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Raumfahrt - Crew safety during an early lunar return

3.06.2019

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The Apollo Lunar Module, an expendable spacecraft shown here on the Moon, can serve as a model for a much safer reusable lunar spacecraft. (credit: NASA)

During the Apollo lunar missions and landings 50 years ago, there was a real risk of losing a crew during each mission. For the launch phase, the risk was relatively small due to an effective launch abort system with an escape tower. During the passage to the Moon, the crew would have had the option of using the Lunar Module’s engine to return to Earth, as was done so successfully during Apollo 13. However, once a crew was in orbit around the Moon, or had landed on the Moon, the risk level was multiplied: a loss of vehicle control or failure of the propulsion system for Earth return or, worse, a failure during descent to or ascent from the lunar surface.

The danger to the program comes from the strong possibility that whoever is president after 2024 would cancel work on the reusable set of vehicles.

In one respect, the Apollo lunar module design gave crews the ability to abort a lunar landing by separating the ascent stage from the descent stage and returning to the Command and Service Module, even after committing to land. As long as the lander could reach orbit or was still in orbit, the service module could have probably rescued astronauts stranded in a non-operative ascent stage. However, if the crew had already landed and the ascent stage would not lift off, the crew could only survive a few days. They could not survive a lunar night unless the lander was designed for it. Worse, if the ascent stage had failed in any major way during an ascent, there was no backup system to reach a minimal lunar orbit and to save them from impacting the lunar surface within an hour or less.

On March 26, 2019, Vice President Mike Pence announced a revised lunar initiative, to attempt to return humans to the lunar surface by the end of 2024, instead of by 2028. In April, the picture gradually clarified, as there are now two program phases: to have a new human lunar landing (the exploration phase) by 2024 followed by a more permanent presence (the operational and sustainable phase), probably at the lunar south pole, by 2028. The plans and designs continue to evolve rapidly, as five years is a very short time span and the decision makers can feel the pressure. In early May, this accelerated lunar landing program was named Artemis. The ongoing development program for the lunar vehicles and hardware is called Next Space Technologies for Exploration Partnerships, or NextSTEP. NASA had also planned a Moon to Mars Mission Directorate to coordinate that overall effort, which included Artemis, but Congress refused to allow the organizational change. As a result, Mark Sirangelo, a NewSpace-oriented executive who had been picked to lead it, resigned from NASA on May 23. This means that the existing NASA bureaucracy will be in charge of the program.

There are both good and bad aspects to the new lunar initiative. The significant national prestige or geopolitical aspects of human space operations during a declared “space race” should not be ignored or denigrated, but any acceleration of a lunar effort would also move the day forward to when we can conduct actual water mining operations on the lunar surface. The main drawback (and danger) is that a two-phase plan sets up the possibility that it will include building two sets of lunar vehicles, one set expendable for use by 2024, and another set reusable for later missions. Considering how hard it is to get funding for space programs, it would be very wasteful to build expendable vehicles to use for just a few years. (This assumes that planners still think that expendable vehicles can be built and qualified faster. Hopefully they have paid attention to how fast SpaceX was able to develop its reusable vehicles.)

The danger to the program comes from the strong possibility that whoever is president after 2024 would cancel work on the reusable set of vehicles. The plan also sets up the similar danger that any flow of extra money from Congress in the near term might be cut off after a few lunar landings with the expendable vehicles are accomplished. It also seems unlikely that the House today would even provide such extra funding for the accelerated plan to support a goal backed by Trump. House hearings on the NASA budget earlier this month ignored Artemis and the need for a supplemental budget item to support it.

Assuming that the lunar operational phase, due to be under way by 2028, will include a larger orbiting base or Gateway, from which multiple reusable lunar landers or ferries can operate, the problem of enabling crew survival after 2028 with a subsequent Earth return during an emergency in lunar orbit is probably covered. (Operating a lunar landing program entirely from Low Earth Orbit with no near-lunar base would require a huge vehicle similar to the SpaceX BFS stage).

