First time - Rocket help needed, is it going to 6027 feet?

Hello,
I am building my first rocket and I am using some calculators so check performance before I start actual testing, the results of my inital calculations tell me that this little rocket is going to fly to 6000 feet!

Could anyone help me by checking to see if I have made an error. When I say "if I have made an error" I really mean where I have made an error.
I have made my rocket from aluminum tube, it has the following specifications-
Weight 190 grams or 0.41lb excluding propellant
Rocket o.d. 0.875” or 22.22mm
Now my plan is use the i.d. of the aluminum tube as the motor, my plan is to simply partition the tube with a machined plug.
So my motor will have -
Motor diameter 19.1mm (751 thou)
Nozzle diameter 3.7mm or .145”
Core diameter 0.205” or 5.2mm
With a single grain 3 inches long

So I punched these values into a fantastic calculator I found called the Bates Grain Burn Rate Simulator
This motor will have -
Propellant weight 0.0793lb (36 grams)
Burn time 0.4234 seconds
Initial thrust 15.4964 pounds force
Total impulse 10.54 pounds force

So I took this data and punched it into another calculator -
https://www.unm.edu/~tbeach/flashstuff/RocketAltitudeFixedSize.html

But first I needed to convert total impluse to newton seconds,
So I divide the total impluse by the burn time and get 24.89 pounds force second
I then multiply it by 4.44822 to get 110.7 newtons

Which gives me 6027 feet. I suspect I haven't converted the total impulse correctly?
For my first rocket I am actually aiming for the 5 to 700 foot range.

Hi As this is a site mainly aimed at users of commercial products you won't get much help here. That is until you attain Tripoli 2nd Level Certification and can get into the restricted section of the forum. That is where they deal with Motor Research. You do appear to be on the right track. 2nd level is fairly easy to attain even with a "test" of your knowledge. Go to the Tripoli web site for info on that and see if that might be of some help.

There are some "Real" rocket scientists here that can help...with the right credentials....

Where exactly do you plan on launching this aluminum rocket?

Where exactly do you plan on launching this aluminum rocket?

Good question - I am not in the states so I don't think in my corner of the world there are laws against aluminium rockets. My goal is to keep the altitude low so it doesn't effect air traffic, where will be at my workplace which is out in the country well away from potential hazards. The big question is when which I am working on including the legality side of it, for now its a learning experience.

For what its worth my co worker is also building a motor out of ali, however his is around 2" diameter, single grain. I am a touch concerned about the pressure which I will look at once I get my own problems sorted out.

His plan is to put the ali motor inside a cardboard rocket.

For a motor with these specifications.

Propellant weight 0.0793lb (36 grams)
Burn time 0.4234 seconds
Initial thrust 15.4964 pounds = 15.4 pounds * 4.45 Newtons/pound = 68.5 N
Total impulse 10.54 pound-seconds = 10.5 pound-seconds * 4.45 Newtons/pound = 46.7 Ns.
Average Thrust = Total impulse / burn time = 46.7 Ns / 0.42 s = 111.2 N.

According to your simulator, your rocket could reach ~900 Meters if you could put the correct body tube diameter into the simulator, but the rocket is not likely to survive.

1. This motor is highly progressive so the grain dimensions are not optimum and the motor is likely to CATO.
2. The motor should have (2) 1.5" long propellant grains instead of (1) 3" propellant grain to make the burn more neutral.
3. Also you need a motor liner to prevent the aluminum from melting, and a casting tube to confine the propellant grain.
4. That would reduce the propellant weight significantly. Probably to not more than 30 grams which would reduce the total impulse to not more than 39 N-s and reduce the calculated apogee to ~750 meters.

TRF policy does not allow for the discussion of amateur motor manufacturing in the public forums, however the discussion so far has not got into the details of the propellant so it has not crossed our boundaries, however from your questions here, it is clear you do not have the knowledge to be making your own motors safely, and IMO are likely to hurt yourself if you try to.

Bob

Bobrech, thankyou very much for helping me with the calculations, could we agree that the reason I am learning the math first is to avoid hurting myself? I am of course, going to assume that you were assembled at NASA from the spare parts of other scientists and implanted with an A.I. unit thereby avoid the human condition we call "learning"?

I have already performed a stress analysis of the material, I am sure, very sure that the burst pressure is at least 1886psi, this calculation assumes that the ali is annealed which it is not, but heat can reduce strength so that has been considered, even if I used this motor size (which I am not) the pressure will only get up to 1242psi.

I also mentioned that I am planning on making the motor smaller in size to limit the altitude! So that renders your argument regarding CATO and my point about pressure null and void. I can't stress enough that the data I gave was simply a starting point.

Regarding the melting - I completely disagree, we have already ground tested larger motors than what I have mentioned with no melting. I seriously doubt it would withstand multiple firings though, aluminium is totally unsuitable for multiple use.
The nozzle would be a different story, however I have found 416R stainless to be ideal - it is very free machining and is designed to withstand hi temp erosion (in gun barrels) luckily I have found a source where I can obtain off cuts for free.

Thanks again.

Matthew, just a simple observation here. You've come into this forum to ask for advice and assistance, but when some is offered to you your response is to be offended and insulting.

Do you think that makes people more, or less, willing to help you?

MatthewMachinist said:

Bobrech, thankyou very much for helping me with the calculations, could we agree that the reason I am learning the math first is to avoid hurting myself? I am of course, going to assume that you were assembled at NASA from the spare parts of other scientists and implanted with an A.I. unit thereby avoid the human condition we call "learning"?

