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Day 52 - Polaron IV Launcher
Progress and Booster Tests
Launcher base with slots cut out. The
central release head mounting plate is sitting
on top prior to attachment.
Booster nozzle seat supports are soldered to
the sliding base plates.
Nozzle sliding base plates mounted in the launcher
base.
Two interchangeable release heads. The two
components on the right belong to the bleed
valve assembly. This can be swapped from one
head to the other.
The left release head has plastic tabs for
lower pressures and plastic nozzles. The one on
the right uses ball bearings and is used with
higher pressures with aluminium nozzles.
Release head fitted to the base plate. The
booster nozzle seats here are also mounted on
the sliding base plates.
The air distribution manifold under the
launcher. Note the two quick release couplings
on the left.
The main stage air supply has a non-return
valve mounted just behind the quick release
coupling.
We uses braided steel flex hoses to get the
air to the sliding nozzle seats.
Detail of where the flex hose enters the
bottom sliding base plate. Four spacers between
the top and bottom sliding plates keep the two just
the right distance apart.
12mm launch tubes fitted inside the nozzle seats.
Sometimes a pitchfork is just the right tool
for the job. Here the major elements
of the Polaron IV rocket sit on the launcher.
A booster fitted with experimental winglets
to help guide it to a safe landing. Fitted here
with a 9mm nozzle for the test launches.
The winglets are designed so that the center
of pressure is aligned with the center of
gravity for unstable flight back to Earth.
Setting up the booster for it's maiden test
flight.
With such an unstable rocket we were happy
to see it tumble safely back to Earth.
Another booster is tested without the
winglets so that we could get a good comparison.
Setting up the medium launcher's extension.
This gives 2m guide rails to steer the rocket
while it accelerates.
The kids are getting to be real pros at
helping with the launches and experiments.
Date: 22nd January 2008
Location: Workshop & Denzil Joyce Oval
Conditions:
Windy and overcast, mild
temperatures Team Members at
Event:
GK, PK, John K and Paul K
Polaron IV Launcher Progress
We are concentrating at the moment on
finishing the launcher and the Polaron IV
rocket. It was a busy weekend in the
workshop doing all the heavy duty cutting,
soldering and machining work for the new
launcher. With all that out of the way we
now only have a few minor assembly tasks
that need to be completed and the launcher
will be ready.
Launcher Design
Hold Down
The launcher is based on a similar design
to the Acceleron III launcher. The main
difference is that the central retaining
mechanism has been replaced with a Gardena
mechanism to allow the main stage to fire at
the same time as the boosters.
Because the Gardena mechanism needs to
not only hold down the main stage but also
the boosters which are free to move out of
the launcher, we've had to switch to a brass
mechanism that uses ball bearings instead of
the plastic tabs of regular Gardena
mechanisms. The main stage nozzle is also
made from aluminium because the ball
bearings would damage a plastic nozzle
with the forces involved.
We have also made a second release head
that utilises the plastic tabs for
retention. This will allow us to use plastic
nozzles in other configurations and at lower
pressures.
Dual Air supply
This is our first launcher to use a dual
air supply. We will be able to supply a
different pressure to the boosters compared
to the main stage. This gives us the ability
to optimise the performance of both the main
stage and the boosters. This will also allow us to use this
launcher for rockets without boosters.
The manifolds and air distribution hoses
are all rated to at least 500psi. This will
allow us to use this launcher in the future
for heavily reinforced rockets.
The nozzle seats with the launch tubes are removable
for easier transportation.
Sliding Nozzle Seats
The booster nozzle seats are designed to
freely slide in and out
from the center for two reasons:
a) We can use different sized main stage and
different sized boosters without the need to
change the launcher.
b) During pressurisation as the boosters and
main stage pressure vessels expand, the
sliding nozzle seats move with them. This
prevents the nozzles being placed under
lateral stress if they were fixed in place.
( This was also a feature of
the Acceleron III launcher.)
