Ablatives Thermal Testing

Overview:

To match conditions similar to those expected from the high speed flight, a propane blowtorch will be used to heat small samples of different materials with different ablative coatings (see test matrix below) while a thermocouple will monitor the surface temperature of the materials. The mass and thickness of the samples will be measured before and after the test to determine the effectiveness of the ablative coating used. A test stand has been built to hold the propane blowtorch at a specific distance from the samples (which can be adjusted as necessary). 

The tests will be performed in a secure environment (blast chamber) and the operators will wear respirators during the tests. This document from Apogee Components was used as inspiration for this test procedure. Apogee Newsletter

Materials Used:

• Fiberglass
• West Systems 105 epoxy (resin + hardener)
• Phenolic microballoons
• POR 15 High-temp Heat Resistant Paint
• 400-grit sandpaper
• Small popsicle sticks
• Mixing boat for epoxy
• Dremel 
• Worthington propane blowtorch (14.1oz)
• Arduino Uno

• Thermocouple Amplifier MAX31855 breakout board (MAX6675 upgrade): https://www.adafruit.com/product/269

• K-type thermocouple: https://www.adafruit.com/product/270

• Jumper wires

• Breadboard
• Test stand
• Calipers
• Mini scale
• PPE: Gloves, respirators, safety goggles

Safety Notes:

• Use a respirator when working with phenolic microballoons because the tiny particles will get into your lungs otherwise. Make sure you have the proper canisters for your respirators as well (contact EHS for details)
• Use gloves whenever you are working with fiberglass because the fibers are very splintery. Additionally, the samples will have sharp edges before they are cut and sanded.
• Use safety goggles when using the Dremel (sanding or cutting) so your eyes don't get irritated by the dust or so small pieces don't fly into your eyes when cutting the samples!
• Make sure to use a respirator with proper canisters when doing the blowtorch testing because off-gassing will occur when the samples are heated. 

Below is the matrix for samples used in this thermal test. All samples measured ~2" x 2" (with thickness depending on the sample), and there are three samples for each combination (for three trials). Each sample is labeled by "series" (1-9) with a hyphen, followed by the number of the trial. (so the nomenclature is series-trial number, for example: 4-2)

Fiberglass Sample Preparation:

1. Used West Systems epoxy to make 4 of 8" x 8" sheets of fiberglass (2 x 5 layers, 2 x 10 layers), making sure to put Mylar beneath to keep the samples from sticking. 
2. Let the samples cure for at least 24 hours. 
3. Sanded with a Dremel, then with 400-grit sandpaper. Make sure to sand the shiny side as well. 
4. Mark and cut into 1.5" x 1.5" squares using a Dremel
5. Recorded dry mass of each sample (series 1-9)
6. Took three measurements of thickness on three different sides of the sample, and used this to calculate/record average thickness (series 1-9)
7. Wearing respirators, mixed 50 g. West Systems resin + 9 g West Systems hardener (100:18 ratio for West Systems, but this depends on the epoxy you choose to use)
8. Mixed in phenolic microballoons using a popsicle stick until a consistency slightly thinner than peanut butter (like melted ice cream) was reached.
9. Applied an even but thin layer of phenolic mixture to samples (series 4, 5, 8, 9) using popsicle sticks
10. Let samples cure for 36 hours.
11. Wipe samples with acetone to remove any dust or debris.
12. Recorded average thickness of samples with phenolic coating (series 4, 5, 8, 9)
13. Sanded with 400-grit sandpaper then recorded mass (and wiped with acetone)
14. Applied high-temp paint to fiberglass samples (series 6,7) and fiberglass + phenolic samples (series 8,9)
15. Let paint dry for at least 24 hours (depending on what paint you use)
16. Wipe samples with acetone to remove any dust or debris.
17. Record mass of each sample (series 6, 7, 8, 9)
18. Record average thickness of each sample (series 6, 7, 8, 9)

Test Procedure:

1. Testing in blast chamber
2. Heat for approximately 12 seconds

Below are some pictures of the test stand, thermocouple used, Serial Monitor showing that the thermocouple works, and the fiberglass samples used (before they were cut and ablative coating was added)

Test #1: 

The first thermal test we conducted was on a single 5-layer control sample of fiberglass (using West Systems epoxy). The purpose of this test was simply to see what improvements needed to be made for the next test, during which more samples will be tested. Below are photos of the sample after the test, as well as a graph of temperature vs. time. 

Test Details:

• A thermocouple was attached to the back of the sample with Kapton tape
• The blowtorch was positioned 10cm away from the sample
• The sample was heated for approximately 12 seconds
• The exhaust in the blast chamber was turned on

Observations:

• The sample caught fire during heating, and remained on fire after the blowtorch was turned off (eventually self-extinguished)
• The epoxy holding the layers together was burned (and off-gassing was observed) but the epoxy within each layer seemed to be intact.
• The thermocouple (attached with Kapton tape) separated from the sample during heating. This is the likely cause of the small step observed in the graph below.

Improvements for Next Time:

• Videotape each sample the entire length of the test (in case anything interesting happens after the blowtorch is extinguished, for example)
• Bring an IR sensor to detect the flame temperature
• Attach the thermocouple more securely/consider a different method of attachment so it stays on the sample
• Have a camera on both sides of the sample
• Note the start time on the serial monitor (Arduino) to keep track of when the flame was turned on/off
• Increase the distance between the blowtorch and the sample (measure temperature of air vs. distance of thermocouple from blowtorch, and temperature of flame itself)
• When preparing the samples (fiberglass layup), use a flatter surface so that the average thickness is more consistent
• If testing high-temp paint, use more than one coat so that there is enough paint to show results
• Find some way of remote-triggering the blow torch for added safety
• Use a more accurate scale so that initial and final mass is more accurate
• When measuring thickness (for average thickness calculation), be consistent when choosing points to measure on the sample, and take more than three measurements for a more accurate estimate
• Use thermocouple with thinner tip for more accuracy

Test #2: 

Final Results:

Below is a table showing the results of the thermal tests. Consult the test matrix above to see what each "series" corresponds to (for example, "Series 1" means 1/4" phenolic sheet). All samples measured 2" x 2" in area. For a more detailed spreadsheet, consult the attached Excel spreadsheet. (ATTACH SPREADSHEET) In the table below, since some samples have multiple layers of ablative coating, a key is used to denote the mass/thickness of each layer, followed by the total mass/thickness.

F = fiberglass

P = phenolic sheet

PM = phenolic microballoons

H = high-temperature paint

*Mass of sample measured after hole drilled (to attach sample to test stand) and after all ablative coatings have been added, which is why there is a discrepancy between the total mass and the sum of the masses of each layer. In some cases, the mass measures zero for the high-temperature paint because the scale was not accurate enough and the layer was thin.

Useful links:

https://tfaws.nasa.gov/TFAWS12/Proceedings/Aerothermodynamics%20Course.pdf