Conference 7.286::space

    At a GUESS, I'd say it was a pressure effect. A low pressure region
    forms at the base of a rocket moving through the atmosphere. An
    exhaust plume helps relieve that but with most engine designs the
    plume does not eliminate the low pressure region (one of the advantages
    of plug  nozzles is that they often do eliminate the low pressure
    region).
    
    If the pressure differences are great enough, this low pressure
    region will drag some of the exhaust plume backwards. From here
    it gets caught up in the turbulent airflow around the base of the
    vehicle and could appear to move forward. The F-1 engines expanded
    to roughhly sea level pressures so the pressure within the exhaust
    plume would be higher than ambient, contributing to this effect.
    If you have a tape of a Saturn V ascent up through S-II ignition
    (some of the later Apollo doc's have sequences like this) try watching
    it on slo-mo paying attention to the exhaust plume.
    
    The shuttle SRBs probably expand to near sea level pressure and
    I suspect the airflow around the base of that vehicle gets really
    bizarre.
    
    gary

    Thanks!  The dynamics are sure complicated.
    Joe

    The plume at the base of the ET is insulation burning away from the
    tank due to the radiant heat plus friction.  It's been there since
    STS-1, but it's amazing how many people noticed it after STS-25!  If
    you've ever seen "Hail, Columbia!", they have an excellent scene where
    they show the ET separation from a camera at the business end of the
    orbiter.  Sure enough, the entire bottom of the tank is charred.
    I've talked with an engineer at MSFC about this, and he indicated
    that even though the insulation is not truly an ablative, it does
    a perform well that way.
    
    Regards,
    
    Aaron 

    BRIGHT GAS FLOW ON STS-26 IS OLD PHENOMENON - can881228.txt - 12/8/88
 
    Flashes of flame seen between the solid rocket boosters during the
STS-26 launch were caused by a well-known "plume recirculation" effect
that has been known since the launches of V-2 rockets in the 1940's.
The flashes of light were reminiscent of the flashes that preceded the
breakup of Space Shuttle Challenger in 1986 and caused some
apprehension among observers until booster separation. 
 
    "The effect is visible -- if weather conditions and camera angles
permit -- from around 92 seconds into flight until just before
jettison of the boosters," said Aerospace Engineer Lee Foster of
NASA's Marshall Space Flight Center.  "Even if it's not visible, we've
used instrumentation to determine that the effect does indeed occur on
every flight." 
 
    Plume recirculation, Foster explained, is due to the relative
location of the booster and main engine nozzles, the expansion of the
rocket jets at high altitudes, and the airflow around the Shuttle
vehicle and its plumes. 
 
    "As the Shuttle ascends, atmospheric pressure becomes lower,"
Foster said.  "As a result, the exhaust plumes grow wider as the
Shuttle gains altitude.  Then, as they get wider, they begin to
intersect with each other and this intersection region gets closer and
closer to the base region of the vehicle. 
 
    "Usually around 92 seconds," Foster said, "the pressure in this
intersection becomes higher than the pressure in the base region and
some of the exhaust plume gases from this intersection are reversed,
or recirculated, into the base region.  The high-temperature gases heat 
the aft dome of the external tank and the aft sections of the boosters." 
 
    By itself, that effect wouldn't result in anything visible. However,
another ingredient--oxygen--is added to the picture at this point.
 
    "Around the outer boundary of the recirculation region, oxygen
from the airflow around the Shuttle mixes with some of the
recirculated exhaust gases, allowing residual fuel in the plume to
burn.  That's what results in the flames seen in the flight
videotapes," said Foster. 
 
    Foster said the plume effect is not a safety issue: it was
predicted before the first Shuttle mission and the vehicle thermal
protection system designed to allow for it. 
 
    "We've been building our data base on this subject ever since the
mid-1950s," he said. "In fact, plume recirculation occurred from the
very beginning, even with the German V-2 rockets. 
 
    "During the first six flights of the Shuttle, we put a variety of
gauges on the external tank and boosters to measure the heating from
plume recirculation.  These measurements showed that the heating was
basically the same on each flight and that the vehicle is well
protected from the heating." 
 

    I have a related question on the exhaust from the SSMEs. During
    operation (and in particular noticeable at ignition during launch) in the
    exhaust from each SSME can be seen a bright ring. From memory the ring
    for each SSME moves slightly downwards just after ignition presumably
    as the engines throttle up to the launch power setting.
    
    What are these rings called?
    
    Are they discussed elsewhere in this conference? (I couldn't find a ref.)
    
    What causes them? (A full answer might be beyond the scope of my brain.)

Are you thinking of the sort of pyramid-shaped blue phenomenon behind each
engine cone?  It is actually there from ignition, when you can kind of see it
form as the combustion stabilizes, till the shuttle is far enough away so you
can't see it any more.

I don't know what it is called, but I always assumed that it is just the part of
the exhaust flow which is hot enough to glow.  Could also be some interaction
with the supersonic gas flow intersecting with the relatively slow-moving air
around, I guess.

Burns

RE: <<< Note 478.5 by AUSSIE::GARSON "achtentachtig kacheltjes" >>>

In the aviation world they are called shock diamonds. Some jet engines produce
a long series of them while running in afterburner. One of the best examples is
the J-58 engine used by the SR-71. It produces probably ten or so shock
diamonds, from close to the exhaust nozzle stretching out perhaps 20 or 30 feet
to the last one.

I don't know the thermo/hydrodymanics that cause them, really, other than that
they correspond to shock waves in the exhaust flow.
					-- Tom