Author Topic: NASA defends decision to restart RS-25 production, rejects alternatives  (Read 115679 times)

Offline Propylox

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NASA defends decision to restart RS-25 production, rejects alternatives
Re: Is it fair to question the decisions of NASA/Congress? May we reject their decisions and offer superior alternatives? Is it likely NASA would/did agree with said alternatives, but chose to build a Congressional rocket while cowardly keeping silent as to this mistake?

“A recent, parametric estimate performed by NASA suggests that just the design cost for creating and certifying an RS-25 equivalent engine would be approximately $2.23 billion, which is 40 percent greater than the total estimated cost of this procurement action to acquire six RS-25 flight-ready engines.”
Re: That's $2.23 billion down the drain by selecting the RS-25 over RS-68. Additionally, the RS-25 is a brand new engine with zero flight record, both increasing risk of LOM/LOC and requiring EOC. It's lower-performance, less safe, considerably more expensive (both development and operation) and possibly the worst design decision in SHLV's history. Thank you Congress, Augustine and Obama.

Since higher thrust, regenerative nozzle RS-68s are presumably out of the question...
Re: Regen is not only out of the question, it's unnecessary and always has been unless the core is designed specifically to make regen necessary. For example; Too many RS-68s together heat each other, and the core's base. Spread them apart and they're too close to the SRBs. No argument to the studies that professed such. So use less RS-68s with a bit of thermal mitigation and suddenly they don't heat up, each other, the base or roast by SRBs. You can mail me the $2.23 billion check now.

Offline MATTBLAK

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Don't tell me; tell the guys in charge of Constellation! Oh; that's right...
« Last Edit: 07/08/2017 04:07 am by MATTBLAK »
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Offline MATTBLAK

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...Besides: 3x RS68A engines, although having about the same thrust as 4x RS-25 engines, mass about 16,000 pounds more than 4x RS-25s! That, and their approximately 10% lower specific impulse would appear to make their use a bit pointless in any other context than Delta IV-Heavy. If SLS used 3x RS68A's in a wide-spaced manner like that suggested by the 'Direct' launcher guys, the thermal crosstalk issues with the SRBs could be mitigated somewhat. Couple that with shifting most of the main corestage structures to aluminum/lithium and composites, and the heavy engine penalty of the RS68s could be offset some.

And I don't suppose that changing the RS68 main nozzle to a regenerative cooling structure would save a great deal of mass, would it? And would the 5-to-10% specific impulse improvement of regenerative be deemed worth it?

Somehow; I feel a sense of deja-vu coming on; writing and reading this stuff...
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Offline Hog

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NASA defends decision to restart RS-25 production, rejects alternatives
Re: Is it fair to question the decisions of NASA/Congress? May we reject their decisions and offer superior alternatives? Is it likely NASA would/did agree with said alternatives, but chose to build a Congressional rocket while cowardly keeping silent as to this mistake?

“A recent, parametric estimate performed by NASA suggests that just the design cost for creating and certifying an RS-25 equivalent engine would be approximately $2.23 billion, which is 40 percent greater than the total estimated cost of this procurement action to acquire six RS-25 flight-ready engines.”
Re: That's $2.23 billion down the drain by selecting the RS-25 over RS-68. Additionally, the RS-25 is a brand new engine with zero flight record, both increasing risk of LOM/LOC and requiring EOC. It's lower-performance, less safe, considerably more expensive (both development and operation) and possibly the worst design decision in SHLV's history. Thank you Congress, Augustine and Obama.

$2.23 billion would be required to create and certify and RS-25 equivalent engine. Since the decision is to stay with RS25, that $2.23billion isn't required therefore not "money down the drain".

Not sure your assertion that the RS-25 is a new engine without any flight record males sense, 14 of the current inventory have actual flight experience, actually 3-4 flights on some of them.  Surely the risk of LC/LV risk would be markedly lower with RS-25, than with a new RS-25'equivalent engine?
Paul

Offline oldAtlas_Eguy

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My main concern over the proposed build rate and the limitations of AJR's infrastructure to only be able to manufacture 2 engines per year is what that does to SLS flight rate and ultimately its per flight costs. With the first set of new engines available for assembly onto the flight frame in Q4 2025 which would support a flight in 2027 the next flight SLS FLT#6 would be NET Q4 2029!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

This is not good.

This makes this LV cost $$2B+ per flight for just an equivelent 100mt of LEO payload. That is a cost evaluation of $/kg for comparison to other LV's costs of $20,000/kg. Compare that to FH at $2,000/kg for all new boosters EXPD flight costs/price. NASA could purchase 16 FH flights for the money spent on just 1 SLS.

