CAPE CANAVERAL, Fla. - SpaceX stands ready to launch a much-needed load of supplies to the International Space Station this weekend on the heels of a failed supply run by Russia.

Besides food and experiments, the Dragon cargo ship ordered up by NASA holds a new docking port, or parking place, for future commercial crew capsules.

Liftoff is scheduled for 10:21 a.m. Sunday. Good flying weather is forecast for SpaceX's unmanned Falcon rocket.

This shipment is especially critical because the space station has lost two deliveries since fall.

A Russian supply ship spun out of control shortly after liftoff in April and burned up on re-entry with all its contents. In October, an Orbital Sciences Corp. cargo carrier was destroyed in a Virginia launch explosion.

Once again, SpaceX is picking up the slack. This will be the eighth station supply run for the California-based company; the first was in 2012.

"Dragon has been super reliable," SpaceX's Hans Koenigsmann told reporters Friday.

Nearly 5,300 pounds of gear is packed for the trip, including replacements for science experiments lost in the Orbital launch accident, some of them designed by students.

Stored in the capsule's unpressurized trunk is the first of two new docking rings for the station. Spacewalking astronauts will hook up the 1,160-pound port built by Boeing later this summer.

The twin ports eventually will be used by astronauts arriving in new American-built capsules. NASA is paying billions of dollars to SpaceX and Boeing to develop the crew capsules. The SpaceX version is a souped-up Dragon. Boeing's model is the CST-100, short for Crew Space Transportation; the 100 represents the beginning of space at 62 miles up, or 100 kilometers.

In both cases, manned flights are still at least two years off. Until then, Americans will continue to hitch rides to the space station on Russian rockets for tens of millions of dollars per seat.