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42 votes
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How are the SpaceX Falcon 9 Mod 3 and Mod 4 grid fins different?

Ok, I asked so I could use these awesome photos in an answer... I confess. The material has changed from Aluminium with an ablative paint to bare titanium. The specific shape, size, and mount points ...
geoffc's user avatar
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20 votes
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Why didn't Saturn V have grid fins? Long March and Soyuz do!

The Apollo capsule was allowed to tumble after being pulled away by the LAS. This tumbling would continue until the first drogue parachute was deployed, which would stabilize the capsule. The LAS ...
Hobbes's user avatar
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18 votes
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Why did the grid fin of the CRS-8/SES 10 booster burn?

According to Elon Musk (during the post-flight press conference): New design coming for Grid Fin. Will be largest titanium forging in the world. Current Grid Fin is aluminum and gets so hot it ...
Hobbes's user avatar
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15 votes
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What do the Teeth on the SpaceX Falcon Mod4 do?

tl;dr - Better aerodynamics at high speeds. There is a paper that compares the plain grid fins with locally swept "toothed" grid fins: Novel High-Performance Grid Fins for Missile Control at High ...
Floern's user avatar
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14 votes
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What are these four "waffle-like" structures on top of the Long March 2F just launched?

From Spaceflight101: For Shenzhou Missions, a stretched version of the typical Long March fairing is used ... Four aerodynamic stabilizers are attached to the upper part of the fairing that is part ...
Russell Borogove's user avatar
14 votes
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Why not jettison the nose-cone from Falcon Heavy side boosters for better control authority?

Getting rid of the nose cones would make things worse, not better. The reason the nose cones allow less control than the typical F9 (or FH center core) is because the cylinder of the rocket usually ...
CBHacking's user avatar
  • 1,542
13 votes

Why not jettison the nose-cone from Falcon Heavy side boosters for better control authority?

They needed to upgrade the grid fins anyway, this is one of the modifications intended for Block 5. So the new grid fins were available. This meant the choice is between: switching to more ...
Hobbes's user avatar
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13 votes
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CZ-4B Grid fins

TLDR the main purpose of the grid fins is to steer the spent 1st stage away from populated areas when it's falling to the grond. CZ-4B targets SSO and is mostly launched from T-SLC (Taiyuan Satellite ...
user3528438's user avatar
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11 votes
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Is SpaceX grid fin material flammable?

As far as I know, the RCS uses nitrogen cold gas thrusters. The grid fins are covered in an ablative material, they are heated by atmospheric friction. The landing is badly filmed because this is a ...
Hobbes's user avatar
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10 votes

On the Falcon Heavy, why are the side boosters using Ti Grid fins, but not the center core?

I know, I know, I am asking, so I can answer. But these are such great photos, and they are interesting questions... From Facebook a possible answer (Not authoritative, I get that) the following: ...
geoffc's user avatar
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10 votes
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What happens if the Falcon 9 launches with its grid fins open?

I can think of three principal issues raised by launching with fins deployed; I would guess that none of them would endanger a mission, but I don't have any inside information to support that. ...
Russell Borogove's user avatar
9 votes

What happens if the Falcon 9 launches with its grid fins open?

If you have noticed, the fins deploy by opening upwards. This makes me wonder if they considered your case. When in normal use, the force vector upon them pushes upwards. In your case, the force ...
geoffc's user avatar
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9 votes
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Why Did Starship 3 launch with its Hypersonic Grid Fins fully deployed?

The four grid fins on the Starship booster weigh 3 metric tons each (6,600 lbs), which is about twenty times heavier than the Falcon 9 grid fins, based on an estimate that I found of 140 kg for Falcon ...
Steve Pemberton's user avatar
8 votes
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Are these statements about "a first use of grid fins by China" simply wrong?

It's pretty rare to see popular news articles get technical details right, and this is no exception. The grid fins referred to in earlier QAs here are, as Polygnome notes, not maneuverable; they ...
Russell Borogove's user avatar
8 votes
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What's behind the design of SpaceX grid fins?

