Does Heat Warp Quantum Tube?

Abstract

We've learned that poop pipe has a significant amount of thermal expansion when compared to other rocketry materials. This is evidenced by the tests done on the “Effects of Temperature on HPR Materials” page. Could this amount of thermal expansion be the culprit in squirrelly rocket flights? I set up an experiment to see what happens if a rocket is left in the sun during a launch. One side of the rocket gets full sun, while the other side is left in shade.

Very few rockets in our hobby don't have at least some spin on the way up. As a bent or curved rocket spins in flight it corkscrews.

Take Eric Salerno’s PML Endeavor. This rocket has been known to fly “squirrelly” on hot sunny days. On at least two occasions, the trajectory of the rocket has exhibited a corkscrew pattern. On one occasion, a bend in the rocket could actually be seen while sitting on the pad. It was hypothesized that when sitting in the sun, one side of the rocket heats up, and the shade side stays relatively cool. This would cause the tube to expand on the hot side and warp the airframe.

Postulate

Does Quantum Tubing warp in the sun, causing a temporary warp in the airframe?

Investigation

Test Outline

A rocket would be place on a fixture. It would be rotated and measured for any deviation from straightness using a calibrated dial indicator. External influential factors would be measured while conducting the test such as rocket temperature on both the sun side and shade side, along with several ambient weather conditions. All measurements would be taken prior to conducting the test in the sun to establish a baseline.

Fixturing

The fixture was made with a 1 x 6 pine board. One inch holes were drilled to accommodate a V-block on each end. The V-blocks were placed 18” apart. This distance was chosen because there was a coupler and a PML CPR 3000 in the center. I believe that the coupler would stiffen the center 4” of the tube and reduce the effective tube length to 14”, or 1.16 feet. A stop was secured to the plywood to ensure that the dial indicator would always trace the same circumference location.

The fixture assembly was placed on a table and was checked for stability. A dial indicator was secured to a magnetic base. The magnetic base was secured to steel for stability.

The whole assembly was tested for stability and repeatability. The forward section of a maroon painted PML Endeavor was placed in the V-blocks and butted up against the stop before exposure to the sun. This is the actual rocket mentioned in the abstract of this paper. A mark was placed on the tube to ensure that the same location was always up after rotational measurement.

At the start of the test

Equipment Used

First Test Conducted

The skies were partly cloudy with a light breeze from the WSW. The ambient temp ranged from 73°F to 75°F. The relative humidity started at 63% and steadily fell to 50%.  The clouds caused problems during testing as the sun kept going behind the clouds and the surface of the rocket kept cooling off. I thought of scrapping the test for the day until I realized that this would eliminate the possibility of some other influence causing the tube bending. It quickly became apparent: more sun, more bend, less sun, less bend. The test was concluded when the clouds became increasingly abundant.

During the test

Measuring the temperature

The results were clear: the tube bent upward when heated by the sun.

Figure 1
Graph showing plotted data

Second Test Conducted

Since the weather conditions were variable, a second test was conducted. The skies were clear with a light breeze from the SW. The ambient temperature remained constant at 75°. The relative humidity ranged from 62% to 55%.  In the interest of uniformity, the test was conducted at the same time of day.

Figure 2
Graph showing plotted data

Discussion

When most any material is heated, it expands. This is true for Quantum Tubing. As demonstrated in the “Effects of Temperature on HPR Materials” test, Quantum Tubing expands and contracts greater than other rocket building materials such as wood or fiberglass. That test demonstrated this, and why pistons get stuck when it is cold outside.

As the sun heats up the tubing, it expands. Since the ½ of the tube is in the shade, the sunlit side gets hotter than the shaded side. While the material expands on the sun heated side, the shaded side stays relatively constant. This causes the tube to bend.

Figure 3
Illustrating one side heated resulting in bending

The graphs clearly show that as the surface of the rocket heats up, the tube becomes more bent. The greater the temperature, the greater the bend.

Looking at the graphs, we also notice that the greatest difference in “Tube Temp” to “Deflection” correlates with the difference in “Sun Side” to “Bottom Side”. This shows that the greater the temperature differential, the greater the bend. This is most evident in the 12:04 PM EST data points on the Aug 15 graph, Figure 1.

Since we measured the deflection at ½ of the distance between the V-blocks, we can assume that the total deflection, as measured from one end to the other is 2 times the deflection. With an effective test distance of 14”, we can formulate the following:

Def x 2 / 1.16 = Total Deflection per foot

The greatest deflection was 0.030”. Thus:

0.030 x 2 / 1.16 = 0.052” per foot

This means that a 6 foot tall rocket can bend at least 5/16”.

On the second day of testing, the temperature of the nose cone was measured. When the test area read 126.5°F, the nosecone was 142.8°F.

Conclusions

1.   The evidence shows that when one side of a rocket made from Quantum Tubing, the sun side expands more than the shaded side.

2.   The evidence suggests that for every ~52°F differential heating on a Quantum Tube, a 4” diameter rocket will bend ~0.052” per foot.

3.   To reduce the amount of “heat bending” and for straighter flights, keep your rocket in the shade, and paint it a light color.

Appendix A

Raw Data collected: