A NAR L3 Certification Rocket
Copyright © 2008 Bdale Garbee
- This document is released under the terms of the - - Creative Commons ShareAlike 3.0 - - license. -
| Revision History | |
|---|---|
| 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. -
Table of Contents
- 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. -
-
- Planning
-
- - Pick a club launch with suitable waiver and facilities to - support flight. -
- - Confirm L3CC member(s) available to attend selected launch. -
- - Confirm that required loaner motor hardware will be available at launch. -
- - Notify launch sponsor (club president) of intended flight. -
- - Notify interested friends of intended flight. -
- - Perform final pre-flight simulation with as-built masses, etc. -
-
- Gather consummables and tools required to support flight
-
- - fresh 9V batteries -
- - black powder -
- - e-matches -
- - motor retainer and adapter parts -
- - small nylon wire ties -
- - cellulose wadding material -
- - masking tape -
- - screwdriver for phillips-head avionics bay screws -
- - small straight-blade screwdriver for power switches -
- - motor reload kit (or arrangements to procure at launch) -
- - high temperature grease -
- - long small diameter dowels for igniter insertion -
-
- Before Leaving Home
-
-
- program altimeters for suitable mach delay and recovery deployment
-
-
- MAWD
-
- - no mach delay -
- - 1300 foot main deploy -
-
-
- miniRRC2
-
- - no mach delay -
- - 1000 foot main deploy -
- - 2 seconds apogee delay -
- - no main delay -
- - dual deploy -
- - ops mode 16 (default) -
-
- MAWD
-
- - assemble all recovery system components and ensure everything fits -
- - confirm wiring and operation of altimeter power and safe/arm - switches -
- - Ground test recovery system to confirm suitable black powder - charge sizing -
-
- program altimeters for suitable mach delay and recovery deployment
-
-
- Pre-Flight
-
- - confirm payload batteries in good condition, bay loaded, - power switch works -
- - confirm reception of signals from transmitter(s) -
- - install fresh 9V batteries for altimeters on avionics bay sled -
- - inspect altimeters and associated avionics bay wiring for - visible faults -
- - close up avionics bay -
-
- build and install BP charges
-
- - Drogue Primary Charge - 3.5 grams 4F BP -
- - Drogue Backup Charge - 4.0 grams 4F BP -
- - Main Primary Charge - 1.5 grams 4F BP -
- - Main Backup Charge - 2.0 grams 4F BP -
- - fold main chute, connect recovery harness to piston and airframe, - install in MMT and tape paper over the front end -
- - fold drogue chute into a kevlar pad, connect recovery harness to - nose cone and airframe, install in airframe -
- - power up payload using switch on base plate in nose cone, then - install nose cone, using masking tape to adjust fit as required -
- - safely power up altimeters, operate safe/arm switch, - and confirm e-match continuity -
- - safe and power-down the altimeters -
- - load motor per manufacturer instructions -
- - install motor in motor mount -
- - install motor retention -
- - prepare igniter using e-matches, 1/8 inch dowel -
- - confirm all screws in place, avionics off and safe -
- - fill out a launch card -
- - notify RSO/LCO of readiness for inspection and launch, obtain - a rail assignment and permission to move rocket to launch pad for - final prep -
- - coordinate readiness with support team members, photographers, - observers -
-
- Final Prep
-
- - move rocket to launch area -
- - clean and lubricate launch rail if necessary -
- - confirm reception of signals from payload transmitter(s) -
- - mount rocket on launch rail, rotate to vertical -
- - power up primary altimeter, confirm expected beep pattern -
- - power up backup altimeter, confirm expected beep pattern -
- - arm ejection charges -
- - confirm altimeters both giving expected beep patterns for - igniter continuity -
- - install igniter and connect to launch control system -
- - capture GPS waypoint for rail location -
- - smile for the cameras, make sure we have enough "foil Murphy!" - shots taken -
- - retreat to safe area behind LCO -
- - confirm continued reception of transmitter signal(s) from - payload bay -
- - confirm photographers and observers are ready and know what to - expect -
- - make sure binoculars and backpack with water and recovery tools - are at hand -
- - tell RSO and LCO we're ready to launch -
- - try to relax and enjoy watching the flight! -
-
- Recovery
-
- - track rocket to landing site -
- - capture GPS waypoint of landing site, take lots of photos -
- - note any damage -
- - gather up and roughly re-pack recovery system for return to - flight line -
- - bring the rocket to observers for post-flight inspection -
- 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. -
- - The first is descent mass. Pre-flight calculations used - 25 pounds. - The actual flight weight was 25.2 pounds plus the burn-out - weight of the M1297W, which should be about 4.5 pounds. - That yields 29.5 pounds total. All pre-flight calculations - were done using 25 lbs, with the thought that the motor mass - might cancel out against the drag provided by the drogue. - In flight, it appeared the drogue supported the nose and the - main supported the fin can with very little interaction between - the two. -
- - Second, the dimensions given by Team Vatsaas' spreadsheet - for the pattern grid seem small. For a 10 foot chute, they - suggest a grid size of 5 inches, which looks more like an 8.5 - foot finished chute size to me. -
- - Finally, the Cd in the spreadsheet is 1.5, which may be overly - optimistic. -
- 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... -
- - the main chute may be too small -
- - switch to a piston to cap the main chute bay -
- - beef up the battery retention on the avionics sled -
-