Author Topic: Atlas V and Centaur Q&A  (Read 138787 times)

Offline vda

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Atlas V and Centaur Q&A
« on: 02/04/2007 01:40 PM »
Trying to understand ELV design better here...

"Atlas growth" diagram: http://www.lockheedmartin.com/data/assets/12480.jpg

As I understand it, optimal propellant tank size is when it is cylindrical (easier to manufacture than, say, spherical) and cylinder's length and width are in the same ballpark (better volume/surface ratio). Length is typically greater than width because it makes rocket cross-section smaller, reducing air resistance, and it's much easier to stretch the tank than widen it.

However, you don't want to stretch tanks too much because rocket gets too long and narrow and also you do not realize weight savings from shorter & fatter cylinder of the same volume.

Going to wider tank diameter looks like rather simple operation - some tooling change at the factory and appropriate changes in engine attachment, but it doesn't look like difficult or costly thing to do. Especially when your upper stage is _already_ wider than first stage - you already pay "bigger cross-section" penalty.

Why LM does not do it with Atlas V first stage? Look at the picture. Do I understand it right that they want to widen Centaur tank(s) first? Why not 1st stage first, then Centaur?

Another question - why phase 2 first stage has 2 engines? It is not just "widened & shortened first stage tank" then? Is it heavier (more propellant)?

Online Chris Bergin

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Re: Atlas V and Centaur Q&A
« Reply #1 on: 02/04/2007 01:51 PM »
Actually, we've been meaning to have a Q&A for this vehicle, like we do for the Shuttle. So seen as that's a great opening post, we'll do that with this thread.

We'll do one for the Deltas and others as we go.

Offline Mark Max Q

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Re: Atlas V and Centaur Q&A
« Reply #2 on: 02/04/2007 02:07 PM »
Yeah good idea. Maybe stick on the US Umanned Vehicle section?

Offline Kayla

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Re: Atlas V and Centaur Q&A
« Reply #3 on: 02/04/2007 02:18 PM »
Your comments and 2 questions are very appropriate and to the point.

Q1) If you look at the recent history of Atlas evolution the Atlas booster has grown in capability much faster than has the Centaur.  This includes the introduction of the RD180 on Atlas III and V, and the increase in booster tank diameter (from 10 to 12.5 feet), and propellant load of Atlas V.  The current Atlas V velocity split between booster and Centaur is skewed toward the booster doing more than the optimum amount of work for most missions.  

Increasing the Centaur capability is what is needed to better serve the Atlas fleet capability.  This includes increasing the Centaur propellant load (50% and 350% growth options are included in Phase 1) as well as thrust by increasing the number of RL10's from 1 to as many as 6.  The intent of the wide body Centaur (WBC) is to provide flexibility in the propellant load and thrust to satisfy mission needs.

WBC is also being designed to provide much greater flexibility for long duration and cryo storage to satisfy the needs of the in-space market, such as for exploration.

Q2) The wide body Boosterís of Phase 2 propellant load will increase from Atlas Vís by 70%.  To accommodate this growth the booster thrust must also be increased.  To maintain commonality with the RD180 engines Atlas V is already flying and to provide engine out capability it was decided to utilize 2 RD180 to provide this greater thrust.


As you correctly notice, the Atlas evolution story hinges on developing increased diameter tanks for the booster and Centaur.  This results in a relatively straight forward development, primarily centered on reintegration of existing components to the new tanks, to significantly increase capability for all customers.

Offline edkyle99

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Re: Atlas V and Centaur Q&A
« Reply #4 on: 02/04/2007 04:40 PM »
Quote
vda - 4/2/2007  8:40 AM
Why LM does not do it with Atlas V first stage? Look at the picture. Do I understand it right that they want to widen Centaur tank(s) first? Why not 1st stage first, then Centaur?

