Here is way to estimate or "guesstimate" descent impact loads. First, get an estimate of the descent rate. One way to do this is to go into Rocksim and choose a tube. Just use the "mass over-ride" option to assign the mass of your rocket. Then attach a parachute to the tube and look at your descent rate. This is how fast your rocket will fall with your chosen parachute diameter. Now, you can estimate the equivalent drop height (H) for your rocket without a parachute by using the following formula:
H = 0.5 x ( (velocity)^2 ) / 32.2
"H" is the drop height in feet
"velocity" has units ft/sec.
32.2 ft/sec^2 is the acceleration of gravity.
As an example, if your rocket has a descent rate of 20 feet per second, the impact will be the same as if the rocket was dropped from 6.2 feet. Imagine dropping your rocket from 6.2 feet on the salt flats or the Pony Express without a parachute! This will be the same impact load. If you can make your parachute large enough to slow the descent rate to 10 feet per second, your equivalent drop height will be only 1.55 feet.
Originally printed in Extreme Rocketry Magazine
Rocketry is one of those things you do in life that has no in-between. You have either a complete success, or an unmitigated disaster. Every flight, including failures, is a new andunforgettable learning experience. While some of the disasters can be attributed to bad or defective equipment or materials, a lot of failures can be attributed to incorrect preparation.
You certainly feel bad when you forget wadding in your Big Bertha, but it pales in comparison to forgetting something when flying your Big Kahuna. And the more we pay attention to the successes and failures of others and ourselves, the more we learn and the better our chances are for successful flights in the future.
Flights with a regular model rocket are basic. Wadding-parachute-motor-igniter-plug and you're off to get a launch pad. Mid- and high-power rockets are more complex, so more things can go wrong. The lack of proper preparation reminds me of one flight I saw. The rocket represented a considerable investment of time, effort and money for this person. The lift-off, boost and coast were perfect, and separation charge fired at apogee. However, during separation, everybody saw the one little "oops" this rocketeer forgot: to fasten the shock cord to both sections of the rocket. The upper part of the rocket came in under parachute, but the booster came in ballistic. Ouch.
Trying to document all of the possible ways to go wrong would fill a James Michner novel. Here is a small list of failures what I have either witnessed or been guilty of myself: All it takes is something like a forgotten O-ring in the motor and you get a CATO. Or there's not fastening the shock cord correctly and you get more pieces coming down than went up. Forgetting to arm the recovery electronics gets you a ballistic rather than parachute recovery. Using the wrong size launch rod will send your rocket off in unwanted directions, if it cleared the rod at all. Forgetting wadding turns your parachute into either a melted wad or the equivalent of a screen door, both bad for future flights. Not verifying your CG on assembly can turn your vertical flight into a horizontal one. That's not a good way to get the crowd to do the Wave.
Since we are all rocket scientists, I decided to take a "page" from the professional rocket scientists and write check-off lists, or "procedures" as they call them, for rocket preparation and launch evolutions. Even in the middle of the Apollo 13 disaster, everybody had a procedure for everything. If there wasn't one, you wrote it to make sure everybody was clear on what they needed to have and what they were supposed to do. This made sure everybody was "on the same page."
Procedures are essential to a person like me. I would forget my head, as the saying goes, if it wasn't permanently attached. I run down a procedure to make sure I don't forget something every time I leave the house. If I didn't, I would leave at least one essential thing behind, every time. I started using procedures years ago when I was SCUBA diving. It is embarrassing to get to the dive site and discover you forgot your weight belt, regulator or fins (or all of them) as I did on several occasions.
The source of my organization comes from my Palm Pilot. Not only do I use it to help keep me organized, I can also recover flight data from my onboard computer into it while on the flight line as well. I mention Palm specifically because there is a shareware program called HandyShopper that I use for these lists. I use the Aisle #'s as step #'s so that I can easily adjust the order of things in a procedure if I have to. In practice, after completing a step, I merely check it off, just like if I had just grabbed the bread or eggs. If you don't have a handheld computer, clipboards and paper served the professionals for years.
The best way to develop your own procedures is to sit in a quiet area and go through everything in your mind, start to finish. After you have imagined them, write them down and go through the list again. Then go and perform the procedure, adding notes and adding/changing steps as you go. As with all endeavors in our lives, your mileage may vary. The standard that you should aim for is that anybody can understand and complete your procedures. Imagine yourself in a full body cast with your jaw wired shut. A fellow rocketeer of approximate experience should be able to get you to the range, prep and fly your rocket without any "input" from you.
