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Each flight log entry usually represents a launch or test day, and describes the events that took place.
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Day 136 - MicroLab 'Gravity' Mechanisms - Part I

Date: 11th August 2013
Doonside, NSW, Australia
 Sunny, very light winds < 5km/h early, 20C
Team Members at Event:
 PK, GK, and Paul K,.


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.

Rocket Configuration

MicroLab from day85 - (Jan 2010)

MicroLab - Experiment setup

Launch @ 110psi


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.


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.


See also Pendulum rocket fallacy for more details: (Thank you to PK for this reference)

Launch Day

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:

Well done guys! :)

Flight Details

Launch Details
Rocket   Polaron VIIIx
Pressure   110psi
Nozzle   16 mm
Water   2600mL
Flight Computer   ST II - 6 seconds
Payload   Microlab, MD80 clone, zLog altimeter
Altitude / Time   347 feet (106m) / 30.8 seconds
Notes   Good launch with rocket tilting to one side a little. Good late deployment as designed. with good landing. Good video and data.
Rocket   Polaron VIIIx
Pressure   110psi
Nozzle   16 mm
Water   2600mL
Flight Computer   ST II - 6 seconds
Payload   Microlab, MD80 clone, zLog altimeter
Altitude / Time   331 feet (101m)  /  10.1 seconds
Notes   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.
Rocket   Flygon
Motor   D12-7
Altitude / Time   ? / ? seconds
Notes   Good burn and straight flight. Parachute opened well and rocket landed safely.
Rocket   Axion IV
Pressure   120psi
Nozzle   9mm
Water   1400mL + foam
Flight Computer   ST II - 5 seconds
Payload   None
Altitude / Time   ? / 26.4 seconds
Notes   Good flight with good landing.
Rocket   Black Thunder (Paul's Xarconian Criuiser)
Motor   C6-3
Altitude / Time   ? / ?
Notes   Good bboost. Parachute failed to inflate but good safe landing.
Rocket   Axion IV
Pressure   120psi
Nozzle   9mm
Water   1400mL + foam
Flight Computer   ST II - 5 seconds
Payload   None
Altitude / Time   ? / 28.3 seconds
Notes   Good flight with good landing.



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