Hey guys, here is a bit of a draft introduction that I have been working on. I have a lot more work to do, but I thought I'd share this early so that anyone who follows might get a better idea of what the MP4 concept is all about. Feedback is welcome.
P.S. I recognize that I've probably over-used Astronautix material and original research here. Not being familiar with academia, I have no idea what is prudent. I guess I should probably seek permission (and also a secondary source) if I want to use those sections in the final document?
Perhaps the greatest challenge of designing a Humans to Mars mission architecture is the task of getting large payloads through the “bottleneck” of Mars’ precariously thin atmosphere. While thick enough to cause an approaching spacecraft to quickly break up and vaporize, Mars’ atmosphere is too thin to slow a heat-protected spacecraft from orbital or interplanetary velocities (4500 to 6000 meters per second) down to a safe velocity for landing (1 or 2 meters per second) using conventional methods. To date, the few small robotic landers which have been successfully landed on Mars’ surface have been able to meet this challenge through a combination of unique strategies: Very low ballistic coefficients (i.e. low-density lightweight spacecraft) which allow the craft to slow in the high atmosphere as much and as early as possible, supersonic parachutes to efficiently bring the craft from supersonic to subsonic speeds, and a final powered descent – which scrubs off the last bit of speed and permits a safe vertical touchdown. The above challenge is here-on referred to as “EDL” (Entry, Descent, and Landing).
EDL on Mars is commonly seen as a “bottleneck” because of the inherent difficulty in slowing high-mass payloads from these tremendously high entry velocities to safe touchdown speeds using only Mars’ tenuously thin atmosphere as a speed-brake. Previous robotic probes have been able to deliver at least some useful mass to the surface only because of their lightness. To date, only lightweight surface payloads of less than 1 tonne have been achieved. This can be seen to be in sharp contrast to the very large surface payloads commonly perceived to be required to support Human crews on Mars’ surface. A casual census of Humans to Mars architecture studies reveals that the lightest landed mass specified is on the order of 24 tonnes (Mars for Less) and the heaviest; over 60 tonnes (NASA’s DRM5). Such a high surface payload mass requirement – being around two orders of magnitude above that which has been successfully achieved to date – imposes a great challenge on the science and engineering of EDL. At least one commentator has likened the difficulty of this challenge with that of “herding cats in a room filled with smoke and mirrors, where the floor is covered with apples and oranges.” Indeed some mission architects apparently prefer to dismiss the problem altogether – perhaps in the hope that someone else will solve this EDL problem at some future date.
Nevertheless, many different kinds of theoretical solutions to the problem have been offered. One of the most common of these is some kind of large deployable heatshield. A large heatshield area can be used to lower the ballistic coefficient, creating higher drag in the high atmosphere, and allow the craft to come into the supersonic range at a higher altitude – at which point some other device is used to bring the craft into the subsonic range for the final phase of powered descent. Most commonly, this would be a single large high-mach supersonic disk-gap-band (DGB) parachute, or alternatively, a more difficult method of deceleration termed “supersonic retro-propulsion” - a tricky alternative which involves firing high-thrust engines directly into the incoming supersonic air stream. In addition, some mission architects specify a type of entry vehicle with a high lift-to-drag ratio (L/D), such as some kind of lifting body, which allows the vehicle to use what is called a “lofted” entry trajectory – allowing more aerodynamic deceleration to occur in the high atmosphere before the craft plunges into the thicker lower atmosphere. It should be noted that some method of deceleration through the transonic envelope (either DGB parachutes or supersonic retro-propulsion) is still required.
However, all of these purported solutions entail their fair share of difficulties. Large deployable heatshields, of any kind, remain untested (a single suborbital test of a very low capability inflatable shield not withstanding). For inflatable shields, there remain problems with finding a suitable material – tough enough to withstand harsh acoustic vibration, deformation, and heat loads, yet flexible enough to be folded up into a reasonably sized package. Mechanically folding heatshields pose the issue of how to protect the seams from the harsh entry environment. Due to the tremendously high aerodynamic loads, both types end up being quite heavy in their construction, compared with a single-piece rigid heatshield. Large inflatable aerodynamic decelerators of other kinds have also been proposed. However, these pose a similar suite of difficulties, and likewise also do not solve the problem of providing adequate deceleration through the transonic envelope. (Terminal velocity for a reasonably sized entry vehicle in Mars’ thin atmosphere is supersonic.)