Air Supply Components
A bleed valve is fitted to the Gardena
release head to allow the rocket to be
depressurised if a launch needs to be
aborted. The bleed valve is located where it
is because it needs to sit behind the
non-return valve fitted under the launcher.
The non-return valve in the main stage
air supply line is there so that water is
not forced back into the pressure regulator.
Having the bleed valve so close to the
rocket is less than desirable, but it is
only used very rarely, and for high
pressures we will connect it via a string so
it can be operated from a distance.
The booster air supply line does not need
a non-return valve because this line is
always dry. As a result the booster bleed
valve is located near the pressure regulator
away from the rocket. The boosters are
filled with air through the launch tubes
which fill the boosters above the water
line. This allows the launcher to equalize
the pressure in all boosters without
transferring the water between them.
The launcher base has a pair of quick
release fittings to for easy
connection to the air supply hoses.
Diagram of the air
supply components (Click to Enlarge)
Gluon Boosters
Each booster is made from a pair of 1.25L
bottles spliced together using PL Premium
and the symmetrical splicing technique.
These pairs are designed to be extended in
the future using Tornado couplings where the
neck of one booster is connected to the neck
of the next spliced pair.
The 13mm nozzle is extra long to allow the booster
to move upwards a little while it is being
pressurised without coming out of the seat
and breaking the seal.
Booster Recovery
In order to simplify the design, the
boosters use a passive recovery
system. The recovery system consists of a
pair of winglets centered on the Cg of the
booster, but angled to one side. This
causes the booster to fall mostly sideways much
like a back gliding rocket.
Normally this sort of system would not
work on a regular rocket because the rocket
would be unstable. In this case, however, the main
stage keeps the boosters aligned in the
right direction during flight and only when
they separate do the boosters fall in
an unstable manner.
This recovery system allows the boosters
to remain lightweight and simple with no
moving parts. Because the boosters do not
fly very high and they do not weigh much the
landing on grass is sufficient to
prevent damage.
The winglets also help to steer the
booster away from the main stage just after
separation.
Booster Coupling
The booster is loosely attached to the
main stage. The booster coupling mechanism
is made from a coat hanger wire and bent to
allow it to be glued directly to the rocket
body. This fits into small
tubes attached to the main stage. I will
cover this in detail the next update.
Booster Testing
In order to understand how the boosters
will behave in flight we set up our regular
launcher and fired them into the air this
weekend. These tests were strictly to
understand how well the booster will float
back to Earth and how well it would survive
a landing.
This was actually the first time we
fired a spliced rocket so it was good to see
it get off the pad. We increased the
pressure from 60psi on successive launches
to 120psi to make sure it still held. We
made sure we stayed well away as we did not
know if the glue would hold up. (We had
previously pressure tested these to 115psi)
(If the video does not play, try the latest
Flash player from Macromedia)
We also launched the booster by itself
without the fins to compare its performance.
For a high performance flight we may remove
the winglets to save a little on the drag as
well as the weight.
This was also the first time we used the
launcher's 2m extension. With the booster
being so unstable it helped get it up in the
air a bit higher so we could watch it tumble
back.
We were pretty happy with the results
and no real design changes are necessary.
The next step will be to make a mock-up
of the main stage simply by taping a number
of 2L bottles together. The mock-up will not
be pressurised and will not have any payload
or recovery system but will have equivalent
dimensions and weight distribution of the
actual main stage. We will boost that at our
local park using the three boosters. This
test will hopefully prove to us that the
booster arrangement works before we fit it
on the actual main stage. The payload on the
main stage costs around $300 so we are
taking the development in stages.
The reason the mock-up will be
unpressurised is to make sure it does not
leave the park. We expect the final booster
and main stage combination to reach above
600 feet and that could easily put it
amongst the houses near the local park. We
hope to launch the actual rocket at a much
bigger launch site at one of the NSW
Rocketry Association (NSWRA) launch events.
