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Day 138 - Dual Thrust Rocket
30th November 2013
Location:Doonside, NSW, Australia
Conditions:Overcast, winds ~15km/h
Team Members at Event: PK, GK,
and John K.
It's been a while since the last update.
There have been three total fire bans in
Sydney during the past couple of months so the NSWRA
launches had been cancelled. I've also had
to put development on hold for a few weeks
while my wife was in hospital and then at
home recovering. Even though we weren't
launching there have been plenty of things
keeping us busy. This week we flew a small
dual thrust rocket a number of times in
The main idea behind a dual thrust rocket is to give a rocket
a large acceleration on launch and then a longer prolonged burn
for a sustained flight. This concept has been around
for many years. There are many examples of this in the pyro
rocket world not only in HPR but also in weapons systems. (See
reference section) It is similar to
how a T-nozzle works in water rockets, although the T-nozzle only has a large
nozzle during the launch tube phase. Here is a dual thrust water rocket
example that uses a sliding nozzle that allows for more water to
be ejected through the large nozzle before switching to the
The implementation we used in our tests is just the
Spacer we have been using on our rockets. The difference is that
when we normally use the jet foaming spacer the hole in the spacer is
comparable to the size to of the hole in the nozzle. This ensures that
the water doesn't drain from the main nozzle too quickly and allows
the foam to form in the lower part of the rocket. The dual
rocket uses the opposite principle and makes the main nozzle
large and the internal nozzle small. This allows the bottom bottle
to drain very quickly giving the large initial boost, and at the same
time the internal nozzle produces a much weaker but longer burn so that the rocket sustains the flight. The internal
nozzle is simply directed out of the main nozzle. In this
configuration the spacer is longer to create a bigger air
pocket at the top of the boost chamber.
The spacer also serves the same function as in the Jet
foaming configuration which creates a constant size air gap at
the top of the lowest bottle. So all you have to do is fill the
rocket as normal and then when pressurised the water will sort
itself out to the right levels in the top and bottom chambers.
You can tune the system by varying the length of the bottom
pressure chamber to increase or decrease volume for the boost
phase. You can adjust the length of the spacer to change the
water to air ratio and finally you can adjust the size of the
internal nozzle to change the duration of the sustain phase. One
added benefit is that for the sustain burn, the water is stored
a lot further up the rocket aiding in the rocket's stability.
As we wanted to try several different configurations we made the
internal nozzle swappable. We made 3, 5 and 7mm inserts for the
Simulations predicted that this kind of rocket is going to
perform a little worse than a normal rocket with a large nozzle. A good
opportunity to test the simulators.
There have been similar full length tubes
used in water rockets previously to try to
mix the air and water for example Jelo's,
Thunderrocket's and Todd's experiments: (http://wrocket.hampson.net.au/?p=1196)
but the aim with the dual thrust rocket is
not to mix the air and water.
3, 5 and 7mm
Spacer fitted inside rocket
(22 / 9 / 2013) Before launching this rocket we wanted to see what the entire
system will do, so we fired it on the static test stand a couple
of times. We had
to make a couple of modifications to the stand to fire a full
bore nozzle. We are using a sawn-off Clark cable-tie launcher to
launch this rocket because the spacer gets in the way of a long
launch tube. We suspended the release head so it
would not impact the ground after release. With a full bore nozzle the
gets ejected from the rocket at a good pace. We did not connect
the rocket to the load cell this time.
For these tests the rocket consisted of
two 90mm spliced-pairs and a single bottle
joined with tornado tubes giving a total
capacity of around 5.45L.
We filled the rocket with 1.7L of water and
used the 5mm insert for the inner nozzle.
The lower chamber is divided into
approximately 50:50 water to air ratio as
this is what the simulator suggested.
Generally the optimum water fill for
boosters is a higher proportion of water
when compared to normal rockets. This means
that about 1L was in the boost chamber and
about 0.7L in the upper chamber.
We pressurised the rocket to 40psi and
fired. We wanted to try a lower pressure
first to see how the large bore nozzle
release head behaves. Having the release
head suspended meant that no damage was
Short Clark-cable tie launcher
Static test stand
shortly after release
The boost phase looked like it worked
well except that some air mixed in as a
result of the blow-through caused by the inner
nozzle.The exhaust plume out of the main nozzle
was quite turbulent. This is likely reducing
the efficiency of the boost as air is being
ejected before all the water is ejected.
Ideally you would not want any air to mix
with the water in the boost chamber. In
the sustain phase the water stream was quite
even and emerged from the main nozzle
cleanly. Once the air pulse started the
exhaust from the main nozzle became quite
turbulent again, and a small amount of water
was getting pushed up against the sides of
the boost chamber. We have
seen this before.
The slow motion video showed that as the
sustain phase air pulse started, it generated
enough of a vacuum that it collapsed the
lower bottle briefly. This is not too much
of an issue though, but with fins fitted to
that part of the rocket it may briefly move
them in flight?
We repeated the experiment and this time
fired the rocket at 110psi. Again the two
thrust phases were clearly visible and the
results were similar to the first test. The
bottle again collapsed during the sustain air
Blow-through is evident
Too much air in exhaust
The tests showed that the dual thrust
arrangement created the distinct two phase
thrust but actual flight performance was
still unknown. We had to wait a couple of
months before we could test fly it.
The following video shows these two
Although it was a little windy on launch day, the wind was
blowing away from the rocket eating trees and so there were no
issues with launching. The first rocket was set up with a 5mm
inner nozzle in the dual thrust configuration. We launched it at
120psi and you could see the quick boost up to speed and then a
prolonged stream of water for the gentle sustained flight. The
rocket didn't go very high, only around 253 feet. It was a very
nice flight to watch though.
