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According to Wikipedia https://en.wikipedia.org/wiki/Star_48#Use_on_New_Horizons a solid fuel Star 48 “kick stage” was chosen as the 3rd stage in the launch of Europa Clipper.

Solid fuel rockets have less precise burns than liquid fueled rockets since they cannot be throttled, stopped or restarted. Solid fuel rockets can only be "throttled" by making self-cancelling wiggles in trajectory. This "unthrottlability" is irrelevant for boosters.

The burn of a liquid fueled top stage can be fine tuned to correct small errors in the burns of previous stages.

Why was the Star 48 chosen over a liquid-fueled 3rd stage? The Space Shuttle used Star 48 for launch of GSO satellites to avoid the safety hazards of liquid fuel in the cargo bay, but this obviously doesn’t apply to an uncrewed Falcon Heavy launch.

Edit: Barry's answer points out that the 3rd stage of Europa Clipper's launch vehicle has been eliminated due to a change in mission profile. So the question could be changed to "Why would a solid fuel rocket be chosen as the 3rd stage for the Europa Clipper launch, or any similar uncrewed launch?"

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  • $\begingroup$ Must be a typo, Star 48 is to be used, not 44. $\endgroup$
    – blobbymcblobby
    Nov 25, 2022 at 17:32
  • $\begingroup$ @blobbymcblobby ... right you are. Duly edited. $\endgroup$
    – Woody
    Nov 25, 2022 at 18:13
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    $\begingroup$ Feels like 2 questions. Extended 48B gives more kick than standard 48 originally used, which goes some way to compensate for loss of direct flight that SLS would have given. Thrust vectored seems unlikely as its just not a common option. There was something about SLS being too wobbly for Clipper and some dampening was required, so may be a spin-stabilized 48B (and not a thrusting V) is more.. stable.. less vibration.. $\endgroup$
    – blobbymcblobby
    Nov 25, 2022 at 18:29
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    $\begingroup$ @blobbymcblobby Since the spin stabilization is likely to start after the separation of the craft from SLS, SLS being wobbly might not be the reason spin stabilization is used. $\endgroup$
    – AJN
    Nov 26, 2022 at 3:14
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    $\begingroup$ Related: space.stackexchange.com/a/14554/195 $\endgroup$
    – Russell Borogove
    Nov 28, 2022 at 4:00

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I'm not sure whether a version was even chosen because it was removed over 2 years ago with a Mars gravity assist route. The 6065kg to 41.69 km²/ sec² is within regular FH capabilities. This is the fifth time I've had to explain this from this https://www.youtube.com/watch?t=112&v=KK74a6FAnSg&feature=youtu.be

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