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'[PIC]: Which Oscillator in a Rocket?'
2001\04\21@034543 by Tony Goetz

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Hey everyone,

I posted a few months back about building an altimeter based on pressure and
accelerometer data acquired by a PIC microcontroller and stored on an EEPROM
chip. Some good points were brought to my attention, including the use of a
resistor/capacitor oscillator as opposed to a crystal. Looking at my design
now, I'm thinking I'd like to use a crystal because of its accuracy. Back
when I first posted someone said the crystal may not hold to its frequency in
flight because of the G's it will be subjected to. However, taking a look at
commercial altimeters (like the R-DAS. pictures at
http://www.fortunepaint.com/aerocon_interim/rdas.htm ) I notice that they
often use crystal oscillators. Does this mean it's generally okay to use them
without much frequency drift?

Thanks,
-Tony

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2001\04\22@140441 by Alan B. Pearce

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>However, taking a look at
>commercial altimeters (like the R-DAS. pictures at
>http://www.fortunepaint.com/aerocon_interim/rdas.htm ) I notice that they
>often use crystal oscillators. Does this mean it's generally okay to use
them
>without much frequency drift?

They may be using ruggedised oscillators. I do not know what is done to make
them more rugged, but you may get away with ordinary ones by being careful
about which axis you mount them in.

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2001\04\22@141053 by David VanHorn

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At 08:57 PM 4/21/01 +0100, Alan B. Pearce wrote:
> >However, taking a look at
> >commercial altimeters (like the R-DAS. pictures at
> >http://www.fortunepaint.com/aerocon_interim/rdas.htm ) I notice that they
> >often use crystal oscillators. Does this mean it's generally okay to use
>them
> >without much frequency drift?
>
>They may be using ruggedised oscillators. I do not know what is done to make
>them more rugged, but you may get away with ordinary ones by being careful
>about which axis you mount them in.

You'd definitely not want the thrust axis through the sides of the case.
RC osc or resonators would be safer.
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2001\04\22@142142 by Sean H. Breheny

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When I built my rocket altimeter (http://www.rocket-roar.com then follow
the link to the Rocket Altimeter Project), I first asked on the PICLIST
about crystal oscillators being affected by acceleration. The rocket I was
using experienced 15 g of acceleration. I was advised that it would
probably work fine but that I should orient the long axis of the crystal
case parallel to the acceleration direction. Someone else (sorry, I don't
recall exactly who right now) told me that he has made boards with standard
crystals and placed them on 3-axis shakers which apply large vibrational
accelerations (>15 g if I remember correctly) and they maintain correct
operation all through the test.

In the end, my altimeter's clock seems to have worked fine throughout the
ride :-)

Sean

At 08:57 PM 4/21/01 +0100, you wrote:
{Quote hidden}

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2001\04\22@182242 by Matt Bennett

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David VanHorn wrote:
>
> At 08:57 PM 4/21/01 +0100, Alan B. Pearce wrote:
> > >However, taking a look at
> > >commercial altimeters (like the R-DAS. pictures at
> > >http://www.fortunepaint.com/aerocon_interim/rdas.htm ) I notice that they
> > >often use crystal oscillators. Does this mean it's generally okay to use
> >them
> > >without much frequency drift?
> >
> >They may be using ruggedised oscillators. I do not know what is done to make
> >them more rugged, but you may get away with ordinary ones by being careful
> >about which axis you mount them in.
>
> You'd definitely not want the thrust axis through the sides of the case.
> RC osc or resonators would be safer.

In a rocket the acceleration is relatively low.  Extreme acceleration
for a rocket is ~50G which is what you get in the small rockets of the
type that are shot from pods on attack helicopters.  50G is no problem
for a crystal oscillator, though you may have some frequency deviation
on initial acceleration, but it will quickly die down.  It is the delta
that really hurts.

I made a PIC controlled device that was launched from a howitzer, 15,000
G is the maximum acceleration there.  For a direct fire gun like a
tank's, you will get accelerations of about 50,000 G.  I did a number of
tests at the 15,000 G level, and resonators have only slightly better
survival rates than crystals, - both were bad.  at 50,000 G you're
pretty much out of luck if you want a device that relies on mechanical
oscillations (crystal or resonator).

If you need very hardened crystals, Statek will make them for you- I had
some made- the price was reasonable- about $20/ea for 15,000G parts (in
1998).

Matt Bennett

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2001\04\22@212041 by Alexandre Domingos F. Souza

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>I made a PIC controlled device that was launched from a howitzer, 15,000
>G is the maximum acceleration there.  For a direct fire gun like a
>tank's, you will get accelerations of about 50,000 G.  I did a number of
>tests at the 15,000 G level, and resonators have only slightly better
               ^^^^^^^^^^^^^^^^

       Man, how did you get this?????????

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2001\04\22@212457 by Sean H. Breheny

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He probably shot it from a howitzer :-) Doesn't everyone have one of those
in their backyard ? (Helps to show it off when you have disputes with your
neighbors).

