Looking at pictures of Titan rockets with solid rocket boosters, the bell is at an angle. Why would they do that?
https://historicspacecraft.com/Rockets_Titan.html
https://en.wikipedia.org/wiki/Titan_%28rocket_family%29
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asked Feb 22 at 15:07
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3$\begingroup$ "The nozzle is canted 6° from the axis of the SRM so that the un-deflected thrust is in the direction of the centre of gravity of the Titan 3 stack" Source:repository.tudelft.nl/islandora/object/… $\endgroup$– OhsinFeb 22 at 15:39
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2$\begingroup$ Not just Titans, Deltas and many others nick-stevens.com/wp-content/uploads/2016/11/… Probably pretty much any that don't have a gimbaled nozzle. $\endgroup$– Organic MarbleFeb 22 at 16:27
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1 Answer
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As Ohsin comments above, the SRB nozzles are canted to direct the thrust approximately through the rocket's center of mass. This prevents any imbalance in thrust between the SRBs from creating a significant rotating torque -- instead, thrust imbalance will tend to make the rocket sideslip.
The downside of doing so is that some of the SRB thrust is wasted, the horizontal component of the booster's thrust being canceled out by the one on the opposite side. The vertical component of the thrust is given by the cosine of the cant angle (and so the effect is often called cosine loss). For a 6° cant, the vertical thrust is about 99.5% of the total thrust.
answered Feb 22 at 15:57
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8$\begingroup$ The fact that the Titan SRB nozzles don't gimbal, but use fluid-injection TVC, was also a reason for setting them up that way. If they gimbaled, they would just move to point through the CG automagically. $\endgroup$ Feb 22 at 16:28
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$\begingroup$ related: Are side boosters sometimes angled even if they are symmetrically arranged? and some more angled boosers $\endgroup$– uhohFeb 23 at 1:40
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$\begingroup$ So interesting and such a simple solution. I noticed the angle, but never questioned it. And I always wondered how difficult it is to keep a rockets centre of gravity centred above the thrust to avoid rotating sideways. $\endgroup$– felixmpFeb 23 at 8:01
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1$\begingroup$ The center of gravity does shift over the course of the ascent as fuel is consumed, so the SRBs are rarely pointed precisely through it. As Organic Marble mentions above, the boosters have a fluid injection system for thrust vector control, so corrections can be made if necessary. Normally the thrust from the boosters is very closely matched, so there's very little torque in any case. $\endgroup$ Feb 23 at 15:30