THE GREEN MONSTER
A Propane Tank Steam Rocket
Propane cylinders are lighter than CO2 tanks because they operate at lower
pressure, but they can still take 600 PSI. CO2 tanks can take up to 3000 PSI.
Outlet valve and pressure relief seen from inside.
The outlet valve (left) is pushed open when the tank is screwed into the
propane stove. The fitting on the right is the pressure relief valve. Empty
the cylinder by allowing the propane stove to stay BURNING until all the gas
is gone. And leave it connected for an hour after the flame goes out. DO NOT
RELEASE PURE PROPANE! Propane mixed with air will EXPLODE if ignited!
The paint on top of the pressure relief fitting needs to be filed off down
to bare metal. A disk of sheet steel needs to be silver-soldered there.
The tank valve must be drilled out to 1/2" in a drill press. Leave the
plastic base on for now. Hold the tank with a ViseGrip chain wrench. Put the
chain at the bottom of the tank where it's strongest. Adjust the chain claws
as close together as possible to avoid denting the tank with the claws when
you clamp down on it for drilling.
When you drill into the empty tank, you will smell ethyl mercaptan. This
is the oil put inside all propane tanks so that (odorless) propane leaks can
be detected. It is neither flammable nor explosive, it just stinks really bad.
The chain wrench on the tank.
After drilling the 1/2" throat, the outlet fitting may or may not have
fallen free. If it hasn't, use a nail to move it back and forth until it
does break free. Next, turn the tank upside down and keep shaking it until the
fitting, which is now loose, is seen in the hole.
The outlet fitting.
Now get pliers on the pin and pull it out. Holding it upside down, use an
air gun to blow out any metal chips. Remove the label and plastic base.
Sketch of the hold plate.
The 3/16" steel hold plate needs to have a hole 7/8" in diameter hole-sawed
(the bimetal type are best) into it. Or it can be drilled 29/64".
The modified tank with exit cone and hold plate.
The 1/4" x 1/4" fin support brace was TIG-welded to the hold plate after
being bent to shape. The hold plate was TIG-welded onto the straight part
above the threads on the tank. The exit cone is 24 gauge stainless, tapering
from 1/2" to 1 & 5/8" diameter, shaped over a pipe reamer. The overlapped
seam was silver-soldered. The tank fitting was tapered inside with the pipe
reamer in the drill press. The exit cone was then silver-soldered into the
tank fitting/hold plate and the pipe reamer was used again to ensure a smooth
transition. A metal disk is silver-soldered over the relief valve fitting.
The nozzle plug, hold-down arm and pressure gauge.
The nozzle plug was turned from a 3/8" brass flare plug. The part where
the threads were is now 1/2" wide to fit into the 1/2" nozzle. The 1/16" wide
O-ring groove was cut using a 1/16" cutoff blade in the lathe. The silicone
(only!) O-rings are Dash 012, MSC #06840128. The nozzle plug was drilled 1/4"
for the copper tubing which is silver-soldered in. The hole in the pipe where
the copper tubing exits is 5/16", drilled at a steep angle.
The GREEN MONSTER.
The hold-down force required is 98 pounds minus the dead weight of 4 & 1/2
pounds. The water required is 2 pints (2 lbs), so the weight empty is 2 & 1/2
pounds. The 5/16" guide rod is 8 feet long. The body tube is a 3" mailing
tube. The 3" Estes nose cone has a How High altimeter by Quest inside.
The external battery and chute delay switch.
The switch arm swings out and burns the fuse ignitor when the rocket starts
up the launch rod. The 3-foot chute is deployed after a 7-second delay
(2 1/2 " pyro fuse, 2 1/4" TNT green fuse or 2 3/4" of green Phantom fuse).
The yellow wires go to the ignitor through a hole in the bottom of the body
tube. The chute charge is a slightly rounded 1/8 tsp of FFFFg.
The continuity tester.
A battery clip has been modified to expose the battery snaps and the black
wire has been cut off. The (-) LED lead (flat) was soldered to the (-) snap
and the (+) LED lead has a 330-ohm resistor used as one probe.
The primary and secondary chute savers.
The primary chute saver is an aluminum muffin tin. The secondary uses a
coffee bag cut to 8 & 1/2" by 12" and then formed over the spray paint can or
beer bottle into a cup 2 & 3/4" wide x 6 & 1/2" deep and taped on the bottom
(only). The two chute savers are then taped bottom to bottom.
The Green Monster is 31 inches, or 2.6 ft., long. Video frames are always
1/30 sec. apart.
Frame 212 of the video.
Measurements from the video frames printed out full page indicate that the
rocket has gone 3.44 ft. from the starting-point frame (207) to the next,
which is 103 ft/sec or 70 MPH. Then 94 MPH, 141 MPH, 153 MPH, 188 MPH, 211 MPH
and finally 223 MPH (!!!) before going out of the camera's view.
According to the Quest How High altimeter, the apogee was 557 ft.