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Atlas V 411 - Just why ?


Gaarst
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  • 1 month later...
On 11.10.2015 at 3:54 AM, Gaarst said:

I mean, even if thrust is still towards the centre of mass, the rocket would have to fly sideways. And I know that one booster is cheaper than two, but wouldn't it be simpler to just put two of these and launch the thing straight up instead of having some over-complicated gimballing or trajectory control to avoid too strong aerodynamic forces ?

But you never want the thing to fly straight up. Rockets start pitching over very quickly to build up horizontal velocity, and if you take into account that they are already asymmetric (mass distribution of LV components and payload doesn't always work out in a nice straight line with thrust), it turns out you need this over-complicated gimballing and trajectory control anyway. You have to do all that CoT and CoM math and gimbal your engines accordingly to stay on course. So no matter how many SRBs you have and where they are, they're just variables in the equation, what the numbers are doesn't matter (as long as your gimbal range allows it, and Atlas' RD-180 boasts an 8° TVC). Guidance algorithm doesn't get simpler if you remove one SRB, but your launch price does get about $10M better :)

Edited by Reddy
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3 minutes ago, Reddy said:

But you never want the thing to fly straight up. Rockets start pitching over very quickly to build up horizontal velocity, and if you take into account that they are already asymmetric (mass distribution of LV components and payload doesn't always work out in a nice straight line with thrust), it turns out you need this over-complicated gimballing and trajectory control anyway. You have to do all that CoT and CoM math and gimbal your engines accordingly to stay on course. So no matter how many SRBs you have and where they are, they're just variables in the equation, what the numbers are doesn't matter (as long as your gimbal range allows it, and Atlas' RD-180 boasts an 8° TVC). Guidance algorithm doesn't get simpler if you remove one SRB, but your launch price does get about $10M better :)

Agreed. Symmetry only matters if you can't counteract asymmetry.

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  • 1 month later...

So in a month, OSIRIS-REx is launching on top of an Atlas V rocket. This Atlas V rocket uses the 411 configuration. Now I don't exactly understand what the four means since the RD-180 has 2 nozzles. Nor do I know how the interior of the RD-180 looks so that might mean four of something else in the RD-180. I don't know! Anyway the 411 configuration uses 1 SRB on the side. This messes up the alignment of the center of thrust and the center of mass. (COT & COM) To compensate for this, the RD-180 gimbals to re-align the COT and COM.

 
 

Now the reason they use just one SRB and not two to make it less complicated is that the one SRB gives them just enough oomph to get OSIRIS-REx to Bennu.

What are your thoughts on this? One SRB? Two SRBs? Or No SRBs? :) 

@IonStorm made a challenge to replicate OSIRIS-REx. Check it out!

Also take a look at this link for more info on 411 config!

http://spaceflightnow.com/2016/08/08/buildup-of-unusual-launcher-begins-for-nasas-asteroid-sample-return-mission/

 

 

 

 

Edited by Firemetal
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2 minutes ago, shynung said:

The Atlas V rocket wasn't originally designed to have strap-on solid boosters, so the oxygen piping and control wires get in the way. That's why the SRB configurations has to be offset in some cases.

atlas-5_config.png

 

Agreed however the Space Shuttle's main engines were also gimbaled. They did so due to the bad alignment of the COM and COT. This happens with the Atlas V's SRB too. In fact all of those diagrams are unbalanced.

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Just now, VaPaL said:

What I don't get about this is why it isn't symmetrical with 2, 3 and 4 SRBs. It could be done. With 5 I don't know and with 1... well...

My personal theory is money.

 

18 minutes ago, DDE said:

See also

 

Yeah sorry. I didn't know if this had been discussed before.

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On August 12, 2016 at 3:32 PM, Firemetal said:

My personal theory is money.

Exactly.  SRBs are expensive, so buy as few as you need.  Yes, every Atlas V SRB configuration is asymmetric so they all rely on the RD180 gimbaling.  

On August 12, 2016 at 3:39 PM, DDE said:

 

As to what "4" is, that's the fairing size.

Perfect.  Atlas V nomenclature is 1st digit is fairing size in meters (4 or 5), 2nd digit is number of SRB (0-5), 3rd digit is number of motors on the Centaur (1 or 2).

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20 minutes ago, IonStorm said:

Yes, every Atlas V SRB configuration is asymmetric so they all rely on the RD180 gimbaling.  

Yeah I never noticed the asymmetry.  Ugh I wish we had that gimbaling in KSP. You may be able to recreate it by tilting the current engine but once the SRB separates, the COM and COT are unbalanced again.

Hey squad! We need more realistic gimbaling!

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On 12 August 2016 at 3:31 PM, VaPaL said:

What I don't get about this is why it isn't symmetrical with 2, 3 and 4 SRBs. It could be done. With 5 I don't know and with 1... well...

When you look at the diagram you'll notice that the mounting spots for the srb's don't change in the various configurations. I'm sure there's value in having a single design instead of changing wiring and framing structure in between designs.

if the rocket can handle the assymetry of a single srb, it surely can handle the odd arrangement (pun intended) of an even number of srb's.

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  • 4 weeks later...

SRB_2.jpg

The Atlas V SRB a.k.a AJ-60A nozzle is fixed at a 3 degree cant away from the attachment point, so it will thrusting towards (near) the center of gravity of the rocket, pretty much like Space Shuttle did. For MUOS 5 launch, someone actually ask but for 551 configuration, the RD-180 is gimbaling 1.5 degree off center, it may higher with 411 config as it has max gimbal range of 8 degrees. BTW let's wait until tomorrow for OSIRIS-REX launch that also use the 411 config. I'll be ask in the social media (psst, you guys should ask too! :wink:) and hope they will answer it. ^_^

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 SRB_2.jpg

The Atlas V SRB a.k.a AJ-60A nozzle is fixed at a 3 degree cant away from the attachment point, so it will thrusting towards (near) the center of gravity of the rocket, pretty much like Space Shuttle did. For MUOS 5 launch, someone actually ask but for 551 configuration, the RD-180 is gimbaling 1.5 degree off center, it may higher with 411 config as it has max gimbal range of 8 degrees. As 4 in 411 stands for fairing size. 4 means 4.2 meters that come from the original Atlas II with diameter size with three different length: 9 (original), 10 and 11 meters. While 5 means 5.4 diameter (with 4.57 usable room) with 3 different length: 10, 13 and 16, developed and built by RUAG Space that uses carbon fiber composite based on the flight proven Ariane 5 fairing. Note that this fairing also encapsulated the centaur upper stage.

atlas_V_payload1.jpg

500 series 

atlas_V_payload2.jpg

400 series 

Hope that helps you ^_^

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2 hours ago, asmi said:

You can do it with MJ + RO. It handles asymmetric thrust pretty well in most cases.

Hey I tried it and created a miniature 411 atlas. With vectors, it works. No mech jeb. Just need to keep your fingers on the right keys... :confused:

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