Here is a diagram from a recent Ars Technica article about the H3:

enter image description here

You can see at the bottom that the main engines are not 90 degrees off from the side engines. I'm sure that this has been accounted for and the rocket will not fail because of this, but why was the design chosen to be this way? Why not go with a cross pattern at exactly 90 degrees?

  • $\begingroup$ As per hobbes answer this rocket does have symmetric thrust, but many in fact do not and have to change the thrust vector to keep it through center of gravity, including space shuttle and Atlas V youtube.com/watch?v=f-o6BLBzsUA $\endgroup$ – GremlinWranger Dec 22 '18 at 8:57
  • $\begingroup$ The engine configuration is not off center. All pairs of engines or boosters have equal distance to the central axis of the rocket and are on a line through the axis. An angle of 90 degrees is not necessary. There is no 90 degerees angle at a symmetric configuration of 3 or 6 engines. $\endgroup$ – Uwe Dec 22 '18 at 16:20

There is a difference between 'off-center' and 'not in a geometric pattern'. The engines are not off-center. The axis through the 2 core engines just does not line up with the boosters.

Why not a cross pattern at 90º: because there is no reason to do so.

The core engines are placed to provide a thrust vector that goes through the center of mass. The boosters are also placed to provide a thrust vector that goes through the center of mass. So the rocket is balanced in both phases of flight (before and after booster separation).

This is not the only launcher that does not line up all engines in a regular pattern:

F9 coordinate frame

(image from the F9 user manual) This is the coordinate frame for the Falcon 9. The 8 engine bells are rotated a bit in relation to the Y and Z axes.

The strongback is in the +Z direction. In a Falcon Heavy, the 3 cores are along the Y axis, so the FH is not symmetrical around the XY plane.

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    $\begingroup$ Even given this there is probably a reason for it, I do not belive they choose this at random or for astetical reasons. $\endgroup$ – lijat Dec 22 '18 at 14:46
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    $\begingroup$ Doesn't address "but why was the design chosen to be this way? " $\endgroup$ – Organic Marble Dec 22 '18 at 15:18
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    $\begingroup$ At a guess, it has to do with optimizing the engine bay layout for both the 2-engine and 3-engine cases. Look at where the propellant lines enter the engine bay. But I haven't found a source for this. $\endgroup$ – Hobbes Dec 22 '18 at 15:41
  • $\begingroup$ Yeah, I'm still curious as to why they actively made this choice. Also with the F9, isn't it still perfectly symmetrical? The M3 only has rotational symmetry. $\endgroup$ – David says Reinstate Monica Dec 22 '18 at 16:09
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    $\begingroup$ The FH only has rotational symmetry too. $\endgroup$ – Hobbes Dec 22 '18 at 16:33

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