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="id2560049"></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="id2815458"></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="#id2822757">1. Introduction</a></span></dt><dt><span class="chapter"><a href="#id2828645">2. Design</a></span></dt><dd><dl><dt><span class="section"><a href="#id2838502">Overview</a></span></dt><dt><span class="section"><a href="#id2815988">Rocksim File</a></span></dt><dt><span class="section"><a href="#id2842401">Drawing from Rocksim</a></span></dt><dt><span class="section"><a href="#id2823397">Motor Retention</a></span></dt><dt><span class="section"><a href="#id2833680">Nose Cone Electronics Bay</a></span></dt><dt><span class="section"><a href="#id2812400">Electronics</a></span></dt><dd><dl><dt><span class="section"><a href="#id2842095">Avionics</a></span></dt><dt><span class="section"><a href="#id2819216">Stability Evaluation</a></span></dt><dt><span class="section"><a href="#id2826797">Expected Performance</a></span></dt><dt><span class="section"><a href="#id2845875">Recovery System</a></span></dt></dl></dd></dl></dd><dt><span class="chapter"><a href="#id2847645">3. Construction Details</a></span></dt><dd><dl><dt><span class="section"><a href="#id2845740">Airframe</a></span></dt><dt><span class="section"><a href="#id2813144">Nose Cone</a></span></dt><dt><span class="section"><a href="#id2834138">Avionics Bay</a></span></dt></dl></dd><dt><span class="chapter"><a href="#id2831717">4. Recovery Systems Package</a></span></dt><dd><dl><dt><span class="section"><a href="#id2847622">Recovery System Description</a></span></dt><dt><span class="section"><a href="#id2830883">Recovery Initiation Control Components</a></span></dt></dl></dd><dt><span class="chapter"><a href="#id2846528">5. Checklists </a></span></dt><dt><span class="chapter"><a href="#id2842033">6. Flight Summary</a></span></dt><dt><span class="chapter"><a href="#id2842161">7. Analysis and Conclusions</a></span></dt></dl></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2822757"></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="id2828645"></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="#id2838502">Overview</a></span></dt><dt><span class="section"><a href="#id2815988">Rocksim File</a></span></dt><dt><span class="section"><a href="#id2842401">Drawing from Rocksim</a></span></dt><dt><span class="section"><a href="#id2823397">Motor Retention</a></span></dt><dt><span class="section"><a href="#id2833680">Nose Cone Electronics Bay</a></span></dt><dt><span class="section"><a href="#id2812400">Electronics</a></span></dt><dd><dl><dt><span class="section"><a href="#id2842095">Avionics</a></span></dt><dt><span class="section"><a href="#id2819216">Stability Evaluation</a></span></dt><dt><span class="section"><a href="#id2826797">Expected Performance</a></span></dt><dt><span class="section"><a href="#id2845875">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="id2838502"></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="id2815988"></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="id2842401"></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="id2823397"></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="id2833680"></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="id2812400"></a>Electronics</h2></div></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2842095"></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="id2819216"></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="id2826797"></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="id2845875"></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="id2847645"></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="#id2845740">Airframe</a></span></dt><dt><span class="section"><a href="#id2813144">Nose Cone</a></span></dt><dt><span class="section"><a href="#id2834138">Avionics Bay</a></span></dt></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2845740"></a>Airframe</h2></div></div></div><p>
106 The airframe tubing provided in the Polecat kit is thick cardboard tube
107 with a thin exterior fiberglass wrap. To increase airframe strength,
108 and particularly to prevent zippers, additional reinforcement seemed
111 The inner layer of paper was removed from the front 9" or
112 so of the tube. The tube was soaked with West Systems epoxy diluted
113 with about 20% by volume with acetone, and then a carbon fiber wrap was
114 applied to the interior front of the tube and held in place during
115 curing by an inflatable child's bounce toy inside a plastic garbage
116 bag. The result is a substantially strengthened tube, with carbon
117 fiber lining from the leading edge back past the first centering ring.
118 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2813144"></a>Nose Cone</h2></div></div></div><p>
119 The provided nose cone bulkhead was replaced by a custom centering
120 ring cut from 3/8 inch birch plywood. The ring's outer diameter was
121 adjusted put place the ring approximately an inch forward of the end
122 of the motor mount tube, and the inner diameter was cut to fit Giant
123 Leap 98mm phenolic airframe tubing. A length of such tubing was cut
124 to fit inside the nose cone and extend back to flush with the trailing
125 edge of the ring. The centering ring was drilled and fitted with two
126 u-bolts for recovery system attachment and four 6-32 T-nuts to hold
127 a payload mounting plate in place over the aft end of the 98mm tube.
