NASA managers are closing in on a mission trajectory plan for the new Space Launch System (SLS) rocket that is set to debut in 2018. Mike Sarafin, NASA’s mission manager for the first Orion/Space Launch System (SLS) mission, recently provided an early look at an outline of launch events and a high-level status of the process of designing the trajectory for the maiden flight.
Both the SLS and the Orion vehicles are still in development, while the trajectory design is in a preliminary phase – as the launch vehicle and spacecraft plans continue to mature.
“We’re in the initial design phase,” Mr. Sarafin noted in a recent interview with NASASpaceflight.com.
“We have three design phases of the mission trajectory; we’re doing the preliminary mission [trajectory design] phase, then we have a ‘final’ analysis [but] it’s not the ‘final’, and then we have what’s called a ‘best-estimate’ trajectory, so we’re in that initial phase.”
Sarafin is a NASA Shuttle veteran, having previously worked in the Mission Operations Directorate (MOD) at the Johnson Space Center in Texas as Lead Flight Director of several Shuttle missions at the end of the Shuttle Program, the last being STS-132 in 2010.
He most recently served as the Flight Director for the Orion spacecraft’s first test flight, Exploration Flight Test-1 (EFT-1).
As EM-1 Mission Manager, Sarafin is now working out of NASA Headquarters in Washington, D.C. He will be the Chair of the Mission Management Team (MMT) for the mission, giving the final go to proceed for launch to Launch Director Charlie Blackwell-Thompson.
The SLS uses propulsion system elements similar to or derived from the Space Shuttle program and the sequence of events in the last few seconds before launch should be familiar to Shuttle watchers.
About six seconds before T-0, the four RS-25 Core Stage engines will start and throttle up to 100% of their original, 1970s rated power level.
Assuming the engines are healthy and the rest of the vehicle remains healthy, the two solid rocket boosters (SRBs) will ignite at T-0, lifting the vehicle off Launch Pad 39B at the Kennedy Space Center in Florida, as umbilical connections from the Mobile Launcher are cut and vehicle access arms swing away.
With liftoff, the Core Stage engines will throttle up to 109% power level to help the SRBs, which are producing their peak thrust in the first approximately twenty seconds, accelerate the vehicle up and away.
“Tower clear is around…seven, eight seconds into flight,” Sarafin noted, “then we hit Mach 1 about a minute into the flight, and we may do a throttle-down around the area of maximum aerodynamic pressure, or Max-Q. So about a minute into flight we may throttle the Core Stage engines.”
As with other ascent events and their timings, Sarafin explained why it remains to be determined whether the Core Stage engines will have a “throttle bucket” in the Max-Q region.
“It really gets into the atmospheric density [and] the actual mass of the vehicle,” Mr. Sarafin explained, “because mass and thrust need to be compared and we haven’t finished building the elements yet. So we’ll need to get an actual weighing [of the vehicle], we’ll need to understand the actual thrust-to-weight ratio of the vehicle.”
A Max-Q Core Stage engine throttle bucket would cover the time from about 60 seconds to 80-90 seconds after liftoff, but the actual timing and how far down the engine throttles might need to step (if at all) is to be determined.
Another area of flight where a short, shallow throttle bucket was discussed was in the pre-SRB separation time-frame to control loads on the SRB forward struts. Mr. Sarafin said whether this will be necessary is still to be determined.
The SLS Boosters burn for about 126 seconds and are jettisoned a little over two minutes into flight.
“So we’ve separated the boosters about two minutes and ten seconds into flight and then we jettison the spacecraft adapters, which are the panels that cover the [Orion] service module and the solar arrays,” Mr. Sarafin continued.
“[At] about three minutes, twenty seconds we’ll jettison those panels. And then twenty-five seconds later, the launch abort system gets jettisoned and that will expose the Orion capsule. So now the crew module and the service module are in their space config.
“About eight minutes, ten seconds or so, we hit MECO — Main Engine Cutoff — and we separate from the Core Stage.
“Now it’s the ICPS [that is in control] – the Interim Cryogenic Propulsion System – the upper stage that provides attitude control and [performs] maneuvers to circularize the orbit and perform the Trans-Lunar Injection.”
Mr. Sarafin also noted that the Core Stage engines will probably throttle back in the last several seconds of their burn to limit vehicle acceleration, but the timing will also depend on more precise determinations of the thrust-to-weight of the vehicle.
After Core Stage separation, both the spent stage and the mated Orion-ICPS stack will be in an elliptical Earth orbit with an apogee of about 975 nautical miles and a perigee of about 22 nautical miles.
At the first apogee, around forty-five to fifty minutes after liftoff, the ICPS will make its first burn to bring the perigee of the orbit up to 100 nautical miles.
Then around 90 minutes after launch as the stack completes its first orbit, the ICPS will perform a Trans-Lunar Injection (TLI) burn to send the vehicle towards the Moon.
“Between the time that we have Main Engine Cutoff and the time that we do TLI, we’re going to deploy the solar arrays on the service module,” Mr. Sarafin added. “We’ll be producing power and [Orion] will no longer be [using] batteries [for power].
“It’ll be producing its own power and charging the batteries; that process, on the long-side takes [about] 12 minutes to deploy the solar arrays. And then we’ll need to position them to an articulated position for the TLI burn because we’re going to put this big load into [the vehicle] with the RL-10 engine on the ICPS.
“So we put it in a load-safe position, that takes like a minute to position the arrays and then we do TLI.”
After TLI, Orion will separate from the ICPS and Mr. Sarafin explained the final work for the upper stage.
“The ICPS after separation will maneuver, out of plane, and do a disposal burn. After the disposal burn it will deploy thirteen cubesats; those cubesats will go on to perform their own science missions.”
(Images: Via NASA and L2 – including SLS renders from L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)
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