Author Topic: NASA defends decision to restart RS-25 production, rejects alternatives  (Read 36524 times)

Online Steven Pietrobon

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Did you ever model a 7 RS-25 core? If so could you point to it?

Never did seven engines. There are diminishing returns with more than six engines. You can find all my simulations at

http://www.sworld.com.au/steven/space/sls/
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline robert_d

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Re: NASA defends decision to restart RS-25 production
« Reply #221 on: 03/07/2017 11:02 AM »
Just reading up on the RS-25, formerly known also as the SSME. This machine is so remarkable as to almost defy belief. The complex plumbing, hydraulic system and helium system all had to come together to work flawlessly over immense temperature ranges. Also to be able to handle the phase change from liquid to gas at the appropriate places and times.

So it amazes me even more that NASA would decide that this highly complex and expensive machine could be thrown away. After the various upgrades to the block II version, it seems they were near to the goal of 60 full duration (shuttle 8.5 minutes) firings before major overhaul.

I could not find a simple description of the start-up sequence, although there were some graphs that seemed to apply. I do remember that early in Constellation they dropped the idea of making it restartable in favor of the J-2X (I think). I guess the complexity of the engine would make that expensive-to-near- impossible. Yet I will still hold out hope.

Online MATTBLAK

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Since higher thrust, regenerative nozzle RS-68s are presumably out of the question; how feasible/plausible is it that a further iteration of RS-25 could be developed that would notably upgrade the engine in thrust, whilst losing the minimal amount of specific impulse or reliability? Is running them at 115%, 120 or even 130% percent of original design thrust a realistic goal? I think I read in old Shuttle literature that original Block-1 engines were tested to destruction at close to 120% percent throttle settings?
« Last Edit: 03/07/2017 11:33 AM by MATTBLAK »
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Offline WulfTheSaxon

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Since higher thrust, regenerative nozzle RS-68s are presumably out of the question; how feasible/plausible is it that a further iteration of RS-25 could be developed that would notably upgrade the engine in thrust, whilst losing the minimal amount of specific impulse or reliability? Is running them at 115%, 120 or even 130% percent of original design thrust a realistic goal? I think I read in old Shuttle literature that original Block-1 engines were tested to destruction at close to 120% percent throttle settings?

I donít think itís been tested past 115% (as shown in figure 7 here), but I could certainly by wrong. There was once supposed to be 120% contingency capability in Block III.

Online Lars-J

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Re: NASA defends decision to restart RS-25 production
« Reply #224 on: 03/07/2017 10:30 PM »
Just reading up on the RS-25, formerly known also as the SSME. This machine is so remarkable as to almost defy belief. The complex plumbing, hydraulic system and helium system all had to come together to work flawlessly over immense temperature ranges. Also to be able to handle the phase change from liquid to gas at the appropriate places and times.

So it amazes me even more that NASA would decide that this highly complex and expensive machine could be thrown away. After the various upgrades to the block II version, it seems they were near to the goal of 60 full duration (shuttle 8.5 minutes) firings before major overhaul.

Two points.
1. Just because it is an amazing technical achievement does not mean that it is good fit going forward. SLS is really a custom design around it - it makes no sense on any other existing or near future launch vehicle. The liquid propulsion market appears to be shifting towards more affordable and reusable (restart-able) engines.

2. Sunk cost fallacy. What is spent on a project is spent. If another choice makes more sense going forward, then it makes sense to put the RS-25 to pasture. (as sad as it sounds)

Offline TomH

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Re: NASA defends decision to restart RS-25 production
« Reply #225 on: 03/08/2017 12:06 AM »
Just reading up on the RS-25, formerly known also as the SSME.

Always known as RS-25. Latest model is RS-25D. SSME is a categorization of all models. Proposed disposable replacement is designated RS-25E.

I do remember that early in Constellation they dropped the idea of making it restartable in favor of the J-2X (I think). I guess the complexity of the engine would make that expensive-to-near- impossible. Yet I will still hold out hope.
Very complex, very expensive. Not at all worth it.
« Last Edit: 03/08/2017 12:10 AM by TomH »

Online brickmack

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Re: NASA defends decision to restart RS-25 production
« Reply #226 on: 03/08/2017 01:48 PM »
Always known as RS-25. Latest model is RS-25D. SSME is a categorization of all models. Proposed disposable replacement is designated RS-25E.

