Closing Document

CLES-FACIL 2008http://cles-facil.insa-lyon.fr

cles-facil@insa-lyon.fr

LEIA

Table of ContentsProject Goals ................................................................................................ 4

1.1 Cansat.................................................................................................4

1.2 Recuperation System..........................................................................5

1.3 In-Rocket Measurements....................................................................5

1.4 Integration of the New Motor..............................................................5

Developed Solutions .................................................................................... 6

1.5 Cansat.................................................................................................6

1.5.1 Structure..........................................................................................6

1.5.2 Ejection............................................................................................7

1.5.3 Original Parachute System...............................................................7

1.5.4 Radio Link........................................................................................8

1.5.5 Control.............................................................................................9

1.5.6 Recuperation....................................................................................9

1.6Recuperation system.........................................................................10

1.6.1 Ejection Mechanism.......................................................................10

1.6.2 Double Parachute System..............................................................11

1.6.3 Timer..............................................................................................11

1.7 Measure in the rocket.......................................................................12

1.7.1 Accelerometer................................................................................12

1.7.2 Pressure Sensor.............................................................................12

1.7.3 Camera..........................................................................................13

1.8 Integration of the new motor............................................................13

Results and analyses .................................................................................. 15

1.9 Cansat...............................................................................................15

1.10 Recuperation System......................................................................16

1.11 Measurements in the Rocket..........................................................16

1.12 Integration of the New Motor..........................................................17

1.13 Unexpected Result..........................................................................17

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Project Members

With help of:

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Nicolas PralyProject Lead

Gabriel Arnold-Dulac

President

Nicolas PralyMechanics Lead

Florent BouchouxElectronics

Lead

Gabriel Arnold-Dulac

Software Lead 

Séverine BelloirMichal Ruzek

Sylvain TanguyRafic MezianiJérôme BégelJean-Mathieu

Cousin 

Mathieu Riedinger

André MachadoRaphael Antoine

Bertrand MandrinFernando Mattioli

Damien MalricPhilippe Roudot

Sylvain RouardFormer Project

Lead

Marc Dal-MolinPresident (2007)

Spas BalinovPresident (2006)

Project Goals

This year the project’s goal was to realize a reusable cansat launcher.

The rocket supposed to be built in such a way that it could be launched at least two times during the launch campaign. As a launch vehicle, its payload was supposed to be a cansat with an approximate weight of 600g. The rocket was also supposed to integrate various sensors to better understand the flight path (accelerometer, altimeter) and help in its recuperation (GPS, radio link).

This year the rocket was launched with a new motor never used by the club before. One of the objectives of the year was also to properly integrate this ne motor in the rocket.

1.1 Cansat

The CANSAT (Can- Satellite) is an international competition to promote space-oriented research and engineering activities at the student level. Cansat competitions are very active in Japan and in the United States, but a cansat was launched for the first time in France only last year in our INESS project (please refer to the closing document of CLES FACIL 2007).

The objective is to embark a module of the size of a soda can in an experimental rocket and eject it at the flight’s appex. The final objective is to bring the CANSAT towards a target on the ground by controlling its trajectory.

In our case, we chose, as last year, to conceive a cansat able to control a parafoil with an automatic pilot. The difference between the “traditional” cansats and ours is the decision that our ejected module would not be the same size as a soda can. This choice is justified by the fact that our cansat will carry in addition to the standard payload, a Kiwi transmitter, 2 servomotors, and a camera, the sum of which require substantial additional volume. However we remain in the ‘special category’ range of less than 1kg.

Our previous cansat had an extremely unstable flight, so in hopes of bettering the stability of our flight this year, we chose to go with a 2 stage deployment of the parafoil : once the cansat is ejected from the rocket, a

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first parachute opens. After a few seconds the first parachute releases itself and acts as a drag chute for the parafoil.

