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This diagram of the Delta II 74xx-series shows 4 booster rockets mounted rather asymmetrically. All other things being equal, it looks like the net thrust of the set would push the rocket off-axis.

enter image description here

Why are the boosters not mounted in a radially symmetrical pattern?

@pericynthion points out that as long as all the booster thrust axes point through the center of mass, there's no net rotation. However, center of mass is going to shift as core stage propellant is consumed, so (assuming the booster nozzles are fixed) it's not going to be exactly zero. Furthermore, it makes the net thrust of the whole rocket not aligned with the vertical axis, which is going to produce some (small) amount of aerodynamic shear force.

Is anything done to correct for these imbalances, or are they just small enough to ignore? Do the individual boosters have differently-canted nozzles, or is it corrected by center engine gimbaling or some other mechanism?

Update: TIL there are Atlas V configurations with a single radially attached solid rocket booster! The core engine and SRB both just vector to keep everything upright and it accepts the high AOA. Check it out.

enter image description here

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  • $\begingroup$ Is that really asymmetrical, or is the image rotated? That looks like pairs of boosters on either side. $\endgroup$
    – geoffc
    Feb 9, 2015 at 21:38
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    $\begingroup$ As long as the thrust vectors all point through the vehicle center of mass, there won't be a net moment. Speculation: The non-90-degree spacing might be to allow access for ground support equipment and umbilical towers. $\endgroup$
    – pericynthion
    Feb 9, 2015 at 21:39
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    $\begingroup$ @geoffc There's the top down layout at the right next to the side view. It only says "Solid Rocket Motor Positions" LOL ;) BTW it's not just Delta II that uses such radially asymmetric SRB layout, here's some configs for Atlas V. And here is a nice big photo of Delta II 7420 with a direct view of the "extra spacing". And here another one. $\endgroup$
    – TildalWave
    Feb 9, 2015 at 22:10
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    $\begingroup$ @RussellBorogove The GEM-40 strap-on boosters are available with or without thrust vectoring. I'm not sure which variant is typically used with the Delta II - perhaps it depends on the configuration. $\endgroup$
    – pericynthion
    Feb 10, 2015 at 0:39
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    $\begingroup$ The GEM-40s (or GEM-46s, for Delta II Heavy) flown on DII were not vectorable. However, DIV uses vectorable GEM-60s for every Medium+ configuration - if 2 SRMs, both are vectorable; if 4 SRMs, motors 1 and 2 are vectorable and motors 3 and 4 are fixed-nozzle. $\endgroup$
    – Dan A.
    Sep 14, 2016 at 16:29

1 Answer 1

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The Delta II first stage thrust structure is designed with attach points for up to nine strap-on boosters, of which either three, four, or all nine are used. If we number the attach points sequentially around the circumference of the first stage (they aren't, but let's pretend anyway), three boosters might use attach points 1, 4, and 7, leaving two empty slots in between each. If they use four boosters, they might use points 1, 3, 5, and 7, leaving 1, 1, 1, and 2 slots between them, hence the asymmetric look.

See these images for example:

Two spaces between the boosters visible here

One space visible here

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    $\begingroup$ The second link returns a 403 forbidden. $\endgroup$
    – Philipp
    Jul 23, 2015 at 14:29
  • $\begingroup$ Updated the second link. $\endgroup$
    – Tristan
    Jul 27, 2015 at 22:33

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