RS-25 was the Rocketdyne designation, SSME was what NASA called it. For SLS, NASA calls it RS-25, but no distinction is made between RS-25D and E (just remaining inventory and new production units)

Offline robert_d

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Re: NASA defends decision to restart RS-25 production
« Reply #227 on: 03/08/2017 03:33 PM »
Very complex, very expensive. Not at all worth it.
Tom,
That's the part I am trying to pin down a bit more. What exactly is required for restart? There didn't seem to be any indications of (say) massive amounts of helium required to spin up the turbo pumps. But if spin-up is an issue, could electric spin-up be an option? If they can get a Tesla to go from 0 to 60 in 2.5 seconds, I would think it might not be inconceivable to carry enough power for 4 engine restarts in batteries. (trying to think outside the box). The main tanks would already be pressurized, so that shouldn't be an issue.

Thinking about it a bit more -
Could the COPV's required for the helium system (9 engines) be mounted inside the Hydrogen propellant tank? Helium could be loaded as cold as practical so as dense as possible. Otherwise there should be room outside, above the structural thrust structure.
« Last Edit: 03/08/2017 03:58 PM by robert_d »

Offline robert_d

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Re: NASA defends decision to restart RS-25 production
« Reply #228 on: 03/08/2017 04:21 PM »


Two points.
1. Just because it is an amazing technical achievement does not mean that it is good fit going forward. SLS is really a custom design around it - it makes no sense on any other existing or near future launch vehicle. The liquid propulsion market appears to be shifting towards more affordable and reusable (restart-able) engines.

2. Sunk cost fallacy. What is spent on a project is spent. If another choice makes more sense going forward, then it makes sense to put the RS-25 to pasture. (as sad as it sounds)

Point 1 - SLS is NOT a "design around it" because the RS-25 engine includes reusability which the (horrendously short-sighted) SLS design ignores.

Point 2 - But the RS-25 actually exists and the production line will exist, so we are NOT talking about past costs. And I am just asking if the reusability feature available can be applied to a smaller vehicle, one that would possibly be more likely to be used. This all assumes the 60 flight before overhaul became reality.

Offline Jim

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That's the part I am trying to pin down a bit more. What exactly is required for restart?

The SSME was a head start engine.  As long as the temp and pressures were within the start box, it would start.  Creating the start box conditions two to three minutes after launch (and disconnect from chill down supplies) and during free fall (after staging) are a "challenge". 

. But if spin-up is an issue, could electric spin-up be an option? If they can get a Tesla to go from 0 to 60 in 2.5 seconds, I would think it might not be inconceivable to carry enough power for 4 engine restarts in batteries. (trying to think outside the box).

No, not for any real engine.  The mass of the electric motor rotor would be a drag on the engine.  And don't go into anything about clutches and gears.  Direct drive single shaft turbo pumps are the design points.  Gas spinup only requires tanks and a valve on the engine.


The J-2 carried GH2 and LO2 start tanks that would be replenished during engine operation.  But it was a gas generator engine and not staged combustion.

Online Lars-J

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Re: NASA defends decision to restart RS-25 production
« Reply #230 on: 03/08/2017 04:53 PM »


Two points.
1. Just because it is an amazing technical achievement does not mean that it is good fit going forward. SLS is really a custom design around it - it makes no sense on any other existing or near future launch vehicle. The liquid propulsion market appears to be shifting towards more affordable and reusable (restart-able) engines.

2. Sunk cost fallacy. What is spent on a project is spent. If another choice makes more sense going forward, then it makes sense to put the RS-25 to pasture. (as sad as it sounds)

Point 1 - SLS is NOT a "design around it" because the RS-25 engine includes reusability which the (horrendously short-sighted) SLS design ignores.

Yes, it is. RS-25 was designed as a sustainer engine that burns to almost orbit. It's use in SLS is the same. Most other engines are designed to be either first or upper stage engines - and virtually no engine would do the job as well for SLS.