1.2 Recuperation System

This year we choose to enhance our traditional recuperation system. Since one of our objectives was to launch the rocket twice during the week, we needed to assure ourselves that the landing would cause as little damage as possible. However, since the launch campaign takes place on a military base, we have a constrained space in which we can operate. Therefore, we cannot simply use a bigger parachute, as the slower descent speed could cause us to land too far off course (in a mine field for example). As a compromise, we decided on a dual parachute system for the rocket.

The first parachute deploys after the cansat is ejected. This would be a smaller chute like we used in previous years. The second one would open near the ground (approximately 200 m). We also ameliorated our parachute door opening mechanism to replace the rubber band with a steel spring.

In the hope of speeding up the recuperation process, the rocket would have a GPS receiver and a telemetry system that would allow the ground station to know the rocket’s position at all times.

1.3 In-Rocket Measurements

This year we also wanted to integrate an accelerometer and a pressure sensor in the rocket to know the evolution of acceleration and the culminating altitude. We also decided to transmit the data in real time via the Kiwi transmitter (more on the Kiwi later). Since last year there was also a camera in the rocket, we decided to keep this feature.

1.4 Integration of the New Motor

There is a significant difference between the new Pro 54 motor and the old Chamoix motor. First of all, the Chamoix has a threaded rod at the top of the motor, so the mechanical integration is simple. Secondly, the Pro 54 is much longer and thinner than the Chamoix.

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Developed Solutions

1.5 Cansat

1.5.1 Structure

The CANSAT is a 105 x 115 x 220 mm box (≈2,3 L). It contains:

• 2 servo motors

• a parafoil stored inside a shell

• a system to deploy the parafoil

• a micro-controller

• a GPS

• an accelerometer

• a magnetometer

• a Li-Poly (non-rechargeable) battery

• an FSK modulator

• a Kiwi radio transmitter (custom transmitter provided by the CNES)

• a video camera

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Free zoneThrus t ring

Rocket skin

Pro 54 Retaining s leeve

Retaining tab

1.5.2 Ejection

As described in the previous part, the cansat is ejected at the apex of the flight parabola. Since we see the cansat as a payload (and not a part of the rocket as we did last year), we had to design a generic payload ejection system. We based our work on our traditional parachute ejection system and used it to open a side of the rocket’s fuselage through which we could eject the cansat. In order to eject the cansat properly we put in place four large elastic bands to push the cansat out of the rocket.

Pictures on next page:

The new ejection system will be described more completely in the recuperation system part.

1.5.3 Original Parachute System

Last year we had trouble controlling the cansat once ejected from the rocket. One of our hypotheses was that the parafoil opened while the speed of the cansat was too high. In order to solve the problem we chose to use a first parachute to slow the cansat before opening the parafoil. When the cansat is ejected from the rocket the parachute opens. After five second the first parachute is liberated, acting as a drag chute for the

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parafoil. This way, the parafoil deploys while the speed of the cansat is approximately 10m/s.

In order to liberate the parachute we designed a small system to absorb the opening shock of the first parachute and then liberate the first parachute after 5 seconds.

1.5.4 Radio Link

Due to the dense vegetation on the camp of La Courtine (where the launch campaign takes place), both rocket and cansat are not necessarily recovered, so we cannot rely on onboard memory for data storage. To be sure to get at least essential data from the rocket we used a radio link. Last year this system allowed us to quickly recover the cansat thanks to GPS data that had been transmitted. This year we included GPS devices in both the rocket and the cansat, in the hope of being able to recover them easily.

Aside from the GPS coordinates, some other parameters were also transmitted, such as altitude data for the rocket and acceleration for the cansat.

Planète Science and the CNES lent us an FM radio transmitter called the Kiwi. It has a TX power of 300mW and uses space-reserved frequency bands (137MHz – 139MHz).

