5 ms·

It's not widely known or understood, but there are, in my opinion, quite a few ways to make significantly superior jet engine replacements. Factors of 2 or 3 i

It's not widely known or understood, but there are, in my opinion, quite a few ways to make significantly superior jet engine replacements. Factors of 2 or 3 in specific power and efficiency are possible. Due to the rocket equation, this could yield vastly longer range and more efficient aircraft (1/10 the weight, 10x the range, 10x the efficiency, 2x the speed...). In particular, you point out the limitations of the Brayton cycle and material limitations. However, there are other cycles that are much closer to Carnot efficiency, or match it in the ideal case. There's a lot you can do when isothermal compression is possible. Additionally, if there is a thermal limitation, you can do isothermal combustion, such that much more heat addition is at the maximum temperature of your materials, and you can even design efficient reheating. It's not a jet -- you want to pair this with a driven fan system or at least have high bypass, but it's fundamentally more thermodynamically efficient. For supersonic applications, scramjets remain possible, but challenging. And rocketry + high altitude flight remains extremely exciting and potentially more efficient than standard jetliner transport. Just difficult. So yes, there are physical limitations. But they are not what most people think they are, and they do not mean what people think they mean.

rayiner 13y ago
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.
- kbenson 13y ago
  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/2011/02/space_stasis.html http://www.slate.com/articles/technology/future_tense/2011/0...
  - jholman 13y ago
    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.
    - rayiner 13y ago
      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-merlin-1d-is-not-quite.html 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.
paulsutter 13y ago
The world could use another Elon Musk. A female Elon Musk would be even better. Congratulations on Lightsail and keep thinking even bigger.
- newnewnew 13y ago
  Why would a female Elon Musk be better than a male one? Are males inferior?
  - paulsutter 13y ago
    Same reasons it's awesome to have a black president: 1. There are people it would inspire, and 2. There are people whose expectations would be shaken up The latter group needs as much help as the former.
    - newnewnew 13y ago
      There are few enough Elon Musks in the world that I think we should be grateful for whichever ones we get
    - ekianjo 13y ago
      Wow a black president made certainly a lot of difference. Just like the white ones : lying, deceptive, secretive, and dishonest (See NSA). Sorry if I fail to see much difference.
      - Nursie 13y ago
        Err yeah. Why would you think that a black president would be different to a white one? That's precisely the shakeup the world needs - to realise that whatever skin colour, male or female, people are people and you shouldn't judge them negatively or positively on how much melanin or testosterone they produce :P
      - IdahoEv 13y ago
        He's been mendacious as all of them, sure. That's not the important difference. The important difference is that there are now millions of black American kids no longer growing up in a world where they just "know" as a first principle that "black people can't be president". That sort of example simply changes people's outlook. Inspiration and aspiration matter.
nether 13y ago
> It's not widely known or understood, but there are, in my opinion, quite a few ways to make significantly superior jet engine replacements. > Factors of 2 or 3 in specific power and efficiency are possible. Due to the rocket equation, this could yield vastly longer range and more efficient aircraft (1/10 the weight, 10x the range, 10x the efficiency, 2x the speed...). I'd like to hear about these, assuming they produce comparable amounts of thrust as jet engines while being a reasonable size (so as not to increase drag excessively).

rayiner 13y ago

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.

kbenson 13y ago
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/2011/02/space_stasis.html http://www.slate.com/articles/technology/future_tense/2011/0...
- jholman 13y ago
  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.
  - rayiner 13y ago
    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-merlin-1d-is-not-quite.html 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.

paulsutter 13y ago

The world could use another Elon Musk. A female Elon Musk would be even better. Congratulations on Lightsail and keep thinking even bigger.

newnewnew 13y ago
Why would a female Elon Musk be better than a male one? Are males inferior?
- paulsutter 13y ago
  Same reasons it's awesome to have a black president: 1. There are people it would inspire, and 2. There are people whose expectations would be shaken up The latter group needs as much help as the former.
  - newnewnew 13y ago
    There are few enough Elon Musks in the world that I think we should be grateful for whichever ones we get
  - ekianjo 13y ago
    Wow a black president made certainly a lot of difference. Just like the white ones : lying, deceptive, secretive, and dishonest (See NSA). Sorry if I fail to see much difference.
    - Nursie 13y ago
      Err yeah. Why would you think that a black president would be different to a white one? That's precisely the shakeup the world needs - to realise that whatever skin colour, male or female, people are people and you shouldn't judge them negatively or positively on how much melanin or testosterone they produce :P
    - IdahoEv 13y ago
      He's been mendacious as all of them, sure. That's not the important difference. The important difference is that there are now millions of black American kids no longer growing up in a world where they just "know" as a first principle that "black people can't be president". That sort of example simply changes people's outlook. Inspiration and aspiration matter.

nether 13y ago

> It's not widely known or understood, but there are, in my opinion, quite a few ways to make significantly superior jet engine replacements. > Factors of 2 or 3 in specific power and efficiency are possible. Due to the rocket equation, this could yield vastly longer range and more efficient aircraft (1/10 the weight, 10x the range, 10x the efficiency, 2x the speed...). I'd like to hear about these, assuming they produce comparable amounts of thrust as jet engines while being a reasonable size (so as not to increase drag excessively).