The issue seems to be the need for a single control surface (for simplicity) to work at hypersonic speeds, down to subsonic. Grid fins have been used high speed missles in this speed regime, so why ...
geoffc's user avatar
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8 votes

Why are the new Titanium Grid fins not used on all Falcon 9 landings?

Musk tweeted an answer to a question. Musk Tweet in which he wrote: For now, we only use those on super hot reentry missions. Will go to all Ti with Falcon 9 V5, which is a few months away. So ...
geoffc's user avatar
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7 votes
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Can the grid fins affect the rotation of the Falcon 9 descent?

Yes, the grid fins can control the rotation of the stage -- and they're the primary mechanism for that during descent, since the cold-gas thrusters have little force and the engines are shut down for ...
FLHerne's user avatar
  • 183
6 votes

How are the SpaceX Falcon 9 Mod 3 and Mod 4 grid fins different?

On official spacex website I found dimensions: http://www.spacex.com/news/2015/08/31/grid-fins Falcon 9’s first stage is equipped with hypersonic grid fins which manipulate the direction of the ...
Marcos's user avatar
  • 131
6 votes

Why are the cells on grid fins square in shape?

There are 3 regular geometric patterns you can use: triangle, rectangle and hexagon. All others have irregularities (e.g. very small cells next to larger cells) or space lost (circles). According to ...
Hobbes's user avatar
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6 votes

Is helium used to power the Falcon 9 grid fins?

Helium is a gas at any reasonable temperature, so a system using helium would be pneumatic. Pneumatic systems are less suited for exact position control than hydraulics: because gases are ...
Hobbes's user avatar
  • 128k
6 votes

Is helium used to power the Falcon 9 grid fins?

It was thought, initially that the grid fins used RP1 propellant as a hydraulic fluid, when it was using an open system (The fluid is ejected after use until you run out, which they did). After a ...
geoffc's user avatar
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5 votes

Could SpaceX's grid fins be controlled by electric motors instead of hydraulics?

As a general rule electric motors excel at high speed low loads while hydraulics work best at the low speed high torque end. Especially where the range of motion is small, the forces large and precise ...
GremlinWranger's user avatar
5 votes

How do the F9 grid fins extend and rotate?

The earliest use of the grid fins failed to land successfully because it ran out of hydraulic fluid. They use the RP1 from the main tank to run the hydraulics and they were initially open loop (the ...
geoffc's user avatar
  • 79.6k
4 votes

Why did the grid fin of the CRS-8/SES 10 booster burn?

After the entry burn, the first stage is still going pretty fast - About 8200km/h, or Mach 6.6 or so. Still plenty fast enough for atmospheric friction and compression heating to take their toll. ...
Ryan's user avatar
  • 41
4 votes

How do Falcon 9 grid fins deploy?

They are hydraulics which initially used an open loop, using kerosene as the working fluid. Some of the initial landing attempt failures were because they ran out of fluid. Closed loop would keep the ...
geoffc's user avatar
  • 79.6k
4 votes

Do Falcon 9 grid fins lock up during reentry?

If you look at the webcasts you'll see that they are extended while in free fall (before re-entry) and you can kind of see them bounce and lock into place with a jolt. I believe they are hydraulically ...
BrendanLuke15's user avatar
3 votes

What's behind the design of SpaceX grid fins?

To answer the edited addition: The mechanics to do something like this would: Add considerable mass to the stage This would prevent the use of the stage for the non-recoverable (all fuel used) ...
Baldrickk's user avatar
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3 votes
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What would the reference length be for a grid fin?

A reference "length" can't be a surface area, the units are wrong. I suggest using the major dimension of the fin. Example: For an airfoil, it's the chord length. The introduction to the web page ...
Organic Marble's user avatar
3 votes

On the Falcon Heavy, why are the side boosters using Ti Grid fins, but not the center core?

Part of it is that the shape of the side boosters requires larger fins for maintaining control authority. (The core interstages's cylindrical shape increases it by 30%, according to Musk.) The other ...
SkyRate's user avatar
  • 141
3 votes
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Will the New Glenn have grid fins? Why or why not?

The Falcon 9's grid fins exist to solve a particular problem: Control during engines-first descent. Aerodynamic stability during that phase comes from having the center of mass well below/ahead of ...
Bob Jacobsen's user avatar
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