This just does not make sense. What is the justification for this immense expenditure?

Added: This cost problem is mainly due to low flight rate and not the basic costs of manufacture. At 2 flights per year the $/kg drops <$8,000/kg which is almost the same as that of other current medium and Heavy LVs. So it is the infrastructure limitations in build rate that has been built into a very short shighted build up of infrastructure due to limiting capital equipment costs expenditures that will make the SLS program unaffordable to operate.
« Last Edit: 07/08/2017 03:31 pm by oldAtlas_Eguy »

Offline Khadgars

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NASA defends decision to restart RS-25 production, rejects alternatives
Re: Is it fair to question the decisions of NASA/Congress? May we reject their decisions and offer superior alternatives? Is it likely NASA would/did agree with said alternatives, but chose to build a Congressional rocket while cowardly keeping silent as to this mistake?

“A recent, parametric estimate performed by NASA suggests that just the design cost for creating and certifying an RS-25 equivalent engine would be approximately $2.23 billion, which is 40 percent greater than the total estimated cost of this procurement action to acquire six RS-25 flight-ready engines.”
Re: That's $2.23 billion down the drain by selecting the RS-25 over RS-68. Additionally, the RS-25 is a brand new engine with zero flight record, both increasing risk of LOM/LOC and requiring EOC. It's lower-performance, less safe, considerably more expensive (both development and operation) and possibly the worst design decision in SHLV's history. Thank you Congress, Augustine and Obama.

Since higher thrust, regenerative nozzle RS-68s are presumably out of the question...
Re: Regen is not only out of the question, it's unnecessary and always has been unless the core is designed specifically to make regen necessary. For example; Too many RS-68s together heat each other, and the core's base. Spread them apart and they're too close to the SRBs. No argument to the studies that professed such. So use less RS-68s with a bit of thermal mitigation and suddenly they don't heat up, each other, the base or roast by SRBs. You can mail me the $2.23 billion check now.

Complete non-sense.  RS-25 is one of the most flight proven engines of all time, most of the current stock has flown multiple missions with 100% reliability.  The $2.23 billion was for the design of an equivalent engine to the RS-25, not the RS-25 its self.  NASA spent about $1 Billion re-starting RS-25 production including 6 additional production engines.
« Last Edit: 07/08/2017 08:45 pm by Khadgars »
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Offline oldAtlas_Eguy

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NASA defends decision to restart RS-25 production, rejects alternatives
Re: Is it fair to question the decisions of NASA/Congress? May we reject their decisions and offer superior alternatives? Is it likely NASA would/did agree with said alternatives, but chose to build a Congressional rocket while cowardly keeping silent as to this mistake?

“A recent, parametric estimate performed by NASA suggests that just the design cost for creating and certifying an RS-25 equivalent engine would be approximately $2.23 billion, which is 40 percent greater than the total estimated cost of this procurement action to acquire six RS-25 flight-ready engines.”
Re: That's $2.23 billion down the drain by selecting the RS-25 over RS-68. Additionally, the RS-25 is a brand new engine with zero flight record, both increasing risk of LOM/LOC and requiring EOC. It's lower-performance, less safe, considerably more expensive (both development and operation) and possibly the worst design decision in SHLV's history. Thank you Congress, Augustine and Obama.

Since higher thrust, regenerative nozzle RS-68s are presumably out of the question...
Re: Regen is not only out of the question, it's unnecessary and always has been unless the core is designed specifically to make regen necessary. For example; Too many RS-68s together heat each other, and the core's base. Spread them apart and they're too close to the SRBs. No argument to the studies that professed such. So use less RS-68s with a bit of thermal mitigation and suddenly they don't heat up, each other, the base or roast by SRBs. You can mail me the $2.23 billion check now.

Complete non-sense.  RS-25 is one of the most flight proven engines of all time, most of the current stock has flown multiple missions with 100% reliability.  The $2.23 billion was for the design of an equivalent engine to the RS-25, not the RS-25 its self.  NASA spent about $1 Billion re-starting RS-25 production including 6 additional production engines.
Actually those 6 engines are only 4 flight, one qual/development engine, and one production/qual/pathfinder. The contract also sets the price per engine and options for additional engines in sets of 4. Options contract duration from execution would be 6 years. The optimal time for the execution of engine option contract would be 1 year after the last set of 2 engines start (5 years before those 2's delivery) or around 2021.

