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Copyright © 2008 Bdale Garbee
+ This document is released under the terms of the + + Creative Commons ShareAlike 3.0 + + license. +
Revision History | |
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Revision 1.0 | 15 November 2008 |
Successful certification flight at Hudson Ranch | |
Revision 0.2 | 28 October 2008 |
Revising during flight to DC | |
Revision 0.1 | 23 October 2008 |
Initial content, derived from YikStik |
Table of Contents
+ This is a rocket I'm building for my second attempt at a NAR Level 3 + certification flight. It's basically a Polecat Aerospace Goblin 10 kit + augmented with an additional electronics bay in the nose cone, some + structural reinforcement, and incorporating a few personal build + preferences. +
+ Preliminary analysis suggests that it should reach just under 7k feet + on the Aerotech M1297W reload, and could break two miles on the + Cesaroni M795W moon-burner. This means that a certification flight can + be supported at Hudson Ranch with the standing 8k waiver, at the Tripoli + Colorado site under their higher-altitude window, or at either of the + NCR launch sites under their standing waivers. + The smallest reasonable motor for this rocket would be a Cesaroni + K445 or equivalent, which would yield an apogee of about 2300 feet. +
Table of Contents
+ The Goblin 10 kit is a simple "four fins and a nose cone" rocket + that is short and squat, with a 98mm motor mount. + It supports dual-deploy by + using the forward end of the long motor mount tube to hold the main. + The primary electronics bay is between the forward two motor mount + centering rings, accessed by a side hatch. An additional payload bay + will be built inside the nose cone to carry experimental altimeters, + a tracking beacon, and possibly a GPS position reporting system. +
+ I will include 8-24 T-nuts in the aft centering ring spaced to allow + the use of an Aeropack 98mm retainer and associated 75mm adapter. +
+ Instead of using the supplied nose cone bulkhead, I intend to cut a + custom one that would support installing a length of 98mm motor mount + from the tip of the nose to the bulkhead. With a plate cut to cover + the aft end of the airframe tube, this would form an electronics bay + capable of holding a beacon transmitter, GPS system, or other custom + electronics. +
+ The recovery system will feature dual redundant barometric altimeters + in the main avionics bay between the two forward motor mount + centering rings. +
+ A PerfectFlite MAWD will be flown as the primary altimeter and to + record the flight altitude profile. + A MissileWorks Mini-RRC2 will fly as backup altimeter and to + directly capture max velocity. +
+ Each altimeter will have a separate battery and rotary power switch. + A third rotary switch will be used as a SAFE/ARM switch configured + to interrupt connectivity to all ejection charges in accordance with + NAR certification requirements. +
+ The Goblin 10 kit designers indicate + that the rocket is unconditionally stable with all motors that fit + the motor mount geometry. Since we're adding mass at both ends, by + putting a payload in the nose cone and by glassing the fins, the + overall stability of the design should be retained, but simulation + to confirm that seems prudent. +
+ Thorough analysis using + + RockSim + + with various motors ranging from the Cesaroni K445 through the + Aerotech M1939W always shows the stability as marginal. + This is typical of short fat rockets that don't meet normal length + to airframe diameter ratio expectations. + Given this, I take the fact that RockSim shows the stability as + marginal instead of unstable as strong evidence that the rocket + will in fact be stable in flight. + I also note that the simulated margin of stability + in my as-built configuration is fairly close to the margin of + stability of the as-designed model. +
+ The Aerotech M1297W reload should carry this vehicle to just under + 7000 feet AGL from Colorado Front Range launch sites. It + should reach just over 2 miles on a Cesaroni M795 moon burner + or equivalent. +
+ The recovery system will use dual redundant barometric altimeters + firing 4F black powder charges using commercial e-matches. + At apogee, a drogue chute will deploy with separation of the nose + cone. A Giant Leap TAC-1 36 inch chute already in hand will serve + as the drogue. + At a preset altitude, a main chute will be deployed from the forward + end of the motor mount tube to achieve recovery of the bulk of the + rocket at approximately 20 ft/sec. +
