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WATER ROCKET - FLIGHT COMPUTER
This section describes the details of the water rocket flight computer (FC) as designed and built by the Air Command team.

The purpose of the FC is to co-ordinate various events during the flight of a water rocket. One of its responsibilities is to deploy the recovery system at the desired time.

Version 1.6
Overview
Features
Changes from V1.5
BUY NOW
Operation
Normal Mode
Configure Mode
Functionality
Timing
Timing Periods
Motor Position
Lost Alarm
Triggering
Examples
Scenario 1
Scenario 2
Scenario 3
Factory Reset
Hardware
Circuit Diagram
Component List
Component Overlay
Servo Motors
Power
Power Consumption
Notes

V1.6 Overview           

This flight computer (FC) was designed primarily to provide timing for parachute deployment and staging of multistage water rockets. The FC can drive one or two standard RC servomotors that are used to release latches on recovery systems. The timing can be initiated in a variety of ways such as an acceleration switches, pressure switches or deceleration detection. The FC can be used on both simple or more advanced rockets.

Water Rocket Flight Computer V1.6

Features

The FC has the following main features:

  • Dual RC servo motor control.
  • 7segment LED display indicating status information.
  • Built in launch detect G-switch (optional).
  • External launch detect / burnout / negative-G trigger input
  • Buzzer for indicating status and helping to locate lost rockets
  • EEPROM used to store settings while power is turned off
  • 15 configurable control parameters

These instructions give an example of how to use the flight computer in a parachute deployment mechanism.

Changes from V1.5

  • The altimeter power connector has been removed. The altimeter can still be powered via the second servo connector. If both servos and altimeter are used then the second servo can be connected to the altimeter's pass through connector to connect the second servo motor.
  • The PCB is now double sided resulting in less weight and 25% smaller area. The PCB is also only 27mm wide allowing it to be used inside T-8 and T-12 FTC tubing.
  • The timing parameters have been changed to represent seconds directly rather than the more confusing offset/multiplier technique used in V1.5. Although the direct setting reduces the range of values possible, they should cover the vast majority of situations as used in the real world. The timing for both D1 and D2 phases can now be set between 0.1 and 99.9 seconds in 0.1 second increments.
  • The setting of the two delay parameters in the Normal mode have been removed and the FC now simply requires one press of the ARM button to ARM it. This was done to make it simpler for the rocketeer to use in the field, and prevent accidental timing changes.
  • The servo control pulse width range has been extended to allow driving some non-standard servos their full range.

Buy It Now

V1.6 FC has been sold out

The Assemtech G-switch data sheet is available here:
http://docs-asia.electrocomponents.com/webdocs/0027/0900766b80027e0b.pdf

Disclaimer

In no respect shall Air Command Water Rockets incur any liability for any damages, including, but limited to, direct, indirect, special, or consequential damages arising out of, resulting from, or any way connected to the use of the item, whether or not based upon warranty, contract, tort, or otherwise; whether or not injury was sustained by persons or property or otherwise; and whether or not loss was sustained from, or arose out of, the results of, the item, or any services that may be provided by Air Command Water Rockets.

While we try to ensure the quality of the flight computer, we cannot guarantee a rocket's safe return to Earth since it is fitted in recovery systems beyond our control. This product should always be considered experimental.


Operation

The flight computer can be switched into one of three different modes:

  1. Normal - This is the normal mode for pre-launch and in-flight operation.
  2. Configure – In this mode all 15 parameters can be set to specific values.
  3. Factory Reset – In this mode the configurable parameters are reset to their default factory settings.

Figure 1 shows the flow diagram for the three modes. Each mode will be discussed in more detail below.

Figure 1 - Operational Mode - Flow Diagram

Video describing the operation of the FC.

Normal Mode

To enter normal mode, simply turn the FC on. Before delving deeper into how to configure all the FC parameters let’s have a look at a typical flight profile (Figure 2) and see how it behaves. The flight profile is broken up into a number of different phases. The 15 configuration parameters affect what happens in each of these phases. 

Figure 2 - Typical Flight Profile Operational Phases

Operational Phases
 

  • Initialize – The FC sets both servomotors to their preset positions (Position 1) and reloads all the parameters to the previously stored values.

Initialization sequence display after power on:

1. Setting M1 to default position 1

2. Setting M2 to default position 1   

3. Standby ready to be armed      

Pressing the ARM button arms the rocket.

