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Day 27 - Foam
Tests, Baffle Tests, and Burst Tests
Setting up for four new static fire tests.
We attach the rocket to the launcher with
the 2 meter extension.
The baffle that screws onto the Robinson
coupling, You can see the air holes on the side.
We have to manually fire it. A great way to
cool down on a hot day.
Frame from the video showing just prior to
launch.
Just after launch, there is no blow through.
Rocket configuration for static fire tests
#7 and #8
Don't try this indoors!
Another day at the office. "You did want the
flowers washed dear didn't you?"
I won't tell if you won't. The after math of
a foam test.
Frame from the close up video showing Jet
Foaming in action.
The water level rises as foam is generated
at the bottom.
with a 7mm nozzle, the thrust ended after
7.28 seconds.
Re-enforcing a bottle with another bottle.
They loosely fit together, and then the
outer jacket is shrunk over the inner one.
On the test stand.
The top jacket separated. This will need a
bit more work.
Stress fractures from 220 psi near the top
of the throat. The bottle did not break.
Date: 9th March
2007 1:00pm -
3:00pm
Location:
Back Yard (site #6)
Conditions:
Mostly sunny with a little cloud cover.
Rockets:
(click the name for rocket details)
A newer rocket rebuilt from the
last launch attempt that failed on
the launch pad. Fixed to launcher.
Team Members at
Launch Event:
PK, GK, AK, John K and Paul K. Number
of firings: 4
This week we performed a number of follow
on static tests based on the previous
tests. Last week the tests led us
down two very different paths in controlling
the water inside the rocket. The first led
us to developing a baffle that fits on the
Robinson coupling of the lowest bottle. This
deflects the air preventing the "blow-through"
effect.
The second path of investigation was to
use the blow-through effect to our advantage
to create foam inside the rocket.
Flight Day Events
Static Fire Tests #5 and
#6 - Baffle
This test shows how water behaves inside
the lowest bottle with
the baffle in place. It seems to solve the
blow through problem as well as the water
being pushed up against the sides during the
air pulse as was seen in the previous tests
#1 and
#2.
The baffle (see picture at left) replaces
the coupling nut and is essentially a
plugged tube with four 6mm holes radiating
out from the center. Because the baffle is
so close to the base of the bottle, the
bottle lobes help direct the airflow
downward to the surface of the water.
The baffle looks like it solves the
blow-through problem, and doesn't allow
air to pass directly into the nozzle from
the lowest coupling.
The baffle also eliminated the water
held in the lowest bottle during an air
pulse.
At this point we don't know if and how
much it reduces overall thrust from the
rocket especially during the air pulse. -
More tests needed.
We now have to test fly it to see what
difference it makes during a real flight.
Static Fire Tests #7 and
#8 - Foam
Theory
Foam inside the rocket is ejected to
provide a different thrust profile compared
to just plain water and air. The foam
provides a different density medium that
acts as the reactive mass.
We developed the following technique to
generate high density foam inside the rocket
during launch and throughout the thrust
phase. We call this technique Jet Foaming.
(If the video does not play, try
the latest
Flash player from Macromedia)
A foaming agent is added to the water,
and the water is arranged in the bottles
such that the lowest bottle has a majority
of the water but also has an air-gap at the
top. (see diagram at left) The upper bottle contains some water
also. This water is held above the air gap
due to the closed cavity in the rest of the
rocket. Upon launch water is forced from the
upper bottle into the lower water bottle at
great pressure and this generates foam very
quickly in the lower bottle. The foam
generation continues even after the water
runs out in the upper bottle as air then is
continuously blown into the already generated
foam.
This technique has the advantage that
although it takes a little time to convert
all the water in the lower bottle into foam,
the blow-through effect ensures the first foam
generated is
delivered to the nozzle directly and instantly
while the
rest of the water is turned to foam.
While the use of foam inside of water
rockets is not new (see
Antigravity Research), at the time of writing
we were not aware of others using this
technique for a rocket to continuously
generate its own foam during the boost part
of a flight. [If you have additional
references on foam work in water rockets
please contact us. We would like to add them
to the credits list.]
Tests
In this test the baffle was removed and
the standard coupling nut put back. About
400ml of water was placed in the upper
bottle, and about 1800ml in the lower
bottle. We added about 80ml of kids bubble
bath solution to the water. (See diagram at
left)
During the filling process quite a bit of
foam was generated and pushed up into the
upper bottles, but as you see on the video,
by the time we fired the rocket, the water
was free of foam, with only low density foam
sitting on top.
The jet foaming technique creates a bit
of a mess, so make sure you do it over a
dirty floor that needs cleaning.
Lessons learned
The Jet foaming technique is very
effective at creating foam for the
duration of the boost phase.
Quite a bit of low density foam is
left in the bottle after the boost phase.
We will need to measure the weight of the
remaining foam, and perhaps try to develop
a way of getting rid of this during the
boost.
The thrust profile is changed
significantly from normal air/water mix.
We will need to buy or build a logging
load sensor to get some realistic
measurements.
With the 7mm nozzle the rocket
provided thrust for 7.28 seconds, which
means that take offs are likely to be
slow, but should be nicely sustained.
Therefore this technique may be
particularly useful in the sustainer of a
two stage rocket.
We appreciate all the support and helpful
suggestions from the many people in the
online water rocket community.
Burst Tests
We also tried a bottle re-enforcement
technique based on Richard Wayman's
technique described here:
We used a 1.25L bottle with one half of
the jacket made from a 1.5L bottle, and the
second half (the neck) from another 1.25L
bottle. Because we don't have a heat gun we
used hot water instead to shrink the outer
bottle. It gave a very satisfactory result.
We had three attempts at blowing it up
but on the first attempt the hose released
the nozzle at 160 psi. On the second attempt
a connector broke at a thread on our
pressure regulator at 200psi most of the
water was drained from the bottle through
the pressure bleed valve...oops. We quickly
replaced the connector and tried again.
On the last attempt at 220psi the bottle
again separated from the hose, and although
there was virtually no air in the bottle,
the bottle took off and skidded along the
grass for about 15 meters. We will attach
the hose properly next time through a
different connector.
Looking at the bottle after the tests it
was obvious that this is likely to be a very
good technique for building rockets that fly
at 200+ psi. The top part of the jacket
separated, but that is likely to be improved
with larger overlaps, and perhaps glue.
