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Elon Musk tweeted "Flying with larger & significantly upgraded hypersonic grid fins. Single piece cast & cut titanium. Can take reentry heat with no shielding."

These have been commonly known as a large visible element of the Mod 4 Falcon 9 design. (Falcon 9 1.0 has been retconned to Mod 1, V1.1 to Mod 2, 1.1 Full Thrust/1.2 to Mod 3). The other is fuller thrust running the Merlin 1D at even higher thrust than the current 190,000 lbs thrust. (The Merlin Vacuum has been tweeted to have run at 240Klbs, instead of its current 205Klbs, but that was a test run).

What do we know has changed on these grid fins?

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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 have changed as well.

Let's start with a nice shot of the Mod 3 design on a Falcon 1.1 Full Thrust.

Mod 3 design

You can see the size, shape, and design differences in these side by side photos, or as commonly known rocket porn. (Admit it, you just come here for the articles).

Comparing grid fins

Showing some of the structural differences highlighted. You can see that two support columns within the fin are no longer needed. The new fins are a bit bigger. They are unpainted titanium. The mount mechanism uses the same connectors but a different design on the fin components.

Showing structural differences

Now for a profile shot, look at the teeth on that baby... Betcha they got some bite.

Profile view of Mod 4 grid fins

Elon tweeted after the landing: "New titanium grid fins worked even better than expected. Should be capable of an indefinite number of flights with no service."

The previous fins were clearly ablating hard, and getting quite hot during previous landings. Thus these new ones should make reuse one step easier.

I saw a comment that the camera lens on the descent cam did not get covered in goop as usual, and wondered if it was the absence of the ablative paint; in fact in previous recoveries, it was not atmosphere (read: water) getting on the lens cover, but rather the older-model grid fins shedding material. In other words, all boosters with the new titanium fins should be less vulnerable to ablation and thus the camera feed should remain intact.

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    $\begingroup$ The lens fogging corresponded nicely with the fin (paint) burning on GTO flights. Can be seen well in the recent BulgariaSat webcast. But this was LEO flight with partial boostback and so the atmospheric entry was not supposed to be as violent as these clearly-burning instances on GTO missions. $\endgroup$
    – jkavalik
    Commented Jun 26, 2017 at 5:09
  • $\begingroup$ What are the dimensions of the new grid fins? $\endgroup$
    – andrepcg
    Commented Jun 27, 2017 at 8:32
  • $\begingroup$ Amazing that they've engineered it in such a way that they could simply ditch the ablative! $\endgroup$
    – Ezra Bailey
    Commented Jun 30, 2017 at 13:33
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    $\begingroup$ New grid fins: 159 cm high x 123 cm wide, old grid fins 141 cm high x 123 cm wide (measured from photos) $\endgroup$
    – Hobbes
    Commented Jul 15, 2017 at 11:18
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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 stage’s lift during reentry. The fins are placed in an X-wing configuration and are stowed on ascent and deployed during reentry. While the fins are relatively small – they measure just 4 feet by 5 feet – they can roll, pitch, and yaw the 14-story stage up to 20 degrees in order to target a precision landing.

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    $\begingroup$ Note that those dimensions are for the old grid fins (the URL is from 2015). $\endgroup$
    – Hobbes
    Commented Jun 30, 2017 at 15:20

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