I guess my thought is that sound, at least as heard from our respective
locations, isn't a good indicator.  I had a good feel for the sound delay
from the kids' rockets, and I heard the engine sound and then the two
"booms".  I think I have a pretty good feel for when in flight the sound
occured, as opposed to when I actually heard it.  I figure that if it was
from the sound barrier, that seems reasonable, as the rocket had not
traveled significantly farther from my position when it reached peak
acceleration than when it was on the ground.  I should have heard the engine
first, as it was the first output from a roughly stationary source, and I
should have heard the "boom" last if it was the breaking of the sound
barrier, as it really hadn't traveled further from me until after it could
have gone super-sonic.  But I also should have heard something very similar
if it was "chuffing".  In my thought, the key is that the rocket was not
accelerating towards me, but laterally, and then away.  If the rocket had
been approaching at super-sonic velocity, then the sonic shock would have
certainly been the first thing that I heard.  But the rocket wasn't
approaching me at super-sonic velocity and it couldn't have been.  It may
have been going away from me at a super-sonic velocity, but the sound
pattern wouldn't then be an indication.

I would say that we were about a mile out, and the difference I heard in
volume was significant enough that I expect at two miles it may be
reasonable to have heard only the final booms, with engine thrust attenuated
due to distance.  I also think that if you were to have only heard the two
booms due to super-sonic effects from a side distance of 2 miles, you would
have had to have about 500 feet of elevation or more above the launch
platform for that to be possible.

I also think that the burn time of the engine implies ambiguity as well.  If
the acceleration was in the ballpark of 100m/(s^2) then it should have been
about 3.5 seconds until we broke the sound barrier, and we should have heard
the sonic boom and not much else...  But the cut-out was expected at 4
seconds, so again it's tough to tell.  I think if we had audio from
significantly above the platform or timed audio recordings that we could
compare to video and acclerometer data, then audio might be conclusive.

Of course, it's been a few years since my last physics class, and I haven't
been regularly playing with solid-fuel propellants for over a decade, so I
might be a bit off.

On Tue, Jun 2, 2009 at 12:24 AM, Glenn LeBrasseur <gle...@twaves.org> wrote:

> Well that's just it; I was at the road-station two miles away and I never
> heard the engine sound. All I heard were two cracks about ?15? seconds after
> launch. Would I have heard a chuff sound if I never heard the engine sound?
> Just curious.
>
> Glenn
>
>
> I wrote:
>
>>
>>
>>
>>  That was the motor chuffing, we think.
>>>
>>
>> Yeah, I don't think it's possible to hear a sonic boom if you can already
>> hear the engine (though the chuff sound was impressive). My understanding is
>> that a sonic 'boom' comes from the fact that you hear no sound, then all of
>> the sound at once when the shock arrives at your location.
>>
>>
>>
>>  Glenn LeBrasseur wrote:
>>>
>>>> Is the speed measurement a little off? I am sure I heard a sonic boom.
>>>> In fact I thought I heard two cracks seperated by about 1/2 second?
>>>>
>>>> Glenn
>>>>
>>>> Glenn LeBrasseur,  KJ7SU
>>>> gle...@twaves.org
>>>>
>>>>
>>>>
>>>> Quoting Keith Packard <kei...@keithp.com>:
>>>>
>>>>  Here's the data I captured from the on-board telemetrum data log.
>>>>>
>>>>> The graph is a bit messy as it has so many lines.
>>>>>
>>>>> Here's some numbers too:
>>>>>
>>>>> Max height above ground: 3864m (baro)
>>>>> Max speed:                308m/s (baro) 286m/s (accel)
>>>>> Max accel:                120m/s² (accel)
>>>>> Time to apogee:            28.2s
>>>>>
>>>>> --
>>>>> keith.pack...@intel.com
>>>>>
>>>>>
>>>>
>>>>
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>>>>
>>> --
>>> -------------------------------------------------------
>>> Andrew Greenberg
>>>
>>> Portland State Aerospace Society (http://psas.pdx.edu/)
>>> and...@psas.pdx.edu  P: 503.788.1343  C: 503.708.7711
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