Coasting after motor burnout

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TNmike

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Let's say you have two identical rockets, except for their weights, and both fly with the same motor. The lighter rocket will boost higher, but will coast a more limited amount. The heavier rocket will boost lower, but will coast further than the other rocket because of its higher momentum. What are some of the guidelines or rules of thumb concerning this scenario? I know there is plenty of math behind this, but for now, baby steps.
 
You have it intuitively correct: adding mass always reduces speed off the rail / top speed. There is, as you mention, an optimum mass below which you go faster but not as high and above which you go slower but not as high.

So for a fast one, build as light as practical. For maximum altitude, use a simulator such as OpenRocket, RASAero, RockSim to tune it. For a small field, ramp the weight up and balance it with more surface area at the rear.
 
Yep, forget the baby steps.

Thanks guys, I'll try to peruse these documents in the coming days. It's a lot to take in.
 
If you want the heavier rocket to coast further, converting that higher momentum into altitude, use a longer delay. In your example use a C6-3 for the lighter rocket, and a C6-5 in the heavier one.

kj
 
Use the Optimum Weight function in RASAero II (see Pages 92-93 in the RASAero II Users Manual).


Note from the RASAero II Users Manual:

"Note that for many larger high power rockets, and sounding rockets, the optimum weight is typically the lowest possible weight of the rocket. RASAero II will stop the optimum weight flight simulation trajectory iterations once the weight has gotten down to the motor loaded weight from the rasp.eng file, i.e., there is no weight left for the rocket airframe and other rocket systems."


Once the Optimum Weight is determined, you can do individual RASAero II runs at different weights to make the flight simulation plots and get a look at what is being traded in terms of max velocity and altitude gained during coast.

You can also try different weights with different levels of wind to look a weathercocking off the launch rail, and trajectory effects.

Also, with the Rogers Modified Barrowman Method (Page 36 of the Users Manual), you can look at the forward movement of the Center of Pressure (CP) with wind. (CP is fixed with angle of attack if the default Barrowman Method is used.)


Chuck Rogers
Rogers Aeroscience
 
It also needs to be noted that for model rockets, especially small model rockets, typically the optimum weight is more than the motor loaded weight, but less weight than one could realistically build the rocket.

Many smaller minimum diameter high power rockets will fly higher with weight added. Then for really large high power rockets, they start acting like sounding rockets, and then the lower the weight the higher the altitude.


Chuck Rogers
Rogers Aeroscience
 
A couple weeks ago, I was out with the local High School. One of the projects had to do with this subject. The rocket was 3" LOC tubing with a 38mm mount.
All three flights used the SU DMS H219T...I don't currently have all the data..but in general..
no weight added - 29oz rocket 2200 feet.
Heavy flight- 78 oz 878 feet

All three flight had dual altimeters...The weight was added via an aluminum sleeve with lead bolted to the sleeve. The CG did not change more then .5" between the no weight to the heaviest flight. The heavy flight was very interesting flight. You could see the rocket working to stay on a straight flight, while the large weight mass was trying to 'control' the rocket too.

Chutes were changed to maintain safe decent rates and a min of 30 mph rail exit speeds were maintained as well.

Tony
 
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