LaunchPSU Final Report

June 15, 2004

Prof. Mark Weislogel

Student Team:

Jim Cloer

Ryan Jenson

Dave Roper

Joshua Hatch, AC7ZX

1. Project Overview

Project Description

Yahooo! Launch PSU!

Four PSU undergraduate students in Mechanical Engineering designed and built a balloon launch system to carry an 8lb proprietary payload to over 100,000ft making pressure, temperature, elevation, relative humidity, imaging and tracking measurements all the way. They had 8 weeks to select, procure, design and construct the system which launched the morning following a lightning storm near Millican OR, 22 miles east of Bend, May 21, 2004. I (the course instructor) must brag, the launch was a total success. Funny, heartbreaking, exciting. It had it all, and all for college credit (prospective students think ME406!). The balloon reached over 116,700ft within 2 hours (That’s 22 miles straight up and over 99.5% out of the atmosphere). We lost GPS contact with it over 60,000ft because of the government blackout, but had it within eyeshot when the balloon burst. Good thing because the parachute must have ripped away from the payload which fell to Earth within 8 minutes! It almost reach 1000mph before decelerating to terminal velocity. The payloads hit the soft pumice soil of eastern Oregon ground at about 80-90mph! Hard hit, but not enough to prevent the equipment (i.e. broken and grounded antenna) from transmitting the landing position, and not hard enough to destroy the proprietary payload. Incredible. This is still hard to believe.

The launch video from http://www.me.pdx.edu/~micro-g/balloon2004/Balloon2004.html -- it captures just one of the many cheers that day. The official site for the project is http://www.me.pdx.edu/~lpsu/index.htm and will be updated Fall Quarter 2004.

I am very proud of this team: Josh Hatch, Dave Roper, Ryan Jenson, and Jim Cloer. Just amazing.

The Numbers Maximum altitude: 116,760’

Maximum speed, upper atmosphere: 969 mph

Terminal velocity, lower atmosphere (speed at impact): 62 mph

2. Balloon LaunchPSU used a Raven Industries balloon due to altitude requirements, payload weight, and delivery time constraints due to the academic calendar. The balloon was made from 0.35-mil polyethylene with a thinner “burst panel” designed to ensure the balloon ruptured at the top rather than at the bottom (a bottom rupture allows gas to vent until the balloon reaches neutral buoyancy and floats for an extended period). Final volume of the balloon was 54,000 cubic feet; the balloon was filled with helium.

Compared to latex or totex balloons, the Raven was more expensive but more readily available, and for our payload size it had a much greater chance of reaching at least 100,000’. Free lift capabilities became the deciding factor for the final purchase of the balloon. Other manufacturers were not able to guarantee performance for our payload and/or had unworkably long lead times (75 days+).

Due to the high cost of the balloon, only one was purchased, meaning no practice sessions (or mistakes) were possible.

Contact Information

Supplier Raven Industries Sulphur Springs Balloon Plant 186 County Road 3502 Sulphur Springs, TX 75482 (903)-885-0728 phone Contact: Mike Smith, Aerospace Engineer (903)-885-1032 fax

http://www.ravenind.com/RavenCorporate/eng_films/balloons_small.htm

Other manufacturers Kaymont (Totex balloons) PO Box 348 Huntington Station, NY 11746 Phone: 631-424-6459 Phone:1-800-644-6459 Fax:631-549-3076 Customer Service: paul@kaymont.com

Kaysam Worldwide Inc. (Latex balloons) 55 Shepherds Lane Totowa, NJ 07512 E-mail: info@kaysam.com Phone: 973-790-3366 Fax: 973-790-1275

Cost Cost, including shipping, was approximately $890.

3. Filler Assembly The Raven Industries balloon we chose was 50+ feet in length! It did not stretch like latex or totex – it was similar to a hanging plastic bag and simply filled out to its maximum volume as it ascends and then ruptured. The very large final volume translated to a large initial volume of helium being put in on the ground; the fill took roughly three, 245 cubic foot helium bottles.

