Author Topic: Space Launch System (SLS) Q&A  (Read 10143 times)

Online Chris Bergin

Space Launch System (SLS) Q&A
« on: 11/14/2011 07:00 PM »
As requested a general Q&A thread for SLS.

Resources:

Recent SLS articles (we're the only site covering the vehicle in depth and with this frequence. Yay us ;))
http://www.nasaspaceflight.com/tag/hlv/

SLS/HLV Forum Section:

http://forum.nasaspaceflight.com/index.php?board=37.0

L2 SLS Section (amazingly large for a relatively new vehicle):
http://forum.nasaspaceflight.com/index.php?board=48.0

Offline parham55

Re: Space Launch System (SLS) Q&A
« Reply #1 on: 11/14/2011 09:09 PM »
Has the evacuation plan been looked at?
Will we see slide wire baskets, Ares I style roller-coaster, Apollo era rubber room back in action, and/or something entirely different?

Offline brettreds2k

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Re: Space Launch System (SLS) Q&A
« Reply #2 on: 11/15/2011 02:52 PM »
I do know they are not going with the Roller Coaster style system, that they decided not to go with on the new system.
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Offline Paper Kosmonaut

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Re: Space Launch System (SLS) Q&A
« Reply #3 on: 11/21/2011 05:11 PM »
In most of the available NASA promo material of the new SLS, they show both versions in a kind of Saturn-clad black blocks on white surface appearance. I presume it is just done to win the hearts of all those "I just wanna see them shuttles flying again" people but I think without the tons of black and white paint the payload capacities will increase dramatically. Realistically, will the SLS rockets be pristine white or brown bodied?
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Offline JayP

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Re: Space Launch System (SLS) Q&A
« Reply #4 on: 11/22/2011 12:05 AM »
In most of the available NASA promo material of the new SLS, they show both versions in a kind of Saturn-clad black blocks on white surface appearance. I presume it is just done to win the hearts of all those "I just wanna see them shuttles flying again" people but I think without the tons of black and white paint the payload capacities will increase dramatically. Realistically, will the SLS rockets be pristine white or brown bodied?

The black and white stripes are roll reference marks so that they can tell from tracking camera imagery the roll angle and rate of the vehicle. That trace all the way back to peenemünde and has nothing to do with the appearance of the shuttles.

 It's still to be determined if the tankage will be uncoated SOFI or will require a topcoat.

Offline kevin-rf

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Re: Space Launch System (SLS) Q&A
« Reply #5 on: 11/22/2011 01:39 AM »
Why would it require a top coat? There is nothing below it that can be damaged by falling foam...
I just saw some idiot at the gym put a water bottle in the pringles holder on the treadmill.

Offline RocketmanUS

Re: Space Launch System (SLS) Q&A
« Reply #6 on: 01/24/2012 03:30 AM »
If new SSME's ( or expendable version ) are produced for SLS when will the production start?
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Offline wolfpack

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Re: Space Launch System (SLS) Q&A
« Reply #7 on: 01/25/2012 12:52 PM »
If new SSME's ( or expendable version ) are produced for SLS when will the production start?

Well, if there are 15 SSME's left over from STS, and SLS uses 4 for the first flights, then that's 3 flights at least with the existing stock. First flight 2017, one flight every two years, so add six years = 2024. Maybe they'd want to mix one RS-25E in with the RS-25D's so you might need a flight rated one in 2024. Figure two years lead time, so 2022. That'd be my guess.

Production of non-flight rated, sacrificial offerings to the turbopump and combustion chamber gods would have to occur sometime in this decade.

Offline RocketmanUS

Re: Space Launch System (SLS) Q&A
« Reply #8 on: 01/26/2012 02:56 AM »
If new SSME's ( or expendable version ) are produced for SLS when will the production start?

Well, if there are 15 SSME's left over from STS, and SLS uses 4 for the first flights, then that's 3 flights at least with the existing stock. First flight 2017, one flight every two years, so add six years = 2024. Maybe they'd want to mix one RS-25E in with the RS-25D's so you might need a flight rated one in 2024. Figure two years lead time, so 2022. That'd be my guess.

Production of non-flight rated, sacrificial offerings to the turbopump and combustion chamber gods would have to occur sometime in this decade.
So SSME production would not most likely start until we were within three years of the first SLS block 1A launch?

