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Drt_trkr

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Hello to all!!!
As I'm sure you can see i am new to the forum and fairly new to model rocketry. I have of course built the kits way back when in school but now have found myself interested in building my own. With the rocket about complete I have run into an issue that I have concern about and was hoping i could get some info from the more experienced rocketeers!!

So here is my issue i need advice with... I have built a rocket that is about 3'6" tall and weighs approx. 2.5 lbs. I had the idea to use 4 of the Estes D12-5 engines to power the rocket. Now I am second guessing that the weight may be too great for the 4 engines. Now keep in mind I am new and do not quite understand how to determine the thrust to weight ratio.

I had originally wanted to build my own Engine for the rocket but found that Potassium Nitrate is a little hard to get... (unless I'm not looking in the right place) So I resorted to the smaller pre-made engines and hoping 4 D12-5 will do the trick... Any advise???

Thank you for your help!!

Drt_trkr
 

WillMarchant

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On motor making... Go to http://tripoli.org/ and look under "research". My advice is that beginners need to use commercial motors until they gain experience with rocketry in general.

There is a lot of great information online. Places like http://www.info-central.org/ and http://www.apogeerockets.com/Peak-of-Flight_index.asp are invaluable.

I am also a fan of hardcopy books and two of my favorites are:
http://www.questaerospace.com/itemdesc.asp?ic=9502&eq=&Tp=
and
http://www.apogeerockets.com/design_book.asp

http://www.info-central.org/?article=230 talks about the "5 to 1" rule of thumb for selecting the minimum motor for a successful liftoff. If that doesn't help, please come back. The folks on this forum are always happy to help!
 

FatBoy

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Welcome to rocketry!

You also might want to think about setting that rocket aside for a while and gaining a lot of experience with smaller and simpler kits. Besides Estes and Quest, there are many rocket kit vendors available these days. To name only a few: Semroc, FlisKits, Custom, Red River, Sirius.... the list of top-notch companies goes on.

You can model your big rocket using rocket simulation software, like RockSim. That will tell you if it is stable and if your motor selection can safely fly it.

Like Will said, staying with commercial motors is the way to go.
 

bob jablonski

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Welcome to rocketry!

You also might want to think about setting that rocket aside for a while and gaining a lot of experience with smaller and simpler kits. Besides Estes and Quest, there are many rocket kit vendors available these days. To name only a few: Semroc, FlisKits, Custom, Red River, Sirius.... the list of top-notch companies goes on.

You can model your big rocket using rocket simulation software, like RockSim. That will tell you if it is stable and if your motor selection can safely fly it.

Like Will said, staying with commercial motors is the way to go.
Welcome.
I agree with the above info.
Mr. Bob
Starlight model Rockets LLC
www.starlightrocketry.com
 

Marlin523

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You need to take note of what happens to the CG and CP when you think of clustering. Yes, you will get more thrust, but you may also get a disaster.
 

powderburner

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I sure don't want to "welcome" you by making it look like I have joined the line of guys telling you NO, YOU CAN'T DO THAT
(do you feel like you have a "kick me" sign on your back yet?)
I would like to strongly suggest that you start out with commercial motor products.

I had originally wanted to build my own Engine ...
It's just that it really is harder than it looks to make your own motor if you really want (1) good performance (2) consistent performance (3) safe operation and (4) the same features of the store-bought motors (reliable delay charge, reliable ejection charge, etc).

Note that saving money is nowhere on my list. This is because to come anywhere near accomplishing (1), (2), (3), or (4), you are going to have to invest in some serious equipment, including safety gear. You will need a safe operating facility. You will need paperwork, permits, and land.

You simply will not be able to make reliable,consistent, safe, full-function motors that have any useful performance capability by dabbling with mortars and pestles, DIY home mixtures, and make-do tooling. If you try to do it that way, you will spend a lot of time and money. Especially with blackpowder, there are a lot of "little" steps that make a big difference in gunpowder thermal energy release characteristics and motor performance, and if you skip them you will merely be producing mediocre motors (at best). I am talking about all the silly-looking steps like how to make the best charcoal, how prep the ingredients, how to meal the mix, what to wet it with, how to dry it, how to corn it, how to meal it (again), how to wet it (again), how to store it, and on and on. If you are not interested in all the steps you may as well use factory gunpowder---and there go your savings in money.