In order to be able to transmit digital data, we needed a frequency modulator. At first we developed a modulator built around a DDS-chip, allowing a transmission speed of 480 bytes/sec. Unfortunately, just before the launch, it appeared that the modulator was not working as expected, most likely due to soldering problems of the SMD DDS chip. At the last minute we had to build another modulator using an analog IC FSK modulator: the XR2206.

The reception and demodulation process was done by the CNES. They have an awesome, fully equipped truck with everything we could possibly need for the reception of our telemetry. It has a 3-meter extensible mast with a high-gain antenna, which should normally have received our signal perfectly…

Like every year, we had to develop our own ground station software. The radio link is set up in such a way that it is actually transparent for us at a software level. It is effectively a 9600baud unidirectional serial link. We chose to use Python with wxPython & PySerial to decode the data stream and display it in a simple window. We highly recommend the use of PySerial for working on serial data links, as development was very rapid and the PySerial library quite robust and intuitive.

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1.5.5 Control

This year’s control algorithm was built on top of our previous design. Originally we were using multiple GPS coordinates to derive our velocity (both for direction and speed). The problem with this approach was the fact that our GPS was a 1Hz model; this meant that to have the 3 points necessary to properly derive our velocity we ended up with a measured velocity that was 1-2 seconds behind our actual velocity.

This year we decided to take a new approach: we integrated a 2-axis magnetometer based on a MEMS IC. With this sensor we were able to get our heading in real-time (ground speed being an information that is not very interesting to us), thus allowing us to react much faster to sudden velocity changes, and therefore have a much more stable flight. The GPS receiver was still used for absolute positioning and ideal heading calculations. We also integrated a 3-axis accelerometer (also a MEMS IC) simply to gather additional data. All data was sent back every second over our Kiwi radio link.

The control loop ran on the onboard microcontroller (AtMega 128) and was composed of these 5 major steps:

1. Gather all TWI sensor data (Magnetometer, Accelerometer).

2. Gather GPS data (UART).

3. Calculate ideal heading (based on current GPS coordinate and target GPS coordinate).

4. Calculate angle between ideal and current heading. This is the necessary correction.

5. Apply necessary correction on the servomotors for one second (until next iteration).

This control system has absolutely no flight model, and is a simple closed-loop directional control. We either pull left or right on the parafoil for a set amount of time t (t < 1s), and the next iteration will correct the remaining offset. We have a threshold in place so that if our heading is almost correct, we will not turn.

Aside from the addition of the magnetometer, the control system is identical to last year’s. We considered adding a 2nd level of control that would stabilize an unstable flight, but we are still not decided as to the necessary actions to take to stabilize ourselves, nor are we completely sure of what our stability problems really are.

1.5.6 Recuperation

The cansat, due to its control system, would already transmit its GPS coordinates to us. Recuperation is therefore quite simple, as we already know approximately where it landed.

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1.6 Recuperation system

1.6.1 Ejection MechanismTo not reinvent the wheel, we used our old ejection system with a slight evolution. The previous version used elastics to keep the latch in place. As we had to change the elastics quite often due to wear during our tests, we designed a system with a spring. Otherwise, the system functions identically to the old model.

When the ejection door is latched shut, the system is in the same configuration as the picture. When the parachute or the cansat must be ejected, the motor is turned on, thus turning the cam and lifting the latch.

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1.6.2 Double Parachute System

As is explained in the project goals (p. 3-4), we used a dual parachute this year to reduce the landing shock while still staying within our authorized operating zone.

The first parachute opens just after the cansat is ejected (right after the flight’s apex). The rocket’s airspeed with this first parachute is approximately 15m/s.

To save place in the rocket we had to put the second parachute in the same compartment as the first one. To avoid opening the second parachute inadvertently, we put it in a bag that is kept shut until we wish to deploy it.

You can see a description of our deployment system to the left. The first parachute is attached to a string (the red string in the diagram). When the rocket is at 200m over the ground this string is cut by a system using an electrical heating wire. When the string is cut, the first parachute (which is also attached to the second parachute’s bag) pulls on the bag. When the parachute pulls the bag away, the second parachute comes out and deploys.

