Skypod was a project of SUNSET, Strathclyde University Near Space Exploration Teams.

Our aim is to develop an intelligent payload, the skyPod v1.0 (to be attached to a weather balloon) which is capable of reaching altitudes up to the stratosphere, while recording environmental data to be logged on board and to be transmitted back to a station on the ground. The payload will record:

• Atmospheric Pressure
• Internal Temperature
• External Temperature
• GPS Location
• Acceleration in 3 axes
• Humidity
• HD images of the earth

Some of the intelligent features include:

• Free-fall detection
• Perimeter detection
• Emergency cut-down
• Landing detection
• Battery level monitor
• Power-save mode

The intention is for the balloon to carry the payload up to a height at which atmospheric pressure causes the balloon to expand and burst. The payload is brought back down to earth safely with aid of a parachute and the payloads dual-tracking system increases chance of recovery, allowing data (which has not been transmitted back) to be extracted and processed for examination and analysis.

Space based equipment:

• Radiometrix NTX2-434.650-10 - Transmitting on 434.65MHZ at 10mW
• Arduino Nano 3.0
• Argent Data Systems GPS
• Sparkfun Electronics Openlog and a 32 GB SDHC Class 10 Card
• Analog Devices - Triple Axis Accelerometer Breakout ADXL345
• Dallas Temperature Sensors - 2 x DS18S20
• Kodak Playsport ZX3
• Rocketstore - 45“ Octagonal nylon parachute
• Honeywell - Humidity Sensor HIH-4030 Breakout
• Bosch - Digital Barometric Pressure Sensor BMP085
• Energizer Lithium AA - x14

Ground Segment:

For all the SUNSET launches, there are two ground stations. One is a mobile unit, one is a satellite tracking station based at the University of Strathclyde, called STAC.

The primary tracking system for HABs at STAC consists of:
* 2 x M2 436CP30 yagi antennas, Vertically stacked, giving an overall gain of 22dBi
* SSB Electronics masthead preamplifiers
* ICom IC-910H
* 2 x M2 RC2800PX Rotators for Azimuth and Elevation control
* Dell PC loaded with the relevant UKHAS software of choice

• Yaesu FT-817
• 12 Element homebrew 'Portable' Yagi antenna
• Laptop running the UKHAS special

An array of sensors was constructed to measure the following:

• Position and Altitude (GPS)
• External and Internal Temperature
• Pressure
• Relative Humidity
• 3-axis Acceleration
• Battery Voltage

A meteorological balloon was sourced to carry the payload to high altitude. Such balloons were only available from a limited number of manufacturers. An established one being the Japanese based company Totex Corporation (1). They produce a popular range of balloons with detailed documentation freely available online. This data was used as the basis of the calculations in the following sections.

The following parameters were important in balloon selection:

• Burst altitude : Determined by volume of gas and burst diameter
• Ascent rate : Higher ascent rate causes the balloon to drift less and therefore increases the possibility of a safe landing, and allows shorter flight time requiring shorter battery life.

These are inter-related and conflicting parameters, which are determined by the gas volume used and burst diameter of the balloon selected. For any one balloon a greater volume of gas results in a greater ascent rate but the balloon will burst at a lower altitude. These parameters, and others, are summarised can be seen online at http://ukhas.org.uk/guides:balloon_data

Some of the terms from this online table require further explanation:

• Payload Mass : The total mass of the payload and parachute.
• Gross Lift : The lift generated by the volume of gas in the balloon
• Nozzle Lift : The Gross Lift minus the Average Weight of the balloon.
• Recommended Free Lift : This is the difference between the Nozzle Lift and the Payload weight, the net force experienced by the payload.

From analysis of the data from the table, the KCI 1200 was selected as the balloon of choice. This balloon offered the best compromise of ascent rate, burst altitude and cost.

For a maximum payload mass of 1.65 kg and a burst altitude of 29 km, the data shown in Table 1 was calculated using a pre-existing spreadsheet which can be found online at http://ukhas.org.uk/_media/guides:burst3.xls.

The balloon was sourced from a UK supplier of Totex sounding balloons, Random Engineering, at a cost of £70.25.

A 9.01m3 canister of Helium gas was sourced from BOC for £91.10. BOC were able to offer a significant discount through the University and thus no alternative suppliers were considered. Welding gas was selected as it was cheaper than party balloon gas and was of a higher purity (99.9%).

To increase the chances of recovering the payload a ‘cut-down’ mechanism was built in to the system. The purpose of the cut-down mechanism was to detach the payload from the balloon by cutting the balloon cord if the payload were to drift into an area which may cause it to land in an inaccessible location (e.g. in a body of water, the middle of a busy city or a mountainous area).

As the payloads live location was being continuously monitored, the cut-down mechanism could have been deployed upon receiving an actuating signal from the ground. However, as it could not be guaranteed that transmission would be successful it was decided to make the payload fully autonomous. Thus, an acceptable perimeter was pre-set in the flight software prior to launch and the mechanism would actuate if the payload crossed the pre-set boundary.

There were two practical solutions to detaching the balloon from the payload:

• Mechanical Actuator : A motor or solenoid to rotate a blade or pull a pin.
• Hot Wire System : A current passed through nichrome wire heating it to high temperature to cut through the rope.

Once the advantages and disadvantages were identified for each, it became clear that the hot wire system would provide a more effective and robust solution as no advantages were identified for the mechanical actuator over the hot wire system.

The location of the cut-down mechanism required some consideration, since its location would affect its performance. Figure 1 shows different arrangements considered.

Cutting the balloon cord inside the payload with the parachute attached to the balloon cord would result in the parachute being detached from the payload, meaning it would be left to fall back to Earth with no parachute.

Attaching the parachute directly to the payload instead of the balloon cord would have a number of disadvantages: · The parachute could become tangled in the payload and fail to open when the balloon is detached. · The hanging parachute could obstruct the camera ports. · While ascending the parachute could open and act against the lift of the balloon.

To prevent the parachute flapping around the payload and obstructing the cameras it can be held further up the balloon cord by feeding the balloon cord through a hole in the parachute canopy and tying a knot to prevent it falling down, as illustrated in Figure 1. However, the lower knot would prevent the balloon chord slipping through the spacer and the hole in the parachute canopy and so the whole system would still be carried upwards by the balloon.

Cutting the balloon cord above the parachute and below the balloon would allow the balloon to be completely detached and the payload to descend with the aid of the parachute. It would, however, require copper wires to run up the balloon cord from the payload to a section of nichrome coiled around the balloon cord.

The next image shows the consequences of mounting the cut-down at each location. As a result of this analysis, location 3 was selected.

The hot wire cut-down mechanism operated by passing a current through a section of nichrome wire coiled around the nylon balloon cord. The nichrome wire was heated to a very high temperature, allowing it to melt the cord. The current was delivered to the small section of nichrome from the payload using normal copper wires which ran up the balloon cord.

Extra care had to be taken when coiling the nichrome around the balloon cord as the nichrome coils needed to be close enough to each other to successfully cut one section of the nylon cord, but not too close that they touched and created a short circuit preventing the nichrome from heating up and cutting the cord.

The nichrome wire used had a 0.1 mm diameter (38 AWG) and had a resistance of 1 Ω/cm.

The system was tested using a 5 cm section of nichrome with a 5 V supply and then with two 1.5 V batteries.

The 3 V solution offered a sufficiently short cut-down time, when compared to the 5 V solution. It was decided to power the cut-down mechanism with the 3 V ancillary power supply as described in power source section. It was decided to keep this separate from the main power supply due to its high current draw (0.55A).

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