There is also a possibility of a separate contract to AJR for additional infrastructure to increase engine production rate for 2/yr to as many as 8/yr. This would be needed to have engine production match the rest of the vehicle manufacture rates. If NASA does this this contract would probably happen about 2020. After the first flight of EM-1. It would be confirmation by the PTBs that SLS is here to stay and that its use in the later half of the 2020's will be 2 flights/yr. Also going to a production rate of 8/yr from 2/yr is liable to reduce unit costs of engines by as much as 50%. If SLS will be used in the late 2020's and into the 2030's then this would be a prudent investment to lower those out year costs making the per flight costs of SLS at 2 per year as low as $600M. That is in 4/kg to LEO equivalent comparison costs of $6,000/kg. Still expensive but not outrageous.

Offline Tomness

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Are these engines going to be RS-25D or RS-25E?

Offline MATTBLAK

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new ones are likely to be 'RS-25E'.
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Offline Khadgars

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NASA defends decision to restart RS-25 production, rejects alternatives
Re: Is it fair to question the decisions of NASA/Congress? May we reject their decisions and offer superior alternatives? Is it likely NASA would/did agree with said alternatives, but chose to build a Congressional rocket while cowardly keeping silent as to this mistake?

“A recent, parametric estimate performed by NASA suggests that just the design cost for creating and certifying an RS-25 equivalent engine would be approximately $2.23 billion, which is 40 percent greater than the total estimated cost of this procurement action to acquire six RS-25 flight-ready engines.”
Re: That's $2.23 billion down the drain by selecting the RS-25 over RS-68. Additionally, the RS-25 is a brand new engine with zero flight record, both increasing risk of LOM/LOC and requiring EOC. It's lower-performance, less safe, considerably more expensive (both development and operation) and possibly the worst design decision in SHLV's history. Thank you Congress, Augustine and Obama.

Since higher thrust, regenerative nozzle RS-68s are presumably out of the question...
Re: Regen is not only out of the question, it's unnecessary and always has been unless the core is designed specifically to make regen necessary. For example; Too many RS-68s together heat each other, and the core's base. Spread them apart and they're too close to the SRBs. No argument to the studies that professed such. So use less RS-68s with a bit of thermal mitigation and suddenly they don't heat up, each other, the base or roast by SRBs. You can mail me the $2.23 billion check now.

Complete non-sense.  RS-25 is one of the most flight proven engines of all time, most of the current stock has flown multiple missions with 100% reliability.  The $2.23 billion was for the design of an equivalent engine to the RS-25, not the RS-25 its self.  NASA spent about $1 Billion re-starting RS-25 production including 6 additional production engines.
Actually those 6 engines are only 4 flight, one qual/development engine, and one production/qual/pathfinder. The contract also sets the price per engine and options for additional engines in sets of 4. Options contract duration from execution would be 6 years. The optimal time for the execution of engine option contract would be 1 year after the last set of 2 engines start (5 years before those 2's delivery) or around 2021.

There is also a possibility of a separate contract to AJR for additional infrastructure to increase engine production rate for 2/yr to as many as 8/yr. This would be needed to have engine production match the rest of the vehicle manufacture rates. If NASA does this this contract would probably happen about 2020. After the first flight of EM-1. It would be confirmation by the PTBs that SLS is here to stay and that its use in the later half of the 2020's will be 2 flights/yr. Also going to a production rate of 8/yr from 2/yr is liable to reduce unit costs of engines by as much as 50%. If SLS will be used in the late 2020's and into the 2030's then this would be a prudent investment to lower those out year costs making the per flight costs of SLS at 2 per year as low as $600M. That is in 4/kg to LEO equivalent comparison costs of $6,000/kg. Still expensive but not outrageous.

Thank you for that, good analysis.  I agree, we're likely to see a follow-on contract with AJR to support 2 SLS flights per year, but like you stated we won't see such a contract until end of this decade.
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Offline Propylox

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... Somehow; I feel a sense of deja-vu coming on; writing and reading this stuff...
I'm not going to wax poetic about butterflies, but decisions often determine the next set of decisions and weight the following choices. In every type of engineering, key junctions exist that must be robust or the entire system/structure can collapse. Well, selection of the RS-68 vs the RS-25 is one of those defining decisions that dictates the rest of the rocket, program and its viability.
With the RS-25, the SLS can only be a money pit, time waster and ultimately a dead end.

