Throughout the on-pad testing sequence, officials have focused on three key challenges created by the Delta 4-Heavy and its trio of Common Booster Cores that generate the 1.9 million pounds of thrust to propel the 23-story rocket off the launch pad.

Delta program chief Dan Collins holds a model of the Delta 4-Heavy rocket during a news conference Tuesday. Photo: Carleton Bailie/Boeing

"We entered into it realizing this is a significantly different booster than a single core Delta 4. We did not believe 'oh this is just the run of the mill.' So we had a lot of plans in place," Collins said.

"We're very proud of the progress so far. We are continuing to work to make sure we don't leave any stone unturned in providing mission success for our customers on this."

• An early issue was the environmental control system's ability to thermally condition the three Common Booster Cores and their hydrogen-fueled RS-68 main engines during the countdown to launch.

"We had proven (the environmental control system) had the ability to pump the right volume of gases into the different compartments of the rocket to get them at the temperatures they need to be for a single core. Now going to do three at one time was certainly a question that we wanted to make sure we attacked early.

"In the first tanking test we paid particular attention to that and we found that we did have we the ability to tune the system for all of the compartments. We found on that first tanking test, however, that while we were able to get three RS-68 engines chilled to the right temperature, we were not able to maintain that chill as long as we wanted to.

"So we modified a couple of lines in the system, putting in larger gas lines so that we could deliver more gas, and made a few adjustments on valves. When we got to the second tanking test, everything was perfect. In fact, we have significant margin vs. our chill requirements."

• The second worry was handling the vast amount of telemetry data from three Common Booster Cores instead of just one.

"The amount of data flowing through our ground system is essentially tripled. So obviously that was one of the questions we wanted to attack -- looking at the ability of the system to process the large amount of data. Again, everything was designed from day 1 for this. We had proven with one CBC out there, everything worked. But now you have to prove that it works with three. So we paid particular attention to that.

"Not only do you have to process the data, you have to make sure you are processing the data in the right order so that you don't get the answer before you've asked the question. That gets down to some very, very complex timing within our ground system computers that you have to make sure you are doing it right. So we put a lot of attention on that, we put a lot of data traps into the system to make sure we understood how data was being processed, that it was flowing correctly."

• The human factors of running the countdown were the third challenge.

"And then finally you get down to the human beings -- the workload on the humans beings. Are they being asked to do the right amount work in the right amount of time? ... That's really why we go through these different tanking tests and the way we go through them in an increasing complex fashion -- first just trying to get it done, then trying to improve on the process based what you learned, then bring the element of time into it.

"But (its) getting the humans used to dealing with the three sets of data and getting our launch team to where we know what we are asking of them is a task well within their ability to perform and that somebody isn't trying to make too many decisions in a short period of time, which could end up leading to a delay in the launch.

"The computer won't let the system launch if it's not ready but the launch team who is monitoring it, if they are not confident, they also have the ability to shut the thing down. So you want to make sure that they have a workload that they can accomplish in the right time period so we when get down inside T-minus 1 minute to go, they've done their job, they've answered all their questions and they're confident everything is moving forward."