[sugpro] Re: Verifying Motor Performance Through Flight Tests
- From: "Michael Monteith" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "michael_r_monteith@xxxxxxxxx" for DMARC)
- To: sugpro@xxxxxxxxxxxxx
- Date: Tue, 17 Feb 2015 07:29:31 -0800
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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