A question about the Atlas V Rocket

[SlabGizor117]

I'm just kinda confused with the SRBs. How do you fit 5 on symmetrically? This picture, I assume, shows all the different SRB patterns, but that thrust would be extremely assymetrical, I don't understand. Could somebody explain? http://www.wired.com/images_blogs/photos/uncategorized/2008/07/09/atlas_v_family.png

Yes, I still don't know how to embed pics, sorry.

Thanks for the help,

-Slab

Edited March 16, 2015 by SlabGizor117

[LethalDose]

Not sure what the source of the pics are, but to fit 5 strap-on boosters around a core, you would need to place them every 72 degrees radially.

5-point radial symmetry isn't supported in stock KSP, but there's a mod that should allow it.

Addendum: The mod is Editor Extensions by MachXXV

[RocketPilot573]

Interestingly, that is NOT any form of symmetry. The Atlas V is weird, I think the thrust on each SRB is different to keep everything balanced.

[SlabGizor117]

But why have it asymmetrical and tweak the thrust in the first place? Wouldn't it be easier to just have it symmetrical like a "normal" rocket? http://i.imgur.com/ZYOTWwT.png

[Deutherius]

Could it be that way to counter asymmetrical payloads?

Quick googling brought up this.

Edited March 14, 2015 by Deutherius

[OhioBob]

It my understanding that they gimbal the RD-180 main engine to compensate for the asymmetry.

- - - Updated - - -

But why have it asymmetrical and tweak the thrust in the first place? Wouldn't it be easier to just have it symmetrical like a "normal" rocket? http://i.imgur.com/ZYOTWwT.png

It's my understanding that the vehicle has fixed attachment points for the SRMs to provide some ease and consistency in vehicle assembly and servicing. To maintain symmetry while attaching varying numbers of SRMs would require additional or moveable attachment points.

(edited to add the following)

To illustrate this point, if 0^o is always the attachment point of the first SRM, then the following points would be needed to provide symmetry for 1 to 5 SRMs:

1 SRM - 0^o

2 SRM - 0^o, 180^o

3 SRM - 0^o, 120^o, 240^o

4 SRM - 0^o, 90^o, 180^o, 270^o

5 SRM - 0^o, 72^o, 144^o, 216^o, 288^o

That's a total of ten attachment points. Plus you would need a service gantry that provides access to all those points. Instead they provide five permanent attachment points and always attach the SRMs in some arrangement to those five points. The arrangement of the points, 2 on one side and 3 on the other, also makes for easier gantry access.

Edited March 14, 2015 by OhioBob

[Daze]

It my understanding that they gimbal the RD-180 main engine to compensate for the asymmetry.

Exactly, like the most of real engine, the RD-180 has a thrust vectoring of various degree, in this case 8°.

Now you will have a question "Why on KSP this kind of thing doesn't works?"

Simply because the actual gimballing ( As i've said in this suggestion thread ) is silly, image that for the S3 KS-25x4 is ONLY 0.5°. which is crazy.

Also IRL in the case of Atlas-V helps the gimbal of the SRB. (OMG gimbal in SRB? Really KSP?)

[OhioBob]

Exactly, like the most of real engine, the RD-180 has a thrust vectoring of various degree, in this case 8°.

The SRM nozzles are also canted outward 3^o, which reduces the length of the moment arm. Performing a quick back of the envelope calculation, I estimate that the RD-180 would have to be gimbaled at and angle of about 2^o-2.5^o to compensate for the off-axis thrust of one SRM.

[SlabGizor117]

Welp, this got complicated fast, so, Basically, other structures on main fuselage block symmetrical SRB placement, SRB thrust and gimbal tweaked?

That, or the designers were high at the time...

Anyways, thanks for the help everyone!

-Slab