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Oct 02, 2019, 02:31 AM
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Hard times in BFR land

Very disappointing watching the BFR becoming more & more complicated over the years. The original concept was the simplest because it had 1 purpose: moving humans & cargo to Mars. For every trip, it landed with everything it took off with. They could load it with Martian regolith for the return trips. This way, the center of gravity was the same for every trip. The nose payload would always balance the weight of the engines, allowing it to reenter as a blunt body. Spaceships need to reenter as a blunt body rather than nose first, to lighten their heat shields.

It had header tanks on the top of the mane tanks. The header tanks fueled the landings. By having them as high as possible, it would reduce the amount of ballast required to weigh the nose down during reentry. By having them inside the mane tanks, they would be insulated for long duration flights.

The problem was to make money, the vehicle had to be repurposed for the core satellite business. Payloads would be dropped off in Earth orbit & the vehicle had to return without a payload. This was the same problem NASA encountered with the shuttle.

Like NASA, SpaceX added wings to try to make it pitch down without a payload. The problem was to still satisfy its original role of moving humans to Mars, the wings needed variable lift. To give the wings variable lift, they had to move forward rather than reenter as a blunt body. This required a heavier heat shield.

The shuttle worked around the center of gravity problem by always having a heavy crew compartment + lead ballast in its nose & having a long payload bay in the center, where payloads could be shifted around. The nose was heavy enough that they always moved the payloads as aft as possible. It didn't have to make money, so it could be very constrained in payloads & devote a lot of performance to ballast.

The BFS has all the payload in the nose because the rear is devoted to fuel & probably doesn't have enough margin to carry lead. It doesn't always have a crew compartment because it needs to make money.

To regain the blunt body reentry, they added canards. They made both the canards & wings able to flip completely out of the way. These surfaces were now more for creating drag than lift. This allowed varying the amount of drag without forward motion, allowing reentry as a blunt body. The canards & wings could now shift the balance anywhere along the fuselage, depending on what payload was in the nose. With no payload, the canards could swing out of the way & provide no drag to the nose while the wings could put drag into the rear end. With a payload, the canards would swing into the airstream & provide drag.

In the latest iteration, Elon has elected to move the header fuel tanks into the nose. It seemed like a temporary measure to fit more propellant in the mane tanks & give the BFS prototype extra performance, but in an interview he said they were going to make the nose tanks integral into the design. They would now need a 3rd set of header tanks inside the mane tanks for long duration flights, tanks in the nose to balance the CG, & the mane tanks, despite having the 4 control surfaces.

It was all incredibly complex & taking more room away from the payload. The payload was now shifting to the center, as it did in the shuttle. They also reduced the weight of the engine section by only having 6 engines. This is the least number of engines of any design.

Most likely, it was driven by the move to a heavy, conventional heat shield made of glass tiles. To save fuel, they would need more drag during reentry, have the canards provide more drag all the time & this required shifting the CG with fuel instead of drag.

They also added many large fins to the booster in exchange for not landing on the launch mount, but this isn't as brutal as the complexity they added to the ship.
Last edited by Jack Crossfire; Oct 02, 2019 at 11:29 AM.
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