My supposition is that Lockheed proposed a long-term, step-by-step upgrade approach that has served Atlas well in the past.  In the late '80s, early '90s, the legacy Atlas was improved with modern electronics, a simplified roll control system, and bigger fairings, etc..  Centaur was improved and stretched a bit as well, and at some point given extend-able nozzles.  In the late '90s, Lockheed married RD-180 to the legacy Atlas fuselage to create Atlas III.  Then Lockheed replaced the old Atlas balloon tank with the new structurally-stable Atlas V booster.  

The next obvious step would be to develop a wide-body Centaur - something that was already partly done with the old Titan IV Centaurs.  The next step after that would be a bigger, more powerful booster.  Etc.

 - Ed Kyle

Offline bombay

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Re: Atlas V and Centaur Q&A
« Reply #5 on: 02/04/2007 04:58 PM »
Quote
vda - 4/2/2007  8:40 AM

Trying to understand ELV design better here...

"Atlas growth" diagram: http://www.lockheedmartin.com/data/assets/12480.jpg


Going to wider tank diameter looks like rather simple operation - some tooling change at the factory and appropriate changes in engine attachment, but it doesn't look like difficult or costly thing to do. Especially when your upper stage is _already_ wider than first stage - you already pay "bigger cross-section" penalty.

To quite the contrary, tooling changes to increase diameter are extremely costly.  You would for all intents and purposes have to acquire all new tooling and quite possibly a whole new building to accomadate growth in tank size.  This amounts to untold $millions.
You have to ask the question, why did Atlas V settle on 12.5 ft dia. versus going to 17 ft like the Delata IV?  Existing Titan tooling and factory size pretty much dictated the choice to use a 12.5 ft. diameter Atlas V booster.

Offline bombay

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Re: Atlas V and Centaur Q&A
« Reply #6 on: 02/04/2007 05:05 PM »
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edkyle99 - 4/2/2007  11:40 AM

The next obvious step would be to develop a wide-body Centaur - something that was already partly done with the old Titan IV Centaurs.  The next step after that would be a bigger, more powerful booster.  Etc.

 - Ed Kyle
WBC has been floating around in some form for the past 20 years but never got off of the ground.  If WBC never came about under GD, Martin-Marrieta, or LM, all of whom had deep pockets to fund the development of a WBC, it's highly unlikely that it will come about any time soon from private sector development, that is, from ULA.


Offline edkyle99

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Re: Atlas V and Centaur Q&A
« Reply #7 on: 02/04/2007 06:17 PM »
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bombay - 4/2/2007  12:05 PM

Quote
edkyle99 - 4/2/2007  11:40 AM

The next obvious step would be to develop a wide-body Centaur - something that was already partly done with the old Titan IV Centaurs.  The next step after that would be a bigger, more powerful booster.  Etc.

 - Ed Kyle
WBC has been floating around in some form for the past 20 years but never got off of the ground.  If WBC never came about under GD, Martin-Marrieta, or LM, all of whom had deep pockets to fund the development of a WBC, it's highly unlikely that it will come about any time soon from private sector development, that is, from ULA.


I agree.  We won't see changes unless there is a need, and funding.  

 - Ed Kyle

Offline meiza

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Re: Atlas V and Centaur Q&A
« Reply #8 on: 02/04/2007 06:50 PM »
What are the biggest diameter tanks that can still be carried by air?

Offline yinzer

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Re: Atlas V and Centaur Q&A
« Reply #9 on: 02/04/2007 07:34 PM »
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bombay - 4/2/2007  9:58 AM
To quite the contrary, tooling changes to increase diameter are extremely costly.  You would for all intents and purposes have to acquire all new tooling and quite possibly a whole new building to accomadate growth in tank size.  This amounts to untold $millions.
You have to ask the question, why did Atlas V settle on 12.5 ft dia. versus going to 17 ft like the Delata IV?  Existing Titan tooling and factory size pretty much dictated the choice to use a 12.5 ft. diameter Atlas V booster.

I would like to request that Chris exercise a firm editorial hand on this thread to keep it Q&A rather than allowing it to degenerate into the mud-slinging that is becoming a permanent fixture of most threads involving EELVs.

