In this level-2 EZI-65 lesson, we will finish the rocket construction and verify our design.

• Finish Airframe and Bay Details
• Check Stability
• Size Recovery System

Finish Airframe and Bay Details

Before finishing, we need to cut the holes for the two toggle switches in the bay and drill the various holes in the bay and airframe. Measure your two toggle switches from the front. Mine ended up needing a 4x9mm slot for the handle to poke through and the mounting holes were 2cm apart. You want to cut as small a hole as possible to allow the switch to travel fully. You also need to figure out how far apart the switches need to be spaced so they won't interfere with each other. (Remember that the tube is curved and the switches will be angled towards each other a little when installed.) Mine needed 9mm separation between the holes.

The altimeter bay has relatively little room to put these controls. We'll line them up 1" from the aft end of the bay to give at least 1 caliper of space between the seam where the bay joins the payload section and the vent holes for the altimeter (static pressure ports). You want three holes spaced evenly around the airframe. The easiest way to do this is wrap a long piece of paper around the airframe and mark where one end overlaps the paper. Unwrap the paper and measure this distance. Mine came out to 12 3/4" which divides easily into three. Make two marks between the far end and the edge mark, dividing the strip evenly into thirds. In the center of the strip, lay out the slots and holes for your switches. Re-wrap the paper around the altimeter bay and transfer your marks to the tube. (I spaced my holes 1" from the aft edge of the body tube.)

Now cut out the two slots for the switch handles with a sharp hobby knife. Take your time and try not to cut outside of the slot as this will weaken the airframe. Once you have the slots cut out, drill the holes for mounting the switches. Test fit your switches from the inside of the bay and make sure they will mount properly later.

Drill three 5/32" holes at the equally spaced marks. These holes provide vents (also called "static ports") to equalize pressure inside the rocket so that altimeter reads the correct outside pressure as the rocket ascends. The instructions usually call for one 1/4" hole or three "smaller holes." I chose three 5/32" holes because they have just a little more area than one 1/4" hole (measured in 32nds: 3 × 5² vs. 1 × 8²).

You also need to drill vent holes in the payload section and main airframe. These holes also allow pressure to equalize, but for a different reason. At ground level, the pressure is higher and as the rocket ascends, the outside pressure becomes lower than the inside pressure. This tends to make the inside pressure act as a force to separate the rocket. Premature separation may occur if enough pressure builds up. However, we don't want to defeat the ejection charge, so we want much smaller holes. I used two 1/8" holes in the payload section and main airframe. Use the paper strip method to get the holes evenly spaced and make sure they clear the nose cone shoulder and altimeter bay coupler. I centered my vent holes on the payload bay and put them 11" down from the forward end of the body tube.

Because the slots and holes are into the soft cardboard tubing inside your fiberglass shell, wick some CA into all switch slots and vent holes. (The CA hardens the edges of the cardboard and allows you sand it.) Once the CA has thoroughly dried, clean out the holes and switch slots with a hobby knife and sand the inside smooth.

Now we'll mount the altimeter to its plate. Mount the plate with two 3/4" 6-32 machine screws and plastic stand-offs to the mounting plate. Use the third hole you drilled to strap the wiring harness to the plate as a strain relief using a small cable tie. Finally, wrap a piece of electrical tape around the base of each of the slide switches so the body can't short the other switch.

Check Stability

We need to make sure our rocket will be stable. Since we added 5 1/2" to the length and didn't change the fin geometry, we're pretty sure it will be, but let's go ahead and check it anyway. The first thing to do is weigh the rocket and measure its center of gravity (CG). Then we must calculate the center of pressure (CP) of the rocket. There are several programs which will do this. I like the WINROC CPCalc program by Stephen Roberson. Once we know the CP, we can make sure the center of gravity (CG) is forward of it. (See the CG/CP Relation section in INFOcentral for more information.)

In order to weigh the rocket, find the CG and take measurements for the CP, you will need to assemble your rocket in as close to finished form as possible. Mount the altimeter into the altimeter bay with the two eyebolts and put the rocket parts together. First of all, weigh the rocket and record the weight. This is the dry weight. (Mine was 3.9#. Interestingly my previous stock EZI-65 weighed 2.6#.)

