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Using Arduino => Project Guidance => Topic started by: gt_aerospace on Jul 30, 2020, 08:17 pm

Title: Speed measurement for rockets
Post by: gt_aerospace on Jul 30, 2020, 08:17 pm
I have an idea for measuring speed of my rockets. I want to measure speeds over Mach One . I just wanted to ask that will this idea work ??

I Want to measure the speed in m/s so I have to know how much distance did the rocket travel in 1 second.
I am using a pressure sensor for measuring the Altitude.

If I give a 1 second delay between the readings of the pressure sensor , I will get to know the distance travelled in 1 second.

I will store all of that information to a SD card.

I have some questions -

1) is bmp180 good enough for this task ?

2) I am going to use the delay function . Is there any better option ?

3) Will this idea give me speeds with accuracy ?


Do anyone of you have a better idea on measuring speed accurately ?
I'm just a beginner so your advice will be helpful .
Thanks


 
Title: Re: Speed measurement for rockets
Post by: pylon on Jul 31, 2020, 05:37 pm
Quote
1) is bmp180 good enough for this task ?
We don't know as you didn't provide any information about your rockets. How high are they expected to fly? What speed do you expect?

Quote
2) I am going to use the delay function . Is there any better option ?
delay() is just a busy wait function, so the processor is waiting while doing nothing. Usually such systems do other tasks while waiting (for example: writing to the SD card), so using millis() might be a better option. But that depends on what you actually program.

Quote
3) Will this idea give me speeds with accuracy ?
The speed will have an accuracy (as every measurement has some accuracy) but we don't know if that fits your expectations. I wouldn't expect much accuracy but that depends on how you build your rocket and where you place your sensor (air resistance also shows some pressure).
Title: Re: Speed measurement for rockets
Post by: gt_aerospace on Jul 31, 2020, 07:36 pm
I am expecting that the rocket should go over 1 km and it should achieve speeds over 350 m/s .
 
It should atleast have an accuracy of 10 m/s .
Title: Re: Speed measurement for rockets
Post by: TheMemberFormerlyKnownAsAWOL on Jul 31, 2020, 07:46 pm
How many seconds do you expect to be flying?
Title: Re: Speed measurement for rockets
Post by: MrMark on Jul 31, 2020, 08:19 pm
I am expecting that the rocket should go over 1 km and it should achieve speeds over 350 m/s .
 
It should atleast have an accuracy of 10 m/s .
According BME180 datasheet (https://cdn-shop.adafruit.com/datasheets/BST-BMP180-DS000-09.pdf) the specified relative accuracy pressure is +/- 1 meter equivalent near sea level. If that were the limiting factor the accuracy would be about 1 m/s.  In such a highly dynamic environment it's probably worse, maybe considerably so, but we can say on that data point that it's not obvious that you can't get better than 10 m/s accuracy.
Title: Re: Speed measurement for rockets
Post by: aarg on Jul 31, 2020, 08:26 pm
You will have trouble obtaining a stable pressure reading with supersonic airflow around the vehicle.
Title: Re: Speed measurement for rockets
Post by: johnerrington on Aug 01, 2020, 09:16 am
I guess we can rule out measurement of air pressure, because of turbulence; ecept that when your rocket reaches apogee it will tell you how high it is, and hence the average speed.

So you need some sort of ASI (air speed indicator).

https://www.skybrary.aero/index.php/Air_Speed_Indicator

the pitot tube method is common to most aircraft - but near ultrasonic speeds may cause errors (that CAN be corrected out).  maybe you could research what is used on supersonic planes?

I also considered a hot wire anemometer, but not very robust and take relatively a lot of power - unless you could use a thermistor?

http://www.resistorguide.com/ptc-thermistor/
Title: Re: Speed measurement for rockets
Post by: TomGeorge on Aug 01, 2020, 09:33 am
Hi,
Welcome to the forum.

Please read the post at the start of any forum , entitled "How to use this Forum".
OR
http://forum.arduino.cc/index.php/topic,148850.0.html (http://forum.arduino.cc/index.php/topic,148850.0.html).

This might help about using barometric altitude.

https://www.youtube.com/watch?v=PD-4fi0MygI (http://"https://www.youtube.com/watch?v=PD-4fi0MygI")

Have you googled       model rocketry measuring speed

Tom.... :)
Title: Re: Speed measurement for rockets
Post by: johnerrington on Aug 01, 2020, 10:26 am


Hi TomGeorge your link is incorrect so here it is.

https://www.youtube.com/watch?v=PD-4fi0MygI (https://www.youtube.com/watch?v=PD-4fi0MygI)

Having watched it I now know all about earwax, and that as the guy admits you cant use a barometric altimeter to measure speed - just the apogee height.
Title: Re: Speed measurement for rockets
Post by: markd833 on Aug 01, 2020, 11:44 am
Does this (https://www.sparkfun.com/news/499) link help you out?

Sorry, I forgot to say that it's a Sparkfun discussion about model rocket instrumentation!
Title: Re: Speed measurement for rockets
Post by: gt_aerospace on Aug 01, 2020, 12:50 pm
How many seconds do you expect to be flying?
The rocket can about 7-8 seconds to hit apogee
Title: Re: Speed measurement for rockets
Post by: wildbill on Aug 01, 2020, 12:57 pm
This can't be a new problem. How do other rocket builders do it? GPS?
Title: Re: Speed measurement for rockets
Post by: gt_aerospace on Aug 01, 2020, 01:33 pm
This can't be a new problem. How do other rocket builders do it? GPS?
Good Idea ! I think I should contact some Rocketeers .
Title: Re: Speed measurement for rockets
Post by: MarkT on Aug 01, 2020, 01:56 pm
GPS receivers are limited in top speed deliberately (except under licence), so that they cannot be used to
make guided missiles, so this might not work here.