The balloon was filled via a neck at the top of the balloon. The neck was about 25’ in length, 6”+ in diameter, and was made of the same thin, fragile plastic film as the rest of the balloon. This raised the issue of how to dump 750 cubic feet of helium into the balloon quickly without damaging the filler neck. The tanks were at 2200 psi initially. We opted not to use a regulator on the bottles, as that would drastically slow the rate of fill. Without a regulator, helium came through the air hose at an extremely high velocity. High pressure was not an issue, since the tank valve essentially creates an enormous pressure drop by choking the flow. To avoid damaging the filler neck with a high velocity stream of air, we built a diffuser that attached to the end of the hose and was inserted in the filler neck. We aided the diffuser by using a large diameter hose which acted as an additional diffuser between the tank and the primary diffuser.

We had a custom hose made locally. It was constructed from 1” inside diameter standard air hose. The tank end necked down to a ¼” NPT female thread which connected directly to the .580 CGA tank fitting (standard on helium bottles). The end connected to our diffuser simply had a standard 1 inch NPT male thread fitting. The fittings were attached to the hose with standard machine tightened hose clamps. The diffuser was built from PVC plumbing supplies. The body was 1½” inside diameter schedule 80 pipe, approximately 1’ long. Schedule 80 was used for the body since there were to be many holes drilled in it and we wanted to ensure the integrity of the pipe. The end attached to the hose was a compound adapter made of schedule 40 PVC fittings. The first adapter, which attached to the body, was 1½” inside diameter female threads on one end while the other end was a smooth wall (non-threaded) 1” inside diameter female fitting. The second adapter, which connected the first adapter to the custom hose, was a smooth wall, 1” outside diameter male fitting on one end and a 1” inside diameter threaded female fitting on the other end. The smooth parts of the two adapters were fitted together using plumbing cement, while the threaded parts were simply screwed onto their respective mates. The end of the diffuser which pointed into the filler neck was simply capped off with a schedule 40, threaded pipe cap. Thread tape was used on all threaded fittings to ensure seal and seating.

The diffuser body was drilled 56 times in random directions using a 3/8” drill bit. 56 was the most it could have without the holes getting too close together and unacceptably reducing structural integrity. Drilling through the PVC left quite a mess of plastic debris dangling inside the body. This was cleaned up beautifully using a 1 3/8 inch “berry” type hone. The outside of the body was sanded lightly to get rid of any burrs and smooth out the surface.

As an added precaution during filling; each time we opened a helium bottle, we “cracked” the valve very slowly so as to not damage the filler neck with a huge rush of fast moving air.

Suppliers

PVC HPS Pipe & Supply 598 Baseline

Cornelius, OR 97113 (503)-357-4217 phone hpspipeandsupply@hotmail.com

Custom Hose Associated Hose Products Inc 4444 NE 148th Ave Portland, OR 97230 (503) 257-4673

Tank Fitting Air Gas 341 SE Baseline St Hillsboro, OR 97123 (503) 640-3644

Hone Baxter Auto Parts Inc 1001 SE Tualatin Valley Hwy Ste A34 Hillsboro, OR 97123 (503) 547-0119

4. Parachute One of the team members had previous knowledge of designing and ordering a custom parachute for an experimental rocket project. Using the idea that our recovery system was to work in exactly the same fashion, we used an online chute calculator to determine what size parachute was needed in order to safely bring our payload back to earth.

Calculator: http://www.washingtonhighpower.com/Chute%20Calculator.htm

Payload weight:15 pounds, including parachute

Descent Rate: 25 ft/sec (amateur rocketry recommends a touch down at 15mph)

Launch Altitude: assumed 1,000’ (final launch site altitude was approximately 4,000’)

We used an X-form style ‘chute for its “softer” opening characteristics compared to a round or hemispherical type. Opening shock was a concern because of the FAA mandated 50 pound breaking strength string. Once the size was determined, we ordered two custom ‘chutes – flight ‘chute plus spare – from a local rocketry supplier (who also supplied the experimental rocket chute) and received them within a week.

