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With February 6th's launch of Falcon Heavy we saw the landing of both side boosters. During the past few weeks and again in the press conference today I've heard over and over about the increased difficulty in controlling these boosters for landing over a plain F9 mission. Specifically because they have nose-cones the aerodynamics are different and apparently the grid fins are only able to provide less control authority in that configuration. As a result the side boosters (but not the center one) got the upgraded Titanium fins with larger control surfaces to make up the difference.

My question is why go to all the trouble and expense of upgrading the grid fins to make up for poor aerodynamic profile? Why not just fix the profile? If the blunt end of the rocket as found on the Falcon 9 after stage separation is better, why not jettison the side booster nose cones after separation and before landing? It seems like that would be a relatively chap and easy part to just ditch.

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    $\begingroup$ SpaceX's is all about reusablity. Jettisoning things is the opposite of that. $\endgroup$ – user23073 Feb 7 '18 at 17:18
  • $\begingroup$ To add to @SkyRate's comment: SpaceX tries/plans to retrieve the fairings, which are similar in complexity to the nose cones. $\endgroup$ – Jochen Lutz Feb 8 '18 at 15:35
  • $\begingroup$ @JochenLutz While it makes sense that it would be a step backwards on the re-usability front, you can't tell me the nose cones have the same complexity and cost that goes into the main fairing. Sorry, but that's more than a bit of an exaggeration. Given that the answer seems like it is going to be "it was simply more efficient to boost control authority rather than improve aerodynamic profile", exaggeration doesn't help here. $\endgroup$ – Caleb Feb 9 '18 at 8:42
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Getting rid of the nose cones would make things worse, not better. The reason the nose cones allow less control than the typical F9 (or FH center core) is because the cylinder of the rocket usually continues well above the grid fins. The interstage (which extends above the grid fins, and does not separate from the first stage at any point) is the same width as the rocket, but the nosecones taper. That taper means there's a lot more empty air above the grid fins than normal, which reduces the fins' ability to control the rocket's descent.

Source for the cylinder (interstage) being important for control: Elon talking about challenges in building FH during the after-launch press conference. Relevant bit:

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  • $\begingroup$ This makes sense, but by the same token if this was the problem couldn't the nose-cone design have been changed to accommodate some empty space and create an aerodynamic profile similar to the interstage? $\endgroup$ – Caleb Feb 7 '18 at 8:59
  • $\begingroup$ Extending the body beyond the fins would reduce the control authority. A boat tail - which the cones have become now the rocket is going backwards - has a negative impact on stability $\endgroup$ – JCRM Feb 7 '18 at 9:18
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    $\begingroup$ @JCRM I'm no hypersonic aviation or fluid dynamics expert (are you?) but I'm inclined to believe Mr. Musk on this one when he says the current F9 design uses airflow guided by the grid fins and reflected off the interstage for a ~30% increase in control authority, and switching to nosecones reduced that. I don't know why they didn't choose to extend the stages with a fake interstage, given that, aside from the usual reasons of "it costs more to manufacture and reduces payload capacity because it weighs more" that apply to almost everything. $\endgroup$ – CBHacking Feb 7 '18 at 9:44
  • $\begingroup$ @CBHacking Can you edit this answer with your source for that statement? $\endgroup$ – Caleb Feb 7 '18 at 10:22
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    $\begingroup$ @caleb I know this is old, but in case that's still a puzzle - The fins cannot be lowered without exceptional increase in complexity, as they currently sit right around the top of the fuel tank. There is no other space to mount them lower, until you get to the engines. They would need to intrude into the tank, removing fuel, adding more structure, and lots of thermal challenges. $\endgroup$ – Saiboogu Dec 10 '18 at 20:46
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They needed to upgrade the grid fins anyway, this is one of the modifications intended for Block 5. So the new grid fins were available.

This meant the choice is between:

  • switching to more expensive (but reusable) grid fins, or
  • throwing away the nose cones, and adding another separation event, making sure the nose cone doesn't hit the stage after separation, etc. This seems to me to be the more complex (=expensive) choice.

Throwing away the nose cones also means another item that needs to be rebuilt every time a stage is reused, and more work in refurbishing the stage. And it's a step away from the long-term goal of full reusability.

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    $\begingroup$ You're probably right about the unwanted complexities of another separation event, but keeping it from hitting the stage seems like a mater of waiting until it's turned around and started its decent and hit a little atmosphere. I suspect that baby would fall behind pretty fast. Maybe hitting the other side stage would be a bigger risk. $\endgroup$ – Caleb Feb 7 '18 at 9:43
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    $\begingroup$ @Caleb In all your comments, you seem to be missing the main point that SpaceX wants to reuse as much as possible. That's the real reason. Everything else is secondary. $\endgroup$ – ell Feb 7 '18 at 19:00

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