Point 2 - But the RS-25 actually exists and the production line will exist, so we are NOT talking about past costs. And I am just asking if the reusability feature available can be applied to a smaller vehicle, one that would possibly be more likely to be used. This all assumes the 60 flight before overhaul became reality.

Just the RS-25 existing (and its production line) does not mean that it is free, or will be a lower cost than an alternative from this point on. (the point of the sunk cost fallacy) RS-25 is saved by being such a good fit for SLS (see point 1 above) - if SLS goes away, RS-25 will as well.

Offline TomH

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Re: NASA defends decision to restart RS-25 production
« Reply #231 on: 03/08/2017 06:47 PM »
Always known as RS-25. Latest model is RS-25D. SSME is a categorization of all models. Proposed disposable replacement is designated RS-25E.

RS-25 was the Rocketdyne designation, SSME was what NASA called it. For SLS, NASA calls it RS-25, but no distinction is made between RS-25D and E (just remaining inventory and new production units)

They call it minimal change, but it is not identical in every way, and they are not going to worry about it being able to fly 30+ times. RS-25F is simplified to be disposable and less expensive.

Offline TomH

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Re: NASA defends decision to restart RS-25 production
« Reply #232 on: 03/09/2017 09:08 AM »


Two points.
1. Just because it is an amazing technical achievement does not mean that it is good fit going forward. SLS is really a custom design around it - it makes no sense on any other existing or near future launch vehicle. The liquid propulsion market appears to be shifting towards more affordable and reusable (restart-able) engines.

2. Sunk cost fallacy. What is spent on a project is spent. If another choice makes more sense going forward, then it makes sense to put the RS-25 to pasture. (as sad as it sounds)

Point 1 - SLS is NOT a "design around it" because the RS-25 engine includes reusability which the (horrendously short-sighted) SLS design ignores.

Yes, it is. RS-25 was designed as a sustainer engine that burns to almost orbit. It's use in SLS is the same. Most other engines are designed to be either first or upper stage engines - and virtually no engine would do the job as well for SLS.

Point 2 - But the RS-25 actually exists and the production line will exist, so we are NOT talking about past costs. And I am just asking if the reusability feature available can be applied to a smaller vehicle, one that would possibly be more likely to be used. This all assumes the 60 flight before overhaul became reality.

Just the RS-25 existing (and its production line) does not mean that it is free, or will be a lower cost than an alternative from this point on. (the point of the sunk cost fallacy) RS-25 is saved by being such a good fit for SLS (see point 1 above) - if SLS goes away, RS-25 will as well.

Robert D, SLS absolutely IS designed around the RS-25, not because it is reusable, but because it is a high ISP sustainer that does not suffer proximity heating issues. SLS is devolved from Ares V which initially was to use RS-25. The designers tried to substitute RS-68 to save cost. Lower ISP and heating issues forced them to expand from 8.6m dia. to 10.0m. Endless problems cascaded and in the end they were back to RS-25 and 8.6m. SLS is a scaled down version of Ares V. There are endless threads in the archives where you can read yourself sick about the whole mess if you care to. The early RS-25 A, B, and C engines did not have the lower maintenance endurance of RS-25D. It took years upon years of continued R&D refinement to get to that engine. It is a very sophisticated engine, is labor intensive to build, and its cost reflects those facts.

You will not see RS-25D, E, or F used in another LV. You will see reusability return in Raptor and BE-4.

Offline ZachF

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What I find disturbing is that the fifth set of engines are only required in 2027, so there will only be four SLS flying in the next 11 years?  :o
I was under the impression that they'd be aiming at a somewhat higher launch cadence.  :-\

Yep, and each of those launches will come in at a nice $5 billion a pop when all is said and done...

 :o

Offline ZachF

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To be more precise. One engine takes about 5 years to make. The factory has the capacity to start working on two new engines per year. This means that after 5 years (and assuming that each year you add two new engines to production), 2 engines are delivered for integration. And two for every year after that.

5 years to build an engine!?!?!

From the JOFOC.



ARJ hand builds their engines (at least, that's what I knew they did in the past).

 :o This is the production process I imagine for a 5 year build: (see image) It's no wonder the surviving RS-25's are treated like precious commodities.

