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I wonder if triangle stack would be a better design to handle thrust and stability of the flight.

enter image description here

Photo source (public domain).

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    $\begingroup$ Wouldn't three in a triangle be more like one single bigger tank with three rockets below ? $\endgroup$ – Criggie Jan 2 '18 at 19:45
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    $\begingroup$ @Criggie that's exactly what it would be... :) $\endgroup$ – RonJohn Jan 2 '18 at 22:05
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    $\begingroup$ What advantages would you expect? I see none but a whole host of problems due to thrust vectoring . $\endgroup$ – Polygnome Jan 3 '18 at 0:18
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    $\begingroup$ I don't understand the question. Can someone who does add a simple image to illustrate the two cases? $\endgroup$ – user985366 Jan 3 '18 at 19:10
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    $\begingroup$ @user985366 - Seen from above (or below), $\underset{\textstyle\text{o}}{\text{oo}}$ versus $\text{ooo}$ . $\endgroup$ – David Hammen Jan 3 '18 at 20:12
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The boosters are in a row because it's easy to assemble them horizontally when they're in a row. If they were stacked, the lower 2 stages would have to carry the weight of the third, so they'd need to be made stronger.

The plan is also to have the center core to run at a lower thrust setting.

In a triangle setup with the 3 cores as they are now, i.e. 1 two-stage rocket with the payload on top, plus 2 boosters), total thrust (as well as weight) would be asymmetric, which is undesirable: to compensate, you'd have to gimbal the engines and thrust at an angle, which is less efficient than thrusting along the velocity vector.

Putting the 3 cores in a triangle has another drawback: the attachment points now overlap each other, making it more difficult to have 2 identical "side" cores.

Underfilling one of the side cores to stagger separation is possible, but reduces performance because the core will shut down earlier.

Back in the 1960s, British company BAC was working on MUSTARD, a two-stage to orbit fully reusable vehicle. This consisted of 3 lifting bodies of identical shape. 2 were the boosters, the third was the spacecraft.

There were 2 ways to stack the 3 vehicles: in line, or in a triangle.

Mustard takeoff

BAC studied both configurations (plus lots of others) and found the in-line configuration was the better one, in part because of the balance problems of the triangle (from 'British secret projects volume 5: Britain's space shuttle', by Dan Sharp, page 149). Given that Mustard was a low-budget R&D program, the fact that different booster configurations have been studied tells me the Falcon Heavy configuration was not arrived at by accident, IOW, SpaceX will have looked at the alternatives and discarded them because they did not offer a benefit over the one they chose to implement.

Finally, consider history: every rocket with 2 boosters has them in line instead of in a triangle. Ariane 4, 5, 6. Titan IIC, 34D, Delta IV Heavy, Space Shuttle, to name a few.

Edit: A configuration with 3 first stages in a triangle, plus a second stage and payload in the center of that triangle would be worse: now you have to run all 3 stages at the same power, and have to ditch them at the same time. This means the first stage has a shorter total run time compared to the real-world configuration.

In RW, the center core is run at a lower thrust setting. So when the 2 boosters separate, the center core still has fuel left and can keep running for a while. This increases performance over running the 3 cores at the same thrust.

So for a triangle with the second stage in the middle, you'd need a new, larger second stage to compensate for the lower performance of the first stage.
This configuration reduces commonality with the F9: second stage is unique, you'd need a new interstage to connect the 3 cores with the second stage, this interstage needs separation mechanisms to separate it both from the cores and from the second stage (while the F9 interstage stays attached to the core). You can't recover the interstage any more. Interstage gets much heavier, reducing performance.

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  • $\begingroup$ But why is asymmetrical undesirable? With 9 Merlin engines in each first stage, couldn't the three cores even in a row as on the FH, compensate for one of the dropping off first and the half a minute later? Why not fill them with different amount of fuel to optimize their drop off one at a time, all three of them. "Because it's easy" sometimes provoke competitors to put in hard work to make it even easier. $\endgroup$ – LocalFluff Jan 2 '18 at 21:01
  • $\begingroup$ Addressed those questions... $\endgroup$ – Hobbes Jan 2 '18 at 21:13
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    $\begingroup$ "Putting the 3 cores in a triangle has another drawback: the attachment points now overlap each other, making it more difficult to have 2 identical "side" cores. " I'm not following this. In a triangle all 3 would be connected to the other two at 2 points 60 degrees apart. That should make the anchoring identical on all 3 1st stage boosters. $\endgroup$ – Dan is Fiddling by Firelight Jan 3 '18 at 16:54
  • $\begingroup$ Check how the real-life FH cores are connected: there are two arms on opposite sides of each other. That arrangement can't be used on a triangle. $\endgroup$ – Hobbes Jan 3 '18 at 16:58
  • $\begingroup$ Either the payload would need to hang over empty space in the middle, instead of being normally stacked on top of the central booster, or it would be off-center, creating a slew of problems as it would shift the center of mass off center of thrust. $\endgroup$ – SF. Jan 17 '18 at 14:11
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The other answers have excellent reasons for why the 3 boosters on the Falcon Heavy are in a line, however they haven't mentioned the asymmetrical weight distribution that making a triangle would cause.

Consider: 2 regular, "unloaded" boosters (no fairing/payload) and one "loaded" booster in a triangle. The booster / corner with the payload would be heavier.

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  • $\begingroup$ I guess I had considered a centered design in the triangle shape as a given, but you are in fact correct. $\endgroup$ – PearsonArtPhoto Jan 2 '18 at 20:53
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The row works better for the intended purpose. The center booster will not be consuming fuel as quickly as the side boosters. This will allow it to continue on for a time after the side boosters have been dropped off.

If one were to do a triangle, it would be more stable, but would not allow for releasing two of the boosters separate from the third.

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  • $\begingroup$ How would a triangular arrangement be more stable? $\endgroup$ – pericynthion Jan 2 '18 at 17:39
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    $\begingroup$ @Hobbes it has to do with the only thing you seemed to have missed in your otherwise excellent answer, and that is that the payload is attached to the center booster. In the triangle configuration, the payload would have to be centered between the three, not attached directly over just one booster. You can't have the payload centered above all three and at the same time any one booster, which is probably the biggest reason for the chosen configuration. See yeah's answer. $\endgroup$ – wedstrom Jan 2 '18 at 23:21
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    $\begingroup$ not necessarily. The Space Shuttle has the orbiter in an asymmetrical position relative to the boosters. The Atlas V 511 has one booster, and the payload is on top of the second stage, not on top of the center of mass. That suggest to me you could build an FH with the payload on top of the center core, and 2 boosters clocked at 60º between them. $\endgroup$ – Hobbes Jan 3 '18 at 6:51
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    $\begingroup$ @Hobbes you could, but the three cores would have to be splayed slightly, so some of your thrust would be lost to lateral forces which cancel out, and the structure would have to withstand those lateral forces, which would make it heavy. You could use gimballing, engines don't like being gimballed for a long time and you'd be limiting your usable gimbal angle because "centred" is already gimballed some. $\endgroup$ – anaximander Jan 3 '18 at 13:15
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    $\begingroup$ @Hobbes Even if you could overcome the structural issues of a splayed triangle setup, you'd run into problems when you drop the boosters. For a start, having the flat sides not parallel to the airflow may cause drag issues, but more importantly, on jettison their natural tendency would be to move towards the remaining core, so you'd need larger separation motors to overcome that. Also, once the boosters are gone, the centre core would be pointing some degrees off what is now "forward", which will cost fuel and velocity to correct, in a maneuver that might be tricky when under full thrust. $\endgroup$ – anaximander Jan 3 '18 at 13:18

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