Some might say money's been spent, so we can't reconsider. Yet we spent $1.4 billion developing, building and testing multiple J-2Xs for flight. Some may say the PoR is set in stone, but that didn't preserve Ares and Nov 8th threw all kinds of presumptions out the door.
It's time to readdress our SHLV - correcting terrible choices, designing a viable program, expediting progress and breath life into human exploration that was taken to the pawn shop in 2008-2010 for "walking around money". For starters; the $1.6 billion already devoted to RS-25s was a waste and it's time for a better, cheaper engine.

... The $2.23 billion was for the design of an equivalent engine to the RS-25, not the RS-25 its self.  NASA spent about $1 Billion re-starting RS-25 production including 6 additional production engines.
Actually those 6 engines are only 4 flight, one qual/development engine, and one production/qual/pathfinder. The contract also sets the price per engine and options for additional engines in sets of 4. ...
(Great info Atlas)

That's $1.18 billion to get 16 RS-25D out of storage and ready for SLS #1-4, or a cool $74mil per engine, plus
“... restarts [Aerojet Rocketdyne’s] production capability, including furnishing the necessary management, labor, facilities, tools, equipment and materials required for this effort”, as well as “implementing modern fabrication processes and affordability improvements and producing hardware required for development and certification testing”.
http://www.americaspace.com/2015/11/24/nasa-contracts-with-aerojet-rocketdyne-to-restart-rs-25-engine-production-for-sls/
That doesn't actually build new engines, just gets ready to. The six new engines cost an additional $413 million, or a cool $69mil per engine (with only four flying) - based on the "... $2.23 billion, which is 40 percent greater than the total estimated cost of this procurement action to acquire six RS-25 flight-ready engines
https://www.nasaspaceflight.com/2016/01/nasa-defends-restart-rs-25-production/

Hmmmmmmmm.
Old RS-25Ds cost $74mil each times four per flight, while RS-68s cost ~$20mil each times three. So the SLS costs $236mil more initially per flight with RS-25Ds. Once the completely redesigned RS-25Es with zero flight history arrive, that drops to $216mil more per flight with RS-25Es (though testing is likely to drive this cost up). Yep, the RS-25 should be cancelled and replaced with RS-68s immediately. Even better if the build-up and production contract can be repurposed for an engine that's actually needed - like restarting J-2X production.

(the following belongs in another thread, but is left here to close and emphasize decision chains)
Switching to RS-68s leaves money on the table for J-2X production; which allows cancellation of the Advanced Booster program as RS-68 and J-2X loft +130mT to LEO and +40mT to the Moon; and no AB-program frees cash for actual missions - with components! The timeline for launching infrastructure toward a manned, lunar program suddenly jumps from the 30's to whenever the first J-2X launches. Maybe the DSG on EM-3?
There's that butterfly!
« Last Edit: 07/11/2017 12:53 pm by Propylox »

Offline MATTBLAK

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There are some truths and interesting 'repacked' summarizing of the SLS engine choices you discuss... Trouble is - we thrashed out all this stuff during the pre-SLS 'Direct vs Constellation' discussions the better portion of a decade ago. This ship has sailed! The SLS; if it flies at all, will fly with RS-25's and RL-10. It doesn't matter if I and some other folk think regenerative RS68, J2X, MB60 or BE3's would be better choices. They don't care what we think. The die has been cast and the pork must flow...
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Offline Endeavour_01

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I'm not going to wax poetic about butterflies, but decisions often determine the next set of decisions and weight the following choices. In every type of engineering, key junctions exist that must be robust or the entire system/structure can collapse.

I agree, which is why your plan would cause the entire SLS/Orion program to collapse. As MATTBLAK said the ship has sailed on what engines to use on SLS. The junction was passed 6 years ago. To change engines now would require a complete redesign of the SLS core and upper stages and would add years plus untold costs to the PoR.

Even then its not like RS-68 and J-2X are a panacea. There are tradeoffs with whatever engine you use. From what I recall J-2X is an excellent 2nd stage engine when you want to go to LEO, but SLS's main purpose is for BLEO flight. For that the RL-10 is superior. As MATTBLAK said RS-68 is much heavier than RS-25 and there are heating issues with more than 3 RS-68s on the core.

Quote
The timeline for launching infrastructure toward a manned, lunar program suddenly jumps from the 30's to whenever the first J-2X launches.

