Author Topic: SLS General Discussion Thread 2  (Read 223717 times)

Offline Robotbeat

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Re: SLS General Discussion Thread 2
« Reply #320 on: 03/13/2016 11:39 PM »
None of these options is under the slightest consideration. Join L2 for the definitive status of SLS. The elephant mastodon in the room is indeed a hydrocarbon fueled 15m diameter monster which will be affordable due to leaner manufacturing processes, the lack of government involvement, and most of all, reusability.
...and only needing to be 2 stages. Vs 2 boosters, 1 core, and an upper stage. And the 2nd stage of the mastadon would also be your lander/ascender, which is honestly just as important as the launch vehicle.

...anyway, there was definitely a small cadre of people pushing for a return to a big kerolox core, but they lost out to the Shuttle-derived folk.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline EE Scott

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Re: SLS General Discussion Thread 2
« Reply #321 on: 03/14/2016 12:40 AM »

...snip...

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

This is not a helpful description. The "best answer" was pushed into view by folks who in my opinion appeared highly politically biased. Does the idea of black zones or perhaps the name Michael Griffin not bring back any memories of those days? If I am remembering that time inaccurately, please let me know.
Scott

Offline Oli

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Re: SLS General Discussion Thread 2
« Reply #322 on: 03/14/2016 09:59 AM »

...snip...

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

This is not a helpful description. The "best answer" was pushed into view by folks who in my opinion appeared highly politically biased. Does the idea of black zones or perhaps the name Michael Griffin not bring back any memories of those days? If I am remembering that time inaccurately, please let me know.

So NASA faked their own cost estimates? SLS is a good TLI launcher and it was considered cheaper in development than the alternatives. Back in 2010.


Offline notsorandom

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Re: SLS General Discussion Thread 2
« Reply #323 on: 03/14/2016 01:14 PM »
It only needs enough thrust to keep positive T/W after the SRBs stop thrusting.
This may be a pretty minor nitpick. The Shuttle had a T/W lower than 1:1 at SRB separation. Isn't the same true of SLS?

Offline edkyle99

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Re: SLS General Discussion Thread 2
« Reply #324 on: 03/14/2016 02:20 PM »

...snip...

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

This is not a helpful description. The "best answer" was pushed into view by folks who in my opinion appeared highly politically biased. Does the idea of black zones or perhaps the name Michael Griffin not bring back any memories of those days? If I am remembering that time inaccurately, please let me know.
"Black Zones"?  That had nothing to do with SLS.  Michael Griffin?  He was gone before SLS was defined. 

A series of studies, including the substantial "Requirements Analyses Cycle", were performed during 2010-2011, months after President Obama sent Griffin packing.  Saturn V-like RP/LOX first stages were considered, but the development costs were an issue.  ORSC and J-2X would have been required.  SLS won in part because the propulsion existed, or nearly existed, minimizing development cost.  NASA can hardly afford SLS as it is.  It never would have been able to fund a full-up new propulsion development effort. 

 - Ed Kyle

Offline EE Scott

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Re: SLS General Discussion Thread 2
« Reply #325 on: 03/14/2016 02:38 PM »

...snip...

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

This is not a helpful description. The "best answer" was pushed into view by folks who in my opinion appeared highly politically biased. Does the idea of black zones or perhaps the name Michael Griffin not bring back any memories of those days? If I am remembering that time inaccurately, please let me know.
"Black Zones"?  That had nothing to do with SLS.  Michael Griffin?  He was gone before SLS was defined. 

A series of studies, including the substantial "Requirements Analyses Cycle", were performed during 2010-2011, months after President Obama sent Griffin packing.  Saturn V-like RP/LOX first stages were considered, but the development costs were an issue.  ORSC and J-2X would have been required.  SLS won in part because the propulsion existed, or nearly existed, minimizing development cost.  NASA can hardly afford SLS as it is.  It never would have been able to fund a full-up new propulsion development effort. 

