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The "standard" Block 2 (about to become Block 3) Starship does its deorbit burn and its flip-and-landing burn on the propellant in the header tanks, so SpaceX says. They also said that those were transferred to the main tanks in the Block 2 design to make the landing burn. (Note that there is really nothing "standard" about Starship until all the experimental flight testing is successfully done.)
That transfer from headers to main tanks may or may not still be true in Block 3, as I saw in some illustrations plumbing direct to the engines from the header tanks. Those header tanks hold something like 20-25 tons at most, not anything like 100 tons! The ship explodes upon toppling over in the Indian Ocean, because not all of that 20-25 tons was used to make the landing. There are still fuel and oxidizer aboard when it falls over and breaks open.
I do not know of any variants yet being seriously considered except the HLS and some sort of propellant tanker. And that will likely be true until after the next round of Block 3 flight tests are done! Then it gets bigger yet again in the Block 4 design, which must again be proven in test before it can be considered as a viable prototype for any sort of real mission work. Excepting maybe HLS and the tanker, any other variants must wait until everything through Block 4 has been tested and found adequate.
As for the ballistic coefficient of a Block 2 or Block 3 Starship, the hypersonic drag coefficient of a round cylinder dead broadside to the wind is about 1.20 based on the cylinder blockage area, coming from my old Hoerner "drag bible". The drag coefficient of a flat plate normal to the wind in hypersonic flow is about 1.84, based on its blockage area. Same data source. I took a good guess for the effects of the pointed nose, and for the relative blockage areas of body and flaps, and determined a CD = 1.22 on a blockage area of just about 462 sq.m normal to the wind.
Starship does NOT enter normal to the wind, it enters at a nominal angle of attack of 60 degrees, although that varies some about that nominal angle. Thus the normal blockage area is not actually normal (90 deg) to the wind, it is about 30 deg off. That's no big impact on CD, but the effective blockage area is 462 sq.m times a cosine factor of 0.866 for that angled entry.
I used Bob's numbers of 120, 160, and 40 metric tons for the inert mass of Starship, even though I thought (and still think) the 40 ton figure is ridiculously unrealistic. To that I added 20 tons landing propellant, and 100 tons payload at Mars, and I added the same 20 tons of propellant and 0 tons of payload at Earth. That puts mass-at-entry 240, 280, and 160 metric tons at Mars, and 140, 180, and 60 metric tons at Earth.
I did entries at Mars at 7.5 km/s off a fast trajectory, and 2 deg below horizontal. I did entries at Earth at 7.9 km/s out of low circular LEO and 2 degrees below horizontal.
That's where my numbers came from! They should be pretty good, as good as the masses at entry are. The whole thing turned into a sensitivity study with ballistic coefficient the independent variable. The low inerts are ballistic coefficient roughly half those of the heavier values.
GW
Last edited by GW Johnson (Today 11:44:45)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Moving fuels from the compressed tanks to the near empty tanks creates boiling due to a much lower pressure. It would be better to use a tank in a tank and a control valve.
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