Moon and Mars are physically and fiscally feasible
by John Strickland
|In my opinion, the extreme expense created by trying to use the SLS operationally, with multiple launches per year in an attempt to set up a Moon and/or Mars base, is beyond both the financial and physical capacity of NASA and its contractors.|
Some of the claims made in these articles are perfectly true, while others are wrong or misleading. These points need to be set straight so that human spaceflight and Mars mission proponents can use them in arguments in support of their goals. My main focus here is on the article about Orfeu Bertolami, the technical paper by him and his colleagues in Portugal, and the op-ed by O. Glenn Smith and Paul Spudis.
To make clear my personal positions: I support all of the significant space goals and destinations, and that one destination can and will support the path to others. I believe that while the physical vehicles and equipment needed to land large crews on the Moon and Mars have not yet been built, and while some parts of the needed technology have not been tested, we nonetheless do have essentially all of the basic knowledge needed to do Mars missions.
The issue of the Space Launch System (SLS) booster does creep into all of these arguments since it has such a central effect on mission costs. In my opinion, the extreme expense created by trying to use the SLS operationally, with multiple launches per year in an attempt to set up a Moon and/or Mars base, is beyond both the financial and physical capacity of NASA and its contractors. An SLS-sized but fully reusable booster, on the other hand, would allow such operations to proceed. It is also practical in the short term to return to the Moon to stay with smaller boosters if they are also fully reusable. The smaller payloads would make some lunar operations less efficient, but the very wide aerodynamic vehicles needed for Mars operations are not needed for lunar ones since the Moon has no atmosphere. In any case, use of lunar-derived propellant has been a key ingredient in the more progressive Mars plans since 1989.
While it is true that Mars One has generated a huge amount of positive pro-Mars publicity and media interest, it is also true that the organization still lacks the funding even to attempt to launch a mission into Earth orbit, much less to fund a Mars mission, a Mars base, or a Mars colony. To create a practical Mars settlement capable of growing will cost many billions of dollars even after a reliable Mars transport system exists.
The main thrust of the article about Bertolami’s research, “I see no way of reaching Mars in 50 years,” is that there is no propulsion system that can get a crew to Mars fast enough so that they sustain a radiation dose low enough for space agencies to approve the mission. Bertolami’s skepticism on near-term Mars missions would push the first Mars mission back to 2065. The writers claim that high speed (created with an advanced propulsion system) is the key method to reduce radiation doses. This is forced by the other claim that sufficient radiation shielding is too heavy to be launched. By too heavy, they really mean too expensive, a cost forced by use of the SLS; the use of reusable boosters is not considered.
It is reasonable to suggest that no human missions to Mars will be attempted until after 2025. By that time, a decade hence, reusable rockets will be a fact of life, and they will be flown by more than one company. It is probable that fully reusable rockets in the Saturn V class will also be flying by then, possibly as soon as 2020. The cost of launching heavy payloads on such fully and rapidly reusable rockets will fundamentally change the thinking about human space missions, as they will lower launch costs by a factor of ten or more.
Up to now, the high launch costs associated with expendable boosters have beneficially forced miniaturization of robotic spacecraft components, but you cannot miniaturize humans. Thus, most mass allocations for human missions have been constrained by, and focused on, the vehicles to carry the crew. These missions have tended to carry very little else. Such missions cannot be considered robust, as the crew cannot do much once it reached its destination. If you compare human-carrying to cargo-carrying vehicles as used for bases like those in Antarctica, the cargo is the vast bulk of what is carried to support those bases. For effective and robust lunar and Mars missions, the same emphasis on cargo must also apply to them.
|Up to now, the high launch costs associated with expendable boosters have beneficially forced miniaturization of robotic spacecraft components, but you cannot miniaturize humans.|
The radiation issue, primarily the heavy nuclei of cosmic rays, can be effectively dealt with by using sufficient shielding mass, which can be any material with a low atomic weight. With the advent of high-mass space operations, that mass can be delivered in sufficient amounts and rapidly enough to support the use of heavy shielding. Even the super-expensive 2009 NASA Mars plan used about 1,200 metric tons per mission, but these were not robust missions, which would have left little or nothing behind for the next crew to use. The crew would also have a very thin safety margin. A water jacket of about seven metric tons per square meter is needed to provide almost complete protection, while a jack of one metric ton per square meter (one meter thick) would probably provide sufficient protection for a Mars crew. A combination of the medium level of one ton per square meter around the activity areas and somewhat thicker protection around sleeping quarters would provide a very good level of protection.
For a truly robust Mars mission, which establishes a permanent orbiting Mars base and one surface base with immediate fuel production capability, my estimate for the mission dry (non-fuel) mass would be about 4,000 tons before departure from L1 in a series of vehicles. It is clear that even with a crew of about 12, very good crew shielding would use up no more than about one tenth of this mass, or 400 tons. Some people say that using this much shielding mass is far too heavy, but they are still thinking in terms of today’s expendable launchers, when launching 400 tons of anything might cost $4 billion or more, even if it was not launched by the Space Shuttle. (When you realize that this much mass is comparable to the entire mass of the existing space station, it is still easy to be a doubter.)