So most likely there will be no more than three SLS block 1 launches?
How many flight ready 5 segment SRB's will be made?
How hard is it to make more 5 seg SRB's if needed after the first production run for the first few SLS block 1 flights?
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Offline 93143

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Re: Space Launch System (SLS) Q&A
« Reply #9 on: 01/26/2012 03:07 AM »
I think without the tons of black and white paint the payload capacities will increase dramatically.

The payload would not be impacted dramatically by paint.  According to Wikipedia, Shuttle saved about 600 lbs after the first couple flights by not painting the tank; SLS probably won't save more than about half a ton.
« Last Edit: 01/26/2012 04:57 AM by 93143 »

Offline wolfpack

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Re: Space Launch System (SLS) Q&A
« Reply #10 on: 01/26/2012 12:34 PM »

So SSME production would not most likely start until we were within three years of the first SLS block 1A launch?

So most likely there will be no more than three SLS block 1 launches?
How many flight ready 5 segment SRB's will be made?
How hard is it to make more 5 seg SRB's if needed after the first production run for the first few SLS block 1 flights?

Really just guessing based on the 2017 inaugural launch date. I'm sure there's a lot more on L2, but my sub ran out. Didn't Jim say they (NASA) have yet to sign a contract with any manufacturer for SLS? Certainly no one's going to build anything without getting paid for it.
« Last Edit: 01/26/2012 12:35 PM by wolfpack »

Offline Zach121k

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Re: Space Launch System (SLS) Q&A
« Reply #11 on: 09/17/2013 05:03 PM »
A couple of days ago, my brother and I, both avid nerds and space obsessed, were debating on the Dual-Launch profile of an SLS Moon landing. Our question was, Wouldn't it be more economically sound to just have kept with the Ares 1 and V, possibly IV, for a dual launch, instead of launching two SLS Heavy Lifters...?
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Offline Jim

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Re: Space Launch System (SLS) Q&A
« Reply #12 on: 09/17/2013 05:30 PM »
A couple of days ago, my brother and I, both avid nerds and space obsessed, were debating on the Dual-Launch profile of an SLS Moon landing. Our question was, Wouldn't it be more economically sound to just have kept with the Ares 1 and V, possibly IV, for a dual launch, instead of launching two SLS Heavy Lifters...?

Trick question.  The answer is neither.   
But two launches of a common vehicle is better than two launches of dissimilar vehicles, which would have higher costs infrastructure.

Offline Zach121k

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Re: Space Launch System (SLS) Q&A
« Reply #13 on: 09/18/2013 03:49 PM »
That makes sense! A small infrastructure would be easier.
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Offline andreaITA

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Re: Space Launch System (SLS) Q&A
« Reply #14 on: 03/30/2015 03:58 PM »
Can someone please tell me how many and what kind of layers the sls core stage is made of?
 In particular I want to know what role has the intertank and if there is any particular external fairing/bulkhead or metal structure that take care of all the loads.
« Last Edit: 03/30/2015 04:08 PM by andreaITA »

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Re: Space Launch System (SLS) Q&A
« Reply #15 on: 01/24/2016 11:17 PM »
What is the dry weight of the core stage, without the engine mass?

or another way to put it

how much for just the empty tanks, skirts, and interstage?
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Offline Budgie

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Re: Space Launch System (SLS) Q&A
« Reply #16 on: 04/14/2016 11:39 PM »
Hey, all! First post, so hopefully I'm doing this in the right place.

NSF has so much content, so I'm not necessarily asking for a response, but perhaps a link to a thread or a nudge in the right area to search.

1. Why is SLS so expensive, and is it too expensive for what it is, or is it exactly what we should expect for a system of its size and capability?

2. Is there a review or discussion about why we need it, here on NSF (even L2) or on the internet (the more recent the better)? I am a firm believer that SLS is the right way to go for NASA for BEO exploration, but I would like to have some more facts or the opinions of those more knowledgeable than me to back this up.

Thanks!
« Last Edit: 04/14/2016 11:40 PM by Budgie »

Offline rocx

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Re: Space Launch System (SLS) Q&A
« Reply #17 on: 04/15/2016 10:14 AM »
Hello Budgie,

Those two questions have started many threads, and caused even more threads to be locked. I'll try to limit myself to answers that most commenters here would agree on.

1. Big rockets, historically, have been very expensive. There is not much to compare with, and the full costs for Saturn V and especially Energia are hard to find, but it is many billions of dollars. On the one hand building something large means that existing facilities may not be adequate, so you have to build new factories/test stands/manufacturing tools/transport infrastructure. On the other hand there are fewer launches to spread the development costs.

