Techniques, Tips, and Tricks

Techniques,

Tips, and Tricks

Our first HPR rocket was a NCR Phantoom 4000. That was the first and last kit that we’ve bought. Since that time we have been doing 100% scratch building. We hope to pass along some of the things that we have learned. Some of the building techniques may have been around along time others may be new to you.

The first step in scratch building a rocket is coming up with an idea or borrowing an idea from someone else. What do you want the rocket to do? Look cool? Stand out at a launch? Extreme altitude? Before your project gets to far along in the planning stages it is best to run a quick SIMMs on it. SIMMs is a computer program that allows you to design rockets on the computer. There are several companies out there who have these programs available. A few even offer a free demo version that will get you started. Why do you need this? For several reasons. There are a few points on a rocket that need to have a certain relationship to each other in order for the rocket to fly safely. You wouldn’t want to spend all that time and money on your first scratch built rocket and have it skywriting all over the place. Would you? These points are the Center of Pressure (CP) and the Center of Gravity (CG). Basically the CP has to do with the fins, the nose cone and the body tubes surface area. It takes all this in to account and calculates them and gives you a certain point on the rocket. The CG is the easy one. It’s the balance point of the finished rocket ready to fly. These two points must have a certain relationship to each other to have the rocket fly. This relationship is referred to as Margin of Stability. Usually it is between 1 and 2.5 calibers. Caliber? A Caliber is the diameter of your body tube. And the CG must be ahead (nose cone end) of the CP. In a nutshell. Now on to the building.

Now that you have a rocket in mind and you have done your homework on the computer what motors are you going to fly it on? The reason we ask is because this will determine how strong you need to build it. If you are using 2.6” Body Tube (BT) and want to fly it on up to G’s no problem. If you want to fly it on the long burning 54’s then you will need to build it stronger. Lets start with the body tube. There are several on the market, Kraft paper, phenolic, Quantum, and fiberglass or Kevlar. They all have their advantages or disadvantages. Kraft paper tubes are not all the same Estes 2.56 BT is about twice as thin as LOC’s 2.56 BT. LOC 2.56 BT is fine up to about H power, after that they need some help. Help in the way of fiberglass or Carbon fiber. The first tube that we glassed was some 2.56 LOC. I sanded them for about four hours because I wanted the resin to soak in. The outer layer of most Kraft tubes is a shinny layer called glassine. This glassine layer is kind of waterproof. If you glass over the top of this you will not end up with a strong BT. the best thing to do is to peel the BT like and onion. Kraft tubes have several layers to them. Take a knife and slowly peel the glassine layer off. It will leave a fuzzy look but not to worry, it lays down nicely when glassed over and there is no sanding at all. Now that you have the BT ready, what size glass are you going to use? How strong do you want it to be and is weight going to be a problem? Fiberglass comes in several weights from .75 oz to 8 oz or so. How many layers are you going to need? One layer of 1.5 oz glass on a 2.6” BT would make it strong enough for an I. We have rockets that have up to 3 continuous layers of 4 oz glass that can take up to K’s. I think that when doing more then one layer of glass it is best to have them continuous, meaning not to do one layer let it set up and then do another. Do them all at the same time. This way there is not a cold joint between each layer. Just cut the cloth long enough to go all the way around three times.

Now to the motor tube (MT) and Centering Rings (CR). Motor tubes are cardboard tubes that also have a glassine outer layer. If this is not removed the glue will not be able to soak into it and will cause a week glue joint. The glassine can be peeled off the same way as a BT is. This gives the glue joints for the centering rings and fins a good place to soak into. Centering rings are easy to make. We use hole saws to make ours. The first step is to get the proper size hole saws. You’ll need one for outside diameter and one for the inside diameter. For a 2.56” BT with a 38MT the hole saws that we use are 2 ľ” and 1 5/8”. To get the outside diameter right we place the centering rings on a Ľ bolt and place it in the drill press and sand them until they are an exact fit. Remember that the MT will have the glassine layer removed and some hand sanding may be required to get it to fit properly.

