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Topic: Speed measurement for rockets (Read 407 times) previous topic - next topic

gt_aerospace

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


 

pylon

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).

gt_aerospace

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 .

TheMemberFormerlyKnownAsAWOL

How many seconds do you expect to be flying?
Please don't PM technical questions - post them on the forum, then everyone benefits/suffers equally

MrMark

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 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.

aarg

You will have trouble obtaining a stable pressure reading with supersonic airflow around the vehicle.
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

johnerrington

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/
I'm trying to help. If I find your question interesting I'll give you karma. If you find my input useful please give me karma (I need it)

TomGeorge

#7
Aug 01, 2020, 09:33 am Last Edit: Aug 01, 2020, 11:41 am by TomGeorge Reason: URL fixed, thanks. @johnerrington
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.

This might help about using barometric altitude.

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

Have you googled       model rocketry measuring speed

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

johnerrington



Hi TomGeorge your link is incorrect so here it is.

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.
I'm trying to help. If I find your question interesting I'll give you karma. If you find my input useful please give me karma (I need it)

markd833

#9
Aug 01, 2020, 11:44 am Last Edit: Aug 01, 2020, 01:24 pm by markd833
Does this link help you out?

Sorry, I forgot to say that it's a Sparkfun discussion about model rocket instrumentation!

gt_aerospace

How many seconds do you expect to be flying?
The rocket can about 7-8 seconds to hit apogee

wildbill

This can't be a new problem. How do other rocket builders do it? GPS?

gt_aerospace

This can't be a new problem. How do other rocket builders do it? GPS?
Good Idea ! I think I should contact some Rocketeers .

MarkT

#13
Aug 01, 2020, 01:56 pm Last Edit: Aug 01, 2020, 01:57 pm by MarkT
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!]
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

markd833

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 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 ....

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