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Model Forum / General / Rockets / March 2007A Rig for measuring Isp
(If this isnt the right group tell me, I couldnt find a group especially for Amateur rockets) Ive been working with rcandy for a while, Ive been experimenting with different mixes to get it to the consistancy I want. But lately, Ive been wondering how to make a rig to measure Isp. The idea came from Jimmy Yawns Site. This is what the rig looks like: http://picasaweb.google.com/Overclocked2300/PrototypeScale The Black part is where the engine goes, the pointer points to the scale. I calibrated the scale by Using water. Since 1g of water =1ml, then 1L of water = 1kg. I just convert kg to pounds and then to newtons, and bam, Instant Isp meter. Now, Today I got to the actual testing (It only took maybe 20 minutes to build), I used a Estes C6-5 Engine to Do the actual testing, so I can be accurate and see how much % error this scale has. Here are The Results...According to Estes, The Max (Ideal) Thrust is 15N, Mine measured 14.75 N, thus giving a 2% Error! I expected Much more, atleast 10% Error between measurements. The Average Thrust Gave the same results, 2% Error. However, I just noticed on the data sheet for the engine that it says they can vary by +/- 10%. Is this 2% Error the Engine Or is it the Scale? Would you call the scale Accurate in measuring Isp? > (If this isnt the right group tell me, I couldnt find a group > especially for Amateur rockets) That OK, neither could we. ;-)  SignatureRocketry Planet — hobby rocketry news, feature articles, news archive, discussion forums, live chat, free auctions, launch calendar and the largest collection of web links anywhere! http://www.rocketryplanet.com > Here are The Results...According to Estes, The Max (Ideal) Thrust is > 15N, Mine measured 14.75 N, thus giving a 2% Error! I expected Much [quoted text clipped - 4 lines] > they can vary by +/- 10%. Is this 2% Error the Engine Or is it the > Scale? Would you call the scale Accurate in measuring Isp? The NAR S&T results are 14.09 N for the C6-5 (with a sigma of 2.94), so the "error" from your test was closer to 5% http://wwwtest.nar.org/SandT/pdf/Estes/C6.pdf Estes (and other motor/engine manufacturers) sometimes overstate the performance. I assume from the picture of your test stand that you read the peak thrust "by eye". I think right there is probably the most significant error, as it is hard to capture the peak thrust when the peak lasts less than a 1/10 second. To answer your question, the 2% (or 5%) error is a aggregate of - the test stand error, which is probably pretty small since you calibrated it with a weight similar to the thrust you're measuring; - eyeball error; - motor variability. You probably know this, but to measure Isp you'll need the total impulse (or average thrust plus thrust duration), not peak thrust. Besides needing some method of integrating impulse over the duration of the burn, you may need to dampen the motion of the test stand "arm". From the picture it looks like that's a furring strip; they're pretty stiff and tend to bounce or "ring" when hit with an impulse. It won't be nearly as bad as with a steel arm, so maybe the ringing won't be large enough to need correcting. Something to think about, though. SignatureSteve Humphrey (replace "spambait" with "merlinus" to respond directly to me) > > Here are The Results...According to Estes, The Max (Ideal) Thrust is > > 15N, Mine measured 14.75 N, thus giving a 2% Error! I expected Much [quoted text clipped - 34 lines] > Steve Humphrey > (replace "spambait" with "merlinus" to respond directly to me) A video will clear up some uncertainty about my rig. http://video.google.com/videoplay?docid=-882349013109587184 Thats How I measure Thrust. I go frame by frame and Observe where the needle goes. > You probably know this, but to measure Isp you'll need the total impulse > (or average thrust plus thrust duration), not peak thrust. Besides [quoted text clipped - 4 lines] > nearly as bad as with a steel arm, so maybe the ringing won't be large > enough to need correcting. Something to think about, though Actually, I didnt know this, Thanks for the info though! So Im guessing to get Peak Impulse, I would add Peak Thrust + Time for that moment. The whole rig is made from Scrap wood I found in the garage. I forgot the mention, In the video the Top Most line