QUESTION:
Does the launcher's second stage have a restartable capability? How are the
cut-off and restart are controlled?

ANSWER from Andrew Petro on December 9, 1997:
Not  all  launch  vehicle  second  stages  have  engines with re-start
capability.   First of  all, many  launch vehicles  have more than two
stages and for those there would  not be any need to re-start  the 2nd
stage.  But it is a good question because most launch vehicles  follow
a path in which  the rocket shuts down  and coasts partway around  the
Earth and then the  rocket has to fire  again to place the  spacecraft
into a circular orbit.  This  can either be done by having  a separate
final stage (it could be the 2nd, 3rd, 4th, etc.) that does this  burn
or the  final stage  must be  re-started to  do this  burn.  The Space
Shuttle uses its small orbital maneuvering engines to do this.

Once the rocket is coasting in space, it is in free-fall  (weightless)
so the liquid  oxidizer and fuel  will just float  around inside their
tanks.  To re-start  the engines, the fuel  and oxidizer in the  tanks
must either be behind  a pressurized bladder so  that it is forced  to
flow to the engine or there must  be a way to move the propellants  to
the end of the  tank that feeds the  engine.  Sometimes small  engines
(with their own pressurized propellant supply) are fired to create  an
artificial gravity force to move the propellants to the big engine  so
that it can be started.  Many satellites have final stages with  solid
propellant which is not affected by the weightlessness.

With some  kinds of  propellant, actually  re-starting the  engines in
space is done the same way  as on Earth, using something like  a spark
plug to ignite the oxidizer and fuel.  Many rockets have final  stages
which use  a special  kind of  propellant which  is hypergolic,  which
means that the oxidizer and fuel are so reactive with each other  that
they ignite  on contact  and don't  require any  spark.  Naturally you
have  to  be  careful  handling  this  stuff.   The Space Shuttle uses
hypergolic propellants for its  small orbital maneuvering and  control
rockets.

There are valves in the lines which carry the oxidizer and fuel to the
engine and these valves  are opened when you  want the engine to  fire
and they are closed to make the engine shut down.

Andrew Petro
Johnson Space Center