Supplier Binder Design P.O. Box 13376 Salem, OR 97309 Phone/Fax: (503) 581-3180 Contact: Mike Fisher

http://www.binderdesign.com

Cost The two parachutes cost $103 including shipping.

5. Boxes Our design criteria for the boxes and their materials were light weight, ruggedness, and a low thermal conductivity value. The box needed to restrict as much heat transfer as possible, but could not be airtight which would introduce a pressure gradient (and popping danger).

Our team decided to make the payload boxes from ½” foam core (Figure 5.1). To hold the box together we epoxied the edges with J.B. Weld and sealed the edges and corners with aluminum tape. This shell material was easy to cut precisely and assemble. Foam core is sturdy enough to respond well to the impact of a normal landing. In fact, the foam core absorbed much of the energy of our crash landing. Note the foam core and bicycle helmets are both made from polystyrene foam.

http://www.sturdyboard.com/accessories/#self

Figure 5.1 - Foam core, edge view; photo from www.sturdyboard.com

The thermal conductivity of polystyrene foam core is different at different temperatures. When the polymer cools down, there is less intermolecular movement which in turn decreases the k value. An average value for k is

k=(0.032 to 0.035) at T= 20°C

6. Set-up Form top down, the balloon and payload assembly was arranged as follows:

- Balloon

- Nichrome line cutting circuit (see below)

- Parachute, attached to balloon through grommets at top

- Fishing swivel to prevent parachute shroud tangling

- Proprietary payload box #1, tied into string with tubular webbing

- Proprietary payload box #2, tied into string with climbing accessory cord

- A chemical light stick (for visibility after the pre-dawn launch)

- Another fishing swivel

- Telemetry capsule

- LED marker light

All portions were connected with 50 pound test kite string. The telemetry capsule was attached by running loops of string through a hole punched in each side. The upper edges of the holes were protected from string abrasion with small patches of aluminum tape. Total weight for the telemetry capsule was a mere 28 ounces.

7. FAA Regulations The FAA Code of Federal Regulations requires prior notification for regulated, unmanned free balloons which exceed the following limits:

- 50 pound maximum breaking strength string

- Maximum length of string of 50 feet without flag

- Maximum total payload of 12 pounds

- For each box, maximum of 6 pounds

- 4 pound maximum box weight if the total box weight divided by minimum box surface area of more than 3 ounces per square inch.

Accordingly, our overall design process focused on staying within these limits and therefore out of FAA regulation. Even so, we planned on launching the balloon far from busy airspace.

8. Site Planning When finding a launch site, there were many variables to take into account. Safety was a main concern, so the launch site had to be secluded. You never knew if the string might break, sending the payload plummet towards the earth…

Recovery was another important parameter. To enhance recovery, we planned for the payload to land in a relatively “clear cut” region. If the payload were to land in a large group of trees, we would be less likely to receive a location transmission. We also wanted to avoid finding the balloon caught in the top of an old growth tree.

Landing in central or eastern Oregon would satisfy the aforementioned criteria. After looking at online balloon projection software (http://www.wrh.noaa.gov/Portland/), there was a high probability that the balloon would have a south-east trajectory. We therefore looked for a launch-site northwest of a desolate area.

We decided to launch from Millican, Oregon for several reasons. Millican, with a population of seven, is contained inside a large bowl shaped landscape. This would reduce crosswinds which could make balloon filling and launching a daunting and unpredictable task. Also, there were a lot of back roads, not very much private property in the area, and the vegetation was mainly scrub.

Our group lucked out by landing on a field of pumice-like rock, which was very porous and bouncy to step on. When our payload landed, this soft material absorbed a lot of the impact, which probably saved our payload. It also landed less than a half mile from a dirt road.

Comparison of the flight path data validated the general prediction of the online balloon projection; our total distance traveled was less than predicted, but this was most likely due to our plummeting and therefore nearly drift-free descent.

9. Camera System The camera system consisted of three main components, a camera, an interval timing circuit, and a battery pack.

