Why does Starship use gridfins instead of flaps or air brakes?

[RFspace]

This is a thought I had that no one really talks about.
But why did Starship go for those flaps instead of grid fins?

Grid fins are supposed to be the most optimal design for hypersonics, and after all, Starship is nearly the fastest moving aircraft of all time, especially on mars reentry from earth. Gridfins give excellent control authority at those conditions, which is crucial for trying to guide a craft to a precise landing point from orbit or another planet.

How will it slow down without flaps, you might be wondering?  Well, its not like the flaps really are most of the surface area compared to the entire craft belly-flopped, and you could just pitch into a deeper belly-flop. Also, coincidentally, grid fins are terribly performing at low supersonic and transonic speeds, and pool up air, which is actually a good thing because it acts as a massive brake. Once it naturally gets to the lower speeds, subsonic, it flows better and regains its precision, as we need for landing.

Building off of that, gridfins offer control from all angles, 360, as they can rotate to fit the need. This means that Starship wouldn't need any header tanks, as it can fully make its flip, so the fuel falls into its position, then after that light its engines. The flaps need the engines help do the flip, so it needs header tanks while its horizontal or angled.  Not only will it not need header tanks, but it also saves propellant, because it wont be expelled to make the flip, it will just fire immediately doing work.  Additionally, it means Starship could flip down with its ass facing down sooner, and still be able to use the fins to guide it too the site.  Flaps cant really do anything at all once Starship goes into that position. So we could mostly ditch the heavy RCS system, (a very small one for small adjustments in space might still be wise though).

Finally, it seems like it would be way simpler. I dont know what they did/are going to do to protect that joint from the heat. A gridfin could probably be simplified to some rod sticking out, and the twisting mechanism can be inside the crafts body. A full joint seems way more complicated.

This is my take on the situation. I'm really interested and curious to hear input from others.

[tyrred]

No one is really talking about your idea because it frankly does not do the job.

Form follows function.

Grid fins on boosters help guide a booster only during certain aerodynamic regimes, mind you boosters traveling only a small percentage of orbital velocity.

The elonerons/flaps/whatevermacallits on Starship are designed to guide a Starship through entry/descent from orbital or even faster velocity coming in hot from Mars.

Not even the same ballpark of function.

[deltaV]

Convective reentry heating is inversely proportional to the square-root of radius - see Chapman equation (slide 17) in https://tfaws.nasa.gov/TFAWS12/Proceedings/Aerothermodynamics%20Course.pdf. So things with small radius overheat. This is why orbital reentry vehicles never have sharp points or sharp edges in the airflow. Grid fins have leading edges with relatively small radius so I'd expect their leading edges would overheat if used at orbital speeds. Falcon 9's first stage is going an order of magnitude slower than orbital speeds so heating isn't as bad for it. I'm not sure of this though - I'm not an aerodynamics person.

[Stan-1967]

How will it slow down without flaps  .....

The ballistic coeficient drives this.  Flaps are better for slowing down than grid fins, but it will still slow down.  The point is too slow down with control of trajectory.

....
But why did Starship go for those flaps instead of grid fins?

Grid fins are supposed to be the most optimal design for hypersonics,
....

...Also, coincidentally, grid fins are terribly performing at low supersonic and transonic speeds,

...Once it naturally gets to the lower speeds, subsonic, it flows better and regains its precision, as we need for landing.

...Building off of that, gridfins offer control from all angles, 360, as they can rotate to fit the need.

I think you need clarity on your own understanding on what gridfins can & cannot do within specific aerodynamic regimes.  Also, gridfin rotation looks to be limited to something like +- 10 degrees.   Watch a F9 landing at an accelerated frame rate & see very little deflection is needed.  Too much deflection will stall the airflow through the grids.

This means that Starship wouldn't need any header tanks,

Header tanks are needed to manage the CG.  The flap placement manages the CP, as well as pitch, yaw, & roll during the bellyflop, which controls the movement & trajectory during the bellyflop maneuver

Additionally, it means Starship could flip down with its ass facing down sooner, and still be able to use the fins to guide it too the site.  ...

Once Starship does the flip, the engine gimballing has all control authority needed for the precision landing.  This is true on F9 as well.  Once the landing burn starts, the engines do more to affect precision landing than any flaps or grid-fins. 

Flaps cant really do anything at all once Starship goes into that position. So we could mostly ditch the heavy RCS system, (a very small one for small adjustments in space might still be wise though).

Grid fins provide very little control as the speed rapidly declines during the landing burn.  So not much different than the flaps.  Again, the engine takes over the bulk of the control.  RCS is needed for fine adjustment if Starship is to land in the Mechazilla arms.

Finally, it seems like it would be way simpler. I dont know what they did/are going to do to protect that joint from the heat. A gridfin could probably be simplified to some rod sticking out, and ......

Any metal sticking out unprotected by some heatshield into the airflow during reentry from orbital speed will become melted slag.  Gridfins present more problems than solutions for orbital reentry.

[dondar]

Convective reentry heating is inversely proportional to the square-root of radius - see Chapman equation (slide 17) in https://tfaws.nasa.gov/TFAWS12/Proceedings/Aerothermodynamics%20Course.pdf. So things with small radius overheat. This is why orbital reentry vehicles never have sharp points or sharp edges in the airflow. Grid fins have leading edges with relatively small radius so I'd expect their leading edges would overheat if used at orbital speeds. Falcon 9's first stage is going an order of magnitude slower than orbital speeds so heating isn't as bad for it. I'm not sure of this though - I'm not an aerodynamics person.

Aluminum grid-fins of Falcon 9 boosters were regularly cooked up, so SpaceX in the end has  replaced  aluminum by significantly more expensive titanium. And Falcon 9 booster enter atmosphere at pedestrian speeds.
There is another thing, grid-fins are effective brakes only in the transonic region which makes them pointless in Starship case. You want to brake early and in the high atmosphere where the air friction is less of a bitch. And you need to shed a lot of energy away preferrably keeping the shock-wave detached from you vechicle, you do not want to focus plasma along ship surface (lol) (observe Falcon 9 body above grid-fins for vivid illustration)..