In this picture it shows that the side boosters of the Falcon heavy will land. However, the middle booster is not going to land. Why is this the case?
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asked Nov 1, 2022 at 11:39
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$\begingroup$ Mostly a fuel thing. Depending on the weight/speed they need to achieve, there might not be enough fuel left in the boosters for them to decelerate and attempt to land again. $\endgroup$– KazCommented Nov 2, 2022 at 4:08
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4 Answers
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The Falcon Heavy is a flexible system. Depending on how much you are willing to pay, in order to get the proper payload and orbit.
To date (3 launches + one today in theory as I write this) they have tried to land the center core downrange. 2 out of 3 failed to land due to a variety of issues, and the third did actually land, but the Octograbber robot, which is designed to drive under the landed booster, grab it on the launch mounts, and stabilize it until a crew can come aboard to check it out, could not grab the booster, since a F-9 has different undersides than the F-H (They use the connectors to connect the side boosters).
The Octograbber needed updates to its 'grabbers' to be able to grab a Falcon Heavy core stage, and there were heavy seas, and in the swish slosh of the waves, it slid off the deck.
In this launch (Nov 1, 2022) they need the performance for a direct to GEO launch, which requires expending the center stage.
There is a mode, where all three boosters burn to exhaustion and none are recovered for max payload/performance but no customer has yet requested this configuration. (But that could change!)
There is a mode, where the center core is still expended, but the two side boosters land on a pair of ASDS barges for additional performance.
This flexibility allows SpaceX deliver on many possible orbits and performance targets. With different costs, since a booster is now kind of valuable, if it can be reflown 15 or so times.
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8$\begingroup$ Up until a week or two ago, they were still targeting a dual-droneship landing for the side boosters, i.e. your second alternative mode. Current speculation as to the reason for the change is that some super-secret payload was removed from the manifest on short notice, thus reducing the payload mass / energy requirements. $\endgroup$ Commented Nov 2, 2022 at 13:21
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2$\begingroup$ @JörgWMittag I had thought they were doing dual down range landing as well, earlier, and was annoyed as it takes two ASDS ships out of the rotation and slows down the cadence. I did not think about when they switched it. Interesting. A real sign of the flexibility of SpaceX to turn ona dime like that. (And a change within a year is considered a dime in this industry). $\endgroup$– geoffcCommented Nov 2, 2022 at 14:26
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It's perhaps worth noting that Falcon Heavy is difficult to operate in a fully-reusable configuration. Consider that it basically offers two use cases relative to the Falcon 9...
- Payloads which are heavy (more power!)
- Payloads which are going to high-energy orbits (more speed!).
It's the latter we're dealing with in this case - the payload isn't super-heavy, but the Falcon is taking it all the way to geosync orbit. To do that, they need to get as much velocity has possible out of both first and second stages, and that means the centre core is moving about twice as fast as a normal Falcon 9 booster would be at stage separation.
And that's hard to recover from. The faster it's going, the harder it is to slow down for a landing - more fuel needs to be saved for a re-entry burn, because otherwise you're dealing with a lot more heating, and your booster is getting cooked. And more fuel saved for landing means less fuel available to accelerate the payload.
Basically, there's a narrow window of flight profiles where a full-reuse Falcon Heavy is useful. E.g. it worked for ArabSat because the Falcon only needed to send that to GTO, not all way to GEO. But for a direct-to-GEO mission like USSF-44, full recovery just isn't worth the effort.
(A small addendum after actually watching the launch... the centre core was travelling at about 4km/s when it shut down, and was already past the Bahamas and well on the way to Bermuda. Good luck bringing that down safely...)
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5$\begingroup$ Not only is the center core going twice as fast as a normal booster, it is also heavier. Since energy goes up linearly with mass and with the square of the velocity, the center core will have have more than four times as much energy to shed than a normal F9 booster. And then it also takes twice as long to bring it back to port, during which time the drone ship is unavailable for other launches! Once you get to launching weekly, you get into some really interesting economics, where it might actually be cheaper to expend a rarely used rocket (center cores cannot be converted to normal boosters) $\endgroup$ Commented Nov 2, 2022 at 13:29
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3$\begingroup$ … than to block your recovery assets for a week. $\endgroup$ Commented Nov 2, 2022 at 13:29
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1$\begingroup$ @JörgWMittag agreed, the economics of recovering multiple boosters at sea are iffy, given impact on scheduling other flights around drone ship availability. I suspect we'll see this model for many of the future Falcon Heavy launches... RTLS for the sides, expending the centre. $\endgroup$ Commented Nov 2, 2022 at 20:00
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1$\begingroup$ @JörgWMittag - I'm curious what you mean that the centre core is heavier, though. Can you elaborate on that? I'm aware that it has some structural strengthening, but not sure that's a major factor. $\endgroup$ Commented Nov 2, 2022 at 20:01
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$\begingroup$ @SimonGeard A more likely scenario for many of the scheduled future Falcon Heavy launches, is to move them to Starship. Just like some Heavy launches were moved to single-stack F9s (as that rocket gained performance). $\endgroup$ Commented Nov 3, 2022 at 16:21
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The Falcon Heavy is essentially a 2½ stage rocket, just as the Space Shuttle was a 1½ stage rocket. In both cases, the side boosters are the first half stage. In the case of the Falcon Heavy, the side boosters do most of the heavy lifting during the initial phases of launch. The central booster is intentionally throttled down so that it can retain a good amount of propellant after booster separation. The central booster is then throttled up so as to add a needed extra kick. The central booster will "land", if by "landing" one counts burning up on reentry or dropping into the ocean as "landing".
Or as Meatloaf once sang, "Two out of three ain't bad".
answered Nov 1, 2022 at 12:03
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3$\begingroup$ nit: Conventionally, a "half stage" consists of engines that are dropped without dropping any fuel tank. For instance, the original Atlas rocket had three engines with a common fuel tank; two of the engines were jettisoned partway while the middle engine continued to draw on the common fuel tank. (This is very much a terminology nit, and to be fair, I can't in short order find any good reference for how you should count side boosters. But a "half stage" generally refers to something different.) $\endgroup$ Commented Nov 1, 2022 at 21:53
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1$\begingroup$ @CharlesStaats I counter that "half stage" is any intentional detachment of hardware that is less than a full stage - a full stage being all the engines and tanks used up until the detachment event. The early Atlas flights were one form of half-stage, the Soyuz and Falcon heavy are another. $\endgroup$ Commented Nov 3, 2022 at 16:18
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$\begingroup$ I feel like if the Shuttle is considered to have a half stage, the Falcon Heavy should as well, given that they both drop a pair of side boosters. $\endgroup$– SkylerCommented Nov 3, 2022 at 16:24
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$\begingroup$ As a side note, I want to see what a Falcon Heavy-Centaur can do. (Take that wimpy upper stage off and fit a Centaur there instead.) $\endgroup$– JoshuaCommented Nov 3, 2022 at 18:10
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$\begingroup$ @Skyler As far as I can tell, the Shuttle isn't normally considered to have a half stage. (In fact, the only rocket for which I can find any reference of a "half stage" is the original Atlas rocket that dropped two of its engines.) I do think it would be reasonable to consider the Shuttle's big orange fuel tank (but not its side boosters) as a half stage. $\endgroup$ Commented Nov 3, 2022 at 21:40
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In this particular mission's ( USSF-44 ) launch profile, the centre core ( middle falcon 1st stage ) is expended and wont be recovered. It also does not have grid fins and landing legs to save cost and increase performance.