The camera of choice was a film-based Canon Elph LT APS. It is a relatively common camera and can be found at most large electronic stores (i.e. Best Buy, Fry’s, etc.). The Elph LT was chosen because of its compact, lightweight structure, along with its ability to produce quality photos. In order for us to use the camera a few alterations had to be made. Altering the camera was easily achieved due to its electronic based operation (opposed to mechanical). Fortunately many cameras today rely on electronic triggers/shutters, which are easily controlled using 555-based timing circuits.

Only four solders were needed to modify the camera. The first and easiest is shown in Figure 9.1 below.

Figure 9.1: First solder location

When the two copper prongs are separated the camera will not take any pictures. This is because the camera “thinks” the lens is covered. By soldering the prongs together a closed circuit is created and the camera then “thinks” the lens is open. This ensures it will always take pictures and reduces the chance of malfunction during flight.

The final solders were done near the shutter trigger. There were a total of three which are indicated by the arrows in Figure 9.2 below. Wires were attached to points A, B, and C. Special care was taken when soldering point C. The wires in parallel above the solder point control the LCD. Accidentally soldering the wires will cause the LCD to fade out and possibly fail altogether. Once the wires were soldered to the camera, the ends of wires A and B were twisted together. Touching wires A, B, and C resulted in a picture being taken, as expected. After the soldering was complete the wires were directed outside the camera and taped against the side of it to prevent undue stress on the solder joints. At this point a total of three wires extended from the camera with wires A and B connected to one another. Connecting wires A and B has no noticeable affect on the camera, but in order for a picture to be taken all three wires must be simultaneously connected. The final step was to put the camera back together, install a CR2 lithium battery, and put the film in.

Figure 9.2: Final solder locations

An adjustable interval timer was used to control the timing between pictures. A kit for the timer can be found at www.allelectronics.com and at a few other online stores. It requires a minimal amount of soldering and the instructions were pretty easy to follow. Local electronic stores did not have the kit, but RadioShack had all the components needed to replicate it. The kit was slightly modified with the addition of a 1.67 MΩ resistor, which replaced one of the 1 kΩ resistors. The components plus the additional resistor are listed below, which are also shown in Figures 9.1 to 9.3. The circuit diagram is reproduced in Figure 9.4.

• 1 - 100 nF capacitor (C1) • 2 - 100 μF capacitors (C2, C3) • 3 - Diodes: 1N4148 (D1, D2), 1N4007 (D3) • 1 - 555 IC (IC1) • 1 - 8-pin IC holder (ICS) • 2 - PC board connector: 3-pin, 2-pin (J1, J2) • 1 - 3mm LED (LD1) • 1 - PC board (P1) • 3 - 1 kΩ resistors (R1, R3) • 2 - Potentiometers: 0.470-1 MΩ (RV1), 22-47 kΩ (RV2) • 1 - 1.67 MΩ resistor (Rx) Note: not in kit • 1 - 28 VDC 10A relay (RY1)

Figure 9.3: Interval timing circuit

Figure 9.4: Circuit diagram from kit

The interval timer operated by using a RC-type circuit along with a 555-IC to control the pulses. With the circuit above a pulse caused the relay to switch to position “NO.” Basically the capacitor was charged over a period of time. It then discharged causing the relay to switch to the position “NO” as shown in Figure 9.4. During a “pause” the relay was at position “NC” and during a “pulse” it momentarily switched to position “NO”. This cycle effectively mimicked a person periodically pushing the camera button to take a picture.

The interval between pulses was controlled by C2, R2, and RV1. R2 and RV1 were in series so their total resistance was R1+ RV1. The modified circuit had a maximum pause resistance of 2.67 MΩ (1 MΩ + 1.67 MΩ) and a C2 capacitance of 100 μF. The time between pulses was 2 minutes and 57 seconds. Notice the time relation was not simply R·C. An exact equation for the time was not formulated and was not necessary for such a short time interval. It was known that a pause resistance (R2 + RV1) of 1 MΩ and capacitance of 100 μF created a 60 second pause. The addition of a 1.67 MΩ resistor increased the resistance to 2.67 times the original, giving a projected time of 2 minutes 47 seconds. With such a small resistance increase a linear relation was assumed, which turned out to be quite accurate. As stated above the pause created was 2 minutes and 57 seconds. With a 25 exposure roll of film the shooting time was projected to be 74 minutes. It turned out that the time was far too short with our ascent time having been close to 104 minutes.