+ I intend to sew the main parachute from scratch with my wife's help + using a design documented by + + Team Vatsaas + + using 1.9oz rip-stop nylon and 550 lb parachute cord. The anticipated + build weight implies that a 10 foot parachute would be appropriately + sized. +
+ The recovery system attachment points will all use 1/4 inch u-bolts + with nuts, washers, and backing plates through bulkheads. +
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+ I have collected all of my + + build photos + + in one place, they may show better than I can explain how various + aspects of the Goblin went together. +
+ The airframe tubing provided in the Polecat kit is thick cardboard tube + with a thin exterior fiberglass wrap. To increase airframe strength, + and particularly to prevent zippers, additional reinforcement seemed + warranted. +
+ The inner layer of paper was removed from the front 9" or + so of the tube. The tube was soaked with West Systems epoxy diluted + with about 20% by volume with acetone, and then a carbon fiber wrap was + applied to the interior front of the tube and held in place during + curing by an inflatable child's bounce toy inside a plastic garbage + bag. The result is a substantially strengthened tube, with carbon + fiber lining from the leading edge back past the first centering ring. +
+ The provided nose cone bulkhead was replaced by a custom centering + ring cut from 3/8 inch birch plywood. The ring's outer diameter was + adjusted put place the ring approximately an inch forward of the end + of the motor mount tube, and the inner diameter was cut to fit Giant + Leap 98mm phenolic airframe tubing. A length of such tubing was cut + to fit inside the nose cone and extend back to flush with the trailing + edge of the ring. The centering ring was drilled and fitted with two + u-bolts for recovery system attachment and four 6-32 T-nuts to hold + a payload mounting plate in place over the aft end of the 98mm tube. +
+ The airframe tubing was glued into the tip of the nose cone with West + Systems epoxy using both milled glass and microlite filler to thicken + the mix. The centering ring was then epoxied in place using a similar + mix around the outer edge to form a heavy fillet and 5-minute epoxy to + the piece of airframe tubing. After the epoxies cured, a rotary tool + was used to cut the airframe tubing off flush with the aft surface of + the centering ring. +
+ The avionics bay walls were installed approximately 90 degrees apart + prior to installation of the motor mount assembly in the airframe. + The airframe wall was marked for a 3.5 x 6.5" access hatch centered + over the bay 90 degrees from the rail button line. This allows + sufficient room to install the switches on one side of the hatch yet + still inside the bay, and to place the static vent on the other side + of the hatch so that there will be minimal effect from air disturbed + by movement over the hatch cover edges. +
+ Rails were fabricated from 3/8" birch plywood and 6-32 blind nuts to + allow for a removable avionics sled, rectangular, with 4 screws to + hold the sled in place. + A suitably sized avionics sled should be possible to install and remove + through the avionics bay hatch allowing for possible future experiments + with alternative avionics. +
Table of Contents
+ This rocket uses dual deployment. +
+ The apogee event separates the nose cone from the + airframe. The nose cone is attached to the airframe with a length + of heavy-duty tubular nylon shock cord. A drogue chute protected + during ejection by a kevlar blanket is attached to the shock cord + close to the nose cone end. +
+ The main is a 10 foot chute sewn from the design documented by + + Team Vatsaas. + + It is held in place prior to ejection by a layer of paper taped over + the front of the motor mount tube. At ejection, a piston pushes the + chute forward through the paper and ejects it from the rocket. + This chute is attached to the airframe through an additional length of + heavy-duty tubular nylon shock cord. +
+ Depending on the results of ground testing, the main chute may be + packed in a Giant Leap kevlar deployment bag attached at the main + chute apex, with a smaller drogue chute deployed to pull off the bag + and cleanly deploy the main. The primary motivation for this is to + prevent the main chute shrouds from tangling during ejection. +
+ The main avionics bay between the forward two centering rings is + populated with two commercial altimeters, a PerfectFlite MAWD + and a Missile Works miniRRC2. + Each is powered by a dedicated 9V alkaline battery, and has a + dedicated on/off power switch mounted for access from outside the + rocket. Additionally, a single safe/arm switch with two poles is used + to interrupt the return circuits from the igniters to each altimeter. + See the attached schematic of the avionics bay contents for more + details. +