The first list is the material preparation procedure. You make sure your rockets are ready, double check you have everything, test electronics, dip a few igniters, whatever you need to do to make yourself ready. This will prevent the proverbial running around like a headless chicken the morning of the launch, which cuts into flying time. Doing this over an evening or two during the week gets you 90% ready. All you have to do the night before is quickly check everything before packing it into the car to make sure no one has "borrowed" something. I verify my range box, motor box, etc. are properly stocked by writing in the bottom or on the cover of every compartment what is supposed to be there, so anything missing jumps out you.
Next you can concentrate on the family. Lay out clothes for everybody, make sure your club ID's, cash for range fees and so on are on hand (preferably packed in your range box).
The next procedure is car-packing. The order that I use to pack the vehicle is the opposite of what I will need on the range. Things that have to come out first (tables, chairs, etc.) go in last. If you pack everything but food and drink the night before, you can do it calmly and you have the time and leisure to double-check and properly secure the items. You also make sure the vehicle is up to the job. Check the fluids, tires, gas and so on. If your alarm doesn't go off and you wake up late on launch day, you can jump into your clothes, dash out to the car and drive off, with the worst consequences being you have forgotten food, drinks and family members.
Once you are on the range and set up, you can relax a bit and take a break. Fly some model rockets, catch up with club members, volunteer as RSO/LCO for a shift, whatever. Your prior planning has given you this break.
Once you are ready to launch a big rocket, pull out its' pre-flight procedure. You will probably need an individual procedure for each of your HPR rockets. This procedure should take your rocket from cold (unprepared) to warm (ready for RSO and the launch pad). The number of individual steps is not important. Clarity of the steps is important. Thirty-seven steps to load and secure the motor into the rocket might be a bit of overkill, but you don't want to have just "stick it in and tape it down" either. Make sure your flight card is filled out, electronics are installed and ready, your CP/CG ratio is good and everything is connected and ready to go.
Now comes the final countdown. Get your rocket approved by the RSO, draw a pad from the LCO, and head out with the rocket and your final preparation procedure. Verify the launch pad can handle your rocket, put the rocket on the pad, insert the igniter, arm the electronics, take the rocket from warm to hot (ready) and head back to the range head to ready your cameras.
If you have invested the time in developing your procedures, you have eliminated 98% of human error on your part. You have done everything you could to ensure a safe flight that ends in a recovered rocket.
The investment of time you spend at home developing these procedures will save countless hours and rockets on the range. As the military puts it, "The more you sweat in peacetime, the less you will bleed in wartime."
Safe and successful flying!
Remember, the things you need FIRST go in LAST.
I enjoy painting plastic models and pewter miniatures used in games. I've used many different kinds of paints, so when it came time to paint another rocket, I decided to try some of them, rather than another can of Krylon. I've settled on using artist grade acrylics for my miniatures, so I decided to start there.
I chose acrylics for a variety of reasons. Typically they are non-toxic, they clean up with soap and water, can be thinned with water, are permanent, durable, and are readily available.
I purchase most of them from either Robert's or Michael's craft stores (don't forget the 40% off coupons in the Sunday newspapers).
The brand I like best is Liquitex, but I have had good luck with Golden and Winsor & Newton. The Liquitex paints come in a little wider color range, and are a little cheaper. Experiment yourself and see what works best! The model acrylic paints (like Testor's Model Master Acryl line) do come in a wider range of colors and are already thinned, but they seem fragile-any little scratch seems to take them right off.
Most of the artist paints can be heat-treated to improve their durability. Usually baking them at 200 F for about 30 minutes works, however, my wife would not appreciate an Alpha in the oven, so I put them on the dash of my car, and park in the sun.
Acrylic paints consist of two parts: a pigment and a binder. Usually the pigment is a naturally occurring mineral that is finely ground, and the binder is a plastic resin. Most artist acrylic paints across brands are compatible, but not all. Some very interesting colors can be mixed together. Get a color wheel and experiment. Also, there are a wide variety of pearl colors, or interference colors. Pearl colors appear as metallics, and interference colors change color depending on the angle you look at them
Make sure to buy the artist-grade, and not the student-grade paint. The student-grade paints are about 2/3 the price, but have less pigment in them, and come in fewer colors.
