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Day 130 - Inverter flight and Shadow II CATO
10th February 2013
Location:Doonside, NSW, Australia
Conditions:Sunny, light winds < 10km/h
Team Members at Event:GK,
PK, Paul K and John K.
Today was a bit of a mixed day again with
some good successes, but also a spectacular failure. The
conditions were great with nice blue skies
and fairly calm conditions.
The Shadow II rocket was set up first as normal
while conditions were still calm.
We decided to keep the camera we used last
time as tests showed that it was
working properly. We shook the electronics
section vigorously dropped it several times
to test if anything would interrupt the
video, but all was good.
Assembling the payload bay
Filled with 1.7L of water
Loading it onto the launcher
Launch pressure: 440psi
We pressurised the rocket to 440psi (30
highest pressure this rocket has ever been
and launched it. The rocket beautifully CATOed about
8 feet in the air, The payload
section and the upper foot of the rocket
kept going a little ways up. The parachute was ejected as a
result of the explosion and gently guided
the whole payload section back down. You can
hear the servo motor activate about 1 second
before it touches down. The delay was set
for 9 seconds.
Here are some frame grabs from the video
of the explosion.
The bottom 3 feet of the rocket also
survived and landed right next to the
launcher. The middle section of the rocket
completely disintegrated in a shower of
fiberglass confetti. This is why you want to
stay completely well away from these kinds
of rockets when they are pressurised. The debris field was well within
the exclusion zone. The sound was quite
impressive and you could hear it echo off
Top and bottom landed right next
to each other
Inspecting the payload
We weren't too disappointed because we
knew this day was coming sooner or later.
The plan was always to keep pushing this
rocket to see how far it will go and is the
reason we have been increasing the launch
pressures incrementally. Personally I was
expecting one of the bulkheads to give way
before the pressure chamber.
Lower rocket section in bottom
Rocket eating trees are starving
Lower section already on the
Okay let's go to
the video tape ....
We believe this was a very unusual
failure as once a rocket launches the
starts dropping rapidly so there should be less
and less stress on the pressure chamber as
the rocket climbs. One would expect that either the
rocket blows up on the pad or it flies, but
not explode mid flight. We
believe there was an additional source of
stress in the pressure chamber during the
flight which caused it to fail. At the time
the rocket was 2.4m (8 feet) off the ground
and travelling at 42m/s or 151km/h (94mph).
Potential sources of additional stress:
The large acceleration (~44G at the
may have added enough compression stress to the
pressure chamber that caused it to fail
at a weak point. At 44G you have about
15Kg (33 lbs) pushing down on the
top of the pressure chamber due to
weight of the payload section,
parachute, deployment mechanism and
nosecone. If you include the weight of
the pressure chamber up to the failure
point, that's a 29Kg (64lbs) load. The interesting thing is that
at the time of the explosion the
rocket hadn't hit peak acceleration yet.
There was also water left in the lower
section as can be seen in the video.
The rocket may have been undergoing
a bending stress which may have put more
stress on one side of the pressure
chamber than on the other.
A shockwave may have travelled up
the rocket as it cleared the launch
tube. The reason this may be a
real possibility is that the simulation
results show there is a large spike in
the acceleration just after it
clears the launch tube. On the
right is an example from Dean
Wheeler's simulator. At first we thought
this spike was an artefact of how the
simulation calculates the acceleration,
but from real data captured with an
accelerometer we see that this
acceleration spike is a real phenomenon.
We really need to do more investigation
in this area as to how much the air heats up
inside the pressure chamber at these
pressures and how that impacts on the
strength of the pressure chamber. The rocket
was pressurised in about 1 minute for this
Weak Point Location
It is hard to find the exact point of
failure, but it looks like it was just below
the top coupler.
The top coupler was added
after the crash we had
a year ago. Although we cut away the
damaged section there may have been an unseen stress
fracture in the rocket below the replaced
Stress also tends to concentrate at
transitions and there
is a sharp transition from the main body
tube to the coupler.
On the right is an example of a
pipe and coupler stress analysis.
While not dimensionally related to
this rocket it does give you an idea where the stresses are
Below is a shot from the
high speed video that shows the approximate
location of the failure. It appears to be
right near the top coupler. The distance
from the tip of the nosecone to the end of the
coupler is 92cm. From the
video we measured the distance from the tip of the
nosecone to the location of the puff of
vapour and got approximately 99cm with a
margin of error +/- 10cm. This is pretty
close to the coupler.