(Meanwhile, another domestic liquid engine producer builds over 100 engines per year)

Remember, many of these contractors are paid in time + materials.... There is literally 0 reason whatsoever for them to increase productivity.

Offline robert_d

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So what about this,
A six RS-25D, three BE-3 booster, firing for about 2.33 minutes so that it can return. Restart on 3 BE-4's for boostback/re-entry, then one or two BE-3's for landing. Use a new BE-3 vacuum optimized engine as the upper stage engine.

Current inventory is 16 rs-25's. Enough for 2 boosters and 4 spare engines. Proposal would be to build 16 more to get potential total of 4 boosters with 8 spare engines. No modifications except for production efficiencies. Buy 18 BE-4's from B.O. for boosters and spares. Contract for 12 vacuum optimized versions for the upper stage expendable, but put out a contract for a long term second stage/depot development that anyone could bid on that would give good performance to lunar regions using the BE-3 powered second stage. That way nothing is wasted, and a returnable second stage is not required.

Since the BE-3 will be used for human suborbital, I would think the entire system could be human rated.
So could this system lift a fully fueled Orion to LEO? Could a hydrolox propellant depot be developed?

Would be ironic if the supposedly BLEO Orion stayed in LEO while a descendant of the originally intended suborbital BE-3 went on to be part of a reusable cis-lunar transfer system.


Offline envy887

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So what about this,
A six RS-25D, three BE-3 booster, firing for about 2.33 minutes so that it can return. Restart on 3 BE-4's for boostback/re-entry, then one or two BE-3's for landing. Use a new BE-3 vacuum optimized engine as the upper stage engine.

Current inventory is 16 rs-25's. Enough for 2 boosters and 4 spare engines. Proposal would be to build 16 more to get potential total of 4 boosters with 8 spare engines. No modifications except for production efficiencies. Buy 18 BE-4's from B.O. for boosters and spares. Contract for 12 vacuum optimized versions for the upper stage expendable, but put out a contract for a long term second stage/depot development that anyone could bid on that would give good performance to lunar regions using the BE-3 powered second stage. That way nothing is wasted, and a returnable second stage is not required.

Since the BE-3 will be used for human suborbital, I would think the entire system could be human rated.
So could this system lift a fully fueled Orion to LEO? Could a hydrolox propellant depot be developed?

Would be ironic if the supposedly BLEO Orion stayed in LEO while a descendant of the originally intended suborbital BE-3 went on to be part of a reusable cis-lunar transfer system.

Reserving 10% of core stage fuel for RTLS (enough for 3500 m/s dv) and using a dual BE-3U EUS, this reusable booster would be pretty similar to SLS Block 1 performance. I get about 90t to LEO and 30t to TLI, even with fairly conservative assumptions about the extra dry mass needed for engines, legs, etc. The extra thrust in the core stage and EUS helps performance a lot.

Matching Block 1B would take upgraded boosters or a upper stage redesign; both together could get close to Block 2. Of course, designing the EUS to accept refueling (e.g. like ACES) would obviate the need for that kind of performance to LEO.

Offline Hog

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Re: NASA defends decision to restart RS-25 production
« Reply #237 on: 05/11/2017 06:01 PM »


Quote
Point 1 - SLS is NOT a "design around it" because the RS-25 engine includes reusability which the (horrendously short-sighted) SLS design ignores.

Yes, it is. RS-25 was designed as a sustainer engine that burns to almost orbit. It's use in SLS is the same. Most other engines are designed to be either first or upper stage engines - and virtually no engine would do the job as well for SLS.


[/quote]

They lit the 3 SSMEs at 100%RPL, then lit the 2 SRBs, up to 104.5% at "tower clear", then both liquids and solids decrease thrust, "in the bucket" for Max.Q, then liquids back to 104.5 and solids increase their thrust, then staging at approx. MET-122seconds with Booster Sep, approx. 5 minutes at 104.5%RPL with some OMS firing during OMS-1(depending on mission), then 3 g throttling, then the seats snap forward at MECO.

Yes, the RS-25 is a sustainer engine, and it burns during every second of that 0-17,500mph drag race, but as soon as the 25's shutdown at MECO, assuming OMS-2 wasn't required and aside from some minor orbital corrections, is the Orbiter on fully on-orbit?

