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Model Rocket Thrust

  1. Jan 3, 2005 #1


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    I was wondering how I could find out the thrust, any any other interesting facts about my model rocket motor, but maily the inpulse/thrust.

    Here is what I know/can measure:
    -mass of propelant and whole motor assembly
    -type of propelant (aswell as full expected reaction of propelant as it burns)
    -size of rocket motor, diameter, volume, nozzle diameter
    -theoretical total energy output/heat of the reaction of rocket
    -burn time

    As of right now, I dont think this particular rocket will move anywhere on its own power, so I cant really measure its distance moved (horizontal or vertical), or its velocity.

    from this information here, what can I find? (probably not much)
    What could I do to imporve so that I have more valuable info so as to find the thrust?

    Here is my idea for finding the thrust force the rocket exerts,
    hand the rocket from a string with the nozzle pointing down, have the other end of the string tied to a [spring] scale. Light the rocket and then throughout the time that it is burning, measure its appearant weight on the scale and then its final weight. You know the initial weight and time it took to burn, and then you can just assume it burns evenly the whole time. From that you can then find that at 'such and such' second it wieghed this amount, even though it really had a mass of 'whatever' (excuse the technical terms), and then you can calculate the thrust upward from that. This methold should work, in theory, since the rocket isnt strong enough to 'fly' upward under its own power, and if you like, it wouldnt have to be a sping scale, it could be a normal scale and then face the nozzle upward, in this way, even if the rocket could 'fly' it would still work.

    How does that sound, or is there a better way?
  2. jcsd
  3. Jan 3, 2005 #2


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    Each pack of Estes engines comes with a pamphlet with performance data. HERE is a typical one. The number right after the letter, though, is the average thrust.

    Or do you actually want to do a static test? http://members.aol.com/ricnakk/static.html [Broken] guy put a motor upside-down on top of a kitchen scale for a simple test rig...
    Last edited by a moderator: May 1, 2017
  4. Jan 3, 2005 #3
    The following was taken from "How to Make Amateur Rockets 2nd Edition" by John H. Wickman of CP Technologies. I have changed it slightly in terms of style but none of these ideas or examples are mine. They emphasize citing sources to avoid plagarism at my high school to a great extent, probably a good idea:biggrin:

    Thrust = (chamber pressure) * (Throat area) * (Nozzle Coefficient) = Pc * At * Cf

    for avg Thrust:

    Average Thrust = Favg = (propellant weight flowrate) * Isp [Isp = specific impulse]

    So lets take an example, you have a motor design that has 2lbs of propellant and burns for 10 seconds with a specific impulse of 200 seconds.

    propellant weight flowrate = weight of propellant / burn time
    =0.2 lbs/second
    Favg = propellant weight flowrate * specific impulse
    = 0.2 lbs/second * 200 seconds
    = 40 lbs

    the average thrust is 40 lbs.

    To find the specific impulse divide the propellant ejection velocity by g, which is 32.2 because we are talking about weight of the propellant in the above equation and not mass, this is somewhat of a conversion. Dividing the weight in pounds by 32.2 will reveal the mass in slugs which is what is needed for the above equation for Favg to work.

    Isp = (Propellant Ejection Velecity) / 32.2

    So overall, you could combine these equations to get this one:

    Favg = [(weight of propellant) / (burn time)] * [(Propellant Ejection Velocity) / (32.2)]

    ***i'm not sure how to calculate the propellant ejection velocity when the spcific impulse is not already known because Isp = Propellant Ejection V / 32.2

    Hope this helps...any questions and I will make an attempt to explain or figure out with the much needed help of my trusty sidekick J. Wickman :tongue2:
  5. Jan 3, 2005 #4


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    The rocket I am speaking of is still theoretical, and when it is made (soon), it will be homemade, not estes, so I cant use thier charts or figures unfortunatly.

    as for Quantum_Prodegy's reply, all those formulas will help alot, but as of right now, I dont think I know most of those variables like chamber pressure, nozzle coefficient, or specific impulse, yet, I still need something more basic to find those things in the first place.
    Interesting that you should bring up John H. Wickman, because, I actually know him. I attended his 'Rocket Camp' (course 2) This past summer in Casper Wyoming where we build 9 foot tell rockets and some pretty big engines as well.

    So it seems like my original idea of setting the rocket upside down on a scale and then taking readings throughout the rocket firing isnt such a bad idea afterall, the only problem is that I now need a way to take the readings continusly and semi-acuratly, to avoid any more in acuracy that has already been brought in by doing it in this manner.
  6. Jan 3, 2005 #5
    Really? Small world! What makes it more coincidental is the fact that I have never been to Wyoming, and I live in Ottawa Canada. My Dad ordered his book (came with a video and computer programs to assist in the design) from his website.

