From: "Jeff Greason" <jgreason@hughes.net>
Newsgroups: sci.space.tech
Subject: Re: Rocket sound levels
Date: Tue, 22 Feb 2000 18:32:57 GMT

Scott,

If you want more details, drop me a line -- but the question is
of general interest and I thought I'd post a reply here.  This
is an area of ongoing interest to me, due to the critical effect
of rocket noise on rocket vehicle operating patterns.

To first order, you need the thrust, chamber pressure, exhaust
velocity, throat pressure, and throat diameter.  The other
factors can make quite a difference, but make the calculations
quite a bit more involved, and a lot of the data is empirical.
(Unless you pick a combination which has poor stability and
a fluctuating chamber pressure, which can *really* drive up
your noise (as seen, for example, on solid rockets, or on the
XLR-99 ammonia/LOX engine).

The question as asked leaves out an important detail, which is
the frequency of the noise.  Bigger engines are not only louder,
but have lower-frequency noise which attenuates more slowly
with distance from the source, and which is harder to control
with water-spray techniques.

As a rule of thumb, sound output power will be between
0.5% and 1% of the mechanical power output of the
engine (0.5*Thrust*Exhaust Velocity).  However, the source
is *not* omnidirectional, and the sound is concentrated
in a wide conical region "downstream" of the nozzle.

Directivity can increase sound levels by 20-40dB in the
worst locations.

Ignoring directivity, ignoring the frequency-dependence of
atmospheric absorption,  and assuming noise output of 1%
the mechanical power, you get:

Sound Pressure (dB) =
      10 log T + 10 log Isp - 20 log R + 123dB

Where T is thrust is pounds, Isp is in seconds, and R is
distance from the source in feet (this is just the inverse
square law rearranged in dB units, equation 5.13 in
reference [2])

Using this, then adding 15-30% (of the dB) for the worst-case
direction, can give you a first approximation if doing
something like siting a test stand or doing a "first look" at
possible launch facilities.  Again, peak noise is about 50-60
degrees "off-axis" from the exhaust direction.

When you get the results, if the resulting noise levels don't
seem awfully darned loud, you're not doing it right :-(.  Rocket
noise is a real problem, and I think controlling it is going to
be a big area for commercial rocket research (when there is
such a thing).  For test stands, this is why water-spray systems
are so desirable.

Some references:

[1] S. McInerny, AIAA-90-3981, "Rocket Noise -- A Review"

    Can be ordered from AIAA (if you're a member) for
    about $20.  Technical, but the best modern source on rocket
    noise I know of

[2] _Lunar Missions and Exploration_, University of California
     Engineering series, Wiley, 1964, p. 257-260

   Hard to find, but a nice few-page summary for siting launch
   facilities.  Coincidentally, this is the book Henry Spencer was
   touting here a while back.

[3] NASA TN D-21 "Near-Field and Far-Field Noise Surveys
   of Solid-Fuel Rocket Engines for a Range of Nozzle Exit
   Pressures"

  Easy to find in a library, limited, but a good "first glance"

[4] NASA CR-566, "Acoustic Prediction Methods for Rocket
     Engines, Including the Effects of Clustered Engines and
     Deflected Exhaust Flow", R.C. Potter and M.J. Crocker

    Massive, definitive, and sleep-inducing.  A mid 1960's summary
    of the state of the art in rocket noise at the time.  Hard to find,
    and not worth it unless you're going to really get down and dirty
    with the calculations -- unless you're clustering engines.  For
    single engines, the methods in [1] above are much easier to
    apply.

----------------------------------------------------------------
"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <www.xcor-aerospace.com>                <jgreason@hughes.net>

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Who would buy a Roton?
Date: Thu, 15 Apr 1999 03:04:44 GMT

In article <37153CD2.DAB1E395@ibm-pc.org>,
Jorge R. Frank <jrfrank@ibm-pc.org> wrote:
>> (b) they can
>> legally and routinely operate from metropolitan airports
>
>This would seem to be an advantage for Kelly and Pioneer over Rotary.
>Since their vehicles ignite their rockets at altitude, they would
>generate no more ground-level noise at takeoff than an ordinary jet.  I
>just can't see a ground-level rocket ignition ever meeting FAA noise
>specs.

