Engine Construction
The typical engine is designed to operate at 1000 psi. The casing of the
engine should be able to withstand at least 3000 psi as a safety factor. A
low carbon seamless steel tube with 1/16" walls can withstand that sort of
pressure. If the tubing has a welded seam, test fire a few engines to see if
the tubes can take the pressure. One drawback to using steel as an engine
casing is if the engine explodes you have some very lethal shrapnel flying
around. If you use a high strength/high heat plastic you can eliminate some
of this danger. Epoxy can be used to wet down a mat of fibreglass then the
fibreglass is rolled around a large dowel to form a casing. The dowel has to
be coated with a lubricant to keep the epoxy from gluing the casing and
dowel together. Or you can obtain a heavy cardboard tube with the correct ID
and coat it with epoxy then wrap epoxy/fibreglass around it. If the tubes are
constructed properly they can take quite a bit of pressure before splitting
apart.
A rocket engine is equipped with a nozzle to accelerate the exhaust out of
the rocket at a high velocity. A nozzle has a convergent section that does
this. A divergent section of nozzle is used to lower the exhaust pressure
so the exhaust gases accelerate out of the engine at high speeds.
The nozzle of the engine can be machined out of metal or made of a fireproof
ceramic. If the nozzle and the casing are metal, they can be brazed together
before the engine is loaded. The nozzle can also be screwed into place by
using 4 - 6 screws going through the side of the casing into the nozzle. Care
must be used to see that the screws don't break through the inside of the
nozzle. On smaller rockets, you may be able to get away with plaster of Paris
nozzles or for more powerful motors try pressing a mixture of 90% kaolin and
10% aluminium oxide into a nozzle shape in the casing. Dampen the mix with a
little water before pressing. You can make a nozzle die by turning 2 pieces
of hardwood into divergent/convergent sections. This die should be fitted
with a dowel guide pin at the mating points to help keep the die straight.
A nozzle can be made from just a divergent section. This can be easily made
by drilling the required hole in a section of nozzle. Then by drilling out
the first hole with larger drills without completely breaking through, a
diverging nozzle is formed. Smooth out the ID of the nozzle after drilling
the holes. This type of nozzle is pretty good on smaller engines with a 1" ID
or less. By using some ingenuity, you should have no problem in making a
serviceable nozzle. A rule of thumb to use for the ID of the nozzle is to use
a hole that has an area (repeat-area, not diameter) 1/3 the area of the ID of
the rocket engine casing.
Most propellants burn unsteadily at low pressures. Solid rocket engines are
equipped with a blast plug that allows the pressure to build up in the engine
before being blown out like a cork in a bottle. Sometimes the igniter is
combined with the blast plug in a single unit. A stiff plastic disk makes for
a good plug. It should have a thickness of about 1/16".
The engine is sealed with a plug in the fore section. Depending on the construction of the engine this plug may be made of wood, plastic, or metal. It
is held in place with either screws or epoxy. This plug must make the casing
gas tight. Remember most rockets develop 1000 PSI.
The igniter is simply an electric match. It can be made with nichrome wire
or a small light bulb. The match is used to ignite a small charge of black
powder that in turn ignites the propellant. The igniters leads should be
shunted together to eliminate premature ignition. A fuse can be used instead
of electric ignition. If you go this route, be sure of the burning time of
the fuse and allow yourself enough time to retreat to safety after igniting
the fuse. I cannot recommend using a fuse because you cannot stop a fuse
from burning if someone walks into your launch area. With electric ignition,
everything is under your control until the time of launching.
ENGINE CONSTRUCTION