Back to
Air Command Rockets

 

Find Data

Pressure Chambers

Caps

Adhesives

Experiments

Test Procedures

Test Equipment

Products

Conversions

psi to bar

mm to inches

feet to meters

grams to ounces

Request Experiment

Current Requests

Submit Experimental Data

FAQs

Contact

About Labs

Test Equipment - Static Thrust Test Stand II

Overview

The test stand was designed to measure the static thrust produced by a water rocket. It is based on a single point load-cell connected to a load cell amplifier and the amplifier in turn is connected to a data logger. The data is captured through a laptop.

Load Cell

The load cell used in this stand is a 70kg load cell from a Chinese manufacturer whose cost and delivery were far cheaper than local distributors. Purchase price was US$24 + $9 delivery. Delivery was around 8 days.

The 70Kg load cell is a good compromise between the upper and lower ranges of thrusts for regular water rockets.

Data Logger

The data logger used is from DATAQ. The DI-148U was chosen as it already had a USB connection. The DI-194RS is a cheaper serial port alternative.

The logger was ordered through a local distributor called Total Turnkey Solutions (www.turnkey-solutions.com.au). All up delivered the DI-148U Starter Kit was AU$118.

Load Cell Amplifier

The data logger reads in the -10V to 10V range with a 10-bit accuracy. The load cell only produces millivolts over the entire deflection range which is too low for the data logger to get enough resolution directly. The amplifier basically converts these small voltage changes to a voltage over the full dynamic range of the logger. It also provides the necessary excitation voltage for the load cell.

You can buy these off-the-shelf but you will pay upwards of $100. Being on a limited budget, we decided to build our own. The amplifier is based on an instrument amplifier IC INA125P. The circuit is relatively straight forward and these IC's are designed for exactly this task.

Because it is not easily sourced within Australia, we ordered them from Texas Instruments in the US.

 
Data-logger, load cell and load cell amplifier.
The load cell amplifier is housed in a metal box to reduce noise.

The load cell amplifier is located close to the load cell and enclosed in a metal box. Coax cable is used for the long run to the data logger. The noise level is down to around 0.02V over the -9V to 9V range.

On the 70Kg load cell this allows one to resolve down to ~5 grams. This means we can get thrust measurement accuracy down to about 0.05 Newtons.

The expected thrust range of most rockets under test will be about 30N to 150N. That range applies for our typical rockets with pressures of around 120psi with nozzles under 10mm. The same setup will also be used for full bore tests of up to around 600N.

Circuit Diagram

You can adjust the amplifier's gain using the multi-turn trim pot in order to get the required thrust range.

Because we use two 9V batteries as the power source we don't quite get the full +/-10V range, but it is close enough. The excitation voltage used for the load cell is 5V.

Parts List

Since this was going to be a one-off we didn't bother with a PCB and just built the circuit on a strip board.

Part Designation Quantity Source
INA 125P Instrument Amp U1 1 Texas Instruments
16 Pin IC socket - 1 Jaycar
50 ohm Coax Connector JP2 1 Jaycar
5 DIN socket JP1 1 Jaycar
DPDT switch S1 1 Jaycar
Power LED LED1 1 Jaycar
500 Ohm multi turn pot R1 1 Jaycar
1.5KOhm R2 1 Jaycar
0.1uF Capacitor C1, C2 2 Jaycar
9V battery B1,B2 2 Jaycar
9V battery clip - 2 Jaycar
Metal enclosure - 1 Jaycar

Software

The data logger came with free "lite" software for capturing and viewing the data. However, being free there are some limitations in terms of exporting the data and maximum allowed sample rate. The max sample rate is 240Hz which is more than ample for us. The full software with higher allowed sample rates (14,400 samples/sec) is another $200. You can also buy a $99 software add-on that lets you save the captured data into Excel friendly format.

The free software, however, does record the data into their own proprietary binary file format. A software application was written to read this data and convert it to thrust curves.

Test Stand

At the top of the test stand the load cell is bolted to a heavy steel plate which in turn is bolted to the stand. Above that is the load cell amplifier. The entire rocket is just suspended from the load cell. 

The arrangement at the bottom of the test stand stops the rocket moving side to side when thrusting. It consists of 3 adjustable wheels on ball bearings that allow them to be brought closer together for narrower rockets and further apart for wider rockets. The rocket only lightly touches these (a few mm clearance) so there is essentially no friction between them and the rocket during the test.

The rocket under test can be quickly disconnected from the load cell with the single pin that goes through the bracket connected to the load cell. This lets us take the rocket off, fill it with water and re-attach it back to the load cell.

The release mechanism consists of a brass Gardena mechanism with the spring removed and rubber bands are used to provide a retracting force. We added a non-return valve inside the release head as well as a hose quick connect adaptor to the bottom. We use a thick piece of plastic with a string attached wedged under the collar. To release the nozzle we simply pull on the string and the whole mechanism falls away.

The total cost of the test stand not including the laptop was about AUD$230.

 

Complete test stand setup with load cell, logger, laptop and support framework.
The amplifier is mounted on top ...
... with the load cell underneath. The bottle is connected to the load cell here.
The rocket is supported by three wheels that barely touch it. This allows the rocket to move with very little friction.
The wheels can be adjusted out or ...
... in depending on the size of the rocket.
First tests set up outside.
Nozzle release head. Pulling the string causes the release head to drop away.

References

The original source of the article is from Air Command Water Rockets website here:
 
The following references were used in the construction of this test stand.
Other sources of load cells, amplifiers and loggers we looked at:

 



Copyright 2006-2010 Air Command Water Rockets

Total page hits since 1 Aug 2006: