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The function of a rocket's airframe may vary slightly by the design of the rocket, but predominantly, the airframe is the central structural member of the rocket. The motor mount and the fins transfer thrust to the airframe, and everything else - electronic bays, nose cone, bulkheads, etc. - is pushed by the airframe. Rockets can have multiple airframes like the Estes Trident, or Estes Ram Jet, of just one like most typical rockets. Regardless, the airframe is susceptible to all of the stresses involved in the entire flight from thrust, mach transitions, early or late ejection events, the recovery harness tugging on it in different directions, and the impact of landing in often awkward attitudes.
Because of the stresses involved, the airframe's material plays an important role in the performance and in some cases, the motor selection for a rocket. Take a look.
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The most common rocket airframe is made of cardboard. These range from the pre-made tubes from Estes or Quest, to cardboard mailing tubes, or the cores of paper towels, or Christmas wrapping paper. These tubes are created by laminating several layers of thin cardboard together using glue to wrap it around a form at an angle, creating the typical spiral pattern commonly found on these tubes. These tubes are made in a very wide variety of sizes from very small, like Estes' BT-5 to 10" diameter like the Polecat Aerospace Thumper. Using pre-made tubes usually makes building a bit easier because you can find stock parts like centering rings, bulkheads and more importantly, nose cones. One down-side, is if your rocket lands in a pond, or is dragged through the mud, its chances of survival without special treatment is going to be significantly decreased. Most tubing made for model rocketry comes with a thin glassine coating to reduce or eliminate the spiral groove commonly seen in these sorts of tubing, and to make finishing easier, however using these tubes with a covering/reinforcement layer of fiberglass for example, requires removing this protective layer.
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Phenolic tubes are manufactured in the same manner as Kraft cardboard tubes, except instead of glue, the paper strips are given a thin coating of phenolic resin. This encapsulates and soaks into the paper, making it resistant to moisture and quite a bit stiffer than Kraft tubes. This also results in more weight, but this is usually of little consequence for the benefit of added strength and durability. Another detractor, is that phenolic tubes are usually more brittle than Kraft tubing, where a hard landing or impact can sever the tube, rather than causing it to "unwind" like kraft tubes tend to do. Phenolic tubes have been known to survive landing in a pond, or being caked with mud and rinsed out with water without an adverse affect on performance. They also stand up well to wet-sanding if you're looking for a flawless finish without having to add a composite reinforcement layer.
While Phenolic tubes typically have a higher compression strength than kraft tubes, they can shatter easily. Some companies are developing tubing that can tolerate up to 50% more compression without breaking. More recently, there have been developments in materials that allow products like seamless linen phenolic tubes, which offer the same or better strength, but with no spiral seam. This reduces the amount of time it takes to finish a rocket, or prep for a composite reinforcement.
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A 'composite' is a combination of any number of materials to form one cohesive unit. Most high-performance rocketeers prefer to use composite tubing. This can mean that they use fiberglass, carbon fiber, or even kevlar to wrap standard kraft or phenolic tubing to make it stronger, or they go the whole nine yards, and make or purchase tubes made entirely from fiberlgass, carbon fiber or kevlar, or even a mix of the three. Composite tubes can be made in any size, and can even use honeycomb, foam, or other core materials sandwiched between the layers to alter the characteristics of the finished tube. Depending on the desired goal, composite tubes can run from just a couple of bucks for several tubes, to several hundred dollars per tube.
Composite reinforcement - Using a standard Kraft tube, Phenolic tube, or even concrete forming tubes found in your hardware store or local building supply, and protective layer is removed from the side to be reinforced, and either fiberglass, carbon fiber, kevlar, or a mix of these materials is laminated to the tube using a laminating epoxy resin. The resulting tube is a bit heavier than it started out, because the epoxy is soaked into the base tube, and forms a much more rigid part than the original. The type of material, its weight, weave, and even the resin that you use can play a large role in the strength and weight of the finished part. Often it makes sense to use this sort of construction, because you can usually find a nose cone that will fit the base tube, as well as centering rings and bulkheads that will easily fit inside it.
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Full-composite Tubing - Making your own tubes can be done in several different ways, but for each, you have to start with a form called a mandrel. You can use a standard cardboard or phenolic tube as a mandrel, covering it with a sacrificial layer to prevent it from sticking to your layup, or you can find, say an Aluminum tube that matches the ID of the tube you want to make. There is a lot of prep-work, and post-processing involved in making your own tubes, but the end result is not only custom-tailored to your needs, but it is also very light, and very strong. Composite tubes can take advantage of the ability to form anything you want, like an integrated transition, or motor retention to core materials like Nomex or Aluminum Honeycomb, or foam.
As well as choice of material, and material weight, you can also choose how your material is shaped. Carbon Fiber and Fiberglass can be found with different weaves, each with different pros and cons. Some manufacturers have just cloth, with a 2x2 weave, or a twill weave, while others sell a uni-directional cloth with sparse rovings of say kevlar to keep things in place, while others may blend carbon fiber and a colored kevlar for decoration, or a fiberglass cloth woven in a spiral, similar to a "Chinese Fingercuff" allowing one weave size to be used on a range of tube sizes. Using different materials and weaves in layers, you can make an incredibly light tube that will stand up to whatever you can think of throwing at it, provide for a flawless base for a paint job, or to just look cool.
You also have the option to purchase these components from a small variety of vendors already made for you. Filament-Wound Fiberglass tubing is a very popular alternative to making your own tubing, and is one component very few people are prepared to make at home. Filament winding involved wrapping a single gathered strand of fibers, dipped into resin around a mandrel, and coursed back and forth over the length of the mandrel to fill the voids and complete the tubing. This process usually involved the use of specialized and very expensive tubing, so is usually accomplished by manufacturers of other, similar products to be sold cost-effectively.
Whichever material or process you choose for your tubing, It's a good idea to know what is available to you, and what you might want to spend in order to get the results you are looking for. Most often, it's as simple as picking up some tubes at a hobby shop, or a local launch, but sometimes, the project you're working on warrants more attention to detail than that.
Archive Info-Central article, revised by Tim Scott |
