ARLISS Data Logger

For the last few years there has been continuing discussions regarding the increased number of student payloads that seemed to have been damaged while being deployed from ARLISS rockets. (ARLISS is an annual international launch event sponsored by AEROPAC. For more information see www.ARLISS.org.) Following the 2012 ARLISS event, a team was assembled to create the ARLISS Data Logger, a device that could be used to instrument the current fleet of AEROPAC ARLISS rockets and measure the forces that the rockets were inflicting upon their payloads.

The ARLISS Data Logger is an electronics module that resides in the bottom bulkhead of the ARLISS payload carrier. Both the carrier internal payload compartment space and the carrier external dimensions were maintained so that these instrumented carriers were compatible with existing payloads and rockets. This made it possible to deploy the instrumented carriers widely across the fleet and collect sufficient data to begin to profile AEROPAC’s ARLISS fleet.

For a more in depth description of the ARLISS Data Logger Project click here.

The Loggers were first used at the 2013 ARLISS event. The data from this the 2014 and 2015 events can be viewed here. The ARLISS Data Loggers used for this event had a maximum accurate measurement range of +/-50Gs. The sensors saturated just beyond that range.

The Loggers used for the tests described on this web page have been modified to measure +/-200Gs. The Logger firmware was also modified to have the option to start recording after a fixed delay, rather than to trigger by the detection of a launch event.

Carrier Drop Tests

The high acceleration forces measured by the logger during the ARLISS 2013 event surprised us. It also made us wonder whether payload carrier acceleration was a good standalone indicator of stress exerted upon a payload. To explore this, I conducted a series of "carrier drop tests" to provide a reference for the measured accelerations.

The measurement data and a video of the drop tests are below. Click on the graph's image for a high resolution chart. The link to the Excel spreadsheet is below each image.

Drop Test Results

Spreadsheet (.xls) link. (Not captured on the video record.)	Spreadsheet (.xls) link.
Spreadsheet (.xls) link.	Spreadsheet (.xls) link.
Spreadsheet (.xls) link.	Spreadsheet (.xls) link.
Spreadsheet (.xls) link.	Spreadsheet (.xls) link.
Spreadsheet (.xls) link.	Video Record of the Drop Tests MP4 WMV (for Windows)

Dropping the payload carrier from a height of 2 feet above a lawn resulted in peak acceleration of about 140Gs!
It's clear that raw acceleration alone is not a good indicator of stress on payload.

It seems that a better indication would be impulse (the area under the acceleration curve) normalized to one second. More analysis needs to be done here.

Deployment Ground Tests

I also performed ground deployments tests varying the number of nosecone shear pin s and the the size of the CD3 CO2 cartridge (my ARLISS rocket uses CD3 deployment). The results of these tests are presented below.

The rocket upper airframe was positioned horizontally about 18" above the ground. Recoil resistance wa provided by a lawn shrub, but some recoil is evident in the videos.

Test #1 - CD3-24g, two #2-56 nylon shear screws

Test #2 - CD3-24g, four #2-56 nylon shear screws

Test #1 - CD3-16g, two #2-56 nylon shear screws

Test Procedure Comments

The test procedure ran smoothly. The Logger was set to delay 5 minutes from power-on, then to record for 8 minutes. From the time the Logger was powered-on until I was ready to trigger the test took 4.5 minutes, 5.75 minutes, and 3.75 minutes respectively. The MicroSD-card was removed after the first test and examined. After successful recording was verified, the MicroSD-card was reinserted and not removed again until all the tests were performed. Locating the portion of the xls file that contained each test interval was simple.

Results Analysis

The CD3 deployment mechanism seems to have a out/in/out deployment acceleration signature (at least in my rocket). However, the video did not show the out/in/out movement of the payload carrier that was captured on other test using black powder deployment. Since the carrier oscillation is believed to be cause by the cooling of the combustion gasses, it makes sense that CD3 deployment would not have this oscillation. (CO2 gas starts off cold rather than hot.) I suspect that the out/in/out acceleration profile may be caused by the payload carrier impacting and bouncing off the nose cone while the nosecone is still inside the rocket. The inward part of acceleration oscillation takes only about 5 milliseconds, so its not surprising that the camera, running at 30 fps, didn't record anything.

The increase in peak acceleration force that Test #2 exhibited over Test #1, seemed to verify that more or larger shear pins cause an increase in acceleration upon the payload. Nominally, the pressure below the carrier would need to increase by about 3 psi to sever the two additional shear screws. However, Test #3 should have resulted in a lower peak acceleration than either of the other two tests. That Test #3 resulted in the highest peak acceleration is perplexing. Perhaps evaluating the area under these curves or performing FFT may present an explanation.

If you have suggestions of comments on the research, please contact us by going to www.rafresearch.com/contact.html.