What was this?

I was watching an episode of Space Voyages on the SMithsonian Channel last night
and early in the show, they showed the launch of a modern era rocket. Could have
been an Atlas or Delta, couldn't really tell. But as soon as it started to take
off, there was very visible rocket exhaust coming from the top, at the payload
area. I never saw anything like that before. It wasn't an emergency escape
system, as it didn't pull the top off the rocket. Any idea what this could have
been?

On Fri, 17 Jan 2014 14:30:45 -0500, JF Mezei
wrote:

On 14-01-17 02:07, wrote:

been an Atlas or Delta, couldn't really tell. But as soon as it started to take
off, there was very visible rocket exhaust coming from the top, at the payload
area.

My guess would just be venting of gases. When you have tonnes of liquid
hydrogen and oxygen in tanks not designed to widthstand much pressure,
as those gases boil off, they have to vent the excess so that pressure
in tanks does not build up too much.

After fill up, they use vent tubes attached to the rocket to vent the
excess O2/H2. Just before launch, they close off those valves and
retract those vent tubes and allow a bit of pressure to build in the tanks.

It is possible that vents re-open automatically for a short while at
about launch time to vent excess pressure.

At one point, the engines consume the fuel at a rate which matches or
exceeds pressure build-up, so there is no longer need for venting. But
this venting may be needed in the very first moments of launch.

That is my guess.

Thanks. I could very well be. Do they ignite the venting gasses? Because it
clearly looks like fiery rocket exhaust. I saw it again tonight when I
accidentally started to watch the same episode again, thinking it was a new one.
It also looked for a second or two like it was to start the rocket spinning, but
they wouldn't do that before it even cleared the tower, would they? And for what
purpose?

On 1/17/2014 2:07 AM, wrote:
I was watching an episode of Space Voyages on the SMithsonian Channel last night
and early in the show, they showed the launch of a modern era rocket. Could have
been an Atlas or Delta, couldn't really tell. But as soon as it started to take
off, there was very visible rocket exhaust coming from the top, at the payload
area. I never saw anything like that before. It wasn't an emergency escape
system, as it didn't pull the top off the rocket. Any idea what this could have
been?


If what you saw matches the first second of this video trailer for the
series:

http://www.smithsonianchannel.com/sc.../space-voyages

Then I think you are watching a computer simulation of an SLS launch. It
looks to me to be gasses venting off the top. But in reality I would
suspect there would be little venting. Usually venting is stopped just
prior to lift-off to maintain flight level pressures within the the
LOX/LH2 tankage.

Dave

On Sat, 18 Jan 2014 01:43:57 -0500, David Spain wrote:

On 1/17/2014 2:07 AM, wrote:
I was watching an episode of Space Voyages on the SMithsonian Channel last night
and early in the show, they showed the launch of a modern era rocket. Could have
been an Atlas or Delta, couldn't really tell. But as soon as it started to take
off, there was very visible rocket exhaust coming from the top, at the payload
area. I never saw anything like that before. It wasn't an emergency escape
system, as it didn't pull the top off the rocket. Any idea what this could have
been?


If what you saw matches the first second of this video trailer for the
series:

http://www.smithsonianchannel.com/sc.../space-voyages

Then I think you are watching a computer simulation of an SLS launch. It
looks to me to be gasses venting off the top. But in reality I would
suspect there would be little venting. Usually venting is stopped just
prior to lift-off to maintain flight level pressures within the the
LOX/LH2 tankage.

Dave


Yup, that's the shot I saw and the one I'm refering to.