Awhile back, for a class project, I dug into figuring out what the most efficient rockets were by payload mass fraction. Although I surveyed only a limited field, I was surprised to find the biggest rockets were almost always the most efficient. The bigger the rocket, the better overall it tended to do. For instance, my chart was topped by large rockets like the Saturn V, Energia and Falcon Heavy, itself more efficient than the smaller Falcon 9. So I've always wondered, if one did a comprehensive chart of rockets, which wind up top, and are larger rockets usually more efficient if you control for differing propellants? If so, is there a law of physics that would explain bigger rockets being more efficient? If you're doing a list, please state each rocket's payload mass fraction in percent and list off the propellants it uses if you can. One last thing I noticed was that over time, the R-7 family has steadily gotten ever more efficient. Is this trend seen everywhere over time or have we plateaued?
Quote from: Hyperion5 on 02/04/2013 06:25 pmAwhile back, for a class project, I dug into figuring out what the most efficient rockets were by payload mass fraction. Although I surveyed only a limited field, I was surprised to find the biggest rockets were almost always the most efficient. The bigger the rocket, the better overall it tended to do. For instance, my chart was topped by large rockets like the Saturn V, Energia and Falcon Heavy, itself more efficient than the smaller Falcon 9. So I've always wondered, if one did a comprehensive chart of rockets, which wind up top, and are larger rockets usually more efficient if you control for differing propellants? If so, is there a law of physics that would explain bigger rockets being more efficient? If you're doing a list, please state each rocket's payload mass fraction in percent and list off the propellants it uses if you can. One last thing I noticed was that over time, the R-7 family has steadily gotten ever more efficient. Is this trend seen everywhere over time or have we plateaued? There isn't a "law" of physics regarding this AFAIK. I believe it's just that when you double the amount of fuel mass, you don't double the amount of structure mass. The fuel volume (and thus the mass) scales roughly cubicly while the structure mass scales roughly quadratically. Thus as you get larger and larger rockets your fuel mass faction gets better and better, thus increasing your payload mass faction. The ideal rocket is a payload sitting on top of a giant lump of fuel.
The fuel volume (and thus the mass) scales roughly cubicly while the structure mass scales roughly quadratically. Thus as you get larger and larger rockets your fuel mass faction gets better and better, thus increasing your payload mass faction.
Let's face it, it should be easier to get a higher t/w ratio on a bigger engine than a smaller one, shouldn't it?
Quote from: Hyperion5 on 02/04/2013 08:44 pm Let's face it, it should be easier to get a higher t/w ratio on a bigger engine than a smaller one, shouldn't it?Why?
I propose that there are at least two additional factors, not related to cube-square law, which enable bigger LVs to have larger payload fraction.1. big, really big LVs like Saturn V and Energia were government projects with almost unlimited funding. Using more expensive but lighter material here and there made sense because target was maximum payload size, not necessarily best economy.
2. big LVs tend to use hydrogen.
But there definitely is an advantage of upping the scale from small engines.
Spacex uses some pretty light material in their rockets, namely lithium-aluminum alloy, and they're not exactly funded on anything close to that scale.
The Saturn V beats a Saturn IB, a Falcon Heavy beats a Falcon 9, the Atlas V tends to beat its smaller predecessors and so on.
Also, thrust scales as throat area, not as chamber volume. Thus, to get a good expansion ratio, at some point your engine is too big and you have to make your base wider to fit your engine...
Saturn V is not just scaled up Saturn IB. Major differences in first stage.F-1 outperforms H-1 in overall Isp (don't be fooled by the similar sealevel figures, F-1's higher pc and area ratio inevitably takes the lead at higher altitudes)Bundling several long but thinner tanks (8 Redstones and 1 Jupiter) weighs more than equal volume on one big tank. This seems to contradict the pressure vessel scaling law present earlier in this thread, but does not. There's a geometry based constant in the mass equation. Also support structures become more challenging with a tank cluster.F9H performance awaits actual demonstration.I'll rather leave Atlas details to Jim, but clearly they enjoy even more impressive first stage engine performance upgrades.
I'd bet my house you could make a Saturn V-class rocket more efficient in terms of payload mass fraction than something the size of a Saturn IB.
Quote from: Hyperion5 on 02/05/2013 04:52 pm I'd bet my house you could make a Saturn V-class rocket more efficient in terms of payload mass fraction than something the size of a Saturn IB. With similar clean sheet designs for both, of course. As said the cube/square...favor bigger....
Not /universally/. Again, because thrust is proportional to throat area, as you scale up too big, you run into constraints forcing you to either increase chamber pressure or become a squat rocket (negating any further benefit to proportional drag).
Quote from: Robotbeat on 02/06/2013 02:13 pmNot /universally/. Again, because thrust is proportional to throat area, as you scale up too big, you run into constraints forcing you to either increase chamber pressure or become a squat rocket (negating any further benefit to proportional drag).True, I was responding to discussion of specific classes where that is not yet a problem. Advancements since F-1 mean that using existing high pressure engines (RD-family etc) you'll get into ludicrous big class before available thrust/area becomes serious problem.
I was just pointing out that the scaling laws can cut both ways. And there lots of them.