It behooves everyone in a position of authority over the Artemis program to work to provide reasonable protection for lunar crews during the first phase so that crews are never again put into the (now) unnecessarily risky position of those that flew on Apollo.

But what about the period from 2024 to 2028? We have no idea if there are any plans to place an early crew refuge habitat in a low lunar orbit reachable by a lander or ascent vehicle, versus the high lunar orbit where the Gateway will be located. Since each mission launched on Orion will cost billions and is scheduled years in advance, there are no plans for another standby rescue mission in case of a problem. The production capacity for the SLS booster is limited, and funding is always tight, making a set-aside rescue mission via an SLS very unlikely. The overall cumbersomeness of the SLS-based mission concepts also would result in a very long response time, probably too long to effect a rescue.

It behooves everyone in a position of authority over the Artemis program to work to provide reasonable protection for lunar crews during the first phase (2024–2028) so that crews are never again put into the (now) unnecessarily risky position of those that flew on Apollo. To more fully protect the new lunar crews, a couple of additional steps need to be taken besides the lander design. The three main risks to the crew after they have entered lunar orbit are (1) inability to lift off from the lunar surface, (2) failure of the ascent vehicle or its engine during the ascent from the lunar surface or after an aborted landing, and (3) inability to leave lunar orbit and return to the Earth. All of these would require both backup systems and a rescue mission to deal with. Since crews are most likely to be stranded on the lunar surface or in lunar orbit, it makes sense that emergency equipment should be placed in both locations before any human missions. This would consist of either a second return vehicle with storable propellant or a crew refuge habitat with a solar radiation storm cellar and supplies enough to last for several months while a rescue mission is mounted.

The current accelerated lunar plan is one promulgated by the White House. In my opinion, some of the news media unfairly blamed then President Reagan in 1986 for pressuring NASA to launch the shuttle Challenger “on schedule” ahead of a State of the Union speech that night, (a hypothesis which has never been proven), when the provable cause was clearly NASA management’s decision to ignore engineers’ stark warnings, as in “take off your engineer’s hat and put on your manager’s hat.” However, in the case of a future lunar crew lost or stranded with no hope of rescue on a lunar mission before the operational phase, the onus would be at least partly on the White House, since it is a program they have initiated with a clear deadline. Part of the onus would still be on the mission and vehicle designers.

The clear intention for the new plan seemed to be that a lunar landing expedition would first go, in a vehicle like Orion, to a minimal Gateway station with a small habitat module and docking node in a high lunar orbit, where they would board a composite lunar vehicle. A transfer module acting as a space tug would move the lander and ascent sections to low lunar orbit, where the lander would carry the crew in the ascent section to the lunar surface. On ascent, the lander would be left behind and the ascent stage would rendezvous with the transfer tug, which would carry the ascent stage with crew back to the Gateway; alternatively, the ascent stage could go directly to the Gateway. These two modules could possibly have been reusable in the earlier plan.

On May 16, NASA issued NextSTEP contracts to 11 companies to begin studies of lunar vehicles. There are suggestions that the previously planned three-part vehicle system could have reusable ascent and transfer elements. In the contracts, there is not a single mention of the ascent vehicle component, which was previously assumed to be built in-house by NASA but will now be considered by a separate NextSTEP procurement. Thus, in this set of contracts, the integrated lunar vehicle still seems to be in two or three parts, so It is not at all clear if there is now a desire by NASA for it to be designed as a reusable lander or not.

For a vehicle using hydrogen-oxygen propellants and only needs to perform a total of about 4 to 5 kilometers per second of delta-V per round trip, and in an environment with no air drag and low gravity, you simply do not need a three-segment vehicle to perform a lunar landing and return, even from a high lunar orbit. It is not clear why the vehicle would consist of “a complete integrated lander that incorporates multiple elements such as a Descent Element, Ascent Element, and Transfer Vehicle” (NASA wording). Such phraseology seems to be contradictory as it does not describe an “integrated vehicle”, unless the “elements” are fully and physically integrated into a single vehicle. Some companies have since proposed a two-vehicle version of the system which omits the transfer vehicle.

