Author Topic: Differences between Direct and SLS  (Read 7804 times)

Offline Odaniv

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Differences between Direct and SLS
« on: 02/02/2012 10:03 PM »
Could the informed please enlighten me on the differences between the last version of Direct and SLS? As I understand it, the Direct team claimed victory, but there are those who were Direct fans who are not fans of SLS.

Thank you!

Offline Lars_J

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Re: Differences between Direct and SLS
« Reply #1 on: 02/03/2012 03:33 PM »
Boosters:
Direct = 4 segment SRB
SLS = 5 segment SRB

Core:
Direct = STS-sized core with 3 or 4 SSME
SLS = Stretched core with 5 or 6 SSME

Upper stage (for evolved version):
Direct = powered by 4-6(?) RL-10
SLS = powered by J-2X

Offline Ben the Space Brit

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Re: Differences between Direct and SLS
« Reply #2 on: 02/03/2012 03:54 PM »
Ground weight support:

DIRECT: Boosters via thrust beam
SLS: Core thrust structure
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Offline Downix

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Re: Differences between Direct and SLS
« Reply #3 on: 02/04/2012 03:31 AM »
DIRECT: upper stage handles both orbital insertion and earth departure duties

SLS: Upper stage and earth departure duties handled by different stages.
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Offline 93143

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Re: Differences between Direct and SLS
« Reply #4 on: 02/04/2012 03:56 AM »
Core:
Direct = STS-sized core with 3 or 4 SSME
SLS = Stretched core with 5 or 6 3, 4, or 5 SSME (may not bother with 3)

Where did you get 6 engines?

Also, DIRECT had an option to use an upper stage with one J-2X; in fact, that was the option they presented to the Augustine Commission.  SLS, on the other hand, has two upper stages; the first is an in-space-only stage using a yet-to-be-determined engine, and the second uses three J-2Xes to maximize LEO payload and is apparently not intended for EDS work...

Oh, and the Jupiter Upper Stage was based on the Wide-Body Centaur, so it had an excellent mass fraction (balloon tank, I believe, with a common bulkhead).  In practice they expected it to be basically a large ACES stage, with a similarly good mass fraction.  As far as I can tell from the conceptual work, the SLS upper stages are MSFC battleship stages right now - CPS Block 1 (the initial version of the small in-space stage, without long-duration equipment added) has about the same dry mass as the JUS and holds about 40% as much propellant...

It should be noted that the SLS design is still fairly early (pre-SRR) and may change.

The 4-engine Jupiter Upper Stage used RL-60s, not RL-10s.  The recommended configuration had 6 RL-10B-2 engines, but they had options for either 6 or 7 RL-10A-4-2 engines, as well as the single J-2X option.

DIRECT also had configurations with 5-segment boosters and even stretched tanks, though they preferred to de-emphasize those.  The de-emphasis was sometimes said to be because they were overkill, and sometimes (IIRC) said to be a strategy to let NASA claim the general idea of Jupiter as their own without having to admit that DIRECT had been right all along.

...

Also, DIRECT's plan involved getting started a lot quicker, which combined with a Shuttle extension was supposed to result in a lot less damage to NASA's human resource pool than SLS has to deal with...

Ground weight support:

DIRECT: Boosters via thrust beam
SLS: Core thrust structure

First I've heard of this.  Reference?  Or was I just not paying attention during an article?
« Last Edit: 02/04/2012 06:31 AM by 93143 »

Offline Odaniv

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Re: Differences between Direct and SLS
« Reply #5 on: 02/04/2012 07:02 AM »
I'm pretty sure I've seen options of Direct with the 5 segment boosters?

Why the stretched core? ( besides the politics.)

What are the advantages of a third stage?

Overall, in terms of mass to orbit, which is the better rocket?

Are the added expenses of SLS worth it?

Offline gospacex

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Re: Differences between Direct and SLS
« Reply #6 on: 02/04/2012 11:59 AM »
Could the informed please enlighten me on the differences between the last version of Direct and SLS?

Direct was designed to minimize both Shuttle->Direct transition costs, and operation costs.

SLS wasn't - it is a compromise between those who want something which actually does have a chance of flying (as opposed to 'being cancelled'), and those who want to milk as much $$$ from NASA as possible, and don't care one iota whether this thing ever flies.

Online Mark S

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Re: Differences between Direct and SLS
« Reply #7 on: 02/04/2012 02:00 PM »
DIRECT designed the Jupiter vehicle to be a general purpose workhorse that could fill many roles for the US space program, while also minimizing development time and cost and operational cost. Jupiter would have had a high flight rate due to its moderate size, lower cost, and more diverse missions including the CxP lunar goals.

