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Comment by rayiner | Hacker News Reader

I didn't mean to imply that, say, the Brayton cycle was an absolute physical limit to aerospace vehicle performance that cannot be worked around. Of course, you can switch from jet engines to scramjets, you can switch from optical lithography to x-ray lithography, you can avoid the realities of supersonic drag by going into space, etc.

But: a) all that is risky and extremely expensive (and as the semiconductor example shows us, often exponentially so); and b) whatever technology you use, you will always be pushing up against physical limits--that's just the nature of the physical engineering disciplines.

These are novel concepts in a world where we see "technological progress" as the evolution of the internet from AOL in 1995 to Facebook in 2010 and wonder why other areas don't improve so fast.

PS) I'd be quite interested to see any specific papers you had in mind vis-a-vis alternative cycles for jet engines.

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In time I'll publish a paper and technical report. In a little more, I'll show a prototype. But I do have my hands fairly full!

I don't usually work from the literature (it is hard to access papers outside of academia, and they're usually weird, abstract, hard to read, and miss the point anyway) so I work from the first principles.

But at a high level, imagine either an Ericsson or Carnot cycle, (dependent on materials), where high temp, high pressure heat addition is contiguous combustion during expansion, and the isothermal compression is a secret sauce :-)

> I don't usually work from the literature (it is hard to access papers outside of academia, and they're usually weird, abstract, hard to read, and miss the point anyway) so I work from the first principles.

Amazing.

Amazing will be 30 years from now when I'm watching a plane fly over using a Fong-type engine ;).

> In a little more, I'll show a prototype

Say what you will about HN, but comment threads about seemingly impossible technical challenges that end in this make me immensely happy to be here :D

I think the point is to retreat farther from the idea "a better jet engine" to "a better way to fly" or "a better method of propulsion". By shopping around in that larger idea space you may be able to trade in your current physical limitations for something less constraining, or even just less known. Discussions on this topic always make me think of Neal Stephenson's article on rocketry, which in my mind is closely linked to local maximums of innovation/invention[1].

[1]: http://www.slate.com/articles/technology/future_tense/2011/0...

It's a good article, thanks for reminding me to re-read it.

I note that in it, Stephenson makes a prediction, as if it were a known fact, that I think we now have significant reason to doubt.

He says "Rockets are as close to perfect as they're ever going to get. For a few more billion dollars we might be able to achieve a microscopic improvement in efficiency or reliability, but to make any game-changing improvements is not merely expensive; it's a physical impossibility."

I think we now have reason to believe that for around a billion dollars ($1b is the estimated total R&D budget as of a year ago), SpaceX has already made better-than-microscopic improvements in efficiency. But more interestingly, there's reason to believe that with little or no additional outside investment (i.e. working on profits), SpaceX is pretty likely to soon make a very large improvement in efficiency, with Grasshopper &c. Rockets are probably not as perfect as they're ever going to get.

Without taking away from SpaceX's accomplishments, it's important to keep the performance of the Merlin engines in context. The Merlin 1d is arguably the most efficient rocket engine using a gas-generator cycle and RP-1 (kerosene) fuel. That's a very specific claim. Hydrogen-burning engines like the one on the Space Shuttle are much more efficient (measured by specific impulse), and some Soviet kerosene rockets came very close both in specific impulse and thrust to weight ratio, though they used a somewhat different cycle. Relevant discussion: http://www.flightglobal.com/blogs/hyperbola/2012/06/comment-....

Still a very impressive achievement, but looking at the performance of SpaceX's engines circa 2012 and comparing them to what the Soviets built in 1960 or 1970 helps put the pace of the industry into context.

I had assumed, in my ignorance, that the Merlin engines were not an important advancement. I don't think that specific impulse is necessarily the important sense of "efficiency" (though I do assume that it correlates with the important sense).

The relevant efficiency, to me, is "awesomeness achieved per human effort and non-renewable resource", and the useful proxy for this is "awesomeness per price". Arguably SpaceX rockets are cheaper (per given mass to given orbit, obviously) that most or arguably all of the competition. I say "arguably cheaper" because it's a little tough to figure out how to price the shoulders of giants on which they stand.

And really I only picked the word "efficiency" in the context of Stephenson's "efficiency or reliability".

Anyway, hopefully this explains why I think that true reusable rockets, Musk's often-stated ambition, qualify as a "very large improvement" in efficiency. Specific impulse is not expected to significantly improve, right?

Also, I apologize in advance for my legion failings in the area of comprehending aerospace engineering.

Not exactly efficiency, (though they have high combustion efficiency) but low cost and very good mass ratio. If they make it reusable, that where the killer improvement is. Elon says they're getting there; I'm crossing my fingers.

I utterly agree in substance, see my cousin-comment.

Do you think think it's unreasonable to call "payload per cost" a relevant "efficiency"?

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