First Flights

 The Thrifty Rocketeer blog continues...

Most of us have flown an Estes Rocket kit before, and so we are familiar with the small blurb on the packaging and certainly in the instructions that cite two or three motors that the model will fly on.  

It's my understanding that these motors are: 

  1. Currently in production (at the time the kit is produced)
  2. Designed to "fit" in the unaltered kit, and
  3. Tested in "Open Rocket"  or similar RocSim program 
The  company has done all the hard work for you... taking into account the area of the fins, the location of the center of propulsion, the center of gravity, the length of the body tube, the weight of the constructed model... everything assumed to be as they designed and intended the kit to be assembled.
Testing a Model Rocket Launch Controller - Robot Room
Now, if you've deviated from the plans, or made a special modification, you're gonna have to factor those changes into the rocket simulation program and see what it recommends for your motors to effectively fly the bird safely.

At least, that's my understanding.
And I believe that all kit manufacturers have a listing for those two or three motors that will work in their rocket.   (Only twice have I see a kit that specifies only ONE motor value and once only TWO motors were specified.)

But recently, a question was posed on a rocketry forum on Facebook that gave pause.

The author stated a short version of what I started with above... "There are always three motors listed, one one is always indicated as the "first flight" motor. Why is that?  What happens if you don't follow that recommendation, and say, go right for the largest, most powerful motor first?"

It's a thought I had never had before, but it must be common.  Why start with the 'first flight' motor?

To my thinking, the company knows that the majority of rocket builders are going to be kids, and maybe their building technique isn't quite up to snuff yet.  That is, while their instructions are complete, has the builder followed all the steps?  Did they fillet the fins?  Did they remember to glue that part inside?

To my thinking, this is why the instructions recommend the weakest motor for the "first flight" it will stress the rocket the least, and reveal any flaws or mistakes first... at a low level than the stronger motors.

But another responder also pointed out another very good reason for starting with the "first flight" motor.  You want the rocket to be recovered, especially if this is your first trip to the rodeo.

When I launched my first few rockets, I was alone in a golf course, and they went well.  But I wanted a video to post, so I drew my son into the launch and gave him my phone to record the flight.  Having never seen a flight, he missed it... that is, he stated "I had no idea that it would go that fast...that high..."

The responder pointed out that you want to be able to track your rocket and recover it.  

You learn things from your first launch, even if you don't realize it.... how high it goes...what the open chute looks like... how far it will drift... what it looks like on the ground.

If you go for the upper limit right out of the box, you may loose sight of the rocket, and may never recover it.  I know this doesn't seem likely, but a second respondent also agreed, saying they lost their first two rockets by ignoring the first flight recommendation and going for the limit.  By their third rocket, they had learned their lesson, and never have lost a rocket since. 

Are there other reasons for these three initial motors?  Probably.   One reason is that you want the rocket to slow as it reaches apogee, and for the ejection charge to puff out that parachute or streamer at the top of the a slow enough speed that it doesn't strip off the cord.

But, are there other reasons for going with the initial "first flight" motor?   I'd like to know your thoughts.  Please share your ideas in the comments.


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