Is it possible to access and climb half dome without a reservation in the park? Am thinkging of flying into SFO and then working my way as close to an access point outside the park (no advance reservations at hotel) and then getting up very early and try to go up and down before driving back to SFO?????

You can enter Yosemite without reservations. You can climb Half Dome without a reservation or permit. Check www.nps.gov/yose/trip/valleyhikes.htm.
You may have to access trailhead via the park shuttle service. Not sure if can park nearby.

There are frequent hotel cancellations so try to get inside the park for the night before. Here's a link:

http://www.yosemitepark.com/lodges2c...D=22&PageID=36

It's a strenuous hike that will take 10 to 12 hours round trip. You wouldn't want to drive from outside the park to the trail head as that would probably add another hour to the beginning of your day.

From your lodging you can take the shuttle to Happy Isles shuttle stop. There is also some parking there as we parked in the lot to take the Vernal Falls hike.

besides what suzie says about not staying outside the park - there is no guarantee you would find a room outside the park either. Places do book pretty solid . . .

While there is no parking at Happy Isles (it is serviced by the trams only), you would want to park at Curry Village. It is only a 10 minute tram ride. If you get off the mountain late (after the trams stop running for the day) it is less than 1/2 hour walk back to your car. But be safe and make sure you bring a flashlight and rain gear. We did this hike in May and encountered rain and snow at the higher elevations. We were also rushing to get back down the mountain before dark because we didn't bring flashlights (dumb me).

Are you considering driving back to SF the same day you go to the top of Half Dome? I hope you are in tremendously good shape because that is REALLY ambitious.

Do you mean the trail or the rock face?

<< then getting up very early and try to go up and down before driving back to SFO?????>>

Um...we did the half dome hike last year, starting at 6:30am, hiking up to the shoulder by 1:00pm (I'm the turtle in the family, lol), waiting for the shoulder to open because of construction that they were doing last year, starting up to the top by 3:00, and got back to Happy Isles by 9:00pm. We're middle aged so we weren't scooting along as fast as some of you young whippersnappers, but driving back to SF afterwards would be WAY past my ability level. Hate to admit it, but I was exhausted after that "hike".

We climbed only to the top of Nevada Falls, leaving Happy Isles around 1 pm and got back down by 8 pm. My sis is out of shape and we went really slow. Then we drove back to So. Calif that evening after dining in Yosemite Valley. She needed to be back on a plane to Vegas by 6 am, so we drove most of the night. I was a complete zombee, not exaggerating. Very tired. But the worst part was, that after doing a hike like that (of which the top of Vernal is way closer than Half Dome)- and then driving for 4-5 hours, one's legs tend to cramp up really bad. But maybe that was just me. I couldn't walk the next day and I am in pretty good shape. I think it was because I wasn't moving and couldn't stretch as I was driving. But if it is either do your trip driving home that night OR not go at all, I'd say go for it.