Author Topic: SLS vs. STS - why is SLS payload capacity lower?  (Read 13138 times)

Offline deltaV

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #20 on: 01/16/2012 09:39 PM »
The SLS core is a lot more than just a few SSMEs and an ET. It's essentially the whole back end of the Shuttle, minus OMS pods and TPS. There's a bunch of expensive thrust structure, Auxillary Power Units (or whatever equivalent), avionics, plumbing, hydraulics for gimballing the engines, the gimbal mounts, etc.

Why does the SLS core need Auxillary Power Units? It doesn't need to operate when the engines are off, so why not just put generators on the engines?

Offline Jim

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #21 on: 01/16/2012 10:12 PM »
The SLS core is a lot more than just a few SSMEs and an ET. It's essentially the whole back end of the Shuttle, minus OMS pods and TPS. There's a bunch of expensive thrust structure, Auxillary Power Units (or whatever equivalent), avionics, plumbing, hydraulics for gimballing the engines, the gimbal mounts, etc.

Why does the SLS core need Auxillary Power Units? It doesn't need to operate when the engines are off, so why not just put generators on the engines?
APU's provide hydraulic power and not electrical power.
Because that would require a redesign and rebalance of the SSME to account for the losses.  The SSME needs an external source of hydraulic power for TVC and valve actuation.
« Last Edit: 01/16/2012 10:13 PM by Jim »

Offline Ronsmytheiii

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #22 on: 01/16/2012 10:16 PM »
BTW, SLS Block I (70mt) is 5 seg SRB with 3 RS-25E

Minor correction, SLS block I has 4 SSME's:

Quote
Block I uses a Core Stage Propulsion of LO2/LH2 with Four SSMEs (RS-25Ds) now sported by the configuration, an advance on the three RS-25Ds, as previously noted.

http://www.nasaspaceflight.com/2011/10/sls-trades-opening-four-rs-25s-core-stage/
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Offline Lars_J

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #23 on: 01/17/2012 02:04 AM »
OK, all this equipment maybe explains the weight difference. And if adding that "small" dv thrust is so easy/cheap - then why does nobody propose using the core+engines+all auxillary equipment? Depot, habitat, etc.?

Such usage should be envisioned from the start so that all design decisions are taken accordingly...
It was never done with Shuttle and when they started to do it for Skylab, they found it wasn't worth it and just launched the habitat completely dry on a Saturn V. It's been thought of before and found to be impractical and just simply not worth it. That doesn't keep every single space cadet on the internet proposing the same idea at one point or another.
Indeed - there have been hundreds (if not thousands) of proposals to use the Shuttle external tanks as on-orbit habitats or dry/wet storage. None of them practical. A simple google search would illustrate that this futile idea is far from new.

Online Robotbeat

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #24 on: 01/17/2012 02:06 AM »
OK, all this equipment maybe explains the weight difference. And if adding that "small" dv thrust is so easy/cheap - then why does nobody propose using the core+engines+all auxillary equipment? Depot, habitat, etc.?

Such usage should be envisioned from the start so that all design decisions are taken accordingly...
It was never done with Shuttle and when they started to do it for Skylab, they found it wasn't worth it and just launched the habitat completely dry on a Saturn V. It's been thought of before and found to be impractical and just simply not worth it. That doesn't keep every single space cadet on the internet proposing the same idea at one point or another.
Indeed - there have been hundreds (if not thousands) of proposals to use the Shuttle external tanks as on-orbit habitats or dry/wet storage. None of them practical. A simple google search would illustrate that this futile idea is far from new.
Yes, I'm sure if you look back at our full post histories, both of us may have proposed a similar idea at one point. ;)
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Online TomH

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #25 on: 01/17/2012 03:10 AM »
If SLS core is really reaching LEO - this offers many possibilities (after some rework/redesign of course) - empty tanks to be used as fuel depot or deep space habitat and it even has engines (RS-25 are already multiple-times re-startable in vacuum)...