Sean

At 10:21 PM 4/22/01 -0300, you wrote:
{Quote hidden}

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2001\04\22@214418 by Alexandre Domingos F. Souza

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>He probably shot it from a howitzer :-) Doesn't everyone have one of those
>in their backyard ? (Helps to show it off when you have disputes with your
>neighbors).

       Oh bad english, I think I gotta find what the hell is a howitzer :o)

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2001\04\22@222613 by Sean H. Breheny

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A howitzer is a type of cannon, designed to fire at high angles, similar to
a mortar except that it is a breech-loader rather than muzzle loading. :-)

Sean

At 10:46 PM 4/22/01 -0300, you wrote:
> >He probably shot it from a howitzer :-) Doesn't everyone have one of those
> >in their backyard ? (Helps to show it off when you have disputes with your
> >neighbors).
>
>         Oh bad english, I think I gotta find what the hell is a howitzer :o)
>
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2001\04\22@222752 by Brandon Fosdick

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"Alexandre Domingos F. Souza" wrote:
>
> >He probably shot it from a howitzer :-) Doesn't everyone have one of those
> >in their backyard ? (Helps to show it off when you have disputes with your
> >neighbors).
>
>         Oh bad english, I think I gotta find what the hell is a howitzer :o)

Lots of different variations over the years but in general a Howitzer is
a really big gun.

A quick search of http://www.defenselink.mil turns up lots of pictures.

An M109 mobile Howitzer in Croatia.

http://www.defenselink.mil/photos/Jan1996/960102-A-6435A-002.jpg

An M198 firing a round.

http://www.defenselink.mil/photos/Dec2000/001120-M-6514O-001.html

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2001\04\22@223002 by Dale Botkin

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On Sun, 22 Apr 2001, Alexandre Domingos F. Souza wrote:

> >He probably shot it from a howitzer :-) Doesn't everyone have one of those
> >in their backyard ? (Helps to show it off when you have disputes with your
> >neighbors).
>
>         Oh bad english, I think I gotta find what the hell is a howitzer :o)

An indirect fire artillery piece, usually fairly large bore, high angle
of fire.  The US Army uses 155mm and 8" guns that I know of.

Dale
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discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
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2001\04\23@034407 by Roman Black

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Alan B. Pearce wrote:
>
> >However, taking a look at
> >commercial altimeters (like the R-DAS. pictures at
> >http://www.fortunepaint.com/aerocon_interim/rdas.htm ) I notice that they
> >often use crystal oscillators. Does this mean it's generally okay to use
> them
> >without much frequency drift?
>
> They may be using ruggedised oscillators. I do not know what is done to make
> them more rugged, but you may get away with ordinary ones by being careful
> about which axis you mount them in.


My preference is to use solid ceramic resonators, like
the Murata CSA series. 16MHz is commonly available and
they handle g force shocks much better than any can crystal
in my experience. They are often cheaper too, although
less accurate timing. But I have found that their accuracy
is a lot better than listed in the official spec though,
maybe due to improved manufacturing techniques.
-Roman

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2001\04\23@194640 by Peter L. Peres

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>> tests at the 15,000 G level, and resonators have only slightly better
>                ^^^^^^^^^^^^^^^^
>        Man, how did you get this?????????

It is not very hard to get. Throwing a concrete slab or steel plate onto
another from a certain height is a good start.

Peter

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2001\04\23@225252 by Matt Bennett

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"Peter L. Peres" wrote:
>
> >> tests at the 15,000 G level, and resonators have only slightly better
> >                ^^^^^^^^^^^^^^^^
> >        Man, how did you get this?????????
>
> It is not very hard to get. Throwing a concrete slab or steel plate onto
> another from a certain height is a good start.
>

It's actually harder to get than you would expect.  The most common way
to do it is with a device called an "air gun"- which is a long tube
(100-300 feet) with a free floating piston in it held in place at one
end- the other end is covered with thin plastic, a lot like cellophane.
Draw a vacum in the tube and let the piston go.  Air pressure will push
the piston down the tube, accelerating it as it goes.  The real secret
is at the far end- the piston (called the "bird") flies into the
decellerating medium- we used a specially cut piece of honeycomb
aluminum.  The pattern cut into the aluminum will give you the
acceleration profile (actually decelleration, but it's the same G's).

I don't work there anymore, but I think we could get about 15,000 G from
a 100' air gun, which is equivalent to a 155mm howitzer at max zone
(biggest charge).  We also had a 300' air gun for tank simulations,
which gave us the 50,000 G.  You could use a howitzer to generate these
acceration levels, but if you ever want to do some analysis on your test
parts, getting them back is pretty hard, and you have two directions of
acceleration to worry about.  With an air-gun, you only have strong
acceration in one direction.

A standard, straight from Digi-Key 16C71 in a SOIC package easily takes
15,000 G as long as the force of the acceleration is down onto the
board.  You can do that level of acceleration if you hold a board
aligned with the acceration, as long as you use thin surface mount parts
(think lever arm).  Passive (SMT) components survived without a problem,
as well.  If you want to work with high accerations, you must go with
surface mount because they are lower mass, and the holes of through-hole
components seriously weaken your PCB (which was about 1/8" in this
case).  This device was part of an GPS artillery fuze, the GPS signal
was sent back to us to track the bullet in flight.  Pretty neat, and it
actually worked.