For SLS specifically the first argument was supposed to be invalid, because the tools and infrastructure from the Space Shuttle supply chains are reused. The amount of reuse may have been exaggerated, or deciding you are going to reuse the same contractors before starting contract negotiations may have lead to bad deals. There is also a significant group who believe the reuse requirement was only added in to secure employment and contracts for the existing aerospace manufacturers, by the senators and congressmen in whose districts they are located.

2. The two unique qualities of SLS are heavy payload delivery and a very large fairing. In-orbit refueling could reduce the need for the first, in-orbit assembly could be an alternative for the second. But both bring along their own technical challenges which need their own threads or forum sections to discuss.
Any day with a rocket landing is a fantastic day.

Offline Proponent

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Re: Space Launch System (SLS) Q&A
« Reply #18 on: 04/15/2016 11:09 AM »
Hey, all! First post, so hopefully I'm doing this in the right place.

NSF has so much content, so I'm not necessarily asking for a response, but perhaps a link to a thread or a nudge in the right area to search.

1. Why is SLS so expensive, and is it too expensive for what it is, or is it exactly what we should expect for a system of its size and capability?

I'd say it's too expensive for the capability it provides, especially since there is no clear plan for using the capability.  See the critique of SLS's (and Orion's) costs by the Space Access Society attached to this post.

Quote
2. Is there a review or discussion about why we need it, here on NSF (even L2) or on the internet (the more recent the better)? I am a firm believer that SLS is the right way to go for NASA for BEO exploration, but I would like to have some more facts or the opinions of those more knowledgeable than me to back this up.

There's lots and lots of discussion of the topic here and all over the internet.  I think it's very revealing, though, that, to best of my knowledge, there are just two official attempts at justifying SLS.  One is in the Senate authorization act that created and defined  SLS in 2010 (attached).  The senators just said SLS was a good idea without being very specific how they'd come to that conclusion, without saying what alternatives they had considered and without any visible input from technically-qualified experts.

The other attempt at an official justification that I'm aware of was made at a seminar entitled "Removing Barriers to Deep Space Exploration" in November 2013.  A NASA official--I can't remember whether it was Bill Gerstenmeier--pointed to the interest expressed by Dennis Tito's project Inspiration Mars in using an SLS launch as evidence that an SLS-like vehicle really is needed (some discussion here).  This was such an incredibly weak justification that all by itself it's almost enough to convince me that SLS is a bad idea.  I mean, if NASA, wouldn't you think that NASA would rely on its own very capable engineers to assess launch vehicles rather than relying on statements from a very marginal third-party effort?  We don't even know whether Inspiration Mars's interest an an SLS launch might have been entirely motivated by the hope that it could get the launch for free, since it would be SLS's first flight.

Contrast the decision to build SLS with the Apollo mode decision of 1962, when NASA decided to go to the moon with lunar orbit rendezvous rather than with earth orbit rendezvous or with a direct flight to the lunar surface.  Back then, the [ir]engineers[/i] analyzed the options, argued about them, and finally chose lunar orbit rendezvous.  You can read all about it.  The politicians reviewed the decision and could have overturned it, but the didn't.

SLS, in contrast, seems to have been selected by politicians.

By the way, a long ago I opened a thread for studies concluding that SLS-like heavy lifters really are a good idea for exploring beyond Earth orbit.  Nobody's been able to find such a study!  Meanwhile, quite a number of studies have suggested that using smaller launch vehicles would enable more exploration given under realistic budgets

Offline Hog

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Re: Space Launch System (SLS) Q&A
« Reply #19 on: 04/15/2016 02:28 PM »
If new SSME's ( or expendable version ) are produced for SLS when will the production start?

Well, if there are 15 SSME's left over from STS, and SLS uses 4 for the first flights, then that's 3 flights at least with the existing stock. First flight 2017, one flight every two years, so add six years = 2024. Maybe they'd want to mix one RS-25E in with the RS-25D's so you might need a flight rated one in 2024. Figure two years lead time, so 2022. That'd be my guess.

Production of non-flight rated, sacrificial offerings to the turbopump and combustion chamber gods would have to occur sometime in this decade.
They just built a 16th engine last year from service parts of the STS program.  The newest engine is ME-2063, its sister ME-2062 was built back in 2010.  Neither engine has have been fired yet.  This will occur this year.  Both of these engines are scheduled to be used together on EM-2/SLS-2.
The other 14 Main Engines are veterans all of which have flight time under their belts(or bells).

Here is a video of ME-2063 being built.