Stagnation pressure at the top dead centre of the rocket cone would give a guide to supersonic
velocity (like a pitot tube), assuming shock waves are kept out of the sensor.

However you might want to back this up with integrated accelerometer data - flights are short so drift may be
managable.  You can calibrate the accelerometer drift at prelaunch and apogee points if you have a way to
determine altitude at both points.

[BTW ultrasonic means above 20kHz, not faster than sound!]
Title: Re: Speed measurement for rockets
Post by: markd833 on Aug 01, 2020, 01:58 pm
Commercial GPS receivers like the ones we can buy have a speed limit built into them so that they can't be used for undesirable purposes - if you know what I mean....


A quick check on Wikipedia  (https://en.wikipedia.org/wiki/Coordinating_Committee_for_Multilateral_Export_Controls#:~:text=In%20GPS%20technology%2C%20the%20term,18%2C000%20m%20(59%2C000%20ft).)says:

In GPS technology, the term "COCOM Limits" also refers to a limit placed on GPS tracking devices that disables tracking when the device calculates that it is moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).[3] This was intended to prevent the use of GPS in intercontinental ballistic missile-like applications.

With a speed exceeding 350m/s, that's 1260km/h (782mph) - you may well be ok.

However, from my days working with the military, it was generally considered that the GPS height channel was not very accurate. However, things may have improved since then ....
Title: Re: Speed measurement for rockets
Post by: MarkT on Aug 01, 2020, 02:11 pm
Altitude accuracy from GPS depends on having satellites acquired both high in the sky and close
to the horizon - being 1000m up in the air helps with the latter, but I doubt a GPS would acquire new
satellites in the timescale involved!

Not sure if many GPS's expose vertical error estimates like they do for CEP?  You could time the launch
when the vertical error estimate is lowest.

If you manage to get to hypersonic speeds (ie Mach 5 +), then you have to worry about shock-plasmas
killing the GPS signal :)

This is where I learnt my rocket science BTW - recommended! https://www.edx.org/course/hypersonics-from-shock-waves-to-scramjets-2 (https://www.edx.org/course/hypersonics-from-shock-waves-to-scramjets-2)
Title: Re: Speed measurement for rockets
Post by: srnet on Aug 01, 2020, 03:46 pm
Ublox GPSs can be configured for a range of 'platform' settings, and can for instance be set to work at up to 50,000m altitude.

However all the platform setting significantly restrict vertical velocity, the max setting varies from 20m/s for 'pedestrian' mode up to 100m/s maximum for 'airborne < 4g' mode.
Title: Re: Speed measurement for rockets
Post by: MrMark on Aug 01, 2020, 10:37 pm
Altitude accuracy from GPS depends on having satellites acquired both high in the sky and close
to the horizon - being 1000m up in the air helps with the latter, but I doubt a GPS would acquire new
satellites in the timescale involved!
The reason GPS altitude accuracy is worse than horizontal accuracy is because the accuracy is a function of the relative geometry of the visible GPS satellites (referred to as Geometric Dilution of Precision or "GDOP").  The optimum geometry in a particular dimension is achieved when there is a visible satellite in either direction along that axis.  For altitude that would mean a satellite directly above and directly below.  There will never be a visible satellite directly below in any normal scenario because there is a planet in the way.
Title: Re: Speed measurement for rockets
Post by: SteveMann on Aug 01, 2020, 10:49 pm
The US military, who build and own the GPS, has placed a speed and height limit so no receivers can be used in hostile guided weapons - good idea of course. Consumer GPS stop working when moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).

GPS altitude measurements are so inaccurate that aircraft flying a GPS approach use barometric altitude for decision heights.

Why not use an accelerometer? Take a reading once a second and then you can calculate speed, altitude and acceleration.
Title: Re: Speed measurement for rockets
Post by: TomGeorge on Aug 02, 2020, 12:20 pm
The US military, who build and own the GPS, has placed a speed and height limit so no receivers can be used in hostile guided weapons - good idea of course. Consumer GPS stop working when moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).

GPS altitude measurements are so inaccurate that aircraft flying a GPS approach use barometric altitude for decision heights.

Why not use an accelerometer? Take a reading once a second and then you can calculate speed, altitude and acceleration.

[sarcasm]
A subsonic airborne drone, flying under the radar loaded with C4...



[/sarcasm]
Or is it easier said than done?

Tom.... :)
Title: Re: Speed measurement for rockets
Post by: GoForSmoke on Aug 02, 2020, 01:45 pm
Fly the rocket at night with strobe lights and take long-exposure pictures from 3+ cameras with grids over the lenses.

How well you know where the cameras are and view angles is how well you can reconstruct the flight from the data.

Title: Re: Speed measurement for rockets
Post by: SteveMann on Aug 02, 2020, 02:32 pm
[sarcasm]
A subsonic airborne drone, flying under the radar loaded with C4...
[/sarcasm]
Or is it easier said than done?

Tom.... :)
That only works in the movies.  But that's what the USAF was thinking when they designed GPS