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represents a launch or test day, and describes the
events that took place.
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Day 25 -
Polaron III, J4 II, Acceleron and more underwater launches
Trimming the fins so that there is enough
clearance for the launcher.
Preparing Polaron III for its maiden flight.
Last minute consultations ...
Launch. We are investigating why the water
column looks the way it does.
A diagram of Polaron III's twin parachute
deployment mechanism.
A well lit view of Polaron III on the launch
pad.
Did the rocket camera finish recording?
A video frame taken from the onboard camera,
with superimposed calibration measurements. This
image was used to estimate the rocket's
altitude.
A kids' hard life. ... Sometimes you have to
get up pretty early to chase rockets.
J4 II pressurised and ready to ....
.... go. J4 II also exhibited the same sort
of water column that Polaron III did.
We didn't use the launcher extension as
there was no cross-wind.
Descent shots of the three rockets flown on
the day.
John said that the coloured water tasted
good. He is hoping for a little more during
launch.
Preparing Acceleron for its one flight.
Good lighting on the water columns.
A bit blurry, but nice colour.
Getting ready to submerge the rocket. The
tank stays above the surface.
At a depth of ~2m pressurised to 130psi.
Launch. It is hard enough to catch the
rocket above the surface. The video shows the
launches.
A booster under development. New
fins have been fitted since the last
launch day.
Team Members at
Launch Event:
PK, GK, AK, John K and Paul K.
Number
of launches: 7 (part 1), 3 (part 2)
This was an excellent day for flying
rockets, nice and mild, and no wind
whatsoever. We had an early start today
because we didn't want to be chased off the
field before we were done. We managed
to get seven launches in on the day, and we
were really happy with the performance of
the rockets.
Later in the day we launched the Cena rocket
a few times from underwater again but from a
greater depth.
Flight Day Events - Part 1
The launcher has been working really
well for us, but we haven't needed to use
the launcher extension yet. That will be
handy when there is going to be a cross
breeze. It provides a very stable base for
the rocket launches.
When we loaded Polaron III for the
first time on the launcher we discovered
that the fins just fit inside the launcher
ring. There was only a few millimeters to
spare all around, so we got out the
scissors and took about 1 cm off the end
of each fin. Better to have a little less
weight and drag ( read stability ) than
catch a fin on the launcher.
We loaded up the parachutes and inserted
the camera. This was Polaron's first time
to 120psi so we were happy nothing leaked.
We wanted to make sure we got one good
launch in with a camera rather than risk
damaging the rocket with too high a
pressure.
The rocket flew a beautiful flight with
the rocket deploying the parachute right
at apogee. The onboard video shows that as
the rocket just stops ascending, you can
hear the release motor and chute ejection.
(If the video does not play, try
downloading the latest
Flash player from Macromedia)
From the video we were able to measure the
altitude of the rocket, because the camera
looked straight down at apogee as it was
over a baseball diamond on the field, which is a known
size. We measured the diamond later in the
day with a tape measure. It was 26.87
meters diagonal.
Then at home we measured the camera's Field of
View (FOV) (by taking a picture with the
camera of A4 paper at a distance of 1 meter) using the similar triangles technique,
we got an estimate of 121 meters ( ~397
feet). See diamond image on left.
We are confident that the measurement is
quite accurate, as entering the rocket
parameters into a simulator gave an apogee
of 119 meters.
That's close enough in our book.
We flew Polaron III two more times
after that at 130 psi, and both times it
performed very well. We are surprised that
the rocket doesn't spin very much, but
that is likely due to the low
acceleration, low top speed and its the
fairly large diameter. Its fins are just
eyeball aligned as they are held under
rubber bands. This aspect of the fin
design seems to be working very well and I
think we will use more of it.
J4 II has had a new nosecone fitted
that sits much better on top of the
rocket. J4 II's first two flights were
excellent with the single parachute
deploying right around or just past
apogee. It was too high to see when it
deployed. On J4's last flight the
parachute opened perhaps a little early,
but the flight still looked great.
Acceleron was also launched once, but
with a few modifications this time. The
water holes for the dummy payload were
closed up about 2/3 to prevent the water
emptying out too soon after burnout. If
the water empties too early then the
rocket also looses stability early. The
other major mod was the use of new larger,
rubber band attached fins. These allow the
rocket to hit the ground without snapping
a fin off.
We launched it with its highest pressure
to date of 120 psi. You can sure see the
difference in acceleration and flight at
the higher pressure. The take off was
great and it went mostly vertical, the
water in the dummy payload looked as if it
emptied later than normal because the
rocket went quite high. The parachute
deployed as the rocket was still ascending
at a fairly rapid rate, and the dummy load
had separated from the rest of the rocket. It obviously still had
enough water in it with sufficient
momentum. The dummy payload is secured
with two wire loops (for redundancy)
that should prevent this from happening,
and I remember checking that these were
hooked on, but during takeoff it looks
like they unhooked, because there was no
damage to the loops nor to their
attachment points.