However, with future launch costs of one tenth or less of current ones, 400 tons of water would cost about $400 million or less to launch, a tiny fraction of the total expedition cost, or about what a single current military satellite costs to launch with a cost-plus booster. What would be in effect a large water tank would have a sleeping room in the middle of it, effectively surrounding the crew sleeping area. The (presumably segmented) water tank does not even have to be cryogenically cooled. Some of the water could even be used in an emergency for conversion to propellants. The rest of the crew’s work area would be surrounded by a water jacket about 1 meter thick. If the mass of one tenth of the total expedition is too expensive to launch, maybe Mars is not that important.
The total amount of propellant for a 4,000-ton expedition to leave L1 for Mars would average about 1,600 metric tons, if both an Oberth Earth flyby and an aerocapture maneuver at Mars are used. Without these advanced maneuvers, the propellants to reach Mars orbit from L1 would mass more like 16,000 metric tons. So, by using advanced but practical navigation, trajectory, and braking methods, we’ve saved far more mass in propellant than used for shielding.
In the opinion of many, reducing radiation exposure is a major factor in determining if human Mars expeditions are feasible. By simply adding this level of radiation shielding, and burying the crew’s habitats on the surface of Mars, Bertolami’s entire argument is negated. This is allowed by the probability of greatly reduced launch costs (which allows the use of heavier radiation shielding) before any Mars expedition could possibly be ready.
The other main issue raised by Bertolami is equipment reliability and distance from rescue. Most of the Mars missions proposed so far have been by necessity “light” missions, usually with only one crew module, one propulsion system for transit to Mars and back, one crew lander, and one Mars Ascent Vehicle (MAV). Failure of any of these can mean the loss of the crew or the mission, depending on when the failure happens. Bertolani says ,“I don’t believe one can reach Mars through a brute force approach.”
While it is not completely clear what he means by “brute force”, the impending launch cost reductions will allow a huge amount of redundancy in robust, high-mass Mars missions. Extra crew modules, propulsion systems, reusable Mars ferries instead of a one-way lander and a one-way MAV, and, especially, creating a crew refuge and logistics base in low Mars orbit before any landing attempt, can greatly reduce the risk to the crew and to the mission from any single failure.
|Yes, a crew going to Mars will need a habitat module with a mass comparable to that of the space station, as mentioned above. No, that should not be a problem unless you assume reusable rockets will forever be a fantasy.|
Bertolami does support a return to the Moon, but his idea of first creating a lunar mission architecture and then, years later, a different Mars mission architecture, would be very wasteful of funds and could easily delay Mars missions by a decade or more. Other than the time involved for the mission, lunar and Mars missions can be very similar. A round trip to the lunar surface from L1, for example, takes almost exactly the same velocity change or delta-V of 5.2 kilometers per second as a round trip to the surface of Mars from low Mars orbit. Surface crew habitat modules that are buried for radiation shielding and systems to produce propellants from local ice would be very similar on both types of missions. Both mission types really need reusable ferries that have a backup crew survival capsule that can detach and take over if the main vehicle fails. Both the Moon and Mars have dust that would need to be kept out of the crew habitats. Both have physiological vacuum conditions on the surface.
The other recent major challenge to the concept of human Mars missions is the SpaceNews op-ed “Mars for Only $1.5 Trillion”. This article focuses primarily on cost, and seems to insist that the only way to mount Mars expeditions is with the same super-expensive expendable launchers like the SLS, which the first article assumes will be used. In fact, the extreme costs described in the Smith article do a very good job of arguing against the use of the SLS.
The article also covers the extreme complexity of a Mars mission, and mentions much of the equipment that must all function properly for it to succeed. In this point the authors are correct, but we do have at least 15 or more years to test such systems in orbit before they are used on an expedition. The authors also point out the impossibility of rescue for a crew that could be on the opposite side of the solar system for years. The provision of spare parts, duplicate equipment, and modules and extra supplies that a high-mass Mars mission allows would also go a long way to reduce the problems caused by the certainty of multiple equipment failures. One major point missed in this article, in spite of the attention given in the article to the high equipment failure rate, is the pressing need to reduce the large proportion of crew time spent on habitat module maintenance by improving the module and life support equipment designs. This time and money needs to be spent now, on Earth in design labs, not later, at Mars by crew members, or they will have no time at all for exploration.
Yes, a crew going to Mars will need a habitat module with a mass comparable to that of the space station, as mentioned above. No, that should not be a problem unless you assume reusable rockets will forever be a fantasy. A crew in such a well-shielded habitat module would receive much less radiation than the crew does in the current station. There would be at least two habitat modules, each capable of holding the entire crew for the whole mission. 3-D printers would also be brought along that could rapidly fabricate entirely new parts if that were needed.