Matt

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2001\04\23@231712 by Dale Botkin

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On Mon, 23 Apr 2001, Matt Bennett wrote:

> This device was part of an GPS artillery fuze, the GPS signal
> was sent back to us to track the bullet in flight.  Pretty neat, and it
> actually worked.

And I thought the Copperhead was pretty cool.

Dale
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2001\04\24@155604 by Brandon Fosdick

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Matt Bennett wrote:
> case).  This device was part of an GPS artillery fuze, the GPS signal
> was sent back to us to track the bullet in flight.  Pretty neat, and it
> actually worked.

I once looked into using GPS to guide a sub-orbital rocket and now I'm
using it to guide R/C airplanes. One of the difficulties I've found is
the 1Hz update rate is just too slow for high speed flight. How did you
deal with that on something as fast as an artillery shell?

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2001\04\25@205600 by Alexandre Domingos F. Souza

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>I once looked into using GPS to guide a sub-orbital rocket and now I'm
>using it to guide R/C airplanes. One of the difficulties I've found is
>the 1Hz update rate is just too slow for high speed flight. How did you
>deal with that on something as fast as an artillery shell?

       Millitary GPS has faster update rates :o)

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2001\04\30@181708 by Tony Goetz

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Sorry for that delay in response; haven't had much time to e-mail! I
certainly wouldn't let the responses go without answer.

Thanks all who gave input on this. There seems to be a little bit of argument
as to which would be great, adequate, etc, but a few voices of experience
have said that crystals work (for them) without any trouble. So, since they
work fine and generally have good accuracy, that's what I'll go with. Of
course, the real test is actually sending the thing up in a rocket! We'll see
when that day comes....

Again, thanks for your input and sorry it took so long to get back.

-Tony

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2001\04\30@185210 by David VanHorn

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At 06:14 PM 4/30/01 -0400, Tony Goetz wrote:
>Sorry for that delay in response; haven't had much time to e-mail! I
>certainly wouldn't let the responses go without answer.
>
>Thanks all who gave input on this. There seems to be a little bit of argument
>as to which would be great, adequate, etc, but a few voices of experience
>have said that crystals work (for them) without any trouble. So, since they
>work fine and generally have good accuracy, that's what I'll go with. Of
>course, the real test is actually sending the thing up in a rocket! We'll see
>when that day comes....


Another thing to watch is battery connections. Springs can fail at launch,
or in the opposite direction on burnout.

Good luck.

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2001\04\30@215046 by Tony Goetz

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Oh absolutely. It all has to be secured in there- I just hope everything
holds. It looks like the battery (23A) will fit not just well, but tightly in
an 'N' holder from Radio Shack. I'll have to make sure the battery holder
stays in place too!


-Tony


<< >Thanks all who gave input on this. There seems to be a little bit of
argument
>as to which would be great, adequate, etc, but a few voices of experience
>have said that crystals work (for them) without any trouble. So, since they
>work fine and generally have good accuracy, that's what I'll go with. Of
>course, the real test is actually sending the thing up in a rocket! We'll
see
>when that day comes....


Another thing to watch is battery connections. Springs can fail at launch,
or in the opposite direction on burnout.

Good luck. >>

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'[PIC]: Which Oscillator in a Rocket?'
2001\05\01@015618 by David VanHorn
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At 09:49 PM 4/30/01 -0400, you wrote:
>Oh absolutely. It all has to be secured in there- I just hope everything
>holds. It looks like the battery (23A) will fit not just well, but tightly in
>an 'N' holder from Radio Shack. I'll have to make sure the battery holder
>stays in place too!

Mount this across the long axis of the body, or the spring may compress and
disconnect you momentarily. Crossways, all you have to do is keep it in the
holder.

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2001\05\01@020236 by Tony Goetz

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That looks to be the most space efficient way anyways, although that's a good
thing to point out!


<< >Oh absolutely. It all has to be secured in there- I just hope everything
>holds. It looks like the battery (23A) will fit not just well, but tightly
in
>an 'N' holder from Radio Shack. I'll have to make sure the battery holder
>stays in place too!

Mount this across the long axis of the body, or the spring may compress and
disconnect you momentarily. Crossways, all you have to do is keep it in the
holder. >>

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2001\05\21@120118 by Alan B. Pearce

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>Thanks all who gave input on this. There seems to be a little bit of
argument
>as to which would be great, adequate, etc, but a few voices of experience
>have said that crystals work (for them) without any trouble. So, since they
>work fine and generally have good accuracy, that's what I'll go with. Of
>course, the real test is actually sending the thing up in a rocket! We'll
see
>when that day comes....

Just make sure you vibration test it first. It should not be too hard to
come up with a vibration table that you can calculate the G force it
provides, and have it at a similar value to what the rocket is likely to
provide - even if it provides only sine vibration it will at least verify
operation is likely to be OK.

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