ME 4090 Summer 2017

Steps for building a dual-deploy, high-power rocket

Here are the basic steps for building a rocket, listed in the approximate order in which you will want to do them (though some things, like building the airframe and building the av-bay, are basically independent so could be done in either order or, better still, at the same time). To get all the steps done in time you will need to have different team members work on different things at the same time. There isn’t enough time for everyone to work together in order on everything.

1. Use OpenRocket to make some overall design decisions, starting with a basic model.

a. Study the “basic” OpenRocket file that was provided. It doesn’t include all the parts, but enough parts to make reasonable design decisions. Take note of the length of the airframe top and bottom sections, the number, location, and shape of the fins, and the (imposed) weight of the av-bay. Also notice the value of the static margin (when an I-140 motor is loaded). Simulate a launch and notice how high the rocket goes and when during the flight the parachutes come out.

b. Modify the OpenRocket file by choosing the lengths of the airframe top and bottom sections. (Remember that airframe tubing comes in 4-foot sections and I even have some extra. You can cut sections shorter than 4 feet if you wish). Also select the number of fins (I recommend either 3 or 4) and their exact shape and location. Be sure the fins have tabs/tangs on them – these will be through-hole fins. Leave the weight of the av-bay alone for now (but eventually you change that to the actual weight of the av-bay that you build). Leave the sizes of the parachutes alone – 18” drogue and 36” main – at least for now. Make sure the static margin stays in an appropriate range (with the motor loaded).

c. Simulate a launch and notice how high the rocket goes and when during the flight the parachutes come out. The rocket should leave the rail going at least 45 ft/sec and not hit the ground going faster than 25 feet/second. The table (or the plot) will tell you how high the rocket goes, what is the best delay time for the motor, how fast it leaves the rail, how fast it hits the ground, etc. These performance values, as well as drawings of the rocket, can be used in your PDR slides. (Try “View Type: 3D figure” then use your cursor to drag the rocket to any angle then take a screenshot – you will need to add labels to that figure manually.)

2. Use CAD software to draw, then fabricate, parts from plywood and G10 fiberglass.

a. Use CAD software (ask TA Ryan for advice, if needed) to draw centering rings (2), double-thick, self-centering av-bay bulkplates (2), and an av-bay sled (1) plus attachment parts with holes for threaded rods (2). Laser-cut all of these (with TA Ryan) out of 1/4 inch thick plywood in the ME Anderson Lab (AKA Maker Space). Also have the laser cut a 1/4 inch diameter hole in the center of all 4 bulkplate pieces (for eyebolts). You can drill additional holes in pieces (for threaded rods to go through, for example) with an electric drill later. Bring your airframe, coupler, and motor-mount tubing with you to the laser cutter to ensure that the pieces you cut will in fact fit. If they do not, edit the CAD file and re-cut.

b. Use CAD software (ask TA Ryan for advice, if needed) to draw the fins you designed with OpenRocket. Make sure that you can lay out all 3 (or 4) of your fins, including the fin tabs/tangs, on a single piece of 12 inch by 12 inch (AKA 12” x 12”) G10 fiberglass, also known as garolite. Make an appointment with TA Ryan to cut your fins with the water-jet cutter in the ME Student Machine Shop. If the water-jet cutter isn’t available for any reason, we can also cut G10 fins with a band saw and sand them (in a stack) to a final, identical shape.

3. Prepare additional parts for the rocket.

a. Cut your airframe top and bottom sections to the designed length. This can be done (with TA help) on a band saw in the ME Student Machine Shop.

b. Decide how long the lower (drogue) recovery harness should be – about 3 times the length of the rocket – then cut that length of harness. For the upper (main) recovery harness, you can borrow from a kit rocket tote.

c. Cut an access hole into the shoulder of the nose cone with a Dremel (power) tool. Wear safety goggles whenever you are using power tools. See sample rocket in the lab to see what this might look like. Attach a forged eye-bolt to the nosecone and reach through the access hole to put a lock-nut on it. Make this hole as small as possible so that it doesn’t interfere with the nose cone fitting into the airframe.

