When more than one side booster is added to a first stage to increase thrust and they are distributed uniformly around the core (as opposed to those shown in links below) thrust does not seem to need to be angled. They could all point straight down (except for dynamic thrust vectoring for steering) due to symmetry.

In the two images below, both from the recent NASA Spaceflight article Beidou-3 MEO-5 and MEO-6 launched by Long March 3B it looks to me like the exhaust from the boosters is angled outwards, I've added an annotated, cropped, and enlarged version to better show what I think that I'm seeing.

Is this in fact the case for this launch, and does this happen frequently?

Beidou-3 MEO-5 MEO-6 Long March 3B Beidou-3 MEO-5 MEO-6 Long March 3B

![Beidou-3 MEO-5 MEO-6 Long March 3B]https://i.stack.imgur.com/vJJZQ.jpg

Beidou-3 MEO-5 MEO-6 Long March 3B

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    $\begingroup$ Thrust vectors are probably oriented to intersect the mean center of mass. That way, if one engine fails, you don't get a sudden pitch or yaw moment from the asymmetry. $\endgroup$
    – Anthony X
    Jul 29 '18 at 4:50
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    $\begingroup$ @AnthonyX I see, so approximately "torqueless". Losses due to $\cos(\theta)$ are small, and resulting radial loading due to $\sin(\theta)$ is manageable? $\endgroup$
    – uhoh
    Jul 29 '18 at 4:54
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    $\begingroup$ That configuration with the nozzles angled out was also used for the Ariane 1-4. $\endgroup$
    – Hobbes
    Jul 29 '18 at 8:10
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    $\begingroup$ @Jack Tangential gimbaling would be for steering control -- swing two opposing nozzles in the same direction and the vehicle pitches or yaws; swing them in opposite direction and it rolls. $\endgroup$ Jul 29 '18 at 17:08
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    $\begingroup$ And the Snark $\endgroup$
    – user20636
    Jul 30 '18 at 9:21

Caveat: This is a somewhat speculative answer based on reasoning alone

The side booster nozzles appear to be oriented such that their thrust vectors intersect what appears to be the vehicle's nominal center of mass. The rationale for this would be to avoid or at least mitigate the effect of a thrust asymmetry if a booster fails, underperforms, or burns out prematurely. The idea would be that thrust will always be applied through the vehicle's center of mass (except where commanded as a correction or steering maneuver). Any off-center thrust would apply a torque in the pitch and/or yaw axis, which would cause the vehicle to pitch or yaw.

  • $\begingroup$ Would you think (also based on speculative reasoning, you have more of a handle on it than me) that this angle would/should be maintained through the entire first stage ascent? Would it make sense to stop the angle after max-Q or at a specific point of ascent? $\endgroup$ Sep 17 '18 at 19:35
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    $\begingroup$ @MagicOctopusUrn I believe the engines on the Long March 3 boosters are only tangentially gimbaled (as pointed out in comments above), so the outward angle is maintained for the entire booster burn. Ditto any fixed-nozzle solid boosters. Gimbaled booster engines might be mounted so that their neutral position is canted outward like this, but during nominal flight they could steer directly aft to claw back the cosine loss. $\endgroup$ Sep 17 '18 at 19:45
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    $\begingroup$ Adding some speculation to yours: If you lose a strap-on booster early, the mission is over, catastrophically, so who cares about preventing a yaw moment then? I'll bet that they aim the strap-on's thrust through near where the expected COM is at burnout, because that's where preventing an unexpected yaw moment (from slightly early burnout on one side) could save the mission. $\endgroup$ Sep 17 '18 at 20:55
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    $\begingroup$ @WayneConrad Loss of a single strap-on booster isn't necessarily a loss-of-vehicle/loss-of-mission situation, particularly with a liquid booster, and particularly if it's not too early in the burn. A number of launches have lost an engine on ascent and continued on to partial or full success (e.g. AS-101, Apollo 6, Apollo 13, Falcon 9 CRS-1). If a booster failure causes a large yaw moment, though, it will be a loss-of-vehicle situation. $\endgroup$ Sep 17 '18 at 21:59
  • $\begingroup$ @RussellBorogove If you lose an engine in a multi-engine liquid stage, the remaining engines can continue to burn the fuel intended for the lost engine: you lose thrust, but you don't lose delta-V (I think). But if you lose a strap-on solid motor, you're not getting anything out of its fuel anymore. Now it's just weight. Plus, how do you lose a strap-on solid motor in a benign way? Don't they tend to burn until they either burn out more or less normally, or stop in some way that ruins your day? $\endgroup$ Sep 17 '18 at 22:49

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