129 The airframe tubing was glued into the tip of the nose cone with West
130 Systems epoxy using both milled glass and microlite filler to thicken
131 the mix. The centering ring was then epoxied in place using a similar
132 mix around the outer edge to form a heavy fillet and 5-minute epoxy to
133 the piece of airframe tubing. After the epoxies cured, a rotary tool
134 was used to cut the airframe tubing off flush with the aft surface of
136 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2834138"></a>Avionics Bay</h2></div></div></div><p>
137 The avionics bay walls were installed approximately 90 degrees apart
138 prior to installation of the motor mount assembly in the airframe.
139 The airframe wall was marked for a 3.5 x 6.5" access hatch centered
140 over the bay 90 degrees from the rail button line. This allows
141 sufficient room to install the switches on one side of the hatch yet
142 still inside the bay, and to place the static vent on the other side
143 of the hatch so that there will be minimal effect from air disturbed
144 by movement over the hatch cover edges.
146 Rails were fabricated from 3/8" birch plywood and 6-32 blind nuts to
147 allow for a removable avionics sled, rectangular, with 4 screws to
148 hold the sled in place.
149 A suitably sized avionics sled should be possible to install and remove
150 through the avionics bay hatch allowing for possible future experiments
151 with alternative avionics.
152 </p></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2831717"></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="#id2847622">Recovery System Description</a></span></dt><dt><span class="section"><a href="#id2830883">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="id2847622"></a>Recovery System Description</h2></div></div></div><p>
153 This rocket uses dual deployment.
155 The apogee event separates the nose cone from the
156 airframe. The nose cone is attached to the airframe with a length
157 of heavy-duty tubular nylon shock cord. A drogue chute protected
158 during ejection by a kevlar blanket is attached to the shock cord
159 close to the nose cone end.
161 The main is a 10 foot chute sewn from the design documented by
162 <a class="ulink" href="http://www.vatsaas.org/rtv/systems/Parachutes/Chute.aspx" target="_top">
165 It is held in place prior to ejection by a layer of paper taped over
166 the front of the motor mount tube. At ejection, a piston pushes the
167 chute forward through the paper and ejects it from the rocket.
168 This chute is attached to the airframe through an additional length of
169 heavy-duty tubular nylon shock cord.
171 Depending on the results of ground testing, the main chute may be
172 packed in a Giant Leap kevlar deployment bag attached at the main
173 chute apex, with a smaller drogue chute deployed to pull off the bag
174 and cleanly deploy the main. The primary motivation for this is to
175 prevent the main chute shrouds from tangling during ejection.
176 </p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2830883"></a>Recovery Initiation Control Components</h2></div></div></div><p>
177 The main avionics bay between the forward two centering rings is
178 populated with two commercial altimeters, a PerfectFlite MAWD
179 and a Missile Works miniRRC2.
180 Each is powered by a dedicated 9V alkaline battery, and has a
181 dedicated on/off power switch mounted for access from outside the
182 rocket. Additionally, a single safe/arm switch with two poles is used
183 to interrupt the return circuits from the igniters to each altimeter.
184 See the attached schematic of the avionics bay contents for more
187 Details of ejection charge design goes here.
190 <a class="ulink" href="http://www.info-central.org/recovery_powder.shtml" target="_top">
191 Info Central Black Powder Sizing
193 page is the most authoritative site I've found on this topic.
194 The formula they suggest is diameter in inches squared times
195 length in inches times a coefficient in grams of black powder.
196 For the main charge, which will be in the 98mm motor mount tube, a
197 pressure of 15psi is appropriate giving a coefficient of 0.006.
198 For the drogue charge, which will be in the main airframe, a
199 pressure of 5psi is more appropriate, leading to a coefficient
202 The drogue bay is 10 inches ID at the widest point, but contains
203 the protrusion of the main bay and a decreasing radius in the
204 nose cone. Thus some fudging on the length is appropriate, and
205 we will use 18 inches. That works out to 3.6 grams of BP. This
206 rocket will not fly high enough for there to be a significant
207 effect on BP burn characteristics, so no special compensation
210 The main bay is 3.9 inches ID and perhaps as much as 24 inches long
211 depending on which motor is selected.
212 That works out to 2.2 grams of BP.
214 Ground testing yielded 3.5 grams for the apogee charge and 1.5 grams
216 Backup charges will contain additional BP in accordance
217 with the "blow it off or blow it up" philosophy.
219 With a 10 foot Team Vatsaas design parachute and our
220 anticipated build weight, the descent rate under main
221 should be just over 20 feet per second.