Do you have a simulation worked out for the motor internal conditions? If you don't have a properly sized nozzle figured out, and balanced internal conditions for pressure/temperature/nozzle area/chamber size, and a burn face that regresses at the right geometry, you are going to have motor problems than range from no thrust to over-pressurization. What do I know about this? I've tried it, and it doesn't work very easily. Blackpowder is very squirrely stuff.

It really is a whole lot easier to start out with pre-made motors. There are gobs of them out there in many sizes, and if the blackpowder motors are not big enough there are more composite-propellant-motors too.

But do come back, and ask more questions. We will be happy to give help and advice and maybe a little more grief---all in fun. (You should see what they do around here to the people they really like.)
 
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Handeman

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So here is my issue i need advice with... I have built a rocket that is about 3'6" tall and weighs approx. 2.5 lbs. I had the idea to use 4 of the Estes D12-5 engines to power the rocket. Now I am second guessing that the weight may be too great for the 4 engines. Now keep in mind I am new and do not quite understand how to determine the thrust to weight ratio.
The thrust to weight ratio is, as a "rule of thumb" 5 to 1 minimum for a safe flight. This is 5 times the average thrust to the weight of the rocket. The ratio is to ensure sufficient speed off the rod or rail so the fins will keep the rocket stable. Looking at the actual thrust curve of the motor may result in slightly different results.

The first thing you have to do to convert the D12-5 thrust to pounds is convert the 12 Newtons to pounds. divide by 4.45 to convert from Newtons to pounds. That works out to 2.7 pounds of thrust. You're using 4 motors so that becomes 10.8 lbs of thrust. Divide that by 5 and you end up with 2.16 lbs. A little less then you should have to safely fly a 2.5 lb rocket.

If you look at the actual thrust curve for an Estes D12 motor here, you will see that it spikes to 30 N thrust during the first 0.50 seconds and then levels out about 9 N for the duration of the burn.

At 30 N initial thrust, you have 6.7 lbs of thrust to get the rocket off the pad and up to speed. Times 3 motors, in case one doesn't light, you would have 20.1 lbs, divided by 5 and you have a max lift off weight of 4 lbs for three motors. That and a 50 ft safe zone should be an acceptable safety margin.

The 4 D12-5 motors should be safe for lift off of your rocket.

The 5 second delay might be another issue. You really need to simulate the flight, or do the math long hand, to determine how long it will take to reach apogee. I would suspect D12-3 motors would be a safer choice for that rocket. The D12-5 might not eject until the rocket was already a lawndart.
 

Commonwealth.Net

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Hey the advice given is great!
You many want to look a single Aerotech F motor.
One motor is easier to work with and the cost will also be less.

The 5 to 1 rule is correct, look at the thrust curves on the Aerotech motors to pick the correct one, as they make several F type motors.
 

powderburner

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The first thing you have to do to convert the D12-5 thrust to pounds is convert the 12 Newtons to pounds.
Handeman posted a whole bunch of good advice,
I just want to point out a slightly different version of this one step.

Instead of doing the math, based on the motor rating (that is part of the motor label), it is probably a little better to just go to the thrust curve data.

Look at the NAR motor list at:
http://www.nar.org/SandT/NARenglist.shtml

Find your motor, click on the "main" entry (in the case where there are multiple versions of the same basic motor, click the blue one) such as the data for the Estes D12 series:
http://www.nar.org/SandT/pdf/Estes/D12.pdf

Scroll down through the data and you will usually find a plot of thrust-time data, and below that you will see the numerical values (from the motor test stand) used to make the plot

Why go get all this extra data? Sometimes our motor manufacturer friends tell little white fibs. Maybe they do this to "sell" their motors, or to make them look like more than they really are, or who knows, but the point is that there have been a few motors in the past that were slightly "mis-labeled"

So, if you want to have the actual thrust number to put into the operations that Handeman suggested, it is just a little safer/better/more accurate to go to the data source and use the actual thrust numbers.

Now, your rocket design with four D12 motors will probably fly great if all four motors ignite like you want them to. However, if you have one dud, do you have a "plan B"? You need to re-run your numbers with a rocket carrying a dud motor weight and flying on only three operating motors. It might not go as high, but it would be good if your design got high enough to deploy the recovery gear and land safely without doing the lawn-dart thing. You might want to check into using D12-3 motors, if they still work OK with a "normal" launch with all four motors firing.

You can probably improve your chances of getting four lit motors by using the new Quest Q2 igniters. I have not experimented with them yet but they are reported to be EXCELLENT for getting clusters lit with ****very**** high probability of success.
 
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