Rocketry Planet

Lesson 5: Preparing to Fly

In this lesson, we will install the recovery system, check stability and choose motors.

Weight and C.G.

Now that you have the rocket painted and assembled, you can get the final weight. Install the recovery system and assemble the rocket, including everything necessary for flight. Weigh the rocket at this point. This is your "dry weight." The weight is critical for determining the parachute size and for accurate simulations.

Then, determine the center of gravity (C.G.) of your rocket by balancing it. This is the dry C.G. Balance again using a weight which simulates the loaded weight of the heaviest motor you plan to fly. This is your liftoff C.G. You now have the numbers you need to verify the stability of your rocket.

Recovery System

Now that you have the final weight, you can determine the size parachute needed. Add the dry weight of the rocket to the burn-out weight of the largest motor you plan to fly. To determine the burn-out weight, subtract the propellant weight from the total weight of the motor. (For high-power motor information, see ThrustCurve.org.)

See the Main Chute Sizing article in INFOcentral for guidelines on parachute sizing. You can usually use the parachute which comes with your kit as long as you didn't add too much weight. West Coast fliers should be warned that many kits are designed for the smaller and softer fields in the East, which may result in a smaller than ideal 'chute.

If you decide to replace the 'chute with an after-market one, you can use the size guidelines of the manufacturer to determine which size 'chute to use. Obviously, larger 'chutes will bring your rocket down more softly, but will cause it to drift farther. If you have plenty of recovery area, round up to the next larger size. If you have a soft grassy field, you can round down.

Computer Simulation

If you rocket is of normal rocket proportions with reasonably large fins, as my Tomahawk is, there will be little question of stability. However, it's always a good idea to make sure. To do this, we need to calculate the center of pressure (C.P.) of our airframe. There are many programs which can do this. My personal favorite is RockSim by Apogee Components. (See the ThrustCurve.org simulators page for more options.)

Above you can see a screen shot from RockSim with the airframe entered into the program. You can see that I've already run simulations on the two motors I plan to fly, an I357 and a K550. The 38mm I357 is a nice level 1 motor for getting large rockets off the pad and the 54mm K550 is one of my all-time favorite motors. You can see the calculated C.P. is well behind the measured C.G (almost three calibers) even with the K550.

RockSim will calculate the weight of the rocket based on the materials you specify. However, it can only be as accurate as its parts database and the amount of detail you enter. I prefer to override the weight and C.G. it calculates with the actual ones once I have them. The menu item Rocket|Design... will bring up a tabbed dialog and the last page allows you to specify these values. Note that you should specify the dry weight and C.G. of the rocket and RockSim will calculate the flight values based on the selected motor.

If you fly electronics, you don't really need to figure out the delay times, but it's still nice to know more or less how high your rocket is going to fly. If you are flying with motor ejection, you definitely need to run simulations to figure out your delays.

RockSim will show you the optimal delay length when you test fly a motor. To choose the best delay, you should run a simulation with the ones just shorter and just longer than the optimum. Look at the Velocity at Deployment column to choose the delay which gives you the lowest speed at opening.

You are now ready to fly! I hope you have enjoyed this series and have many great flights on your rocket.

Reader comments:

#1

I like the format, and the fact that the article is in both the forums and the web site, both in HTML format. I think people will be able to follow the classes easier that way.

I notice that this class has all the lessons posted at once, is that the way the upcoming Tomahawk class will be run or will you post a lesson at a time?

crontab on 08-02-2007 09:06 PM

#2

Since this was a pre-existing How-To Class that had lost its home, I published in its entirety. Future classes that are being built in process will have the classes posted a lesson at the time, with each lesson added to the original posting. On the web site front end, each lesson will be on its own individual page, just as this class article is currently posted. Comments posted will be visible both in the web site front end and here in the forums.

ddmobley on 08-02-2007 09:22 PM

#3

It's nice to see this again! It's still a good introduction (if I say so myself), even if some of it is dated after half a decade.

JohnCoker on 08-03-2007 11:10 PM

#4

John, you always did a good job with your classroom material. Thorough, detailed, good photos, great word smithing. And the format is perfect for getting new people interested and, more importantly, involved. I am excited about the feature and glad to showcase your work as the inaugural entry.

ddmobley on 08-04-2007 12:09 AM

#5

I enjoyed the write up here and used some of the techniques on my own Tomahawk builds shown here.

[image remove: no longer available>

I guess next I'll have to add a 3 & 4" examples to the collection.

fox_racing_guy on 08-19-2007 08:02 AM

#6

Beautiful! What size is the large one? Who made the kit?

ddmobley on 08-19-2007 03:00 PM

#7

Barry at LOC/Precision made up the parts for me to build both of these. The smaller one is a 2" example and he now has turned this into a regular catalog kit for sale. Anyone wanting to build the smaller one can call or E-mail Barry and ask for the PK-29 Tomahawk kit. He tells me he has them all made up and ready to ship. If you would like to build the 5.5 model I'm sure he still has the fin pattern on file and would be happy to sell that model as well. The 5" kit has a 75mm motor mount so I should be able to put some stupid power motors in and have some fun!
A big thumbs up to whoever runs THIS forum, there sure don't seem to be the same "restrictions" like the other forum.
ATTENTION FRANK KOSDON! come join this forum, I think they will let you post whatever you like here.
I sure hope I just didn't ban myself over there

fox_racing_guy on 08-19-2007 06:06 PM

#8

I've always liked the IQSY and started a 6" dia (2/3rd scale) scratch build in 2002 for my L3. After moving twice, I finally got around to finishing it... in flyable form but not with final paint. Here are some pics of my L3 flight and the build...
http://photos.thrustgear.com/cpg/thumbnails.php?album=8

And reposted from the way-back machine, my initial start at construction...
http://thrustgear.com/IQSY/html/

The 5:1 Ogive from Peformance Rocketry is very scale-like compared to the 4:1 PML cone. I'm glad I made the switch a while back.

Now, I need an excuse to finish up the painting and detailing!

-John DeMar

jsdemar on 08-20-2007 01:21 AM

#9

Sweet build and flight pics you have there! I put a 75mm motor tube in mine as well and can't wait to send her up.

fox_racing_guy on 08-20-2007 04:35 AM

#10

Cool, nice rocket. I still have the top half of my 4", I really need to remake the lower section....

DumasBro2 on 08-20-2007 09:37 AM

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