Here is a list of the missions that these  engines last flew on. This is copied from another one of my posts, so is not new info to the board.

Block II/RS25D Engines with flight experience (listed with last mission flown)
1)  2044  STS-133
2)  2045  STS-135
3)  2047  STS-135
4)  2048  STS-133
5)  2050  STS-120
6)  2051  STS-132
7)  2052  STS-132
8)  2054  STS-131
9)  2056  STS-121
10) 2057 STS-134
11) 2058 STS-133
12) 2059 STS-134
13) 2060 STS-135
14) 2061 STS-134

Unflown Block II/RS25D engines
15) 2062 (circa 2010 build)
16) 2063 (2015 build)

Development Engines Block II/RS25D (these are the 2 current Development Engines that have been/areto be fired at Stennis.
17) 0525
18) 0528

Here is a graphic showing the RS25 engine assignments for SLS missions 1 through SLS-4
Paul

Offline mcdouble

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Re: Space Launch System (SLS) Q&A
« Reply #20 on: 04/08/2017 06:24 AM »
Is the configuration of SLS/Ares V (large hydrolox core with large solid boosters) mostly a result of the need to use existing shuttle infrastructure, or is it a reasonable HLV design in it's own right?

I have been reading various threads on this site about possible timelines in which the shuttle had not been developed, and instead there had been a continuation of Saturn IB or the various INT proposals, building Skylab style stations in LEO etc. What I'm wondering is, in such a timeline if NASA were to want to develop a new HLV, would they ever consider something like SLS? Or would it make more sense to just start making Saturn Vs again?

I would assume a lot would depend on exactly what infrastructure was in place, so if F-1 and J-2 engines were already being produced, Saturn V would seem like a better idea than developing huge new SRBs and powerful hydrolox engines like the RS-25 or RS-68. But let's assume that those engines are available, (or maybe 4xUA1207s could be used instead of Shuttle SRBs etc), would there be any reason to favour one design over the other?

Sorry for the convoluted nature of this question, I guess what I'd really like to know is how much of the SLS design is based on the political necessity of using Shuttle technology and how much is it an actual good solution, all other things being equal?

Offline Proponent

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Re: Space Launch System (SLS) Q&A
« Reply #21 on: 04/08/2017 03:03 PM »
NASA considered Saturn-like HLVs during the requirements analysis cycle (RAC); snoop around, and you'll find lots of discussions about that on this site.  There were three separate efforts, RAC-1, looking at Shuttle-derived HLVs, RAC-2 for Saturn-like HLVs, and RAC-3 for "modular" HLVs, i.e., those using multiple small-diameter cores tied together, like a Falcon Heavy on steroids.

NASA found that while RAC-1 designs had lower development costs, RAC-2 vehicles had better economics in the long run, if I remember correctly.  RAC-3 designs, each involving multiple independent cores and numerous separation events, were found to be insufficiently reliable and powerful.  (By the way, it's a common misunderstanding that the RAC-3 designs were evolved EELVs: they were not).

So, I think the answer is that an SLS-like design results from the desire to use Shuttle infrastructure.

It seems to me that a basic problem with the core-plus-boosters style (e.g., SLS) is that, because the core is carried virtually all the way to orbit, maximum performance is critical.  It is crucial that it be as light as possible and that its engines function efficiently both at sea level and in a vacuum.  Every extra kilogram on the core is a kilogram of payload lost.  That may not be so bad, at least in the long run, for a reusable system, but if you throw the core away on each flight it makes for poor economics.  With a classic two-stages-to-orbit design, on the other hand, a kilogram of extra mass on the first stage might take only 0.1 kg off the payload to orbit.  Performance is not so crucial, and there ought to be more scope for making things cheaper.  A while ago, I took a stab at demonstrating this quantitatively.

Aside from the Shuttle and its offspring, SLS, the only other rocket I can think of that carried its core to orbit is Energia.  It too was designed to carry a shuttle.

EDIT:  The original stage-and-a-half Atlas carried its core all the way to orbit as well.  That design was driven by a USAF requirement in the early 1950s that all engines be ignited on the ground.
« Last Edit: 04/15/2017 12:34 PM by Proponent »

Offline baldusi

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Re: Space Launch System (SLS) Q&A
« Reply #22 on: 04/17/2017 03:42 PM »
Sputnik rocket (8K74PS) also ignited everything at launch. Originally air lighting an engine was not trivial (still isn't) and ground lit was the safest choice. Just look at the Molnyia rocket issues with the upper stage.

