Tunnel Vision Part III

The terminal and smells of hydraulics and motors are now well behind us and the status console indicates NORMAL operating conditions. It is no longer possible to look at the tube structural support rings as they pass at a rate faster than the flapping of hummingbird wings.

As the tube slowly turns to go around and through mountains, the pod rotates like a banking roller coaster keeping our sense of gravity directed down through our seats.

The on-board stereo is softly playing the soundtrack from a popular science fiction space trek show and my mind links up with a scene from an episode where the space ship is propelled to a distant part of the galaxy by mental control of thought, time, and energy. I am awakened from this brief daydream by a yellow WARNING sign above each seat and a soft professional automated voice instructing passengers to place their heads against the head rest for the upcoming increase in acceleration.

Just moments ahead is the greatest design challenge of the whole system: analysis of the geometric, thermal, and dynamic parameters of the wind tunnel throat such that this high volume of air will indeed transition from subsonic to supersonic speeds as the tunnel re-expands. While reflecting on the many details studied during the design phase, my mind wanders for an instant to the many potential applications to use this mode of high speed transportation. A ski lift in the Wasatch mountains, underwater transoceanic links from San Francisco to China, satellite launchers, postal payload and letter delivery networks, and large scale, 100 plus passenger models, like a commercial aircraft or train fuselage with no wings, tail, or landing gear. My reflection is interrupted by a subtle vibration caused by a shock wave forming on the leading edges of the pod…

Tunnel Vision Part II

Just before stepping through the hatch of our five foot diameter bullet, I look behind the craft at the 165 foot diameter fan stretching above and all around me and I realize more than I had ever before, that I would be sharing space with this huge mass of turbulent energy as we both accelerate into the common 5 foot diameter tunnel just ahead. Although invisible, I could feel the airstream converging in on itself, and me, creating a pressure difference across the pod.

We take our seats, which are highly reclined to minimize the pod and tunnel diameter, and fasten our seat belts for the non-stop trip. This is no ordinary craft. There are no wheels, no bearings, no engine, and no brakes. This pod is a pressure vessel with forward and aft regulators to balance the inside environment at a comfortable pressure, temperature and air circulation. Forward and aft circumferential hydrodynamic seals turn the cylinder into a piston, a very fast piston, driven at cruse by pneumatic pressure.

As the hatch is closed, I sit uneasy. I look out through the mostly transparent tubular walls at those who we leave behind and it occurs to me that in a moment, they will be many, many miles away. My eyes scan from the right, over head and to my left, taking in the exciting moment, and then after a brief wink at each member of my family, I look straight ahead and become acutely aware of the small  hole which is to be our immediate destination. The ALL CLEAR indicator illuminates on the front console, a small bump sounds from below, and we move out at a brisk 1/2 the acceleration of gravity, 15 ft/sec2.

v = a*t

x =  ½ a*t^2

(constant acceleration equations with zero initial velocity and position)

At this rate we reach 100 miles per hour in 10 seconds (not exactly a drag race car), having traveled a quarter mile. After 20 seconds, we pass 200 miles per hour and just over one mile. This constant acceleration continues as the air behind us converges and we approach the throat, minimum tunnel diameter, and 640 miles per hour…

#engineerclips

Tunnel Vision Part I

(Here is a short short story I wrote many years ago, for your engineering imagination, vision, and assessment. Is this story possible? Can such a wind tunnel really be operated? If not, why? Does it violate some law of physics or thermodynamics? What are the challenging design issues? What are the environment and safety issues? I look forward to your comments.)

Tunnel Vision – Part I

Here I am, finally, standing in the entrance of a life-long dream, ready to climb aboard a four-seat cylindrical pod and fly through a 600 mile long wind tunnel at speeds approaching Mach 3. It is a beautiful spring morning here in Salt Lake City, Utah. The sun has just risen over the Great Salt Lake, and in 15 minutes, my family and I will be in Anaheim, California, to see it rise again, this time over the Pacific Ocean. We plan to enjoy two full days at our favorite theme park before returning home having missed only two days of work.

As we enter the terminal, to my left are the huge turbo-fans providing the large mass-flow-rate of air to the slowly converging tunnel stretching to the right. My heart accelerates as I witness the party ahead of us enter their pod, secure themselves and their belongings, and look forward down the launch rails in anticipation.

Upon positive detection of all enable circuits, the pod’s onboard computer engages its drive cam with the variable lead, threaded shaft rotating at constant angular velocity and extending a mile into the tunnel. The pod in front of us moves out at a moderate, but aggressive constant acceleration and in a matter of 10 seconds is out of sight.

For a moment, my mind jumps back to a time when I was a child and I went with my mother to the drive-up window at the bank. She put a check or some money in a small plastic canister, placed it in a machine outside the car window, pushed a button, and after 3 seconds and a vacuum like sound, the canister was in the hands of the teller in a nearby building. Oh how fun, I thought, to go on such a ride.

Then it was our turn. We stepped into the airfoil shaped bridge protecting us from the high winds. From here I could see clearly the variable-lead screw drive shaft just below our pod. Its threaded helix was designed with an initial small gradual angle thus importing a small forward velocity to any vehicle engaged to it. This design concept provides for continuous, unchanging operation, fewer moving parts, and less start and stop action. The acceleration of the engaged pod is directly proportional to the rate-of-change of the helix angle with respect to distance along the shaft. The angle increases steadily until at its high helix end, the pods velocity will be approaching Mach point three (0.3).

Just before stepping through the hatch…

(Click Part II below) #engineerclips

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