15:19 11-02-2012
Spugpow: There are too many variables in there for me to weigh them:
- Air is compressible, which helps to increase pressure, and that in turn helps eject it at greed speed. What I do not know -not now anyway...) is how the energy spent compressing it weighs against the propulsive forced gained.
- The opening s something that works as well with water as with air. Generally such openings are small to increase the speed of the ejected mass.
- Heating increases the pressure of a gas and thereby will increase the ejection speed, but again heating air also costs energy; without quite a bit of additional study I cannot compare the two.
Of these factors, I would say that the heating one strikes me -without firm evidence- as the best explanation of why jetting works better for man-made machines in air than in water. But if the temperatures needed to make it really efficacious would turn out to be above the boiling point of water, I see problems with any 'biologification'.

Jan: that is in fact what squid on Earth do: they combine their two modes of propulsions. It seems that people are only now starting to investigate the energy benefits and costs between the two.

El Squibbonator: thank you. One of the things I may do during my sabbatical is to update the site. As for the film, I thought that, even though scenes have actually been shot, I know too little about the film business to predict where it will end. But when the film comes out, I will definitely announce it on my site and in the blog.
02:35 10-02-2012
El Squibbonator
First off, I'd just like to say that I LOVE your site. I'm a member of the Speculative Evolution forum, and I even have my own "alien planet" project going there; I call it Valhalla. You should check it out. I also saw you mention that film that Furaha's going to be in. Would you mind telling me about it. What's it going to be called?
20:28 09-02-2012
Anthony Docimo
ahh, but if you run out of energy in the water, you drift. if you run out of energy in the air, you plummet. (that's the advantage to water-based drag: it applies in all directions)
18:08 08-02-2012
William Stephens
could you check your email please?
17:24 08-02-2012
Maybe the best use for jet propulsion in water is to combine it with flapping fins to add acceleration when needed. Something like pengui-squid.
03:59 08-02-2012
That's the thing though: with aircraft, jet-propelled flyers actually <i>outperform</i> "flapping" flyers (with propellers). With boats, the opposite is generally true. The author says that this is because of the three factors of air's compressibility, the ability of a specially shaped opening to increase the speed of the ejected air, and the ability to heat the air--adding energy.
14:58 05-02-2012
Spugpow: I am not yet quite convinced about the arguments regarding compressibility of air. It may well be true, but I do not immediately see why compressibility makes such an impact. All jet propulsion and most flapping propulsion is about the product of speed and mass of the displaced material. Air has little mass, while water has high mass. That seems to give water an advantage, but that greater mass carries the dual burden of more inertia (meaning it is more difficult to propel) and, as there is water outside a swimming animal, more resistance to movement.

But perhaps a comparison within a medium is more relevant. Within water, swimming with fins wins over jet propulsion in water because the fins manage to propel a much larger volume of water than jet propulsion did (I could find the precise pages in one of Alexander's books if you would like them). Jet propulsion in water really is not the best solution if energy efficiency is the only consideration.

I would not be surprised to learn that the same applies in air: animals with flapping wings might be able to displace a much larger volume of air than jet-propelled fliers do, for the same amount of energy spent. If so, that would be the most straightforward explanation for the lack of jet-equipped flying animals.
08:35 05-02-2012
I've been thinking a lot about animal flight recently, specifically, the viability of jet propulsion in air. I came across this interesting passage in my research ( from here: ):

"An interesting observation is that biological jetters do not have any speed advantage over flapping swimmers. This is in contrast to mechanical propulsion where turbojets can achieve much higher top speeds than propellers. The result for mechanical propulsion is, however, based on propulsion in air whereas the biological case refers to aquatic propulsion (there is no known aerial biological jet propulsion). Limiting ourselves to underwater propulsion, mechanical jets and propellers perform similarly, although pumpjets do have some advantage related to the delay of cavitation on the pump propeller. Thus, the lack of a speed advantage for biological jetters is related primarily to the fluid environment, not to a fundamental difference in the physics of biologically generated jets vs. mechanically generated jets.

The speed advantage for mechanical jets in air is due to the compressibility of air. Because air is a gas, a great deal of energy can be added to it simply by compressing it (reducing its volume by increasing pressure). Additional energy can be added by directly heating it. Finally, high energy air can easily be accelerated to enormous velocities simply by pushing it through a specially shaped nozzle. A high jet velocity allows net thrust to be generated for very large vehicle velocities. The same results are very difficult to achieve with liquids because they are very nearly incompressible unless extreme measures are employed."

I wonder what implications this has for creature design. Could a jet-propelled animal overcome the disadvantages of reduced density in air by compressing the air, heating the air, using a specially shaped outlet, or some combination of these approaches?

Here's another idea: perhaps there could be a creature that uses jet propulsion not to increase its forward speed, but to more directly increase its lift. There are aircraft that do this by blowing air over the top of the wing, or over a special aerofoil ( ) I'm imagining a creature with pulsing jet pods on its wings whose slit-shaped outlets direct the flow of air over a specially shaped area to increase lift.
18:57 04-02-2012
Is there some posts or other sources on how the different parameters affect the climate and biological variability?
More specifically, what about greater axialt tilt, say 40° or even 90°? What would the climate in different latitudes be (if we count on different overall temperatures of the planet)?
If the winter temperatures are close to the arctic and the summer ones to the tropical (in fact even more, because of longer days, I think), what would it mean to fauna and flora?
14:51 04-02-2012

My pleasure. I know it is not my business, but if you wish to draw a world map that looks as if it actually depicts a planetary surface, I would advice you to use a 'plate carrée' projection with a 2:1 proportion. With that and with Celestia you can easily check your polar areas for unwanted distortions:
17:42 03-02-2012
William Stephens
Thanks for the map! I won't be moving onto the other more complex maps for quite a while - I just draw my planet.
22:35 02-02-2012

Sorry about that; it was merely the result of my mood at the time of answering. Actually, people do read older posts, which is nice. As for maps, have a look here:

That is an equidistant cylindrical map, made with Matlab, and I can make such blank maps easily. Is that what you mean? I can turn the bold frame off if you want, then you get longitude and latitude lines only. To choose which projections you want, have a look here:

That will show you a very large number of projections, all with a Earth map and with what on a sphere would be circles; their shape shows how that particular projection distorts the map.

Mind you, getting a line drawing of your planet like that in matlab would involve making a list of hundreds of not thousands of coordinate pairs for each point defining a coastline (lots of work...). If you are thinking about a pixel map instead of a vector map, there is a program that can distort existing maps from one projection to another, but I cannot reach it right now.

Spugpow: Thanks for that link; it looks like a fun project
17:47 02-02-2012
William Stephens
aaaanyway, I was wondering if you could post a link to a blank Plate Carrée projection map, a blank equidistant cylindrical map and the other types of map you used in the 'about planet furaha' page. They woul really speed along mapping my planet, Artemis. I searched the net and could not find any blank maps to draw my continents on :/
06:09 02-02-2012
Thanks .

Here's something I think you'll like:
22:33 01-02-2012
William Stephens
No, it's just that I was a little disheartened when you said 'Who reads posts that are two years old?' when I told you I did to look at the rock sucking cliffhangers and pose my question about the awesome yellow gliders
Berichten: 196 t/m 210 van de 930.
Aantal pagina's: 62
Nieuwer11 12 13 [14] 15 16 17Ouder