NASA studying practice rendezvous options for Artemis 2 Orion

by Philip Sloss

NASA is studying the addition of an Orion rendezvous demonstration to the Artemis 2 test flight to reduce overall mission risks to the subsequent Artemis 3 lunar landing from first-time operations. The Human Exploration and Operations Mission Directorate (HEOMD) is working with its Artemis Programs to evaluate some of the trade-offs from a range of options for an early rendezvous test for the spacecraft on its first crewed mission, such as what the rendezvous target would be and where in the mission the demonstration would take place.

Taking another look at Artemis 2 mission objectives is a part of a recent re-examination of how to meet the goal of landing of U.S. astronauts on the Moon again by the end of 2024. No decisions have been made yet, but favored options would stage the demonstrations early in Artemis 2 with minimal impact to existing, baselined objectives.

Early concepts for the Artemis 3 lunar landing mission included a complex sequence of Gateway, Human Landing System (HLS), and Orion launches and dockings, with multiple mission-critical activities being executed for the first time. HEOMD is looking to reduce mission dependencies, spacecraft complexities, and first-flight operations from Artemis 3.

Moving rendezvous and proximity operations (RPO) development up to be tested on Artemis 2 would provide additional opportunities to test the system and to incorporate lessons learned from the demonstration before it is essential to mission success on Artemis 3.

Considering rendezvous demonstration for Artemis 2 crewed test flight

NASA plans three Artemis missions to achieve the goal announced in March, 2019, of landing two U.S. astronauts on the Moon by the end of 2024. Two test flights of the agency’s Orion spacecraft and Space Launch System (SLS) rocket would precede the landing that would culminate the third mission.

Currently targeting launch late next year, Artemis 1 is a four to six week long uncrewed test that includes the first SLS launch which will send Orion on its first solo flight into a Distant Retrograde Orbit (DRO) of the Moon. Artemis 2 is currently baselined as a week and a half long crewed test flight for Orion.

Likely to fly no earlier than 2023, Orion would stay in Earth orbit on Artemis 2 for the first forty-two hours of the mission to fully demonstrate the capabilities of the spacecraft’s environmental control and life support system (ECLSS) that will be flying for the first time with a crew of four astronauts. A lunar flyby would follow one short and one long orbit around Earth.

Credit: NASA.

(Photo Caption: Planned or proposed changes to Artemis Phase 1 are highlighted in green balloons in a slide from Douglas Loverro’s presentation to the NASA Advisory Council’s Human Exploration and Operations Committee on May 13. Loverro was NASA HEOMD Associate Administrator at the time; one of the action items from a programmatic assessment was to try to better balance mission risks across all the flights manifested to land a U.S. crew on the Moon by the end of 2024.)

Those two test flights would be followed by several launches for the Artemis 3 lunar landing. The concept of the mission last year included six launches of Gateway modules, Human Landing System (HLS) elements, and Orion spacecraft with crew.

The complicated sequence of Artemis 3 events included separate rendezvous and docking of all the elements in a cislunar halo orbit as a prerequisite for starting a crewed descent to the lunar surface in the lander. Near the start of Douglas Loverro’s short tenure as Associate Administrator of HEOMD, a broad programmatic reassessment of the 2024 Moon landing plan was also started; one of the efforts was to look at how to maximize the overall chances of success for the three mission sequence.

“We had the opportunity to go off and look at a number of options,” Marshall Smith, NASA’s Director of Human Lunar Exploration Programs, explained in a June 17 interview. “[We] left it open, [to look at] ‘how can we balance risk and how can balance risk to Artemis 3?’ If we could take some things we could do on Artemis 3 and move them to an earlier mission what would be involved in that.”

“One of the options is to do some of the rendezvous, proximity operations, and docking maneuvers early.” Changes to the Artemis 1 mission baseline were considered and have been ruled out.