 - Ed Kyle

Good points. I was not thinking about the RAC studies, I thought you were referencing ESAS, etc., when you stated that this issue was studied to death a decade ago, which I took to mean that you are stating that any doubts about the superiority of SDLV solutions vs. non-SDLV for NASA's manned BEO program was basically put to bed long ago.
« Last Edit: 03/14/2016 02:39 PM by EE Scott »
Scott

Offline EE Scott

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Re: SLS General Discussion Thread 2
« Reply #326 on: 03/14/2016 02:40 PM »

...snip...

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

This is not a helpful description. The "best answer" was pushed into view by folks who in my opinion appeared highly politically biased. Does the idea of black zones or perhaps the name Michael Griffin not bring back any memories of those days? If I am remembering that time inaccurately, please let me know.

So NASA faked their own cost estimates? SLS is a good TLI launcher and it was considered cheaper in development than the alternatives. Back in 2010.



No I don't mean to imply that, sorry if it came off that way. I wasn't thinking specifically of SLS.
Scott

Offline edkyle99

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Re: SLS General Discussion Thread 2
« Reply #327 on: 03/14/2016 02:43 PM »
It only needs enough thrust to keep positive T/W after the SRBs stop thrusting.
This may be a pretty minor nitpick. The Shuttle had a T/W lower than 1:1 at SRB separation. Isn't the same true of SLS?
At SRB sep, STS T/W was probably 0.91-0.93 or thereabouts, so generally speaking the design was for a nearly 1:1 ratio at staging.  T/W went positive within 15-20 seconds and of course remained positive for the subsequent ~355 seconds of the SSME burn.  I'm not sure about SLS at the moment, but I would expect it to also be ballpark 1:1.

 - Ed Kyle
« Last Edit: 03/14/2016 02:47 PM by edkyle99 »

Offline TomH

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Re: SLS General Discussion Thread 2
« Reply #328 on: 03/14/2016 03:35 PM »
It only needs enough thrust to keep positive T/W after the SRBs stop thrusting.
This may be a pretty minor nitpick. The Shuttle had a T/W lower than 1:1 at SRB separation. Isn't the same true of SLS?

If you were doing a perfectly vertical shot toward GSO or escape, that ratio would cause gravity losses which are greater than acceleration. The thing is, the W part of that T/W is a function of gravity times mass. When you factor your velocity into separate vectors according to your trajectory, some of the V is vertical climb and some of it is downrange velocity. The downrange velocity is canceling out part of the gravitational pull. Sitting on the ground, mass and weight are equal. Once you are in a semi-orbital trajectory, the weight is less than the mass, so what seems to be < 1:1 ratio actually is not.

Think about STS. At main tank separation, there is not enough velocity for orbit. The tank reenters and burns up. The thrust of the two OMS engines is technically far less than 1:1 T/W if you are considering the mass of the vehicle to also be the weight of the vehicle. However, because the vehicle is almost in orbit, the forward velocity is cancelling out almost all of the pull of gravity (and remember, weight is a function of mass times gravity). So though the mass is still pretty much the same, the weight is now much lower. The low thrust OMS engines are quite adequate to increase the velocity to orbital velocity.

The same thing is true after SRB separation. The downrange velocity is cancelling part of the effect of gravity, so the weight is actually lower than the mass and the T/W is greater than 1.

Offline oldAtlas_Eguy

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Re: SLS General Discussion Thread 2
« Reply #329 on: 03/14/2016 03:43 PM »

...snip...

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

This is not a helpful description. The "best answer" was pushed into view by folks who in my opinion appeared highly politically biased. Does the idea of black zones or perhaps the name Michael Griffin not bring back any memories of those days? If I am remembering that time inaccurately, please let me know.

So NASA faked their own cost estimates? SLS is a good TLI launcher and it was considered cheaper in development than the alternatives. Back in 2010.



No I don't mean to imply that, sorry if it came off that way. I wasn't thinking specifically of SLS.
In order to understand the "best" of the study, you must also investigate what were the assumptions made for the  evaluation models. These assumptions can create their own set of biases funneling you to a specific design as best when it is not. SpaceX is obviously using a different set of assumptions in their models to determine "best" that result in the BFR/MCT configuration. Each different set of assumptions result in a different "best".