These paints have different viscosities between brands, and even between lines in the brand. High-viscosity paints have almost a peanut butter type consistency, and need large amounts of thinning for our purposes. The lower viscosity paints typically have the same amount of pigment as their high-viscosity counterparts, but need less thinning, so they go further. I use the medium-viscosity Liquitex, or Golden's Most of these paints have 2 ratings on them: Lightfastness and Opaqueness. Lightfastness refers to the pigment's ability to withstand fading. The highest grade of Lightfastness means the paint will stay it's original color for 100 years when exposed to museum-quality lighting. Most paints fall into this category, however many of the fluorescent colors shouldn't be used-they will break down in a few days of outdoor light.
To thin these paints, a variety of items can be used. Water is the most common, but don't use too much. As the water dries, the paint shrinks, and can crack. Add a drop or two of dishwashing detergent to the water to lower the surface tension. Windshield wiper fluid is another thinning agent, however I have had the alcohol react with the plastic binder in a few brands. Do not use mineral spirits or turpentine! Unfortunately, the best thinner for acrylics is not the cheapest--mediums. Mediums are sold for different types of thinning, from brushing to airbrushing, to giving a very glossy or dull finish without any overcoat. Most mediums are the plastic resin in a nonvolatile suspension. One thing I have learned, is to use the same brand of medium for the paint - different resins can react with each other. You can use several mediums, like combining airbrush medium with gloss medium. As a rule of thumb, do not user more than 25% medium in the paint. Try thinning to different levels, but remember, it is easier to thin the paint more than thicken it back up. A little medium goes a long ways.
I have applied these paints using brushes, rollers, and airbrushes. The best results have been with an airbrush, although a roller or brush can be used equally well with a few cautions. For brushing, use the highest quality, softest, synthetic fiber bristle you can find. Natural bristles tend to absorb the moisture in the paint, so it doesn't dry naturally, and can crack. Softer bristles leave less of a brush mark. Don't skimp on a quality brush, you wil thank yourself later.
If you don't have access to an airbrush, a spray bottle (like the kind for misting plants) can work too. Find the finest spray pattern you can, the finer the spray, the more uniform the paint will be. Pump-style hairspray bottles can work, but very little paint will go down per pump. Even though the paints are labeled non-toxic, I would still recommend using a NIOSH approved paint mask for any airbrush work. To use this paint in an airbrush, thin the paint to the consistency of heavy cream. Set your air pressure to about 20-23 lbs., and hold the airbrush as perpendicular to the rocket as possible. Start the spray before the rocket, and use a smooth stroke, ending the spray after the rocket. I practice on an old piece of cardboard, so that I can get the color and consistency that I want.
If you have spirals in the body tube to fill, I've used a product called Gesso with some success. Gesso is used to prime canvas for acrylic or oil paints, so that the paint does not soak through the canvas. It looks like a thick, syrupy material. I thinned it using water, then applied it using a brush directly onto the spirals. I had to let it sit overnight for complete drying, though. After that, I sanded the Gesso smooth. You can apply masking tape about 1/16" from each side of the seam, to make a guide if you like. It may take more than one coat, try to apply thin coats. If you want to use a primer, I've used whatever I had handy-spray cans of Krylon, airbrushing model paint primer, or an acrylic primer made by Designs From the Heart, or thinned Gesso. Each seemed to work equally well, but I let the oil-based primers dry for about 48 hours before I tried to apply acrylics over them. The solvents in the primer should be completely outgassed, or the paint will bubble later.
As a top coat, I have used either Liquitex's Solvar gloss varnish, the Gloss medium, Future floor wax, or simply left it alone. Some of the paints leave a little rough surface, and do need topcoating. Future has worked well, and it is cheap. The varnishes are a little on the expensive side, but very durable. Solvar is also removable with turpentine, so you can repair the paint easily. Solvar also has some UV inhibitors in it, which helps the paint resist fading.
Liquitex has a website with good technical information at www.liquitex.com. Golden's is www.goldenpaints.com. Winsor & Newton's website is www.winsornewton.com, but it seemed to concentrate more on their oil paints. Each has technical contact email, which I have found to be fast, friendly and expert advice (Liquitex put me in contact with their chemical engineer when I had a question on heat curing!). Acrylics have been a good alternative for me. They are more versatile, cheaper, easier to use than oil based enamels, and I because of their non-toxic nature, I can paint inside my house with them. Experiment with them, and see if they work for you.
Have fun, and keep the pointy end up! This article originally appeared in the December, 2002 issue of the Ballistic Beehive.