Engineering Aspects of
The Human-Powered Submarine
Team of Virginia Tech
Hull Design and Construction
Hull design is one of the most important factors in the ability for a submarine to attain high speeds. The Phantom 4 hull has already been designed and constructed but modifications will need to be made after the lessons learned at last year's competition. The dome on the front of the sub will be lengthened; although this will create a more unstable design, there will be benefits in increased speed. More importantly the extended nose will create more room, increasing the pilot's ability to power the sub.
The design of the next propeller-driven boat, Phantom 5, will also be a large project for the hull crew this year. Working from the lessons learned at competition, Phantom 5 will have the smallest volume possible to minimize weight thereby increasing acceleration. The design will also attempt to take advantage of laminar flow over the length of the hull to reduce drag. With the design completed almost a full year in advance, great improvements in construction and testing will be available the following year.
Propulsion
Speed will be the focus of the propulsion crew this year. In 2001, the team experimented with a counter-rotating propulsion system, and was very pleased with its performance. The torque placed on the hull by a single propeller was eliminated, even with a gear ratio of 4:1. This year that success will be incorporated into the most advanced propulsion system ever designed by any HPS team: a computer-controlled, counter-rotating, variable-pitch propeller system. This system will use the advantages of a counter-rotating system, but will continuously vary the pitch of the blades to increase thrust for greater acceleration and higher top speeds. Research is also being done on a linear drive system for future submarines. This system will allow the pilot to provide power using a more efficient stair-stepping motion instead of a cyclic pedaling motion. If it performs well, this system could be incorporated as early as this summer and has the potential to increase the speed of the boat even further.
Life Support and Safety
Life support and safety encompasses the design and construction of the pilot emergency beacon, maintenance of scuba equipment, building of the launch and recovery vehicle (LRV), and the installation of a pilot restraint harness. The crew will work to integrate the pilot emergency beacon into the control system joystick as a trigger button and have a 5-second delay on the release of a buoy. After the maintenance of the dive equipment, the team hopes to acquire dive gear to prevent renting equipment in the future. Air tanks and other essential dive gear must be inspected before testing and competition next summer. A new LRV for Phantom 4 will be designed for easy transport of the submarine. It will incorporate some storage capacity and flip-up shelves for use as work surfaces. A pilot restraint harness will be integrated into the hull to allow the pilot to be safely secured to the submarine with easy egress in case of emergency. The harness will be made of aluminum and provide a solid frame for the pilot to push against to maximize pedaling efficiency.
Controls
The controls crew has three primary goals this year: to improve the current control system on Phantom 4, design and build a speed brake system for Phantom 4, and begin researching and testing possible alternative control systems for future submarines. While the current control system in Phantom 4 performed well at competition this summer, there is still room for improvement, especially in drag reduction. The system will be simplified internally to allow for more clearance of the control arms, and externally the fin size will be reduced and streamlined to reduce drag. To improve underwater operations and diver safety, a braking system will be incorporated into the control fins of Phantom 4. The pilot will have the ability to deploy a set of drag scoops to stop the submarine. The final activities of the controls crew will be to begin investigating the possibilities of alternative control approaches. Research will be done in areas of non-fin based control systems, such as momentum wheels and moving masses, which would reduce the drag on the boat by eliminating fins or other control appendages.
Electronics
In 2001, the HPS team successfully added electronic controls to Phantom 4. This system converted electronic signals from the joystick into control surface motion as well as added the ability to automatically control the depth of the sub. This year the team will expand the use of electronics by adding data logging, computerized directional control, a pilot display, and a computer-controlled variable-pitch propeller, which will require a significant increase in computer processing over last year's design.
The data logger will allow the team to monitor the effectiveness of the systems as well as the performance of the pilot. This data will be used in setting control parameters and will guide future sub designs. For this data to be useful during competition it will need to be easily accessible after each run. This will be accomplished by using IR or wireless LAN.
From last year's experience, it is apparent that reducing the pilot's control effort increases the sub performance. The team has already accomplished automatic depth control and the next step is automatic directional control. This will be achieved by using a digital compass unit to sense the magnetic field of the Earth and output a signal relative to magnetic north. This signal will be used to control the direction of the sub, eliminating all control effort required by the pilot.
A pilot display will be added to provide performance information such as torque, rpm, and speed. The propulsion system is designed for a specific input rpm and maintaining this rpm will increase the overall propulsion efficiency. A display will improve the ability of the pilot to maintain the correct peddling rpm during a timing run to hopefully reach greater speeds.
Several teams have used computer-controlled, variable-pitch single propellers at previous competitions and have shown these systems to be effective in improving performance. By using the torque of the output shaft as a control signal, the pitch of the propeller blades can be changed to produce greater thrust. This will require accurate measurement of the shaft torque and the ability to precisely control blade angle, which can be achieved by using a torque transducer and linear actuator.
A single computer will control all of these systems. A 300 MHz computer and A/D converter will be used with LabView software as the primary control program. These improvements will be a substantial undertaking and require many costly components but will help maintain the Virginia Tech Human-Powered Sub Team as world champions and move it closer to breaking the world speed record.
Testing and Competition
The team's goal for the 7th ISR is not only to perform well in the speed competition but also to repeat as world champions by winning the Overall Performance Award. This year's team is highly experienced and very motivated to uphold the standard set by previous Virginia Tech teams. The HPS team has been very successful at the previous competitions because of its rigorous testing and safety procedures. In addition to conducting static tests in Virginia Tech's campus swimming pool, the team will do safety checks and full-speed testing at the David Taylor Model Basin. Testing must be done to ensure the performance and reliability of the various systems underwater. Testing also provides a controlled environment for the team's safety divers to work together with the boat. Efficient underwater operations at competition are critical, so divers must feel comfortable working as a team. Through testing and design innovation, the HPS team hopes to be successful at Carderock in June.