# Why are the Falcon 9 Stage-1 engines placed with offsets in the octaweb compartments?

Pictures of the octaweb show that the engines are not placed at the center of each octaweb compartment---they are closer to one side than to the other.

See the picture below for an example. Focus on just one section of the octaweb---any of the white panels at the top will do. The engine is much closer to the right side of the panel than to the left of it. You can trace the same pattern all the way around the octaweb.

Why? Is this just to make space for other components that must go in the same compartment? If so, then what might those components be?

• This is an odd, and I think incorrect, use of the word 'asymmetrically'. Clearly the layout has rotational symmetry. Commented Jun 5, 2020 at 11:56
• I think you're confusing symmetry with "how the enclosing panels are designed for optimum assembly and strength" Commented Jun 5, 2020 at 12:13
• Consider the radial centerline of an octaweb section. If the engine were on that centerline, then the right side of the section would have mirror symmetry relative to the left side. But the engine is offset from the centerline, so that symmetry is broken. This is splitting hairs, but you can talk about symmetry here. That said, you're right that I was casual about my terms, though I think you can still get the gist of the question?
– user36480
Commented Jun 5, 2020 at 20:45
• Keep in mind, these are bypass engines. Those 8 smaller circles also spit fire. Commented Jun 8, 2020 at 20:14
• As a former mathematician, I think it's perfectly reasonable to ask why each individual panel is not bilaterally symmetric. As a rocket enthusiast, I acknowledge this is not the kind of symmetry that is most significant in rocket design. Commented Dec 16, 2023 at 20:01

The engine placement looks symmetrically to mee in this image.

Outwardly, the biggest change is an arrangement of eight engines in a circle around the center engine. Previously, the nine engines were built in three rows of three. SpaceX says the octaweb reduces the length and weight of the Falcon 9 thrust structure and streamlines manufacturing of the first stage. While this is the first NASA mission with an octaweb first stage, the engine pattern flew successfully during several previous SpaceX missions launching commercial payloads.

To place each engine not in the center of each octaweb section would reduce the thrust structure mass and improves the access to the engines during manufacture.

This graphic shows the thrust structure for the offset version:

And this is the centered version:

The additional brown parts are needed for a stable centered mount of the engines. So the centered thrust structure weighs a little bit more.

• There is rotational symmetry, but you can see that each engine is offset to the side of its octaweb section. The offset is clockwise so that the top engines are closer to the right than to the left of their octaweb compartments. You can see a smaller hole just below and to the side of each engine, which is the gas generator exhaust, so my guess is the offset is there to accommodate that---but it would be nice to know for sure!
– user36480
Commented Jun 5, 2020 at 10:06
• @Alex I don't think your downvote is justified Commented Jun 5, 2020 at 12:14
• I didn't downvote, Carl. I appreciate all the comments and answers I can get, yours included. Why do you say I downvoted?
– user36480
Commented Jun 5, 2020 at 20:40
• Makes sense, Uwe, but how would this asymmetry reduce mass?
– user36480
Commented Jun 5, 2020 at 20:49
• But Uwe, the engines are gimballed. The gimbal joints attach to plates directly above the engines, as do the thrust-vectoring cylinders. There are no connections to the radial side plates anywhere in the octaweb. In fact, shifting the engines so close to those side plates limits the maximum gimbal angle you could apply to the engines. Very nice graphics, by the way :D
– user36480
Commented Jun 8, 2020 at 20:54

This article uses the same picture and seems to confirm your guess that the offset is for the gas generator exhaust (the smaller holes next to the Merlin nozzles)

https://www.thespacetechie.com/octaweb-structure/

The engines are placed in offset

All the Falcon 9 engines apart from the center, are placed in offset with respect to the compartment. This is to make additional space for the exhaust of the gas generator. (Merlin engine runs on a gas generator engine cycle.)

Hypothetically they could also have made space for the gas generator exhaust to fit symmetrically in one compartment by making the whole octaweb structure bigger. However the picture with the compartment covers removed seems to show something close to the smallest possible structure, which is to put the nozzle and the gas generator exhaust along the diagonal of the compartment. This means the nozzle ends up in one of the outer corners and the gas generator exhaust ends up in the opposite inner corner.

https://blogs.nasa.gov/spacex/2014/04/18/meet-the-octaweb/