Author Topic: SLS LEO capacity doesn't make sense to me.  (Read 3531 times)

Offline Brusion

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SLS LEO capacity doesn't make sense to me.
« on: 02/12/2017 01:15 PM »
So please help me understand this, because I can't find a straight answer anywhere on the internet.

I have never understood how to shuttle could put so much mass into LEO, whereas SLS so little.

The shuttle launched 122 tonnes on STS-93, the Chandra launch.  I believe this was the heaviest shuttle launch ever.
Excluding the OMS burn, the shuttle was nearly in LEO after SRB and main engine exhaustion.  Just a little OMS burn was required to circularize the orbit.  I understand that the shuttle took the 3 RS-25s to orbit with it.  They are 3.5 tonnes each.  Lets call it 15 tonnes of of total dead weight dragged to LEO.  Plus about 5 tonnes of OMS fuel and deadweight.  That means the shuttle put 102 tonnes(including the shuttle) into LEO.  If there was no shuttle and just payload, that would be 102 tonnes payload with all the same engines.

Now compare this with Block 1.  Block 1 has 20% more SRB thrust and total impulse.  It has 25% more LOX/LH2 and 25%(well slightly higher with RS-25 improvements) more main engine thrust.  By that reasoning shouldn't it be able to put 20-25% more mass into LEO that the shuttle?  Like 120+ tonnes?

Shouldn't it put the 120+ tonnes into a low earth orbit, with a short burn by the ICPS to circularize the orbit?
« Last Edit: 02/12/2017 01:16 PM by Brusion »

Offline Jim

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #1 on: 02/12/2017 01:26 PM »

Shouldn't it put the 120+ tonnes into a low earth orbit, with a short burn by the ICPS to circularize the orbit?

Have you included the mass of the ICPS?

Offline Brusion

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #2 on: 02/12/2017 01:42 PM »
A fueled ICPS, Orion and service module with fuel comes in around 65 tonnes.  This is lower than the capacity of the 68 tonnes to LEO Shuttle-C concept(a DIRECT style shuttle derived launcher, basically replacing the shuttle with a 3xRS-25 expendable payload launcher).

Still makes no sense.  SLS should have a higher capacity than what they are saying...or someone is going to hit me with some logic here so I get why it doesn't.

Offline muomega0

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #3 on: 02/12/2017 02:01 PM »
So please help me understand this, because I can't find a straight answer anywhere on the internet.

I have never understood how to shuttle could put so much mass into LEO, whereas SLS so little.

The shuttle launched 122 tonnes on STS-93, the Chandra launch.  I believe this was the heaviest shuttle launch ever.
Excluding the OMS burn, the shuttle was nearly in LEO after SRB and main engine exhaustion.  Just a little OMS burn was required to circularize the orbit.  I understand that the shuttle took the 3 RS-25s to orbit with it.  They are 3.5 tonnes each.  Lets call it 15 tonnes of of total dead weight dragged to LEO.  Plus about 5 tonnes of OMS fuel and deadweight.  That means the shuttle put 102 tonnes(including the shuttle) into LEO.  If there was no shuttle and just payload, that would be 102 tonnes payload with all the same engines.

Now compare this with Block 1.  Block 1 has 20% more SRB thrust and total impulse.  It has 25% more LOX/LH2 and 25%(well slightly higher with RS-25 improvements) more main engine thrust.  By that reasoning shouldn't it be able to put 20-25% more mass into LEO that the shuttle?  Like 120+ tonnes?

Shouldn't it put the 120+ tonnes into a low earth orbit, with a short burn by the ICPS to circularize the orbit?
Space Shuttle orbiter Discovery is commonly referred to as OV-103, for Orbiter Vehicle-103.
Empty Weight was 151,419 lbs (69mT) at rollout and 171,000 lbs(77mT)  with main engines installed.
SLS without the upper stage is 70 mT and its Blocks (upper stages) shall be 130mT.   
The gain is using separate engines for the upper stage. 

The huge amount of lift is the issue with HLV and SLS.
On the Lower End, no one would launch a 6mT capsule to LEO on a 70/100/130mT LV that costs 1B+ per launch + 1B+ caspule.
On the Higher End, Mars missions only average 200mT/yr  -  So flight rate cannot bring down SLS/Orion costs.

During the 2005 ESAS, LV24/25/"Direct" {the two launch solution to 'mooning'}  lost out to CxP's Ares I (to close the gap by 2009) and Ares V (140mT) because at one time it was to support both LEO and BEO with exclusion of all other options.
SLS then missed its Dec 31, 2016 operational goal because it has no missions.  So a mission in 2025 would be 20B in launch costs alone.
By not completing the LV, it can be referred to as 'development' or sunk costs.
The original goal of complete reuse and $100/lb would not be realized and this was known during its preliminary design stage.

