[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: Mon, 23 Feb 2015 09:50:36 -0800
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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