To find the CG, we'll need to select the largest motor we plan to use. I picked a K550, but a smaller motor would be OK for the more cautious. Tape the appropriate casing and reload to the back of the rocket, more or less where the motor will be when installed and find the point where the rocket balances. Measure the distance from the tip of the nose cone and record it. This is the center of gravity (CG). (Mine was 46"). If you don't have a reload handy, a AeroTech K550 and Dr. Rockets 54/1706 casing weigh 3.4#, and you can use something to simulate that weight.

Now we find the center of pressure (CP) through calculation. I used CPCalc which determined the CP at 49.5" from the front (see the CPCalc Screen). With the heavy K550, this isn't stable enough (49.5" - 46" = 3.5" which is just under the 4" diameter). With a high impulse motor like the K550, this is probably OK, but I'd still add some nose weight to fly the rocket on this motor.

The motor one is more likely to use for level-2 certification would be a J180. (See the INFOcentral article on Motor Selection for a more detailed description.) For this motor, my CG came out to 44" (the J180 + 54/852 case weighs 1.8#), which is stable (49.5" - 44" = 5.5", which is greater than the 4" diameter). The lift-off weight for this configuration will be 5.7# (3.9# rocket + 1.8# motor), which requires 127N/s average thrust motor, well within the J180s specs. (See its TMT test data.) For level 1 certification, we might choose an I154, which would be even lighter. (See its data.)

Size Recovery System

We need to choose parachutes capable of bringing our rocket down safely. Since we're using dual deployment, we can aford to oversize the main parachute slightly since it won't be falling that far. For those launching in the desert or other hard surface, this is a nice option.

First of all, read the INFOcentral article on Drogue Sizing to determine the size of parachute we need for the drogue. Our rocket is very like the example and my calculations come to 42 ft/sec, which is a reasonable descent rate. I calculated: A = 2.7475ft² and W = 4.25#: (3.9# plus a little extra for the spent motor). I decided to add a 4" mylar streamer anyway, since it is an aid in tracking, but it isn't necessary to slow down the rocket.

Next read the Main Chute Sizing article. I'm planning to use a SkyAngle parachute (made by The b₂ Rocketry Company which is sort of between a circular and an X-form 'chute. For a 15ft/sec descent rate, a circular 'chute would need to be 55" in diameter and an an X-form 'chute would need to be 72" in the longest dimension. b₂ recommends 3 ft² per pound of rocket so at 4.25# we'd expect to use their 36" parachute. Since their parachutes are more like X-form parachutes and measure twice their rated size this seems to match nicely with my calculations.

We need to make our recovery harness as well. You should use tubular nylon or Kevlar instead of the elastic provided by LOC/Precision. (Hopefully you just threw it away.) Tubular nylon can be purchased at well-equipped sporting good stores in the climbing department. I like the 1/2" wide nylon webbing. it comes in many colors and is very strong. You need a nice long piece between the altimeter bay and main airframe and an even longer piece between the payload section and the nose cone. I rescued a strap from a larger deceased rocket and ended up with 8' and 16' straps. You need to sew a loop in each end to attach to the quick links. I used a local shoe repair store which charged $5 per end. You can do it yourself, but it's a pain, believe me.

We also need to consider how we'll protect our recovery system from the ejection charge. You can sort of use wadding, but for dual deployment, you either need to use a deployment bag or a Nomex cloth. Pratt Hobbies sells Nomex parachute protectors which I always use. For the EZI, I used two of the X-large size, since that's what I had laying around.

The last thing we have to do is make sure the parts fit properly together. My couplers and nose cone were a little small for the body tubes. You want the sections to fit together so that you can just lift the rocket without them separating. Much tighter and you'll have to increase the ejection charge too much. The traditional way to do this is by wrapping tape around the the coupler/nose cone shoulder until you get the right fit. My favorite technique is to bond a piece of 220 grit sandpaper to the shoulder. Either way, make sure the pieces won't separate too early. In particular, you want to make sure the main doesn't come out at apogee! You might also consider using shear pins, see the INFOcentral article on Shear Pin Design.

You've now finished construction of your rocket, fitted your electronics, verified the stability and designed the recovery system. The rest of it is just finishing and final assembly. If you've gotten this far, pat yourself on the back and start planning your paint scheme!