At around the same time, Stephenson experimented with front-mounted cylinders.
The unsuccessful 0-4-0 Invicta, built in 1829 immediately after Rocket, still had them at an angle.
The successful 2-2-0 locomotive Planet (1830) had internal front-mounted cylinders set to the horizontal.

Engines built to the Planet design and the subsequent 2-2-2 Patentee design of 1833 made the design of Rocket obsolete.
At around the same time, Stephenson experimented with front-mounted cylinders. The unsuccessful 0-4-0 Invicta, built in 1829 immediately after Rocket, still had them at an angle. The successful 2-2-0 locomotive Planet (1830) had internal front-mounted cylinders set to the horizontal.
Engines built to the Planet design and the subsequent 2-2-2 Patentee design of 1833 made the design of Rocket obsolete.
Stephenson's Rocket