1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Goblin 10</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="book" lang="en"><div class="titlepage"><div><div><h1 class="title"><a name="id2481338"></a>Goblin 10</h1></div><div><h2 class="subtitle">A NAR L3 Certification Rocket</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Bdale</span> <span class="surname">Garbee</span></h3></div></div><div><p class="copyright">Copyright © 2008 Bdale Garbee</p></div><div><div class="legalnotice"><a name="id2736747"></a><p>
2 This document is released under the terms of the
3 <a class="ulink" href="http://creativecommons.org/licenses/by-sa/3.0/" target="_top">
4 Creative Commons ShareAlike 3.0
7 </p></div></div><div><div class="revhistory"><table border="1" width="100%" summary="Revision history"><tr><th align="left" valign="top" colspan="2"><b>Revision History</b></th></tr><tr><td align="left">Revision 1.0</td><td align="left">15 November 2008</td></tr><tr><td align="left" colspan="2">Successful certification flight at Hudson Ranch</td></tr><tr><td align="left">Revision 0.2</td><td align="left">28 October 2008</td></tr><tr><td align="left" colspan="2">Revising during flight to DC</td></tr><tr><td align="left">Revision 0.1</td><td align="left">23 October 2008</td></tr><tr><td align="left" colspan="2">Initial content, derived from YikStik</td></tr></table></div></div></div><hr></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="chapter"><a href="#id2744045">1. Introduction</a></span></dt><dt><span class="chapter"><a href="#id2749934">2. Design</a></span></dt><dd><dl><dt><span class="section"><a href="#id2759790">Overview</a></span></dt><dt><span class="section"><a href="#id2737277">Rocksim File</a></span></dt><dt><span class="section"><a href="#id2763689">Drawing from Rocksim</a></span></dt><dt><span class="section"><a href="#id2744686">Motor Retention</a></span></dt><dt><span class="section"><a href="#id2754969">Nose Cone Electronics Bay</a></span></dt><dt><span class="section"><a href="#id2733689">Electronics</a></span></dt><dd><dl><dt><span class="section"><a href="#id2763384">Avionics</a></span></dt><dt><span class="section"><a href="#id2740504">Stability Evaluation</a></span></dt><dt><span class="section"><a href="#id2748086">Expected Performance</a></span></dt><dt><span class="section"><a href="#id2767164">Recovery System</a></span></dt></dl></dd></dl></dd><dt><span class="chapter"><a href="#id2768933">3. Construction Details</a></span></dt><dd><dl><dt><span class="section"><a href="#id2749141">Airframe</a></span></dt><dt><span class="section"><a href="#id2754017">Nose Cone</a></span></dt><dt><span class="section"><a href="#id2771414">Avionics Bay</a></span></dt></dl></dd><dt><span class="chapter"><a href="#id2745741">4. Recovery Systems Package</a></span></dt><dd><dl><dt><span class="section"><a href="#id2740673">Recovery System Description</a></span></dt><dt><span class="section"><a href="#id2752914">Recovery Initiation Control Components</a></span></dt></dl></dd><dt><span class="chapter"><a href="#id2748101">5. Checklists </a></span></dt><dt><span class="chapter"><a href="#id2750187">6. Flight Summary</a></span></dt><dt><span class="chapter"><a href="#id2764884">7. Analysis and Conclusions</a></span></dt></dl></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2744045"></a>Chapter 1. Introduction</h2></div></div></div><p>
