On 4/16/2010 9:57 AM, Neil Fraser wrote:
On Apr 16, 8:30 am, Pat wrote:
Orion can carry six astronauts, Soyuz only three.
So you need two Soyuz to evacuate the whole ISS crew, but only one Orion.

Once Dragon (or one of its competitors) comes online, Russia would
presumably scale back its Soyuz flights so that there is only one
Soyuz docked to the station at any given time. Between one Dragon (7
crew) and one Soyuz (3 crew) I don't see a problem with evacuating the
six people on ISS.

No, Dragon would make a great lifeboat, and remove the need for using
Orion for that mission.
The problem is that neither Dragon or Orion exist at the moment, and
considering that Orion needs to be modified and lightened to make it fly
on Atlas V or Delta IV Heavy, there's a very good chance that Dragon
will be ready to go before Orion Light is.
Then comes the question of whether Orion Light has any utility for a
manned Moon or Mars mission. Because if it doesn't, all it is is a ISS
crew taxi/lifeboat...and again there is Dragon to do that.

Once upon a time there was a requirement for a dedicated ISS lifeboat
since crew transfer was originally to be handled by Shuttle. Since
the orbiter can't loiter at the station for six months, a separate
vehicle was needed for emergencies. Concepts ranged from (seriously)
confiscating two unflown Apollo vehicles from museums, refurbishing
them and docking them to the station, to the X-38. With the
replacement of Shuttle with vehicles that can be parked at the station
for the duration of its crews' mission, there doesn't seem to be a
need for a dedicated lifeboat.

The Russians had a concept also:
http://www.astronautix.com/craft/alpeboat.htm

Pat