d. Use a piece of angle-aluminum to draw lines on the bottom airframe tube for fin slots. Be sure these fin slots are exactly parallel to the axis of the airframe tubing and also that they are equally spaced around the perimeter of the tubing. Use a utility knife, a Dremel (power) tool, or a router (power tool), to cut the fin slots. Hold the airframe tubing in a fin-cutting jig which you are doing this. Since you will be covering these slots with epoxy, your cutting doesn’t have to be perfect (but it should be close). Dry-fit the fins in the slots to be sure they fit. Be sure the fins fit between the centering rings and won’t interfere with the centering rings.

e. Use fine sandpaper to roughen up any parts of the rocket that will be touched by epoxy or paint (almost the whole thing). Plywood and “Blue Tube” are rough already, but be sure to lightly sand the G10 fins and the plastic nosecone entirely. After sanding them, wipe off any dust.

4. Assemble the motor-mount tube / fin can. Note: This section of the rocket cannot be modified once assembled, so it is critical to get these steps right! Dry-fit all the parts and make sure you understand all the steps before beginning this section.

a. Drill a hole then attach a forged eye-bolt to the top side of the upper centering ring using a lock-nut. Make sure the eye-bolt is oriented so that the centering ring can still fit into the airframe.

b. Drill holes for two “t-nuts” on either side of the lower centering ring. They should be close enough to the motor-mount tube so that washers on the machine screws that fit into these t-nuts can be used for motor retention. The t-nuts fit in from the top (and should be glued down, but don’t get any glue in the threads!) and the screws and washers screw in from the bottom.

c. Use epoxy (West Systems (with powder) is fine for all inside epoxy tasks, like this) to attach the upper centering ring to near the top of the motor-mount tube. Put an epoxy fillet both above and below the centering ring. Be sure that the ring dries perpendicular to the axis of the motor-mount tube.

d. Tie one end of your drogue recovery harness to this eye-bolt using a rocketry-approved knot – instructions provided. You won’t be able to reach this knot after the rocket is assembled, so get it right the first time!

e. Feed the drogue recovery harness up through the lower airframe tube. Dry-fit the entire assembly as follows: Slide the motor-mount tube, upper centering ring first, into the airframe tube until the bottom of the motor-mount tube is flush with (even with) the bottom of the lower airframe. The upper centering ring should now be above the fin slots. Insert the fins until they touch the motor-mount tube. Looking up into the airframe tube from the bottom, you should be able to see where you will be putting internal epoxy joints on all the fins. Temporarily slide the lower centering ring on too, with screws and washers inserted into the t-nuts, to show that you can retain a motor. To extract the lower centering ring, pull it out by the screws. Pull out the fins and then extract the motor-mount tube as well, but leave the recovery harness through the tubing.

f. Epoxy in the top of the motor-mount unit as follows: Put epoxy on the end of a dowel (i.e. a stick) then reach into the lower airframe tube and apply a bead (i.e. a ring) of epoxy inside the airframe, just where the upper centering ring will be. (Avoid getting epoxy on the recovery harness or smearing it elsewhere inside the lower airframe tube.) Slide the motor-mount tube, upper centering ring first, up into the airframe. This part should still be dry at this point. Slide it in until the upper centering ring reaches the bead of epoxy then twist it slightly to form a continuous fillet of epoxy (mostly above the centering ring). Put the lower centering ring on the motor-mount tube too BUT DO NOT PUT ANY EPOXY ON IT, to hold the assembly exactly centered while it dries.

g. Once the epoxy on the upper centering ring is dry (several hours, at least – overnight is better), insert and epoxy the fins (one at a time is best, waiting to dry in between applications of epoxy) using DP420 “no sag” for the outer fin fillets. Use blue painter’s tape to mask off parts of the airframe and the fins that you don’t want epoxy to touch. Hold each fin exactly perpendicular to the tube using a notched angle-aluminum piece and more blue tape. Your team has two tubes of DP420 epoxy so use it sparingly (and only on joints that need to be smooth).

h. Once all the fins are epoxied externally, remove the lower centering ring and apply West Systems epoxy (with powder) fin fillets to the fins on the inside, for added strength. Add six more fillets per fin, to complement the two external fin fillets.

i. Epoxy a small wood block between two fins just above where the lower centering ring will go and screw the lower rail guide into it. Leave that loose enough that it can still spin.

j. Now that you are completely done working in the space between the motor-mount tube and the airframe tube, add a bead of epoxy around the inside of the bottom of the airframe tube and push the lower centering ring up against it, twisting slightly to form a continuous fillet (mostly above the centering ring). Add another epoxy fillet below the lower centering ring, to hold it from both above and below. Don’t let any epoxy get into the t-nut screw threads!

