douglas423 Posted September 24, 2015 Posted September 24, 2015 I'm needing help figuring out how powerful my rocket is and i can't seem to figure out how to get it. If tried using a scale and the rocket upside down on it but it didn't read the thrust, I think there is an issue with the scale. Please tell me how you guys test your rockets newtons per second. I'm needing to figure this out in order to use other calculations for height and speed.Here is a video I tried measuring it
Oinikis Posted September 24, 2015 Posted September 24, 2015 you need to make a thrust time-graph, and you'll see the progression of the thrist, plus the area underneath it is you total impulse (newtonsecs). Now for the measurment. the most basic thing you can do is to take a mechanical scale-dynamometer, strap your rocket, and film the measurments, than watch it frame by frame, and draw your graph on a piece of grid paper, so you can easily count the squares and get the total impulse. that is exactly what I did in this video here:https://m.youtube.com/watch?v=Mts5fvc58VY There is also a good read in this topic here:http://www.nakka-rocketry.net/static.html This type of measurment is not very accurate, because you are suffering from dynamic effects like friction and inertia, althou it is still very useable, and a good tool for learning. For something more advanced you could look into digital test stands. I have one built from wood, load cell obtained from bathroom scale, INA125P opamp and an arduino, plus some amazing software provided by Stix. It allows your for much better and faster measurment, and you can work the numbers on excel. Sorry for the formating and spelling mistakes, I'm on a phone 1
stix Posted September 24, 2015 Posted September 24, 2015 I agree with what Oinikis posted - especially the bit about Stix's software . With a basic set of kitchen spring scales, video and some graph paper (or ms excel) you'll get very satisfactory results. Firstly, did you actually test the scales just prior to igniting the motor? What I normally do is mount the motor then put a known weight on top and do a final check, remove the weight, then ignite the motor. There's little point saying "I think there is an issue with the scale" Is there an issue?... well CHECK IT!! and if there is, throw it away and get a new one. What resolution are the scales? ie. the smallest weight increments it can measure. Just with a general observation of your video, it looked like the motor burned for about 8-10 seconds. Combine that with the sound and the look of the flame exiting the nozzle - my conclusion is that the motor would never have even been able to alleviate it's own weight and if inverted with a stick attached, it would have just sat there. Therefore the harsh reality is that it's possible there was not enough thrust to measure in the first place - although you would think there would be something registered, but as I said, that would depend of the scale resolution. What fuel are you using? What type of configuration - end burner or core burner, nozzled or nozzle-less? Some more info and we'll be able to get an understanding and point you in the right direction. People on this forum are always willing to help, but you have to help yourself first. Cheers.
Col Posted September 24, 2015 Posted September 24, 2015 I agree with whats been said. Iwould redo the test with a spring type kitchen scale just to rule out any lag issues with the digital. I`ve tested fountains that had almost no thrust but they still managed to move the needle a tiny bit.
douglas423 Posted September 25, 2015 Author Posted September 25, 2015 Okay thank you guys. I tried out the scale before but its not that accurate. It is used for shipping wieght for boxes. I used a 65/35 ratio of kno3 and table sugar, I disolved it into water then boiled it out . I also used a wax/clay nozzle and it was a core burner. I had about 100 grams of fuel but one issue was it did not fully evaporate the water and was still very soft inside making it not burn as good. What methods and ratio would you guys recomend?
stix Posted September 25, 2015 Posted September 25, 2015 (edited) A scale used for shipping weights would likely have large increments, with a small amount thrust never reaching the first mark. As suggested by Col, most domestic digital scales have lag and therefore not suitable for rocket motor testing. The easiest testing for you to do would be spring scales. Alternatively you could make your own load cell which is what Oinikis and many others have done, or purchase one - I'm sure there would be people on this forum who could assist. 65/35 is pretty much the standard. Too much water left in the fuel certainly slows it down as you have witnessed. With R-candy it's important to drive the water out. A 'snap' test is a good measure of where your fuel is at - flatten out a small amount, let it cool - if it bends it's not done, keep cooking until the test piece snaps. I used the r-candy dissolved method like yourself but wasn't happy with the consistency and repeat-ability, so I purchased a coffee grinder to grind up the KNO3, mix in icing/confectioners sugar then melt. In my view the "melted" method delivers more consistent results. There are quite a few other methods a well. Keep persevering and you'll get there. Cheers. Edited September 25, 2015 by stix
FloridaCracker Posted September 25, 2015 Posted September 25, 2015 Stix, Do you have a thread on making a DIY load cell to go with your software code? Something with part numbers/description or alternatives like the bathroom scales and a diagram of how it all fits together would be awesome and I'm sure greatly appreciated by many! Just a thought and maybe it's already been done and I haven't found it yet?
