Data Acquisition

Collecting data from test motors is less complicated than it may seem. There are many options to go about collecting motor test data spanning using a few inexpensive components all the way up to dedicated data collection devices. The process outlined here is intended as a starting point and has many options for expansion. There are dedicated motor testing data collection devices that combine all the below components into a single device.

There are 2 types of data most often collected when motor testing, chamber pressure and thrust. Other data may also be collected, such as case temperature, burn through wires, etc.

Chamber pressure is the most relevant data in a motor test, as it is needed for propellant characterization as well as confirming simulation results on a motor being tested. Further, thrust can be calculated from pressure using nozzle dimensions. BurnSim will do these calculations for you when importing pressure test data.

Equipment

Basic equipment needed for measuring chamber pressure includes a pressure transducer, analog/digital converter, a laptop and a power source. The laptop does not need to be anything special.

Pressure Transducer

Chamber pressure is measured with a pressure transducer. While there are different types of pressure transducers, the type most useful here is one with a range suitable for measuring motor chamber pressure (typically in the 1000-2000 psi range) and 3 wires with an output in the 5V range. The 3 wires are Ground, Excitation Voltage, and output. These can be purchased on ebay for relatively cheap, as well as other sources.

Analog / Digital Converter

An Analog / Digital converter does exactly what the name implies. It takes an analog signal and converts it into a digital signal suitable for recording on a computer. Inexpensive A/D converters are available from www.Dataq.com for as little as $50. Their least expensive starter kit includes free software and is suitable for our purposes here.

Laptop

A laptop is used to record the data from the a/d converter, running the free software that comes with the Dataq unit. Nothing special is required here other than a USB port and the capability to run a recent version of windows. There may also be linux versions of the software available, although i've not tried that myself.

Pressure Transducer Calibration Manifold (optional)

This is an optional component but recommended for getting the most accurate data. Pressure transducers come with specifications and their output voltage correlates to a range of measured pressures. You can generally rely on this to be correct, but a calibration manifold allows you to confirm your numbers are accurate.

The calibration manifold consists of a block of aluminum with a threaded port each for a pressure gauge, grease gun, the pressure transducer as well as a pressure bleeding screw. A grease gun is used to create pressure, and a pressure gauge is used to show actual pressure being generated.

[image needed]

Power Source

A 9v battery is suitable to power the pressure transducer, although anything that provides the appropriate voltage will work.

Putting it All Together

Once you have all the equipment listed above its time to put it all together.

First, install the dataq software on the laptop and follow all instructions there.

Connect the transducer output to the positive (+) input for channel one on the a/d converter. Connect both the pressure transucer ground and the negative (-) input for channel one on the a/d converter to the negative side of the voltage source (battery). Connect the positive side of the battery to the exitation input of the pressure transducer. Connect the a/d converter to your laptop.

Example of a pressure transducer, battery, and A/D converter connected for pressure data collection

At this point you should be able to see the dataq software showing 1 volt (or zero volts depending on the transducer).

Testing and Calibration

If you have a calibration manifold, connect the pressure transducer to the manifold along with the gauge and the grease gun. Skip forward to the next section if you do not have a test manifold.

The manifold should be filled with grease by pumping the grease gun until grease comes out of the bleeding port. Then close the bleeding port and you should be able to pump up the pressure with the grease gun and see that pressure reflected on the gauge. If it does not hold steady, you need to bleed all air out of the manifold. You may need to loosen the pressure gauge and pump in grease until the air escapes and then tighten the gauge back up. The same process may be required on the pressure transducer.. If everything is working correctly, you'll see the voltage rise in the Dataq software as you pump up the pressure with the grease gun. It will hold steady until you release pressure with the pressure bleeding screw.

If you are successful in making the data rise and fall by increasing the pressure and releasing it with the bleeding screw, you can confirm the calibration of your pressure transducer. For example, if the data sheet that came with your transducer specifies output as 5v at 1000psi, you can pump the test manifold up to 1000psi and confirm the voltage in the dataq software.

[TBD process for calibration and setting units in dataq software]

Running a motor test

NOTE Testing rocket motors is DANGEROUS. Always do so in a safe location, and observe from a safe distance not in clear sight of the motor being tested. You should always assume the absolute worst will happen, and that should not cause any serious problems. For example, you should assume the motor being tested will fail catastrophically, and your testling location/setup should allow for that possibility without potential for injury.

When you are ready to test an actual motor, the pressure transducer should be connected to a pressure port on the motor and connected to your a/d converter. A long usb cable can be used to attach the recording laptop and have it located some distance from the actual test motor. Utilize a remote firing system.

TBD more details to come...