Artemis 2 mission objectives and trade-offs

Rendezvous, Proximity Operations, and Docking (RPOD) capabilities for Orion were planned to be introduced on the flight following Artemis 2, which would demonstrate the spacecraft’s initial, crewed, operating capability. Orion’s Guidance, Navigation, and Control (GNC) software would be expanded with RPOD functionality, a package of rendezvous sensors would be added to the spacecraft, and a docking system would be integrated to allow Orion to link up with other vehicles.

Rendezvous operations for Orion can be divided into far range, mid-range, and close-range segments. Orion and crew would proceed through those segments of the rendezvous phase of the mission as it closed in for docking with the Gateway in its planned cislunar Near-Rectilinear Halo Orbit (NRHO).

The spacecraft could also perform proximity operations and docking in a close range segment to extract a secondary payload carried by an SLS Exploration Upper Stage.

The trade studies that Smith is overseeing are still underway, looking at different combinations of options and settings between orbits of the Earth or Moon and between targets from small satellites to Gateway modules to upper stages.

Credit: NASA.

(Photo Caption: Some of the flight test options that HEOMD was looking at for possible inclusion in missions that would fly before launches of Artemis 3 lunar landing mission hardware. In addition to looking at the trade-off of risks for benefits for the two Orion missions planned before Artemis 3, NASA is also looking at Human Landing System (HLS) flight test benefits against the limited time remaining before the 2024 deadline.)

“We haven’t made a final decision on how we’re going to exactly do this activity, but I think right now we’re on the track of going ahead and doing some kind of rendezvous and prox. ops (proximity operations) activities to give us some understanding about Orion’s manual control capability and those types of things to the crew as well as looking at some of the software work that we have to go do on Artemis 2,” Smith said.

“The options on the table were obviously you could do it with a CubeSat or a smallsat we carry with us or we launch one separately. Or maybe with the Gateway Logistics Services systems or even with the ICPS, the Interim Cryo[genic] Propulsion Stage that we carry with us as well.”

“We even looked at some options about do we continue with the highly elliptical orbit design of the mission as compared to maybe changing it and doing some other type of mission to capture some additional options there,” he added.

The studies are considering the proverbial forest and the trees and there are trade-offs to each of the options. “It turns out in the short term and from a cost and schedule perspective we can accomplish all of our goals by using something that we carry with us like an ICPS and doing a rendezvous and docking activity with that,” Smith said.

The scope of a rendezvous activity is also a part of the study, how far away to start the demonstration and how close to approach the target. “Now we would not dock with that, we would actually do the rendezvous and prox. ops,” Smith noted.

“We would do far-field and maybe a mid-field [rendezvous], depending on how close. We’ll have to talk about what we would do with the system and how far we would get [from] that [and] how close we would get to that.”

Credit: NASA.

(Photo Caption: Composite of graphics from a NASA presentation on Guidance Navigation and Control for Orion RPOD operations. Current plans call for Orion to rendezvous and dock with the Gateway in NRHO for lunar landing missions, and the approach is divided into far, mid, and close-range segments. For Artemis 3, it is now expected that Orion will dock directly to the HLS spacecraft in either NRHO or another lunar/cislunar staging orbit.)

Docking is not a favored option because it carries schedule risk that looks to outweigh the benefit of performing it on Artemis 2. “If we were going to have to put the whole docking system in on Orion 2 that would have been a pretty big schedule hit because Orion is kind of past the point where it would have been easy to integrate,” Smith said. “It was going to undo some work to put that in.”

“Plus we’re pretty comfortable with the docking because it’s based on the standard we’re using on ISS (International Space Station) and we’re pretty comfortable [with] the actual docking part of it.” The recent docking of the SpaceX Dragon Commercial Crew vehicle on Demonstration Mission-2 was the second docking of the NASA Docking System (NDS) on the ISS, with both docking and undocking demonstrated during the Dragon’s uncrewed Demonstration Mission-1 last year.

The studies also currently favor performing the RPO demo during the early part of the mission in Earth orbit. “I think we’re pretty set on staying within the high Earth orbit (HEO) and that’s basically from a crew risk perspective,” Smith explained.

