070224_002 - Thermal tests of skin temperature and
This experiment is carried out in order to understand what
happens inside the lowest bottle of a Robinson coupled rocket.
In particular what happens when some amount of water is left in
the upper bottles.
III rocket was set up on the
launcher and tied down for this experiment. A number of cameras
and lighting were set up around the rocket.
Setting up cameras for the experiment.
... and a couple more.
It is a good idea to wear ear protection, as
it can be pretty loud.
Firing the rocket by hand.
Static Fire Test #1
We filled the rocket with 2.5
L of water. There was about 600ml was in the top
bottle, and about 1.9L in the bottom bottle.
With an air gap in the top of the lower
This water configuration is quite easy to
achieve, just pour the water in and let it
partially fill the next bottle. As the top
bottle is closed off the small
Robinson coupling hole mostly keeps the
water coming through to the bottom bottle
and hence leaving an air gap. The nozzle was
a 9mm Gardena nozzle. The rocket was
pressurised to 110psi before being fired.
Following are frames from
the slow motion video.
Just prior to launching test #1.
Initial bubble blow-through, extends all the
way to the nozzle.
Jet of water from upper bottle creates a lot
of bubbles in the lower bottle.
Lots of bubbles causes the spray to widen.
When bubbles reduce then flow from the
During the air pulse, some remaining water
is forced up the sides of the bottle.
We pulled the hose off the rocket and
watched what happens to the water inside the
bottle and the corresponding spray from the
nozzle. The video clearly shows the jet of
water from the top bottle dragging air with
it penetrating all the way through the water
in the lower bottle. This starts a process of
generating a great many bubbles in the water
against the nozzle and progresses up
the bottle until the entire bottle is filled
with bubbles so that it is milky in
appearance. The nozzle spray also correspondingly
widens the same way as we saw on the last
launch day. The air bubbles in the water
must be expanding as they exit the nozzle,
and hence the wide spray.
After a while when the water runs out in
the top bottle and water is only being
pushed out of the bottom bottle, the nozzle spray
narrows again and the water becomes a little
more clear, although the surface is very
rough and there is a substantial number of
bubbles in it.
As the water is about to run out, the and
pulse happens, the airspeed inside the
rocket increases significantly. The
air flow is such that the remaining water actually gets pushed up
the sides of the bottle and is kept there
until the air runs out and the water then
just drains out of the nozzle. It wasn't a lot
of water but it was an unexpected result.
Static Fire Test #2
The second test involved only filling the
lower bottle with 1.9L of water so that
there was no water in the upper bottle. We
used exactly the same pressure (110psi) as the
On launch the blow-through also happened,
but the blow-through bubble quickly
retracted again to the surface and the water
stayed relatively bubble free for the
duration of the burn. The nozzle spray was also relatively
Our experiment confirmed that the
wide water spray observed last week was
due to expanding bubbles rather than a
faulty nozzle/seal as previously believed.
Designing a better coupling might mean
that the rocket does not produce this "blow
through" and allow the water to come out
more evenly and hence more efficiently.
Water flows up the sides of the lowest
bottle during the air
pulse. Minimising this should also allow
better flight efficiency.
To get a more efficient burn there
should not be any water in the upper
bottle above the coupling. Water often
ends up there if you fill the rocket too
quickly with air.
Things to consider
If the bubbles are produced in an
efficient manner it may be possible to use
that to an advantage with an expanding
Research used detergent
to create foam inside the rocket with an
expanding nozzle to make the rocket more
efficient. When perfected a technique based
on the static fire test #1 may be used to
generate the same effect without the
viscosity penalty of using detergent.
If the temperature of the air inside the
rocket drops below freezing at higher launch
pressures, it may be likely that the during
the air pulse tiny water droplets exiting
could be freezing and exiting as ice or
These were static tests that do not take
into account the acceleration of the rocket,
so the water may behave differently, but
from last weeks tests we see that even
accelerated rockets behaved in a very
We purposefully recorded the rocket
video sideways so that we could see more
of the action.