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Do six or seven RS-68's amount to roughly twice as much thrust as the shuttle stacks 3 SSME's? If they do, then I think we have your 200MT booster.
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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Whoa there, just upping the thrust doesn't mean anything, it might increase payload a few percent since you get less gravitational losses. The critical part is the efficiency of the engines, which if you just stack more of them together doesn't improve.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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You can see where I'm going with this though? Perhaps I could ask it like this: What is the best equivalent to 6 SSME's?
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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Six SSMEs
NASA is making a conscious decision, just as Boeing did with Delta-IV, to intentionally trade lower efficiency but much simpler and bigger engines for bigger cheap fuel tanks (which also bumps up the tankage efficiency a tad).
Five RS-68's have much more thrust than six SSMEs. If you put six SSMEs under the big 10m tank, the payload would suffer since the full fuel tank is so heavy.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Okay, could we just add 2 more SRB's to the Ares-V then?
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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That, and maybe another J-2X on the upper stage, buys you 150MT. This comes at the cost of more significant changes to the launch table and VAB bays, longer prep times, and less reliability of launch. Oh and another $80-100M per flight give or take. Development wise, the EDS will share less in common with the Ares-I upper stage (which really are very similar), much heavier-duty structural design for the core, which will no longer even resemble the Shuttle tank, and more difficult boost-phase attitude control.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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What I'm suggesting here is 2 shuttle stacks melded into one. 4xSRB's on the outside, a main tank twice as heavy, and the equivalent of 6 SSME's underneath. The Space Shuttle doesn't have an upper stage, so I guess without one this 'duality'-booster should lift exactly twice as much as the shuttle stack, right? Whats that then?
If we added an upper stage, even more?
Your payload estimates seem too conservative to me.
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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Why does it seem conservative? The SRBs are not very efficient, in fact they are the worst kind of rocket for efficiency (low propellant velocity, very high empty mass). The only good things about them are their sheer size and pretty good reliability. The 150MT figure is what I recall from a NASA Ares-V study brochure if memory serves.
Such an arrangement is too far from the original dimensions of Shuttle to be practical, you might as well start over and make a 12m wide NOVA-style behemoth.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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No no, your not getting it (or maybe I'm missing something?).
What is the fundamental difference between 2 shuttle stacks melded into one, and 2 seperate shuttle stacks?
So thats twice as much payload, right?
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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I don't think I get it. Do you mean a single cylindrical core stage with similar volume as two Shuttle external tanks?
Slightly more than double actually, with increased cube/square law tankage efficiency.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Exactly. So how many tonnes is that?
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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Probably in the region of 200MT, but the size of rocket you are talking about is a behemoth, a 12m class core stage puts this rocket in the NOVA category of monster megarockets.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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The wikipedia article says the ET is 46.9m by 8.4m diameter. Thats 2600m^3. To get 5200m^3 out of a 10m diameter tank, a tank length of about 66m is required.
Actually, the main tank of the Ares V is nearly there at around 60m, not including the upper stage tanks. Infact, I'd imagine you could, pad logistics permitting, mount two extra SRB's onto the Ares V, without increasing Max-Q or max heating over the space shuttle during launch.
That should be around 200T then.
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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You didn't mention making the thing longer
But you forget that the Shuttle is powered by the most efficient production large Hydrogen engine, the SSME (aka RS-25). Ares-V on the other hand will be pushed by the less efficient RS-68 to reduce costs. So even if you did add another pair of SRBs, no, it won't get anywhere near 200MT. The 150MT figure is entirely reasonable for just slapping on more boosters. To get the same thrust as a cluster of five RS-68's, you'd need seven and a half SSMEs. You would probably also need a wider, heavier upper stage too, particularly if the main tank is going to be longer and payload heavier.
All in all, you are looking at a 300% increase in engine costs for more payload than you need for the Moon, but still not enough for Mars in one shot.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Also,
I recommend picking up a used copy of 'Rocket Propulsion Elements' by George Sutton. Most of the back of the envelope calculations you are looking for can be founded there. You could probably ignore most of the derivation and skip to the algebra (those this will give you greater understanding of the mechanics). You'll probably pay $30 from barnesandnoble.com and would be a good investment.
To address so of the suggestion you brought up. As you scale up your tank, some of the mitigated factors will become much more significant. Structural imperfections will have a much greater likelihood of popping up and you'll quickly begin to increase weight as you try to reinforce the structure. Deformations and the influence on adjacent structures is also of consideration. There are also a host of combustion related issues that can influence the design of the rocket.
Anyhow, good luck and hope you able to get a look at Sutton.
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Thanks for that suggestion, ftlwright. I'd love to read such a book.
From what I understand, the RS-68b will attain very nearly SSME efficiencies. Also, we're talking 5-segment SRB's in place of the old 4-segment, in keeping with the Ares development plan. I'd imagine the combination to at least achieve 200T, and thats as a 2-stage vehicle, like the shuttle.
Another way to look at it is imagining a [purely theoretical] 'two-fold' Ares V as a starting point: 4 SRB's, ~120m tank, 10 RS-68b engines... Thats 300T. Then halve the main tank volume.
Now that I've brought the idea up, how about a 'one-and-a-half fold' Ares V: 3 SRB's, a main tank 1.5x60m = 90m. 7 RS-68b's. That should amount to 1.5x150T = 225T. Actually, 3 SRB's arranged equi-spaced might be more accomadatable than 4. Not sure about a ~90m tank though.
- Mike, Member of the [b][url=http://cleanslate.editboard.com]Clean Slate Society[/url][/b]
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Not so fast, its impossible for a low-pressure engine like RS-68 to attain the same specific impulse as the high-pressure SSME. It will not have comparable performance with the same volume of tankage.
The "magic number" is still 250MT or so for single-shot DRM sized missions to Mars.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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