SLS for Return to the Moon by the 50th Anniversary of Apollo 11, page 4: further on lightweighting the SLS core.

                                             Copyright 2013 Robert Clark

 NASA has decided to revert to the original Al 2219 aluminum alloy that was first used on the shuttle external tank for the SLS core:

SLS takes on new buckling standards, drops Super Light alloy.
February 18, 2013 by Martin Payne 
http://www.nasaspaceflight.com/2013/02/sls-new-buckling-standards-drops-super-light-alloy/

 This is due to the greater brittleness of the lighter aluminum-lithium alloys used on the later super lightweight ET tank (SLWT). And because the later alloys were not available in the greater thickness needed for optimal lightweight performance. 
 However, NASA itself estimated the Al-li alloys could save 25% off the weight of a propellant tank over the Al 2219 alloy:

RELEASE : 09-096
NASA Uses Twin Processes to Develop New Tank Dome Technology
http://www.nasa.gov/centers/langley/news/releases/2009/09-096.htm

 Still NASA estimated in regards to the SLS tank, reverting back to the Al 2219 alloy would only cost 3,000 kg in lost payload, much smaller than 25%. Apparently, the reduced thickness of the plates available for the aluminum-lithium alloys used on the SLWT results in reduced weight efficiency. 
 However, a new aluminum-lithium alloy Al-Li 2050 has similar strength at lightweight to the SLWT alloys and is available in thicker plate sizes:

Shell Buckling Knockdown Factor (SBKF) Project Update.
http://www.nasa.gov/offices/nesc/home/Feature_ShellBuckling_Test.html

 Then we could recover the ca. 25% saving over using the Al 2219 alloy. This now is a significant increase in payload, beyond just 3,000 kg. The original ET tank using Al 2219 alloy weighed 35,000 kg. The new SLS tank is scaled up 33%, so under the same Al 2219 alloy would weigh in the range of 46,000 kg. Then the new Al-Li alloy saving 25% off this would be a saving of 11,500 kg. 
 NASA made an assessment of cost benefit analysis and decided on the older Al 2219 alloy. But this is Apollo era, 1960's, technology. This is going backwards not forwards in our technological development. 
 Further weight saving can be achieved by using composites for the intertank. NASA with Boeing is investigating large cryogenic composite tanks. This is still a research project. However the intertank is an unpressurized structure. Structures like this have been made of composites for decades. 
 To estimate the weight that can be saved, note the intertank in the al-li SLWT weighed 5,500 kg:

Single Stage To Orbit Mass Budgets Derived From Propellant Density and Specific Impulse.
John C. Whitehead
32nd AIAA/ASME/SAE/ASEE Joint Propulsion ConferenceLake Buena Vista, FL July 1-3, 1996

http://www.osti.gov/bridge/servlets/purl/379977-2LwFyZ/webviewable/379977.pdf

 Then the intertank of the SLS of 33% larger size may be estimated to weigh 7,300 kg. A new composite material known as an isotruss saves significantly on weight:

 It weighs less than 1/7th that of aluminum at the same strength. This would reduce the intertank mass to less than 1,000 kg. This would subtract off an additional 6,000 kg from the tank mass to bring it down to 28,500 kg. This is nearly 18,000 kg in total off from the original SLS tank weight, which could go to extra payload.
 As I mentioned in the blog post SLS for Return to the Moon by the 50th Anniversary of Apollo 11, page 3: lightweighting the SLS core, internal NASA estimates put the actual payload of the SLS as significantly above the 70 mT mark often cited by NASA. Then an additional 18,000 kg added to this payload capability would put the SLS payload to LEO at ca. 100 mT. This is important because it would mean the SLS would have the capability to do manned lunar lander missions, not just lunar flybys.
 NASA administrator Charles Bolden has said NASA, meaning the administrators, has no plans on a Moon mission, being more focused on a mission to an asteroid. However, the public in general, space advocates, industry, and even NASA's own ranks have shown no interest in the asteroid mission:

Back to the Moon? Not any time soon, says Bolden.