We repeated the flight with 100mL less of
water to make it lighter during launch and
again the rocket performed similarly to the
first flight but went up to 268'.
We then replaced the inner nozzle with
the 3mm insert and set the rocket up again.
At 3mm the nozzle is less than 2% of the
cross sectional area of the main nozzle. We used 1.6L of water in the rocket. The
launch again was quite quick, but the
sustain was very gentle and just kept the
rocket moving up slowly. The air pulse from
the sustained flight came quite late and
near apogee. The rocket only reached 203'.
This was definitely a fun one to watch. The rocket was stable after boost even with so much water
still in it.
For comparison we flew the same rocket
@120psi with a standard configuration and a
9mm nozzle and the rocket went up to 373'.
For the last launch of the day, we flew
the dual thrust rocket again with the 3mm
inner nozzle but pressurised to 125psi. The
rocket flew the gentle sustain flight again
this time to 219'.
In all four dual thrust flights it was
evident that the boost phase exhaust
contained a lot of air which would have
reduced the efficiency of the flight. When
air escapes past the water then it's not
being used to accelerate the remaining water
out of the nozzle.
Setting up for first launch
Launched at 120psi
Water spray shortly after launch
Downloading data between flights
Ready to go again
Recovering respective rockets
Launched with red food colouring
Standard 9mm nozzle
As can be seen from the results, not all
experiments lead to better performance. The
simulators were right in that the rocket
would not perform as well as a regular
rocket, and in fact they were a little
optimistic as the rocket flew lower than
expected. Some of this may be attributed
to the loss of pressure in the booster
to the blow-through effect. One option
to solve the blow-through is by putting the
nozzle a lot further down in the water and
then adding a small non-return valve half
way up the spacer. This should allow the water
to equalize up to the valve, but once
launched keep the water inside the spacer.
This was a fun experiment to see what
would happen in flight with this
configuration. While this particular
configuration didn't lead to better
performance, it does offer a different
flight profile for a number of in-flight
experiments. This experiment should be
repeated with higher pressures.
The rocket also remained stable throughout the
entire flight despite a considerable amount
of water in the rocket during a major part
Here you can see the long narrow
water column during the sustain
is a highlights video from the day.
have questions or comments about the dual
thrust experiments please jump to the
bottom of the page.
There have been a few more developments
and events that have happened in the past
couple of months.
Fin Alignment Jig
The last few launches have used older
fin-cans that aren't terribly well aligned
and as a result we've had a few spinning
on-board videos. We made up a simple fin jig
to make a new fin-can that is hopefully
better aligned. We'll make up a few more
fin-cans and retire the old ones. We flew
these new fin cans this weekend and although
they didn't completely stop the roll they
have significantly reduced it. Some of the
roll may have been due to other protrusions
on the rocket like the camera and parachute
Both sets of fins are removable from
the pressure chamber.
PVC Rail Buttons
After testing the
rail buttons a couple of months ago,
we weren't quite happy with their
strength. This time we made the rail
buttons in the same profile from a PVC
extrusion used to join Hardiflex panels.
These are a lot stronger and hold the
rocket securely under larger loads. The
small profile also means they are more
aerodynamic than our normal machined
rail buttons. Each weighs 1.5 grams. We
heat the bottom wider section in hot
water and then form it on a roller to
the bottle profile.
The rail button is just attached with
electrical tape to the side of the rocket. At 2.4m
length for the extrusion, you can make
about 50 rail buttons at a cost of a few
WARAausKIEL also uses a
similar rail button system but built
around an plastic I beam.
Cut to size
We also did a load test
on the rail buttons by filling a pair of
spliced-pairs completely with water and
suspending them horizontally by
the guide rail. This gave a total weight
hanging off the two buttons of 4.2Kg
(9.2 lbs). That's strong enough for our
Attached to bottle
Undergoing load testing
We test flew these rail
buttons on one of the rockets this
has continued on this rocket over the last few weeks.
We've been focusing on getting the
boosters made and attached.
See the updates on the
Polaron G2 build
You Have Been
Warned - Series 2
In December last year I was
contacted by October Films in the UK
working on a Discovery Channel show
called "You have been warned - Series 2"
about using one of our YouTube clips.
Over the last few months they have asked
many technical questions as I think they
will have a presenter describing how our
2-stage water rocket works. I've also
sent them copies of the clip without any
of the text overlays, as I believe they
want to add their own.
I don't know how the
final presentation/clip will look, so it
should be interesting. I recently
received a tentative date for when the
episode should air in Oz.
Episode 11: Backyard
Boffins (provisional date 30 Dec 2013)
I believe the show is
also called "Outrageous Acts of science"
in the US, but I am not sure it is the
same content or not.
We have added
an article on
nozzles that describes some of the
differences between the nozzles commonly in use.
Powerhouse Museum Mini Maker Faire
Last week as a part of the NSW Rocketry
Association we were at the
Mini Maker Faire at the Powerhouse
Museum. We had a similar setup to what
we normally have at the Macquarie
University Astronomy open nights, There
was a good turnout of people and it was
fascinating to see what other Makers do
in their spare time. We also managed to
static fire a number of G class motors
which are always fun to watch because
they are loud and produce a lot of flame
& smoke. It was also great to
catch up with Dave Jones from his
EEVblog. Dave and I worked together for
a short time back in 1994 and this was
the first time since then that we had
met up again.