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Day 136 - MicroLab 'Gravity' Mechanisms - Part I
Date:
11th August 2013
Location:Doonside, NSW, Australia
Conditions:Sunny, very light winds < 5km/h
early,
20C
Team Members at Event: PK, GK,
and Paul K,.
Introduction
This is the first of a 3 part series that
looks at how "gravity" based mechanisms
behave in real flight. These mechanisms are often used
by rocketeers as a basis for their early
parachute deployment designs and are based on the
following expected behaviour:
"A weight hangs down inside a
rocket while on the
launch pad, during launch and all the way up
to apogee. At apogee the rocket tips upside
down in relation to this weight and with the
weight now pointing towards the nosecone triggers a parachute deploy mechanism."
The incorrect assumption is that no matter what the
orientation is of the rocket, the weight
will always continue to hang down towards
the ground.
In real life the weight will 'hang'
towards the nosecone shortly after burnout,
and continue to do so through the coast phase,
through apogee and all the way back to the
ground. Mechanisms based on this
principle typically deploy a parachute soon
after burnout at maximum speed!
This week we look at two mechanisms in
flight:
A hanging weight at the end of a solid arm
that is free to rotate around one
axis.
A free weight allowed to move only in one
dimension inside a tube.
The experiment below demonstrates this
behaviour in actual flight. This is a
follow-on experiment from
Day 85 where we
demonstrated the behaviour in mercury
switches.
Experiment Setup
The MicroLab was again housed in the body
of a 2L bottle. Due to the size of the
subject being studied the camera this time
was set up on a short boom away from the lab
and a wide angle lens was placed in front of
the camera. The view was also set up to show
the horizon in the background to help
indicate
the rocket's orientation (attitude) in relation to the
mechanisms.
The flight profile was
set up so that the parachute would deploy
well after apogee so that we could observe the mechanisms
from launch, through apogee and a little bit
after.
Parachute deployment was achieved using an
electronic timer (STII)
and set to deploy 1 second past the predicted
apogee time. The rocket's recovery was
completely independent of the experiment.
The rocket also carried a barometric
logging altimeter so that we could correlate
the altitude to the mechanism's behaviour.
The following
video shows the behaviour of the two
mechanisms.
Flight #1
The rocket was launched at 110psi. The rocket tilted a little after
launch which was likely due to the added weight and
drag of the camera and lens. It reached an
altitude of: 347 feet (106m) which was close
to what was expected. The parachute opened
well past apogee and rocket made a safe
landing.
From the on-board video you can clearly see
that both the weights moved up towards the
nosecone right
after
burnout at an altitude of around 70-80 feet
(22m), and remained there until the
parachute deployed. Although the hanging weight
was swinging the centerline of the swing was
pointing towards the nosecone.
As air drag on the rocket decreased near
apogee the swing period increased as the
weight was essentially in zero G in relation
to the rocket.
On the launch pad
Just after Burnout
Near apogee
Heading back towards the ground
Flight #1
Altimeter Plot showing the point
at which the weights moved up.
Flight #2
The rocket was launched again at 110psi
and reached an altitude of 331 feet (101 m).
The parachute failed to deploy and the
rocket continued along its ballistic path
until it hit the ground. Although the rocket
was damaged, good video and altimeter data
was obtained that showed the mechanism's
behaviour over the entire ballistic flight path.
Again as
can be seen from the video, after burnout
both weights continue to 'hang' towards
the nosecone for the rest of the flight to
apogee and back down to the ground.
On the launch pad
Just after burnout
Near apogee
Heading back down
Flight #2
Altimeter Plot showing the point
at which the weights moved up.
Why does it happen?
In order to explain why this happens we
need to consider the forces acting on the
rocket and on the weight.
After burnout the rocket has two main
forces acting on it: Gravity and Drag. A
free weight inside the rocket only has
gravity acting on it, but not drag. Because
gravity acts the same way on both the weight
and the rocket it essentially cancels itself
out from the point of view of the
rocket-weight system. The result is that
only the drag force on the rocket is left.
Because the rocket is slowing down faster
than the weight, momentum carries the weight
forward. The same way you move forward in a
car when the breaks are applied.
As the rocket approaches apogee it slows
down and since drag is proportional to the
square of the velocity, drag also decreases
significantly. At this point the weight and
rocket are essentially in zero - G and both
are just floating relative to each other.
Gravity doesn't all of a sudden act
magically just on the weight to move it in
relation to the rocket. As the rocket
continues to fall from apogee drag again
starts to increase and the weight again
starts hanging towards the nosecone.
Conclusion
As has been shown, parachute deployment mechanisms based on
this principle are not suitable for
detecting apogee. They may, however, be
used for detecting burnout to trigger other
events.
It was another ideal launch day with blue
skies and almost no wind. We had planned to
fly the MicroLab experiment several times. For these
launches we set the parachute deployment delay
for 6 seconds, with expected apogee at 5.2
seconds. We wanted to deploy the parachute
late so that we could observe the experiment
through apogee. The rocket was launched at
110psi as the rocket used older bottles and
we didn't want to push them too far.
The launch went great with the parachute
opening when expected. I was a little
concerned about the shock of opening the
parachute late, but luckily all turned out well.
The altimeter looked like it stopped
recording when we retrieved the rocket, but
later we found that it did actually record
the whole flight. So we got both good video
and altimeter data.
We set the rocket up again to repeat the
experiment. This time the parachute did not
open for some reason and the rocket crashed
heavily. The timer power switch was in the
off position and the servo still in the
stowed position. I know I double checked the
timer was armed prior to launch. We will
need to look at this further. A couple of
culprits could be a dodgy power switch or
battery clip as we have seen these issues
before. The battery was ripped away from
it's mounting position which was right next
to the power switch so it is likely that it
caused it to move to the off position on
impact.
The camera thankfully recorded the entire
flight and so did the altimeter. From past
experience we mounted the altimeter half way
down the rocket to help protect it during
crashes. Other than the top two bottles
buckling and the nosecone getting crushed,
all the important electronics and components
were fine, and the whole rocket can be
easily repaired.
The crash actually did have one very good
outcome though. It showed exactly what
happens to the experiment inside the rocket
over an entire ballistic path. Something we
would normally never attempt.
Later in the day we flew the Axion IV
rocket again a couple of times with foam.
Both flights put in a great performance and
had safe landings. We filmed the 4 launches
using the GoPro's 240fps mode from ground
level. The video turned out quite well.
Paul also flew a couple of his pyro
rockets so all in all it was a great day
despite the crash.
Here is a
highlights video from the day's launches.
10 Challenges
We wanted to congratulate
Team Lucrockets for achieving Level 1 on
the Strength
Challenge. That's a great rocket they
have there. Here are links to their videos
of the attempts:
Good launch with
rocket tilting to one side a little
again. No parachute deployment. Top
of rocket damaged, and the top 2
bottles. Good video and data.
3
Rocket
Flygon
Motor
D12-7
Altitude / Time
? / ? seconds
Notes
Good burn and
straight flight. Parachute opened
well and rocket landed safely.