By Jeff Foust on 2013 April 5 at 1:05 pm ET

A week from Monday marks the third anniversary of President Obama’s speech at the Kennedy Space Center where he formally announced the goal of a human mission to an asteroid by 2025. While that is an official goal of NASA’s human space exploration program, there remains some opposition or, at the very least, lack of acceptance of the goal by many people, including some with NASA, as a report on NASA’s strategic direction concluded last December.
At a joint meeting of the Space Studies Board and the Aeronautics and Space Engineering Board in Washington on Thursday, the head of that study, Al Carnesale of UCLA, reiterated those concerns. “Since it was announced, there was less enthusiasm for it among the community broadly,” he said of the asteroid mission goal. “The more we learn about it, the more we hear about it, people seem less enthusiastic about it.”
Carnesale suggested that, in his opinion, it might be better to shelve the asteroid mission goal in favor of a human return to the Moon. “There’s a great deal of enthusiasm, almost everywhere, for the Moon,” he said. “I think there might be, if no one has to swallow their pride and swallow their words, and you can change the asteroid mission a little bit… it might be possible to move towards something that might be more of a consensus.”

http://www.spacepolitics.com/2013/04/05/back-to-the-moon-not-any-time-soon-says-bolden/

 The SLS even by its first mission in 2017 can do manned lunar landing missions by incorporating well known and relatively low cost weight saving methods to its core and upper stages.

 This would go a long way towards garnering support both among the public and those in  industry to know that a return to the Moon is in the offing and in the very near term.

  Bob Clark


Update, Sept. 28, 2013:

 Finally, NASA has acknowledged that the Block 1, first version of the SLS to launch in 2017 will have a 90+ mT payload capacity not the 70 mT always stated by NASA:

SLS Dual Use Upper Stage (DUUS).
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130013953_2013013757.pdf

 This is important since it means we will have the capability to do manned lunar landing missions by the 2017 first launch of the SLS:

SLS for Return to the Moon by the 50th Anniversary of Apollo 11, page 5: A 90+ metric ton first launch of the SLS.
http://exoscientist.blogspot.com/2013/09/sls-for-return-to-moon-by-50th.html

SLS for Return to the Moon by the 50th Anniversary of Apollo 11, page 3: lightweighting the SLS core.

Copyright 2013 Robert Clark

 In the blog post SLS for Return to the Moon by the 50th Anniversary of Apollo 11, page 2: Orion + SEV design, I made the argument that the SLS Block I, scheduled to launch in 2017, should have significantly more payload than the 70 mT cited by NASA, which is no more than that of the smaller, less powerful Block 0 SLS. Recent news reports also indicated that a Freedom Of Information Act (FOIA) request to NASA on the SLS specifications was denied:

NASA MSFC Says That SLS Performance Specs Fall Under ITAR.
http://spaceref.com/news/viewnews.html?id=1697

Report: NASA in Huntsville won't release performance specifications for new rocket.
By Lee Roop | ****@al.com
on January 25, 2013 at 3:23 PM, updated January 25, 2013 at 3:51 PM
blog.al.com/breaking/2013/01/report_nasa_in_huntsville_wont.html

 Rand Simberg had suggested to me that the reason why NASA did not want to release the actual capabilities of the Block I SLS is that it would negate the need for even proceeding with the expensive Block II SLS. Thus it was an attempt, he argued, to maintain the "pork" of the expensive Block II development.

 However, I have been informed by those in the know that the Block I will indeed likely have a greater payload capacity than the 70 mT of the Block 0 version. However, a problem with providing such specifications for a new rocket is there is always weight growth beyond that which was originally expected.

 I had argued that scaling up a rocket should result in increased payload. But an additional factor to consider is that the new SLS core will not be scaled up in all dimensions. It is to be kept at the same width of the shuttle external tank (ET) while its length is stretched 33%. The same diameter is maintained to use the same tooling as that used to build the ET. However, stretching the length while maintaining the same diameter means additional strengthening members have to be attached to maintain its strength against bending and buckling loads. So it's not just a straight-forward matter of scaling up the mass of the core stage to estimate the payload capacity of the Block I. So I fully believe as the SLS core stage comes closer to completion then more accurate values for the payload capacity will be released.