stix Posted September 26, 2015 Posted September 26, 2015 You're correct - there is posts somewhere on this forum that I read a while back, so it's not new. The only person I know of on this forum who's recently made their own load cell amp is Oinikis (post #2). The 'thrust meter' that I made myself some time back was based on an AD converter kit connected to spring scales with a potentiometer connected to the central spindle - this worked fine but wasn't as accurate as I would have liked. My brother who's into electronics made a device which is very accurate (1 gram resolution) this is what I now use if I want to do some proper testing. Unfortunately it uses the parallel port on a computer which is becoming redundant and the electronics side is complicated but I could ask him about the circuit diagram. From my side, I wrote some simple software which is loaded into a Freetronics Eleven (Arduino Uno equivalent) development board. These are readily available and cheap. I then wrote some software to extract the data for use in excel or another program. Like I said, there is some good info on this forum somewhere. This from Richard Nakka's site http://www.nakka-rocketry.net/strainlc.html. may also help. He also has other great info regarding sugar rocket motors - worth a read. Cheers.
dagabu Posted September 27, 2015 Posted September 27, 2015 Rocket Test Rig petehand@gmail.com makes and sells them. There is no other thrust test stand out there that matches it!
Peret Posted October 4, 2015 Posted October 4, 2015 (edited) Thanks for the endorsement, Dave! Check's in the mail https://www.youtube.com/watch?v=YwhQQY1UuFY The accompanying software will calculate the height for you, taking atmospheric drag into account, accurate to a couple of percent. Edited October 4, 2015 by Peret
stix Posted October 4, 2015 Posted October 4, 2015 Having read the original post about this device - which was conceived a few years back (I can't remember when/where right now) it's good to see that it's still going. Have there been any mods since?
Peret Posted October 5, 2015 Posted October 5, 2015 Some minor changes to the rig. It's USB now, no serial adapter needed. The analysis software has improved a lot.
stix Posted October 5, 2015 Posted October 5, 2015 Rocket Test Rig petehand@gmail.com makes and sells them. There is no other thrust test stand out there that matches it! Well, not yet Some minor changes to the rig. It's USB now, no serial adapter needed. The analysis software has improved a lot. Seriously though, it sounds like a good device. I've always mucked around making thrust meters and enjoy a bit of programming. I've looked at making a "stand alone" device similar to yours. The obvious benefits being able to do multiple tests without attached to a laptop etc., but that would be way too much work. So, much kudos to yourself and others who were involved in getting something together that's easy to use and actually works. Cheers.
MrB Posted October 5, 2015 Posted October 5, 2015 I don't even make much rockets, but i SO want one of these. The geek in me is screaming, and have been, for... ages. But it's just to darn expensive for a toy that wont see enough use to justify it's cost.I'm quite certain that it isn't that expensive compared to building the equivalent setup on your own, but if you don't get enough use out of it, it still makes it a bad investment. So it remains a christmas gift item, i will never get. (Nobody would know to get me one, so...) Great to hear there is ongoing development.B!
douglas423 Posted October 9, 2015 Author Posted October 9, 2015 Thanks for posting that! I might end up making or buying one.
Peret Posted October 9, 2015 Posted October 9, 2015 Well, not yet Go for it, stix. I assure you, I don't make these for the money. There's 8 to 10 hours work in one, CNC milling and turning and 22 holes to tap by hand. When I break a tap in a blind hole on a body that's got 4 hours work in it you can hear me scream and shout in the next county. Dave would know about that too He used to make them for me before his accident.