    How was the camp? Stupid question, it must have been awesome. Do you know if they're doing one again? I think that's a summer trip my parents would gladly send me on. :tongue2:
  7. Jan 3, 2005 #6


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    The course was GREAT, if you dont mind waking up early (9:00 am) on your summer vacation, I highly recomend it.
    I couldnt believe that we were actually building these gigantic (compared to estes) rockets and motors from scratch and them flying them on an army reserve base in Wyoming. Now I have the remains of my rocket laying on the floor of my room as a souvenier.
    this is the one I went to, http://www.space-rockets.com/course2.html [Broken]
    Next summer, I hope to attend course 3 (and 4 hopefully after that).
    Attached are two pictures, one if of the 2 halves of my rocket before launch, and the other is a picture of what our static launch looked like.

    Attached Files:

    Last edited by a moderator: May 1, 2017
  8. Jan 3, 2005 #7
    Wow, nice. I went to the website earlier today (its on the cover of the book and so far it doesn't have any dates for this years' courses. My friend in school and I along with our chem teacher are building a rocket using the book as a guide, so I'm not sure which course I would take if I am able to take one. I am definately going to look into it further though!

    Great pics, how high did it go? Also, how did you start off the design of the rocket?

    hehe, sorry i'm getting off topic from your original post...
  9. Jan 3, 2005 #8


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    it went about 4000 feet high. it was a very successful launch, not so successful a landing though, there was a parachute problem and it crashed back to the ground.
    We started the design on the first day, he had some software that we used to design the fins, and we already knew how long, wide, and heavy (about) it would be, then a day or so later, we made our nose cones on the lathe.
    As far as to which course to take, you will need to ask yourself, 'Do I want to make a rocket or an enigine?' I myself really wanted the experience of having the finished product launched at the end (course 2 and 4), rather than a static fire (course 3). my understanding is that course 3 uses the book that you have as part of the class, I think they cover very similar topics (at least on engine design).

    As far as my original question,
    If anyone wants to join in on this, that is fine, I am still taking suggestions on what to use to measure thrust. as it is now, I have a crude way to measure average thurst, but no way to impliment it yet (how will I record all the numbers, and the set up, i'll need to find a scale,...)
  10. Jan 5, 2005 #9


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    someone suggested to me that I use the Logger Pro software and force sensor to do this. I should be able to set it up so that the engine pushes against the sensor and then the computer will capture the data and it will allow me to analyze it.
    This will be very accurate and will allow me to do lots of stuff with it, it is also easier then having to build anything eleborate.
  11. Jan 13, 2005 #10


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    I tested it successfully yester day, and I must say, I am very pleased with the results.

    Although I didnt get massive amounts of force (only a max around .35 N), and it didnt burn very long (compared to previous experiments, much shorter), the dat6a collection went very well.

    I attached a picture of what the force graph looks like and what set up and rocket engine look like too incase anyone is interested. It is kinda hard to read the tesxt on the force graph picture, but if you enlarge it it is easier to read.
    the propelent was Potassium Chlorate and sugar (sucrose).

    Attached Files:

  12. Jan 13, 2005 #11
    You should be able to figure the specific impulse. If it won't use up the engine too quickly, I'd suggest you might want to check your formula a bit- try to get the best specific impulse by getting just the right proportions.
  13. Jan 13, 2005 #12


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    isnt the specific impulse the area under the force * time curve, the integal of that graph I uploaded? I think it came to .223 N.

    On the static fire, I had pretty close the the most exact ratio of chloate to sugar as I could get, I measured it out on an electronic scale (in .1 gram increments), this was my best formula yet, and it shows, there us hardly anything left over after I fired the rocket. The only byproducts of the reaction are: Water vapor (that gets blown away), Carbon Dioxide (thats blown away), and Potassium Chloride (some gets blown away), so the remaining empy rocketr is pretty empy, just some white chloride residue leftover. It got so how that a puddle of moltent/liquid potassium chloride (mp. 772 degrees C, 1422 degrees F) formed under the nozzle of the engine and later solidified.

    The reason I think it should have burned much much longer is (to avoid going into a long story), I have done similar tests like this before with less chlorate and it burned much long, half as much chlorate burned 5 times as long ans that had ALOT more surface area to burn on. So I guess my explanationis that the temperatures and pressures were much greater in this more recent rocket which made it burn much hotter and faster than any previous rocket.
  14. Jun 10, 2011 #13
    An accurate way in which to record the data from the thrust measuring device is to set up a camera mount so that the camera is focused on the ticker that shows the amount of thrust. Hence, you have all of the data recorded, accurately, and can be viewed repeatedly. This method works great. One can see that exact force at any given interval. It is a much more accurate recorded method than writing down data as the motor is running.
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