Attempt to predict the fourth iteration of the project SpaceX BFR

    In one of the recent tweets Ilona Mask was literally the following:
    By the way, SpaceX no longer plans to upgrade the second stage of the Falcon-9 to ensure reusability. Instead, BFR development is accelerated. New design is impressive! He is really counterintuitive.
    Unfortunately, there are no technical details of the “radically new design” yet.

    Since 2016, this is the fourth revision of the BFR design (formerly ITS). Prior to this, the rocket diameter and Raptor parameters in 2017 were reduced and advanced aerodynamic rudders appeared on the second stage (also known as BFS) in 2018. What can change now? And are these changes counterintuitive? Attempt to answer the first question under the cut, and the answer to the second, we learn only after a while.

    Moor did his job

    Mask has repeatedly demonstrated that any, even the most beautiful, technology for him is only a means to achieve a goal (multiplanetary humanity or the capture of the American launch market - decide for yourself). Under the knife went Faolkon-1 (not so unpromising, given the boom of nanosatellites), the rocket landing of Dragon-2, the overflow of fuel on the Falcon-Hevi, the salvation of the second stage of Falcon. Now, it seems, it’s time to retire the first-step rescue method worked out on the Falcone-9.

    More precisely, the actual rocket landing on its engines may well remain. But braking before entering the atmosphere, it seems, will remain in the past along with the "Falcon". Let me remind you that Entry Burn was a response to the destruction of the first stage when entering the atmosphere. That's just for the Martian mission had to start developing a second stage capable of aerodynamic drag with hypersonic speed. Apparently the work on it is developing successfully - next year it is planned to begin the jump tests of that second stage, the production of carbon fiber structures has already been launched. Only the second stage needs the first one, but it is about four times as large, and the Japanese who ordered the flight over the moon, most likely, want to quickly. Yes, and SLS at the very least is nearing completion, and it should be overtaken in order to increase the chances of closing that SLS in favor of BFR.

    And then the question arises: why is the first stage at all? The prototype BFR, which was supposed to carry a private lunar mission, and so far it is built on the atmospheric "Raptors" (vacuum do not have time to finish) and has a very good Ve delta of about 5-7 km / s. So let him work the first stage at least temporarily! The decrease in starting mass is compensated by the absence of braking before entering the atmosphere. Now the stage can be braked about the atmosphere having a reserve of fuel only for landing on the barge. Or even going to the aircraft landing. But the second stage at the beginning can even be the second stage of the Falcon-9 with the Dragon-2.

    According to last year’s presentation of the Mask, the second stage of the BFR (also known as BFS) should have a dry weight of 85 tons with a fuel mass of 1100 tons. The ratio of the total and dry mass is 13.9, which is quite good, although at the first stage of the Falcon-9 this parameter is at 20. But the Falcon-9 has to spend fuel on braking before entering the atmosphere, and BFS has thermal protection. According to the same presentation, a fully fueled BFS with 150 tons of PN in near-earth orbit will have a delta of 6 km / s. Our ship will have to accelerate from the surface on atmospheric engines, but recalculation using the Tsiolkovsky formula gives 6.45 km / s of delta with PN of 120 tons without taking into account aerodynamic and gravitational losses. To account for them we subtract 1.5 km / s.

    And so, we have 120 tons, accelerated to 5.95 km / s. With a specific impulse of vacuum “Merlin” and 100 tons for fuel, we have an increment of 6.09 km / s. With a dry weight of the second stage "Falcon" within 5 tons, we have over 15 tons for Dragon-2 and almost 1 km / s in reserve. In fact, we obtain a load capacity slightly higher than the one-time version of the Falcon-Heavy, but with first-degree salvation. In my opinion quite logical step.

    What's next?

    Of course, the solution described above is by and large palliative and is designed to achieve a specific goal (flyby of the moon) with minimal costs while maintaining the possibility of modernization to achieve the same Mars. You can still build the first step with the already traditional SpaceX landing with a braking impulse. You can use the old project of the “Triamsk twins” proposed by General Dynamics as a concept for the future of the Shuttle. But the most interesting option is to use the insights of the “sworn friends” from the ULA on the market economy.

    In the original project, Mask was supposed to be filled with tankers launched from Earth. But the Moon, in the sense of a gravitational well, is much more accessible and there is water on it from which hydrogen and oxygen can be produced. Upgrading "Raptor" under hydrogen, or the creation of hydrogen from scratch instead of methane is not seen as something fantastic. In the end, BlueO successfully coped with the second task. At the same time, water on Mars is also available.

    An interesting result of the transition from a fully rocket landing of the first stage to an aerodynamic one is to reduce the speed increment of the necessary second stage to enter low-Earth orbit. So in the example above, this increment was slightly more than 2 kilometers per second. At the same time, the concept of the Interplanetary Transport System was originally based on the fact that the second stage when rescuing the first one on engines required a delta of 6 to 7 km / s, which made it possible to refill the stage in orbit and fly to Mars along a fast trajectory. But at a higher speed of separation, the first and second stages will have to either cut the maximum possible increment of the ship’s speed, or go to the LEO with incompletely developed tanks, which brings the BFR closer to the ULA project even more.

    A hydrogen interplanetary ship with a total mass of 120 tons, accelerated by the first stage to the same 5.95 km / s, will only spend 45 tons of fuel to reach the NOU. After replenishing the expenditure, he will be able to add 3.85 km / s with a final mass of 50 tons. This is not 85 + 150 tons at a speed of +6 km / s which Ilon promised, but the initial mass of the system when starting from the Earth is only 1305 tons, against ~ 5000 tons for the “old” BFR. Unfortunately, starting from Mars to Earth will be obtained only with a final mass of about 35 tons. Total we have 20 tons of Mon, which we leave on Mars before leaving home. And so, PN is almost 8 times less, the rocket is less than 5 times. Winnings are not noticeable. Unnoticed until we consider tankers. BFR required 6 pieces, but our “not too big” rocket - only 1. Because we switched to hydrogen and sacrificed the speed of the flight to Mars.

    Again, the above was only a hypothetical scenario for the development of the Interplanetary Transport System project. What exactly happens in SpaceX time will tell.

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