http://www.australianrocketryclub.com.au/index.php?pr=Education

I was reading through the Education section of the website and got a bit confused reading the section headed "Selecting a motor". Here's the section:

The average thrust is more important. Let's say you have a rocket and you have the choice of using a D12 or an E9. The D12 has greater average thrust at 12 N/sec compared to the 9 N/sec of the E motor but the E motor has more initial thrust. What this means to you is that the E9 can launch a heavier rocket than the D12 because the higher initial thrust will help get the rocket stable before it leaves the launch rail, but the D12 will average more thrust. You can think of it as being the difference between a push and a flick. Sometimes one is more desirable than the other.

This doesn't seem right to me. The E9 has a lower average thrust. It does have a greater total impulse but because of the lower average, it should also have a low initial thrust too, shouldn't it? I would've thought you'd choose a D12 over an E9 if your rocket was heavier as the D will get the thing flying straight off the rail, not the E.

Checking the thrust curves, it has the D12 peak at almost 30N and the E9 peak at almost 20N.

all of the above is correct   as always it comes down to size/weight of rocket and the motor you wish to use when i launch my estes Executioner it fly`s higher on a E9 than a D12

Length 38.5 in. (97.8 cm)
Diameter 2.6 in. (66 mm)
Weight 8.1 oz. (230 g)

The weight of the Executioner is such that it will fly OK (and higher) with an E9 than a D12.
However, an E9 in a slightly heavier rocket may be inappropriate due to its lower initial thrust.
For example, do not use an E9 in the "Grappler":
"Important Note: DO NOT use the Estes E9-4 in this rocket. It doesn't have enough thrust to safely lift this bigger rocket."
A D12 or even a C11 can be used
This is according to the manufacturer (Apogee): http://www.apogeerockets.com/Dynastar_Grappler.asp

Quoted from wolflair

all of the above is correct   as always it comes down to size/weight of rocket and the motor you wish to use when i launch my estes Executioner it fly`s higher on a E9 than a D12

Absolutely! It wasn't the height I was questioning. As Kryten pointed out, some heavier rockets can be launched on a D12 but not an E9 due to the intitial thrust of a D12 whereas the quoted section from the website seems to give the opposite impression.

Think you may have a point Mr F. In Wolflairs defence his knowledge in model rocketry is not as extensive as it is in high power rocketry! Would suggest a D12 does have much more punch than an E9. The E9 is a longer burning motor than the D12 which is why it would fly a rocket higher however the average thrust is lower. Its total impulse is higher than a D12 though.

The D12 would not get a rocket of the same weight as high but it certainly would make it more stable due to it having more average thrust than an E9!    

Does that make sense?  

all of you have said the same thing but with diffrent words  

I wouldnt launch the grappler on a C or D they dont fly all that well on E or F motors either maybe its the reverse fin design.

From reading the first post I am sure you can find rockets that reach a point that cannot fly on the E9's but could go on a D12 and I am sure you can find rockets that fly on E9's but not on D12's.

Sometimes one is more desirable than the other.

thats great because i am in the house rocketballsa.........lo0l

So the "Selecting a motor" section needs to be revised

The average thrust is more important. Let's say you have a rocket and you have the choice of using a D12 or an E9. The D12 has greater average thrust at 12 N/sec compared to the 9 N/sec of the E motor but the E motor has more initial thrust. What this means to you is that the E9 can launch a heavier rocket than the D12 because the higher initial thrust will help get the rocket stable before it leaves the launch rail, but the D12 will average more thrust. You can think of it as being the difference between a push and a flick. Sometimes one is more desirable than the other.

this is based on a kit that recommends both motors not on kits that doesnt recommend it key word is choice this is why i used the Executioner as an example as it is one of the heaviset kits ESTES makes and recommends to use both motors but it does fly 100% more better on an E9 than the D12 I know this cos i have 2 of them and have pushed them to the limit in the windiest conditions

the grappler isnt a good example to this as the rocket does not give you the choice of the E9 it clearly states to not use it so falls outside of the above example

read the enitre section on the website and it will make more sense not just a paragraph

without confusing the issue if i have a kit that recommends either a d12 or e9 motor going with the bigger motor is not always the better option however if the kit is heavier an e9 would be better at pushing it of the pad not a gentle flick

if anyone can explain it better than whats on the website please send me an email and i will update the website members input into the website is always welcomed

This has turned in to quite a fastenating debate!   Ok wolflair we are all wrong and you are right!   Did miss the term choice!  