Offline edkyle99

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Re: Space Launch System (SLS) Q&A
« Reply #23 on: 04/18/2017 10:24 PM »
Is the configuration of SLS/Ares V (large hydrolox core with large solid boosters) mostly a result of the need to use existing shuttle infrastructure, or is it a reasonable HLV design in it's own right?
SLS is the design you end up with when you start with RS-25 engines.  They are such high-performers that the optimum design wants them to burn a lot of propellant.  High-thrust boosters are needed to get the resulting "sustainer" stage started. 

If NASA didn't have a stockpile of RS-25 engines, it wouldn't have developed SLS.  As Proponent mentioned, the RAC-2 kerosene/LOX designs had lower long-term costs, but, critically, they would have cost more to develop because the needed engines didn't exist.  RS-25 did exist.  Thus SLS.

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Offline Space Ghost 1962

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Re: Space Launch System (SLS) Q&A
« Reply #24 on: 04/19/2017 12:47 AM »
Is the configuration of SLS/Ares V (large hydrolox core with large solid boosters) mostly a result of the need to use existing shuttle infrastructure, or is it a reasonable HLV design in it's own right?
... the RAC-2 kerosene/LOX designs had lower long-term costs, but, critically, they would have cost more to develop because the needed engines didn't exist.
... and Aerojet/Rocketdyne, the maker of the RS-25, is building AR-1, at government expense, which was one of the "needed engines", that will soon exist ... and might not have a use.

We've come full circle.  ::)

Offline ArbitraryConstant

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Re: Space Launch System (SLS) Q&A
« Reply #25 on: 04/19/2017 02:19 AM »
SLS is the design you end up with when you start with RS-25 engines.  They are such high-performers that the optimum design wants them to burn a lot of propellant.  High-thrust boosters are needed to get the resulting "sustainer" stage started.
Wouldn't it make quite a good upper stage engine for an HLV? More than double the thrust of J-2 used on Saturn V, and 30s better ISP.

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Re: Space Launch System (SLS) Q&A
« Reply #26 on: 04/19/2017 02:44 AM »
SLS is the design you end up with when you start with RS-25 engines.  They are such high-performers that the optimum design wants them to burn a lot of propellant.  High-thrust boosters are needed to get the resulting "sustainer" stage started.
Wouldn't it make quite a good upper stage engine for an HLV? More than double the thrust of J-2 used on Saturn V, and 30s better ISP.
Unlike the J-2, the SSME is a booster engine as it can't be air-started. It relies very much on GSE to be kept inside its start-box of temperatures and pressures. NASA found this out a decade ago when they wanted a common upper-stage engine for the Ares 1 and Ares 5. When they lost the SSME, they were forced into an expensive redesign of Ares 1 from its intended 4-segment RSRM/SSME US design to the 5-segment SRB/J-2X US we now know.
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Offline MATTBLAK

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Re: Space Launch System (SLS) Q&A
« Reply #27 on: 04/19/2017 03:20 AM »
SLS is the design you end up with when you start with RS-25 engines.  They are such high-performers that the optimum design wants them to burn a lot of propellant.  High-thrust boosters are needed to get the resulting "sustainer" stage started.
Wouldn't it make quite a good upper stage engine for an HLV? More than double the thrust of J-2 used on Saturn V, and 30s better ISP.
Cannot be airstarted without an expensive, major redesign.
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Offline ArbitraryConstant

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Re: Space Launch System (SLS) Q&A
« Reply #28 on: 04/19/2017 03:35 AM »
SLS is the design you end up with when you start with RS-25 engines.  They are such high-performers that the optimum design wants them to burn a lot of propellant.  High-thrust boosters are needed to get the resulting "sustainer" stage started.
Wouldn't it make quite a good upper stage engine for an HLV? More than double the thrust of J-2 used on Saturn V, and 30s better ISP.
Cannot be airstarted without an expensive, major redesign.
Ah cheers. I guess that also contributes how it lends itself to being used as a sustainer engine.

Offline baldusi

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Re: Space Launch System (SLS) Q&A
« Reply #29 on: 04/19/2017 12:17 PM »
Is the configuration of SLS/Ares V (large hydrolox core with large solid boosters) mostly a result of the need to use existing shuttle infrastructure, or is it a reasonable HLV design in it's own right?
... the RAC-2 kerosene/LOX designs had lower long-term costs, but, critically, they would have cost more to develop because the needed engines didn't exist.
... and Aerojet/Rocketdyne, the maker of the RS-25, is building AR-1, at government expense, which was one of the "needed engines", that will soon exist ... and might not have a use.