  • Armed – The FC waits for the rocket to launch. Typically one would arm the rocket first and then pressurise it. While in this phase the rocket beeps once a second and toggles the display showing the values set for the D1 Tens Delay and D1 Units Delay parameters. This allows for quick visual and audible check to see if the rocket is ready for launch.

Example: Display alternates between: <-- -->

The number displayed when the tone is heard represents the Tens, and the number shown when there is no tone represents the units.

A launch trigger (such as from a G-switch) will cause the rocket to enter the D1 phase.

  • D1 – Typically the first delay (D1) is activated at the time of launch. D1 usually operates in the ascent stage of the rocket flight.

Display shows:

When this period expires the FC goes to the next phase.

  • M1 On Time – At the end of D1 servomotor M1 moves from position 1 to position 2. The amount of time it spends doing that is set through the M1 On Time parameter. The servo remains in this position until power is turned off and turned back on again. This is the time that a parachute or the next stage is released.

Display shows:

At the completion of the servo repositioning the FC enters the D2 phase. 

  • D2 – The FC waits a second period of time before the second servomotor is activated.

Display shows:

At the conclusion of the D2 delay the FC switches to the next phase.

  • M2 On Time – Servo M2 moves from position 1 to position 2. The amount of time it spends doing that is set through the M2 On Time parameter. The servo remains in this position until power is turned off  and turned back on again.

Display shows:

When the motor finishes moving the FC switches to the lost delay phase.

  • Lost Delay – If enabled, the FC waits for another period of time before starting the lost rocket alarm. Typically the rocket is found by the time the alarm sounds and power is turned off. The period can be set from 0 minutes to 31 minutes.

Display shows:

  • Lost Alarm – When the Lost Delay period expires, the alarm starts sounding and continues indefinitely until either power is turned off or the batteries run flat.

Display shows:

 

Flight Profiles

Following are examples of a few different flight profiles that can be configured with the FC. Please refer to the examples section for more details on these: 

  • Single stage with launch detect or burnout detect
  • Two stage – Staging and main parachute with launch detect or burnout detect
  • Dual Parachute (Drogue/Main) with launch detect or burnout detect
     

Configure Mode

In the configure mode the user can cycle through all parameters and change their values. All values are automatically and permanently stored in the EEPROM. Table 1 describes each of the parameters and the range of values that can be set for each. Table 2 describes the values themselves. To avoid confusion a parameter name on the display is designated with a “.” (decimal point) and the parameter's value does not have the decimal point. 

Since the values can range from 0 to 31, alpha-numeric characters (0 – 9, and A – V) are used to display all values.

To enter configure mode and change the parameters do the following:

  1. Make sure power is OFF.
  2. Hold down only the ARM button and turn the power ON.
    The “S”
    symbol appears on the display indicating you are in “Set” or configure mode
  3. Press the ARM button repeatedly to cycle through the parameters.
  4. Press the PGM button repeatedly to cycle through the parameter values.
  5. When finished changing the parameters switch the power OFF.
     

Parameterer

Mnemonic

Default

Range

Display

Description

D1 Tens

D1Tens

0

[0 – 9]

Represents the number of 10's of seconds to wait in the D1 delay.

D1 Units

D1Units

3

[0– 9]

Represents the number of seconds to wait in the D1 delay.

D1 Tenths

D1Tenths

5

[0– 9]

Represents the number of 0.1 seconds to wait in the D1 delay.

D2 Tens

D2Tens

0

[0– 9]

Represents the number of 10's of seconds to wait in the D2 delay.

D2 Units

D2Units

5

[0– 9]

Represents the number of seconds to wait in the D2 delay.

D2 Tenths

D2Tenths

0

[0– 9]

Represents the number of 0.1 seconds to wait in the D2 delay.

M1 Position 1

M1P1

0

[0– V]

Motor 1 Position 1. (prior to launch)

M1 Position 2

M1P2

V

[0– V]

Motor 1 Position 2 (after delay D1)

M1 On Time

M1On

F

(2.4 sec)

[0– V]

0.16 sec / step

M2 Position 1

M2P1

0

[0– V]

Motor 2 Position 1 (prior to launch)

M2 Position 2

M2P2

V

[0– V]

Motor 2 Position 2 (after delay D2)

M2 On Time

M2On

F

(2.4 sec)

[0– V]

0.16 sec / step

Lost On Time

LOn

1

[0– V]

How long the buzzer sounds
0.25 sec / step

Lost Off Time

LOff

4

[0– V]

