Poorman's Space Program

Testing BalloonSats (Part 1)

Paul Verhage

The near space environment and flight into near space are difficult at times, and the BalloonSat must function properly in this environment without losing parts or being difficult to launch. The six performance tests described in this chapter simulate many of the aspects of the near space mission. By successfully completing them, the BalloonSat Program Manager can be confident that each BalloonSats will be successful. However, since the BalloonSat Program Manager must accept the risks associated with the BalloonSat launch, he or she must decide to accept or modify the standards for each test.

Some of these tests are not appropriate for rejecting a BalloonSat for a particular mission. Instead, they provide evidence for needed improvement. In addition, the BalloonSat Program Manager can use the results of these tests to generate a score for BalloonSat Team evaluations. The recommended six tests are Weight, Functional, Thermal, Drop, Shake, and Prep.

4.1 WEIGHT TEST

One requirement for the BalloonSat Program Manager is to give several requirements that each BalloonSat must meet. The maximum allowable weight for each BalloonSat is one of them. Therefore, during construction, each BalloonSat Team should be weighing their creation. The final results of the weight test should not surprise any of the BalloonSat teams.

4.1.1 Weight Test Background

Since the Federal Aviation Administration imposes weight limits on untethered balloon flights, the combined weight of the mission's BalloonSats is limited. To be equitable about this requirement, the total available payload weight for the missions is divided equally amount the BalloonSats.

4.1.2 Weight Test Procedure

Use an inexpensive digital hobby scale for this test. Then do the following:

Load each BalloonSat with its internal experiments, including batteries.

Seal the BalloonSat hatch with rubber bands.

Record the weight of each BalloonSat in writing.

Pass Criteria

Any BalloonSat weighing less than the allowed maximum is ready for the next test.

Note: The traditional maximum weight allowed for BalloonSats is 450 grams (one pound). BalloonSat teams should be encouraged to create the lightest weight BalloonSat capable of carrying out the most science by developing a scoring system that grants higher scores to lower weights. The FAA regulations governing unmanned balloons and rockets are provided under PART 101—MOORED BALLOONS, KITES, UNMANNED ROCKETS AND UNMANNED FREE BALLOONS

Figure 1. A middle school student weighing his BalloonSat.

4.2 FUNCTIONAL TESTING

Functional testing ensures the BalloonSat is capable of carrying out its mission. Because of the costs associated with a near space launch, it's not justified to launch a BalloonSat with a high risk of functional failure.

4.2.1 Functional Testing Background

The costs associated with a near space launch can be broken into three categories: risk, flight, and time.

Risk Cost

On every near space mission, there's a small risk that the near spacecraft will not be recovered. Redundant back-up trackers onboard the near spacecraft reduce, but do not eliminate, this risk, particularly if the trackers should fail, the predicted flight path has significant error, and the recovery zone is located in a desolate or low populated region. A near spacecraft consisting of just two independent APRS trackers can cost over $300.

Not only does the owner of the near spacecraft face this risk, so does the BalloonSat Program Manager who purchased the items to construct the BalloonSat. The cost of a BalloonSat can approach $100.

Flight Costs

To carry out a near space mission typically requires a $60 weather balloon and $100 of helium. An additional cost is the gasoline for the launch and chase crew. Including the driving to the launch site, possibly four hours of chase and recovery time, and driving home, chase crews may drive over 160 kilometers (100 miles). In all likelihood, the chase vehicle is capable of off-road driving and probably has poor gas mileage.

Time Cost

Not all costs relate to money, the time spent planning, launching, and recovering a near space mission can be substantial. The launch crew will spend at least two hours prior to the launch preparing for the flight and making predictions. An hour or more of traveling to the launch site may be necessary, depending on wind conditions. To fill a balloon and launch the near spacecraft requires at a minimum of one hour. Three or four hours may be necessary to chase and recover the near spacecraft and its payload of BalloonSats. Then chase crews must drive home. All told, launch crews will spend at least nine hours in support of the mission.

Combining these costs together and dividing them among five BalloonSats, we see each BalloonSat requires approximately $135 in cost, 1.5 hours of time, and a share of the risk that a $300 near spacecraft could be lost. These costs and risks are acceptable if the BalloonSat has a high probability of functioning properly for the duration of the mission. Therefore, the Functional Test is designed to verify each BalloonSat is capable of collecting its data for the duration of a typical mission (pre-launch to touchdown).

4.2.2 Functional Testing Procedure

Program the data logger and prep all experiments and cameras.

Load the BalloonSat with its batteries, programmed avionics, and experiments.

Start the BalloonSat and let it sit for three hours.

Review the data collected.

Pass Criteria

Did the experiments record data for the entire three hours?

Did the experiments record the expected data?

Did the camera have an unobstructed field of view?

4.3 THERMAL TESTING

The air temperature in near space can be as low as -68 degrees C (-90 degrees F). That's a temperature that most items (snowmen being an exception) don't like.

4.3.1 Thermal Testing Background

Some electronics, like data loggers, have minimum recommended temperatures. Levels for most industrial items range from -40 degrees C to +85 degrees C (-40 degrees F to 185 degrees F).

As the temperature of a material drops, so does it molecular activity. This is a factor for batteries, which are chemical devices that produce a voltage based on their internal chemical reactions. So as its temperature drops, its ability to produce voltage under a given load decreases. Therefore, batteries have a minimum rated temperature. Going below this temperature risks the battery will fail to function.

Alkaline: -18 degrees C (0 degrees F)

NiCd/NiMH: -20 degrees C (-4 degrees F)

Lithium: -55 degrees C (-67 degrees F)

4.3.2 Thermal Testing Procedure

A thermal test requires a thermal test chamber. Therefore, construct the thermal test chamber (TTC) as explained here.

Figure 2. Loading a BalloonSat into a thermal test chamber.

Follow these steps to conduct the temperature test:

Charge the chamber with dry ice and let it chill.

Measure the internal temperature and record.

Program one or more temperature sensors.

Load a temperature sensor into each BalloonSat.

Load the BalloonSats inside the chamber and close the lid.

Let the BalloonSats remain for 30 minutes.

Remove them and download their temperature data.

Evaluate the temperature data.

Pass Criteria

How fast did each BalloonSat cool?.

How cold did each BalloonSat ultimately become?

Note: Thirty minutes is probably long enough to let the thermal test chamber cool before beginning the test. More than one BalloonSat can fit inside the thermal test chamber, so prepare more than one data logger. The slower a BalloonSat cools, the better. In addition, the warmer the BalloonSat remains at the end of the test, the better. Unless an item inside a BalloonSat is severely temperature sensitive, the thermal test is not necessary or sufficient reason to prohibit a BalloonSat from flying. However, each BalloonSat can receive a score based on the above criterion.

A list of all of Paul Verhage's BalloonSat articles is provided elsewhere in this installment of The Citizen Scientist. Part 2 of "Testing BalloonSats" will be published in the December The Citizen Scientist.