Col Posted October 9, 2015 Posted October 9, 2015 Sods law dictates the tap will always break in the last hole, not the first A great bit of kit and worth every penny but at maybe £500 with customs/tax its a fair old wedge
stix Posted October 9, 2015 Posted October 9, 2015 (edited) Go for it, stix. I assure you, I don't make these for the money. There's 8 to 10 hours work in one, CNC milling and turning and 22 holes to tap by hand. When I break a tap in a blind hole on a body that's got 4 hours work in it you can hear me scream and shout in the next county. Dave would know about that too He used to make them for me before his accident. I'm under no illusions. I'm sure the reasons you and your partners developed such a device was not motivated by money. You also forgot to mention the hours of programming that must have gone into the software. After being involved in making various thrust meters over the years (including the programming), I appreciate the work the goes into making one and then evaluating the data - especially if you're making it for the use of others. That would be the most difficult part. It certainly wouldn't be a money making exercise unless they were mass produced but it would be doubtful the market would be big enough. I'll reiterate what I said above: "much kudos to yourself and others who were involved in getting something together that's easy to use and actually works". I'm in the process of making my latest version, but it's been sitting there less than half done for months. I keep getting sidetracked. Cheers. Edited October 9, 2015 by stix
mikeee Posted October 9, 2015 Posted October 9, 2015 My wife usually lets me know on the thrust factor of my rocket performance...........................(Rimshot)! Just like other fireworks plenty of Ooh's and Aah's!
stix Posted October 10, 2015 Posted October 10, 2015 (edited) A reduction in the nozzle diameter (orifice) will also increase the thrust factor, resulting in overall increased performance but with the possibility of an unfortunate premature cato. mikeee made me say it. Edited October 10, 2015 by stix
Peret Posted October 10, 2015 Posted October 10, 2015 at maybe £500 with customs/tax its a fair old wedge Shouldn't cost you that much, Col. I sold one to a guy in Scotland and even with the shipping his outlay was only in the 300 pound region. I'll do you a load cell and electronics board for $100 US if you want to make your own metalwork. With a declared customs value of about 65 pounds they probably won't bother you for duty and VAT.
Col Posted October 10, 2015 Posted October 10, 2015 That would make it much more economical, at £65 HM Customs would tack on £13 in VAT (Total £78). They will take shipping cost into account for VAT so if shipping is £20, they`d want £17 VAT (Total £102) Import duty applies for anything over £135, import VAT applies to anything over £9.Import duty rate is 1.7%, Import VAT rate is 20%. Something else to trap the unwary is Royal Mail will pay the import taxes on your behalf without asking which incurrs a handling fee of £18
dagabu Posted October 16, 2015 Posted October 16, 2015 Go for it, stix. I assure you, I don't make these for the money. There's 8 to 10 hours work in one, CNC milling and turning and 22 holes to tap by hand. When I break a tap in a blind hole on a body that's got 4 hours work in it you can hear me scream and shout in the next county. Dave would know about that too He used to make them for me before his accident. LOL! Yes, I have a test stand still that has the tap end broken off in the final hole on the end cap! I didnt tap by hand though, I have a tapping stand and used a high speed spiral bottoming tap for the holes but its still easy to push the drill/forget the Tap-Magic and snap it off. I'm hoping to rebuild mine and get some motors tested this winter.
Peret Posted October 17, 2015 Posted October 17, 2015 I knew it must have happened to you I have twice broken taps in the end holes. A friend of mine milled off the corners, filled them with aluminum weld and milled them square, and I redrilled. It wouldn't have been cost effective if I had to pay to have it done. Now I drill the end tap holes one drill over size and use short screws, so I don't have to tap more than five or six turns, and it's been a while since I broke another.
jessoman Posted October 20, 2015 Posted October 20, 2015 I too, would love one of these test stands, to compare to the rest of the world where my motors are at. I have wanted to contact Pete for years as I have admired his design, unfortunately I don't have that kind of money. I have just bought an arduino (bit of time off work) and am going to have a tinker, I have no idea what I'm doing, so I'm sure I'll waste plenty of time on it . I suppose that will stop me from burning my fingers for a little while
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