8 This is a rocket I'm building for my second attempt at a NAR Level 3
9 certification flight. It's basically a Polecat Aerospace Goblin 10 kit
10 augmented with an additional electronics bay in the nose cone, some
11 structural reinforcement, and incorporating a few personal build
14 Preliminary analysis suggests that it should reach just under 7k feet
15 on the Aerotech M1297W reload, and could break two miles on the
16 Cesaroni M795W moon-burner. This means that a certification flight can
17 be supported at Hudson Ranch with the standing 8k waiver, at the Tripoli
18 Colorado site under their higher-altitude window, or at either of the
19 NCR launch sites under their standing waivers.
20 The smallest reasonable motor for this rocket would be a Cesaroni
21 K445 or equivalent, which would yield an apogee of about 2300 feet.
22 </p></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2749934"></a>Chapter 2. Design</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="#id2759790">Overview</a></span></dt><dt><span class="section"><a href="#id2737277">Rocksim File</a></span></dt><dt><span class="section"><a href="#id2763689">Drawing from Rocksim</a></span></dt><dt><span class="section"><a href="#id2744686">Motor Retention</a></span></dt><dt><span class="section"><a href="#id2754969">Nose Cone Electronics Bay</a></span></dt><dt><span class="section"><a href="#id2733689">Electronics</a></span></dt><dd><dl><dt><span class="section"><a href="#id2763384">Avionics</a></span></dt><dt><span class="section"><a href="#id2740504">Stability Evaluation</a></span></dt><dt><span class="section"><a href="#id2748086">Expected Performance</a></span></dt><dt><span class="section"><a href="#id2767164">Recovery System</a></span></dt></dl></dd></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2759790"></a>Overview</h2></div></div></div><p>
23 The Goblin 10 kit is a simple "four fins and a nose cone" rocket
24 that is short and squat, with a 98mm motor mount.
25 It supports dual-deploy by
26 using the forward end of the long motor mount tube to hold the main.
27 The primary electronics bay is between the forward two motor mount
28 centering rings, accessed by a side hatch. An additional payload bay
29 will be built inside the nose cone to carry experimental altimeters,
30 a tracking beacon, and possibly a GPS position reporting system.
31 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2737277"></a>Rocksim File</h2></div></div></div>
32 This is the current working design in Rocksim format:
33 <a class="ulink" href="Polecat_Goblin_10.rkt" target="_top"> Polecat_Goblin_10.rkt </a></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2763689"></a>Drawing from Rocksim</h2></div></div></div><span class="inlinemediaobject"><img src="Polecat_Goblin_10.jpg" height="450"></span></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2744686"></a>Motor Retention</h2></div></div></div><p>
34 I will include 8-24 T-nuts in the aft centering ring spaced to allow
35 the use of an Aeropack 98mm retainer and associated 75mm adapter.
36 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2754969"></a>Nose Cone Electronics Bay</h2></div></div></div><p>
37 Instead of using the supplied nose cone bulkhead, I intend to cut a
38 custom one that would support installing a length of 98mm motor mount
39 from the tip of the nose to the bulkhead. With a plate cut to cover
40 the aft end of the airframe tube, this would form an electronics bay
41 capable of holding a beacon transmitter, GPS system, or other custom
43 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2733689"></a>Electronics</h2></div></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2763384"></a>Avionics</h3></div></div></div><p>
44 The recovery system will feature dual redundant barometric altimeters
45 in the main avionics bay between the two forward motor mount
48 A PerfectFlite MAWD will be flown as the primary altimeter and to
49 record the flight altitude profile.
50 A MissileWorks Mini-RRC2 will fly as backup altimeter and to
51 directly capture max velocity.
53 Each altimeter will have a separate battery and rotary power switch.
54 A third rotary switch will be used as a SAFE/ARM switch configured
55 to interrupt connectivity to all ejection charges in accordance with
56 NAR certification requirements.
57 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2740504"></a>Stability Evaluation</h3></div></div></div><p>
58 The Goblin 10 kit designers indicate
59 that the rocket is unconditionally stable with all motors that fit
60 the motor mount geometry. Since we're adding mass at both ends, by
61 putting a payload in the nose cone and by glassing the fins, the
62 overall stability of the design should be retained, but simulation
63 to confirm that seems prudent.
65 Thorough analysis using
66 <a class="ulink" href="http://www.apogeerockets.com/rocksim.asp" target="_top">
69 with various motors ranging from the Cesaroni K445 through the
70 Aerotech M1939W always shows the stability as marginal.
71 This is typical of short fat rockets that don't meet normal length
72 to airframe diameter ratio expectations.
73 Given this, I take the fact that RockSim shows the stability as
74 marginal instead of unstable as strong evidence that the rocket
75 will in fact be stable in flight.
76 I also note that the simulated margin of stability
77 in my as-built configuration is fairly close to the margin of
78 stability of the as-designed model.
