Space Sunday: looking at the Artemis HLS vehicles

As is well-known, the US hopes to make a return to the surface of the Moon with astronauts in 2028. This has been, and remains, a questionable time frame for a number of reasons. As I recently reported, NASA’s own Office of Inspector General (OIG) issued a report indicating the new xEVA suits Axiom Space is developing for use on the International Space Station (ISS) and in lunar missions might not be ready for lunar operations until 2031.

Another bump in the road for 2028 is the availability of a vehicle to actually get crews from lunar orbit down to the surface of the Moon and back to orbit again. Again as I’ve oft mentioned, two companies are in the running to supply this vehicle – called the Human Landing System (HLS) in NASA parlance: SpaceX and Blue Origin. The two systems are very different to one another, and each has built-in complexities, some of which are down to NASA’s decision making, others are due to the choices being made by the two companies.

The biggest NASA-defined challenge is that both HLS vehicle must utilise cryogenic propulsion using either liquid oxygen and liquid hydrogen (Blue Origin) or liquid oxygen and liquid methane (SpaceX). The problem here is twofold: mass, and the fact that cryogenic propellants, as the name indicates, require very low temperatures and relatively large volumes in order function, otherwise they will simply (and dangerously) “boil-off”.

The mass of the propellants means that neither HLS system can be launched with the propellant load needed to reach the Moon, enter orbit and then deliver a crew to the surface of the Moon and back to orbit. They have to launched sans propellants and “refuelled” in space. This is turn brings up two issues.

The first is that no-one has ever performed the large-scale (100+ tonnes) transfer of cryogenic propellants in zero gravity (“refuelling” of the International Space Station is commonplace, but uses hypergolic propellants, which are completely different in nature and handling). Thus, both companies must develop and test mechanisms for the transfer of propellants from one vehicle (the “refuelling tanker(s)”) to another, and test then well before 2028 and Artemis 4.

The problem of boil-off is potentially more significant. As noted, cryogenics require extremely low temperatures if they are to remain liquid. Should they rise above the required temperatures they will sublimate to gas (boil off), drastically increasing their volume. Thus, if some of this gaseous propellant is not vented from the tanks, it could end up rupturing them completely, destroying the vehicle. Hence why rockets using cryogenics are seen venting clouds of propellants between fuelling and launch.

In space, any vehicle using cryogenics will spend the majority of its time in temperatures of around 121ºC. Even with tank insulation, this means there is likely to be significant boil off, meaning one of three things (or a possible combination of two of them):

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  The excess gases must be vented to space (and the inevitable thrust they cause countered), which in turn will require further propellants to offset such loss prior to the vehicle leaving orbit.

• Or, the vehicle must include some means of capturing the gas, and refrigerating back down and cycling it back to the tanks – all of which increases vehicle complexity and mass.
• Or the vehicle must be equipped with some passive means of keeping the propellants as close as possible to their desired liquid temperatures, minimising boil-off, again potentially increasing vehicle mass and complexity.

Thus, both SpaceX and Blue Origin must both find a way of minimising this propellant loss. In the case of SpaceX, this appears to be primarily in the form of loading as much in the way of propellants as possible into the vehicle so that the overall venting does not impact the vehicle’s capabilities; hence the estimates that 8-16 Starship “refuelling” launches might be required for the SpaceX HLS to carry out its mission.

Rather than relying on a massive HLS vehicle with huge propellant tanks, Blue Origin have opted for a much smaller, lighter vehicle (45 tonnes when loaded with propellants compared to the approx. 238 tonnes of the SpaceX HLS when loaded with propellants). However, it needs to be supported by an additional vehicle: Cislunar Transporter.

The latter is a combination of propellant tanks (which will incorporate some form of “zero boil-off” capability Blue Origin has apparently developed) and space-going tug. Following launch, it is designed to be refuelled by a number of New Glenn launches with around 100 tonnes of propellant. It will then dock with the Blue Origin HLS, once launched, and deliver it to lunar orbit, transferring some of its propellants to the lander’s own tanks so it can carry lout its mission.

