To investigate what happens when a foaming agent is
added to the water.
To investigate how foam could be generated in flight.
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.
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 below) 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
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
delivered to the nozzle directly and instantly
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.
III rocket was set up on the
launcher and tied down for this experiment. The standard static
test fire procedure was used. A video camera was set up to film
the lowest bottle to see the water's behaviour.
Setting up for static fire tests.
The rocket is attached to the launcher with
the 2 meter extension.
Rocket configuration for static fire tests
#7 and #8. This diagram shows the distribution
of water within the rocket.
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.
Another day at the office. "You did want the
flowers washed dear didn't you?"
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
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 lot
of foam on the ground around the test area.
Conclusions / Analysis:
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
The thrust profile is changed
significantly from normal air/water mix.
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.