[sugpro] Re: Verifying Motor Performance Through Flight Tests
- From: Jon Cherba <joncherba@xxxxxxxxx>
- To: sugpro@xxxxxxxxxxxxx
- Date: Tue, 17 Feb 2015 09:31:15 -0600
Which pressure sensor would you recommend for using during flights?
On Feb 17, 2015 9:29 AM, "Michael Monteith" <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
> Great! Something non-trivial. lol I like that. As one suggested using
> a pressure measurement on the chamber. I found a very small one that would
> work. Plus I do a lot of programming so that's a distinct possibility to
> work
> on. All my books and all will be here in a month so I can tackle that
> more then.
> Now to decide which altimeter. lol My to-do list is growing. Thanks
>
> I'd like to make a contribution to rocketry as I know so many people have
> been so
> helpful to me. So much I've learned and so much to be learned. That never
> grows old.
>
> Michael
> --------------------------------------------
> On Sun, 2/15/15, Steve Peterson <steve_peterson@xxxxxxxxxxxxx> wrote:
>
> Subject: [sugpro] Re: Verifying Motor Performance Through Flight Tests
> To: sugpro@xxxxxxxxxxxxx
> Date: Sunday, February 15, 2015, 7:11 PM
>
> 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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