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 parachute deployment systems. The timing can be initiated in a variety of ways such as acceleration detection (G-switch), burnout detection (zero pressure sensor), or deceleration detection (inverted mercury switch). Each of these happens during different periods of a flight. The FC can be used on very simple rockets or more advanced ones.

Each servomotor has two configurable positions. The FC moves the servomotors to their first position when it is turned on, and moves them to their second position based on the configuration of the flight computer parameters.

 

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
• External launch detect / burnout / negative-G trigger input
• Buzzer for indicating status and helping to locate lost rocket in tall bushes
• EEPROM used to store settings while power is turned off
• 15 configurable control parameters
• Altimeter/auxiliary power connector

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

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. Two of the 15 parameters can be configured in this mode.
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 now be discussed in more detail.

Figure 1 - Operational Mode - Flow Diagram

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.

Display shows:

• Set D1_Delay (“2.”) – In this phase the user can change the D1_delay parameter by repeatedly pressing the PGM button. Any changes made are automatically stored in the EEPROM. See the Configure Mode section for explanation of the D1_Delay parameter.

Display alternates between:  <-- -->

Pressing the ARM button moves to the next phase.

• Set D2_Delay (“5.”) – In this phase the user can change the D2 Delay parameter by repeatedly pressing the PGM button. Any changes made are automatically stored in the EEPROM. See the Configure Mode section for explanation of the D2 Delay parameter.

Display alternates between: <-- -->

Pressing the ARM button again 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 Delay and D2 Delay parameters. This allows for quick visual and audible check to see if the rocket is ready for launch.

Display alternates between: <-- -->
A trigger will cause the rocket to go to the next 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. When this period expires the FC goes to the next phase.

Display shows:

• 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. This is the time that a parachute or the next stage is released.

Display shows:

At the completion of the servo repositioning the FC moves to 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.

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 are run flat.

Display shows:

 

Flight Profiles

Following are 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
• Single stage with burnout detect
• Two stage – Staging and parachute with launch detect
• Two stage – Staging and parachute with burnout detect
• Dual Parachute with launch detect
• Dual Parachute with burnout detect

 

Configure Mode

In the configure mode the user can cycle through all the parameters and change their values. All values are automatically 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 value belonging to the parameter does not have the decimal point. 

Since the values can range 0 to 31, and there is only a single 7 segment display, 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 switch is set to OFF.
2. Hold down only the ARM button and set the power switch to ON.
   The “S” symbol appears on the screen 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 switch to OFF.
    

Parameterer	Mnemonic	Default	Range	Display	Description
D1 Multiplier	D1Mul	4	[0 – 7]		0 = 0.01 sec / step 1 = 0.02 sec / step 2 = 0.05 sec / step 3 = 0.1 sec / step 4 = 0.2 sec / step 5 = 0.5 sec / step 6 = 1.0 sec / step 7 = 2.0 sec / step
D1 Offset	D1Off	F	[0– V]		D1 = (D1Dly + D1Off) * D1Mul
D1 Delay	D1Dly	0	[0– V]		D1 = (D1Dly + D1Off) * D1Mul
D2 Multiplier	D2Mul	4	[0– 7]		0 = 0.01 sec / step 1 = 0.02 sec / step 2 = 0.05 sec / step 3 = 0.1 sec / step 4 = 0.2 sec / step 5 = 0.5 sec / step 6 = 1.0 sec / step 7 = 2.0 sec / step
D2 Offset	D2Off	0	[0– V]		D2 = (D2Dly + D2Off) * D2Mul
D2 Delay	D2Dly	0	[0– V]		D2 = (D2Dly + D2Off) * D2Mul
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]		0.25 sec / step
Lost Off Time	LOff	4	[0– V]		0.25 sec / step
Lost Delay	LDly	5 (5min)	[0– V]		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. The first of these parameters is the multiplier. The multiplier defines the “granularity” of the timing of the other two parameters. The multiplier can be set to increment the delays in steps as low as 0.01s or as high as 2s. This allows the timing delays to be set anywhere from 0.01seconds to 128 seconds depending on the requirement. Both D1 and D2 are configured in the same way. 

The formula below gives the delay period:

Dx = (DxDelay + DxOffset) * DxMultiplier

Where x = 1 for delay D1 and x = 2 for delay D2.

The second parameter is the Offset. The Offset allows you to set a zero point, or minimum time for the delay. The last parameter is the variable delay (DxDelay). This delay can be varied while in the normal mode and is intended to be adjusted between launches to finetune the recovery system release time or if the pressure, water volume or nozzle size change. Both the offset and delay can be set to one of 32 values.

Short delays are useful for staging rockets, and long delays are useful for opening main parachutes late into the flight.  

Timing Periods

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

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 po