In addition, and unlike the SpaceX HLS, the Cislunar Transporter will be capable of returning to Earth, where it can be loaded with further propellants and thus service additional flights of the Blue Origin HLS to / from the lunar surface.

But even with smaller, lower-mass vehicles, Blue Origin faces pretty much the same challenges as SpaceX in terms of propellant loading the storage. So, leaving these issues aside, how is the general development of both systems going and which is likely to get the prestige of returning astronauts to the surface of the Moon first?

On paper, both companies appear to be pretty neck-and-neck in terms of vehicle development. SpaceX for example, has completed around 50 target milestones with its Starship-derived HLS. These include land testing of an airlock test article; the development (with NASA) of an elevator system to be deployed when the vehicle is on the Moon in order to get crews two and from their facilities on the vehicle (roughly 45 metres above the lunar surface) and “ground level”; a “full test” of the life support systems; testing the Raptor engine’s ability to re-light in a wide range of temperature environments; development and testing of the SpaceX-Orion docking system and the vehicle’s avionics, flight and navigation software; mock-ups and testing of pre-launch ground support infrastructure, etc.

Blue Origin has also completed a similar number of tests on both software and hardware, including vacuum testing of the BE-7 engine to be used by their HLS, their cargo lander and the Cislunar Transporter. However, their testing is potentially ahead of SpaceX in some areas, and liable to quickly move ahead in others.

For example, where SpaceX has been testing its airlock design on land, Blue Origin has completed testing their airlock system within NASA’s Neutral Buoyancy Laboratory at the Johnson Space Centre. This has allowed space suited astronauts to test the airlock in similar circumstances to those they will experience on the Moon.

As well as this, the company has an integrated, full-scale mock-up of their HLS vehicle. This has allowed Blue Origin and NASA to collaborate directly on the design of the vehicle, including accessibility to critical systems, placement and operation of manual flight control systems, data displays, life-support systems, and the layout of essential crew facilities (toilet, food preparation air, food and beverage storage, personal spaces, etc.), in readiness for the manufacture of the initial HLS craft.

Further, later this year Blue Origin is due to launch the first of its Blue Moon Mark 1 cargo landers to the Moon. Whilst much smaller than the Blue Moon Mark 2 HLS, and only capable of delivering up to 3 tonnes to the Moon’s surface (no “refuelling” required), Blue Moon Mark 1 uses the same automated flight control, space navigation, landing guidance, data communications and propulsion management software as will be used on the Blue Moon Mark 2 HLS. Thus this first Mark 1 mission, featuring the lander Endurance, will be both a practical mission delivering two NASA experiments to the lunar surface and serve as a “pathfinder” test of these automated systems and the capabilities of the BE-7 engine.

If successful, Endurance will be followed in early-to-mid 2027 by a second cargo mission to deliver NASA’s cancelled-then-resurrected VIPER lunar rover mission to the Moon. Assuming either or both of these missions perform as expected throughout, they will pretty much indicate the flight software and BE-7 are fit-for-use within the Blue Moon HLS.

Currently, Endurance is at Blue Origin’s facilities at Kennedy Space Centre, Florida, where it will be integrated with its launch vehicle. Prior to arriving at KSC, Endurance had undergone extensive thermal vacuum chamber testing at NASA’s Johnson Space Centre, exposed the thermal and pressure environments it will face during its mission, and testing its overall readiness to fly.

The commonality of systems is also seen with the Cislunar Transporter. This was originally going to be developed by Lockheed Martin, but is now an in-house project at Blue Origin. This means that as well as utilising the same BE-7 engine, the overall design of the Transporter borrows heavily from the New Glenn upper stage, greatly reducing its development cycle and allowing it to use the Tanks and engine mounts, etc., from the New Glenn upper stage, greatly simplifying its design whilst enabling it to be manufactured on the same production line.

Like Endurance, an initial Cislunar Transporter prototype spent mid-2024 undergoing extensive vacuum and thermal testing at a facility at Edwards Air Force Base, California. As a result, production of the Transporter is due to start at Blue Origin’s primary plant at Kennedy Space Centre.

