Slashdot Mirror
Researchers To Build Underwater Airplane
coondoggie writes to tell us that DARPA seems to still be having fun with their funding and continues to aim for the "far out." The latest program, a submersible airplane, seems to have been pulled directly from science fiction. Hopefully this voyage to the bottom of the sea is of the non-permanent variety. "According to DARPA: 'The difficulty with developing such a craft come from the diametrically opposed requirements that exist for an airplane and a submarine. While the primary goal for airplane designers is to try and minimize weight, a submarine must be extremely heavy in order to submerge underwater. In addition, the flow conditions and the systems designed to control a submarine and an airplane are radically different, due to the order of magnitude difference in the densities of air and water.'"
Fossett was building a submarine that 'flew' underwater, maintaining it's boyancy but overcoming it by using forces similar to an airplane (lift, control surfaces). He was going to pull a stunt to fly to the bottom of the world (Marianas trench) for a record. The vehicle is finished.
Before they go too far with the designs, DARPA might want to check their figures for the densities of water and air. Last time I checked they differed by a lot more than "an order of magnitude" and I'd think this might be important.
Can I buy some pot from you?
"We're taking over 150 atmospheres of pressure!"
"How many atmospheres can this ship take?"
"Well, it's a spaceship, so I'd say anywhere between 0 and 1."
Eek!
since when is "fuckofftags" a useful tag?
They're saying how the requirements for submersibles and aircraft are diametrically opposed. That's good! If they were only kinda in opposite directions, that'd be a challenge. But calling on my vast electrical engineering knowledge (and what is mechanical engineering but electrical engineering with molecules instead of electrons?), I can tell you this is easy. What do you do if you discover that your current is diametrically opposed to what you want? That's right, you flip the terminals around, and bam your current is spot on!
So, using the same principle. In air you want the plane light and lift high because gravity means the natural tendency of the plane is to go downward and you want to go up. Underwater, gravity turns into buoyancy and your plane would naturally want to go up when you want it to go down. This sounds like our current problem -- we have a plane that flies perfectly in air, but in water goes the opposite direction of what we want. So what do we do? Yeah, we just flip it. Now the "lift" of the wings is pointed down. All you need then is an engine that works in air and water, and either a crew compartment that rotates to stay vertical, or sturdy straps and training for pilots to maneuver while upside-down. Done!
I just but the reversed-wing thing is actually used in some high speed submersible. Exercise on how to make it work in either direction above/below water left as an exercise for the DARPA grantee.
The enemies of Democracy are
All submarines 'fly' underwater. Take a look at the control surfaces of a submarine sometime. The cross section is similar to that of the wing of an airplane. A submarine is similar to a dirigible in that it can either adjust its ballast or use its control surfaces with a propulsion system to control its depth.
Unfortunately, the *massive* power requirements of such operation mean that they require a rocket engine for propulsion -- and rockets have a rather substantial appetite for propellant. (BTW, the Shkval only goes at ~200 knots, not supersonic.) The shkval is largely rocket propellant, and even so only gets 7-13km range. There are almost certainly improvements possible, but I'd be surprised if you could build a sub that supercavitated for a long enough range to be useful as a sub and not just a missile / torpedo.
What about very small ones?
The deepest diving subs do not use air to surface but lighter than water hydrocarbons, as at higher pressure the air compresses more, displaces far less water, so they can only make one trip to the bottom, then drop ballast and rise to the surface. An incompressible positively buoyant craft can make many trips to the bottom, the catch is the deeper it want to go, the more force is required ie. the faster in must fly through the water, oddly similar to planes.
Of course to correct your other error, you could of course stop a plane in mid flight and blow the tanks and remain airborne, as long as they were tanks of helium and you contained the output in a aerodynamic balloon (for the transition from forward flight to displacement floating). Just not your day is it ;).
Chaos - everything, everywhere, everywhen