piclist 1999\10\22\002530a >
Thread: Survey... What is an acceptable failure rate?
www.piclist.com/techref/index.htm?key=survey+what+acceptable
BY : Russell McMahon email (remove spam text)

>On the same sort of topic, has anyone seen a book or web page devoted to
>making a circuit design bulletproof? I would love to hear a few tips from
>anyone willing to share!

As it seems to be pertinent I'll crosspost something I just sent to the
ARocket list - the subject is the survivability of electronics etc under
impact. Has some relevance to current topic.

Russell McMahon
_____________________________

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=================================================

Electronics, properly designed, can survive decelerations of upwards of
1000g.
Dave Hall notes that the military fire ordnance containing GPS units inside
at closer to 10,000g.
Nothing sensible will survive a full accelerated descent but if you assume
maximum terminal velocities in the 100 - 150 mph range (a less than fully
streamlined body falling vertically) then the degree of crumple afforded by
your late lamented rocket plus some purposeful internal decelerating
material should get accelerations down into the above range.

Getting even overall deceleration of the subcomponents  in your equipment is
vital. What is required is either encapsulating or encasing in a homogeneous
"stiff" medium which is able to transfer the force applied to the outside OR
having individual parts able to handle the deceleration by themselves
(usually harder).

What does it take to limit deceleration to 1000g at 100mph>
Lets see, swapping to pretending units.
100mph ~= 45m/s say 50 m/s
9 ~= 10 m/s/s
Deceleration Distance = Vel^2/20/g's
= 2500/20/1000 = 0.125m

ie if you can decelerate LINEARLY over 0.125m = 5 inches from 100 mph
deceleration will be around 1000g.

Stopping time = Vel/Accn = 5mS at 1000g!

G force goes up as the square of the impact speed
G force goes down in inverse proportion to stopping distance.

To put some perspective on what 1000g means -

A typical resistor (SFR16x) weighs about 0.1 gram (1/300 oz) and small
capacitors etc well under 1g (1/30 oz).

A 10g (1/3 oz) component will weigh 10Kg during this (brief :-)) period.
It's not hard to imagine designing so that eg resistors will withstand this
sort of force - imagine trying to pull through hole resistors off a circuit
board by hand given a good grip
.
A 100g (3 oz) module will weigh 100kG during this period - imagine a
heavyish person standing on your equipment with just the ball of their
foot - a not inconceivable design objective.

Translating this into people terms is not so attractive - a 100kG person
will weigh 100 tons - attempting to survive the imposition, however
temporary, of an evenly applied 100 ton weight is not an attractive thought.

The main tricks, after building it properly, are:

Achieving linear deceleration.
Getting the direction right - if the design is for a lawn dart but it
turns sideways just before impact and decelerates at 90 degrees to the
design direction, failure is "more likely" :-)

I build talking communicators which  are often used by children. It is not
unknown for these to be thrown off tables etc - often without 0.125m being
available for deceleration :-) Actual stopping distance is ill defined but
on a concrete floor is only the pcb mounting flexure distance. "Sensible"
construction ensures that these (usually) survive such treatment.

regards

Russell McMahon

From: We <rrgpthefaultline.net>

Brings up another topic: Survivability. Have not heard that in the civilian
world of rocketry, yet. In the event of a land dart or core sample recovery,
there are things you can do to ensure greater (not certain!) survivability
of
expensive electronics and other  items of sentimental or economic interest.
Sectional density is one key; any item withing the rocket with a high
sectional
density (read longer than wider and heavy) becomes a spear on impact and
will do
just that to anything  in the rocket both  forward and aft of the object.
Most
altimeters are not made to be potted (cast in  a strong matrix to hold parts
in
place) but could be placed in machined aluminum thick walled containers.
Kind of
odd for an HPR vehicle, not so for an Experimental one and 180 bucks is not
trivial.
A friend called just last night to tell me about an impact in which his
allthread in his nose cone penetrated completely through the accordion
pleated
kevlar webbing in his recovery system! Hey, that stuff is supposed to be
bullet
proof!
Food for thought and I know the great thinkers on the list will have many
more
suggestions.
Bill Colburn

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