Finally the battery pack was constructed. The 555-IC needed between 5-15 volts to operate, while the relay needed between 8-10 volts. It may seem that an 8-10 volt battery pack would work fine, however this was not true. When the voltage “exited” the 555-IC it cut it down by one-third, thus 9 volts going in came out and “entered” the relay at 6 volts. The relay would not operate with only 6 volts potential. Using some simple arithmetic it was concluded that a 12-15 volt battery pack was perfect. 12-15 volt lightweight battery packs are not easy to come by. The same desired voltage was achieved by putting two 6 volt lithium batteries in series. The batteries and battery holder were found at RadioShack. Note the battery pack is designed to hold one AA battery but two 6 volt batteries fit snugly as shown in Figure 9.5.

Figure 9.5: Batteries in single “AA” holder

The battery pack was completed with the insertion of a simple slide-switch. Figure 9.6 shows the battery pack, circuit, and switch.

Figure 9.6: Battery pack, sliding switch, and interval timer

Finally all the components were brought together into a functioning system. To complete the system the combined wires A and B from the camera were inserted into the connector (J2) labeled “COM”. The final wire, C was inserted into the connector (J2) labeled “NO.”

Finally the date and time on the camera was set and the flash turned off.

Cost The cost of the camera system is summarized in Table 9.1.

Item CostCamera Battery $8.39Battery Pack $20.27Switch $2.49Interval Timer $9.95Camera $56.99Misc./Backup/Test Equip. $30.00

Total $128.09 Table 9.1: Camera system cost summary

10. Line-Cutting System The balloon that was ordered was unfamiliar and larger then was needed to carry our 12 pound payload. There was a concern that the balloon might “hang” and not burst if under inflated. For this reason a time activated cutting system was implemented. The line cutter operated by electrically heating a nichrome wire, which then burnt through the line and released the payload.

A nichrome heating element, “cutting” battery pack, an adjustable interval timer, and a circuit battery pack were all part of the cutting system. The timing circuit undertook a few modifications as shown in Figure 10.1. The most striking modifications are the large capacitors. C2 and C3 are 1000 and 470 μF capacitors, respectively. Notice the odd looking resistor R1. Resistors greater then 2 MΩ are hard to come by, so a little creativity was necessary to get the desired resistance. Resistor R1 is actually two 10 MΩ Resistors in parallel, which is equivalent to a single 5 MΩ resistor. The combination of resistors and capacitors resulted in a pulse of 1 hour and 57 minutes. The cutting system was constructed such that a “pulse” effectively kept the cutter off. When the pulse terminated the relay switched and the cutting was initiated.

The cutting procedure was powered by three 9 volt batteries in series. The combined voltage totaled 27 volts. Note that alkaline batteries must be used. Lithium 9 volt batteries do not produce the work necessary to heat the nichrome wire significantly. Finally a 4-inch nichrome wire and switch were placed in series with the 27 volt battery pack. The nichrome wire was wound around a string which was later tied off to the payload and balloon.

The same circuit battery pack and switch as shown in Figure 9.6 were used to power the cutter circuit.

Figure 10.1: Modified interval timing circuit

A type of fail-safe was built into the line cutter, which increased the chances a cut would occur if the battery voltage dropped significantly due to the low temperatures. The circuit was built such that a pulse was necessary to keep the nichrome line cutter off. A pulse simply meant that power was being consumed to keep the relay switched to “NO”. This also produced a few watts of power being dissipated, which helped to keep the circuit warm. If the voltage to the circuit dropped significantly below 12 volts the relay would switch back to the “off” position, “NC.” At this point the 27 volt battery pack would initiate, heat the inside of the box, and cut the line.

The flight ready line-cutter is shown in Figure 10.2 below.