As we don’t want to lose the first parachute and the bag. They are attached to the second parachute. With the two parachutes open, the speed of the rocket is around 5m/s.

Note: the system is not exactly identical to the one represented on the picture but it follows the same general principle.

1.6.3 TimerThe timer system is responsible for deploying the parachute and the cansat at the correct points in the flight (the times are based on calculations). The timer is based on an NE555 and can be set with a potentiometer.

A push button switch with a a weighted lever is triggered at liftoff with the force of the acceleration. This switch starts the timer by sending a rising edge to the 555. After a set time, the 555 triggers with a leading edge a flip-flop that closes the door motor's power supply circuit. With the intent of conserving power (in case we take time to locate the rocket, or want to relaunch it without replacing batteries), a second 555 is triggered when the motor starts, and takes care of cutting the power supply to the motor after a couple seconds.

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1.7 Measure in the rocket

1.7.1 AccelerometerA 3-axis MEMS IC accelerometer was installed in the nosecone of the rocket. The intended purpose was to measure the new motor’s acceleration curve and perhaps get some attitude information during the later stages of the flight. The accelerometer was set on a +-20G scale to avoid saturation.

1.7.2 Pressure SensorBefore the GPS era, one of the easiest ways to know the rocket’s altitude was to use a pressure sensor. Knowing the relationship between the pressure and the height – which is a quite linear function at low altitudes – you could determine not only the altitude of your rocket, but also its approximate vertical speed.

Even nowadays there are very few digital pressure sensors. So it’s a great way to use some OP-amps, differential amps, filters and DACs.

The sensor we used (MPX5100) is an absolute pressure sensor that does not need any amplifier, so it was directly connected to the microcontroller DAC.

But due to a lack of time (and importance to the project) it was calibrated at the last second, and we didn’t expect great results.

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1.7.3 Camera

2 small camcorders (FlyCamOne V.2) were taken onboard the rocket and the CANSAT. They’re lightweight camcorders usually used by RC planes, obviously optimized for such a use and consequently able to start recording by themselves after a delay you can define. They store compressed movies on a standard SD card you can easily read using any computer.

Only the cam in the rocket worked ; the CANSATs one stopped recording at the very moment of the launch, probably due to the proximity of the Start/Stop button with the structure.

However the quality of the movie is far from regular camcorders movies quality. This may be caused by the size of the cam, using bad quality optical components. We’d not recommend using such cameras on a rocket flight, it would need suitable cams that can quickly adapt their contrast to the light sources encountered in the sky.

1.8 Integration of the new motor

In order to integrate the motor we use a system with a thrust ring, a carbon tube, a threaded retaining ring and a threaded ring. As we can see the alignment of the motor with the rocket is made with the thrust ring and the threaded ring. In order to make easier the integration of the motor there were strip of foam along the carbon tube. The mass of all the system is under 750g.

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Results and analyses

1.9 Cansat

The cansat was well ejected from the rocket. The first parachute deployed correctly, but due to the shock the parafoil seem to have also deployed itself prematurely as we can see in the following picture:

The MEMS sensors ended up being extremely sensitive to the onboard radio, and were rendered inoperable. With the hope of at least getting some interesting data we chose to remove the flight control algorithm, and created a predetermined set of commands. The reason is that our previous cansat had a completely unstable flight (somersaults, 360s etc.) and we wanted first and foremost to verify that we had at least conquered that problem with our new mechanical structure. We therefore chose to have the cansat fly straight for a dozen seconds, then turn right, then left, then straight again, then repeat. We hoped we could therefore correlate the actions with the GPS coordinates and verify that we were indeed having an effect on the flight path of the cansat.