 I was interested to note that, according to what I have been informed, that the current internal NASA estimates of the SLS payload to LEO would still allow my Orion+SEV lunar landing proposal, though not with as much leeway. Then to the end of increasing the payload and increasing the mass growth margins, I have some suggestions. Though having the propellant tanks being composite might be a bridge too far for a 2017 launch some of the structural strengthening members in the tanks might be. I was struck by this image while researching composite structures:

Isotruss

 It shows a new carbon composite structure called an isotruss. It just looks like it would be lightweight for the strength, doesn't it?

 SpaceX also uses composites to reduce weight in the interstage between the first and second stage of the Falcon 9. They could be used for the interstage for the SLS as well. An even more weight saving application of composites might be in the intertank though. This is the component of the external tank that supports the oxygen tank above the hydrogen tank. It's actually a heavy component of the ET, weighing more even than the oxygen tank.
  
 The tank mass of the ET and other rocket stages is discussed in the report:

Single Stage To Orbit Mass Budgets Derived From Propellant Density and Specific Impulse.
John C. Whitehead
32nd AIAA/ASME/SAE/ASEE Joint Propulsion ConferenceLake Buena Vista, FLJuly 1-3, 1996
http://www.osti.gov/bridge/servlets/purl/379977-2LwFyZ/webviewable/379977.pdf

 I consider this by the way to be the best aerospace engineering paper never published because of the importance of its conclusions. If the validity of its arguments had been recognized then, then we would already have by now routine manned space access.

 On page 8 in Fig. 6 is shown a diagram giving the mass of the intertank in the ET. It's mass is listed as 5.5 tons:

 For the tank stretched 33% in the SLS this might be 7.3 tons. Estimates put the weight savings in the structural mass in the 40% range by using composites. 

 A greater weight saving strategy would be to eliminate the intertank entirely. This is known as common bulkhead design. It also eliminates the weight of one of the bulkheads. This was used very successfully on the Saturn V cryogenic upper stages. Currently it is used on the Centaur upper stage, the Falcon 9 first stage, and the Ariane 5 core. The plan now is to use separated tanks with an interstage to maintain commonality with the current ET design. Still it might be advantageous to do a trade study on the increased payload in comparison to the increased cost of the common bulkhead design.

 Another possibility to save weight might be inflatable payload fairings. For such a large rocket, intended to carry such large payloads, the payload fairings would be quite heavy. There was a NASA RFI for innovative proposals on fairings and adapters, which has already expired. However, perhaps NASA can do a trade study of the weight saving possible under this method. Bigelow has been in the news for his inflatable habitats so this would not be so unusual for aerospace applications.

 Also, I want to argue again as I did in my Orion+SEV post for funding the ULA suggestions for lightweighting the Centaurs, to the extent of getting a 20 to 1(!) mass ratio. With a propellant size of 40 metric tons(mT) this would allow a round trip lunar landing mission with a single in-space stage. In fact such a lunar lander could be reusable, and it would be so small it could even be launched with a 70 mT sized launcher.

 It needs to be mentioned that many knowledgeable industry insiders do not believe the final Block II version of the SLS will ever fly. This is because of the long time frame, 20 years from now so over several presidential administrations, and because of its high cost. The SLS Block I scheduled for 2017 on the other hand very likely will.

 Wouldn't it be great for the public to learn as we more closely approach the completion date for this SLS in 2017 that it already will have the capacity for lunar landing flights and moreover we already have, by then, the in-space stages to accomplish it?

 NASA considering the very legitimate possibility that the Block II will not be funded should have as a contingency some plans of accomplishing the BEO explorations with the Block I SLS. This can be done with no new technology and not really very much extra cost, but just by using known methods of lightweighting the core and in-space stages.


    Bob Clark

Update, Sept. 28, 2013:

 Finally, NASA has acknowledged that the Block 1, first version of the SLS to launch in 2017 will have a 90+ mT payload capacity not the 70 mT always stated by NASA:

SLS Dual Use Upper Stage (DUUS).
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130013953_2013013757.pdf

 This is important since it means we will have the capability to do manned lunar landing missions by the 2017 first launch of the SLS:

SLS for Return to the Moon by the 50th Anniversary of Apollo 11, page 5: A 90+ metric ton first launch of the SLS.
http://exoscientist.blogspot.com/2013/09/sls-for-return-to-moon-by-50th.html