But let's check the numbers if SLS core tank is reaching LEO - STS core tank is ~26,5t and 4 RS-25 engines are ~13,5. So, it's about ~40t. So, a mere 5t advantage from 2 more SRB segments, 1 more RS-25 engine and an upper stage... still doesn't fit.


The cost to convert it to a habitat in space by astronauts is considerably more than constructing a habitat on earth and sending it up on another vehicle.  They considered the wet hab when first designing Skylab, but realized it was impractical.

It is not of use as a depot because you would send up yet another vehicle with the propellant in that vehicle's tanks.  Why transfer the liquids from vehicle b to vehicle a? You still have a vehicle with empty tanks, and you cannot return and reuse vehicle b. Depots will go up preloaded. If a spacecraft with empty tanks comes by, you refill it.  Once the depot is empty, there is no practical way to refuel the depot.  You'd need a super-shuttle to act as a tanker for that concept to work.

Offline Nascent Ascent

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #26 on: 01/17/2012 03:24 AM »
I would think even more so today with the potential of the Bigelow units on the horizon.  Why bother?
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Offline alexw

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #27 on: 01/17/2012 05:21 AM »
The core of SLS Block I (70mt to LEO) is around 200klbs.  So that and payload is around 340klbs (170t).  This is the number to compare with the shuttle.    Orbit is 30 x 130 nmi, so subtract a little for a circularization burn.
SLS also has a fairing that weighs 25klbs that reduces performance
BTW, SLS Block I (70mt) is 5 seg SRB with 3 RS-25E
     Out of curiosity, Jim, for STS and its derivatives do you instinctively *think* (not convert) in tonnes or klbs, or equally well in both? Do you find one more helpful? Did that change at some point?
     I would guess that klbs was universal in the 80s and much of the 90s, since the system is thoroughly designed in FPS-R, and its only relatively recently (Phase I? ISS? ESAS?) that STS is commonly spoken of in metric for ease of comparison. Any truth to that? Did anything similar happen during the Atlas III/V program?
     (Oddly, metric for the most part, I think only in klbs-thrust, not kN.)
                   -Alex

Offline Jim

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #28 on: 01/17/2012 01:21 PM »
Once the depot is empty, there is no practical way to refuel the depot.  You'd need a super-shuttle to act as a tanker for that concept to work.

No, there are resupply vehicles that refill the depot.  That is the key to it.

Offline Jim

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #29 on: 01/17/2012 01:23 PM »
The core of SLS Block I (70mt to LEO) is around 200klbs.  So that and payload is around 340klbs (170t).  This is the number to compare with the shuttle.    Orbit is 30 x 130 nmi, so subtract a little for a circularization burn.
SLS also has a fairing that weighs 25klbs that reduces performance
BTW, SLS Block I (70mt) is 5 seg SRB with 3 RS-25E
     Out of curiosity, Jim, for STS and its derivatives do you instinctively *think* (not convert) in tonnes or klbs, or equally well in both? Do you find one more helpful? Did that change at some point?
     I would guess that klbs was universal in the 80s and much of the 90s, since the system is thoroughly designed in FPS-R, and its only relatively recently (Phase I? ISS? ESAS?) that STS is commonly spoken of in metric for ease of comparison. Any truth to that? Did anything similar happen during the Atlas III/V program?
     (Oddly, metric for the most part, I think only in klbs-thrust, not kN.)
                   -Alex

English units are still prevalent in the business.   

Offline 93143

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #30 on: 01/17/2012 09:01 PM »
Once the depot is empty, there is no practical way to refuel the depot.  You'd need a super-shuttle to act as a tanker for that concept to work.

No, there are resupply vehicles that refill the depot.  That is the key to it.

I think what he means is that it would be better for a resupply vehicle to dock directly with the spacecraft that needs the propellant, since it's going to be expended either way.

I think part of the answer is that the depot will have significant mass and expense invested in low-boiloff technology that the tankers don't need.

