9mm vs 22mm

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RaZias
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9mm vs 22mm

Post by RaZias »

I have seing people saying that 22 mm has more drag and that is why it goes less high.

I agree it goes less high but I don´t agree it´s due to the drag.

It goes less high because it has less impulse, it´s like this:
The rocket releases all the air at once and it will fly the rest of the time by momentum.
A rocket fighting air resistance by momentum is not good.

The 9 mm will release the air during more time, so the rocket has more impulse, it will fight the air resistance with its air propulsion, and spend less time fighting drag with momentum.


So don´t say that 22mm goes less high due to drag, this is not correct. It goes less high because it flies more time by momentum.

I once tested the same rocket with a good cone and with a crashed cone, it almost didn´t gived a big difference in height.

Anyway, 22mm has the best advantage, because bigger the rocket more heavy it will be so it will need more trusth, and that is only gived by the biggest nozzle.
A 9 mm has difficulties for big rockets, it demands rails and boosters...more complexity.
And for a car speed record, a open nozzle is better !
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air.command
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Re: 9mm vs 22mm

Post by air.command »

RaZias wrote:I have seing people saying that 22 mm has more drag and that is why it goes less high.

I agree it goes less high but I don´t agree it´s due to the drag.

It goes less high because it has less impulse, it´s like this:
The rocket releases all the air at once and it will fly the rest of the time by momentum.
A rocket fighting air resistance by momentum is not good.

The 9 mm will release the air during more time, so the rocket has more impulse, it will fight the air resistance with its air propulsion, and spend less time fighting drag with momentum.


So don´t say that 22mm goes less high due to drag, this is not correct. It goes less high because it flies more time by momentum.

I once tested the same rocket with a good cone and with a crashed cone, it almost didn´t gived a big difference in height.

Anyway, 22mm has the best advantage, because bigger the rocket more heavy it will be so it will need more trusth, and that is only gived by the biggest nozzle.
A 9 mm has difficulties for big rockets, it demands rails and boosters...more complexity.
And for a car speed record, a open nozzle is better !
Hi Razias and everyone,

I think the issue is more complex than a 9mm vs 22mm nozzle argument. There are many factors that affect the final altitude of a rocket and in some situations a 9mm nozzle will outperform a 22mm one and in other situations its the other way around. For each rocket launched at a particular pressure and water fill amount there is an optimal sized nozzle to reach maximum altitude. Not specifically 9mm or 22mm. You can use your favourite simulator and play with the numbers and let the simulator optimize the nozzle size for you. Sustainers for example will work better with smaller nozzles than bigger ones.

You are right Razias in that a large nozzle is essential for getting a heavy rocket off the ground, so 9mm vs 22mm argument doesn't make much sense in that case. A 1/2 mm nozzle will leave a puddle at the pad but will not lift the rocket.

The total impulse will be more or less the same regardless of the nozzle size. The main difference there being in the nozzle's efficiency. The nozzle size however does affect the peak thrust and average thrust. Again you need to make sure the nozzle is big enough to make sure the peak thrust is high enough to get the rocket moving off the pad.
All things being equal a larger nozzle will result in higher peak velocity. Always remember that drag is proportional to the square of the velocity, so a higher speed rocket will have more drag. This is why high velocity rockets need to pay special attention to reducing the drag coefficient. i.e. a long skinny rocket.

Larger nozzles produce higher G-forces and as a result you need to make sure everything is secured better like fins and the payload.

- George
http://www.AirCommandRockets.com
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rockets-in-brighton
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Re: 9mm vs 22mm

Post by rockets-in-brighton »

air.command wrote: I think the issue is more complex than a 9mm vs 22mm nozzle argument. There are many factors that affect the final altitude of a rocket and in some situations a 9mm nozzle will outperform a 22mm one and in other situations its the other way around. For each rocket launched at a particular pressure and water fill amount there is an optimal sized nozzle to reach maximum altitude.
Razias/George: Yes, I'm happy to backpedal altogether on open nozzle = lower altitude, having been messing with simulators again. I had myself convinced that drag proportional to the square of the velocity must surely always outweigh the higher burnout velocity due to an open nozzle, but the sims say otherwise. Every configuration I tried says the bigger the nozzle the higher the apogee. I'm now wondering if I should experimenting with Lucozade bottles (30mm spout).

In defence, my only practical experience with reduced nozzles was modelling launch tube + T-nozzle, and I had only one launch using it which went wildy out of control and blipped into a tree. Not much hard data to work with there.
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Tim Chen
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Re: 9mm vs 22mm

Post by Tim Chen »

I find that a full mouth bottle opening nozzle will take off with an explosive force and that a smaller gardena nozzle will take off with a more gradual thrust motion. In general, people seem to be more impressed with the slowly lifting off rocket than the one that vanishes from the launch pad. Even if the fast rocket goes higher, it is less impressive because it makes no visual show for spectators. I wish there was a way to get the best of both worlds in water rockets, but everything is a trade-off.
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air.command
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Re: 9mm vs 22mm

Post by air.command »

rockets-in-brighton wrote: Every configuration I tried says the bigger the nozzle the higher the apogee. I'm now wondering if I should experimenting with Lucozade bottles (30mm spout).
Hi Steve, The larger nozzle may indeed be the better option for your rocket. But be careful with the sim at the more extreme ranges of pressure and nozzle size. The time slices the simulator uses may not be short enough in those instances and give unrealistic predictions.

For smaller rockets try for example the default settings for a 2L rocket in Clifford's simulator and enter the range 2-22 for the nozzle size.

When you run with a mass of 100 grams and drag coefficient of 0.3 the optimal nozzle is: 4.63mm

Increase the mass to 200 grams with same drag coefficient and the optimal nozzle is 22mm (in the 2-22mm range)

Leave the mass at 200 grams and increase the drag coefficient to say 0.5 and the optimum now is 6.63mm.

Run the same simulation again (200 grams) with drag coefficient of 0.3 and give the rocket an initial velocity of say 10m/s (such as from a launch tube or as a sustainer separating) and the optimal nozzle size becomes 5.22mm.

The factors that affect a rocket's altitude are ALL interelated, if one is going purely for altitude, it is best to run a sim first to figure out what the optimal parameters are for a particular rocket setup/pressure/water fill. Even though the predicted altitude may be different to the actual altitude due to external factors, the simulator can still give you the right relationship between the parameters.

- George

PS. Tip#14: To maximize altitude fly on a very humid day.
http://www.AirCommandRockets.com
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Jelo
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Re: 9mm vs 22mm

Post by Jelo »

personaly, i think 9mm rockets are more like real rockets, i preffer them because the onboard videos are a lot smoother, with 22 mm nozzles the bottle looks like it came out of a canon, and when you look at the onboard videos all of a sudden you jump from the launch pad to 50 meters high in not even an image and it looks strange...but an advantage with 22mm nozzle rockets is that they are way more stable due to the fact the CG is way higher.
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Alien Space Agency
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Re: 9mm vs 22mm

Post by Alien Space Agency »

Yup, it does look realistic


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