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spudgun range, are we falling short?

A place to ask general spud cannon related questions.
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Re: spudgun range, are we falling short?

Unread postAuthor: jackssmirkingrevenge » Tue Nov 12, 2013 6:49 am

POLAND_SPUD wrote:Wouldn't the readings be affected by air flowing around the projectile ?


Apart from that, you would need atmospheric data to compare to. Accelerometers to provide INS type data makes more sense - but $$$...
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Re: spudgun range, are we falling short?

Unread postAuthor: POLAND_SPUD » Tue Nov 12, 2013 7:51 am

ohh you don't understand
https://www.sparkfun.com/datasheets/Com ... 000-05.pdf
and
https://www.sparkfun.com/products/11282
It's calibrated and can accurately measure 0.25 meter difference in height.

Accelerometers are expensive ??
https://www.sparkfun.com/search/results ... t=products
Depends what you call expensive.
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Re: spudgun range, are we falling short?

Unread postAuthor: jackssmirkingrevenge » Tue Nov 12, 2013 8:09 am

What about the gadgetry needed to transmit the data?
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Re: spudgun range, are we falling short?

Unread postAuthor: Ragnarok » Tue Nov 12, 2013 8:16 am

...I'm a little at a loss regarding why a barometer would be useful. They can determine altitude pretty accurately, but you either need to record the data on board (and then find the projectile, essentially invalidating the data) or transmit it - at which point you'd be trying to extrapolate the range of the projectile from altitude vs time.

A similar thing applies to an accelerometer - you either find it, or have to integrate the data (and remember that accelerometers cannot measure gravitational acceleration while they're in free-fall) to get range.

In any case, I wouldn't be sure a lot of sophisticated electronics will want to withstand being fired - the kind of electrickery they use in artillery or cannon rounds is not the kind of thing that's going to be easy to make or purchase.

We're talking about several thousand G and exposure to high pressures as well.
Is something that can measure the low accelerations in flight (mostly sub-G, if your projectile is aerodynamic enough) accurately going to have survived taking a smack of four or even five figure G forces first? Is your 20 Pa accurate barometer going to survive exposure to muzzle blast of Megapascals?

If you're transmitting something, just a generic signal to triangulate would be a lot more cost efficient (and a basic transmitter like that could be kept pretty dang simple electronically, so could probably be made hardy enough to survive the launch). A triangulating receiver would be a bit more complex, but doesn't need to survive a launch - and would be usable for multiple launches.
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Re: spudgun range, are we falling short?

Unread postAuthor: POLAND_SPUD » Tue Nov 12, 2013 8:24 am

you can use rfm69hw

It's fairly cheap - more or less 4$ per one module. The added advantage is that it has RSSI os that can be used for determining location.


Though if you're using a barometer the idea is that you can pretty much fire it upwards. Or better yet make it trigger a parachute or some other retardation device once it senses that the projectile is falling

Can't tell really what to expect from a spudgun but modern electronics is very resistant to G forces. Typically if you drop a phone it can experience up to several hundreds of Gs. As far as the barometer goes the idea was to shield it from launch pressures. Not sure how to do it yet but it can be done. I wonder what G forces can we expect when the projectiles flies downrange
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Re: spudgun range, are we falling short?

Unread postAuthor: jackssmirkingrevenge » Tue Nov 12, 2013 10:31 am

Them GPS trackers are getting pretty small and cheap: http://www.ebay.com/itm/Mini-GPS-GPRS-G ... 58a744f15a

This RF tracker claims a 500 metre range, i had bought something similar but this one has a more sophisticated transmitter: http://www.ebay.com/itm/Guardian-Angel- ... 58a4995af5
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Re: spudgun range, are we falling short?

Unread postAuthor: Ragnarok » Tue Nov 12, 2013 11:47 am

POLAND_SPUD wrote:Can't tell really what to expect from a spudgun but modern electronics is very resistant to G forces. Typically if you drop a phone it can experience up to several hundreds of Gs.

A hundreds of G for fractions of a millisecond is a very different prospect to thousands of G for several milliseconds.

I wonder what G forces can we expect when the projectiles flies downrange

Well, to try and recall some old research of mine...

Back when I was actually designing aerodynamic darts, the approximated terminal velocity was around about Mach 0.93-0.94; now, I wouldn't normally state velocities in Mach, but the main limiting factor was the large increase in drag co-efficient at transonic velocities (mostly because the tail shape became inefficient at those speeds), so it's actually more accurately stated as Mach # than m/s.

Taking Mach as 343 m/s, this would give it a terminal velocity of about 320 m/s. In comparison, the muzzle velocity was about 285 metres per second.
Now, terminal velocity is the velocity at which drag equals an object's weight - on earth, this is 1G of drag. So a muzzle velocity of less than the terminal velocity (easily achievable at subsonic velocities) means that all drag is going to be sub-G.

In that particular case, peak drag forces were around about ~0.1G. However, that was strongly dependent on the temperature that day. As the projectile was skirting transonic velocities, a drop in the speed of sound would make a dramatic difference to the aerodynamics. 20 degrees C difference on the temperature could easily double that drag.

In fact, a strong temptation was to scrub about 10 m/s off the muzzle velocity, either through a heavier projectile or simply less pressure, as this would make the aerodynamics considerably less sensitive to temperature or wind (primarily head winds that could have put the air flow transonic).


