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.'"

start with mother nature

[circletimessquare]

study up on flying fish [wikipedia.org] and flying squid [wikipedia.org]

then dabble in cormorants [wikipedia.org] and water beetles [everythingabout.net]

once again, mother nature was here first and has a lot to teach us about where to start

Re:maybe it would be easier

[smellsofbikes]

Would the nuclear B-36 [archive.org] count? It wasn't nuclear-powered but it did have an operational power-producing nuke running in it when it was flying, with the intent to develop it into a fully nuclear-powered aircraft using a General Electric HTRE nuclear aircraft engine. It was as heavy as many subs and you had to crawl around through it, using a rope-pulled trolley to get from the front to the back.

Re:Water is 830 times more dense than air

[qkslvr]

Thank you. I hate it when someone uses "an order of magnitude" as a synonym for "a lot" just to sound smart.

My inexpert take.

[MaxwellEdison]

How about this, a rear propulsion planwhich the pilot operates in a counterbalanced globe at the nose. The airplane slows down to a minimum airspeed and inserts the nose and flips over, the pilot's globe rights itself and the planes control surfaces are inverted. The fusalage takes on water to neutral bouyancy and the plan controls as if it were upside down. I'm sure this is completely unfeasible and I hope someone will explain why. The main problems I see are 1.) Slowing the plane enough that 'insertion' doesn't rip it apart. 2.) The pilot seizing up during this maneuver which would go against all of their piloting instincts. 3.) Control systems designed for air travel would be completely inefficient/infeasable in an aquatic environment. Did I forget anything else?

The concept may date back a long time

[Kupfernigk]

I went for a job interview in the early 80s at a military systems company. I had been working on real-time hardware/software for control of a system in two and half dimensions. They wanted someone to work on a control system which was vaguely similar, but they were very cagey about exactly what. I had a major disagreement with the interviewer about some questions he asked me. On the way out we passed a room where someone was coming out of the door and I saw briefly on the wall a wooden mock up of something that looked very like a manta ray.

I then put two and two together and got five, because I realised that the disagreement arose because the interviewer did not expect the operating temperature range of the hardware to exceed more than about 25 degrees C - which made sense if it was for use in sea water.

Re:The scifi version would be supercavitating subs

[Chris Burke]

Supercavitation would allow submarines to move at supersonic (with reference to water) speeds while submerged

Wait, you mean faster than the speed of sound under water? If that's what you meant, no way, we can't make things move that fast in air, so how can an object basically traveling in an underwater air bubble move that fast? The fastest torpedo listed on the wiki page is a 2004 German torpedo which it says reaches 800km/h, which is 3/4ths the speed of sound in air, which is itself around 1/5th the speed of sound in water.

Regardless, though, supercavitation is pretty awesome. =D

Re:Crazy DARPA

[Anonymous Coward]

Who needs to imagine [vttbots.com] anything?

i've seen them fly in the philippines

[circletimessquare]

http://www.susanscott.net/Oceanwatch2002/jan04-02.html [susanscott.net]

Re:maybe it would be easier

[Talderas]

For extra credit, create a vehicle that can fly, go into space and submerge to the deepest portions of the ocean.

Re:Crazy DARPA

[badran]

You might want to take a look at this WW2 tech:
http://en.wikipedia.org/wiki/Type_AM_Japanese_submarine [wikipedia.org]

Not so implausible...

[w0mprat]

I don't understand the assumption that a submarine-aircraft would have to be made heavy and thus be too heavy to fly. Therefore the idea is suggested to be not likely to work at all or will be too compromised to be good at either role.

Rather than resisting water pressure with a heavy pressure hull, it is only necessary to equalize pressure internally and externally. Also consider that humans can readily withstand pressure to a depth of 20-30 metres (just don't try surface in a hurry).

It then becomes not necessary to reinforce all but a few sensitive systems against pressure at depth. That said it's not as simple as taking a F-22 and filling the avionics full of scotch guard and drilling some drain holes. But landing say a helicopter on water, flooding it and having it perform adequately underwater is not a monumental engineering challenge.

In such a craft you could still have a small pressure vessel for crew and sensitive systems, while the rest of the vechicle is filled with ballast water and the resulting compressed air. We are still talking about weight penalties in additional systems and design, so it's still a vehicle that's neither a good aircraft nor a good submarine.

I also think Darpa would be better off with a some VTOL design considering the difficulties in taking off from water. Something like a submarine apache would be quite an achievement. I'm forgetting that the F-22 design is VTOL capable, now that would be a scary machine for any enemy to go up against.

What is a flight?

[pato101]

The definition of flight, as I studied at flight mechanics, is the motion through a single media or lack of media.
Airplanes fly through the air
Zeppelins and balloons fly through the air
Submarines fly through the water
Space-ships fly out of atmosphere
Ships do not fly because their motion is in between two media
Cars do not fly (except for some instants, as it is the case of Rally cars jumping :P ), since their motion is between two media.

From an engineering point of view, submarines use buoyancy forces mainly besides hidrodynamic lift at controls. There are interesting projects around about small submarines using mostly hidrodynamic lift to move around. However, being buoyancy so cheap and independent of the speed of the sub, makes it so interesting for a sub. In aviation, however, in order to obtain good buoyancy you need so much volume that makes it impractical (Zeppelin) and smarter approach is required (aerodynamic lift) with a penalty (minimum speed required).

Re:The scifi version would be supercavitating subs

[CodeBuster]

Supercavitation would allow submarines to move at supersonic (with reference to water) speeds while submerged, and dogfight underwater like WWI aircraft did in the air.

The last I heard any vehicle making use of supercavitation is not able to turn (or at least not very quickly) at supercavitating speeds while underwater. It would be like a car turning suddenly in a long, narrow, and straight tunnel through solid rock (with similar results). The Skvahl [wikipedia.org] torpedo, for example, is said to have this limitation (i.e. it is a straight shot weapon with no ability to correct course once it has been fired and is up to speed).