+ Details of ejection charge design goes here. +
+ The + + Info Central Black Powder Sizing + + page is the most authoritative site I've found on this topic. + The formula they suggest is diameter in inches squared times + length in inches times a coefficient in grams of black powder. + For the main charge, which will be in the 98mm motor mount tube, a + pressure of 15psi is appropriate giving a coefficient of 0.006. + For the drogue charge, which will be in the main airframe, a + pressure of 5psi is more appropriate, leading to a coefficient + of 0.002. +
+ The drogue bay is 10 inches ID at the widest point, but contains + the protrusion of the main bay and a decreasing radius in the + nose cone. Thus some fudging on the length is appropriate, and + we will use 18 inches. That works out to 3.6 grams of BP. This + rocket will not fly high enough for there to be a significant + effect on BP burn characteristics, so no special compensation + should be required. +
+ The main bay is 3.9 inches ID and perhaps as much as 24 inches long + depending on which motor is selected. + That works out to 2.2 grams of BP. +
+ Ground testing yielded 3.5 grams for the apogee charge and 1.5 grams + for the main. + Backup charges will contain additional BP in accordance + with the "blow it off or blow it up" philosophy. +
+ With a 10 foot Team Vatsaas design parachute and our + anticipated build weight, the descent rate under main + should be just over 20 feet per second. +
+ A successful level 3 certification flight occurred on 15 November 2008 + at the SCORE Hudson Ranch launch facility. The motor was an Aerotech + M1297W provided by Tim Thomas of Giant Leap Motors, the igniter was + assembled by James Russell using his special thermite mixture, and + numerious SCORE, COSROCS, and NCR members were present to assist with + the launch! Great weather for November... mostly clear and sunny, + light winds, dry ground, temps above freezing. +
+ The motor came up to pressure very quickly and the rocket leapt off + the pad, climbing smoothly under power and then doing about two slow + rolls during the coast phase. Deployment of the nose cone and drogue + occurred as planned when the primary apogee charge fired. + Unfortunately, the main deployed around the time the backup apogee + charge fired, so the descent was under main from apogee. Fortunately, + the winds were low enough and the descent rate high enough that the + rocket touched down without damage within the waiver area for a + successful certification! +
+ The rocket weighed 25.2 pounds prepared for launch without the motor. + The motor weighed about 10.25 pounds, which included about 6 pounds + of propellant. Thus the descent mass under chute was just over 29 + pounds. + The miniRRC2 altimeter reported 5949 feet apogee, 980 feet per second + max velocity, and 19 seconds to apogee. The MAWD reported 5953 feet + apogee. +
+ The ascent was straighter than expected... very smooth during + the motor burn, then a couple slow rolls during coast. The two + altimeters agreed within 4 feet on the apogee. The max + velocity recorded is a little higher than predicted by simulation, + but the accuracy of that measurement is likely limited since it is + based on pressure data. +
+ I was able to watch the apogee events through binoculars, and could + clearly see the main deploy as the backup apogee charge fired. I saw + some evidence of tearing of the paper taped over the motor mount to + retain the main chute during ground testing, so assume this was the + root cause of the early deployment. When the backup apogee charge + fired, the shock cord was not yet in tension, and thus the charge + probably kicked the airframe backwards hard enough to allow the main + chute to slide out through the torn paper and deploy. The best fix + for this might be + to fabricate a second piston to use as a cap and retain it with two + shear pins. This would be much less likely to prematurely deploy than + the current taped paper approach. +
+ The most significant variance from expectation was the descent rate. + The spreadsheet provided by the Team Vatsaas folks for their design + suggested we'd see around 21 feet per second. Analysis of the flight + profile from the MAWD shows that our actual descent rate was about + 32 feet per second. There are three possible sources of error to + consider. +
+ My calculations show that if we assume a chute size of 8.5 feet and + a Cd closer to 1, we can get to a descent rate of 32 feet per second. +
+ So, overall, this was a successful flight, but with three things to + change before we fly the airframe again... +
+