Joe Peck has extended the pursuit of realism in model rocketry to the launch environment. Think Poseidon here — this rocket launches from underwater. Initially, it's sealed watertight from nose to nozzle. Ignition is triggered by bringing a magnet close to a sensor contained in the upper section. The motor ignites, blowing off the bottom end cap and we have liftoff!

Overview

One aspect of model rocketry is to create flying models that visually replicate the appearance of actual rockets.  A less common aspect is to simulate the launch environment of an actual rocket, such as a submarine launched missile.  This project involves the creation of a model rocket that is launched from underwater, breaks the surface of the water, and continues flying along a traditional trajectory.  It is accomplished with the help of a magnet, some vaseline, and a bit of electronics.

The launch electronics will be fully contained in the rocket itself, and will be activated via a magnetic switch.  The rocket will be sealed against any water penetration, keeping the electronics, parachute, and motor dry.  Once the motor is ignited the rocket only has to travel a short distance to break the surface of the water and the threat of water penetration will be over.

The schematic is very straightforward.  A normally open magnetic switch controls the gate of a high current FET.  When no magnetic field is present, the gate of the FET is held low by a pull down resistor and is therefore off.  When a magnet is held close to the magnetic switch, it closes and turns the FET on.  When the FET is turned on, it connects the 9 volt battery to the igniter, which will cause the motor to ignite.

The Electronics

The electronics are shown here attached to the bulkhead that will be permanently attached into the upper section of the rocket.  Once installed, the electronics can not be removed from the rocket.

The Schematic

The resistance of common igniters ranges from about 0.5 to 2 ohms, so a low Rdson FET is used so that most of the power is dissipated in the igniter.  The FET must be able to sustain several amps for a short period of time.  A strong battery must be used, such as a 9V nicad. 

Bill of Materials

The Rocket

The rocket consists of two main sections.  The upper section contains the electronics and the battery.  The lower section contains the motor, screw terminals, and the parachute.  The FET output in the upper section is cabled to the screw terminals in the lower section via a short cable.  This cable includes another connector pair that separates upon motor ejection, which allows the parachute to come out.  The upper and lower sections remain connected together as well as to the parachute by some Kevlar string. 

To prevent water from penetrating the joint between the upper and lower sections, a small amount of vaseline is applied to the coupler prior to launch.

The Upper Section

The nose cone can be removed to replace the 9V battery.  The nose cone is then taped to the body tube to prevent water from getting inside. 

The Lower Section

The End Cap

The bottom of the rocket needs to be protected from water penetration as well.  A small section of tube with one end blocked off is placed over the bottom as a cap.  This cap also provides a small amount of air for the motor ignition.  Vaseline is used again to seal the joint between the cap and the body tube.  To keep the rocket from floating in the water, this cap includes a length of Kevlar string that can be tied to the bottom of the launch tank.

The Launch Tank

The launch tank is essentially a large cardboard tube with a Plexiglas front panel.  A 1/4" diameter launch rod is fixed to the bottom of the tank, and guides the rocket during liftoff. 

A small eyebolt is attached to the bottom near the launch rod, to which the rocket cap will be tied.  An "ignition stick" needs to be made, which is essentially a small magnet on a short stick.

Once the rocket is prepared (motor and igniter installed, cap placed on end) it is placed on the launch rod.  Water is carefully poured into the tank with care being taken to minimize jostling the rocket. 

In order to launch the rocket, move the magnet close to the rocket where the magnetic switch is located.  This will turn the FET on and ignite the motor.  If everything works properly, the rocket should burst from the surface of the water and fly high in the sky.
 
Launch Photos

Launch time: Using a magnetic contact mounted to a wooden dowel, the writer launches the rocket.

Epilogue

The rocket for this project was constructed with parts readily available from a local hobby shop.  I selected a standard kit and modified it for this design.  Selecting a kit with a larger diameter body tube would make the construction a little easier, and let you avoid complications such as the offset motor mount tube.  (Don't worry though, the slight offset of the motor won't affect a rocket with this much mass and length.)  Due to the extra weight of the battery you will want to use the largest motor recommended for the kit, or even upsize the motor mount tube and use a larger motor.

A Project Dragonfish instruction document (3256KB) is available in Microsoft® Word format. 

Editor's Note: Joe pointed out that the first improvement to his launch system should be to get the person further from the launching rocket.  This could range from simply using a longer stick to a more sophisticated pivoting arm remotely actuated with a string.  Many people on the Design News site were concerned about launch safety, safety code violations and similar themes.  This is just one individual's venture into the realm outside of purchase and fly.  Although Joe may sometimes run with scissors, he doesn't recommend that everyone follow suit.

For Joe's contribution of this article, he will receive a free one year subscription to LAUNCH Magazine. This sponsorship is made possible by our friends at MM Publishing, Inc., the producers of LAUNCH Magazine. Want your own free subscription? Read the program details page for complete information.

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03-04-2007 02:18 PM  #1
crontab Certified Level Three   Joined: Aug 2006 Posts: 204	Hey, me too! I too run with scissors, and definitely don't recommend it for everyone. Great work, Joe. And congrats on the magazine writeup. Keep 'em coming!

03-09-2007 10:57 PM  #2
Raider Rocketry Low Power Enthusiast   Joined: Mar 2007 Posts: 128	The next step? Maybe it would be building a scale Ohio Class SSBN launching a Trident or the SSGN version launching a Tomahawk. Perhaps the Russian Typhoon with a RPK-2 or a RSM-52. Now that I would pay money to see. Perhaps encapsulating the rocket in an air chamber would help. It could be capped with a balloon or some other rubbery type of waterproof material. The only limit on the water traveled through I could see is getting a launch rod that reaches 3' above the surface of the water. That was fun to watch, but I agree with the launching system requiring a bit more forethought in regards to safety. Yikes!

07-10-2007 03:38 PM  #3
R2K Certified Level Three   Joined: Jul 2007 Posts: 145	I would love to see a project to break mach underwater with a rocket, perhaps with cavitation port that bleeds motor gas out of the nosecone. I think that would have to be on the order of 5500 fps. What type of dynamic pressure would you see under those conditions without cavitation? With? Base drag would be huge also after burnout with massive parasitic cavitation, that might prove highly destructive, as we know turbines can break due to cavitation. What motor would you use? Very high thrust would probably be best, but I think the power in a sprint AMB or Hibex could be used to make a modified rocket hit 5500 FPS under water for a short time.

07-10-2007 04:11 PM  #4
aerostadt Certified Level One   Joined: Feb 2007 Posts: 28	rocket-fired torpedoes Rocket-fired torpedoes are no fantasy. Scientific American had a big article on this subject about 6 years ago. Some information can be found at this website: http://www.newsmax.com/arc...s/2001/4/23/220813.shtml According to the Scientific American article cavitation can actually become an advantage. The idea is to get the rocket torpedo going fast enough so that the entire torpedo is enclosed in a cavitation bubble, thus reducing drag. Obviously, the speed of such a device gives the user an tremondous advantage over an opponent that has only conventional torpedoes. Somewhere I remember seeing on the web a few years ago, someone who launched a HPR rocket from deep underwater. Bob

07-10-2007 06:18 PM  #5
ddmobley Administrator   Joined: Jul 2006 Posts: 2555	Quote: Somewhere I remember seeing on the web a few years ago, someone who launched a HPR rocket from deep underwater. I recall seeing this: http://www.inverseengineering.com/Pages/2005/2005.html Click the first link Sea Launch. It doesn't meet the "deep underwater" definition, but it was in the water...