“Plan D for Outer Space” — NASA updates EM-2 mission baseline

by Philip Sloss

Coming off this year’s manifest changes to the early missions in NASA’s Exploration campaign, the outline and major parameters for the first crewed Orion flight were formally updated to reflect those big picture updates. Exploration Mission-2 is still a test flight to check out Orion with crew system upgrades and circumnavigate the Moon, but the early part of the mission was reworked to take into account the change in configuration of the Space Launch System (SLS) launch vehicle.

The SLS Boosters and Core Stage will leave its upper stage with Orion attached in a higher insertion orbit on EM-2. After a revised post-insertion sequence, the upper stage will take Orion and crew to an even higher and more elliptical orbit than earlier plans. Following separation from the upper stage, Orion will spend almost a day longer in Earth orbit than previously planned at the start of the mission before leaving Earth for a Lunar flyby.

Transferring higher SLS performance to higher MECO orbit, ICPS

The Exploration Mission-2 (EM-2) mission has always been the first Orion crewed flight, but the mission profile has gone through multiple revisions over the years, from a high-lunar orbit mission to a cislunar rendezvous with an asteroid redirect spacecraft to the current “hybrid triple” outline.  Even within that outline, the upper stage for the mission has gone back and forth between the Delta upper stage derivative Interim Cryogenic Propulsion Stage (ICPS) and the Exploration Upper Stage (EUS) that is still in development.

With early SLS launches moving back to the Block 1 vehicle configuration, the mission baseline was formally revised recently. “It was an update to switch from the EUS to ICPS, so in that switch because of the performance differences in the stages we had to re-optimize part of the mission around the ICPS,” Nujoud Merancy, NASA Exploration Mission Analysis Lead, explained.

“So this is the updated baseline for EM-2.”

In terms of launch and insertion, the first few minutes of EM-2 will look similar to the first SLS launch on the Exploration Mission-1 (EM-1) test flight of an uncrewed Orion spacecraft; however, in switching back to the Block 1 ICPS the new EM-2 baseline also changes some significant mission parameters from EM-1. The first change is to raise the high-end of the insertion orbit from 975 nautical miles to 1200.

In its Block 1 configuration the SLS Boosters and Core Stage will take a little over eight minutes to deliver the fully-loaded ICPS and its crewed Orion payload to an insertion orbit that is just below the velocity needed to stay in Earth orbit; this allows the empty Core Stage to safely breakup away from land and populated areas when it reenters the atmosphere.

Revised Exploration Mission-2 (EM-2) outline, as presented by NASA officials to the NASA Advisory Council (NAC) Human Exploration and Operations (HEO) Committee in early December. Credit: NASA.

“We had performance on the Core Stage, so did we want to raise the insertion off the Core to push that performance uphill,” Merancy said explaining the trade-off. “We did end up taking that option, it was a trade between a 975 nautical mile drop off — it drops off lower than that but the coasting apogee would be 975 — but we raised that up to 1200.”

“We have that performance in the Core Stage on EM-1 as well, but being the first flight and we didn’t need to push it uphill, we opted to keep that performance on the Core Stage for the first flight and it increases your engine-out capability on SLS,” she elaborated.

“For EM-2, the first stage would have been flown and because the ICPS and the in-space portion is more the performance driver, we’re opting to raise that Core Stage insertion so we can have more margin on ICPS, which gives us more launch opportunities for ICPS.”

Increasing the apogee of the vehicle at Core Stage Main Engine Cut Off (MECO) also provides some improvement to the overall period of days each month where Orion can reach the Moon on SLS.

“The elliptical parking orbit limits your launch opportunities in the month because you can only go when the Moon is in the direction of the apogee, so pushing performance on the ICPS increases the number of days,” she said. “I think basically we’re between eight to ten days with the difference pushing that uphill.”

First upper stage burn moved up

The second change reorders the sequence of post-insertion events after Orion and ICPS have separated from the Core Stage following MECO. Both the Orion/ICPS mated combo and the Core Stage are still in an orbit with a perigee or low point of around 20 nautical miles that the ICPS must raise to a higher altitude to stay in orbit.