Quoted from wolflair

if anyone can explain it better than whats on the website please send me an email and i will update the website members input into the website is always welcomed

Email sent Or should I post it here? That way everyone can see I'm no better at rewriting this damn thing seriously it took me a fair while for not much change.

I guess my main beef with that paragraph is the statement that an E9 has a higher initial thrust. Just to be anal (sorry it may be genetic) I'll post the two offending thrust curves under discussion at the moment

D12: http://www.nar.org/pdf/Estes/D12.pdf
E9: http://www.nar.org/pdf/Estes/E9.pdf

The D12 has a higher initial thrust. The information that follows the statement about the E9's thrust is (I feel) incorrect as it is based on an incorrect fact that the E9 has a higher initial thrust.

I hope I'm not ruffling feathers too much here... the Education section as a whole is very good reading: clear, easy to understand and informative. But I'm a teacher and I can help but pick

post it here for us all to read

ps i did not wright the current one it could do with a tidy up and we will see if it can be made better  

Well, here goes! I'll repost the first two paragraphs of the section in question to highlight my changes.

SELECTING A MOTOR

When selecting the right motor to use, many people who are new to rocketry only tend to look at the total impulse of the motor - after all, a G has to be better than an F right? Not always. As you can see from the chart above, a G impulse motor has a power range of 80 - 160 n/sec. Most motors don't come anywhere near the top range of the motors potential power - 80 - 160 n/sec is quite a range and it gets far worse, to the point of being totally meaningless, as you get into high power motors such as L and M impulse.

To me, this first paragraph implies that sometimes it's better to choose an F over a G, which it can be when considering motor for a heavy rocket that needs an initial burst of guts. I wouldn't change anything about this 1st paragraph.

The average thrust is more important. Let's say you have a rocket and you have the choice of using a D12 or an E9. The D12 has greater average thrust at 12 N/sec compared to the 9 N/sec of the E motor but the E motor has more initial thrust. What this means to you is that the E9 can launch a heavier rocket than the D12 because the higher initial thrust will help get the rocket stable before it leaves the launch rail, but the D12 will average more thrust. You can think of it as being the difference between a push and a flick. Sometimes one is more desirable than the other.

what do you reckon about the following revision? I'm not saying it's the final version... just an idea

"The average thrust is more important. Let's say you have a rocket and you have the choice of using a D12 or an E9. The E9 motor will have a much greater total impulse, almost double that of the D12. The D12 however, has greater average thrust at 12 N/sec compared to the 9 N/sec of the E motor. What this means to you is that in some situations the D12 will launch a heavier rocket more safely than the E9. This is because the higher initial thrust the D12 supplies will help get the rocket stable before it leaves the launch rail. Once in the air, though, you can guarantee that the E9 motor would lift your rocket higher. "

This paragraph is completely open to further revision by anyone who chooses to

Oh. and I didn't bother posting the third and final paragraph from that section as it recommends following the manufacturer's instructions on what the ideal motors are to use. No need to change anything there!

That looks good Mr F. Makes perfect sense!  

The other area in the education section that needs updating is the motor to casing area. There are quite a few new products available which are not in this section. As it appears many Australian government departments come to our site looking for info it is important we try and maintain as much up to date info as practicle!

well mr f looks like you are going on the web site well what you wrote anyway  

Aha! And next, I take over the world!  

well its offical