Moment of failure
Delaminated coupler stuck to
The Shadow now fits into a
We had a close look at a couple of the
pieces that were in contact with the top coupler. You can see that at least these
two pieces had the outer layers of the
coupler still stuck to them implying that
the bond between the coupler and the tube
was good, and it was the coupler that
delaminated during the explosion. None of the cracks in the body
tube propagated very far along the coupler
(as can be seen especially on the lower
coupler). If the two couplers hadn't stopped
the crack propagation we would have had a
lot more confetti.
It is also interesting to note that from the on-board video you can see how
much the camera turned while the rocket was
pressurised. This seems to imply that
the tube must have been stretching
considerably and the twist comes from the
cloth "unwinding" in the opposite direction
it was laid up. This is only a theory and we
can't be sure. Was this partially the
result of weakening epoxy, or just the
normal stretching of the composite under
load? I believe we saw a similar rotation in
earlier flights. The rotation was perhaps 20
degrees on this flight.
Camera direction before
Good information did come out of the CATO
though. This was the first time we had a fiberglass
pressure chamber burst since we started
making and testing them so it was good to
see what happens when it actually does
break. It's also a good wake up call for how
much energy is stored in the rocket and why
safety is so important. We also learned that
a CATO mid flight IS possible
and the compression forces need to be taken
account during acceleration. A hydrostatic
test may not reveal all the potential
failure modes when acceleration is not taken
into account. Applying an axial load to the
pressure chamber under test may be required
in future tests.
The launcher also didn't leak after the
quick connector was properly tightened and
the video camera recorded the flight without
problems unlike last time.
We are considering what we'll do next
with the Shadow series, but most likely
investigate new pressure chamber materials
and improving their strength. We are likely to
put the Shadow on the back burner for now,
and concentrate on some of the other
projects we have lined up for this year.
We repaired the rocket during the last
two weeks, with most of the repairs only
involving splicing new bottles. See the Inverter build
log for more details. We also made some
modifications to the parachute release mechanism to
prevent it from opening early.
After the Shadow debris shower, we
swapped the release head again and set
up the Inverter rocket on the pad.
When we tried locking the nozzle into the
release head it would
go in fine, but would get wedged and would
not release. This is what happened during
the last launch as well. We tried a second nozzle
with exactly the same
problem. So we
disassembled the release head and used sandpaper to file down the little plastic
tabs by about 1/2mm. On re-assembling the
release head we found that it was still
sticking. It turned out to be the wider
section at the top of the nozzle that was
getting wedged not the little tabs. So we
used the sandpaper to sand that down and
then the nozzle released easily.
We had to sand down this section
to make it not stick in the
With field surgery over, we set the
rocket up on the pad and filled it with
6.5L of water. We used a little
food colouring this time to make it stand
out better on video.
Filling with water
Paul looking forward to
As we approached ~110psi (target pressure
was 125psi) something started leaking.
Leaving the air on we decided to launch the
rocket anyway guessing it should have enough
pressure to fly. The rocket took off very
slowly again before accelerating nicely. It
arced over and deployed the parachute.
Here are a few shots of the launch from
different camera angles.
From the ground it looked like the
parachute opened late, but the on-board
video showed that it deployed right at
apogee, and took 2 seconds to fully inflate.
The modifications we made to the release
mechanism worked well, so no more early
The rocket came in for a gentle landing
because the large surface area of the rocket
helps to slow it down. The altimeter gave a reading of 315
feet which was not unexpected because of the
lower launch pressure and the fact it arced
over rather than going straight up.
You can see how stable the rocket was on the
way up with almost no roll.
Here is a
highlights video of the flight:
While swapping the release head to fly
one of Sam's rockets we found the
cause of the Inverter leak. We forgot to re-tighten the release head on
its seal which we
loosened while trying to work out why the
nozzle was wedging. Doh!
We were going to launch the Inverter
again, but by then the wind had picked up to around
30km/h with stronger gusts and so we decided
to abort the flight. Better to save it for
the next launch. This rocket has a very
large surface area and so cross wind can
easily affect it.
We were happy that the rocket finally
flew well and we're looking forward to
flying it again. We only had the two
launches on the day, but it was fun
watching all the other pyro rockets as there
were quite a few new faces again.