Even after numerous integration tests and a full flight simulation that was successful, our telemetry system failed catastrophically. We think the cansat’s antenna may have been to blame (small home-made helical ¼ band that couldn’t transmit far), but there is no conclusive evidence as to what happened. We know the system worked up the FSK modulator, so something in the actual radio link or demodulation is to blame. Due to this failure we have absolutely no sensor data from the whole flight, which was a great disappointment for us. We have opted to move to a POTS radio data link for our future projects, since the KIWI demodulation truck’s system is not set up by default to handle multiple frequencies in parallel and the overall size of the KIWI makes integration sometimes difficult.

The experience nevertheless had positive results: we succeeded in stabilizing the cansat’s flight. We also validated the rocket’s ejection mechanism. The rocket’s current design allows us to integrate any cansat with the following constraints: less than 105 x 115 x 300 mm and a mass of less than 1kg.

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First parachute

Parafoil

CANSAT

1.10 Recuperation System

The recuperation system didn’t work because the parachute bay doors didn’t open. However when we found the rocket, the string holding the first parachute in place was nearly cut due to the heat wire.

Since we have not had such a problem in at least 20 years, we aren’t sure of the causes. We know the timer was properly triggered, so we are left with the following hypotheses about the door:

• As we can see on some pictures, the cansat’s door was already open and may have blocked the parachute’s door.

• The timer had some problem to provide sufficient power to the motor. Even with new batteries, the power may not have been sufficient to unlatch the door.

• As the cansat’s door was open in mid flight, there was too much pressure on the parachute bay doors from within the rocket. This, coupled with the potential power problems, may have been enough for the door not to unlatch.

As we can see on the pictures, the bottom half of the rocket resisted quite well to the crash landing, which will allow us to reuse the bottom part to build another rocket.

1.11 Measurements in the Rocket

We actually have no measurement results at all since both the rocket and cansat radio links failed. With our inability to get the onboard memory to function correctly, all measurements are unfortunately lost.

The problem with the radio link seems to be with the CNES radio receivers not receiving a strong enough signal from our transmitters to be able to demodulate it.

Several problems may have occurred:

Most probably, batteries were not powerful enough to supply the radio transmitter, which consumes a lot of power. Indeed, tons of tests were made before the launch, and the batteries – lithium-ion camera cells – were not changed before the flight. Using a transmitter near the receiver lead us to think everything is OK, while the signal couldn’t be received when we were too far away from the receiver.

The cansat antenna, which should normally have been 50cm long, was shortened to a pigtail antenna, making it less efficient. Nevertheless, with a 300mW TX power, this should not have been a real issue.

The CNES radio RX equipment was not prepared for receiving our 2 radio transmitters in parallel. It seems that one of the receivers had a sensitivity problem, which could explain why it didn’t receive the rocket’s radio signal. At least one radio link could have been received if we had chosen

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to discard the other one, but at the time of the launch we didn’t realize this.

Both rocket and cansat were able to be recovered even without any GPS data. Our rocket was recovered by a Japanese team (many thanks to them!) while they were searching for their own rocket, and the cansat was recovered thanks to a triangulation process done by Planète Sciences’ volunteers, and visual data.

1.12 Integration of the New Motor

The system was a success. As we can see on the video the motor is integrated and blocked in the rocket in less than 15s without using any tools.

1.13 Unexpected Result

As we can see on the video the rocket acted like a glider during the descent. The only explanation we can come up with is that the cansat bay’s door was acting like a wing and caused the rocket to glide.

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ConclusionThe launch of Leia was not a total success, however we were able to validate some important aspects of the project:

• The system to eject the cansat laterally from the rocket worked as planned.

• The use of a first parachute in the cansat permitted us to stabilize the cansat’s flight.

• The system designed to contain the new engine worked very well, and was easy to use by the pyrotechnician.

In conclusion, we can say that even if the project did not work entirely as hoped, we have discovered good solutions to recurring problems which will allow the club to concentrate all its effort on the cansat and its control for the 2008-2009 year.

The CLES-FACIL team right before launch.

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