For similar first stage performance, using kerosene instead of hydrogen as fuel means that you need a much smaller propellant volume.  The 12.5 foot Atlas first stage is proving adequate for missions people are willing to pay money for; over half of the flights scheduled are in the 401 configuration alone, and there have been no further customers for the 551.

If a 12.5 foot diameter booster is adequate for all likely payloads, and you have the tooling for it, why go for something bigger?
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Online Chris Bergin

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Re: Atlas V and Centaur Q&A
« Reply #10 on: 02/04/2007 08:11 PM »
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yinzer - 4/2/2007  8:34 PM

I would like to request that Chris exercise a firm editorial hand on this thread to keep it Q&A rather than allowing it to degenerate into the mud-slinging that is becoming a permanent fixture of most threads involving EELVs.

You got it. Fastest way to alert me of such mudsliging is to use the alert button on the offending post, which myself and the site's moderators get to see. Should be a good thread, as we get a large amount of LM people here, thanks to LM linking articles from the news site on their internal news site:

Figures of just the news site last week by network location (www.nasaspaceflight.com) - NASA is third.


Offline bombay

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Re: Atlas V and Centaur Q&A
« Reply #11 on: 02/04/2007 08:23 PM »
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yinzer - 4/2/2007  2:34 PM

For similar first stage performance, using kerosene instead of hydrogen as fuel means that you need a much smaller propellant volume.  The 12.5 foot Atlas first stage is proving adequate for missions people are willing to pay money for; over half of the flights scheduled are in the 401 configuration alone, and there have been no further customers for the 551.

If a 12.5 foot diameter booster is adequate for all likely payloads, and you have the tooling for it, why go for something bigger?
After the fact it's easy to say that the 12.5 ft diameter was the smart choice when looking at what's been launched, and I don't disagree with that.
I'm simply saying that at the time, with a clean sheet of paper in hand, and with cost of tooling and factory size not weighing in on what the tank diameter should be, the Atlas V diameter would have likely been larger.

Offline bombay

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Re: Atlas V and Centaur Q&A
« Reply #12 on: 02/04/2007 08:55 PM »
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edkyle99 - 4/2/2007  11:40 AM

The next obvious step would be to develop a wide-body Centaur - something that was already partly done with the old Titan IV Centaurs.  The next step after that would be a bigger, more powerful booster.  Etc.

 - Ed Kyle
Another thing that comes to mind is the Titan IV Centaur, though wider in diameter than the Atlas Centaur was of the stainless balloon variety.

No doubt if a WBC were to be developed today, it would be of the structurally stable variety.  The common bulkhead methodology unique to the stainless balloon would not/could not be used.  I would expect mass ratio to take a hit as well.

Offline Jim

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Re: Atlas V and Centaur Q&A
« Reply #13 on: 02/04/2007 09:12 PM »
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bombay - 4/2/2007  4:55 PM

Quote
edkyle99 - 4/2/2007  11:40 AM

The next obvious step would be to develop a wide-body Centaur - something that was already partly done with the old Titan IV Centaurs.  The next step after that would be a bigger, more powerful booster.  Etc.

 - Ed Kyle
Another thing that comes to mind is the Titan IV Centaur, though wider in diameter than the Atlas Centaur was of the stainless balloon variety.

No doubt if a WBC were to be developed today, it would be of the structurally stable variety.  The common bulkhead methodology unique to the stainless balloon would not/could not be used.  I would expect mass ratio to take a hit as well.

Look at the documents.  It uses the same common bulkhead design.   And it states that is not structural stable but can still maintaint mass ratio of the shuttle centaur

Offline vda

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Re: Atlas V and Centaur Q&A
« Reply #14 on: 02/04/2007 09:17 PM »
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bombay - 3/2/2007  6:58 PM
To quite the contrary, tooling changes to increase diameter are extremely costly.

It probably depends on what you compare it to. My uninformed ranking of update costs:

1. Stretch tank
2. Widen tank
3. Switch to different engine (incl. engine development/testing)
4. Switch to different engine and propellant

(All of the above implies that you don't need significant pad modifications)

Are you saying that "widen tank" shouldn't be at '2' in this table?

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