Congress also conceived SLS to fill that same role, if you read the NASA Authorization Act of 2010. But NASA seems to be taking SLS in the opposite direction, "optimizing" it for BLEO missons only, extending the development time and cost, minimizing its role for any routine operations in LEO and cis-lunar space, adding components not called for in the Act (ICPS, CPS, DSH, MMSEV), dragging development out into the 2020's ("advanced boosters"), dropping any lunar lander, landings or base, and just generally doing their best to minimize the chances of SLS ever flying.

The best spin I can possibly put on SLS is that NASA knew that they only had one chance to build SLS, so they wanted to make it as capable (read "big") as possible. But not all of the blame for that approach belongs with NASA. Congress has had many opportunities to put NASA straight between the passage of the Act, through the HEFT and RAC studies, and the grand SLS announcement circus. DIRECT would actually have met the constraints of the Act better than SLS, even the mandated 130 ton HLV, if Congress had clarified that "tons" meant short (2000 lb) tons, not metric tonnes, and that "total lift capacity" meant IMLEO, not strictly just payload. Alas, those pushing SLS in Congress were of the "bigger, later, and more expensive is better" persuasion. That despite many of us imploring for the voice of reason to those close to the political process.

Now the die has been cast, and we will have to just hope for the best. Good luck, NASA. I would love to see some American HSF before I start pushing up daisies, but I have to admit that the chances seem pretty slim. Actuarily speaking, I only have 25 years or so left. :(

Mark S.

Offline renclod

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Re: Differences between Direct and SLS
« Reply #8 on: 02/04/2012 02:24 PM »
Ground weight support:
...
SLS: Core thrust structure

False !
 see snapshot attached.

Offline woods170

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Re: Differences between Direct and SLS
« Reply #9 on: 02/04/2012 06:50 PM »
Ground weight support:
...
SLS: Core thrust structure
False !

Yup. See attached descriptions from the draft NRA sollicitation for the Advanced Booster. They give great insight into how the SLS core will be very much like the Shuttle ET. It will 'hang' in between the boosters. No support to the ground. The DIRECT vehicle took the same shuttle design. The DIRECT core also 'hung' in between the SRB's.
« Last Edit: 02/04/2012 06:51 PM by woods170 »

Offline Odaniv

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Re: Differences between Direct and SLS
« Reply #10 on: 02/05/2012 07:10 AM »
Thanks for the responses guys this is really clearing up a lot!

I'd like to know what the reason for the ET tank extension on SLS is? ( Besides politics.)

And what are the advantages of a third stage on SLS? When will development of motor for this stage begin?

Offline 93143

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Re: Differences between Direct and SLS
« Reply #11 on: 02/05/2012 07:58 AM »
The tank extension improves performance.  It wants an extra engine (3, 4, or 5 instead of 2, 3, or 4), but the result is a bigger rocket, with more payload.  And Congress said it had to hit 130 tons, which NASA read as 130 tonnes, leading to the requirement for a tank stretch.  The mass target was probably chosen due to a bunch of Mars architectures all showing a largest launch close to that number...

I believe the tank stretch is probably cost-effective, when you divide the additional performance by the additional cost.  It does, however, result in a more expensive system overall, and DIRECT expressed doubt that the extra capacity was necessary...

Some have argued that a tank stretch is necessary to use 5-seg boosters, but it isn't true.  All you have to do is attach the boosters at the second segment from the top...

The separation of the JUS into the LUS and the CPS allows the LUS to be optimized for very heavy loads and the CPS to be optimized for deep space work and long-duration cryogen storage.  Unfortunately this means you now have to pay to develop two stages instead of one, and the CPS now lacks excess tank volume on orbit that could be filled at a depot...

The CPS will most likely use an existing engine.  This does not mean that the engine necessarily exists now, but it will exist by the time the stage is ready to use it.  The Air Force's Next-Generation Engine project is a candidate.  So is the RL-10, which does exist now.  Sources indicate (so says edkyle99) that the J-2X is favoured, and it is already in development because it's wanted for the LUS.  There will not be a separate engine development project for the CPS.

Offline gospacex

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Re: Differences between Direct and SLS
« Reply #12 on: 02/05/2012 10:49 AM »
I believe the tank stretch is probably cost-effective, when you divide the additional performance by the additional cost.  It does, however, result in a more expensive system overall

"However"? The cost is the _biggest_ problem of SLS. It is way too expensive. Trying to increase an already quite respectable performance by becoming even more expensive is akin to buying a 10-room house instead of 8-room one when you can barely afford 6-room one.