Actually, this plan would push a manned lunar program past the 2030s. Rockets are not Legos. The core stage is already being fabricated. To go back now means we have to throw away all the design work and tooling that took 6 years to make.
I cheer for both NASA and commercial space. For SLS, Orion, Falcon 9, Falcon Heavy, Dragon, Starship/SH, Starliner, Cygnus and all the rest!
I was blessed to see the launch of Space Shuttle Endeavour on STS-99. The launch was beyond amazing. My 8-year old mind was blown. I remember the noise and seeing the exhaust pour out of the shuttle as it lifted off. I remember staring and watching it soar while it was visible in the clear blue sky. It was one of the greatest moments of my life and I will never forget it.

Offline Propylox

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...
There is also a possibility of a separate contract to AJR for additional infrastructure to increase engine production rate for 2/yr to as many as 8/yr. This would be needed to have engine production match the rest of the vehicle manufacture rates.
...
With infrastructure for two engines/yr, AR is well positioned to transition that manufacturing capacity and contract from RS-25Es to J-2X, which only need two/flight. With RS-68s powering the core the RS-25Ds could go back into storage as emergency engines to avoid stand down if a future problem occurs with RS-68s.
If intransigence prevails we could delay RS-68s until EM-3, but that reduces the emergency stockpile of RS-25s while paying to fly a rocket there's no intention to use (like Ares-1X). It'd be better to delay EM-1 until the new thrust structure and RS-68 configuration, which shouldn't affect EM-2's timeline. It has its own issues.

...
To change engines now would require a complete redesign of the SLS core and upper stages and would add years plus untold costs to the PoR. ... The core stage is already being fabricated. To go back now means we have to throw away all the design work and tooling that took 6 years to make.
Changing the core isn't a bad idea, as long as the tooling and techniques remain the same. The structural limits are knows and needn't change much, even with new main engines. Fortunately, that's all that's been accomplished so far; knowledge and manufacturing capability - not actual production. Nothing gets "thrown away" other than SLS's poor design and all the time/money wasted on it to date.

For example; The LH tank has a humongous and heavy thrust structure to position the main engines low enough for use and far enough away from the LH for safety. That tank could go from five 22ft barrel sections to six (without changing the jig or raising the roof) with an internally-buttressed common bulkhead. That becomes around 1.9mil lbs propellant with main engines and abbreviated thrust structure flush to the LOX tank. A whole lot of weight and cost just got shaved with payloads (single-stage) or US sitting atop the solid's crossbeam.

For example; If AR produces J-2X instead of RS-25E we don't need specific tooling for the upper stage. The existing 8.4m LOX jig (no longer in use) can produce an upper stage holding around 420,ooo lbs propellant with a single 22ft barrel section and common bulkhead - both shared with the aforementioned first stage. The budgeting for the Exploration Upper Stage, like the Advanced Booster program, can now go towards these redesigns and/or missions.

Offline Khadgars

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...
There is also a possibility of a separate contract to AJR for additional infrastructure to increase engine production rate for 2/yr to as many as 8/yr. This would be needed to have engine production match the rest of the vehicle manufacture rates.
...
With infrastructure for two engines/yr, AR is well positioned to transition that manufacturing capacity and contract from RS-25Es to J-2X, which only need two/flight. With RS-68s powering the core the RS-25Ds could go back into storage as emergency engines to avoid stand down if a future problem occurs with RS-68s.
If intransigence prevails we could delay RS-68s until EM-3, but that reduces the emergency stockpile of RS-25s while paying to fly a rocket there's no intention to use (like Ares-1X). It'd be better to delay EM-1 until the new thrust structure and RS-68 configuration, which shouldn't affect EM-2's timeline. It has its own issues.

...
To change engines now would require a complete redesign of the SLS core and upper stages and would add years plus untold costs to the PoR. ... The core stage is already being fabricated. To go back now means we have to throw away all the design work and tooling that took 6 years to make.
Changing the core isn't a bad idea, as long as the tooling and techniques remain the same. The structural limits are knows and needn't change much, even with new main engines. Fortunately, that's all that's been accomplished so far; knowledge and manufacturing capability - not actual production. Nothing gets "thrown away" other than SLS's poor design and all the time/money wasted on it to date.

For example; The LH tank has a humongous and heavy thrust structure to position the main engines low enough for use and far enough away from the LH for safety. That tank could go from five 22ft barrel sections to six (without changing the jig or raising the roof) with an internally-buttressed common bulkhead. That becomes around 1.9mil lbs propellant with main engines and abbreviated thrust structure flush to the LOX tank. A whole lot of weight and cost just got shaved with payloads (single-stage) or US sitting atop the solid's crossbeam.