On EM-1, the Perigee Raise Maneuver (PRM) is performed by ICPS when the vehicle reaches the apogee of the insertion orbit, about 40 minutes after liftoff and over thirty minutes after MECO, raising the perigee to 100 nautical miles. For EM-2, the PRM burn was moved up in front of Orion’s solar array deployment with ignition ten minutes after MECO, in part to provide a bigger window of time in between major ICPS burns for initial Orion checkouts.

“The apogee on EM-1 had been picked [for PRM] to maximize performance, but that puts it in sort of an awkward position for Orion because the solar array deploy occurs between MECO and PRM and then you have to park them for PRM and then you coast again,” Merancy explained. “So we really wanted to look at whether we could move that PRM earlier.”

SLS Block 1 Crew vehicle configuration. The EM-2 mission baseline has formally moved back from Block 1B Crew to Block 1 Crew, which will be used for both EM-1 and EM-2. Credit: NASA.

The PRM burn on EM-2 will raise the perigee and apogee to an orbit of 100 nautical miles by around 1450 nautical miles. Performing it earlier provides more time for Orion’s solar array deploy and initial spacecraft checkouts between the two major ICPS burns with Orion. The resultant orbit also has a slightly longer period, which provides a little more time before the Apogee Raise Burn (ARB) that ICPS does at the end of the first orbit when the vehicle is at the 100 nautical mile perigee.

“It de-optimizes the PRM so there’s a little bit of a performance hit, but it’s a much more I’ll say stable period then for Orion to deploy the solar arrays after PRM and perform checkout in a quiescent, coasting LEO (Low Earth Orbit). So it’s really an operational consideration to move it forward to make the rest of the Orion checkout more ops-friendly.”

The EM-2 mission profile now also baselines a single low-altitude Earth Orbit, which reduces MMOD risk. In the prior baseline when Orion was flying with EUS, it stayed in the low-altitude “parking” orbit for two revolutions; in that case, Orion and EUS were flying in a circular orbit around 100 nautical miles, which is largely below the altitudes with high MMOD risk.

“The MMOD environment starts at around 500 nautical miles from those satellite breakups that occurred,” Merancy explained. “When we were staging on EUS we can do a couple of orbits and there’s no real difference in MMOD risk; however with ICPS and the elliptical orbit, once the apogee gets above 500 nautical miles you have MMOD concerns.”

“So the other advantage to the early PRM is it boosts up that apogee, we have a slightly longer orbit,” she added. “We went through the whole thing with flight ops (operations), they went through and made sure they could finish Orion checkout, and so we are opting for the first rev (revolution) to reduce MMOD risk but there’s still sufficient time to perform the checkout needed prior to TLI, or in this case ARB.”

ICPS will do a full TLI burn with the uncrewed Orion on EM-1; for the first crewed Orion on EM-2, ICPS it will do an Apogee Raise Burn that takes Orion to the second orbit of the hybrid triple. The profile has Orion flying in three orbits: a low Earth orbit still attached to the booster upper stage, a high Earth orbit (HEO) by itself that was going to last for about one day, and a single solo lap around the Moon before returning to splashdown on Earth. NASA has been considering variations of this mission for EM-2 going back several years.

“The ICPS is capable of doing the full TLI burn, we’re only utilizing part of it,” Merancy said. “Both are well within what ICPS can do, that’s what it’s doing on EM-1.”

Increasing High Earth Orbit altitude and duration

The third change uses the ARB to burn to an even higher apogee than previous planned, increasing the duration of the High Earth Orbit from 24 hours to 42 hours by almost doubling an already very high apogee. “The HEO is around 205 by 59260 nautical miles,” Merancy noted.

“They’ll have a really good view from the HEO and then they get to go by the Moon,” she said of the future EM-2 crew.

The long orbit allows an extended evaluation of the performance of new spacecraft systems close to home, especially the crew systems that will be making their first flight.  “The whole point of the HEO is to provide a checkout near the Earth where you don’t need big burns to come home, we’re one to two days from home,” Merancy said.