is 0, all other lines are graduations of .5kg . >> ... to measure Isp you'll need the total impulse >> (or average thrust plus thrust duration), not peak thrust. > So Im > guessing to get Peak Impulse, I would add Peak Thrust + Time for that > moment. What you want is the total impulse. Go back to Jimmie Yawn's web page where he made the thrust stand similar to yours. Notice he makes a graph of thrust vs time? The area under the curve gives the total impulse. You could plot the graph on graph paper and count the number of squares under the curve as a good approximation. Peak thrust is fun and interesting, but it doesn't help you characterize your motor--it's not useful for calculating Isp, for instance. (Peak impulse is the same as peak thrust ... impulse being another word for thrust in this case.) If you knew the average thrust of the motor, then you could multiply it times the thrust duration to get total impulse. But usually the average thrust is calculated from the total thrust. Ave thrust (or ave impulse) = total impulse / thrust duration SignatureSteve Humphrey (replace "spambait" with "merlinus" to respond directly to me) Speaking of Isp, how do you calculate how high the rocket will go using Isp? Ive read on many sites (including one by Nasa) That Isp is used to calculate how high it will go. > >> ... to measure Isp you'll need the total impulse > >> (or average thrust plus thrust duration), not peak thrust. [quoted text clipped - 24 lines] > Steve Humphrey > (replace "spambait" with "merlinus" to respond directly to me) >Speaking of Isp, how do you calculate how high the rocket will go >using Isp? Ive read on many sites (including one by Nasa) That Isp is >used to calculate how high it will go. Without a lot more information, about the only thing you can get from knowing Isp is exhaust velocity. Keep reading. > Speaking of Isp, how do you calculate how high the rocket will go > using Isp? Ive read on many sites (including one by Nasa) That Isp is > used to calculate how high it will go. You need more than the *just* the Isp. Generally you need to know at least the average thrust, thrust duration, rocket size, and weights of the rocket and the propellant. But if you've calculated the Isp you already must know most of what you need. Since Isp is the total impulse divided by propellant weight (or mass), and since you probably figured the total impulse using a thrust vs time graph, you will know or can calculate the thrust duration, propellant weight, and average thrust. The Fehsken-Malewicki equations are a good way to estimate apogee without using a computer simulation: http://my.execpc.com/~culp/rockets/rckt_eqn.html These equations account for atmospheric drag. Refining the apogee estimate--getting more accurate--means employing a simulation and accounting for changes in thrust, drag changes with airspeed, air pressure change with altitude, temperature, etc. Depending on how high the rocket goes some of these may or may not be important. SignatureSteve Humphrey (replace "spambait" with "merlinus" to respond directly to me) > (If this isnt the right group tell me, I couldnt find a group > especially for Amateur rockets) [quoted text clipped - 10 lines] > then 1L of water = 1kg. I just convert kg to pounds and then to > newtons, and bam, Instant Isp meter. Why would you convert kg into pounds and THEN into Newtons? F=ma (ie N= kg x 'g') (where 'g' is acceleration due to gravity, usually taken as being 9.807 m/s/s) Just trying to be helpful.... :-) PaulB > > especially for Amateur rockets) > > Ive been working with rcandy for a while, Ive been experimenting with [quoted text clipped - 18 lines] > > PaulB Ok, Using what you said for 1.5kg, I get 14,700 N. Its pretty much the same thing, but x1000. So whats the Difference? Both are relatively the same answer. I used google to do the original conversion instead of a calc. If Im only going to measure 50N Max for all my engines, I can get a 50N Scale for 2.95 at American Science and Surplus. I just gotta thing of how to measure it using a scale you would usually pull. Oh wait, I used 1500g, not 1.5kg, Still it equals out to 14.7N...One just has more significant digits. I wonder if Google uses F=MA for lbs to newton conversion. Hmm, It must, Since W=M*G, and Google is On earth, It must find Mass, Then Use F=MG to find Newtons... |
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