79 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2748086"></a>Expected Performance</h3></div></div></div><p>
80 The Aerotech M1297W reload should carry this vehicle to just under
81 7000 feet AGL from Colorado Front Range launch sites. It
82 should reach just over 2 miles on a Cesaroni M795 moon burner
84 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2767164"></a>Recovery System</h3></div></div></div><p>
85 The recovery system will use dual redundant barometric altimeters
86 firing 4F black powder charges using commercial e-matches.
87 At apogee, a drogue chute will deploy with separation of the nose
88 cone. A Giant Leap TAC-1 36 inch chute already in hand will serve
90 At a preset altitude, a main chute will be deployed from the forward
91 end of the motor mount tube to achieve recovery of the bulk of the
92 rocket at approximately 20 ft/sec.
94 I intend to sew the main parachute from scratch with my wife's help
95 using a design documented by
96 <a class="ulink" href="http://www.vatsaas.org/rtv/systems/Parachutes/Chute.aspx" target="_top">
99 using 1.9oz rip-stop nylon and 550 lb parachute cord. The anticipated
100 build weight implies that a 10 foot parachute would be appropriately
103 The recovery system attachment points will all use 1/4 inch u-bolts
104 with nuts, washers, and backing plates through bulkheads.
105 </p></div></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2768933"></a>Chapter 3. Construction Details</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="#id2749141">Airframe</a></span></dt><dt><span class="section"><a href="#id2754017">Nose Cone</a></span></dt><dt><span class="section"><a href="#id2771414">Avionics Bay</a></span></dt></dl></div><p>
106 I have collected all of my
107 <a class="ulink" href="http://gallery.gag.com/rockets/goblin10" target="_top">
110 in one place, they may show better than I can explain how various
111 aspects of the Goblin went together.
112 </p><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2749141"></a>Airframe</h2></div></div></div><p>
113 The airframe tubing provided in the Polecat kit is thick cardboard tube
114 with a thin exterior fiberglass wrap. To increase airframe strength,
115 and particularly to prevent zippers, additional reinforcement seemed
118 The inner layer of paper was removed from the front 9" or
119 so of the tube. The tube was soaked with West Systems epoxy diluted
120 with about 20% by volume with acetone, and then a carbon fiber wrap was
121 applied to the interior front of the tube and held in place during
122 curing by an inflatable child's bounce toy inside a plastic garbage
123 bag. The result is a substantially strengthened tube, with carbon
124 fiber lining from the leading edge back past the first centering ring.
125 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2754017"></a>Nose Cone</h2></div></div></div><p>
126 The provided nose cone bulkhead was replaced by a custom centering
127 ring cut from 3/8 inch birch plywood. The ring's outer diameter was
128 adjusted put place the ring approximately an inch forward of the end
129 of the motor mount tube, and the inner diameter was cut to fit Giant
130 Leap 98mm phenolic airframe tubing. A length of such tubing was cut
131 to fit inside the nose cone and extend back to flush with the trailing
132 edge of the ring. The centering ring was drilled and fitted with two
133 u-bolts for recovery system attachment and four 6-32 T-nuts to hold
134 a payload mounting plate in place over the aft end of the 98mm tube.
136 The airframe tubing was glued into the tip of the nose cone with West
137 Systems epoxy using both milled glass and microlite filler to thicken
138 the mix. The centering ring was then epoxied in place using a similar
139 mix around the outer edge to form a heavy fillet and 5-minute epoxy to
140 the piece of airframe tubing. After the epoxies cured, a rotary tool
141 was used to cut the airframe tubing off flush with the aft surface of
143 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2771414"></a>Avionics Bay</h2></div></div></div><p>
144 The avionics bay walls were installed approximately 90 degrees apart
145 prior to installation of the motor mount assembly in the airframe.
146 The airframe wall was marked for a 3.5 x 6.5" access hatch centered
147 over the bay 90 degrees from the rail button line. This allows
148 sufficient room to install the switches on one side of the hatch yet
149 still inside the bay, and to place the static vent on the other side
150 of the hatch so that there will be minimal effect from air disturbed
151 by movement over the hatch cover edges.
153 Rails were fabricated from 3/8" birch plywood and 6-32 blind nuts to
154 allow for a removable avionics sled, rectangular, with 4 screws to
155 hold the sled in place.