Another part of the answer might be that a departing vehicle (especially one docked together out of multiple launches) could potentially require more propellant than could be launched in one tanker, thus necessitating multiple rendezvous events in order to execute the mission.  (This problem also mitigates against using small launchers to supply propellant; you want the biggest rocket you can find.)  If the tankers are not low-boiloff, you now have multiple mission-critical launches to execute in a very short period of time.  Even if they are, the mission is now substantially more expensive (due to the more expensive, less capable tankers) and risky (due to the multiple propellant transfer encounters) and takes longer to get going.  Or you could just eat the boiloff, but that doesn't really solve any of the problems.  A depot solves all of them.

RLVs used as tankers would strengthen both these reasons considerably.

Then there's the idea of an L1 or L2 depot.  Good luck launching a useful one full, straight from Earth...
« Last Edit: 01/17/2012 09:55 PM by 93143 »

Online edkyle99

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #31 on: 01/18/2012 04:12 AM »
BTW, SLS Block I (70mt) is 5 seg SRB with 3 RS-25E

Minor correction, SLS block I has 4 SSME's:

Quote
Block I uses a Core Stage Propulsion of LO2/LH2 with Four SSMEs (RS-25Ds) now sported by the configuration, an advance on the three RS-25Ds, as previously noted.

http://www.nasaspaceflight.com/2011/10/sls-trades-opening-four-rs-25s-core-stage/

Four RS-25 engines proposed, being studied, discussed, etc., several months ago, at MSFC, according to that article.  I've heard that five RS-25 engines might also be under consideration, even for the early Blocks, to avoid having more than one core design.  Three or more RS-25 engines was the original call.  Nothing is set in stone yet - not until whatever official design review sets it in stone.  That won't be for months, and the schedule is no doubt already slipping.

 - Ed Kyle
« Last Edit: 01/18/2012 04:14 AM by edkyle99 »

Offline JohnFornaro

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #32 on: 01/18/2012 01:39 PM »
Why does a more capable vehicle with more powerful boosters, engines and upper stage deliver 35t less?

Yes, enquiring minds want to know.  The "specialized" definitions of "payload" all avoid the commonsense suggestion that STS was a 105 mt rocket.  The orbiter was "mass going into orbit".  I know that it's not a good idea to leave junk in orbit, but still SLS doesn't seem as capable as STS. 


OK, all this equipment maybe explains the weight difference. And if adding that "small" dv thrust is so easy/cheap - then why does nobody propose using the core+engines+all auxillary equipment? Depot, habitat, etc.?

Such usage should be envisioned from the start so that all design decisions are taken accordingly...

No, because it is not worth the effort, see above post. Nor is there a need.

This is Jim's perennial "need" argument, which explains only the current state of affairs and not the desired state of affairs.  The current state of affairs is disposable rocketry, with its inevitable unsustainability as a result of expense.  What is needed would be re-usable rocketry, which has two senses.  First, the well-known reusable launch vehicle; and second, the reusability of recycling highly refined materials.

Most of the work has been done in launching all that dry mass to LEO, and minor amounts of delta-vee, combined with a different approach to orbital insertion, could send that dry mass to the lunar surface for re-use, where it would be "needed", if a more complete long term strategy for colonization were to be adopted.

(1) OK, all this equipment maybe explains the weight difference. And if adding that "small" dv thrust is so easy/cheap - then why does nobody propose using the core+engines+all auxillary equipment? Depot, habitat, etc.?

Such usage should be envisioned from the start so that all design decisions are taken accordingly...

(2) It was never done with Shuttle ... Skylab ... Saturn V. It's been thought of before and found to be impractical and just simply not worth it.  That doesn't keep every single space cadet on the internet proposing the same idea at one point or another.
(3) Indeed - there have been hundreds ... of proposals to use the Shuttle external tanks as on-orbit habitats or dry/wet storage. None of them practical. (4) A simple google search would illustrate that this futile idea is far from new.

(1) Q. Why not a sustainable approach, with the design intention of sustainability?
(2) A. Never been done before, stay the unsustainable, disposable course.
(3) A. Can't do it because a finished product is more complex than an unfinished product.  Even tho that's not what's being considered.
(4) A. If it isn't new, it shouldn't be considered.

