerobraking but instead use Falcon 9 style tail first renetry profile.
By designing BFS to handle varying number of sea level Raptor engines (between one, seven, thirteen or nineteen engines) with the same tanks and thrust structure essentially any BFS could be used as lower or upper stage. The change would also make BFS sturdier (at the expense of dry mass weight) and more reusable. For example, each booster BFS could be retroffited at the end of usable life to deep space missions.
Such a stack could (barely) reach the orbit using 100tonnes of cargo). Adding 50mT of landing reserve would easily launch all envisioned LEO payloads in the near future. But another advantage is that booster BFS would naturally evolve into tanker version of BFS by having a second set of tanks in the upper (cargo/living) space. It is approximately 1000m3 of volume that can be used for additional 800mT of propellant, bringing the whole stack performance above Falcon Heavy in fully reusable configuration. Plus it could do GTO missions with current satellite payloads (below 7000kg). And with a few more LEO refueling missions, it could easily perform Dear Moon mission with propellant to spare.
So standard BFR booster would only be needed for extremely heavy payloads, such as frequent LEO refueling flights. It this scenario it will not be needed for at least a decade since first BFS flight. There is simply no need for it right now. While significantly lowering initial BFR launch capacity to essentially Falcon Heavy class it acts as immediate, fully reusable replacement of Falcon rocket family for most payloads. It further shortens time to first LEO flight by using the same hardware twice. Same landing gear. Same ground processing. And in this architecture, nothing prevents BFR evolution to full stack capacity as initially described in IAC 2017 presentation.
What would be the total lift capacity of BFR Booster alone to LEO?
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