I have already performed a stress analysis of the material, I am sure, very sure that the burst pressure is at least 1886psi, this calculation assumes that the ali is annealed which it is not, but heat can reduce strength so that has been considered, even if I used this motor size (which I am not) the pressure will only get up to 1242psi.

I also mentioned that I am planning on making the motor smaller in size to limit the altitude! So that renders your argument regarding CATO and my point about pressure null and void. I can't stress enough that the data I gave was simply a starting point.

Regarding the melting - I completely disagree, we have already ground tested larger motors than what I have mentioned with no melting. I seriously doubt it would withstand multiple firings though, aluminium is totally unsuitable for multiple use.
The nozzle would be a different story, however I have found 416R stainless to be ideal - it is very free machining and is designed to withstand hi temp erosion (in gun barrels) luckily I have found a source where I can obtain off cuts for free.

Thanks again.

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No. I'm not assembled with a number of spare parts from NASA. But I have been working professionally in the rocket propulsion arena for 44 years, and have worked as a contractor and/or consultant for NASA Ames, Glenn, Johnson, JPL, Marshal, ESA, JAXA, USAF, Navy, Marines, Army, DoD, OSD, NSF, ERDA, BMDO and a bunch more government agencies as well as Lockheed, Boeing, Northrup-Grumman, Aerojet, ATK, Mitsubishi, Nissho Iwai, Kawasaki, Fuji Heavy Industries, and a lot more aerospace companies. I am an expert in low earth orbit environment and reentry heating simulation, and have worked on experiments flown on the shuttle and ISS as well as Deep Space 1. I'm well paid as a professional but I guess my free advice isn't worth much.....

Go to https://www.matweb.com/ and get the fundamental material properties on aluminum. https://www.matweb.com/search/datasheet_PDF.aspx?MatGUID=1b8c06d0ca7c456694c7777d9e10be5b By the time it gets to 260 C is like bubble gum, which is why you can't use your rocket twice....

Bob

bobkrech said:

No. I'm not assembled with a number of spare parts from NASA. But I have been working professionally in the rocket propulsion arena for 44 years, and have worked as a contractor and/or consultant for NASA Ames, Glenn, Johnson, JPL, Marshal, ESA, JAXA, USAF, Navy, Marines, Army, DoD, OSD, NSF, ERDA, BMDO and a bunch more government agencies as well as Lockheed, Boeing, Northrup-Grumman, Aerojet, ATK, Mitsubishi, Nissho Iwai, Kawasaki, Fuji Heavy Industries, and a lot more aerospace companies. I am an expert in low earth orbit environment and reentry heating simulation, and have worked on experiments flown on the shuttle and ISS as well as Deep Space 1. I'm well paid as a professional but I guess my free advice isn't worth much.....

Go to https://www.matweb.com/ and get the fundamental material properties on aluminum. https://www.matweb.com/search/datasheet_PDF.aspx?MatGUID=1b8c06d0ca7c456694c7777d9e10be5b By the time it gets to 260 C is like bubble gum, which is why you can't use your rocket twice....

Bob

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Game. Set. And match.

Bazinga!

bobkrech said:

No. I'm not assembled with a number of spare parts from NASA. But I have been working professionally in the rocket propulsion arena for 44 years, and have worked as a contractor and/or consultant for NASA Ames, Glenn, Johnson, JPL, Marshal, ESA, JAXA, USAF, Navy, Marines, Army, DoD, OSD, NSF, ERDA, BMDO and a bunch more government agencies as well as Lockheed, Boeing, Northrup-Grumman, Aerojet, ATK, Mitsubishi, Nissho Iwai, Kawasaki, Fuji Heavy Industries, and a lot more aerospace companies. I am an expert in low earth orbit environment and reentry heating simulation, and have worked on experiments flown on the shuttle and ISS as well as Deep Space 1. I'm well paid as a professional but I guess my free advice isn't worth much.....

Go to https://www.matweb.com/ and get the fundamental material properties on aluminum. https://www.matweb.com/search/datasheet_PDF.aspx?MatGUID=1b8c06d0ca7c456694c7777d9e10be5b By the time it gets to 260 C is like bubble gum, which is why you can't use your rocket twice....

Bob

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Yep.
I think that should make things a little clearer.
Always interesting to see how invested some of these new people posting are in their version of things, and how defensive they get.
Not sure "my first rocket" should be anywhere near what he is describing here.

dave carver said:

"There are some "Real" rocket scientists here that can help...with the right credentials...."

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Yes, I am quoting me, Bob is one of the real rocket scientists I was referring to. We aren't all big kids playing with big toys (like I am ).

Follow his advice and be safe. Make sure of your country's laws on flying rockets, jail sucks and governments have no sense of humor when it comes to rockets.

No no, I am not offended at all - the nasa joke was actually meant as humour, if I was offended I certainly wouldn't have said thankyou - twice in my post. I was very aware of Bobkrech's credentials by how he answered the question in an efficient manner (I am surprised to see just how impressive they are...might have to add them to my c.v.) And the explanation as to why we wouldn't succeed twice makes a lot of sense.

One more question - one person has told me that aluminium is like butter and totally unsuitable, another says it is fine for multiple use?

Steel nozzles are obviously ok - I have spent quite a bit of time on the nakka rocketry website looking at de laval nozzles which are often steel?

Thanks again for all the input, I will probably machine the nozzle with an internal thread and machine a steel insert to contain the motor which will be completely internal. By the time I get to actually making the critical working parts and flying it.

There's an odd thing about steel, the more of an alloy it is the lower the melting point. The steel with the highest temperature melting point is mild steel. The alloys can be much lower.