WASHINGTON — A study by a space propulsion company concludes that a human return to the moon by 2024 will require minimizing the launches needed for the lunar lander and also using storable, rather than cryogenic, propellants.

Aerojet Rocketdyne was one of 11 companies that received contracts in May 2019 from NASA as part of a Next Space Technologies for Exploration Partnerships, or NextSTEP, program to perform initial design studies of lunar lander concepts. Aerojet’s contract focused on the transfer module, which would transport the lunar module’s ascent and descent stages from the lunar Gateway to a low lunar orbit.

“But to do that, we obviously needed to study the overall architecture, so you knew the best way to fit the transfer vehicle element into the overall architecture,” Tom Martin, director of business development at Aerojet Rocketdyne, said in a March 20 interview.

That led the company to perform an extensive analysis of potential ways to get a lander to the moon. Aerojet completed that work and provided the results to NASA at the end of last year, and Tim Kokan, principal engineer at the company, presented the study at a Jan. 30 meeting of the Future In-Space Operations group.

The study looked at more than 326,000 different potential architectures, using varying combinations of launch vehicles, module designs and propulsion options. Aerojet used an approach called “utility analysis” to score the performance of each architecture using various technical and cost criteria, then picked 21 architectures for further study.

“The highest utility scoring configurations are the two-element configurations,” Kokan concluded, with just ascent and descent stages and without a transfer stage. In those scenarios, the heavier descent stage would launch on a Space Launch System Block 1B rocket, with its Exploration Upper Stage (EUS), and the ascent stage on a commercial launch vehicle, such as a Falcon Heavy or Vulcan Heavy.

Another issue, Martin said in the interview, was that lander propulsion systems that relied on liquid hydrogen as a fuel raised issues because there was “a whole lot of risk” about the maturity of cryogenic fluid management technologies.

“If you want to pull off 2024, we really think you need to base it on storable propellant solutions for the ascent and descent elements, and you need to have SLS in the architecture to lift at least the descent element,” Martin said.

Since completing the study for NASA, he said Aerojet has continued to refine the architectures. “The philosophy is, with all the things that need to happen for 2024, you really need to minimize new technology development, because that’s a schedule and programmatic risk, and you really need to reduce the complexity of the architecture,” he said. That latter issue involves reducing the number of “mission-critical events” like launches and dockings.

While the study as presented in January had two-element solutions involving SLS and commercial launch vehicles with the highest scores, Martin said the company has looked at launching an integrated lander on a single SLS.

“Our conclusion that the best way to achieve the 2024 mission on schedule was to go to the [near-rectilinear halo orbit] with an ascent element and descent element both launched on the cargo EUS version of SLS, and launch the crew up on SLS/Orion,” he said. “When you look at the number of mission-critical events and when you look at the total architecture cost, that ended up being the best solution.”

That approach would use the same orbit as the lunar Gateway, but would not require the Gateway, with Orion docking with the lander directly. “In 2024 you could do the rendezvous without Gateway being there,” he said. “The architecture is set up to support Gateway when it becomes available.”

That approach appears to be the one that NASA is favoring, based on public comments in recent weeks by agency officials. At a March 13 meeting of the NASA Advisory Council’s science committee, Doug Loverro, associate administrator for human exploration and operations, said he was “taking Gateway out of the critical path” for a 2024 lunar landing, but would develop the Gateway at a slower pace for supporting later missions.

He also appeared to shift away from the three-element approach to lunar lander development. “Program risk is driven by which things haven’t you done in space before that you would now have to do in this mission,” he said, specifically mentioning the three-element lander concept. “We’ve never done that before, so we’d like to try to avoid doing things we’ve never done before.”

At least one company bidding on NASA’s Human Landing System (HLS) program has proposed an integrated lander. Boeing said in November it submitted a bid to NASA for an integrated lander that would launch on an SLS. By contrast, the “national team” led by Blue Origin that includes Draper, Lockheed Martin and Northrop Grumman offered a three-element design that could be launched on a range of vehicles.

Companies like Blue Origin have advocated the use of cryogenic propellants like liquid oxygen and liquid hydrogen because they could eventually be sourced from the moon, if future missions find it feasible to mine water ice deposits at the lunar poles. Martin said such systems could make sense later, but only after infrastructure is in place on the Moon to turn that water ice into propellants.

“We really need to focus on how we enable longer-duration surface operations,” he said, such as power, transportation and other logistics on the lunar surface. “From the perspective of in-situ propellant production on the surface, that’s probably the step after we have a sustained presence on the surface.”

Martin declined to comment on what proposal or proposals Aerojet was involved with. NASA previously said it plans to announce initial awards for the HLS program in late March or early April.

Martin acknowledged that, while Aerojet has cryogenic engines like the RS-25 and RL10 that will be used on SLS, it doesn’t have any off-the-shelf storable propellant engines that could be used for a lunar lander. “We have multiple products in our inventory that are fairly mature in terms of their development,” he said. “We’re looking at how we can bring those systems forward very quickly.”

One option, he said, would be to use a version of the engine that powers the Orion spacecraft’s service module, which for early missions of that spacecraft will involve shuttle-era Orbital Maneuvering System engines. Another option is the XLR-132, an engine the company worked on with the Air Force Research Lab. Although development of that engine stopped years ago, Martin said the company still has some engine hardware, and personnel who worked on the project are still at Aerojet.

“We’re looking at how we leverage Aerojet Rocketdyne’s advanced manufacturing capability,” he said of the XLR-132, “to basically take that design and get into development and production as quickly as possible.”