This is ridiculous - if we would forget for a second that NASA is not interested/capable of creating economically efficient launch vehicles, and lobby behind SLS is interested only in maximizing their profits - that is, interested in *maximizing*, not minimizing, SLS cost.

Offline 93143

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Re: Differences between Direct and SLS
« Reply #13 on: 02/05/2012 04:26 PM »
That is pure opinion, not demonstrable based on publicly-known facts; I question whether it is on topic.

On the other hand, the original poster did mention "there are those who were Direct fans who are not fans of SLS", and one of the main reasons is rather well stated in your post...
« Last Edit: 02/05/2012 04:30 PM by 93143 »

Offline Odaniv

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Re: Differences between Direct and SLS
« Reply #14 on: 02/05/2012 05:23 PM »


The separation of the JUS into the LUS and the CPS allows the LUS

Thanks for the reply, I'm assuming these are stages but I can't find the acronyms for JUS LUS CPS?

Offline 93143

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Re: Differences between Direct and SLS
« Reply #15 on: 02/05/2012 05:31 PM »
Sorry.

JUS - Jupiter Upper Stage (DIRECT) - combined second stage/EDS
LUS - Large Upper Stage (SLS) - second stage
CPS - Cryogenic Propellant Stage (SLS) - EDS/deep space stage

For SLS, the CPS is supposed to be developed first, and the LUS will follow later.  As mentioned, this plan may change, but I believe that's the plan right now.
« Last Edit: 02/05/2012 05:33 PM by 93143 »

Offline woods170

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Re: Differences between Direct and SLS
« Reply #16 on: 02/06/2012 10:43 AM »
DIRECT: upper stage handles both orbital insertion and earth departure duties

SLS: Upper stage and earth departure duties handled by different stages.

A few refinements to this:
- DIRECT's Jupiter 130 used no upper stage. Orbital insertion was done by the core stage.
- DIRECT's Jupiter 246 used the Jupiter Upper Stage (JUS). With one extra SSME on the core, the core propellant ran out before Orbital insertion. Therefore, the JUS handled orbital insertion. And earth departure, if applicable.
- The Block 1 SLS has an 'in-space' iCPS stage, based on the Delta IVH upper stage. This stage is not used for orbital insertion, but will be used for orbit circulization and as an earth departure stage. Orbital insertion to the delivery orbit is done by the core stage, with 3, but most likely 4 RD-25D's.

And, based on the current information available:
- The Block 2 SLS has an 'in-space' CPS stage, to be newly developed. This stage is also not used for orbital insertion, but will be used for orbit circulization and as an earth departure stage. Orbital insertion to the delivery orbit is done by the core stage, with 5 RD-25E's and the Large Upper Stage (LUS), most likely equipped with J-2X.
« Last Edit: 02/06/2012 10:45 AM by woods170 »

Offline Ben the Space Brit

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Re: Differences between Direct and SLS
« Reply #17 on: 02/06/2012 10:59 AM »
It's not the Plan yet, though, unless ...  is there authorization language and appropriations of funding to pursue it?

IIRC - the Authorisation Act specifically required an integrated US/EDS; I've got the feeling that Congress were specifically thinking of the J-2X version of the Jupiter Upper stage or something similar to this.
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Offline Lars_J

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Re: Differences between Direct and SLS
« Reply #18 on: 02/06/2012 03:53 PM »
Core:
Direct = STS-sized core with 3 or 4 SSME
SLS = Stretched core with 5 or 6 3, 4, or 5 SSME (may not bother with 3)

Where did you get 6 engines?

Oops, my bad... I thought I remembered the "official" SLS reveal showing a 6-engine option. But I guess I was wrong.

Online Mark S

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Re: Differences between Direct and SLS
« Reply #19 on: 02/06/2012 06:28 PM »
It's not the Plan yet, though, unless ...  is there authorization language and appropriations of funding to pursue it?

IIRC - the Authorisation Act specifically required an integrated US/EDS; I've got the feeling that Congress were specifically thinking of the J-2X version of the Jupiter Upper stage or something similar to this.

Yup, you got it. NASA is ignoring that requirement, supposedly in order to meet the 130 ton requirement. Although I would be willing to bet that Congress was more likely to be referencing the existing Ares-V design than anything from DIRECT.

From Section 302(c)(1):

Quote
(A) The initial capability of the core elements, without
an upper stage, of lifting payloads weighing between 70
tons and 100 tons into low-Earth orbit in preparation for
transit for missions beyond low-Earth orbit.
(B) The capability to carry an integrated upper Earth
departure stage bringing the total lift capability of the
Space Launch System to 130 tons or more.

Mark S.

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