“That’s nice to know, but they still need propulsion even to maintain that slow speed, don’t they?”
“True enough. For propulsion, they have a unique system I designed in cooperation with a marine biologist. Compared to the noise we have to make in order to turn that screw back there on the Louisiana, these little guys are silent. They make the “silent service” seem like a drag strip! Ever been to a drag strip?”
“Yeah, don’t go without ear plugs!”
“That’s right, but hey, I have a better comparison. Ever been on the deck of an aircraft carrier during a launch and recovery?”
“Yeah, I was on a carrier for one of my midshipmen cruises at the Academy,” replied Pappy. “After hearing the incredible level of noise on the flight deck, it seemed to me the carrier was a sitting duck. All the enemy had to do was listen. I bet you could hear that thing with the naked ear from twenty miles away! It’s funny, because at one point, I thought I might want to be a fighter pilot!”
“Well don’t give up yet. You still might get your chance — only just a little different from how you pictured it as a midshipman!”
“Tell me more, Captain. Now you have my attention!”
“The hydrodynamic design of these fighters enables the electric propulsion system to push them along at over fifty knots, totally silent. In a way, they’re kind of a throwback to the old World War Two diesel-electric boats, which used electric motors for propulsion when submerged. Those old boats didn’t have today’s battery technology, though, which meant they had to surface a lot or snorkel to run their diesels and recharge their batteries. But any submariner who has ever been in an exercise against a diesel-electric boat knows they were extremely quiet when submerged — and extremely difficult to detect. Well believe me, our sub-fighters make those guys sound like the drag strip!”
“I’m going to have to study these things, Captain, to see how you did it.”
“You’re welcome to do that, but since it’s overcast today and Big Brother can’t watch us from above, let’s launch one instead. You can experience a sub-fighter for yourself, firsthand. I’ll get Dwight to demonstrate, and you can get your first familiarization flight, FAM-1, at the same time.”
“Really? You mean… I can get checked out in a sub-fighter? Now?”
“I’m going to be relying on you to be my lead pilot, Pappy. You and two of the lieutenants will rotate flying duties, so you’re going to have to get checked out sooner or later. And there’s no time like the present. Let’s go see Dwight.”
The captain and XO made their way up the stairway from the conning tower to the main deck of Platform Alpha. The XO, who had never been on an offshore oil rig, looked around in amazement. It was huge, with ladders and machines everywhere.
“This reminds me of being on an aircraft carrier,” he said.
“There are a lot of similarities. This place is like a city at sea. The kitchen and mess hall are on this deck along with the laundry room, the head, the showers, the bunkrooms, and the rec room where the crew can play pool or watch TV. That machinery over there is a desalinization plant for making fresh water from seawater. Up above are the working spaces. We can take this ladder right here up to Dwight’s office.”
The captain led the way, and when they arrived, Dwight was standing at a large whiteboard mounted on the far wall. The board extended the full length of one side of the room and was filled with tasks and timelines for getting the Louisiana back to sea within two days. Dwight indicated the beginning and ending of tasks with small triangles connected by a horizontal line. The length of the line indicated how long the task should take. When a task was started, Dwight colored in the triangle at the beginning of the line, and when the task was completed, he colored in the triangle at the end of the line. In Dwight’s usual “low-tech” manner, a vertical string was tacked at the top and bottom of the board. The string represented current time and was repositioned to the right every half hour. Any tasks to the left of the string should have been completed by now, and tasks to the right of the string were yet to be done.
Seeing the captain and XO enter, Dwight pointed proudly to the board. Thanks to the overcast day, his team was at least six hours ahead of schedule. A number of the triangles to the right of the string were already filled in.
“Very impressive, Dwight. Your team is going like gangbusters!”
“You betcha. We aim to please!”
“Well, since you’re so far ahead of schedule, and we don’t have to worry about satellite snoops today, how about taking Pappy for a FAM-1 flight?”
“I’d love to. I was hopin’ to get another chance to fly one of those babies before you took them away! Let’s go… they’re in the hangar sittin’ on dollies. They’re both charged up and ready to go so we’ll push one out to the exterior hoist and lower it to the water.”
“Uh… one question first,” said the XO. “How are you going to recover us? I understand these things have to keep moving or else they sink.”
“Good question,” said Dwight. “We have a net, which we lower into the water with the hoist. We spread the net out to form a barricade. Then, when we complete our flight, we simply fly the fighter into the net and get hoisted out.”
Pappy gave Dwight a skeptical look, and Dwight further assured him, “Believe me, Pappy, we’ve done dozens of test flights on these babies, and the recovery is a piece of cake!”
Pappy gave him a nod, not really convinced, and the three men walked together into the hangar. Pappy let out a low whistle as he caught sight of the fighters. “Wow, you were right, Captain, these things do look like a cross between an F-104 and a Mirage fighter — this is incredible!”
As Pappy walked around the fighter admiring its sleek lines, he noticed something unusual. “Captain, I think we have a problem here.”
“What’s the problem?”
“These fighters don’t have their screws mounted yet, and there aren’t any shafts to mount them on!”
George laughed and said, “They don’t use screws in the conventional sense. When you say ‘screw’, you mean a propeller. For normal cruising, the sub-fighters have a propulsion system which uses an impeller instead.”
“An impeller?”
“That’s right. It’s kind of like a backward propeller mounted inside a rotating tube. In this design, the impeller gives us two major advantages over a conventional propeller. First, it’s quieter. The impeller is much less likely to cavitate, and if it does, much of the noise is muffled because it’s inside a tube.”
“Why is it less likely to cavitate?”
“Well, as you know, cavitation occurs when an object traveling rapidly through the water creates an area of low pressure on its back side. The low pressure causes air dissolved in the water to form bubbles, and the bubbles make a lot of racket.”
“Sure, Captain, every submariner knows that. But why is an impeller better?”
“A conventional propeller is mounted on a central shaft with the blades radiating outward from the shaft. When the shaft rotates the propeller, the tips of the blades are the farthest from the shaft (the center of rotation) and therefore travel with the greatest speed. Typically, cavitation first occurs at the tips of the blades, where their speed is the greatest. An impeller, on the other hand, is formed by blades radiating inward from the walls of a hollow tube. Water flows freely through the tube from the bow of the fighter to the stern. The tube itself rotates, and the blades, which are mounted on the inside of the tube, rotate with it. Since the blades radiate inward, the tips of the blades are nearest the center of rotation, where their speed is the least. Therefore, the tube and the blade can rotate at much higher speeds without cavitating. The result is a high velocity jet of water being expelled from the rear of the tube and driving the fighter forward.”