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events that took place.
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Day 120 - ARB Test Flights, Improved Splices, New
Narrow sleeve splice (top) showing the
amount of overlap.
Narrow sleeve asymmetric #5 (top) Normal
sleeve asymmetric #5 (middle) Old symmetric
Getting ready to test the narrow sleeve
We test inside an old scuba tank.
Bottles burst at 200psi, but splice remained
Components of the new launcher.
Bottle necks are glued to the launcher so we
can attach ARBs easily.
Partially assembled launcher.
The hose quick connector on the bottom of
Small plastic roller at the bottom of the
launcher guides the launch string.
Detail of the release tube.
Screws with flat rounded heads used for retaining
Bottom of rocket fitted into the launcher.
Release ring in the locked position. This
bit needs improving to prevent self launches.
Launcher with bottle in the locked position.
New hose and a new adaptor for the pressure
Launcher with two 2L ARBs attached.
Full launcher with 70cm launch tube.
We prepared 1000mL reloads in individual
bottles to make it easier on launch day.
Rocket is 10cm into it's flight.
Parachute delay was set for post apogee
"You know what? I'm going to let him get
Launcher with ARBs removed. Normal bottle
caps close the ARB ports.
Prepping rocket for flight
Obligatory pose before launch.
It's a bit cool in winter, but only about
Nice laminar flow from the nozzle.
Flight #3 Landing in the tall grass.
Tall grass assists in gentle landings.
Weird parachute shaped mushroom sprouting.
We really only need to sit back and relax as
the kids launch and ...
...retrieve the rockets.
Getting ready for last flight of the day.
Axion was launched without the boosters.
John captured this launch photo on his iPod.
Great reflexes to catch it. At this point the
rocket is pulling in the order of 40G.
All 10 launches for the day deployed their
Packing up at the end of the day.
This rocket has now been stuck 20m up in the
tree for 7 months. It's lost its fins.
Conditions: 14C, overcast and
mostly calm Team Members at Event:PK, Paul K, John K, Jordan K and
It's been a while since the last update,
but the last two launch windows had been
cancelled due to high winds. This is a bit
of a long write up so you may want to skip
to the relevant sections.
This week we tried to improve on the
splices we have been using for our water
rockets. (Quite a while ago we tested a
1cm overlap splice
without a sleeve that showed some promise.) We normally overlap the bottles by
about 3 cm and join them with a 6cm wide
sleeve so there are 3cm of PL premium on
each side of the join. The amount of overlap
required has always been only a bit of a guess. By
reducing the amount of overlap between the
reduce the weight of the splice due to less
glue and a narrower sleeve, but you also
gain more volume inside the spliced-pair.
The new splices still use the
splice #5 technique, but the sleeve is
only 3cm wide so you get a 1.5cm overlap. The narrower sleeve also
makes it easier to apply the glue and
assemble the splice. We still use Sikaflex 11FC to
seal the splice.
Here is a comparison of the splices we
have been using:
Symmetric - 90mm
Asymmetric #5 -
old sleeve - 90mm
Asymmetric #5 -
narrow sleeve - 90mm
The new splice is 20% lighter than the
symmetrical splices and 12% lighter than the
asymmetric splice we had been using. The
internal volume is also the same as the
symmetrical splice. Using 3 of these spliced
pairs in a rocket saves us about 30 grams
compared to the previous asymmetric splice
and about 50 grams compared to the symmetric splice.
We did a hydro burst test on one of the narrow
splices to find out how much pressure
it could actually hold. The bottles burst at
200psi which is about normal for the 1.25L
bottles. It was a pretty big bang and I
suspect there was quite a bit of air in it
judging by the carnage. We do the burst
tests inside an old scuba cylinder. The good news is that the splice
did not fail which means we should have a
comfortable safety margin. We will keep a
close eye on these splices as we use them in
rockets to see how well they hold up to
regular handling and flights.
