Why do the SRB plumes brighten at tail-off?

"Al Jackson" wrote in message
om...
"Terrence Daniels" wrote in
message rthlink.net...

About six or seven seconds before SRB sep (by my rough count) the
exhaust
plumes from the SRB's suddenly get very large and very bright. What's
happening there? Is it something to do with how the fuel is distributed
inside the case?

We do know that the exhaust plume expands as the vehicle goes from sea level
to a near vacuum at 140K feet, but that does not explain the phenomenon you
are talking about.

The SRB rocket nozzles are recessed, thus Aluminum* Oxide pools near
them when the motors are operating. When the pressure tails off in the
solid rocket motors the liquid Aluminum Oxide boils violently. What
you see are bright globs and shards of Aluminum Oxide being coughed
out of the nozzle. Of course none of this gets into orbit!

(One notes that Aluminum is mixed into the solid rocket motor fuel for
energetic purposes.)

The aluminum is in very fine powder form and it *is* the fuel. It is
oxidized by very fine ammonium perchlorate to form alpha aluminum oxide.
This occurs at high temperatures around 5,900º F, IIRC. I am trying to
envision how oxides of aluminum which are by definition fumes, become a
boiling liquid. Are you saying that they are trapped, condense, and form
slag to some extent at the nozzle exit? Why would they cough at lower
thrust values? Could it be the transition from steady state combustion to
unsteady combustion or even a transition from supersonic velocity to
subsonic velocity during thrust tail-off? I always thought the bright chunks
were just isolated pieces of propellant along the case walls being ejected.

Daniel

"Jon Berndt" wrote in message
...
We do know that the exhaust plume expands as the vehicle goes from sea
level
to a near vacuum at 140K feet, but that does not explain the phenomenon
you
are talking about.

I suspect this is an optical illusion - perhaps caused by camera
auto-exposure transients related to thrust tailoff luminance changes.
(Perhaps? If not, it sounds like a good explanation ! ;-)

See, I don't know if it's an illusion... On the Rocketcam videos for
expendable vehicles, you can see that the plume is much larger as the
vehicle gets higher.

I'm still wondering about the plume hugeinating* and brightening at
tail-off. On page 3-38 of the CAIB's Working Scenario there's a chart of SRB
thrust for STS-107. In the last six seconds or so where the plumes expand
and brighten, each SRB's thrust drops from about 1.6 million pounds to ~500K
pounds. It's a clear plummeting of the line on the graph.

I also watched a video from STS-95 - "SRB Replay", available at insideksc:

http://www.insideksc.cjb.net:8081/ww...ts_95_divx.htm

The video starts with a closeup of the intertank, but the 6-seconds-to-sep
flareup is clearly visible as an increasing glare on the tank surface. I'm
really thinking that this flare-up is something internal to the SRBs. I'd
just like to know what causes it.



* "Becoming Huge". I believe that language is flexible, so I made this word
up.

"Terrence Daniels" wrote in
message link.net...
"Jon Berndt" wrote in message
...

The fact that the exhaust plume of any rocket gets larger as it ascends into
ever thinner air is entirely normal and can be seen on old Apollo launches,
etcetera. The reason is that at sea level the pressure that the rocket
exhaust presses against is 14.7 psi. As the shuttle heads toward the vacuum
of space, the air pressure drops off allowing the exhaust gas to expand
further and further. The effect is quite significant on the Shuttle's main
engines and the thrust output rises from 375,000 to 470,000 ft. lbs.

http://spaceflight.nasa.gov/shuttle/...e/basics/ssme/

So in summary. There is no optical illusion as far as the SRB plumes go. I
gave my thoughts on the change in the intensity of the SRB exhaust gases as
have others. Let me just add that if you look closely as the thrust tails
off, the SRB plumes are also changing shapes. They become thinner as the
pressure drops FWIW. Maybe someone from Thiokol could comment if you are
reading this.

I'm still wondering about the plume hugeinating* and brightening at
tail-off. On page 3-38 of the CAIB's Working Scenario there's a chart of
SRB
thrust for STS-107. In the last six seconds or so where the plumes expand
and brighten, each SRB's thrust drops from about 1.6 million pounds to
~500K
pounds. It's a clear plummeting of the line on the graph.

Do they still say "pc less than 50"? ;-)

I also watched a video from STS-95 - "SRB Replay", available at insideksc:

http://www.insideksc.cjb.net:8081/ww...ts_95_divx.htm

The video starts with a closeup of the intertank, but the 6-seconds-to-sep
flareup is clearly visible as an increasing glare on the tank surface. I'm
really thinking that this flare-up is something internal to the SRBs. I'd
just like to know what causes it.

* "Becoming Huge". I believe that language is flexible, so I made this
word
up.

Nah, I better not sheepish grin LOL. Very good Terrence.

Daniel