222 </p></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2846528"></a>Chapter 5. Checklists </h2></div></div></div><div class="orderedlist"><ol type="1"><li>
224 <div class="orderedlist"><ol type="1"><li>
225 Pick a club launch with suitable waiver and facilities to
228 Confirm L3CC member(s) available to attend selected launch.
230 Confirm that required loaner motor hardware will be available at launch.
232 Notify launch sponsor (club president) of intended flight.
234 Notify interested friends of intended flight.
236 Perform final pre-flight simulation with as-built masses, etc.
238 Gather consummables and tools required to support flight
239 <div class="orderedlist"><ol type="1"><li>
246 motor retainer and adapter parts
248 small nylon wire ties
250 cellulose wadding material
254 screwdriver for phillips-head avionics bay screws
256 small straight-blade screwdriver for power switches
258 motor reload kit (or arrangements to procure at launch)
260 high temperature grease
262 long small diameter dowels for igniter insertion
263 </li></ol></div></li></ol></div></li><li>
265 <div class="orderedlist"><ol type="1"><li>
266 program altimeters for suitable mach delay and recovery deployment
267 <div class="itemizedlist"><ul type="disc"><li>
269 <div class="itemizedlist"><ul type="circle"><li>
272 1300 foot main deploy
273 </li></ul></div></li><li>
276 <div class="itemizedlist"><ul type="circle"><li>
279 1000 foot main deploy
281 2 seconds apogee delay
287 ops mode 16 (default)
288 </li></ul></div></li></ul></div></li><li>
289 assemble all recovery system components and ensure everything fits
291 confirm wiring and operation of altimeter power and safe/arm
294 Ground test recovery system to confirm suitable black powder
296 </li></ol></div></li><li>
298 <div class="orderedlist"><ol type="1"><li>
299 confirm payload batteries in good condition, bay loaded,
302 confirm reception of signals from transmitter(s)
304 install fresh 9V batteries for altimeters on avionics bay sled
306 inspect altimeters and associated avionics bay wiring for
309 close up avionics bay
311 build and install BP charges
312 <div class="orderedlist"><ol type="1"><li>
313 Drogue Primary Charge - 3.5 grams 4F BP
315 Drogue Backup Charge - 4.0 grams 4F BP
317 Main Primary Charge - 1.5 grams 4F BP
319 Main Backup Charge - 2.0 grams 4F BP
320 </li></ol></div></li><li>
321 fold main chute, connect recovery harness to piston and airframe,
322 install in MMT and tape paper over the front end
324 fold drogue chute into a kevlar pad, connect recovery harness to
325 nose cone and airframe, install in airframe
327 power up payload using switch on base plate in nose cone, then
328 install nose cone, using masking tape to adjust fit as required
330 safely power up altimeters, operate safe/arm switch,
331 and confirm e-match continuity
333 safe and power-down the altimeters
335 load motor per manufacturer instructions
337 install motor in motor mount
339 install motor retention
341 prepare igniter using e-matches, 1/8 inch dowel
343 confirm all screws in place, avionics off and safe
345 fill out a launch card
347 notify RSO/LCO of readiness for inspection and launch, obtain
348 a rail assignment and permission to move rocket to launch pad for
351 coordinate readiness with support team members, photographers,
353 </li></ol></div></li><li>
355 <div class="orderedlist"><ol type="1"><li>
356 move rocket to launch area
358 clean and lubricate launch rail if necessary
360 confirm reception of signals from payload transmitter(s)
362 mount rocket on launch rail, rotate to vertical
364 power up primary altimeter, confirm expected beep pattern
366 power up backup altimeter, confirm expected beep pattern
370 confirm altimeters both giving expected beep patterns for
373 install igniter and connect to launch control system
375 capture GPS waypoint for rail location
377 smile for the cameras, make sure we have enough "foil Murphy!"
380 retreat to safe area behind LCO
382 confirm continued reception of transmitter signal(s) from
385 confirm photographers and observers are ready and know what to
388 make sure binoculars and backpack with water and recovery tools
391 tell RSO and LCO we're ready to launch
393 try to relax and enjoy watching the flight!
