I work with Adrian at our day jobs, so let me know if you have any specific questions for him. On Wed, Feb 18, 2015 at 2:56 PM, Steve Peterson <steve_peterson@xxxxxxxxxxxxx> wrote: > Thanks, Richard; I had a further thought on the subject regarding Cd. > > You can use "coast down" information gleaned from the flight data to > calculate CdA. Adrian (IIRC) did this with fairly good success. Basically > the data from burnout to apogee (or thereabouts) can be used as I described > for the thrust data to determine net force on the rocket, except the > coasting data is the result of only gravitational and aerodynamic forces. > Weighing the rocket post-flight will allow you to easily deduct the (now > constant) gravitational force (assuming no parts are lost :-)) leaving only > the aerodynamic force. > > That force will be the result of the Cd and the cross-sectional area, both > of which are not easy to measure, although various programs can estimate it, > as you point out. That leaves only the air density to be estimated and, of > course, flight angle (and angle of attack) to be judged/assumed--although > one might argue that with enough test flights (and not-too-dispersed data) > the aerodynamic force would include the effects of "typical" flight angle > and angle of attack. > > Seems like it's a good excuse for repeated flights :-) > > --Steve > > > On 02/18/2015 11:35 AM, Richard Nakka wrote: >> >> You've described the process and challenges nicely, Steve. >> >> It's been a while since I last harvested motor performance from flight >> computer data, but it can be done with, I expect, reasonable >> "engineering" accuracy. Timely discussion, as I plan to do this soon >> with my latest batch of flight test data. >> >> I recommend using the barometric, rather than accelerometer data, >> integrations (and all the other calcs) can be done relatively easily >> using a spreadsheet software. Adrian seems to have greater faith in >> the barometric data compared to accelerometer ( I fly the Raven3). >> >> Propellant mass as a function of time can be extracted from SRM.xls. >> >> With regard to drag coefficient (Cd), I use AeroLab to obtain this. >> AeroLab gives Cd as a function of mach number, which can be handy for >> higher velocity flights. >> >> Richard >> >> >> >> >> >> On Sun, Feb 15, 2015 at 6:11 PM, Steve Peterson >> <steve_peterson@xxxxxxxxxxxxx> wrote: >>> >>> Michael, >>> >>> The basics: if you have position (altitude) with respect to time, then >>> the >>> change in position over time is the velocity. The change in velocity with >>> respect to time is acceleration. If you have the mass (at the same moment >>> in >>> time that you've calculated the acceleration for, then rearrange Mr. >>> Newton's formula (F=ma) to get the net force. Any decent altimeter will >>> give >>> you altitude (to some precision/accuracy) vs. elapsed time (to some >>> precision/accuracy). After that is when the gremlins get you.... >>> >>> Altitude: change in altitude may not represent a true change in position >>> (that is, distance) because the rocket may be headed off at an angle. You >>> will have to either assume a certain angle of flight and calculate the >>> true >>> distance, or assume that it flew vertically (in which case the change in >>> altitude is the change in distance). >>> >>> Mass: it isn't constant, so you'll have to calculate it based on the >>> grain >>> geometry and your static tests, etc. I don't know if any of Richard's >>> spreadsheets list mass burned vs. time, but if they do, that would give >>> you >>> a good start, assuming your manufacturing is under tight enough control. >>> >>> OK, so you've calculated F--but hang on, because that's *net* F. That is, >>> thrust minus the force of gravity and minus the force of drag. The force >>> due >>> to gravity is just g*mass and we've already dealt with mass. >>> However, the force due to drag is more problematic. As you know, it >>> consists >>> of the Cd of the rocket (which will vary with velocity), the angle of >>> attack, atmospheric conditions (launch pad altitude, altitude of the >>> rocket >>> at any instant in time, temperature, barometric pressure at launch, etc.) >>> and, of course, the square of the velocity. >>> >>> Your question then becomes, will you know all that stuff with sufficient >>> accuracy to give you a meaningful result? And will your altitude be >>> accurate/precise enough to allow you to do all the math on it to get the >>> acceleration with any kind of accuracy/precision? >>> >>> From what I recall, the Featherweight altimeters are about the most >>> accurate/precise out there (although I would also check with the altus >>> metrum guys because I've read that their stuff is pretty darned good, >>> too). >>> Both will record fast enough to get you data with short enough time >>> intervals. I know Adrian Adamson (Featherweight) has done a lot of study >>> on >>> this--you might check the Featherweight forum and also over on TRF. >>> >>> I should also mention that the Featherweight altimeters (or at least the >>> Raven), and possibly the altus metrum products, will also provide >>> acceleration data so that you don't have to do the double differentiation >>> to >>> calculate acceleration from altitude. I haven't looked into how >>> accurate/precise it is, however. But you still have to know the >>> atmospheric >>> info and the aerodynamics of your rocket--and those two are usually the >>> killers. >>> >>> A lot of people have looked into doing this and, as I recall, very few >>> have >>> managed to come up with anything that was very persuasive (and they were >>> using commercial motors), although I am certainly no expert on this >>> stuff. >>> It's pretty easy (especially if you can program) to simulate a few data >>> points and do the calcs to see what you come up with. Munge the altitude >>> data a bit to simulate inaccuracies and see how much it throws off your >>> answer. Vary the Cd by .1, .2, .3 or so and see what happens. Etc etc. >>> You'll soon get a feel for just how hard this is. >>> >>> --Steve >>> >>> On 02/15/2015 10:17 AM, Michael Monteith (Redacted sender >>> michael_r_monteith@xxxxxxxxx for DMARC) wrote: >>>> >>>> I hope this isn't off topic as it has to do with verifying motor >>>> performance really. I was thinking on what I would need to verify >>>> rocket >>>> motor performance during a flight test. So I was thinking of what would >>>> be >>>> the requirements to gather the data in flight. There is so many >>>> altimeters >>>> and ranges of price. Some show they output thrust time. But not sure >>>> exactly if it's what I'm thinking it is or I'd be better off getting one >>>> cheaper and calculating it. >>>> http://data.rocketsetc.com/altimeter_data.html >>>> >>>> So to my question. What data is required and how fast? I see all >>>> the >>>> thrust curves for static testing but trying to figure out how you >>>> backtrack >>>> and figure from a flight test what the thrust curve is for comparison? >>>> This >>>> is what I want to arrive at, a thrust curve for flight test vs thrust >>>> curve >>>> on static testing. >>>> >>>> My initial guess is at least having time and altitude and having rocket >>>> mass etc. From there you can calculate acceleration etc and arrive at >>>> thrust. I don't want to think of the formula right now for this. It >>>> might >>>> be in my pile of books but those are boxed up in Missouri and won't see >>>> them >>>> for about a month now. But don't recall anything like that. >>>> >>>> I figured I might as well buy the right recording altimeter to begin >>>> with. I don't mind spending the money but only if I do it preferably >>>> once >>>> and right. Specifically the right data, accurate, and the right speed. >>>> I >>>> think the more time I spent on it the more confused I was with all the >>>> options on them all. At least until I know the bare minimum. I don't >>>> know >>>> if anyone has gone down this road or not. I saw Richard made mention on >>>> one >>>> of his pages that it was something for a future page. >>>> >>>> If we need to take it offline feel free to email me. >>>> >>>> Michael Monteith >>>> >>>> >>> >> > >