How long the buzzer is silent
0.25 sec / step

Lost Delay

LDly

5

(5min)

[0– V]

How long before the alarm sounds
minute / step

 

Table 1 - Configurable Parameters
 

Value

Index

Displayed as

 

Value

Index

Displayed as

 

Value

Index

Displayed as

0

0

 

B

11

 

M

22

1

1

 

C

12

 

N

23

2

2

 

D

13

 

O

24

3

3

 

E

14

 

P

25

4

4

 

F

15

 

Q

26

5

5

 

G

16

 

R

27

6

6

 

H

17

 

S

28

7

7

 

I

18

 

T

29

8

8

 

J

19

 

U

30

9

9

 

K

20

 

V

31

A

10

 

L

21

 

 

 

 

Table 2 - Parameter Values

Functionality

Timing

The two main timing delays D1 and D2 are configured through 3 parameters each. Both D1 and D2 are configured in the same way. 

Timing Periods

Table 3 lists the minimum and maximum values that each specific phase can be configured to.

Delay

Min

Max

D1

0.1 sec

99.9 seconds

M1On

0.16 sec

5.12 seconds

D2

0.1 sec

99.9 seconds

M2On

0.16 sec

5.12 seconds

Lost Delay

0 minutes

32 minutes

Table 3 - Delay Ranges

Motor Position

Each of the servomotors has two configurable positions Position 1 and Position 2. Position 1 is the position prior to launch. This would typically be the latched position of the recovery system. Position 2 is the position of the servomotor after the expiry of the appropriate period. The motors remain at their Position 2 position until power is turned off.  

RC servomotors are positioned using a specific pulse train on their control line. The FC generates this pulse train for only a short period of time determined by the M1 On Time or M2 On Time parameters. This allows battery power to be conserved when the motors are not required to move. The On Time should be adjusted in such a way that the motor has enough time to move from one position to the other. Sometimes this needs to be adjusted depending on the motor used or if there is a load on the motor and it takes longer.  

The full range of movement of each servo is divided into 32 steps. This means a servomotor that normally has a 90 degree range of movement will be able to be positioned with an accuracy of 2.8 degrees, while a servo that has a 200 degree range of movement can be positioned with an accuracy of 6.25 degrees. 

The positions are made configurable to allow the servomotors to be mounted inside the deployment system in any orientation. Sometimes clockwise operation is needed and sometime anti-clockwise is required.  

The positions can also be adjusted between flights in the Configure Mode if something becomes misaligned or stretched in the deployment system and the servomotor positions need to be updated to compensate.  

HINT:

When configuring the motor positions, set the M1 or M2 On Time delays to something like "6" this will allow you to reposition the servo motors faster as you cycle through their positions. Don't forget to go back and set the appropriate time delay when you are finished.

Lost Alarm

The FC can use the built in buzzer to sound an alarm after a delay to assist the rocketeer to locate the rocket if it is lost in tall grass or hanging in a tree.  

There are 3 configuration settings associated with the alarm. The first is the number of minutes it takes before the alarm is activated (Lost Delay). This is useful for two reasons. Firstly it allows the FC to conserve the battery since in most instances the rocket will be found before the alarm needs to sound. Secondly it allows the FC to remain quiet in flight when used in combination with a video camera that is also recording audio. 

The other two parameters are used to configure the sound duration during the alarm phase. Setting the Lost On Time to a short beep and the Lost Off Time to a long period allows the FC to conserve power, but for more noisy environments the alarm can be set to produce noise more frequently at the expense of power consumption. 

Setting the All three parameters to zero allows the lost alarm functionality to be switched off altogether.  

Triggering

Most of the fun only happens after the FC is triggered from the Armed phase. Triggering can be achieved a number of different ways depending on the rocket design. Some examples include:

  1. Launch detect – This is typically achieved using a G or acceleration switch. Triggering occurs as soon as the rocket leaves the launch pad. Another variation to this is a set of contacts that are closed and an insulator is removed from between them during launch. The insulator is usually attached so it stays with the launcher.
     
  2. Burnout detect - This technique uses a pressure switch to detect when the pressure inside the rocket has reached atmospheric pressure, or some preset value. This is useful for staging rockets. This allows the rocket to deploy the second stage at the correct time regardless of how much pressure or water was used in the booster.
     
  3. Negative G detect – This is another technique used to detect when the rocket has stopped producing thrust and the rocket starts to slow down. An inverted mercury switch can be used here where the mercury floats upwards to make contact as drag continues to slow the rocket down after burnout. Negative Gs happen shortly after burnout.