156 A suitably sized avionics sled should be possible to install and remove
157 through the avionics bay hatch allowing for possible future experiments
158 with alternative avionics.
159 </p></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2745741"></a>Chapter 4. Recovery Systems Package</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="#id2740673">Recovery System Description</a></span></dt><dt><span class="section"><a href="#id2752914">Recovery Initiation Control Components</a></span></dt></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2740673"></a>Recovery System Description</h2></div></div></div><p>
160 This rocket uses dual deployment.
162 The apogee event separates the nose cone from the
163 airframe. The nose cone is attached to the airframe with a length
164 of heavy-duty tubular nylon shock cord. A drogue chute protected
165 during ejection by a kevlar blanket is attached to the shock cord
166 close to the nose cone end.
168 The main is a 10 foot chute sewn from the design documented by
169 <a class="ulink" href="http://www.vatsaas.org/rtv/systems/Parachutes/Chute.aspx" target="_top">
172 It is held in place prior to ejection by a layer of paper taped over
173 the front of the motor mount tube. At ejection, a piston pushes the
174 chute forward through the paper and ejects it from the rocket.
175 This chute is attached to the airframe through an additional length of
176 heavy-duty tubular nylon shock cord.
178 Depending on the results of ground testing, the main chute may be
179 packed in a Giant Leap kevlar deployment bag attached at the main
180 chute apex, with a smaller drogue chute deployed to pull off the bag
181 and cleanly deploy the main. The primary motivation for this is to
182 prevent the main chute shrouds from tangling during ejection.
183 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2752914"></a>Recovery Initiation Control Components</h2></div></div></div><p>
184 The main avionics bay between the forward two centering rings is
185 populated with two commercial altimeters, a PerfectFlite MAWD
186 and a Missile Works miniRRC2.
187 Each is powered by a dedicated 9V alkaline battery, and has a
188 dedicated on/off power switch mounted for access from outside the
189 rocket. Additionally, a single safe/arm switch with two poles is used
190 to interrupt the return circuits from the igniters to each altimeter.
191 See the attached schematic of the avionics bay contents for more
194 Details of ejection charge design goes here.
197 <a class="ulink" href="http://www.info-central.org/recovery_powder.shtml" target="_top">
198 Info Central Black Powder Sizing
200 page is the most authoritative site I've found on this topic.
201 The formula they suggest is diameter in inches squared times
202 length in inches times a coefficient in grams of black powder.
203 For the main charge, which will be in the 98mm motor mount tube, a
204 pressure of 15psi is appropriate giving a coefficient of 0.006.
205 For the drogue charge, which will be in the main airframe, a
206 pressure of 5psi is more appropriate, leading to a coefficient
209 The drogue bay is 10 inches ID at the widest point, but contains
210 the protrusion of the main bay and a decreasing radius in the
211 nose cone. Thus some fudging on the length is appropriate, and
212 we will use 18 inches. That works out to 3.6 grams of BP. This
213 rocket will not fly high enough for there to be a significant
214 effect on BP burn characteristics, so no special compensation
217 The main bay is 3.9 inches ID and perhaps as much as 24 inches long
218 depending on which motor is selected.
219 That works out to 2.2 grams of BP.
221 Ground testing yielded 3.5 grams for the apogee charge and 1.5 grams
223 Backup charges will contain additional BP in accordance
224 with the "blow it off or blow it up" philosophy.
226 With a 10 foot Team Vatsaas design parachute and our
227 anticipated build weight, the descent rate under main
228 should be just over 20 feet per second.
229 </p></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2748101"></a>Chapter 5. Checklists </h2></div></div></div><div class="orderedlist"><ol type="1"><li>