SLS was a separate beast in that the two main "best" concerns controlling its design was development costs and schedule. Actually schedule may have been the more important item of the two. A $15B development cost (the SLS will not be operational until EM-2, $1.5B/yr for 10 years) I would not consider an inexpensive development.
« Last Edit: 03/14/2016 03:47 PM by oldAtlas_Eguy »

Offline Lobo

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Re: SLS General Discussion Thread 2
« Reply #330 on: 03/14/2016 04:16 PM »

Switching wouldn't change the timeframe that much...

Sure would - the first stage engines are one of the most pivotal, complex elements of the whole LV. You switch those out and you have to change the whole design, especially when you're talking different fuel types. ...
Precisely.  If SLS went to a lower-performing hydrocarbon core first stage, a heavier, higher thrust second stage would be needed.  It would mean bringing back J-2X.  It would also mean development of a smaller in-space third stage.

These questions were all studied to death a decade ago.  Multiple studies of innumerable alternative designs.  The best answer nearly every time looked pretty much like the rocket now being built.

 - Ed Kyle

Not quite sure I agree with you here Ed (which is rare, I usually do).

If you mean a decade ago during the ESAS study, I think there were much better options considered (and some not considered) , and then passed up on in favor of basically what's being built now in LV 27.3 (not, exactly, but pretty similar)

By the time we were at CxP's cancellation about 5 years ago, then I think you are correct, what they are building now is probably about the best configuration, given politics, that could be expected.

Offline Lobo

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Re: SLS General Discussion Thread 2
« Reply #331 on: 03/14/2016 04:22 PM »

In order to understand the "best" of the study, you must also investigate what were the assumptions made for the  evaluation models. These assumptions can create their own set of biases funneling you to a specific design as best when it is not. SpaceX is obviously using a different set of assumptions in their models to determine "best" that result in the BFR/MCT configuration. Each different set of assumptions result in a different "best".


Good point. 

I think a lot of the appearance of a "thumb on the scale" of the result of the ESAS study was based on many assumptions by NASA that seemed to be perhaps somewhat unreasonable.   In order to force LV13.1 and 27.3 as the winners, where seemingly there were better options passed up.


Offline daveklingler

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Re: SLS General Discussion Thread 2
« Reply #332 on: 03/14/2016 07:32 PM »

Regarding your assertion that the new first stage would be lower-performance, the tank mass and fuel density difference along with the relatively low difference (~50 seconds) in sea level Isp generally makes kerolox come out slightly better for first stages, which I'm pretty sure you know very well.
I think you know pretty well that the SLS core stage is not a "first stage".  It is a long-burning sustainer stage serving the same purpose as the Orbiter/ET combination.  It provides high specific impulse above all else, much higher than only "~50 seconds" since most of its action time is in vacuum where its advantage over a hydrocarbon engine is in excess of 120 seconds ISP.  It only needs enough thrust to keep positive T/W after the SRBs stop thrusting.

If you replace this high-performing core stage with a hydrocarbon stage, you are going to have to make up the delta-v shortfall with a bigger, more expensive LOX/LH2 upper stage which will require higher thrust than RL10 and the like can provide.  All of the studies showed that result.  The proper application of a hydrocarbon engine would be as part of an SRB replacement.

 - Ed Kyle

I'm trying to avoid blowing an evening with my calculator in an attempt to verify something which only matters as an intellectual exercise.  :)  I did, unfortunately, blow an hour already trying to find decent numbers for the various SLS components, along with an ascent profile, before giving up.

Anyway, without a side booster configuration, I think you'd be very obviously right.  But with a side booster configuration, my intuition is that an ORSC core could be configured in such a way that there would be no delta vee shortfall for the EUS to make up.  I think such an "upgrade" would be faster and easier if the boosters were liquid, and I suspect that with something like a 7xORSC common core for both boosters and center the end result would be quite capable and minimally disruptive.