We've come full circle.  ::)

They wouldn't have passed the NASA's reliability requirements, which put too much emphasis on the turbopumps and number of nozzles. Thus, Even the RD-180 was considered completely unsafe if used on big numbers. But solids were OK since they are so simple, right?
All part of the thumbs in the scale that SLS RAC process was. The only engine that might have passed their "safety" requirements were the F-1B. The AR-500 (as was called at that time) most surely wouldn't.
And yes, I have an issue of claiming ridiculous safety levels and then killing the effective numbers with requirement creep and stack complexity. I'm on the KISS fence.

Offline Space Ghost 1962

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Re: Space Launch System (SLS) Q&A
« Reply #30 on: 04/19/2017 06:59 PM »
Is the configuration of SLS/Ares V (large hydrolox core with large solid boosters) mostly a result of the need to use existing shuttle infrastructure, or is it a reasonable HLV design in it's own right?
... the RAC-2 kerosene/LOX designs had lower long-term costs, but, critically, they would have cost more to develop because the needed engines didn't exist.
... and Aerojet/Rocketdyne, the maker of the RS-25, is building AR-1, at government expense, which was one of the "needed engines", that will soon exist ... and might not have a use.

We've come full circle.  ::)

They wouldn't have passed the NASA's reliability requirements, which put too much emphasis on the turbopumps and number of nozzles.
And that will look moronic in a very short while. Sort of like counting rivets in a submarine ...

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Thus, Even the RD-180 was considered completely unsafe if used on big numbers.
Even given Energia's 4x RD-170 ...  ::)

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But solids were OK since they are so simple, right?
Politically they are extremely simple - binary. "You want SLS?  Yes or no".

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All part of the thumbs in the scale that SLS RAC process was.

The only point of RAC was thumbs on scale. For political "cover". IMHO, ULA "assisted" in this stupidity with RAC-2. A particular sore point with me.

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The only engine that might have passed their "safety" requirements were the F-1B. The AR-500 (as was called at that time) most surely wouldn't.
Unlikely as well. The closer F-1A/B got to practicality, the more opposition and political nonsense grew. (I think there was an even earlier name for it than AR-500 too.)

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And yes, I have an issue of claiming ridiculous safety levels and then killing the effective numbers with requirement creep and stack complexity.
You meant to say "unachievable safety levels" that can be withdrawn when they need to be achieved. As with CC.

It wouldn't be a govt LV if it didn't have creep and complexity. Or perhaps "creeps" ... but I digress.

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I'm on the KISS fence.
There's nothing KISS about SHLV.

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Re: Space Launch System (SLS) Q&A
« Reply #31 on: 05/25/2017 01:59 PM »
A couple of points on this thread.

Earlier there was a point by proponent about other rockets that take their core stage almost to "orbit."  i don't think it's a crticial point to this thread but Arianne 5 has a similar set up and flies a similar profile, and its been an extremely successful commercial vehicle.

Also, going back to the RAC decision, the reason the shuttle derived approach won out despite the superior long term economics of a clean sheet or saturn based kerolox booster was that schedule was considered to be paramount.  IIRC the shuttle derived systems was estimated to be available at least 2 years earlier, if for no other reason because they could skip the competitive procurement process and JOFOC the work to existing contractors. 

At the time NASA was facing a growing realization after so many cancelled efforts that lifecycle cost "doesn't matter" - gov't funding profiles and year-to-year appropriations never allow for the kinds of funding peaks and long term decision-making that lead to efficiency in long term programs, so they decided instead to go for the option that got the nearest term success (equating that to political sustainability) and could be executed under a flat budget profile (political reality). 

Shuttle derived was the only solution that fit that criteria.

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Re: Space Launch System (SLS) Q&A
« Reply #32 on: 05/25/2017 03:10 PM »
...
At the time NASA was facing a growing realization after so many cancelled efforts that lifecycle cost "doesn't matter" - gov't funding profiles and year-to-year appropriations never allow for the kinds of funding peaks and long term decision-making that lead to efficiency in long term programs, so they decided instead to go for the option that got the nearest term success (equating that to political sustainability) and could be executed under a flat budget profile (political reality). Shuttle derived was the only solution that fit that criteria.

SLS is more Constellation-derived than Shuttle-derived. If schedule was really paramount, a DIRECT-like solution would have been faster to launch. We'd still be waiting on Orion though...

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