I am on a quest to figure out how to build large rockets (8-24-inch diameters) that are extremely lightweight. They are not Mach busters or altitude-record setters, but they are fun to watch and cheap-ish to fly.
This note is to report on a little experiment I ran looking for better airframe wall materials. The perfect material would be cheap, light, stiff, strong, readily available, and easy to work. Unobtainium!
Executive Summary
For my application, cardboard wrap is probably a good replacement for honeycomb core.
Raw Materials
I used the following materials to construct test samples.
[F] Fiberglass – 6 oz, plain-weave, E-glass. Commonly available in 50-inch widths for about $5/yd.
[E] Epoxy – U.S. Composites 635 thin resin with 3:1 hardener. $72/gallon kit.
[C] Cardboard wrap – Single faced corrugated cardboard with 1/4-inch A-flutes. I got a 12-inch x 250-foot(!) roll from Staples for about $25.
[H] Aramid honeycomb – 1/4-inch thick, 3/16-inch cells. I can get a 4’x8’ sheet locally from Express Composites for $145.
Test Samples
I prepared the following six samples. Each was a flat plank, 4 inches wide and 12 inches long.
[F] – One ply of fiberglass, saturated with epoxy.
[F+C] – One ply of fiberglass, saturated with epoxy and bonded to the corrugated side of the cardboard wrap. The entire corrugated side of the cardboard was painted with epoxy. The flat side was left untreated. The corrugations run parallel to the long side of the plank.
[F+C+E] – The same as [F+C] with the flat side of the cardboard also painted with epoxy.
[F+C+F] – The same as [F+C+E] with an additional ply of fiberglass bonded to the flat side of the cardboard.
[F+H] – One ply of fiberglass, saturated with epoxy and bonded to a section of honeycomb. A thin layer of epoxy thickened with U.S. Composites Faring Filler was squeegeed on the fiberglass to get decent bonding with the honeycomb.
[F+H+F] – Like [F+H] but with a layer of fiberglass bonded to both sides of the honeycomb.
Test Procedure
Each test sample spanned an 8-inch gap. A bucket was suspended in the middle of the sample. The bucket was filled with water until the sample collapsed and fell through the gap. To prevent the bucket handle from cutting into the sample, a 1.5-inch x 4-inch strip of plywood was put between the sample and the handle. When the sample finally gave way the bucket, water, and plywood strip were weighed. This measured weight is the “breaking weight”.
Results
Here are the measured results. The most interesting results are that adding cardboard [F+C] is 1000 times stronger than just the fiberglass [F] and that honeycomb core is only 3 (or maybe 5) times as strong as cardboard core but it is 45 times as expensive.
The reported costs do not include the epoxy. I only used a few ounces of resin for the entire test.
In the [F] test I did not use the pail and water setup. A single U.S. quarter was enough to break the sample.
In the [F+H] test the sample was severely deformed but did not fall through. I believe the unfished side of the honeycomb snagged the wood that formed the edges of the gap.
In the [F+H+F] test the sample did not break. My bucket did not hold enough water. Adding a bunch of other weights helped but didn’t get there. With the reported 900 oz of weight the sample was sagging a little, maybe 1/4-inch.
Future Work / Exercises for the Reader
I tested the resistance of the materials to bending in one direction. The walls of a rocket experience both lateral and longitudinal forces. Both directions should be tested. My guess is that in the long direction the cardboard cores are actually stronger than the honeycomb.
I tested the asymmetric samples (F+C, F+C+E, F+H) with the fiberglass (in compression) on top of the core material. The results may differ if the fiberglass (in tension) was below the core material.
Rocket walls are pipes, not flat planks. A next step would be to build tube sections (say 8-inch diameter by 12-inch long) and test them.
I did not try to measure partial deformations of the samples. A proper materials testing set up would be able to capture the deformation as a function of weight.
Pictures
Cardboard wrap
Prepared test samples
Test sample with little deformation
Highly stressed test sample
This note is to report on a little experiment I ran looking for better airframe wall materials. The perfect material would be cheap, light, stiff, strong, readily available, and easy to work. Unobtainium!
Executive Summary
For my application, cardboard wrap is probably a good replacement for honeycomb core.
Raw Materials
I used the following materials to construct test samples.