We’ll leave it up to the rocket builder to come up with their own way to trigger the FC. The FC can have a G-switch fitted directly to the PCB, but the FC also provides a connector for external triggers inputs.

Examples

Following are a number of examples demonstrating how to configure the FC for various scenarios.

Scenario 1 – Single Parachute:

You want to use a single parachute on a simple rocket. You want the parachute to deploy at 3.5 seconds.You also don’t want to use the lost alarm functionality. You will also be using the built in G-switch.

Figure 3 - Simple rocket with single parachute scenario

This is how you would configure the computer:

Enter the Configure mode by holding the ARM button down while turning the power ON. Use the ARM button to advance through the parameters, and the PGM button to advance through the values.

Motor M1 is set for full range movement by setting “6.” to "0" and “7.” to "V". If the servo was to move in the other direction the two values would be reversed. The servomotor will be on for 2.4 seconds. This is set by parameter “8.” (value = 2.4 / 0.16 = 15 = F)

We set the “C. D. and E.” parameters to 0 to turn off the lost alarm.

 

D1

D2

M1

D1Tens

D1Units

D1Tenths

D2Tens

D2Units

D2Tenths

M1P1

M1P2

M1On

Param

Value

00 sec

3 sec

0.5 sec

 

0

32

2.4sec

 

 

M2

Lost

 

M2P1

M2P2

M2On

LOn

LOff

LDly

 

Param

Value

When finished setting the parameters, turn the power OFF.

Now if the rocket will fly with the same setup on every launch, all you have to do for each launch is turn on the FC and press the ARM button and the rocket is ready to go. This makes it very simple to use once configured.

Scenario 2 – Rocket with twin parachutes:

Say you have a more advance rocket that will deploy a drogue parachute at apogee and then the main parachute some time later when it has had a chance to descend some way. This technique is often employed to prevent large parachutes from being opened at high speeds as well as to stop the rocket drifting too far in windy conditions.

You also want to use the lost alarm functionality as you are flying in wooded terrain.

A simulation of the rocket predicts that it will reach apogee after 7.25 seconds.

Using drogue parachute descent rate calculations you decide that the main parachute should deploy 22 seconds after apogee.

This is how you would configure the computer:

Set D1 and D2

Since the timing resolution is only in 0.1 second increments we set the first delay to say 7.3 seconds. D2 will be set to 22 seconds. (see table below)

Set M1 and M2

Because of the deployment system configuration we need to set the M1 servomotor to turn clockwise and the M2 servomotor to turn anticlockwise when activated. We set the positions as appropriate for the deployment system. In this example they are just set to full range. The On Time for both servos only requires ~1 second. See the values in the table below. Because the Motor On Time is set in increments of 0.16 seconds, we find that the closest value to 1 second is 0.96 seconds =  0.96/0.16 = 6. So we set the value to "6" for parameters "8." and "b.".

Set Lost Alarm

We choose to set the alarm for 5 minutes after main parachute deployment, and we want the sound to beep for 1 second and stay quiet for 3 seconds. Parameter “E.” sets the delay in minutes, so we set it to 5. The sound on time and off time are given in 0.25second increments so we set “C.” to 4 (1sec/0.25 = 4) and the “d.” parameter to C (3sec/0.25 = 12 = C)

 

D1

D2

M1

D1Tens

D1Units

D1Tenths

D2Tens

D2Units

D2Tenths

M1P1

M1P2

M1On

Param

Value

00 sec

7 sec

0.3 sec

20 sec

2 sec

0.0 sec

0

32

.96sec

 

 

M2

Lost

 

M2P1

M2P2

M2On

LOn

LOff

Ldly

 

Param

Value

32

0

.96

1 sec

3 sec

5 min

When finished setting the parameters, turn the power OFF.

Now if the rocket will fly with the same setup on every launch, all you have to do for each launch is turn on the FC and press the ARM button and the rocket is ready to go.

Scenario 3 – 2 stage rocket with parachute:

In this scenario the FC is fitted to the booster of a 2-stage rocket and will be used for initiating the staging of the next stage and some time later for opening a parachute on the booster. The ideal time to release an upper stage is when the booster is travelling at its fastest which happens right around burnout. It is also the worst time to deploy a parachute since it could be ripped off.