5. Assemble a removable av-bay structure (see example in ME 80) – wiring details later.

a. Assemble the two ends. Each will consist of a “small” bulkhead which fits inside the coupler tube plus a “large” bulkhead which is the same diameter as the coupler tube so will not fit inside it. Put an eyebolt (borrowed from a kit rocket) through the central hole to hold the two bulkhead plates together. Attach that with a lock-nut. The eyebolt faces out, so it will be on the “large” side.

b. Epoxy an airframe-diameter “collar” to the middle (vertically) of the av-bay, like a belt. This will keep the av-bay from falling inside the airframe on either end.

c. Put the sled into the av-bay and make sure both ends close fully. The sled will need to be off-center so it doesn’t interfere with the ends of the eyebolts. If the ends won’t go on fully, shorten the sled somewhat.

d. Figure out where the threaded rods need to go to hold the sled in place. Epoxy the wooden parts to the sled that the threaded rods will go through. Drill holes in the two bulkheads for the threaded rods to go through as well. Once the epoxy is dry, put the sled back in and use threaded rods plus regular nuts, lock-nuts, and/or wing nuts to hold it in place.

e. Optional (but a good idea). Decide which end of the av-bay will come off – usually the aft end – and clamp the threaded rods to the opposite end with extra nuts. Then when the av-bay is riveted into the rocket the bottom end of the av-bay will come off and the sled can slide out, but the threaded rods will stay with the upper end of the av-bay (i.e. the threaded rods will not fall out).

f. Epoxy one pvc cup (to hold an ejection charge) to the outside of each end of the av-bay, ensuring that all nuts or wing nuts can still be turned if need be. Later we will add wires to run from the ejection charges (in the cups) to the altimeter that is mounted, along with a battery and a switch, on the sled inside the av-bay. We will need to drill small holes in the two ends of the av-bay for wires to go through.

6. Finish recovery harness and add parachutes (and flame protectors).

a. Feed the upper (main) recovery harness through the upper airframe and attach its ends to the eyebolts on the bottom of the nosecone and the top of the av-bay, either with knots (carefully!) or with quick-links (borrowed from a kit rocket).

b. Slide the upper airframe down until it sits on the collar of the av-bay. It will be held there with rivets (which don’t break during a flight).

c. Put a loop in the upper recovery harness, somewhat closer to the nosecone so that when the nosecone and the upper airframe hang from the loop they don’t hit each other (the nosecone will be higher up). Attach both the main parachute and the main flame protector to the loop in the upper recovery harness.

d. Temporarily put the main parachute and the upper recovery harness into the upper airframe and make sure the nosecone can fit on the top of the upper airframe. It will be held there with shear pins (which will break when the main ejection charge fires but not when the rocket is shocked by the deployment of the drogue parachute at apogee (earlier in the flight)).

e. Attach the top end of the lower recovery harness to the eyebolt on the bottom of the av-bay, either with a knot (carefully!) or with a quick-link (borrowed from a kit rocket).

f. Put a loop in the lower recovery harness somewhat closer to the av-by so that when the upper half of the rocket (av-bay plus upper airframe plus nosecone) and the lower airframe hang from the loop they don’t hit each other (the lower airframe will be lower down). Attach both the drogue parachute and the drogue flame protector to the loop in the lower recovery harness.

g. Temporarily put the drogue parachute and the lower recovery harness into the lower airframe and make sure the bottom of the av-bay can fit on the top of the lower airframe. This will be a friction fit – we will adjust this fit to be fairly tight on the day of the launch (the tightness may depend on the weather – especially on the humidity) using tape (to make it tighter) or sandpaper (to make it looser) as need be.