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I'm not sure about the total thrust of SLS Block II (Crew Stack). So the math could be wrong. Since the Atlas 5 has a version where 4 SRBs are used, is this also possible for SLS?

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    $\begingroup$ It is not, because the core stage as designed cannot withstand the material stresses nor does it have hardpoints to attach a second pair of SRBs. $\endgroup$ Mar 3 at 21:03
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Such a thing might be possible but it would involve a major redesign of the rocket. All of the forces on the central core would be different so the core would need to be reinforced. As SpaceX found when they produced Falcon Heavy from Falcon 9 it is not a straight forward issue.

There are many other issues as well, not show stoppers but they all add complications. When twin side boosters are exhausted and released they symmetrically fall away to either side. But with four one will have to be ejected with an upwards component or the release will need to happen in two stages with a rotation in between.

It would also mess with the aerodynamics of the whole vehicle, the point it reaches Qmax and the throttling arrangements. All of this can be dealt with but the complexities may not be worth it. Rockets aren't Lego.

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  • $\begingroup$ "But with four one will have to be ejected with an upwards component or the release will need to happen in two stages with a rotation in between." Why? Couldn't the boosters be spaced evenly around the core? Some Deltas had 8 boosters, and they didn't eject anything upwards. It sounds to me like you are just making stuff up. $\endgroup$ Mar 3 at 23:53
  • $\begingroup$ @OrganicMarble I think the those large SRB's need a very large separation force when the SLS is pitching down the range. The SRB towards the earth might impact the rocket on separation. $\endgroup$
    – Ashvin
    Mar 4 at 1:08
  • $\begingroup$ @Ashvin what's the separation altitude? And what's the SRB velocity vector pointed at, at sep? The shuttle ones continued upwards for quite a while after sep. I doubt the traj is much different at sep for the SLS, but I'm willing to be convinced otherwise, if you have data. $\endgroup$ Mar 4 at 1:28
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    $\begingroup$ The Russian Soyuz launcher uses four boosters which are jettisoned at the same time. What's the difference? $\endgroup$
    – GordonD
    Mar 4 at 9:56
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    $\begingroup$ Your comment makes your theory even more confusing. How are four evenly spaced boosters not symmetrical? $\endgroup$ Mar 4 at 14:58

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