NASA had actually started to change direction in April but seems to be creating some confusion about the program is it moves forward by issuing the design contracts in May for the previous design. On April 26, NASA announced in a procurement filing that it was looking for a NextSTEP proposal, for which it may issue contracts in the summer of 2019, for an integrated lunar lander instead of the weird, three-part vehicle originally proposed. What does this mean for crew safety? It would reduce the number of components and the number of needed rendezvous operations, and provide more flexibility in designing vehicles that can survive in-flight damage and still function, just like the World War II airplanes that often landed safely when shot full of holes.

Of the various companies that are interested in participating in Artemis, SpaceX stands out as the one company that may be able to complete development of two independent kind of lunar access for humans within a short time of each other.

A truly integrated, reusable lunar vehicle should consist of a single module, with one major exception. Just like the original Apollo lunar lander’s ascent stage, the integrated vehicle should have a self-propelled crew cabin that can separate from the main lander, if (and only if) the main vehicle fails at any point, and perform an abort to orbit or an abort to surface at any time during a lunar mission. (With the expendable Apollo vehicle, the ascent stage was intended to separate on every mission.) The fuel load for the cabin can also be sized to minimize it so that (depending on the current velocity and altitude), either an abort to orbit or surface, but not both, can be performed. Irrespective of whether the vehicle to be used in 2024 is expendable or reusable, such a self-rescue crew cabin should be included in the design. The cabin should be as small and light as possible, but allow for crew survival long enough to provide a practical rescue time window. It should use new storable, non-toxic propellants for its emergency propellant supply if possible. Once the cabin with crew is either back in orbit or on the surface, a rescue by another vehicle is possible. Thus, in terms of configuration, the integrated lander would look much like the original, but with a much larger propulsion section.

One other design for a safer lunar module is potentially possible. If the integrated lunar module can be designed in functional modular sections so that if a portion of its fuel tanks, engines, or controls are damaged, the vehicle would have enough remaining capacity to continue an ascent from the Moon (or abort a descent and return to lunar orbit), the need for a self-powered crew cabin is reduced. However, the design must take into account the damage that occurred during Apollo 13, where an internal explosion destroyed much of the Apollo Service Module, but left the Command and Lunar modules undamaged. In space, the results of an explosion are reduced since there is no air pressure outside (and often inside) the vehicle to conduct the force of an explosion, resulting in less damage. Good design could help isolate the results of such an event to one portion of the integrated lunar vehicle. Structural partitions between functional modules would add some weight, but this would be balanced by removing the need for a separate crew cabin. All engines should be able to be gimbaled and throttled so that any single remaining functional engine can provide the needed thrust aligned through the center of mass of the vehicle.

If a lunar landing takes place as early as 2024, it is still not clear if the lander system would be delivered to the minimal Gateway in lunar orbit fully fueled or if it would be refueled from an orbiting depot before landing. We should be able to develop a usable cryogenic propellant depot in five years as it is not a complex vehicle: just a set of insulated tanks, cryo-coolers, and fuel transfer pumps. Fuel storage and transfer in high or low lunar orbit would increase the total mass a lander could carry to the surface, and also provide an emergency fuel supply. A fuel depot using lower performance non-cryogenic storable propellant would require more propellant and tank mass and would be more expensive than a cryogenic system, even before lunar-sourced-propellant becomes available. Such a crew refuge needs a solar radiation storm cellar. If a depot is used, a crew refuge/habitat module can be attached to the Gateway depot, possibly surrounded by the depot’s tanks for shielding, to take advantage of the required attitude control and station keeping thrusters such a depot would need. This would satisfy the requirement for a lunar orbit crew refuge.