Let's not embrace SLS.  Instead require all future NASA LVs have a path to complete resuse

Offline Proponent

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #4 on: 02/12/2017 02:09 PM »
Have you included the mass of the ICPS?

But SLS, as originally defined, in the NASA Authorization Act of 2010, SLS was supposed to be able to put 70 tonnes into orbit without any upper stage at all.

Although SLS was marketed as simply a rearrangement of Shuttle components, SLS's core stage is more than three times the mass of the Shuttle's ET: 85,270 kg vs. 26,500 kg in the super-light-weight version.

NB:  The core-stage mass given probably includes the four RS-25 engines and thrust structure.  Take that away, and the tank's mass falls to around 70,000 kg, still close to three times the mass of the ET.
« Last Edit: 02/12/2017 06:16 PM by Proponent »

Offline rayleighscatter

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #5 on: 02/12/2017 02:19 PM »
It also has some additional weight you might not be accounting for (LVSA, MPCV Adapter, SM panels, LAS, etc.) The numbers fudge a little more because I think the current SLS is actually rated a little higher than 70t and the orbit it can put 70t into is higher than shuttle's (although I can't find the numbers offhand).

But another important factor is that SLS isn't being designed for LEO. The jump from stage and a half to two stages really changes the arrangement of mass. SLS is made to send things beyond LEO with the second stage, not to put something into LEO with a stage and a half.

Offline Stan-1967

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #6 on: 02/12/2017 04:09 PM »
So please help me understand this, because I can't find a straight answer anywhere on the internet.

I have never understood how to shuttle could put so much mass into LEO, whereas SLS so little.


It helps me to think of the problem this way:

When Shuttle was done dragging all its mass to LEO with it's 1.5 Stage configuration, the payload it could send on to TMI/TLI or C3 trajectories was around 17 to 20t in the form a IUS ( 14.7t)  + payload.  So basically in my thought experiment, the IUS becomes stage 2.5, or the equivalent of the ICPS or EUS stage 2.5.

So if you were to re-arrange the wasted shuttle mass ( stuff not contributing to getting you to orbit like wings, crew cabin, airframe etc.) into a mass dedicated to a larger stage 1.5 ( core stage) you get  the SLS core stage with 4 RS-25's & stretched tank.  The SRB's basically scale of proportionally to their 5 segment configuration.

However the gain is the the mass of the ICPS/EUS stage 2.5 that gets put into LEO ( or very close to it) is now 30.7t plus payload.  So you see the stage 2.5 grows from 14.7t ( IUS) to 30.7t ( ICPS). 

So to finish this thought, you get a 2X larger stage 2.5 to take you to high energy orbits. ( even though thrust and mass of SRB's and core are only 25% greater ) The SLS vehicle is more mass efficient in staging than STS, and PMF still scales to GLOW. 

TL:DR version....look at what SLS can deliver BLEO not LEO.



Online pippin

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #7 on: 02/13/2017 02:26 AM »
Have you included the mass of the ICPS?

But SLS, as originally defined, in the NASA Authorization Act of 2010, SLS was supposed to be able to put 70 tonnes into orbit without any upper stage at all.
But then you carry the whole SLS core to orbit

Offline Proponent

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #8 on: 02/13/2017 10:30 AM »
Have you included the mass of the ICPS?

But SLS, as originally defined, in the NASA Authorization Act of 2010, SLS was supposed to be able to put 70 tonnes into orbit without any upper stage at all.
But then you carry the whole SLS core to orbit

Yeah, well, I'm certainly not going to try to argue it was a good idea, just that that's what the law says.   imagine that if the 1-and-1/2-stage SLS had actually been built, the plan would have been to for the payload to complete orbital insertion, leaving the SLS core in a slightly sub-orbital trajectory, so that it disposes of itself.  As it is, the actual 2-and-1/2-stage SLS will inject the core into a slightly sub-orbital trajectory--with an apogee of about 900 nautical miles.  It can't be very efficient to lug core up that far.

Online pippin

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #9 on: 02/13/2017 10:34 AM »
Oh, my comment was referring to the original question. It explains the differences, for the two-stage setup see the comments about non-LEO optimization

Offline Ben the Space Brit

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #10 on: 02/13/2017 11:02 AM »
This might be my brain playing me up but I thought that launch of outsize cargoes to LEO (8-10m diameter & >50t IMLEO) for assembly was always a key capability of the SLS. It's one of the reasons why it was funded.
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Online pippin

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SLS LEO capacity doesn't make sense to me.
« Reply #11 on: 02/13/2017 02:45 PM »
This might be my brain playing me up but I thought that launch of outsize cargoes to LEO (8-10m diameter & >50t IMLEO) for assembly was always a key capability of the SLS. It's one of the reasons why it was funded.
Well, and it will be able to do that, so what?
« Last Edit: 02/13/2017 02:45 PM by pippin »

Offline Ben the Space Brit

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #12 on: 02/13/2017 04:35 PM »
Sorry, Pippin, I didn't quite get what you were talking about.

