External tank jettison question

I have a question about the way the external tank is jettisoned. You've
probably guessed that already.

How much fuel does remain in the ex-tank, when the main engines cut off, and
is the tank then sealed?
I suspect that the shuttle's own intakes are sealed. How is the tank then
moved away from the shuttle? Does it have an active separation, with small
rocket thrusters, or does it just drift away? If there is a significant
amount of fuel left in the tank, would it make any sense to leave the tank
open, so outgassing from the residual fuel would move the tank away? Or is
this in any way dangerous to the orbiter?

TIA

Allan F

Allan F wrote:
I have a question about the way the external tank is jettisoned.
You've probably guessed that already.

How much fuel does remain in the ex-tank, when the main engines cut
off,

According to Dennis R. Jenkins's excellent book, Space Shuttle: The
History of the National Space Transportation System (3rd ed), there's
usually around 5-10,000 pounds of LOX and 2-4,000 pounds of LH2
remaining in the tank after separation.

and is the tank then sealed?

The lines that feed the orbiter are sealed to prevent contamination of
the orbiter itself during separation.

I suspect that the shuttle's own intakes are sealed.

Correct.

How is the tank then moved away from the shuttle? Does it have an
active separation, with small rocket thrusters, or does it just drift
away?

Pyrotechnics are used to physically separate the orbiter from the tank.
The orbiter uses its RCS thrusters to move away from the tank.

If there is a significant amount of fuel left in the tank, would it
make any sense to leave the tank open, so outgassing from the
residual fuel would move the tank away? Or is this in any way
dangerous to the orbiter?

The orbiter's RCS can do the job much more precisely than outgassing
could. I'm not a rocket scientist, but I suspect the different levels of
thrust provided by venting the LOX and LH2 feedlines would lead to a
rotation component around the ET's long axis, which may complicate the
end-over-end tumbling used to help the ET break up on reentry.

--Chris