A robust Mars mission would have much more mass than just that in the crew habitats, but the writers focus on the habitat’s cost by comparing it to the space station. They correctly point out that the station took over 20 years and $100 billion to build and launch. They totally ignore the fact that the methods used for designing and launching the station components were dead wrong. If we had launched larger station components at about the same rate as Apollo launches, using a Shuttle-C or Jarvis Booster (boosters proposed in the late 1980’s and similar in capacity to a SLS), the whole station could have been launched and assembled in three years or less. Some of the projected future large launchers could launch a mass equivalent to the station in just two or three launches, using the same rocket, in about two weeks.
Note that the use of SLS-like boosters that I actually supported in 1987 would have greatly reduced the cost of building the space station. However, those boosters would have had a launch rate and costs comparable to the shuttle, but would have placed many times more payload in space than the shuttle. If, however, we had attempted to launch the International Space Station on the current SLS design, with only about one launch per year, it might have still taken 10 to 20 years to finish the job.
|A much more practical and fairer model for costing a Mars program would be based on the available annual level of NASA funding. The entire Mars architecture and funding model thus needs to be designed to fit within the current or future human space program’s annual budget.|
It would be entirely fair to assign the bulk or even the entire development cost of the SLS and Orion to a Mars program if these vehicles are ever used to support one. The extremely high costs given by the authors, however, suggest that any plan to build a lunar or Mars base based on the SLS and Orion will never pass congressional muster, in spite of the fact that it is primarily the Congress that has pushed for those programs. Opponents of a future Mars program would use the total program cost over 20 or 30 years to kill it, just as was done when the “90-Day Report” was released in November of 1989. With the use of fully reusable boosters and spacecraft, the total Mars program cost can be reduced by about a factor of ten, to the point where it would be less expensive than some of the previous and existing programs. By combining a unified Moon and Mars program, the cost can be reduced still further.
When the $400–500 billion Moon and Mars program cost from the 90 Day Report was leaked to the media (even though the report itself had been purged of any cost values to avoid political repercussions), and when, more recently, a trillion-dollar figure for a Mars program was promulgated by some in the media, the numbers seemed staggeringly large, as was intended. The current authors attempt to triple these high numbers. How do they do it? The super-expensive Mars Design Reference Architecture 5.0 of 2009, still the current official NASA Mars plan, assumes only three missions. The current authors assume nine missions, not three, essentially tripling the cost, since all of the boosters and vehicles must be replaced for each mission. The main common cost for a Mars program is the design phase, which is usually the least expensive part of the cost. As I support creating a permanent Mars base with the first expedition, and then continuing expeditions without interruption in a similar manner as the Antarctic science bases, I actually approve of the larger number of missions. These could occur over a period of about 20 years, as windows for mission departures are 26 months apart. For this reason, the individual Mars missions actually cannot be launched at three-year intervals as the article states.
The authors estimate the cost of their first Mars mission at $230 billion including development costs, and the subsequent missions at $142 billion, not including the cost of an expendable lander, but including all of the replacement SLS boosters needed. Thus the following eight missions would each cost about $145 billion, for a total of about $1,400 billion. This overall scenario can only be described as one that slavishly follows the antiquated Apollo mission model. Thus, the authors make a very good case for the extreme expense that would be incurred by using the SLS operationally, which argues forcefully against that very use.
Another issue brought up in the article is the high entry speed of any Mars vehicle as it returns to Earth. It is simply assumed that the crew habitat module must re-enter and be destroyed. This assumption rejects most of the thinking of the last decade, which has shown that crew habitats do not need to be thrown away, but can be used over and over by having them perform an aerocapture and return to their original base at L1 for later reuse by the next crew. The crew, if they were in a separate vehicle, could also return to L1 by the same method, or they could aerocapture into a lower orbit and rendezvous with a low Earth orbit logistics base. The thermal stress on the heat shields used for aerocapture is much less than that needed for full entry from an interplanetary trajectory.
A much more practical and fairer model for costing a Mars program would be based on the available annual level of NASA funding. The entire Mars architecture and funding model thus needs to be designed to fit within the current or future human space program’s annual budget. The use of fully reusable boosters and spacecraft is the key concept to allow this and is mandatory to achieve this, and requires an effective partnership between public and private entities. Support from an international Mars consortium would also reduce NASA costs and allow for even more robust missions. Assuming that commercial development costs would be one fifth of current government program estimates, and operating costs would be one tenth of such estimates, the costs for a 20-year, nine-mission program would be more like $20 billion for vehicle development and $130 billion for the nine missions, or about $14.5 billion per mission, spread out over about three years. Continuing improvements in launch and operating costs between now and 2030, such as production of Mars mission propellant on the Moon, could make these costs even lower. The authors do specifically support the latter concept. A significant portion of development and operating costs could also be shared by a cislunar transport system with an L1 logistics base, which would provide access to the mining bases at the lunar poles as well as a very efficient departure point for Mars missions.