Archive for January, 2011

A Goal for 2011

Tuesday, January 4th, 2011

Accomplishing one of my goals (dual deployment) on the very first day of the year has me in search of a replacement. Something a bit more challenging.  Looking to later in the year when I hope to make my Level 3 certification flight, I realized that flight will result in a number of firsts for me, if I am successful.  It will be the highest, probably just shy of 13,000′, and the fastest, Mach 1.5.  However, that is all being done by sheer brute force of a 75mm M3100 motor.  Certainly there must be a way to finesse some of these.  I started playing around with some simple designs and simulations with the quick realization that I could go a lot faster with far less motor.  So my goal is to do Mach 1.8 with a 38mm motor.  This is actually fairly easy as well so, I added a little twist.  I want to do this with a tube-finned rocket.

I spent yesterday evening searching for good information on the specifications of various materials used to make the airframes.  There really is a bit of a void here and I think I see an opportunity to make a contribution.  I’m going to make several, at least 4, identical rockets that differ only in the materials used and see how they perform.  There is quite a range of specifications too which should make this interesting – perhaps even exciting if material limitations are surpassed in flight testing.  A tube-finned rocket is ideal in this respect for several reasons.  The tube fins create a lot of drag and will be put to the test, structurally, when the rocket is pushed through Mach 1.  I plan to choose a fin geometry to accentuate that. The geometry of tube-finned rockets spreads the loads in such a way that my ability to achieve uniform glue properties and proper bonding of the fin to the main body tube should not be the limiting factor – at least for the weaker materials.

Right now my list of tubes includes: LOC/Precision tube, Blue Tube, G12 fiberglass epoxy, and Carbon Fiber epoxy.  I may add Quantum tubing after a little more study to see if it warrants inclusion. The initial consensus is that CF is the obvious choice – lighter weight than fiberglass and stronger.  G12 will certainly work but is the heaviest.  G12 is a safe bet.  LOC tube is standard thick-walled kraft paper tubing.  It is the lightest and also the weakest.  Most people expect it to fail at high speed.  Blue Tube will be interesting.  It is strong and light, lighter than CF epoxy, but begins to yield much earlier that CF. All of these tubes have the same wall thickness so, this should be a fair test.

I plan to use motors in the Vmax series from Cesaroni (although the White Thunder and RedLine series have interesting thrust curves).  These Vmax motors burn for less than one second with very high thrust.  In these minimum-diameter, high-speed rockets powered by high thrust motors, weight is of the essence.  The lighter it is, the faster it will go with a given motor and rocket geometry.  Lighter rockets will not go as high as the heavier rockets, however, because more of the energy put into the system ends up as momentum in the heavy rocket rather than being “wasted” pushing the air out of the way with the lighter, faster rockets.  I could have decided to use mid-thrust longer burning motors but, that would be mainly a test of the drag differences between the rockets as much as it was about structural stability.  I could select altitude as the metric but, that would favor heavier rockets.  The proper metric must be chosen for this test and I think it will be top speed – fastest rocket wins.  I want to see which material is the lightest that will survive.  You can think of this another way.  The lighter the rocket, the greater the fraction of the total thrust that must be dissipated as drag (i.e., stress to the structure).  The lighter materials will be tested harder than the heavier ones.

This should be fun!

Flight Log: 20110101

Sunday, January 2nd, 2011

January 1 brought good weather for flying rockets.  My club SEVRA had obtained permission to fly at Fentress Naval Auxiliary Landing Field and an FAA waiver to fly to 10,000′ so, I decided to go fly rockets on New Year’s day.  Dana, Eva, Steve, Kelly, and Spear also came along.  Eva and Spear built rockets the night before and flew their Estes Gold Streaks several times.  Eva also flew her HiJinks.  To start the day/year off, I flew my Big Daddy on an E28-7T.  Although the simulations indicated an altitude of 1100′, the rocket only went 835′.

I flew my first dual deployment today. I modified my BDR 4.0 by adding an electronics bay and installed a Featherweight Raven 2 altimeter. The ejection charges were constructed from a 1.5 cc centrifuge vial, a Q2G2 igniter, and 1 gram of FFFG black powder. The Cesaroni H225 White Thunder motor delay was set to 1.5 seconds past the predicted apogee as a back up. The flight went without a hitch, perfectly straight up. The yellow rocket was stunning against the unusually haze-free, deep blue sky.  On the H225 motor, the peak acceleration was 11.5G and the maximum velocity was 346fps.  Apogee was measured at 1264′ and was, once again, higher than simulation. The max speed and altitude both support a CD lower than I’ve been using and much lower than calculated. Because of southerly winds, a lot of the rockets flown were headed for, over, and occasionally into the trees so, I decided to use a smaller parachute. The rocket was flown without a drogue.  The apo charge separated the rocket and the resultant descent rate was 50 fps. The main parachute was set to deploy at 500′. The decent rate on the main chute was 25 fps and the rocket hit a concrete taxiway. Modest damage was sustained but, the rocket was repaired this afternoon and is ready to go once again.  Despite the high descent rate and damage, I think the larger chute would have carried it into the trees. All in all, this was a good start to the new year.

Level 1 and 2 Certifications

Saturday, January 1st, 2011

Time for a rocketry update. I’ve been building quite a few rockets, each intended to be used in my Level 1 or 2 certification attempts. I built a LOC/Precision Vulcanite first but, on an H motor it looked like it would fly at least 2000′ – a bit high for single deploy at our site. I then built a Performance Rocketry Lil’ Rascal. For various reasons, I was still looking for another design, one that could be used for both Level 1 and 2. Two weeks before my planned Level 1 attempt, I saw an article for a BDR 4.0 and decided to build one. The article indicated a very high CD, 2.4, that would result in very low flights on both H and J motors.

I flew the BDR 4.0 on an H90 on November 6, 2010 at our Boy Scout Rocket-Ree event. NAR president Trip Barber was present and I asked him to be my Level 1 certification official. The flight went perfectly but there were indications that the rocket went much higher than expected. Unfortunately, I did not remember to turn on the altimeter and had no flight data. During the following week, there was extensive discussion among club members as to what the CD should be for a rocket like the BDR 4.0. There was a wide range of opinion, simulation results, and a void of published work on the CD of tube-finned rockets.

 

With great uncertainty as to what the CD actually was, I decided to go for my Level 2 certification two weeks later, November 20, 2010. I passed the written portion of the test with a perfect score and was antsy to fly before the winds were predicted to become unfavorable. I had selected a J285 motor and had run many sims. My expectations were for an apogee of between 2000′ and 2500′ with a modest range of optimal delays. I split the difference and went with a delay in the middle of the predicted range, 8 seconds. I should have known something was amiss when the igniter did not go very far into the motor but, I guess I was fairly nervous. With a puff of smoke, the rocket failed to come to life and sat on the pad. Jeff Goldstein, my certification official, offered me a spare and I made certain the igniter went all the way in the second time. This time the motor started and the rocket roared off of the pad. Once again, it seemed to go much higher then expected. The parachute deployed near apogee in a strong wind. I watched the rocket descend slowly and drift far off behind a row of trees. We jumped in the back of a pickup and headed off to the NE corner of the field. After a brief search, we located the rocket in a field of winter wheat and in perfect condition. The successful flight meant I had passed on to Level 2. I did remember to turn on the altimeter for this flight. The BDR 4.0 flew to 3010′! Follow up simulations could replicate this performance only if the CD was a rather low 0.67.