Lets talk about fins now. Fins are what keep our rockets going straight. If we loose a fin in flight the rocket becomes very unstable and they take on a mind of their own. What material do we want to make fins out of? All materials have advantages and disadvantages. Balsa is lightweight and easy to work with. The problem with balsa is that it is not very strong as is. I have seen a rocket with balsa fins with a tissue-covering fly of G power. Aircraft plywood is a better choice. It comes in a bunch of different thicknesses and is fairly easy to work with and comes with most rocket kits. The thing to watch for when using it is that it comes with different number of layers or ply’s. For example some Ľ” ply may only have 3 layers others may have up to 9 layers. The more the layers the stronger it is. Plywood can also be covered with fiberglass to make it even stronger. G-10 is several layers of fiberglass cloth and some type of resin. It comes in several thicknesses from 1/32” up to 3/16”. It is the strongest stuff of the bunch. G-10 is very hard to cut and sand, and is heavy, but is the best choice by far for high performance rockets. Some people have a tough time getting glue to stick to it, which may be a bit of a problem when using it on fins. The trick to getting glue to stick to G-10 is to rough up the surface. Use 60 grit sand paper or score the surface with the blade of a hacksaw. We have even cut small groves on the root edges against the MT to give it some tooth for the glue to bite into. Fins are usually damaged in one of the following ways. During landings where the fins are the first part of the rocket to touch down, the cure for this is to either use a bigger chute to slow down the rockets decent or change the location of the fins, further up the rocket or change the style of the fins so that they don’t hang down below the rocket or use stronger fin material. The next fin problem is shredding fins off the rocket during flight. This is usually from poor fin attachment. Most often the G-10 was not roughed up enough to give the glue a good place to bite. The next cause is a big one. Fin flutter. Fin flutter occurs while the rocket is under boost. The fins begin to flutter in the air stream. They flutter so much that they can break off. Not good! The reasons for this vary. If the fin design has a short root cord and a long span. Or, if the fin material is not strong enough or is too flexible. The cure for this, prior to building is to keep the root cords long and the spans short and use the appropriate size Ply or G-10.

So far we have covered the BT, MT, CR, and Fins. Lets go to an easy one. Nose cones. Can’t I use any nose cone that fits my BT? Depends on the nose cone and what your doing with the rocket. I would not want to use an Estes NC in a rocket with a K1100. The nose cone will implode! Implode? Cave in on itself under the stress of the airflow or drag. Does this mean I can’t use Este’s nose cones for high performance rockets? No. It means you can’t use them the way they come. You have two options to make them stronger. One is to fill them with some two part expanding foam or fiberglass them. AJ’s Estes Fat Boy has 2 layers of 6 oz glass on his nose cone and it has gone over 800 mph! Most nose cones are fine for high performance use just keep this in mind when designing your rocket.

On to the Tube Couplers (TC). Now that we have built a strong rocket, can I just glue in the tube coupler? No. Tube couplers are one of the week spots on a rocket. We know this for a fact. We had a TC failure in a 4” rocket going on a K550. Not a pretty sight! We failed to strengthen the coupler. We had glassed the entire rocket but failed to give the TC any thought what so ever. We now glass the inside of our TCs. If you come up short a TC you can make your own using some scrap BT. Cut it to the desired length and then cut it lengthwise. Over lap the edges and insert it into the BT. This lets you know how much you need to cut off the diameter. The tube can then be glassed on the inside giving you a TC.

Shock cord mounting is up for grabs here. Anyway that you can mount it where it will not fail is allowable. One word of advice here. Repeat after me. “I, (state your name), will never ever use flat elastic for shock cord material. I will only use it to hold up my underwear”. Suitable material is tubular nylon, sometimes called webbing. There are two others that have pros and cons; Bungee cord and Kevlar. Bungee cord has it’s following but way be the weakest link in your recovery system, if the wrong size is used. Kevlar is fairly new and some of the high performance guys are having problems with it because there is zero stretch to it and they are snapping it on a regular basis.

There are just two items left on my list glues and paint. By this time in our building careers we have put down the white glue and picked up epoxy. Epoxy is amazing stuff. We mix it together and in 5 to 30 minutes we have a strong glue joint. Cool. How can we improve this? Lets look at 5,15 and 30-minute epoxy and what their properties are and then we can think of the best application for them. 5 minute has the quickest set time and the lowest work time and is the least flexible of the three. It is a good choice for MT to CR joints. 30 minute has longest set time and the most working time and is the most flexible. This is a good choice for the fin filets where you may want some flexibility. 15-minute is a good combination of the two. To make epoxy even stronger milled fiberglass or just some cut up fiberglass will keep fin filets from cracking as easily.

Painting. I hate painting, you name it, and I hate painting it. But we are told that our rockets look good. So I guess we do a good job at it. Without going into any particular brands. The trick to have a nice looking paint job is simple; elbow grease! We fill or remove any blemish that we can see. We then primer the entire rocket and sand off all the primer that we just put on. This fills any small imperfections. We do this three times. The fourth coat of primer is lightly sanded. Avoid sanding through this coat. The trick part is that we then put on a coat or two of flat white paint. This gives us a light undercoat that improves the color of our topcoat. It brings out colors remarkably. We then apply two coats of clear.

Building strong rockets does not mean that they have to be heavy. It means that the rocket will not fail under normal or intended use. This article turned out longer then expected. We hope you have learned something from our experiences. We are always learning new things by asking questions or learning from our mistakes. And we realize that not everyone will agree with all of our methods and that is OK. We have enjoyed passing along what we have learned.