HUNTSVILLE, Ala. - NASA won't launch the Space Launch System - the big new rocket it is developing in Alabama - for another three years. That's a long time for people waiting for the rocket, but to NASA 2015 is time to put the pedal down.

"Our teams aren't thinking it's a long time," NASA Associate Administrator Robert Lightfoot said in Huntsville Friday. "They're actually thinking we've got a ways to go to get there. It's exciting."

Two key milestones in the rocket's development just passed. NASA launched the Orion spacecraft on its first test flight in December, and it was a big technical and public relations success. And on Jan. 9, NASA test-fired one of the RS-25 engines that will boost the new rocket off the launching pad in 2018. The RS-25 was the space shuttle's main engine, too, but leftover RS-25s that will be used in the new rocket have been reworked with a new "brain" or electronics controller.

Huntsville's Marshall Space Flight Center is NASA's lead center for developing SLS, and engineers from Marshall fired an RS-25 9 on Jan. 9 for nearly eight minutes at the Stennis Space Center in south Mississippi. It was the first of eight test fires that resume in April.

It's all part of a process of design, development and testing for what will be a giant new rocket. SLS will stand 38 stories tall and have more lifting power than anything since the Saturn V. NASA leaders say getting something that big ready to fly just takes time.

"Development is different because we're designing things for the first time," Orion program manager Mark Geyer said in Huntsville this month. "We're adding new systems between these flights. The big things we're adding between what we did in December and 2018 is the rocket, which is a huge amount of work, and for us, the service module and finishing the rest of the crew module. That's a lot of work that needs to get done."

Here's a look ahead at the key SLS milestones this year and when NASA expects to check them off its to-do list:

1. In early March, technicians at ATK Systems plan to fire the company's new five-segment solid rocket motor in a qualification test. Qualificaton tests are different from development tests in that they actually "qualify" what's been developed for flight. Similar to the solid rocket motors that helped lift the space shuttle, the new motors that will help lift SLS are bigger, more powerful and updated in key ways. There's no asbestos in the liner, for example.

2. Sometime this spring, engineers at Marshall Space Flight Center will perform similar qualification tests on the SLS avionics system. Avionics includes navigation, communications and the other key electronic systems that control the rocket's flight. Getting all those systems built and talking to each other was a major task in 2014.

3. In July, the overall SLS program faces its critical design review. That's the review where the "final design is signed, sealed and delivered" by independent engineering experts and NASA officials, SLS spokeswoman Kim Henry said. After that review is passed, NASA has its plan and can accelerate building.

4. In late summer or early fall, construction teams will finish two big new test stands at Marshall that will be used early in 2016 to test the core and upper stages of SLS. The test stages, built at NASA's Michoud Assembly Facility in New Orleans, will be barged to North Alabama and pressure-tested to see how they will hold up in launch conditions..

5. Geyer's team in Houston will continue testing Orion this year and fixing issues that came up in December's test flight. There don't appear to be many, but Geyer said the system that keeps Orion floating upright after splashdown needs work. Only two of its five balloons opened all the way. But Geyer said the Orion team is on track to deliver key flight parts to Cape Canaveral this year.

6. Finally, NASA's partners at the European Space Agency (ESA) will step up work this year on the service module they are building for Orion. That module will sit below the spacecraft and provide power, heating and fuel for Orion during flight. ESA faces the same challenge the American teams faced in designing and developing Orion: Techniques and materials used to build the orbiting space station - the last big space project - need big revisions for a spacecraft that will fly 1,000 times higher.

7. Finally, aside from these major pieces, many smaller, but critical, parts are being built around the nation. Huntsville's Teledyne Brown Engineering is working on one of them - an adapter that will link SLS's core stage to another propulsion stage.

Looking ahead, NASA will be working hard for the next two years to build the complete system. Then its all off to Kennedy Space Center where the rocket, the boosters, the Orion spacecraft and the European support module are "integrated" into one flight package. That will take most of 2017.