wymanthescienceman Posted July 22, 2009 Posted July 22, 2009 I've been recently making small bottle rockets (1/4" ID x 2" long), and I've done a few static tests with them to see how much thrust these little babies can put out. My little rocket motors have roughly 45g of thrust from the tests I've done so far using a digital scale. I used a small board with a hole drilled in it, set on top of the scale, and mounted the small motor in it and lit 'er up, and it seems to work fine. I use a camera to record the screen of the scale so I can see all the changes it goes through. The rocket motors perform pretty consistently so far, and I've found they have a thrust-to-weight ratio (T:WR) of roughly 19:1, as the motors only weigh 2.4g. Anyway, I've been searching the net for awhile now and cannot seem to find out how to calculate what a rocket can lift, given its thrust. The only thing I can do now is strap weight to them and see how they perform, but I want to know how to calculate what a rocket can safely lift when it's thrust is known. So far all I could gather is that a rocket needs a T:WR greater than one to fly, and I think that every increment of 1 over that is equivalent to multiples of acceleration in it's opposition to gravity. I guess that means that a rocket with a T:WR of 5:1 would accelerate at 4x the rate of gravity (9.8M/s2)? Given that, would I just need to multiply that rate of acceleration by the burn time of the motor, to get approximate height when motor burns out? It can't be THAT simple... Does anyone here know how to calculate this? Is there something I'm missing? Dang rocket science...
Seymour Posted July 22, 2009 Posted July 22, 2009 I think for these little babies rocket science can go out the window. Unless of course you are more interested in the rocket science than the result! A rule of thumb that I've heard thrown around the place is that a rocket can take off and fly in a stable manner so long as it can pump out five times it's mass in thrust. If you want more rocket science than exists in a 'rule of thumb', I'm not the one to help
wymanthescienceman Posted July 22, 2009 Author Posted July 22, 2009 (edited) Hmm, that rule of thumb you mentioned does seem to fit my tests so far, I launched a few rockets with 5g dummy headers and when properly balanced the total take-off weight was about 10g. Given the ~45g thrust, these rockets flew well and to a reasonable height so it seems the rule worked out quite satisfactorily. BUT as you said, I am more interested in the science part here, so I would still like to know from any others with more experience how I would go about actually doing the math and getting a result I can use. I have my reasons, mostly just curiosity than anything. Since these little rockets can lift a header to a good height, I know that the calculations aren't totally necessary, but I still wanna know how to do it... Edited July 22, 2009 by wymanthescienceman
derekroolz Posted July 22, 2009 Posted July 22, 2009 Well in the movie "October Sky" the man created a formula for the calculations...but it was never revealed entirly....but when you are trying to calculate your lifting capabilities and such. There are alot more variables coming into play, such as elavation, agles at wich the rocket is launched, the humidity, tempurature, and to calculate where it will land(the most important in high power rocketry) is WIND!!! Although I dont think your calculating for high power rocketry. Basically, there are formulas out there....but you need to find the one that applies to your surroundings. And not use one that works for people at 500 feet below sea level.(as an example) Sorry though I dont know any places to check for these things. But there are alot more variables than you would expect
Seymour Posted July 22, 2009 Posted July 22, 2009 There are numerous and very detailed laws governing the flight of rockets out there. They are however made for dealing with space rockets at one end, and high power model rockets at the other end. They are not frequently used for firework rockets, which are not usually made for efficiency of flight, and usually have features that are very unhelpful in this regard, such as fuels with high solids, even up to 25% material that is just shot out the back with no thrust gain, and aerodynamically, let's face it. Our rockets are umbrellas. I suggest you go to the rocketry forum. (yes, it exists, and by that name). There will be people who have the rocket science skillz here on APC, but I'm sure they will be more numerous (hordes even) on a dedicated rocketry community.
wymanthescienceman Posted July 24, 2009 Author Posted July 24, 2009 Well, I suppose just attaching a header to these rockets and seeing how high they go is going to have to be sufficient in my case. I'll just have to worry about the real science part after I get into this a bit more, and start making real rockets. For now I'll just follow the rule of thumb that Seymour said earlier, I just thought there was a general answer for "This rocket gives X amount of thrust, thus can lift Y amount of weight".... Anyway, thanks for the help.
Swede Posted July 24, 2009 Posted July 24, 2009 Are you familiar with David Sleeter's book, "Amateur Rocket Motor Construction"? Sleeter goes very deeply into the science behind BP and sugar rockets. The book is geared more towards model rocketry than fireworks, but I consider it one of the better "how to" resources for making a rocket. PLENTY of science there, thrust curves, etc.
Aquarius Posted August 8, 2009 Posted August 8, 2009 Sleeters book is excellent, and well balanced. The question you should ask yourself, is what you want or need from your rockets.If you are into model/amateur/high powered rocketry, you probably know of Nakka and Yawns's sites. I built a digital test stand using the guidelines from their sites, and it is as close to rocket science as I'll ever be (apart form CP-calc and the like). But it was fun and challenging. It will show you the differences in performance depending on nozzle diameter, spindle length, composition etc quite good, and it's easy to read the charts. If that's where you want to go, go for it. Before I got there (took a while), I used the "QG-method" (qualified guess): Had a driver with a known comp, a shell or dummy with a known weight and a qualified guess of what height I wanted it to reach. Timed the rocket from start to apogee to get the delay right. Too high and a weaker driver or a heavier payload would fix thing, too low and a lighter payload or a stronger driver would do the trick. Sound easy, but in fact it will take more than a few tries to get right. Good luck!
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