394 </li></ol></div></li><li>
396 <div class="orderedlist"><ol type="1"><li>
397 track rocket to landing site
399 capture GPS waypoint of landing site, take lots of photos
403 gather up and roughly re-pack recovery system for return to
406 bring the rocket to observers for post-flight inspection
407 </li></ol></div></li></ol></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2842033"></a>Chapter 6. Flight Summary</h2></div></div></div><p>
408 A successful level 3 certification flight occurred on 15 November 2008
409 at the SCORE Hudson Ranch launch facility. The motor was an Aerotech
410 M1297W provided by Tim Thomas of Giant Leap Motors, the igniter was
411 assembled by James Russell using his special thermite mixture, and
412 numerious SCORE, COSROCS, and NCR members were present to assist with
413 the launch! Great weather for November... mostly clear and sunny,
414 light winds, dry ground, temps above freezing.
416 The motor came up to pressure very quickly and the rocket leapt off
417 the pad, climbing smoothly under power and then doing about two slow
418 rolls during the coast phase. Deployment of the nose cone and drogue
419 occurred as planned when the primary apogee charge fired.
420 Unfortunately, the main deployed around the time the backup apogee
421 charge fired, so the descent was under main from apogee. Fortunately,
422 the winds were low enough and the descent rate high enough that the
423 rocket touched down without damage within the waiver area for a
424 successful certification!
426 The rocket weighed 25.2 pounds prepared for launch without the motor.
427 The motor weighed about 10.25 pounds, which included about 6 pounds
428 of propellant. Thus the descent mass under chute was just over 29
430 The miniRRC2 altimeter reported 5949 feet apogee, 980 feet per second
431 max velocity, and 19 seconds to apogee. The MAWD reported 5953 feet
433 </p><div class="itemizedlist"><ul type="disc"><li><a class="ulink" href="http://picasaweb.google.com/jamesr2/StealeyMemorialLaunchSiteHudsonRanch" target="_top">
434 Photos of the launch taken by James Russell
435 </a></li><li><a class="ulink" href="http://cosrocs.org/all%20other%20videos/2008videos/11-15hudson/bdale_L3.mov" target="_top">
436 Video of the launch taken by Jeff Lane
437 </a></li><li><a class="ulink" href="http://www.youtube.com/watch?v=xaJnl89wfWU" target="_top">
438 Video of the launch taken by Jason Unwin
439 </a></li></ul></div></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="id2842161"></a>Chapter 7. Analysis and Conclusions</h2></div></div></div><p>
440 The ascent was straighter than expected... very smooth during
441 the motor burn, then a couple slow rolls during coast. The two
442 altimeters agreed within 4 feet on the apogee. The max
443 velocity recorded is a little higher than predicted by simulation,
444 but the accuracy of that measurement is likely limited since it is
445 based on pressure data.
447 I was able to watch the apogee events through binoculars, and could
448 clearly see the main deploy as the backup apogee charge fired. I saw
449 some evidence of tearing of the paper taped over the motor mount to
450 retain the main chute during ground testing, so assume this was the
451 root cause of the early deployment. When the backup apogee charge
452 fired, the shock cord was not yet in tension, and thus the charge
453 probably kicked the airframe backwards hard enough to allow the main
454 chute to slide out through the torn paper and deploy. The best fix
456 to fabricate a second piston to use as a cap and retain it with two
457 shear pins. This would be much less likely to prematurely deploy than
458 the current taped paper approach.
460 The most significant variance from expectation was the descent rate.
461 The spreadsheet provided by the Team Vatsaas folks for their design
462 suggested we'd see around 21 feet per second. Analysis of the flight
463 profile from the MAWD shows that our actual descent rate was about
464 32 feet per second. There are three possible sources of error to
466 </p><div class="orderedlist"><ol type="1"><li>
467 The first is descent mass. Pre-flight calculations used
469 The actual flight weight was 25.2 pounds plus the burn-out
470 weight of the M1297W, which should be about 4.5 pounds.
471 That yields 29.5 pounds total. All pre-flight calculations
472 were done using 25 lbs, with the thought that the motor mass
473 might cancel out against the drag provided by the drogue.
474 In flight, it appeared the drogue supported the nose and the
475 main supported the fin can with very little interaction between
478 Second, the dimensions given by Team Vatsaas' spreadsheet
479 for the pattern grid seem small. For a 10 foot chute, they
480 suggest a grid size of 5 inches, which looks more like an 8.5
481 foot finished chute size to me.
483 Finally, the Cd in the spreadsheet is 1.5, which may be overly
486 My calculations show that if we assume a chute size of 8.5 feet and
487 a Cd closer to 1, we can get to a descent rate of 32 feet per second.
489 So, overall, this was a successful flight, but with three things to
490 change before we fly the airframe again...
491 </p><div class="orderedlist"><ol type="1"><li>
492 the main chute may be too small
494 switch to a piston to cap the main chute bay
496 beef up the battery retention on the avionics sled
498 </p></div></div></body></html>
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