I built a small rocket engine for
demonstration purposes. The engine is built from a 2" diameter acrylic rod
through which I drilled a 0.5" hole. The oxygen at 80 psi or less is
passed through the hole and then is forced through a
convergent-divergent
nozzle at the tail end. The nozzle's throat is about 0.25" and expands to
0.625". I lit the engine by inserting a burning cotton swab (with wooden
stick) while a small amount of oxygen was flowing. The acrylic catches fire
very easily in a pure oxygen environment. The engine can be throttled and shut
off completely, which is a major benefit to hybrid engine designs. Solid-fuel
rockets cannot be throttled or shut off, which makes them difficult to control.
hey everyone I've
built a rocket engine
a hybrid rocket engine out of a piece of
solid acrylic and I'm using gaseous
oxygen as my oxidizer and it's mounted
in the vise right over here so let's
zoom in and take a closer look a hybrid
rocket engine is an engine that uses
solid fuel as well as something else and
so the hybrid aspect of it is that you
don't need pure liquid fuels or pure
solid fuels you're actually using one
solid fuel and then something else so in
this case the solid fuel is acrylic PMMA
poly methyl methacrylate and the
oxidizer is just oxygen gas I built this
engine by starting with a 2-inch
diameter piece of acrylic rod and I
faced the ends off in the lathe and then
drilled a hole all the way through
of course the bit wasn't long enough to
go all the way through I was using a six
inch section rod and so I had to turn it
around in the lathe and drill in from
the other side with a good center drill
it's the holes will meet up just fine
and mine wasn't a problem I made two
aluminum end plates droom up in CAD
first and then used the CNC mill to cut
them out and I cut a ringed gland a
section for the tube to fit into and
then on the nozzle side I counterbored a
hole and then turned a nozzle out of
steel so that the nozzle itself is going
to have to withstand much higher
temperature than the end plate but
really this this engine is only meant to
run for you know 10 seconds at most and
so really this construction isn't going
to last long enough to to make a
reusable rocket let's just say this is
definitely going to be for demo only the
o-rings are high temperature silicone
and I was mostly just concerned with
them not catching fire but again it
probably doesn't matter for the amount
of time that this engine is going to run
I used some quarter-twenty threaded rod
to push the two plates together to make
sure that the oxygen doesn't leak out at
the engine this will keep pressure on
the o-rings and put the whole engine
together and then
- - valve on the back and I wanted sort
of a quick control valve so I opted for
a ball valve so that I could quickly
open and shut it without you know
screwing down a globe valve many turns
to light the engine I just lit a cotton
swab that I soaked in alcohol with a
torch and then with a very very small
amount of oxygen flowing I put the lit
swab in the tail end of the engine and
with all that oxygen flowing through
there and slow rates it's very very easy
to ignite the acrylic and once a tiny
section of it catches the whole engine
takes off and lights up quite easily
I set the oxygen regulator to about 80
psi but I don't think I ever got the
valve all the way open it was just a
little too scary to run this motor
inside with full full 80 psi oxygen
going into it the nozzle itself is has
about a quarter inch diameter throat and
it expands out to almost 5/8 of an inch
I didn't really do any calculations to
figure out what the nozzle shape should
be for the motor round size and the flow
going through it I just kind of
eyeballed it hybrid rocket engines are
actually pretty interesting because you
can throttle them very easily as you can
see in the video when I turn the valve
up and down I can get varying levels of
thrust almost instantly and it's very
easy to throttle the motor up and down
another benefit is that you can turn it
off so with a solid-fuel engine once you
light it there's really no going back
because the oxidizer is is mixed in with
fuel once that thing starts there's
absolutely no way to stop it with a
hybrid rocket engine you can actually
turn off the oxidizer in this case the
oxygen and the fuel will simply stop
burning and then you can light it again
later so being able to throttle and
start and stop the engine is a major
benefit to these hybrid rocket engine -
zion's I didn't measure how much force
this engine is producing again if I were
really concerned about building a decent
engine I would probably spent more time
on the nozzle - the typical shape to a
rocket nozzle is a convergent divergent
nozzle and so the idea is that you want
to speed up the exhaust gas to a
supersonic speed that's what the
convergent part
and then you want to expand that flow to
have a very high volume so the divergent
part of the nozzle that keeps the speed
up near supersonic or above above the
speed of sound and yet has a higher
volume since its cooled the gasses down
so you can design an optimum nozzle for
a specific engine design to get the most
thrust out of it so as you can see the
acrylic has started to melt on the tail
end of this the engine produces a lot of
heat obviously and the nozzle conducts
the heat into that aluminum plate at the
end and since the plate is being pushed
down by the threaded rods it started to
melt the end of the acrylic so again
this this engine is only for quick demos
it obviously wouldn't last very long all
right well I hope you enjoyed that see
you next time bye
s
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