231 <div class="orderedlist"><ol type="1"><li>
232 Pick a club launch with suitable waiver and facilities to
235 Confirm L3CC member(s) available to attend selected launch.
237 Confirm that required loaner motor hardware will be available at launch.
239 Notify launch sponsor (club president) of intended flight.
241 Notify interested friends of intended flight.
243 Perform final pre-flight simulation with as-built masses, etc.
245 Gather consummables and tools required to support flight
246 <div class="orderedlist"><ol type="1"><li>
253 motor retainer and adapter parts
255 small nylon wire ties
257 cellulose wadding material
261 screwdriver for phillips-head avionics bay screws
263 small straight-blade screwdriver for power switches
265 motor reload kit (or arrangements to procure at launch)
267 high temperature grease
269 long small diameter dowels for igniter insertion
270 </li></ol></div></li></ol></div></li><li>
272 <div class="orderedlist"><ol type="1"><li>
273 program altimeters for suitable mach delay and recovery deployment
274 <div class="itemizedlist"><ul type="disc"><li>
276 <div class="itemizedlist"><ul type="circle"><li>
279 1300 foot main deploy
280 </li></ul></div></li><li>
283 <div class="itemizedlist"><ul type="circle"><li>
286 1000 foot main deploy
288 2 seconds apogee delay
294 ops mode 16 (default)
295 </li></ul></div></li></ul></div></li><li>
296 assemble all recovery system components and ensure everything fits
298 confirm wiring and operation of altimeter power and safe/arm
301 Ground test recovery system to confirm suitable black powder
303 </li></ol></div></li><li>
305 <div class="orderedlist"><ol type="1"><li>
306 confirm payload batteries in good condition, bay loaded,
309 confirm reception of signals from transmitter(s)
311 install fresh 9V batteries for altimeters on avionics bay sled
313 inspect altimeters and associated avionics bay wiring for
316 close up avionics bay
318 build and install BP charges
319 <div class="orderedlist"><ol type="1"><li>
320 Drogue Primary Charge - 3.5 grams 4F BP
322 Drogue Backup Charge - 4.0 grams 4F BP
324 Main Primary Charge - 1.5 grams 4F BP
326 Main Backup Charge - 2.0 grams 4F BP
327 </li></ol></div></li><li>
328 fold main chute, connect recovery harness to piston and airframe,
329 install in MMT and tape paper over the front end
331 fold drogue chute into a kevlar pad, connect recovery harness to
332 nose cone and airframe, install in airframe
334 power up payload using switch on base plate in nose cone, then
335 install nose cone, using masking tape to adjust fit as required
337 safely power up altimeters, operate safe/arm switch,
338 and confirm e-match continuity
340 safe and power-down the altimeters
342 load motor per manufacturer instructions
344 install motor in motor mount
346 install motor retention
348 prepare igniter using e-matches, 1/8 inch dowel
350 confirm all screws in place, avionics off and safe
352 fill out a launch card
354 notify RSO/LCO of readiness for inspection and launch, obtain
355 a rail assignment and permission to move rocket to launch pad for
358 coordinate readiness with support team members, photographers,
360 </li></ol></div></li><li>
362 <div class="orderedlist"><ol type="1"><li>
363 move rocket to launch area
365 clean and lubricate launch rail if necessary
367 confirm reception of signals from payload transmitter(s)
369 mount rocket on launch rail, rotate to vertical
371 power up primary altimeter, confirm expected beep pattern
373 power up backup altimeter, confirm expected beep pattern
377 confirm altimeters both giving expected beep patterns for
380 install igniter and connect to launch control system
382 capture GPS waypoint for rail location
384 smile for the cameras, make sure we have enough "foil Murphy!"
387 retreat to safe area behind LCO
389 confirm continued reception of transmitter signal(s) from
392 confirm photographers and observers are ready and know what to
395 make sure binoculars and backpack with water and recovery tools
398 tell RSO and LCO we're ready to launch
400 try to relax and enjoy watching the flight!
401 </li></ol></div></li><li>
403 <div class="orderedlist"><ol type="1"><li>
404 track rocket to landing site
406 capture GPS waypoint of landing site, take lots of photos
410 gather up and roughly re-pack recovery system for return to
413 bring the rocket to observers for post-flight inspection
414 </li></ol></div></li></ol></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2750187"></a>Chapter 6. Flight Summary</h2></div></div></div><p>
415 A successful level 3 certification flight occurred on 15 November 2008
416 at the SCORE Hudson Ranch launch facility. The motor was an Aerotech
417 M1297W provided by Tim Thomas of Giant Leap Motors, the igniter was
418 assembled by James Russell using his special thermite mixture, and
419 numerious SCORE, COSROCS, and NCR members were present to assist with
420 the launch! Great weather for November... mostly clear and sunny,
421 light winds, dry ground, temps above freezing.