For example; If AR produces J-2X instead of RS-25E we don't need specific tooling for the upper stage. The existing 8.4m LOX jig (no longer in use) can produce an upper stage holding around 420,ooo lbs propellant with a single 22ft barrel section and common bulkhead - both shared with the aforementioned first stage. The budgeting for the Exploration Upper Stage, like the Advanced Booster program, can now go towards these redesigns and/or missions.

As already stated up-thread, this ship has sailed, so no, RS-25 will not be replaced on SLS.  RS-25 is perfectly suited for SLS needs.
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Offline MATTBLAK

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Yes. The above is interesting stuff, but old news and more accurately - things that simply are not going to happen. SLS will either fly basically as-is, or will be cancelled. The outcome is binary. There is no third choice.

*If it does fly and gets as far as the Block II configuration operational, the only improvements notionally possible after that would be RS-25E upgrades and changing the huge propellant tankage to lighter alloys. In other words; spending a lot more money to tweak an already very expensive booster.
« Last Edit: 07/11/2017 10:36 pm by MATTBLAK »
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Offline oldAtlas_Eguy

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Yes. The above is interesting stuff, but old news and more accurately - things that simply are not going to happen. SLS will either fly basically as-is, or will be cancelled. The outcome is binary. There is no third choice.

*If it does fly and gets as far as the Block II configuration operational, the only improvements notionally possible after that would be RS-25E upgrades and changing the huge propellant tankage to lighter alloys. In other words; spending a lot more money to tweak an already very expensive booster.
Yes even to tweak the infrastructure to be able to build 2 SLS/yr from the current infrastructure level of just 1 SLS/ every 2 years would require more funding (Probably several $Bs) and time. NASA management, in order to appease Congress, went down the rabbit hole from which there is no exit in regards to engine choices. But at the time they had no time and little choice of available engines RS25 or RS68. RS68 had too many insurmountable problems without a lot of funding and engine development time (make the RS68 regeneratively cooled). So that only left RS25 because of the timeline for first launch. Now they are stuck with the engine. With an available time period of what will be 5 years prior to final assembly of the flight SLS from program start vs the initial timeline of only 2/3 years they could have fixed the RS68 problems.

The complete problem was that management created a 100% success task schedule to get to a 2017 launch of EM-1. When they should have told Congress it was not going to happen NET 2019 just due to all those statistically historical task failures and delays in the execution of a major LV development program under a flat budget with no contingency funding to handle task failures.

Offline Khadgars

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Yes. The above is interesting stuff, but old news and more accurately - things that simply are not going to happen. SLS will either fly basically as-is, or will be cancelled. The outcome is binary. There is no third choice.

*If it does fly and gets as far as the Block II configuration operational, the only improvements notionally possible after that would be RS-25E upgrades and changing the huge propellant tankage to lighter alloys. In other words; spending a lot more money to tweak an already very expensive booster.
Yes even to tweak the infrastructure to be able to build 2 SLS/yr from the current infrastructure level of just 1 SLS/ every 2 years would require more funding (Probably several $Bs) and time. NASA management, in order to appease Congress, went down the rabbit hole from which there is no exit in regards to engine choices. But at the time they had no time and little choice of available engines RS25 or RS68. RS68 had too many insurmountable problems without a lot of funding and engine development time (make the RS68 regeneratively cooled). So that only left RS25 because of the timeline for first launch. Now they are stuck with the engine. With an available time period of what will be 5 years prior to final assembly of the flight SLS from program start vs the initial timeline of only 2/3 years they could have fixed the RS68 problems.

I'm not sure if that is accurate (1 SLS every 2 years).  Everything I'v read shows current infrastructure supports 2 per year with little to no changes, unless you are referring to the RS-25 contract with AJR?  I think we all understand a follow-on contract will be executed probably around 2020.
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Offline MATTBLAK

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Correct - two produced per year, with a 'surge' of three per year with funding ramped-up. But for a Mars Mission buildup of the needed mass, I could see that 4x Block II would be needed, at minimum. It may turn out that the other needed mass would have to go up on Commercial vehicles, such as New Glenn, Falcon Heavy or Vulcan.

3x Block II SLS and 1x each of those other boosters would represent a significant amount of tonnage placed into LEO over a few months or one year.
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Offline TomH

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It is true that all of this has been hashed out many times over the last decade. What has changed, and is being completely ignored in this thread, are the mammoth elephants currently in the womb. All of this will become moot when they are birthed.

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