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5 minutes ago, RCgothic said:

 

 

Wow, that makes a LOT of sense. The more pitch authority they can get, the more aggressive body lift they can get, which means less propellant consumption on the boostback burn. And having them out to the sides keeps them out of the booster’s wake.

Super smart. 

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31 minutes ago, sevenperforce said:

Wow, that makes a LOT of sense. The more pitch authority they can get, the more aggressive body lift they can get, which means less propellant consumption on the boostback burn. And having them out to the sides keeps them out of the booster’s wake.

Super smart. 

Would you mind expanding on this?

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4 minutes ago, JoeSchmuckatelli said:

Would you mind expanding on this?

When the booster is tilted away from the direction of flight it generates lift like a very draggy wing (body lift). As the goal is to slow down, both drag and lift is good.

But body lift creates a very turbulent wake on the lee side of the booster (because it's at an angle to the flow). Fins placed in the turbulence are less effective.

Placing them in the side makes them more effective, which means the booster can be tilted more, which creates both more lift and drag.

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1 minute ago, RCgothic said:

Placing them in the side makes them more effective, which means the booster can be tilted more, which creates both more lift and drag

Ah - so this relates to the 'X' shaped placement over the '+' shape of Falcon? 

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14 minutes ago, Beccab said:

First one was spotted earlier. I guess this could mean S20 stack and Raptor installation today/tomorrow?

There are already thrust simulator rams installed at one of the suborbital test stands that would get in the way of Raptors installed on S20, so I'd assume that this is a fit check for now.

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Just now, RealKerbal3x said:

There are already thrust simulator rams installed at one of the suborbital test stands that would get in the way of Raptors installed on S20, so I'd assume that this is a fit check for now.

They still had the internal goal of 5th august having full stack (at this pace it's probably doable by the end of the week) and there are many rumours of Musk wanting to do a mayor event in front of the full stack asap (Mk1 style), so I wouldn't be surprised if they leave them in and stack S20 and B4 for testing and this event, if it exists

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10 minutes ago, Beccab said:

I wouldn't be surprised if they leave them in and stack S20 and B4 for testing and this event

This. And not a complete waste, either, they'd still get tons of useful data on the minutiae of stacking operations/part clearance, all for relatively little risk. I would expect either way to see multiple stackings/unstackings before launch day just to get the flows down. 

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47 minutes ago, JoeSchmuckatelli said:

Ah - so this relates to the 'X' shaped placement over the '+' shape of Falcon? 

Yes. Everything @RCgothic said. Additionally, both Falcon 9 and Superheavy are designed to use body lift to add range during descent.

Performing a boost back burn requires a lot of propellant. Anything that reduces that propellant consumption means more propellant can be used to push the upper stage higher and faster before stage separation. If the fins were fixed, like the ring fin on New Shepard, then the booster would fall back to the ground on a ballistic trajectory like a lawn dart. However, if the booster is able to lean its “head” forward into the wind as it falls, then the entire booster body generates body lift, converting some of its vertical speed into horizontal motion. It’s not much, but it is enough for the booster to be able to boostback only 90% of the way and then glide the remaining 10% (or something like that).

The grid fans don’t need to provide very much roll authority or yaw authority, because the booster is more or less passively stable. However, it takes a lot of pitch authority to lean into the wind like that. So it makes sense to move the grid fins closer together so that they can work together to provide pitch authority.

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11 minutes ago, CatastrophicFailure said:

I remember reading somewhere that Falcon 9 actually has a "glide ratio" of around 1:1, not bad for a long metal tube. :D

That just means lift = drag. Which would be more or less the default for anything -- the forces in one direction being no bigger or smaller than in the other direction.

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1 hour ago, mikegarrison said:

That just means lift = drag. Which would be more or less the default for anything -- the forces in one direction being no bigger or smaller than in the other direction.

My understanding of the 1:1 L/D ratio is that Falcon 9 is able to “glide” at approximately 45 degrees from an otherwise ballistic trajectory.

That is much lower than most aircraft, even the Space Shuttle, but it is much higher than a ballistic object. Even the Apollo Command Module, which was designed to get as much lift as possible, only had an L/D ratio of 0.37:1.

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27 minutes ago, sevenperforce said:

My understanding of the 1:1 L/D ratio is that Falcon 9 is able to “glide” at approximately 45 degrees from an otherwise ballistic trajectory.

That is much lower than most aircraft, even the Space Shuttle, but it is much higher than a ballistic object. Even the Apollo Command Module, which was designed to get as much lift as possible, only had an L/D ratio of 0.37:1.

It's pretty conceptually simple. Glide ratio = Lift-to-drag ratio. Where it gets more complicated is the question of things like aerodynamic stability and control, airspeed limits, etc.

I would disagree that the Apollo Command Module was "designed to get as much lift as possible". Re-entry capsules are generally designed to be much more focused on managing heat and decel loading than on trying to maximize glide distance.

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7 minutes ago, Minmus Taster said:

S20 seems to have gaps in it's heat tiles, in that the areas between sections seem tile devoid, do you think this is permanent?

 

No. They'll fill those in soon, probably after the nose is stacked. But they won't leave those gaps in. Afaik from others, they may also need access to those sections for now.

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