The cost to convert it to a habitat in space by astronauts is considerably more than constructing a habitat on earth and sending it up on another vehicle. ...

It is not of use as a depot because you would send up yet another vehicle with the propellant in that vehicle's tanks.  Why transfer the liquids from vehicle b to vehicle a?

Here, the first mistake is comparing a finished hab with bulk, highly refined materials, intended for repurposing on even higher ground.  True, cislunar infrastructure would be needed for such relocation to be practicable.  If there is no intent to inhabit the higher ground, such an infrastructure would not be needed.  The second mistake is to object that the depot may be larger than the prop "truck".  On the LV Agnostic Depot/Tank thread, others are suggesting that fairly small 10mt RLV's (or even EELV's) be used to fill a much larger depot.   Altho this would be a smaller "truck" than I would prefer, the refilling concept of a large tank with small trucks is sound.

Earlier, an objection to launching ET's to orbit concerned the popcorning of foam, but I ask if the expedient solution of the 60's is the only way for cryo propellants to be launched for all time?  If any disposable stage gets launched, it will be in the future, and not necessarily an old design.

English units are still prevalent in the business. 

Yes!

Nothing is set in stone yet ...

Huh.  I thought it was mostly Al, Fe, Ni, etc.
Sometimes I just flat out don't get it.

Offline douglas100

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #33 on: 01/18/2012 03:37 PM »


This is Jim's perennial "need" argument, which explains only the current state of affairs and not the desired state of affairs.  The current state of affairs is disposable rocketry, with its inevitable unsustainability as a result of expense. 

No, the current state of affairs is perfectly sustainable and has persisted for 55 years. If nothing changes it can and will continue. Of course this has nothing to do with what you (or I) might desire. The current launch market is too small to warrant the development of RLV's commercially.

Now that doesn't prevent SpaceX, the Air Force or anyone else putting their own money into RLV developments. Maybe they will come up with viable vehicles. We'll see. But the current state of affairs can continue as it is indefinitely.
« Last Edit: 01/18/2012 03:37 PM by douglas100 »
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Offline JohnFornaro

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #34 on: 01/18/2012 04:34 PM »
This is Jim's perennial "need" argument, which explains only the current state of affairs and not the desired state of affairs.  The current state of affairs is disposable rocketry, with its inevitable unsustainability as a result of expense. 

No, the current state of affairs is perfectly sustainable and has persisted for 55 years. If nothing changes it can and will continue. Of course this has nothing to do with what you (or I) might desire. The current launch market is too small to warrant the development of RLV's commercially.

Just to clarify for you first:  I'm not an RLV first guy.  I believe that their time will come.  When I mention "disposable" and "re-usable", I am referring to the idea that since the lion's share of d-v is getting into LEO, and since there is a dearth of refined material on the Moon to work with, that this disposable, empty, highly refined tonnage can find good early use on a lunar base.

I quibble with you that the "current state of affairs is perfectly sustainable", since it is a demonstrated fact that we can't launch people to LEO at the moment, and for the foreseeable future.  By definition, we have "unsustained" our HSF capability.  In a different sense of "sustainability", yes, EELV's could certainly continue to build space infrastructure if Congress evinces a political "sustainability".  But all that's just hypothetical quibbling.

The desirable state of affairs would be an intelligent reuse of valuable assets.  That would require an intentional sustainability.
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Offline 93143

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #35 on: 01/18/2012 09:21 PM »
The "specialized" definitions of "payload" all avoid the commonsense suggestion that STS was a 105 mt rocket.  The orbiter was "mass going into orbit".  I know that it's not a good idea to leave junk in orbit, but still SLS doesn't seem as capable as STS.

You aren't listening.  A lot of the back end of the orbiter was engines and plumbing and APUs and thrust structure.  You're counting it as payload to orbit.  You are not counting the ET, even though it basically makes orbit (the circularization burn is trivial).  Why not?

