By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
This time, weâ€™ll discuss forward bulkhead fabrication, the altimeter and installing rail buttons Wow! Even more rocket science!!
Forward Bulkhead Fabrication
Some of you know writing this monologue takes more time than the actual task. In other words, for those of you who would like to build a rocket, it takes some degree of skill, but not necessarily a lot of time. If I didnâ€™t goof off or get distracted, I could likely build the rocket in one weekend. However, we canâ€™t spend all day in the shop, can we? Moving along. If you look at the photo at right, youâ€™ll see turning the forward bulkhead and extra plywood discs on the lathe. The other photo below shows the completed parts. It doesnâ€™t take much time to cut the plywood on the table saw, cut the corners on the band saw and mount the discs on a lathe mandrel for turning. Afterwards, I installed the bulkhead in the forward body tube with 5-minute epoxy. With the forward body tube now almost complete, I will use one of discs for the altimeter mount. I will then layout and drill the static ports in the payload bay. These ports â€“ three spaced equally around the tubeâ€™s circumference ensure the altimeter is exposed to the changing ambient atmospheric pressure as the rocket ascends. An altimeter you say?
Yes, this particular altimeter has a miniature barometric sensor. Very cool. Itâ€™s a Jolly-Logic “Altimeter OneÂ®â€. The specifications mention this altimeter has a microcontroller that evaluates ambient atmospheric pressure ~20x/second – it simultaneously updates its LCD display 64x/second. The pressure sensor measures altitudes up to 29,500 feet above sea level. The altimeter measurement error is +/- one foot for altitudes up to 10,000-feet. The power supply is an integral lithium polymer battery. Once charged from a USB port (~2-hours), the battery will last ~14 hours. I just charge it in the morning on a launch day and it’s good to go. Its size? It’s less than 2″ long, just over a half-inch wide and about a half-inch thick. Heck, it’ll fit in some of the smallest model rockets available. The altimeter mass is only 6.7 grams too. I think it is well worth the $50 given its performance. For more information, I recommend you visit the Jolly Logic website (www.jollylogic.com). Later, weâ€™ll talk about the static ports the payload bay must have to allow the altimeter to sense the ambient pressure.
Rail Button Installation
Remember the basic rocket diagram in part-2? It shows the â€œrail lugsâ€ or buttons on the side. The two rail buttons slide into the longitudinal launch rail slot. They hold the rocket to the rail, allowing it to slide with very little friction at launch. They look like miniature plastic spools. They are commonly available from rocketry suppliers, but you know me. I had to machine a bunch on the lathe from DelrinÂ® (acetal polymer) bar stock for me and my rocket buddies. Once I devised some rudimentary gauges and set-up the lathe, it only took a minute to machine each button. I then adapted Madcow Rocketry (www. madcowrocketry. com) kit instructions for rail button installation.
First, I located the two rail button holes by using a piece of aluminum angle to draw a thin pencil line from the aft end along the lower body tube. Next, I drilled one hole 2.5â€ in from the lower body tubeâ€™s aft end. Then the forward hole was drilled just behind the center of gravity (CG) point. This second hole went through the tube wall into the shock cord anchor â€“ very solid. I used a #8 (0.1990â€) drill to make the holes for the 8-32 T-nut anchor. Afterwards, I applied medium viscosity cyanoacrylate (CA) adhesive to the holes and the underside of the T-nuts before whacking them in place with a hammer like a maniac pressing them into the body tube wall. I attached the buttons to the T-nut anchors with the 8-32 machine screws and tightened them. Another key point – ensure the screws do not protrude inside the body tube â€“ they could interfere with parachute ejection, the shock cord or motor.
Next time, weâ€™ll talk about how I determined the rocketâ€™s center of gravity and center of pressure. Weâ€™ll also see the result from the first rocket stability simulation. Wow! We love rocket science!!