In hindsight it was probably a good thing,
because if it remained attached it may have
damaged the parachute. Everything ended up
landing well so we were happy.
During the flights and especially
later during analysis of video and
photos we noticed that most of the water
coming out of the nozzle was in quite a
bit of a wide turbulent spray rather
than the normal laminar column of water.
This was happening for both rockets I
was going to blame it on a faulty nozzle
again, but I am beginning to suspect
another cause.
Both rockets had the lowest bottle almost
entirely full. This
means that as the rocket is released the
air pushing on the surface of the water
is unevenly distributed since most of it
is coming in through the 8mm hole in the
Robinson coupling. If this jet of air is
basically blowing into the water it is
likely to produce a lot of bubbles.
Perhaps the water is super frothed by
the time it comes out so that the
bubbles in it are expanding as they come
out. This may be a good experiment to do
on a static test stand to see what
actually happens. If the water is full
of bubbles, then that may be directly
applicable to an expanding nozzle design
for better nozzle efficiency.
(If the video does not play, try
downloading the latest
Flash player from Macromedia)
Flight Day Events - Part 2
We flew the Cena rocket again three
times from under water. We wanted to see
the effect of the increased depth on the
flight above water. The first launch was
at 90 psi the same as the last launch
from 1m. The water depth had a
significant effect on the flight as it
only flew a couple of meters above the
surface.
We increased the pressure to 130psi
and this time the rocket left the water
at a good speed with the tether keeping
it from going next door.
On slow motion underwater video we
noticed that while the rocket was still
moving underwater you couldn't see
anything really coming out of the
rocket, because what was coming out was
water of course. I had half expected to
see a bubble trail. Dad wouldn't let me
use food colouring in the rocket in his
pool. I wonder why?
(If the video does not play, try the latest
Flash player from Macromedia)
If you have questions or comments feel
free to visit our
blog and leave a comment.
Flight Record
Launchh
Rocket
Pressure (PSI)
Notes
1
Polaron III
120
Great flight, with parachute deploy
right at apogee. FC deploy setting
was set to 7. Both parachutes
deployed well, and the rocket also
landed well. The rocket carried a
video camera.
2
Polaron III
130
No camera this time, with FC setting
at 7 again. The flight was
excellent, but ended up arcing over
a bit.
3
Polaron III
130
Again without camera and same FC
setting. Another great flight, good
landing and good deploy.
4
J4 II
130
A perfect flight. Very straight and
very high. The rocket seemed to hang
at apogee, parachute deployed as the
rocket flipped over.
5
J4 II
130
A repeat of the above performance.
An excellent flight.
6
Acceleron
120
A great flight, with
a relatively vertical flight
profile. Parachute opened later than
on other flights. The parachute
opening caused the dummy payload to
separate from the rest of the
rocket.
7
J4 II
130
A good flight, but
the parachute opened a little early
on the way up, and the rocket didn't
reach its potential altitude. Peter
caught the rocket before it hit the
ground.
8
Cena
90
Underwater launch
from around 2m. Rocket didn't fly
very high above the water.
9
Cena
130
Underwater launch
from around 2m. Rocket had higher
velocity when it emerged from the
surface, but the tether stopped it
from flying very high.
10
Cena
130
Underwater launch
from around 2m.Similar flight to #9.
Design and
Development
To date, probably other than Acceleron,
Polaron III with its 4 bottles, 3 couplings,
flight computer, camera and twin parachutes, is our
most sophisticated water rocket. We have
used a few new construction techniques on Polaron III for better performance and
survivability. These include:
Attaching fins with only rubber
bands made from bicycle inner-tubes.
This has the advantage of you being able to
remove the fins for rocket storage and
transportation. It also prevents them
from snapping off when the rocket hits
the ground. The rubber band stretches
with the bottle where tape didn't. Tape
often stretched, the glued surface would
slip and always leave some kind of
sticky residue.
A second parachute loosely held in a
horizontal tube in the payload section.
The main chute line passes around the
second chute in the tube so that when
the main chute opens, there is enough
force to pull the second chute out.
The inter-bottle aerodynamic
adapters are no longer taped in place.
These are allowed to move slightly. A
rubber collar is made from an ordinary
kitchen rubber glove. This is placed
over the join and allows the
bottles at either end to stretch but
without the aerodynamic drag of buckled
tape. Tape was avoided for the above
mentioned reasons. The thin rubber
collar weighs about the same as the
tape.