[sugpro] Re: Verifying Motor Performance Through Flight Tests
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: sugpro@xxxxxxxxxxxxx
- Date: Mon, 23 Feb 2015 17:25:15 -0500
Adrian said he'd use the accelerometer because the baro sensor is
basically crap during the burn. But that you can use the barometer to
essentially calibrate the accelerometer data, i.e. does the accel data
add up to the same apogee as the barometer reports.
I think it can be safely said that you're never going to get as good
of data trying to characterize an engine in flight than you'd get on a
test stand. There are just too many additional sources of error, many
of which are hard to compensate for. But flying two engines on the
same day on the same rocket could give useful comparative data.
On Mon, Feb 23, 2015 at 12:50 PM, Michael Monteith
<dmarc-noreply@xxxxxxxxxxxxx> wrote:
> Richard,
> I sent a note to Ben to pass onto Adrian. I'm curious to see what he says.
> To me
> accelerometer data would be more accurate and higher sample rate. I'm kind of
> anal when it comes to accuracy. I used to work making test equipment used to
> test
> government equipment. lol Depending on what I find out will help me decide
> which
> altimeter to use. While I like the Raven for the same money I can get the
> EggTimer TRS
> which also has GPS and tracking transmitter. It just doesn't have
> accelerometer.
> The EggTimer is a kit but I've soldered this stuff many times so peace of
> cake.
>
> Vendor Model Price Kit Max Alt Baro Accel Sample Rate
> Resolution LCD PC Outputs Config GPS Radio Tele OS
> Datasheet
> Featherweight Raven 3 $ 155 $ 155 100,000 ✓ ✓ 20hz/400hz
> - ✓ 4 Software - - - Win
> only Link
> Eggtimer TRS (DIY) $ 90 $ 140 30,000 ✓ - 20hz
> ✓ ✓ 2 Software ✓ ✓ - Win
> only Link
>
> http://www.featherweightaltimeters.com/The_Raven.php
> http://www.eggtimerrocketry.com/page23.php
>
> Then I may have to ask him about barometric resolution if baro is the way to
> go. From what I gather the Raven's accuracy is +/- .3% and the EggTimer is
> like .01%. Not sure how much that 30ft vs 1ft resolution will cause issues.
>
> I'm not in any rush until my stuff gets shipped to me in a couple of weeks
> for the house. I feel so naked without a house or anything. lol This gives
> me time to iron out what I need as far as altimeter.
>
> I'll be glad to see your results when you get around to it.
>
> Michael
> --------------------------------------------
> On Wed, 2/18/15, Richard Nakka <richard.rocketry@xxxxxxxxx> wrote:
>
> Subject: [sugpro] Re: Verifying Motor Performance Through Flight Tests
> To: sugpro@xxxxxxxxxxxxx
> Date: Wednesday, February 18, 2015, 2:35 PM
>
> 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
> >>
> >>
> >
> >
>
>
>
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