Figure 10.2: Line-cutter ready for flight

Cost The cost of the line-cutting system is summarized in Table 10.1.

Item CostLine-Cutting Battery Pack $6.50Circuit Battery Pack $20.27Switches $2.49Interval Timer $9.95Misc./Backup/Test Equip. $20.00

Total $59.21 Table 10.1: Line-cutter cost summary

11. Tracking System

Problem Statement Since balloons expand to large volumes as they gain altitude, it is possible to track them visually to a considerable height. We were able to see the LaunchPSU as a small dot when it was at 100,000’ plus. However, cloud cover, the need to drive while chasing the balloon, and the need to have a position fix for the balloon’s landing (instead of just a line of possible positions) meant that we could not be rely upon visual tracking.

Any tracking system used needed to be small, light, and reliable. An ideal system would also record position data during the flight for later review. LaunchPSU used an amateur radio Automatic Position Reporting System (APRS) system.

Generic System APRS sends digital Global Positioning System (GPS) location data by radio. An APRS transmitting station consists of a GPS receiver, a Terminal Node Controller (TNC) which packetizes the GPS data and performs modem functions, and a radio transmitter. The typical receiving station consists of a radio receiver, another TNC (to demodulate the received packets), and a computer for displaying the received information. Most APRS activity is on the 2 meter (144 MHz) band.

For more information on APRS, see the web pages of the system’s creator, Bob Bruninga, WB4APR, at http://web.usna.navy.mil/~bruninga/aprs.html.

For more information on amateur radio generally, including information on earning your first FCC license, see the American Radio Relay League at http://www.arrl.org/hamradio.html.

LaunchPSU System

Balloon System

GPS & Antenna LaunchPSU used a Garmin GPS 15H OEM GPS engine (Figure 11.1), programmed to output latitude, longitude, and altitude. This GPS offered several advantages, including small size (roughly that of a matchbook), light weight and the ability to be powered by a 9 volt battery without regulation. It was also Wide Area Augmentation System (WAAS) enabled for maximum accuracy. However, due to restrictions on all civilian GPS units, it did not provide altitude data over 60,000’. We housed the GPS in an Altoids mini-tin, with holes drilled for antenna, power, and data connections, and Velcro to hold the system in place.

The GPS 15H required a separate, active antenna. We used a Micro Mouse antenna (Figure 11.2), which was small, light, and offered respectable gain (25 dB). A better-than-nothing ground plane for the antenna was fashioned by covering the top of the telemetry capsule with aluminum tape, with the antenna Velcroed in place.

Both GPS and GPS antenna were purchased from www.gpscity.com. Cost for both, including shipping, was $185. This represented roughly 60% of the balloon system cost.

http://www.garmin.com/products/gps15h/

http://www.gpscity.com/gps/brados/1513.2.9397419104019376533/oem15h-w.html

http://www.gpscity.com/gps/brados/1513.3.6654060258719376533/micromouse.html

Figure 11.1 - GPS engine in Altoids mini-tin

Figure 11.2 - Micro Mouse GPS antenna on capsule lid

TNC/Transmitter Launch PSU used a Byonics “Pocket Tracker” (Figure 11.3), a simple, dedicated APRS TNC/transmitter combination which is sold in kit form. Construction and alignment of the kit were fairly straightforward, but some previous experience with radio kit building is recommended. The Pocket Tracker was designed to fit in an Altoids tin with its 9 volt battery. Because of the low temperatures we expected to encounter, we used an Ultralife brand lithium

battery (marketed for use in smoke detectors). The power leads were spliced to provide power to the GPS from the same battery, and the two cases were Velcroed back-to-back.