It is this progress within Blue Origin, countered by a perceived lack of significant progress by SpaceX on their HLS through 2025, which led NASA’s former Administrator, Sean Duffy to announce the first Artemis crewed landing on the Moon would not be an SpaceX exclusive, but would feature whichever HLS system was fit-for-purpose and ready for a 2028 launch; a decision since confirmed by the current Administrator, Jared Isaacman.

Under Isaacman’s leadership, there is to be a crewed Earth-orbital test of the HLS vehicles in 2027 under the Artemis 3 banner. This test could be with both HLS vehicles, if both are ready in time, or by whichever is available, and will be used in a final determination as to which vehicle Artemis 4 will use.

However, whether Blue Origin or SpaceX will be in position to meet a 2027 HLS test flight is entirely open to debate. Both companies have already asked NASA to push back the test flight from mid-2027 to late 2027, which the agency has done, but Blue Origin remains somewhat tight-lipped about the overall development status of Blue Moon Mk2 and Cislunar Transporter.

Meanwhile, in promising to accelerate its HLS development, SpaceX has set itself some hefty goals for 2026, especially considering we’re fast closing in on being half-way through the year. These include:

• Actually getting a Starship to orbit.
• Demonstrating Starship can reach orbit with a “useful payload” – thus far, the “version 1” and “version 2” variants have either sacrificed payload lift capability in favour of just getting to sub-orbital velocity, or sacrificed the ability to achieve orbit in favour of carrying a modest payload – Starlink demonstrators – to sub-orbital velocity. Thus, hopes are now pinned on “version 3”, due to make it s first launch attempt sometime in the next month.
• Carry out an on-orbit cryogenic refuelling mission.
• Undertake a “long duration” Starship flight. This was initially defined by the SpaceX CEO as a mission to Mars, now all but abandoned for 2026 (and likely the foreseeable future), leaving the context of the flight uncertain.

There is also the matter of actually recovering Starship vehicles as they return to Earth. This is an essential part of the equation for SpaceX, as the company has indicated it will pay for all of the HLS “refuelling” launches, estimated at up to US $400 million a throw if an entirely new vehicle is used for each if these launches.

Given all that has to be achieved in just 18 months, it may yet ben that the Artemis 3 mission might be further pushed back. If so, then Artemis 4 will likely not occur until 2029 at the earliest (assuming the Axiom xEVA space suits are ready by then). If this happens, then the door to which HLS system is used would again be thrown wide open.

However, there are two additional factors outside of development time frames and general vehicle readiness which could play into Blue Origin’s hands, at least as far as the Artemis 4 mission is concerned: a) vehicle size and mass distribution, b) risk mitigation.

The SpaceX Starship HLS is 52 metres tall and 10 metres in diameter, with a relatively narrow landing leg spread compared to its height. When it comes to landing on the Moon, with the majority of its propellant spent, it also has a very high centre of gravity due to the engines and propulsion systems, crew facilities, power and life support systems, etc., all located in the upper third of the vehicle. Blue Moon Mk2 is only 15.3 metres tall and its centre of mass is in is lower third. It also follows the Apollo lunar lander approach of having a broad spread with its landing legs for increased stability and support.

Whilst it is essential all Artemis missions to the Moon minimise the risks faced by their crews, given the “first time” nature of Artemis 4, the use of Blue Origin Mk2 might be seen as the better choice of lander, simply because its squat, low centre of mass design minimises the risk of it toppling over when landing on a unknown surface. The same cannot be said with certainty for the SpaceX design, where even a minor depression directly under one of its landing legs could result in disaster. As such, use of this vehicle might be better suited until after “eyes on the ground” have been able to more accurately determine relatively “safe” areas where it might land.

So, which vehicle do I think will get to fly with Artemis 4? Allowing for the aforementioned caveat of missions being pushed back and assuming SpaceX don’t find a way of testing an uncrewed version of their vehicle to better assess the risk of toppling-on-landing, I do tend to lean towards Blue Origin. While they face challenges – some of them the same as SpaceX, as noted – their approach just comes across as cleaner, more fit-for-purpose. But then, I don’t work for NASA.