If we have multiple lunar crew landers, one or more of them can be modified to carry down to the surface a nearly identical crew refuge/habitat. If it has a solar storm cellar, the habitat does not even need to be unloaded from the lander after landing, as long as the crew has access to it and can land within a short distance of it. Alternatively, a fresh ascent vehicle could be landed at the intended landing site, well protected against lunar thermal extremes with effective insulation, its own sunshade, and cryo-coolers. The whole intent is to provide coverage of the most likely failure modes without bells and whistles. These can be added as things advance to the operational stage, where mining of lunar water could begin, requiring more frequent and routine lunar flights. Another simple rescue system would be to have a spare lander with ascent vehicle docked at the gateway base, using the base’s power to keep its propellant cold. It would be able to land precisely and rescue a stranded crew very quickly. This is the situation toward which things should evolve to after 2024; the main question is how soon the spares could be built and emplaced.

Of the various companies that are interested in participating in Artemis, SpaceX stands out as the one company that may be able to complete development of two independent kind of lunar access for humans within a short time of each other. A future integrated Artemis lander could fit (probably dry) inside one or more Falcon Heavy launch fairings, and could also be refueled in orbit by tankers launched by Falcon Heavy. At the same time, the company is pushing its Starship/Super Heavy program very hard, and it is quite possible that, due to its huge size, an uncrewed Starship could be refueled in LEO, fly to the Moon, land, take off, and return to Earth before 2024. This timetable is similar to the first scheduled Starship cargo flight to Mars, postulated for 2022 or 2024. If the Starship can land and return safely, it would provide a comprehensive lunar rescue system, even if NASA is unwilling to fund it or participate in its development due to NASA’s continuing political allegiance to the SLS as their primary large booster. By the time the Starship/Super Heavy is flying, it will probably be too late for NASA to switch its support to that system as the primary lunar architecture, but it not only shows great promise as a rescue system, but also as a primary transport system for the period after 2024.

Having at least two ways of getting crews to the Moon and back is as important as having two ways of getting crews to the International Space Station and back down again.

It is possible that, given annual NASA and Congressional budgetary restrictions, the crew safety features, including a depot with a crew refuge in lunar orbit, could slow down a lunar surface program timetable. This is in fact one of the main and legitimate fears of opponents of the Gateway station, which originally was not designed to support lunar landings at all and whose funding, independent of a landing program, would have delayed any lunar landing. To prevent any delays, or even the perception of delays, the safety features should get their own parallel funding so that everyone can see that the landing program is being helped, not harmed. This intent should be clear in any initial proposals for safety-related hardware development.

Having at least two ways of getting crews to the Moon and back is as important as having two ways of getting crews to the International Space Station and back down again. For this reason, it is very desirable to have agreements with commercial companies such as SpaceX, which may be able to conduct their own lunar missions by or before 2024. Thus it is possible that such companies might be able to deliver a crew refuge and depot to lunar orbit and refuges to the lunar surface in advance of human landings, in addition to being able to conduct crew rescues. International partners may also be able to assist.

Taking such a position does not obligate NASA to abandon its own lunar vehicle plans unless Congressional funding fails to materialize, as was the caused during the Space Exploration Initiative almost 30 years ago. There is less than a year to choose between using SLS or private launchers in support of the new lunar program. It is, however, very hard to see how NASA will be able to develop its own lunar hardware without more funding, given the huge financial drain into the SLS program created by Congress. In the event Congress doesn’t provide the additional funding, the Trump Administration always has the option of backing SpaceX or another private company with the capability to develop lunar vehicles by the 2024 deadline, especially if done in cooperation with NASA. The agency is starting to show an inclination to allow vehicle integration by the private companies at their own facilities, which would greatly speed up the process. This private option also offers the huge advantage that the existing NASA plans cannot include for all of its launches: reusable launchers and a much cheaper lunar program.

There are multiple ways to get to the Moon and back relatively safely. Let’s try to reduce risk to crews so that the new lunar exploration phase can quickly evolve into a practical and useful operational and, later, industrial phase. Use of lunar propellant and near-lunar departure points can make access to Mars and asteroids vastly cheaper.

Quelle: The Space Review

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