We used two T intersections that allow two
Air Reservoir Boosters (ARBs) to be connected. These are fitted with
standard bottle threading so that we can
either put normal bottle caps on disable the boosters, or attach the ARBs (bottles /
spliced- pairs / spliced-quads). with
standard tornado tubes. This allows
us to compare the same launcher with and
Instead of using the cable ties, we just
used a piece of PVC pipe with little screws
in the end with flat round heads that hold the
bottle down. The screws heads serve the same
purpose as the tabs on the cable ties. We then cut slits down this
tube to allow the individual segments to
spring back much the same way we did for the
We added a quick-release adaptor to the
bottom of the launcher to make it easy to
fit our scuba hoses.
The release ring is just made out of two
halves of abent steel strip so that we could
get the right size.
Experiment #1 -
To investigate the performance difference
between a launcher fitted with ARBs and one
Background and Theory
An Air Reservoir Booster (ARB) is another
name given to additional volume added to a
launcher in order to gain more performance
from a launched rocket. ARBs have been
around for many years in various
forms. Here is a classic example used by
Robert Youens in his
Insane Air project. His launcher used a 30L ARB.
They have also been popular on a lot of
Thai launchers used for distance
records. Here is a whole
range of launchers that use ARBs in all
sorts of configurations. In Thailand they
call them "Tang Sam Rong". They have also
been referred to as "Big Bang Boosters". Here are some forum discussions on ARBs
ARBs need to be used in conjunction with
a launch tube in order to help the rocket.
When a rocket is launched from a launch tube
the volume inside the rocket increases as
the launch tube is withdrawn and hence the
pressure inside the rocket decreases by a
proportionate amount. The volume increase is
the volume of the cylinder made by the
launch tube that has emerged from the
rocket. Depending on the type of rocket this
can either be a small or significant
proportion of the total rocket volume.
The concept of the ARB is that as the
rocket moves up the launch tube air flows
from the ARB (which is at the initial
launch pressure) to top up the rocket whose
pressure is dropping due to the volume
increase. By the time the rocket just leaves
the launch tube there is higher pressure
left in the rocket than without the
A number of water rocket simulators can
take this launcher volume into account in
their simulations. For example see
Dean Wheeler's simulator.
Simulations indicate that you should be able
to get a measurable performance boost with
We wanted to see how the simulated
results compare with actual real world
flights in a typical small rocket.
We built a small rocket for this
experiment that consisted of a spliced pair
of bottles tornado tube-coupled to a single
bottle. We used our standard deployment
mechanism for recovery. To evaluate
performance we used
the AltimeterOne altimeter to record peak altitude.
In order to get consistent launch parameters we
did the following:
Used the same rocket for all tests.
Weighed the water that went into the rocket
on a digital scale to give exactly 1000mL
On the first launch we set the
target pressure on the regulator and left it on
that setting for all subsequent launches. We
opened the tank valve slowly each time to
fill the rocket to the target pressure.
We tried to alternate between launches
with boosters and without in order to average out any changing weather
We set the parachute deployment delay
to 5 seconds to
ensure that the rocket passed through apogee
cleanly each time without a parachute. Time
to apogee was predicted to be 4.3
Test Rocket Parameters:
The table below shows the results
obtained from 9 test flights. All the
flights were performed with the same rocket.
Forgot to turn on altimeter
On the first test flight the rocket self
launched without reaching the exact target
pressure. So this flight couldn't be used
for comparison. We lowered the launch
pressure to 80psi in an attempt to prevent
further self launches. The cause of the self
launches was later traced to a problem with
the release ring. Self launches again
occurred on flight 5 and 9. We cannot
use these flights reliably for comparison,
though they were very close to the target
launch pressure. After getting distracted, on flight 7 I
forgot to turn on the altimeter so we
couldn't use that one either.
The sample size is too small to give very
accurate results. If we exclude the self
launched flights as these would not have
been at exactly the target pressure we get
an average altitude of 306 feet
(flights 2,4,8) without the booster and an
average altitude of 317 feet (flights
3 and 6) with the booster. This is a
difference of ~ 3.5%.
There are a number of environmental
factors that also affect the measured
altitude. The main one is cross-wind that
may cause the rocket to weathercock and reach a lower altitude
as it flies in an arc. Although on the day
of the tests there was only a slight breeze,
the rocket still landed at various distances
from the launch pad indicating that some
breeze may have played a role.