Now, getting drag that low is unlikely at supersonic velocities - but unless fired seriously fast, these projectiles will probably become subsonic through a mix of drag and gravity (remembering we're probably firing at most of a 45 degree angle) and they should experience sub-G for most of their flight.
In any case, a really aerodynamic projectile design could still get the whole flight within the range of typical accelerometers - less than 10 G is likely, maybe as much as 20 G if it's going like the clappers.
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Re: spudgun range, are we falling short?

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Re: spudgun range, are we falling short?

Unread postAuthor: POLAND_SPUD » Tue Nov 12, 2013 12:01 pm

A hundreds of G for fractions of a millisecond is a very different prospect to thousands of G for several milliseconds.
Well there is just one way to find out.

Jesus JSR you can make your own transmiter if that's all you need. Then just use a received with a loop antenna
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Re: spudgun range, are we falling short?

Unread postAuthor: jackssmirkingrevenge » Tue Nov 12, 2013 12:27 pm

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Re: spudgun range, are we falling short?

Unread postAuthor: Ragnarok » Tue Nov 12, 2013 12:48 pm

Poking around several accelerometer data sheets, it seems it's not uncommon for them to have shock protection into the low thousands of G, but that could still be pretty on the conservative side for a powerful spudgun.

Actually, all this talk of accelerometers brings me back to that old idea about chronographing projectiles based on the recoil of the launcher - basically, a recoil pendulum where the launcher is the pendulum. One fast and accurate enough* could give a decent reading for that, providing a chronograph that can't be shot by mistake or get confused by the muzzle blast (although, of course, you would need to account for the momentum of that, but that should actually be moderately predictable).
*Looking around, it seems there are those with sufficient peak-G, resolution and speed to suffice, particularly in single axis units.

That's getting off topic though.
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Re: spudgun range, are we falling short?

Unread postAuthor: POLAND_SPUD » Tue Nov 12, 2013 2:48 pm

Actually, all this talk of accelerometers brings me back to that old idea about chronographing projectiles based on the recoil of the launcher - basically, a recoil pendulum where the launcher is the pendulum. One fast and accurate enough* could give a decent reading for that, providing a chronograph that can't be shot by mistake or get confused by the muzzle blast (although, of course, you would need to account for the momentum of that, but that should actually be moderately predictable).
Interesting. What's the formula for it? Could make an app that records acceleration values from the phone and does calculations.
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Re: spudgun range, are we falling short?

Unread postAuthor: Ragnarok » Tue Nov 12, 2013 4:18 pm

It's just based on conservation of momentum. If you know that launcher of mass x is recoiling at velocity y (which you can work out by integrating the acceleration), then whatever went out forwards must have had a momentum of -xy.

I don't however think that phone apps would work great. Aside from needing to be securely bolted to the launcher, it'd take a fairly high spec accelerometer - the minimum I'd want for HEAL is at least 30g at a 1 kHz sample rate.
More ideally, I'd want 50g (at least 0.1g resolution, preferably 0.01g) and a 10 kHz sample rate.

However, with such a sensor it should be possible to measure recoil energy to 99% accuracy or better - and from there, an accurate measurement of both gas momentum (which could be found from a "blank" shot at the right pressure) and projectile mass would allow you to accurately calculate muzzle velocity.
Quite literally muzzle velocity, as the measurements wouldn't be affected in the slightest by drag or duped by the muzzle blast.

The caveat is that it can only work if the launcher can be treated as a free body - if the projectile mass were too great a proportion of the launcher mass and the launcher recoiled enough before the projectile leftthe barrel that any (significant) force opposing the free recoil of the launcher started acting, this would cause it to read low.
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Re: spudgun range, are we falling short?

Unread postAuthor: Technician1002 » Wed Nov 13, 2013 7:18 am

Some simple physics.. 1 G = acceleration force of gravity on earth.

From wikipedia http://en.wikipedia.org/wiki/Gravity_of_Earth
It has an approximate value of 9.81 m/s2, which means that, ignoring the effects of air resistance, the speed of an object falling freely near the Earth's surface will increase by about 9.81 metres (32.2 ft) per second every second.


You can use GGDT to find projectle acceleration ranges for your design so you have ballpark values your electronics must withstand.
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Re: spudgun range, are we falling short?

Unread postAuthor: POLAND_SPUD » Wed Nov 13, 2013 11:22 am

Technician1002 wrote:Some simple physics.. 1 G = acceleration force of gravity on earth.

From wikipedia http://en.wikipedia.org/wiki/Gravity_of_Earth
It has an approximate value of 9.81 m/s2, which means that, ignoring the effects of air resistance, the speed of an object falling freely near the Earth's surface will increase by about 9.81 metres (32.2 ft) per second every second.


You can use GGDT to find projectle acceleration ranges for your design so you have ballpark values your electronics must withstand.

The value of earth gravity could be used for determining apogee. So now we have two sensors that could do the same.

Ohh and speaking of electronics and G forces - remember that 40mm granade round that deployed a parachute and had a camera + transmiter for it on board ??
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Re: spudgun range, are we falling short?

Unread postAuthor: Ragnarok » Wed Nov 13, 2013 2:01 pm

POLAND_SPUD wrote:The value of earth gravity could be used for determining apogee.

As I used to keep reminding people, things are (functionally) weightless in freefall!
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