The Pocket Tracker can transmit on either 144.340 or 144.390 MHz (the standard U.S. APRS frequency and the one LaunchPSU used); frequency was selected by a jumper on the circuit board, and so could not be changed during flight. Transmit power was only 220 milliwatts; however, since the balloon was on line of sight until it landed, this was sufficient power if an efficient antenna was used.

http://www.byonics.com/pockettracker/

Figure 11.3 – Completed Byonics Pocket Tracker

Radio Antenna

Design/Modeling The transmitting antenna presented interesting challenges because it had to be effective both when the balloon was airborne and while it was on the ground awaiting recovery. This meant it had to have acceptable characteristics in free space and in close proximity to ground. In addition, it had to be light and could not require an extensive supporting structure. Several designs were modeled in EZNEC (http://www.eznec.com/), including various length vertical antennas and two and three element yagis (with the assumption that a “rolling” capsule could be used for proper orientation on the ground). Antennas were evaluated for both radiation pattern and standing wave ratio (SWR) in free space, at various heights and orientations above ground, and with various radial designs where appropriate. Most designs were acceptable in one situation but completely unacceptable in the other, with nulls covering several square miles below the balloon at altitude,

effectively infinite SWRs on the ground, etc. The antenna which performed the best whether in free space or a very small distance off the ground was the humble half-wavelength dipole.

Construction To eliminate the need for a separate support structure, the antenna was built from aluminum arrow shafts. The shafts were trimmed to length at the fletching (feather) end to preserve the useful threaded insert at the point end. Ping-pong balls were epoxied over the cut ends of the shafts for safety, and the anodizing at the point end was removed with sandpaper to enable an electrical connection with the feed line. A balsa wood support was built for the center of the antenna; each half was slid through a hole in the outer panel and secured to the inner panel with a nylon thumbscrew in place of an arrowhead (Figure 11.4). Small hose clamps were used to hold the coax feed line ends in contact with the shafts, and the balsa wood structure was epoxied to the bottom of the telemetry capsule, and a small hole was cut in the capsule wall to pass the feed line through to the transmitter.

Figure 11.4 - Telemetry capsule immediately before launch; note RTD protruding from left side of

capsule (see section on data loggers)

Chase System

Antenna A mag-mount vertical antenna was used on the chase van roof. The antenna was bent back at an angle of approximately 45 degrees to move (and reduce) the null which would have otherwise

been directly overhead. Such a null was fine for normal ham radio mobile communication, but would be a problem for trying to receive from a balloon at 100,000’.

Receiver An Icom T7H was used, with selection based entirely on what was already “in the shack.” Since the chase station was used just for receiving, a scanner or other 2 meter FM receiver could have been used instead. APRS uses audio frequency shift keying, so transmissions are on a single frequency only.

Sound Card Interface A BuxCOMM Rascal sound card interface was used to connect the receiver to a Dell Inspiron 5150 laptop. Like the Pocket Tracker, the Rascal was purchased in kit form. This kit was beginner friendly.

http://www.packetradio.com/psk31.html

Laptop & Software The laptop’s sound card was used as the receiving TNC, thanks to an array of amateur radio software, most of it freeware. AGWPE enabled packet decoding with the sound card; KipSSPE interfaced AGWPE with the APRS software, APRS+SA; and APRS+SA logged the GPS data and, most importantly, plotted the balloon’s position in DeLorme Street Atlas 8 (APRS+SA is incompatible with the current version of Street Atlas). While the number of links in the receive system was a minor cause for concern, and minor bugs in the set-up took about eight hours to resolve, the system worked flawlessly in the end.

AGWPE: http://www.raag.org/sv2agw/

KipSSPE and APRS+SA: http://www.tapr.org/~kh2z/aprsplus/

An excellent resource for sound card-based packet radio using AGWPE is http://www.qsl.net/soundcardpacket/index.html

System Costs Excluding the laptop and receiving radio, total cost for the tracking system was approximately $475.

Results On launching, the system performed as expected. About 70 minutes after launch, the system stopped sending altitude data as expected (the last altitude sent was 67,016’, and was most probably inaccurate). Six minutes later, however, the transmitter went silent and stayed silent for a heart-rending 102 minutes. This was most likely due to the battery voltage falling below the transmitting threshold in the cold of the upper atmosphere.

