I have never launched a model rocket, but I’m being compelled to do so, for my son’s Cub Scout den. I bought a beginners rocket kit made by Estes, and have a question about engines. The kit says it can tak any one of four types of engines, but recommends a specific type of engine for the initial launch. What’s the rationale behind this–if four types of engines will work, why a specific one for the first time?
(Of course, I bought the engine recommended for the first time launches.)
The reccomended engine is probably the smallest engine. The smaller engine means the rocket won’t go as high or potentially float as far away once the parachute deploys.
Once you get a feel for how the model reacts to wind, you can get bigger engines and longer flights.
One reason is that if you put too weak an engine in it, you won’t get a very impressive launch, but if you put too powerful an engine in, you might lose the rocket. Also, to the extent that model rocketry is dangerous, the more powerful engines are more dangerous than the weaker ones. And there’s probably a price difference, too (no sense in buying the super-expensive engines if you decide you’re not all that interested in model rocketry after all).
You’re probably already aware of this, but the letter ratings indicate how much oomph the engines have. Letters nearer the beginning of the alphabet are weaker than later letters, and I think that there are AA and AAA engines which are weaker than A.
If you launch an untested rocket with a powerful motor you may lose some fins that aren’t attached correctly, leading to some hilarious if somewhat dangerous flight paths. Until you’re sure the rocket is put together well you want to keep the power low.
F.Y.I. Estes rocket engine coding.
What model did you get? A “Big Bertha” is a simple but fun starter kit. It has been a long time since I launched anything but I remember the fun. Go for it.
Just as a humorous aside…
I once calculated that the solid fuel boosters on the shuttle would be an X engine if Estes ever decides to ramp production up that far. 
A few years ago I bought my grandson an Estes beginner rocket kit. We spent that evening assembing it for a morning launch, I figured the wind would be calmer. That morning there was a light breeze, no more than 5 mph. The engine was loading, the rocket placed on the launch pad, wires were hooked up and my grandson launched his first and only rocket. I’m not sure what engine size we used, it was one of 3 that came with the kit. The rocket shot into the air, stalled for a split second, this just long enough for the rocket to change angles just a bit then the engine fired again. It headed east over a large stand of trees then there was a pop and the parachute popped out. It drifted out of sight never to be seen again.
Not knowing a whole lot about model rockets, my assumption as to why several engines are listed as possible but one is recommended for a first flight is marketing.
Basically, if this kit is being used by beginners, make it as easy as possible for said beginner to go to a store and get something specific. Then later they can experiment with other engines.
Model rockets are fun. I once poorly installed the guide rail on one so that it was too snug on the launching platform.It couldn’t take off and the rocket melted the metal launch pad.
Have fun blasting off.
But keep in mind that past a certain point you have to comply with the FAA regs - I once re-routed a weekend flight because a model rocket club had authorization to launch to 5,000 feet, which was about where I was planning to fly 
The recommended beginner motor is probably the one giving most control with the least power. Is it marketing? Well, only to the extent that having the customer have a good first experience is a marketing gimmick.
You could’ve at least been sporting and given 'em something to aim at 
AIUI, those sorts of heights, and the accompanying FAA oversight really doesn’t come into play with the pre-packaged solid rocket motors. That’s for the serious model rocketry nuts who are actually playing with liquid fuels, and much more complex models. I don’t think this is something that the casual user of Estes model rockets will have to worry about.
One caveat for the OP: Model rocketry is fun, but any launch has a chance of resulting in the loss of your rocket. Simply having it fly off into the unknown has happened to me, but far more common are parachute (or other recovery system) failures, where the rocket simply hits the ground too hard. Don’t be surprised if you only get a few launches from your rocket.
While I was in the hobby I had a fleet of 10-15 rockets at one point, and would go out a couple times a summer to launch. It was grand fun. But I never, ever came home with the same number of intact models I started with. Some I could repair, but most of the accidents left me with just pretty trash to dispose of.
In our neck of the woods (er…rocks), we have have these guys with FAA clearance for 8000’ to 16000’ AGL. Very cool to watch them get that high.
Ahhh, model rockets. I highly encourage everyone to get the youngsters involved in this. I got involved in about 6th or 7th grade, and learned a lot about model building, and I also learned a lot about math by trying to calculate launch heights, as well as center of thrust, etc.
My model rocket career ended as I put my biggest engine in my smallest rocket.
Never did find that sucker.
The first flight of my big, 5-foot long or so black rocket (anyone remember the name of that kit?) with its E engine was a loss. Whoever built the mall in the later years probably found it when they ripped up the woods. The rocket went high, and drifted far.
During my childhood, the idea was to build a small, light, cheap rocket and then use the most powerful engine possible. I seldom retrieved a functional rocket, but I had fun. One I remember was a badminton birdie that appeared to leave the earth’s atmosphere.
I always considered the first launch of any newly built rocket a shakedown flight, and as such, always used a lower power engine. Reasons have been cited above, like finding out your fins aren’t as well glued as you thought. (I realize lots of kits come with pre-fab fin sections, but many of us recall the days when you had to cut your own fins and glue them individually. I still do.)
Anybody that has played with rockets and hasn’t lost a few never really did anything. Even our boys at NASA have lost a few.
Think about it, if you have the mentality to want to launch a rocket at some point you are going to put too much unharnessed power in too light of a vehicle. That’s human nature.
That being said, when you go to launch your rocket assume the following:
The field you are in is much smaller than you think it is.
The wind is stronger than you think it is.
You can’t run as fast as you think you can.
Rockets like to get lost and/or malfunction.
The only thing worse than not having a parachute is having a parachute. It’s very tricky to get the right size for the weight of the rocket. You don’t want the rocket to drop so it gets damaged but you don’t want it drifting off to the next county. I used to cut a hole in the top of the parachute so it would descend faster and not drift so far. A ribbon instead of a parachute will sometimes work best for a light rocket.
As others have said, start small and light. Experiment. Then build up. Have fun!
You must have had a Mean Machine.