“This will be the first time that Orion has flown with crew and it is a test flight and by staying on the initial high Earth orbit we’re enabling ourselves the ability to come back to Earth relatively quickly should we have an issue with the ECLSS system or something else in the crew systems.”

On Artemis 2, the SLS Boosters and Core Stage will leave the Orion and ICPS in an orbit with an apogee of about 1200 nautical miles and a perigee of about 20 nautical miles. ICPS will first make a short, perigee-raising maneuver (PRM), followed by a long, apogee-raising burn (ARB) at the end of the first orbit. Following the ARB Orion will separate from ICPS in an orbit with an apogee of over 59000 nautical miles and a perigee of about 200 nautical miles, an orbit that will take 42 hours to complete.

“We’re doing a high Earth orbit while the crew is close to home and we can check out systems, test the ECLSS, the life support systems out very thoroughly, exercise with them, those types of things,” Smith explained. “Then once we’re comfortable with it we can move on.”

The HEO parameters were changed when the mission was rebaselined in 2018 from the EUS upper stage back to the ICPS. The apogee was significantly increased by extending the ICPS ARB, which extends the orbital period from 24-hours long to 42 hours.

The longer ICPS apogee raise burn reduced the amount of propellant needed by Orion to perform a TLI burn on its own following the high Earth orbit, leaving more propellant for the crewed spacecraft to handle abort scenarios on what will be its first flight with astronauts.

“You do the rendezvous and prox. ops activity during the course of that HEO orbit, that makes more sense. If we wanted to change to go the Gateway orbit, the NRHO orbit, the near-rectilinear halo orbit, then we would have to give up on that early checkout orbit that we have and I’m probably not leaning that way right now. I don’t think we’d probably go that direction.”

Credit: Mack Crawford for NSF.

(Photo Caption: The mated Orion spacecraft-ICPS upper stage combo orbits Earth. The upper stage will typically make a perigee raise burn and a trans-lunar injection (TLI) burn on Orion lunar missions. For Artemis 2 the stage will stop short of delivering Orion on a trajectory to the Moon, leaving the crewed spacecraft in a high Earth orbit. The ICPS would then make a third burn with residual propellant, either completing a TLI burn without Orion for eventual disposal in a solar orbit or a deorbiting burn to re-enter Earth’s atmosphere for an ocean disposal.)

The target of the proposed rendezvous exercises on Artemis 2 was also part of the studies; bringing one along for the ride seems to be more advantageous in part because of the high Earth orbit where Orion will be for nearly the first two days of the mission. Staging potential targets such as the Gateway or the Dragon XL Gateway logistics spacecraft in the highly elliptical orbit for Artemis 2 is feasible, but dual-launch Earth orbit rendezvous in the 42-hour long orbit would be challenging.

“We figured out through our analysis and studies that it was feasible but very, very difficult, with very limited schedule margin to do because of the way that the orbits and the orbital dynamics work and the spacing of when you have to launch and how you put those two together,” Smith explained when first discussing Artemis mission trades in a mid-May presentation to the Human Exploration and Operations Committee of the NASA Advisory Council. “It was very difficult to do.”

One of the possibilities that can be brought along with Orion for the ride is the rocket that is giving Orion the ride up to high Earth orbit, the ICPS. “There are some things that we have to work out dealing with the ICPS itself,” Smith explained.

“There are certain things that it has to do to maintain its thermal load properly, so it’s always venting and doing some things, too, so we’re working through that.” After Orion separated from the ICPS and got a safe enough distance away, the current mission baselined the stage performing a third burn that would accelerate the stage and possibly some small CubeSats towards the Moon.

A trans-lunar injection (TLI) burn would aim the stage at a lunar flyby a few days later that would slingshot it into a heliocentric graveyard orbit. The TLI burn for heliocentric disposal would be performed relatively soon after Orion separates because the stage has a limited amount of its own consumables, such as attitude control propellant and power supplies.