Remember that the FAA basically doesn't care about noise *within* the
airport; what really matters is noise heard in the surrounding areas.  A
horizontal-takeoff all-rocket system is unlikely to meet noise specs, but
vertical takeoff helps the situation considerably.  So does the ability to
take off from a little concrete pad at the best point within the airport,
rather than being constrained by runway layout.

And Gary Hudson has been heard to mumble about innovative ways of making
rocket engines quieter.  Nobody's ever really *tried* to do that...
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: gchudson@aol.com (GCHudson)
Newsgroups: sci.space.policy
Subject: Re: Who would buy a Roton?
Date: 15 Apr 1999 14:02:32 GMT

>I would think the most significant noise problem around a spaceport
would the the double sonic boom of a craft returning from space. Ships
with a long crossrange could mitigate the boom somewhat, but the Roton
reenters ballistically, with a mimimal crossrange.<

It's not the returning ships that have a problem, so much as the ascending
ones.  They can produce a double or focused boom about 50 miles downrange which
can be pretty intense in a small area.  We don't have a solution for that one
yet.

Gary C. Hudson

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Who would buy a Roton?
Date: Thu, 15 Apr 1999 14:52:34 GMT

In article <371577B6.68129C71@earthlink.net>,
Kirk Voelcker  <anaxagoras@earthlink.net> wrote:
>I would think the most significant noise problem around a spaceport
>would the the double sonic boom of a craft returning from space. Ships
>with a long crossrange could mitigate the boom somewhat, but the Roton
>reenters ballistically, with a mimimal crossrange.

There is also a sonic-boom issue at launch, which is easy to overlook
because on existing rocket ranges, it usually happens over water in an
area that's cleared anyway.  For a launcher that goes up vertically and
then tips over, there is an area a little ways downrange which gets a
fairly strong boom because the curve of the launcher's path focuses the
boom there.  You might be able to reduce or eliminate this by tinkering
with the ascent path, if you've got a bit of performance to spare.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Who would buy a Roton?
Date: Thu, 15 Apr 1999 23:46:53 GMT

In article <371622B0.16BC2D10@earthlink.net>,
Kirk Voelcker  <anaxagoras@earthlink.net> wrote:
>>It's not the returning ships that have a problem, so much as the
>>ascending ones.
>
>Do you imply that there is a way to mitigate the returning boom (by
>following a certain approach), or that the energies delivered to the
>ground will have dissipated sufficiently to be less annoying?  The boom
>of the Shuttle is pretty impressive to me...

Remember that the shuttle orbiter is very heavy, a fairly solid lump of
equipment.  A returning Roton is mostly empty tanks, and will decelerate
to lower speed at higher altitude.  That may account for Gary's comments.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Date: Thu, 15 Apr 1999 20:18:58 -0700
From: Doug Jones <random@qnet.com>
Newsgroups: sci.space.policy,sci.space.shuttle
Subject: Re: Who would buy a Roton?

anwong@my-dejanews.com wrote:
> Also factor in the noise level of Rotary Rocket's spaceships at
> airports. Anyone remember the clamor of the Concorde landing at
> US airports?  People living around airports back when British
> Airways and Air France wanted to start landing the Concorde in
> US airports were afraid of the noise level that
> the Concorde produced.

It so happens that the only time I wear (or need) hearing protection
is when I'm using a compressed air blowoff nozzle while cleaning
engine parts.  The firings are no biggie, and I'm at the test site for
all of them.  Roton will not be as loud as many people think...

--
Doug Jones, (partially deaf) Rocket Plumber
Rotary Rocket Company