The Environmental Control and Life Support System (ECLSS), crew displays, and other crew systems will be making their debut in Orion on EM-2.

Orion will still do a Trans-Lunar Injection (TLI) burn on its own that provides the remaining change in velocity or delta-V needed to go from the HEO to a Lunar flyby, but the bigger ARB burn by the ICPS at the end of the first orbit reduces the delta-V for Orion’s TLI. “Raising the apogee [means] less demands on Orion’s prop (propellant) system for the upcoming TLI, which leaves more performance should you need an abort post-TLI.”

ICPS fires for a second time to carry Orion away from Earth. On EM-2, Orion’s solar arrays will be deployed between the first two upper stage burns. Credit: Nathan Koga for NSF/L2.

“It’s about a thousand pounds of Orion prop,” Merancy said about the difference in orbits. “So there is an advantage to having that be a bigger orbit, in that then Orion has more prop available post-TLI should other emergencies occur.”

It would have been theoretically possible to pick an orbit with a period between 24 and 42 hours, but practically Merancy said they would not do the TLI burn in the middle of the crew’s sleep period. “We could have gone up to around 27 hours and then you can’t do the orbit period because TLI would have been in crew sleep. Forty-two hours was sort of the next morning for the crew day.”

The Orion TLI burn would be at perigee of the HEO, which would be at the end of that orbit’s period.

“So there is a method to the madness there,” she added. “We got to 42 hours so we can align TLI with the crew being awake and it’s performance that ICPS can get to and leaves more prop on Orion, so all of those factors sort of optimized that orbit period.”

“And we found that the aborts from the HEO were roughly equivalent between the 24 and 42 [hour orbits], so there wasn’t a reason to stay in the 24, comparatively.”

Circumlunar timeline mostly unchanged

After the Orion TLI burn, the rest of the EM-2 mission remains as previously baselined, with the round trip to the Moon taking four days out and four days back. “The Orion in-space portion of the mission looks roughly the same as before, it’s just the HEO that’s different,” Merancy said.“So the TLI, the Lunar flyby, Orion is targeting the same as it was before, it’s just added hours in HEO before it goes there. So it was a nine-day mission before, now it’s a ten-day mission.”

Orion and crew will swing around the Moon at an altitude of about 4800 nautical miles.

Revisiting cubesats

After dropping Orion off in its high, elliptical Earth orbit early on the first day of the mission, ICPS will make its own TLI burn using its flyby of the Moon to head into a heliocentric disposal orbit. “I think we’re in the like 20 to 30 minute range, but Orion will do the separation, do a burn to gather distance from ICPS, and then ICPS will do a disposal burn,” Merancy explained.

“We still baselined a heliocentric disposal for ICPS, so it essentially completes TLI after Orion is separated and will continue on to its disposal. It just has to wait enough time for Orion get safe separation.”

Although the major outlines of the mission are now rebaselined, it will continue to be iteratively refined with subsequent changes generally of smaller degrees. One of the things that will be looked at now is whether there is any secondary payload capability.

Orion Stage Adapter for EM-1 in the Space Station Processing Facility following arrival at Kennedy Space Center in April. The cubesat dispensers can be seen around the top circumference. Following the rebaselining of the basics of the EM-2 mission, NASA will re-examine whether secondary payloads like this can be accommodated. Credit: NASA/Glenn Benson.

On EM-1 thirteen cubesats are being flown in an adapter that sits on top of the ICPS; they will be deployed from ICPS after Orion has separated from the upper stage. Merancy said they set aside some of these questions while focusing on baselining the mission for its main objectives.

“When we were trying to baseline it we just sort of ground ruled out a lot of those secondary questions, so the baseline doesn’t show cubesats,” she said. “That is another item we’re working now — do we want to now basically add cubesats or use performance in other ways.”

“It’s sort of an iterative process, we’ve figured out that much and now we’re going to look to see do we want to add cubesats or not. We’re working with the Science office and SLS on that question, actually.”

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