That's all guesswork.  I'm having fun playing hookey, but I don't dare take a day off to figure out whether it's all impossible.

Offline daveklingler

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Re: SLS General Discussion Thread 2
« Reply #333 on: 03/14/2016 07:44 PM »
SLS won in part because the propulsion existed, or nearly existed, minimizing development cost.  NASA can hardly afford SLS as it is.  It never would have been able to fund a full-up new propulsion development effort. 

My whole point is that it's interesting to contemplate the new possibilities.  Essentially, the lack of any decent hydrocarbon engine a few short years ago has completely changed through the grace of a billionaire and some politicians.  Where the AR-1 is concerned, Congress has essentially reached into the Air Force budget to create something which currently has no useful purpose.  Voila, a relatively cheap (putatively $12.5M) ORSC engine!  What can be done with it?

Offline Prettz

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Re: SLS General Discussion Thread 2
« Reply #334 on: 03/14/2016 07:59 PM »
What was so wrong with using the F-1B?

Offline Coastal Ron

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Re: SLS General Discussion Thread 2
« Reply #335 on: 03/14/2016 08:32 PM »
SLS won in part because the propulsion existed, or nearly existed, minimizing development cost.

It was pretty much a foregone conclusion that the SLS would use what is being used today, since the law Congress wrote pretty much said that NASA must, per Sec. 304 (a) (1), to the extent practicable, use:

"(B) Space Shuttle-derived components and Ares 1 components that use existing United States propulsion systems, including liquid fuel engines, external tank or tank- related capability, and solid rocket motor engines; and (2) associated testing facilities, either in being or under construction as of the date of enactment of this Act."

Plus the quick need dates:

"Priority should be placed on the core elements with the goal for operational capability for the core elements not later than December 31, 2016."

And I'm sure we all recall the many in the space community that wondered why NASA was "dragging it's feet" taking so long to define the SLS.  NASA tried to make it look like they had a choice on the design, but Congress didn't give them enough room to really have any choice.

Quote
NASA can hardly afford SLS as it is.

The cost of development is not the real issue.  The cost of using an HLV every year, for decades, is the real question.  And no one knows the answer to that...
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Zed_Noir

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Re: SLS General Discussion Thread 2
« Reply #336 on: 03/15/2016 04:22 AM »

Quote
NASA can hardly afford SLS as it is.

The cost of development is not the real issue.  The cost of using an HLV every year, for decades, is the real question.  And no one knows the answer to that...

Bah. The folks from Hawthorne will make that moot one way or another soon. Either with the soom to debut HLV lite or the Mastodon SHLV to be announced in September. Either the HLV lite or the SHLV will likely to have at least half a dozen flights per year once in service.
« Last Edit: 03/16/2016 05:33 AM by Zed_Noir »

Offline Steven Pietrobon

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Re: SLS General Discussion Thread 2
« Reply #337 on: 03/15/2016 04:55 AM »
This may be a pretty minor nitpick. The Shuttle had a T/W lower than 1:1 at SRB separation. Isn't the same true of SLS?

For Block IB, its just under 1g at SRB separation. Actual acceleration is 9.5 m/sē.
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline daveklingler

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Re: SLS General Discussion Thread 2
« Reply #338 on: 03/15/2016 05:15 AM »
What was so wrong with using the F-1B?

Congress has determined that the AR-1 shall exist, ye, verily.

That's not to rule out a future such decree on behalf of the F-1B, which I think would have given me more joy, as arbitrary declarations go. Perhaps when SpaceX and/or Blue get around to fielding a really great big rocket, Congress will hold hearings on why the USA doesn't have one yet.

Offline notsorandom

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Re: SLS General Discussion Thread 2
« Reply #339 on: 03/15/2016 02:07 PM »
This may be a pretty minor nitpick. The Shuttle had a T/W lower than 1:1 at SRB separation. Isn't the same true of SLS?

For Block IB, its just under 1g at SRB separation. Actual acceleration is 9.5 m/sē.
Thanks. The minute after I posted that I was wondering if there would be a difference because of the lighter ICPS.

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