SLS puts the engines and plumbing and APUs and thrust structure on the bottom of the ET, just like Энергия.  If you count everything that gets to MECO, including the tank and main propulsion system, the Jupiter-130 is about as 'powerful' as Shuttle (+efficiency from inline configuration, -disposable shroud and possibly LAS).  The SLS Block 1 core configuration is more powerful than J-130, so if you count everything that gets to MECO it will be more.  But the core stage itself weighs more than the ET by a large margin, so if you then subtract that, SLS looks less powerful than Shuttle.

Savvy?

EDIT:  Minor point:  J-130 could launch 77 tonnes into the 30x130 nmi core disposal orbit, not counting the 71-tonne core stage.  SLS Block 1 being capable of 70 tonnes is probably a conservative lowballed number.

Quote
Earlier, an objection to launching ET's to orbit concerned the popcorning of foam, but I ask if the expedient solution of the 60's is the only way for cryo propellants to be launched for all time?  If any disposable stage gets launched, it will be in the future, and not necessarily an old design.

Hmm...

Quote
LV-MLI should have 34% the mass of the SOFI insulation, while providing about 85 times better thermal insulation.
« Last Edit: 01/18/2012 09:47 PM by 93143 »

Offline Jim

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #36 on: 01/18/2012 10:40 PM »


Quote
LV-MLI should have 34% the mass of the SOFI insulation, while providing about 85 times better thermal insulation.

That is targeted for upperstages and not cores
« Last Edit: 01/18/2012 10:41 PM by Jim »

Offline 93143

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #37 on: 01/18/2012 11:29 PM »
Is there a good reason it couldn't be used on a core?  Or the Shuttle ET, if STS were still running?  I suppose it'd be more expensive, but it would have solved the foam shedding issue (and the popcorning issue) completely...

Offline Jim

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #38 on: 01/18/2012 11:41 PM »
Is there a good reason it couldn't be used on a core?  Or the Shuttle ET, if STS were still running?  I suppose it'd be more expensive, but it would have solved the foam shedding issue (and the popcorning issue) completely...

Aero and SRB heating.  The foam also acted an ablator.

Can't say it would have solved the shedding issue. 
a.  don't know how it would handle the shuttle environment with impingement, shockwaves, interference flows, etc.  Upperstage sides are benign.
b.  there might be areas were blankets don't work and foam is still required, such as around struts, attach fittings, feed lines, etc
« Last Edit: 01/18/2012 11:45 PM by Jim »

Offline JohnFornaro

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Re: SLS vs. STS - why is SLS payload capacity lower?
« Reply #39 on: 01/19/2012 02:25 PM »
You aren't listening.  A lot of the back end of the orbiter was engines and plumbing and APUs and thrust structure.  You're counting it as payload to orbit.  You are not counting the ET, even though it basically makes orbit (the circularization burn is trivial).  Why not?

I assure you that one thing I'm good at is listening.  I probably shouldn't have accepted the mass numbers in the OP without further investigation.  Thanks also for that link:

http://sbir.gsfc.nasa.gov/SBIR/abstracts/10/sbir/phase1/SBIR-10-1-X10.01-9382.html?solicitationId=SBIR_10_P1

Quote
A ruggedized [layer ?] of Integrated MLI, high performance lightweight thermal insulation, could be bonded to the sidewalls of LOX/LH2 cryotanks in the Atlas V and Delta IV and might be able to withstand aerodynamic loading during launch ascent.

This development could enable "recycling" efforts to begin.

The desirable state of affairs would be an intelligent reuse of valuable assets.  That would require an intentional sustainability.

Since the additional delta-vee to place the "empties" in orbit is so small, the ruggedized layer of MLI might make it more feasible to park them.  The creation of orbital debris would no longer be an issue.  True, an OTV would need to be tasked with getting them to L1 or the lunar surface, where they could be repurposed.  Again, I quite realize that today, there is no contractural "need" for this effort.  It must be acknowledged that in any lunar exploration effort there will be a need for highly refined materials in useful configurations, and such a tank would fit those uses.  It continues to be my contention that a preference for disposable rocketry is inconsistent with the desire for a sustainable off planet effort.
Sometimes I just flat out don't get it.

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