We entered the rocket and launcher
Dean Wheeler's simulator to
see how the predicted results varied from
The simulator predicted an altitude of
299 feet without the boosters and 325
feet with the boosters or an ~8% gain
in performance. Which is the right ball park
figure for the flights, though the unboosted
flights went a little higher than predicted
and the boosted flights were a little lower than
This experiment was done for a
typical sized rocket, however, ARBs are likely to
be of greater benefit to a narrow body
rocket where the launch tube volume is a
greater proportion of the rocket volume. For
this test the launch tube volume is 266mL
which is only about 8% of the total volume.
More experiments are needed with bigger ARBs
and narrower rockets.
The theoretical graph below compares the remaining
pressure in the rocket as it is just about
to leave the launch tube vs the proportion
of the launch tube volume of the rocket's
volume. The graph shows the values for a
launcher with launch tube only (blue) and one with an
ARB of equal volume to the rocket (yellow). The graph
also shows a solid launch tube (pink) for
From the graph you can see that when the
launch tube volume is a small proportion of
the rocket volume the ARB doesn't provide
much assistance, however, as the launch tube
volume increases the ARB assistance starts
becoming significant. I've also added a plot
line for what a solid launch tube does to
the internal pressure. I had always assumed
that a solid and hollow launch tubes had
identical performance, but this assumption
was obviously incorrect. They behave
similarly for small proportions of launch
tube volume (as is common in normal rockets) but diverge as the launch tubes
take up more space. Of course it is now plainly
obvious once thought about in detail and
plotted. It's good to learn from these
From the graph you can see that for this
experiment, without the ARB the pressure
left in the rocket was ~93% of launch
pressure, and ~96% with the ARB.
Launch Day Events
The weather was overcast with a slight
breeze, but not cold. We set up the launcher
on the medium launcher's base and loaded the
rocket. We used the full bore tornado tubes
for this rocket so that the launch tube
could fit through it. We filled the rocket
with water and then laid it on its side so
we could insert the launch tube without
spilling the water and not getting too much
into the launch tube. This was only
partially successful and some water did
enter the launch tube. After standing the
rocket up we realised that the water that
was in the upper bottle didn't want to drain
to the lower bottle because of the tight fit
between the launch tube and the bottle neck.
So we pressurised the rocket to
around 20psi which forced the water from the
upper bottle around the launch tube to the
lower bottle. This took about 30 seconds.
Once the water was all in the lower bottle
we could finish filling it and launch it.
The first flight self launched as we
approached 100psi. Luckily the servo timer
was armed and the altimeter was on, but it
was a surprise as it made a loud pop as it
launched. At first I thought something blew
up, but it was just a typical full bore
nozzle launch. :) The rocket landed safely. For
all subsequent tests we dropped the pressure
to 80 psi to try to avoid a self launch
event again. The rocket self launched again
on flight 5. We traced the problem down to
the releasing ring slipping off. There is no
spring to keep it up and because it is
sitting on a conical surface it was forced
down as pressure increased. We tightened the
ring, but it still slipped again on flight
9. However, we were happy with the new
launcher's performance. We will fix up the
self release issue before the next test
For the last launch of the day we made a
longer rocket with 3 spliced pairs and a
single bottle. One of the spliced pairs was
a narrow splice. Initially when we put it on
the pad with the water we had a really hard
time getting the water to move to the lower
bottles even at 40psi. The fit between the
internal couplings and the launch tube were
quite snug. As a result quite a bit of the
water drained into the ARBs. We took the
rocket off the pad, reamed out the offending
bottle necks and put the rocket on the pad
again without the ARBs this time. The
reaming did the trick and allowed water to
flow into the lower bottles much more
It was launched at 100psi
and the rocket took off well, but you could
see a bit of a bend in the rocket under the
high acceleration. The rocket flew to 460'
(140m) feet and landed well.
All in all it was a great launch day with
10 good launches and 10 good parachute
deployments. Now having launched our first
full bore nozzles I have to say I still
prefer the slower launches with the