Compared to commercial electronics, avionics systems must endure a harsh physical and electrical environment while meeting a higher standard of reliability. The temperature and vibration limits of the avionics environment are similar to those of industrially rated systems, and like many industrial and medical systems, avioncs systems have a high cost of failure. Avionics failures can become life threatening. In particular, failure of the PSAS avionics system could cause the rocket to behave in unsafe ways, endangering the lives of PSAS members and bystanders at the launch site.
Temperature constraints
The avionics package will be subject to a wide temperature range. During normal transport, storage, and development, temperature will vary over the normal range for commercial electronic equipment, that being 0 to +50 degrees C. However when the avionics package is in operation, the temperature will vary over a wider range. Military equipment is typically specified for -50 to +100 degrees C. This range would be desirable for the avionics package.
We have recorded various temperature extremes in previous LV avionics:
- Approximately -5 degrees C while the avionics rests on the launch pad for 4 hours or more on a very cold desert morning in winter;
- Approximately +40 degrees C while the avionics rests on the launch pad out in the sun on a clear day.
Other factors that can contribute to temperature variations are internal heating of the avionics package due to RF power amplifier heat dissipation, and external heating of the space-frame due to atmospheric friction which is conducted into the interior.
It should be noted that previous LV avionics packages had no active thermal management systems. All thermal control was provided through the use of heat spreaders that warmed up the entire avionics module. Forced air cooling was not used; ambient air heat sinks or heat pipes were not used. These mechanisms for thermal dissipation should be reused in future launch vehicles.
Vibration and acceleration
Previous launch vehicles(LV) flights had been designed with the understanding that the airframe will be subjected to accelerations up to 20 g during the boost phase of flight. This value was determined through simulation by the PSAS mechanical engineering and propulsion teams based on the allowed time duration of boost. 20 g is a maximum static continuous acceleration. It is possible to determine a typical vibration level that this system must withstand using accelerometer measurements recorded during previous LV flights from the on-board IMU.
The vibration characteristics of future LV2 flights (LV2 is the current launch vehicle revision), are expected to be similar since the same airframe and propulsion system will be used in this LV family. The revised avionics system will use simple passive vibration isolation and have minimal additional armoring for survivability.
Standard construction techniques include:
- G10 or FR4 PCB materials at around 1.6mm thick
- mounting non-surfacemount components axially, and additionally supporting them with beads of RTV material
- lacing connecting wires using plastic zip-ties at approximately 50mm or less
- providing strain relief on connectors with head shrink tubing
- consitent fastener retention by threadlocking, washers, or other means as required
Accessibility and usage by developers
Even though the primary design goals are set by the extreme requirements of flight, this circuit must work well in the hands of its developers on the workbench where it will spend 99% of it’s operating life. Because of this, the circuit must be durable enough to withstand the following:
- frequent connecting and disconnecting from the daisy chain without adverse effect to connectors or wires
- transport and handling with moderate care in static resistant bags
- repeated power up and down
- unexpected power shutdown without any shutdown procedure being run
- possible reverse power connection
- possible misconnection of power and communications on the daisy chain
- survive short duration overvoltages, for example 125% for 3 seconds.
The nodes should be tolerant of these conditions, and should not adversely effect the development systems (often laptops) connected to the nodes when these conditions are present. (For example, cross-connecting power and communications should not damage the serial port of the developers laptop.)
Choice of materials to meet environmental conditions
It is completely reasonable to require the materials used for construction of nodes to be avaliable commercially off the shelf (COTS). There should be no need for rare, hard to find materials.
High Radiation Environments
It is outside the scope of the LV2 family to consider the effects of high radiation environments, however for future LV projects this may become necessary. For this reason, it seems important to keep the idea of hardening to high radiation levels, at least those experienced in low earth orbit (LEO) for 14 day missions, somewhere on the design horizion.