It's quite simple: In the STS, the entire mass of the orbiter (around 50t) was part of the inserted mass to Low Earth Orbit. So, the better comparison is between the SLS and the entire mass of the Shuttle Orbiter + Payload. Add on top of that, SLS has a second stage with far better performance that the shuttle's OMS. So you have all the performance advantages of higher thrust in the core with lower dead mass penalties (expending the core tanks at exhaustion) and a better-optimised upper stage (RL-10C is far better optimised for in-vacuum work than RS-25D).

This all adds up. Basically, SLS is designed as a brute-force lifter intended to throw as much mass into LEO as it possibly can. STS had a string of performance compromises associated with requirements for the reusable crew vehicle/mid-stage that meant it never lifted as much as the combination of 3 x RS-25 + 2 SRBs could theoretically manage.
« Last Edit: 02/13/2017 04:36 PM by Ben the Space Brit »
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Online pippin

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #13 on: 02/13/2017 04:51 PM »
But that's not entirely true, as others have pointed out.
SLS has a much heavier core stage (compared to the ET) and that core stage provides a lot of the delta-V for LEO.
With the upper stage it's much better optimized for high-energy missions but it's still good enough to throw 70t or so to LEO. So no problem with that configuration for anyone.

But please keep the original question in mind because that's what I'm focusing on, not all the politics and mission and whether-it's-a-good-rocket-or-not stuff others have been discussing here.
The original question was: why is the payload so LOW compared to STS if you take the orbiter into account and the answer is twofold: in a single-stage configuration you carry that heavy core stage to orbit and in a two-stage configuration especially with EUS it's optimized for more high-energy orbit than STS was which could barely reach LEO

Offline robert_d

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #14 on: 02/13/2017 05:37 PM »
You have a point. And I'm not sure the other replies actually address it directly, although they make excellent points. I think member 'Pippin' gets it.
The Core stage of SLS is large and long. It must also support a stage 2.5 and useful payload. Think about the added weight (as pointed out by others) needed to support that weight over that distance (including the interstage weight).
Compared to STS where the O2 tank did not have to support any additional weight, and the H2 tank only the reduced weight of the O2 tank after 2:08.
The thrust of the 3 main engines had a very short, compact path to pushing on the payload (which was the orbiter itself and enclosed useful payload). So in a way, the side-mount engines and top thrust transfer beam of the SRB's was a true genius of design. 
« Last Edit: 02/13/2017 06:06 PM by robert_d »

Offline Khadgars

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #15 on: 02/14/2017 04:18 PM »
But that's not entirely true, as others have pointed out.
SLS has a much heavier core stage (compared to the ET) and that core stage provides a lot of the delta-V for LEO.
With the upper stage it's much better optimized for high-energy missions but it's still good enough to throw 70t or so to LEO. So no problem with that configuration for anyone.

But please keep the original question in mind because that's what I'm focusing on, not all the politics and mission and whether-it's-a-good-rocket-or-not stuff others have been discussing here.
The original question was: why is the payload so LOW compared to STS if you take the orbiter into account and the answer is twofold: in a single-stage configuration you carry that heavy core stage to orbit and in a two-stage configuration especially with EUS it's optimized for more high-energy orbit than STS was which could barely reach LEO

Exactly.  SLS was designed for BLEO missions, not LEO. 

Online envy887

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #16 on: 03/12/2017 01:16 AM »
Have you included the mass of the ICPS?

But SLS, as originally defined, in the NASA Authorization Act of 2010, SLS was supposed to be able to put 70 tonnes into orbit without any upper stage at all.
But then you carry the whole SLS core to orbit

The SLS Block I core DOES go to orbit, it's just an orbit with a perigee below entry interface, so the core reenters  on the first perigee in a controlled location. The Shuttle ET did the same thing.

So the total mass sent to orbit for SLS Block I is the ICPS+Orion stack, mass ~70,000 kg, plus the burned-out core mass ~95,000 kg, or a total of roughly 165,000 kg.

Shuttle brought the payload, orbiter, and ET to a similar orbit. For STS-93 (Chandra), that was 122,000 kg + 26,500 kg. So that was 149,000 kg, or about 10% less than SLS Block 1.

Block I is ~30% bigger than STS to get 10% mass to orbit. Carrying the sustainer stage from ground to orbit is inefficient and doesn't scale well.
« Last Edit: 03/12/2017 01:21 AM by envy887 »

Offline smfarmer11

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Re: SLS LEO capacity doesn't make sense to me.
« Reply #17 on: 04/09/2017 04:52 PM »
I remember reading that SLS for EM-1 would at core stage jettison be in a 1850x-50km orbit "to ensure safe disposal." So it would have to be able o put the mass of a dry core, Full ICPS, and Orion with its service module. Which is 165,000kg.

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