[F] Fiberglass – 6 oz, plain-weave, E-glass. Commonly available in 50-inch widths for about $5/yd.
[E] Epoxy – U.S. Composites 635 thin resin with 3:1 hardener. $72/gallon kit.
[C] Cardboard wrap – Single faced corrugated cardboard with 1/4-inch A-flutes. I got a 12-inch x 250-foot(!) roll from Staples for about $25.
[H] Aramid honeycomb – 1/4-inch thick, 3/16-inch cells. I can get a 4’x8’ sheet locally from Express Composites for $145.
Test Samples
I prepared the following six samples. Each was a flat plank, 4 inches wide and 12 inches long.
[F] – One ply of fiberglass, saturated with epoxy.
[F+C] – One ply of fiberglass, saturated with epoxy and bonded to the corrugated side of the cardboard wrap. The entire corrugated side of the cardboard was painted with epoxy. The flat side was left untreated. The corrugations run parallel to the long side of the plank.
[F+C+E] – The same as [F+C] with the flat side of the cardboard also painted with epoxy.
[F+C+F] – The same as [F+C+E] with an additional ply of fiberglass bonded to the flat side of the cardboard.
[F+H] – One ply of fiberglass, saturated with epoxy and bonded to a section of honeycomb. A thin layer of epoxy thickened with U.S. Composites Faring Filler was squeegeed on the fiberglass to get decent bonding with the honeycomb.
[F+H+F] – Like [F+H] but with a layer of fiberglass bonded to both sides of the honeycomb.
Test Procedure
Each test sample spanned an 8-inch gap. A bucket was suspended in the middle of the sample. The bucket was filled with water until the sample collapsed and fell through the gap. To prevent the bucket handle from cutting into the sample, a 1.5-inch x 4-inch strip of plywood was put between the sample and the handle. When the sample finally gave way the bucket, water, and plywood strip were weighed. This measured weight is the “breaking weight”.
Results
Here are the measured results. The most interesting results are that adding cardboard [F+C] is 1000 times stronger than just the fiberglass [F] and that honeycomb core is only 3 (or maybe 5) times as strong as cardboard core but it is 45 times as expensive.
| Sample | Weight | Cost | Breaking Weight | Breaking Weight / Weight | Breaking Weight / Cost |
| oz / ft^2 | $ / ft^2 | oz | oz / (oz/ft^2) | oz / ($ / ft^2) | |
| F | 1.10 | 0.40 | 0.2 | v 0.19 | 0.52 |
| F+C | 2.88 | 0.50 | 221 | 76.8 | 442 |
| F+C+E | 3.12 | 0.50 | 276 | 88.5 | 552 |
| F+C+F | 4.18 | 0.90 | 330 | 78.8 | 366 |
| F+H | 3.49 | 4.87 | 95 | 27.2 | 20 |
| F+H+F | 5.53 | 5.27 | 900 | 162.8 | 171 |
The reported costs do not include the epoxy. I only used a few ounces of resin for the entire test.
In the [F] test I did not use the pail and water setup. A single U.S. quarter was enough to break the sample.
In the [F+H] test the sample was severely deformed but did not fall through. I believe the unfished side of the honeycomb snagged the wood that formed the edges of the gap.
In the [F+H+F] test the sample did not break. My bucket did not hold enough water. Adding a bunch of other weights helped but didn’t get there. With the reported 900 oz of weight the sample was sagging a little, maybe 1/4-inch.
Future Work / Exercises for the Reader
I tested the resistance of the materials to bending in one direction. The walls of a rocket experience both lateral and longitudinal forces. Both directions should be tested. My guess is that in the long direction the cardboard cores are actually stronger than the honeycomb.
I tested the asymmetric samples (F+C, F+C+E, F+H) with the fiberglass (in compression) on top of the core material. The results may differ if the fiberglass (in tension) was below the core material.
Rocket walls are pipes, not flat planks. A next step would be to build tube sections (say 8-inch diameter by 12-inch long) and test them.
I did not try to measure partial deformations of the samples. A proper materials testing set up would be able to capture the deformation as a function of weight.
Pictures
Cardboard wrap
Prepared test samples
Test sample with little deformation
Highly stressed test sample
.