423 The motor came up to pressure very quickly and the rocket leapt off
424 the pad, climbing smoothly under power and then doing about two slow
425 rolls during the coast phase. Deployment of the nose cone and drogue
426 occurred as planned when the primary apogee charge fired.
427 Unfortunately, the main deployed around the time the backup apogee
428 charge fired, so the descent was under main from apogee. Fortunately,
429 the winds were low enough and the descent rate high enough that the
430 rocket touched down without damage within the waiver area for a
431 successful certification!
433 The rocket weighed 25.2 pounds prepared for launch without the motor.
434 The motor weighed about 10.25 pounds, which included about 6 pounds
435 of propellant. Thus the descent mass under chute was just over 29
437 The miniRRC2 altimeter reported 5949 feet apogee, 980 feet per second
438 max velocity, and 19 seconds to apogee. The MAWD reported 5953 feet
440 </p><div class="itemizedlist"><ul type="disc"><li><a class="ulink" href="http://picasaweb.google.com/jamesr2/StealeyMemorialLaunchSiteHudsonRanch" target="_top">
441 Photos of the launch taken by James Russell
442 </a></li><li><a class="ulink" href="http://cosrocs.org/all%20other%20videos/2008videos/11-15hudson/bdale_L3.mov" target="_top">
443 Video of the launch taken by Jeff Lane
444 </a></li><li><a class="ulink" href="http://www.youtube.com/watch?v=xaJnl89wfWU" target="_top">
445 Video of the launch taken by Jason Unwin
446 </a></li></ul></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2764884"></a>Chapter 7. Analysis and Conclusions</h2></div></div></div><p>
447 The ascent was straighter than expected... very smooth during
448 the motor burn, then a couple slow rolls during coast. The two
449 altimeters agreed within 4 feet on the apogee. The max
450 velocity recorded is a little higher than predicted by simulation,
451 but the accuracy of that measurement is likely limited since it is
452 based on pressure data.
454 I was able to watch the apogee events through binoculars, and could
455 clearly see the main deploy as the backup apogee charge fired. I saw
456 some evidence of tearing of the paper taped over the motor mount to
457 retain the main chute during ground testing, so assume this was the
458 root cause of the early deployment. When the backup apogee charge
459 fired, the shock cord was not yet in tension, and thus the charge
460 probably kicked the airframe backwards hard enough to allow the main
461 chute to slide out through the torn paper and deploy. The best fix
463 to fabricate a second piston to use as a cap and retain it with two
464 shear pins. This would be much less likely to prematurely deploy than
465 the current taped paper approach.
467 The most significant variance from expectation was the descent rate.
468 The spreadsheet provided by the Team Vatsaas folks for their design
469 suggested we'd see around 21 feet per second. Analysis of the flight
470 profile from the MAWD shows that our actual descent rate was about
471 32 feet per second. There are three possible sources of error to
473 </p><div class="orderedlist"><ol type="1"><li>
474 The first is descent mass. Pre-flight calculations used
476 The actual flight weight was 25.2 pounds plus the burn-out
477 weight of the M1297W, which should be about 4.5 pounds.
478 That yields 29.5 pounds total. All pre-flight calculations
479 were done using 25 lbs, with the thought that the motor mass
480 might cancel out against the drag provided by the drogue.
481 In flight, it appeared the drogue supported the nose and the
482 main supported the fin can with very little interaction between
485 Second, the dimensions given by Team Vatsaas' spreadsheet
486 for the pattern grid seem small. For a 10 foot chute, they
487 suggest a grid size of 5 inches, which looks more like an 8.5
488 foot finished chute size to me.
490 Finally, the Cd in the spreadsheet is 1.5, which may be overly
493 My calculations show that if we assume a chute size of 8.5 feet and
494 a Cd closer to 1, we can get to a descent rate of 32 feet per second.
496 So, overall, this was a successful flight, but with three things to
497 change before we fly the airframe again...
498 </p><div class="orderedlist"><ol type="1"><li>
499 the main chute may be too small
501 switch to a piston to cap the main chute bay
503 beef up the battery retention on the avionics sled
505 </p></div></div></body></html>
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