The external trigger this time is based on a pressure switch that activates when the pressure inside the rocket drops to 10psi above atmospheric pressure. This means that a short time later the rocket will be travelling the fastest as the pressure drops to 0.

This is how you would configure the FC:

Let’s assume that the time between the sensing of the pressure drop to the time that the second stage should be released is only 0.2s. We need to set the D1 delay to 0.2 seconds. This means parameters "0." and "1." are set to "0" and parameter "2." is set to "2".

M1 and M2 positions and Motor On Times are configured as in the previous example. And the lost alarm functionality is turned off.

In this example we set the delay to parachute opening to be 2 seconds after staging. We set the "3." parameter to "0" and the "4." parameter to "2". The "5." parameter is set to "0".

 

D1

D2

M1

D1Tens

D1Units

D1Tenths

D2Tens

D2Units

D2Tenths

M1P1

M1P2

M1On

Param

Value

00 sec

0 sec

0.2 sec

00 sec

2 sec

0.0 sec

0.96sec

 

 

M2

Lost

 

M2P1

M2P2

M2On

LOn

LOff

LDly

 

Param

Value

0.96 sec

When finished setting the parameters, turn the power OFF.

Now if the rocket will fly with the same setup on every launch, all you have to do for each launch is turn on the FC and press the ARM button and the rocket is ready to go.

Factory Reset

To reset the configurable parameters back to the default factory settings do the following:

  1. Make sure the power is OFF.
  2. Hold the PGM and ARM buttons down at the same time and turn the power ON.
  3. The ‘r’ character will be displayed. Followed by the decimal point.
  4. Press the ARM button once more to confirm the reset. And the decimal point will disappear.
  5. After the beep, turn the power OFF.

The default factory settings are set up for a typical single stage 2L water rocket.

Hardware

Circuit Diagram

The circuit diagram for version 1.6 is shown in Figure 4. Central to the design is the PIC16F628A microcontroller from Microchip. The on-board oscillator is used to reduce the external component count.

Figure 4 - Circuit Diagram

Component List

Component

Description

Quantity

IC1

PIC16F628A Microcontroller

1

DIS1

7 Segment common cathode LED display

1

IC2

7805 voltage regulator

1

Q1

BC548 NPN small signal transistor

1

BZ1

Mini piezo buzzer

1

R1-R8

330 Ohm Ό watt resistor

8

R9

1K Ohm Ό watt resistor

1

R10, R11

10K Ohm Ό watt resistor

2

R12

100K Ohm Ό watt resistor

1

C1, C2

0.1uF ceramic capacitor

2

S1

Power switch (slide)

1

S2, S4

SPST momentary push button

2

S3

G-switch Assemtech ASL2 (optional)

1

J1, JP

3 pin connector

2

J3

2 pin connector

1

Table 4 - Component List

The Assemtech G-switch data sheet is available here:

http://docs-asia.electrocomponents.com/webdocs/0027/0900766b80027e0b.pdf

Connections and Component Overlay

Figure 5 - Component Overlay and External Connections

Servo Motors

Standard and micro RC servomotors can be used with the FC and are available from most hobby stores. E-bay also has sets of servo motors at reasonable prices. The typical weight of these is anywhere from 3g to 15g, although larger ones can be used but you must ensure that the battery can deliver the higher currents.

Power

The FC can be powered by a 6V or 9V battery. When a 6V battery is used the voltage regulator needs to be replaced with a power diode such as a 1N4001. This is because the voltage regulator requires at least 7.5V to operate properly. The diode drops the voltage about 0.7V giving roughly 5.3V.

Power Consumption

The flight computer consumes anywhere from ~20mA to ~150mA depending on the number of LEDs switched on, and whether it is producing sound or driving servo motors.

Notes

  1. NOTE: If using the on-board G-switch, you need to ensure that the PCB is oriented vertically and the G-switch is at the bottom. If the PCB is not oriented this way, the flight computer may not trigger.
  2. You need to factor in that the deployment time is not the same as the parachute open time. It may take one or two seconds before the parachute is fully open, so if you are aiming for opening your parachute right at apogee, you need to start deploying it a little earlier. How much time it takes to fully open the parachute depends on the design of the deployment system, the parachute type and the parachute packing technique.
  3. Estimates for timing of events is easiest done with a water rocket simulator that can predict the path of the rocket.
  4. Adjustments to the timing can be easily achieved on subsequent flights.
  5. One does not necessarily need to use the servo motors to activate recovery systems. The outputs can be used to control camera activation or other events on board the rocket.

 



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