“It might change our disposal options that’s one of the things that we’re looking at,” Smith noted. “The other part of it is I think we can get the amount of work done within the life expectancy of the ICPS. We just have to make sure we dispose of it properly.”

Credit: NASA.

(Photo Caption: The three orbit “hybrid triple” mission in design reference form in 2016.  Aspects of the mission such as upper stages, secondary payloads, orbit parameters, and duration have changed for Artemis 2 going back to its Exploration Mission-2 origins, but the three orbits are still the heart of the flight. Orion would make one low Earth orbit, one high Earth orbit over one day long, and one loop around the Moon that would take about a week to complete. If approved, rendezvous demonstrations would occur during the HEO following Orion separation from the ICPS.)

If the ICPS is kept in the high Earth orbit for the life of most or all of its propellant, power, and other consumables to participate in the RPO demonstration instead, an Earth ocean disposal is an alternative to solar orbit disposal.

Staging the RPO demonstrations in high Earth orbit with a “co-manifested” target would have the least impact on the existing mission objectives, which are centered on demonstrating the new ECLSS and crew systems that will fly on the Artemis 2 Orion spacecraft for the first time. The rendezvous demonstrations can be carried out during the first flight day following separation from the ICPS.

“We launch, we do an apogee raise burn, and then what will happen is the ICPS doesn’t do a full TLI, it does a partial burn, [and] puts us into this 42-hour [long] orbit,” Smith explained. “After that burn, we would separate, we would do the RPO and manual control work, and then we would then put the ICPS into a safe disposal trajectory.”

“At that point, the Orion would continue doing the rest of its work in that 42-hour HEO orbit and then if we’re good to go on to the Moon to do the flyby then we’ll do a delta burn with the Orion to give it the extra delta-V (velocity) it needs to go to the Moon and then we’ll do the flyby and return back to the coast of San Diego.”

NASA has not yet indicated when a final decision on whether or not changes to the Artemis 2 mission would be made.

Impacts for Orion Program

While the options for the different changes to the mission plan are studied, Smith is also working with the Orion Program to evaluate accelerating software and hardware development originally planned to debut on Artemis 3. “We’ve asked Orion to go look and come back to us and let us know what those challenges are,” he said.

“It is accelerating work, but the point is we want to shift work from Artemis 3 to Artemis 2, so that’s going to be a natural outcome is you’re going to have to do some additional work to Orion and to the software system, again that’s kind of the point.”

In addition to advancing upgrades to the GNC software for rendezvous and proximity operations, some modifications to the Artemis 2 Orion vehicle would also enable a more thorough demonstration of capabilities. An early, quick look assessment was favorable enough to continue into a deeper evaluation.

Credit: NASA.

(Photo Caption: Orion RPOD state machine diagram. Some of the proposals for demonstrations that could be added to Artemis 2 could exercise portions of far-range, mid-range, and/or close-range rendezvous and proximity operations. One proposal would be for the flight crew to manually perform parts of an RPO without making significant modifications to the spacecraft hardware or flight software, which could provide useful data on Orion handling qualities.)

“Of course we’d have to accelerate that work like I said,” Smith noted. “It’s going to take additional funding to do that work, so we’ll have to work through that if we decided to go down that path.”

“I think we can handle moving the software work up, things like that. The initial stuff that I’ve seen is that we think we can do most of the mods fairly easy I’ve been told that we’d need to go do, except for docking.”

“Putting the actual docking ring on is a pretty big deal,” he added. “It could be done but it’s maybe not something that we’d want to do.”

The study is also looking at demonstrations of Orion’s manual control by the Artemis 2 crew under rendezvous and/or proximity operations conditions, which could form the basis for a smaller set of tests. “We haven’t made a final decision on what we’re doing but there are even options all the way down to making no mods to Orion and just doing manual approaches to get handling qualities out of it, so that’s on the table as well,” Smith said.

“That would be a manual type [demonstration] where they’re turned around, they’re looking at a target, using the ICPS as a visual target and doing those types of work. Which is advantageous, too.”

Lead image credit: Mack Crawford for NSF.

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