When we finally received packets from the balloon again (hooray!), the altitude data was erratic. This may have been due to resting orientation of the GPS antenna, which was pointing towards the ground at a roughly 45 degree angle. Impressively, the transmitting antenna’s balsa structure was crushed and the ping-pong balls blown off on impact. The antenna was still functioning when bent approximately 40 degrees out of line, with one end in contact with ground. Since we were unable to make visual contact with the payload as it plummeted back to earth, location and recovery would have been impossible without the radio system.

The final portion of the APRS+SA log is in Figure 11.5; Figure 11.6 shows the map of the balloon’s flight. Somewhere between Portland and the launch site the GPS re-initialized to 0,0; the straight line running off the right edge of the map came from the prime meridian/equator intersection off the coast of Africa to the launch site. The second, shorter straight line connected the last packet received during flight to the recovery location where transmissions were received again. Figure 11.7 shows the smashed antenna as found.

Figure 11.5 - APRS+SA screen shot

Figure 11.6 - Street Atlas map of LaunchPSU balloon flight

Figure 11.7 - When the wind blows, the cradle will fall...the crashed capsule

12. Sensors & Data Loggers A primary goal of the launch was temperature data acquisition. Since the GPS altitude would cut out well before the target altitude – and because GPS altitude is much less accurate than GPS latitude/longitude – pressure data was also needed to calculate altitude. Three different data loggers were used to make and record the needed measurements, with some redundancy.

Sensors

Hobo H8 An Onset Computer Hobo H8 four-channel logger was used for temperature measurements. The particular model used had temperature and relative humidity sensors built in, plus two external 1-5 volt channels. An RTD temperature probe was installed on one of the external channels, with the probe protruding through the wall of the capsule (see Figure 4 above). It was this temperature data which was used in altitude calculations. The H8 was compact, light, took an easy-to-find lithium coin cell battery, and was easy to use; its only foiblewais that communication between it and the laptop required turning off power management of the USB ports in the computer’s Device Manager.

http://www.onsetcomp.com/Products/Product_Pages/HOBO_H08/H08_family_data_loggers.html

Hobo StowAway Pressure The team had a Hobo StowAway pressure logger on hand. This logger had some drawbacks for our application: it only measured to 0.5 psia/32,000’, and it used an unusual and expensive battery. However, since it was compact and weighed only ~1 ounce, it was included as a backup to our primary pressure sensor. We programmed the primary sensor to use a high sampling rate for a barely adequate duration, based on our anticipated flight time. The Hobo pressure logger was set to a slower rate/longer duration, on the theory that in an extended flight the Hobo would continue recording, hopefully beginning at an (descent) altitude within its range.

http://www.onsetcomp.com/Products/Product_Pages/old_loggers.html

MadgeTech Pressure The primary pressure logger was a MadgeTech PRTemp101. The MadgeTech read to 0 psia in 0.002 psi increments, and also recorded temperature. The logger was mounted with the radio and GPS (see the black box in Figure 1 above); these were bundled with a small heat pack, so temperature recorded by the MadgeTech were those seen by the radio and were not useable in altitude calculations. The pressure port on the logger was positioned with a clear “view” of the interior of the capsule. In turn, the capsule was generously vented around the camera lens and light meter, with additional holes for suspension strings, antenna feed lines, etc., so the pressure inside the capsule was effectively always equal to the local atmospheric pressure.

http://www.madgetech.com/pressure.php#prtemp101

Costs Because the two Hobo loggers and their ancillaries were already on hand, the only instrumentation cost was the MadgeTech logger (approximately $370). For comparison, the Hobo loggers cost ~$100 each, plus $40 for the temperature probe.

Results Sadly, the valiant Hobo H8 gave its life in the crash landing. Its plastic case was destroyed, and the mini jack to which the temperature probe connected was fractured. Luckily, the circuit board itself survived and we were able to download the logger’s data. Both other loggers returned intact, with all data recovered.

Data was analyzed independently by two team members, one using published NASA altitude correlations, the other performing a numerical integration of the ideal gas law. Results by the two methods were within 3% of each other; the correlation results were slightly lower, and so were the source of the numbers in section 1 of this paper.