?s About first and other flights...

Just how important is it to use the suggested engine for the first flight?
What could happen if I don't use the A8-3 and instead used a much higher one on the list such as a C6-5?
Does the lowest suggested engine break in the rocket?

Also, is it OK to use other engines that are not on the list?
If I use a C6-3 in a rocket even though it's not listed.

Thank you for any replies.

The lower powered motors are there for you to be able to see and recover your rocket. If it takes off and is never seen again, then you're out of the rocket, and you'll need to buy or make another rocket to use the remainder [of the motors]. This is a fast way to get out of the hobby.

Motors that are not on the list are use at your own peril. The manufacturers recommendations are there to provide you with a safe flight. You want your rocket's recovery to deploy at apogee when there is the least amount of strain on the recovery devices... To early you shred, too late you zipper, or worse, dart.

K'Tesh said:

The lower powered motors are there for you to be able to see and recover your rocket. If it takes off and is never seen again, then you're out of the rocket, and you'll need to buy or make another rocket to use the remainder. This is a fast way to get out of the hobby.

Motors that are not on the list are use at your own peril. The manufacturers recommendations are there to provide you with a safe flight. You want your rocket's recovery to deploy at apogee when there is the least amount of strain on the recovery devices... To early you shred, too late you zipper, or worse, dart.

Click to expand...

Thank you.

Here's what I don't understand.
Could one still lose a rocket with a bigger engine even after starting with the minimum?

Yes could still lose it. Bigger will go higher. The smaller launch is just to give you an idea how much higher, and make a judgement.

If an A8-3 almost leaves your field.... then a C6-5 is a bad idea.

LaunchPad said:

Thank you.

Here's what I don't understand.
Could one still lose a rocket with a bigger engine even after starting with the minimum?

Click to expand...

If the A8-3 barely gets the rocket above the ground, then a B6-? or C6-? would be in order. It's up to you to decide which motor is the right one. Unfortunately, if you are not launching with a club or someone that has experience with that rocket and those motors, you're pretty much on your own.

As a general rule of thumb, a B motor will go twice as high as an A motor, and a C motor will go twice as high as a B motor.

What you have to learn, and experience is the best teacher, is how high that actually is. Until you've launched a number of rockets, it is difficult to estimate the altitude the rockets actually achieve.

Good luck and let us know how things turn out.

LaunchPad said:

Just how important is it to use the suggested engine for the first flight?
What could happen if I don't use the A8-3 and instead used a much higher one on the list such as a C6-5?
Does the lowest suggested engine break in the rocket?

Also, is it OK to use other engines that are not on the list?
If I use a C6-3 in a rocket even though it's not listed.

Thank you for any replies.

Click to expand...

Using the "suggested engine for the first flight" is not a requirement (there are no "first engine police") but can be very helpful for you to understand the flight characteristics, and to make sure you will be able to recover the rocket for subsequent flights (assuming you are confident your launch site is big enough for the first flight engine).

You could always start out using a larger suggested engine, but again, you may not be familiar with how high the rocket may go, may lose sight of it, and then not know what to expect...

Regarding "Is it OK to use other engines not on the list," it is possible, but you should be aware that they may not be suggested for a reason...for example, a rocket that has suggested engines of A8-3 & C6-5 sounds like a lighter rocket, and the C6-3 would likely set the ejection charge off much too early, and possibly shred the parachute when it deploys while still going up at a high rate of speed (and nowhere near the apogee).

Have you ever tried the OpenRocket 14.11 software? It is free flight simulation software that will calculate things for you, like max altitude, best delay time, speed of rocket at deployment, etc., for all known commercially produced engines/motors. All you have to do is download (or create) the file for the rocket you have, most commercial rockets (like the Estes models) already have pre-existing files you can download. Read the Wiki, or Google "OpenRocket tutorial", or ask for help on this forum if you need it...

Download OpenRocket at:
https://openrocket.sourceforge.net/download.html

It is not important to use the recommended motor for first flight. It IS important to use an APPROPRIATE motor. Maybe that is the recommended motor, and maybe not. The recommended motors assume the rocket weighs a specific amount. Maybe your glue was heavier. Maybe your primer had more solids, and the paint was a couple coats thicker. Maybe your centering ring was a little on the light side, and your nosecone a smidge on the heavy. Any number of variables, including technique, can affect the weight of the rocket once built.

Maybe you built it light, and the A8-3 is too short of a delay, or heavy, were the A8 will struggle to get the thing off the ground.

OpenRocket or Rocksim will help answer these things for you...


Best,

--Coop

After you spend ~$50-$60 to build you first rocket and get a launcher, do you really want to loose your rocket on it's maiden launch? Probably not.

The recommended first motor will usually launch a model rocket to an apogee of 200'-300'. Increasing the motor impulse by 1 letter will approximately double the altitude so the same rocket would apogee at 400'-600'. Up the motor impulse class one more letter and the apogee will double again to 800'-1200'. Most model rocket kits have parachutes that will descent at ~15 feet per second. 10 mph = 15 fps so in a 10 mph breeze, a rocket launched to 300' will lanc 300' downwind from apogee. If the apogee is 1200'. it will land 1200' or nearly 1/4 mile downwind from apogee. The difference is landing in the park you launched from or 2 or 3 blocks away.

To estimate how high your rocket will go use the motor guide at thrustcurve.org All you need to input is the rocket diameter, weight without motor, and the motor mount diameter and length. For a single stage rocket use simple, and for finish average. The guide will simulate how you rocket performs with all the certified rocket motors that will fit in the motor mount. It's easy to use, and the results are accurate enough.

Bob

Another thing to consider is that the recommended engine often represents the best motor to observe your rocket's first flight. This allows you to study it's launch and recovery under what the manufacturer believes is the optimal configuration. If you have an instability caused by fin alignment, or a c/g-c/p imbalance, it can often be discovered during this flight. Allows you the ability to attempt to correct the issue prior to putting it under a higher power motor.

LaunchPad,

You have a lot to think about.

In case it wasn't clear from some of these posts, choosing the right delay time (the "-#") for the motor is critical. Choosing a motor with too short of a delay means the ejection charge will fire during vertical acceleration. This may destroy or zipper (picture a long, jagged rip down the side of your rocket) the rocket. Perfect delay time fires the ejection charge at apogee (vertical apex of the flight parabola). But better too long a delay than too short.

As suggested, use OpenRocket or ThrustCurve.org to determine the apogee time and ideal delay time for an A8, B6 and C6. Choose the delay time accordingly.

And here is a little formula for determining how far away the rocket will come down. Go here and determine the time to land for your rocket and motor. Look at a weather forecast for the expected wind speed for your launch time.

Then do this: desc. time X wind spd. in MPH X 1.47 = drift in feet

And like someone said, this is the amount of drift from the moment the chute comes out and opens (close to apogee). Launch a little into the wind. Compare your landing amount above with the size of your field.

Have fun!