Just clarifying the "RS-25 burns almost to orbit" wording.

Millions of engine-seconds during development, certification and flight qualification ending up with an engine that performed well under flight conditions.  It's demonstrated flight record of the actual hot parts/turbomachinery was backed with 100% mission success.  There were a couple sensor issues, and a maintenance issue(STS-51F and STS-93), but with so much historical data and 14 flight tested engines and 2 new un-fired engines already in the engine stands, hopefully NASA didn't have to defend its decision to restart production too hard. 

SLS- go for launch!
Paul

Offline ennisj

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Re: NASA defends decision to restart RS-25 production
« Reply #238 on: 05/11/2017 07:30 PM »
Quote


Quote
Point 1 - SLS is NOT a "design around it" because the RS-25 engine includes reusability which the (horrendously short-sighted) SLS design ignores.

Yes, it is. RS-25 was designed as a sustainer engine that burns to almost orbit. It's use in SLS is the same. Most other engines are designed to be either first or upper stage engines - and virtually no engine would do the job as well for SLS.



They lit the 3 SSMEs at 100%RPL, then lit the 2 SRBs, up to 104.5% at "tower clear", then both liquids and solids decrease thrust, "in the bucket" for Max.Q, then liquids back to 104.5 and solids increase their thrust, then staging at approx. MET-122seconds with Booster Sep, approx. 5 minutes at 104.5%RPL with some OMS firing during OMS-1(depending on mission), then 3 g throttling, then the seats snap forward at MECO.

Yes, the RS-25 is a sustainer engine, and it burns during every second of that 0-17,500mph drag race, but as soon as the 25's shutdown at MECO, assuming OMS-2 wasn't required and aside from some minor orbital corrections, is the Orbiter on fully on-orbit?

Just clarifying the "RS-25 burns almost to orbit" wording.

SLS- go for launch!

Can I ask for a point of clarification? It was my (possibly incorrect) understanding that at MECO, the shuttle was still suborbital, so that the ET would reenter upon separation, and that the OMS was used to give the orbiter that extra kick into orbit. 

"Almost to orbit" is more of a mission constraint rather than performance constraint if my understanding is correct. The various "wet workshop" reuse ideas would have the ET go to orbit with the orbiter, in which case the RS-25s could continue burning provided there was still LH/LOX in the tank.

Offline Hog

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Re: NASA defends decision to restart RS-25 production
« Reply #239 on: 05/15/2017 11:25 AM »
Quote


Quote
Point 1 - SLS is NOT a "design around it" because the RS-25 engine includes reusability which the (horrendously short-sighted) SLS design ignores.

Yes, it is. RS-25 was designed as a sustainer engine that burns to almost orbit. It's use in SLS is the same. Most other engines are designed to be either first or upper stage engines - and virtually no engine would do the job as well for SLS.



They lit the 3 SSMEs at 100%RPL, then lit the 2 SRBs, up to 104.5% at "tower clear", then both liquids and solids decrease thrust, "in the bucket" for Max.Q, then liquids back to 104.5 and solids increase their thrust, then staging at approx. MET-122seconds with Booster Sep, approx. 5 minutes at 104.5%RPL with some OMS firing during OMS-1(depending on mission), then 3 g throttling, then the seats snap forward at MECO.

Yes, the RS-25 is a sustainer engine, and it burns during every second of that 0-17,500mph drag race, but as soon as the 25's shutdown at MECO, assuming OMS-2 wasn't required and aside from some minor orbital corrections, is the Orbiter on fully on-orbit?

Just clarifying the "RS-25 burns almost to orbit" wording.

SLS- go for launch!

Can I ask for a point of clarification? It was my (possibly incorrect) understanding that at MECO, the shuttle was still suborbital, so that the ET would reenter upon separation, and that the OMS was used to give the orbiter that extra kick into orbit. 

"Almost to orbit" is more of a mission constraint rather than performance constraint if my understanding is correct. The various "wet workshop" reuse ideas would have the ET go to orbit with the orbiter, in which case the RS-25s could continue burning provided there was still LH/LOX in the tank.
You cold be correct, I was asking for clarification myself.
Paul

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