Of the two most studied methods of deceleration through the transonic envelope, supersonic parachutes and supersonic retro-propulsion, the latter has apparently been given more attention as of late. This may be largely due to the realization that parachutes do not scale well: Their mass increases faster than their effective area – and larger parachutes also take longer to inflate. Increasing their deployment mach number (e.g. from mach 2.1 to 2.5 or above) vastly increases both the aerodynamic and heat load that they must sustain – thus requiring thicker and heavier materials. (Mach 2.5 is generally seen as the extreme upper limit of current materials.) Unfortunately, supersonic retro-propulsion isn't very mass-efficient either: Its parasitic propellant mass requirement goes up rapidly as the ballistic coefficient of the entry vehicle is increased (resulting in a feedback-effect which severely limits its usefulness). It is also rife with technical challenges: Figuring out how to light the engines while subjected to a supersonic air-stream is one problem, dynamic stability is another.
So we can begin to build a picture of the overarching trend of the EDL problem: As payload mass requirements go up, the technical challenge of EDL increases (perhaps exponentially), and more relevantly, the mass efficiency (the ratio of achievable payload mass to entry mass) decreases. In other words, when it comes to negotiating the bottleneck of EDL; smaller is definitely better
With this simple criterion in mind, the matter of ascertaining the smallest practical “biggest smallest piece” required to be brought through the EDL bottleneck, as an integral part of a practical Humans-to-Mars mission architecture, is henceforth explored.
The history of practical Humans to Mars (HTM) mission architecture concepts perhaps begins with what is called the “all-up” approach. Boeings 1968 HTM concept, the Integrated Manned Interplanetary Spacecraft, or “IMIS”, stands out as an example because it was the first to incorporate the newfound knowledge of the precise density of Mars’ thin atmosphere. (Mission concepts developed before the Mariner 4 probe made its measurements assumed an atmospheric density nearly 20 times what is actual.) The IMIS concept involved sending a nearly 60 tonne 10 meter diameter capsule-shaped “Mars Excursion Module” or MEM, with crew, through the bottleneck of EDL. It is not certain what proportion of the initial mass at aero-entry would end up as final surface payload. But this MEM was envisioned as a means to deliver 3 crewmen to the surface, and, after a 30 day stay, return them to Mars’ orbit to rendezvous with the interplanetary habitat and spacecraft from which they had originated. Because aerocapture of the main craft was not considered, the total propulsion requirements for this mission concept were quite high. No less than 5 modular nuclear-thermal NERVA propulsion stages were required to be assembled together in LEO, each of which would be delivered by a thrust-augmented version of the mighty Saturn V. An additional Saturn V as well as a Saturn I were required to deliver the main spacecraft and crew, respectively. This type of architecture is termed “all-up” because everything that is required to complete the mission is assembled and sent to Mars as a single unit. http://www.astronautix.com/graphics/z/zmisf04.gifAbove: Baker's large IMIS spacecraft. An example of the "all-up" mission profile.
The next noteworthy mission plan came from Von Braun shortly after in 1969. Von Braun envisioned a very similar but slightly smaller interplanetary craft, comprising a space habitat and a similar 3-person MEM (this time only 43 tonnes), resulting in the requirement of 3 instead of 5 NERVA propulsion stage per craft. Uniquely, instead of only sending one of these assembled craft at a time, Von Braun envisioned sending two identical and independent craft side-by-side. That way, if one failed at almost any stage of the mission the crew would be able to simply transfer to the other and still survive the entire mission. Likewise, it was envisioned that both MEMs could be landed nearby each other on the surface – thus allowing the crew of 3 each (6 total) to offer mutual support during surface operations, and also providing redundancy in case one of the ascent vehicles failed to launch back into Mars’ orbit. Such a mission plan was touted as being extremely robust and safe – and just such redundancy proved invaluable for the crew of Apollo 13 only a year later. Such a ‘paired’ mission architecture might be called a “horizontally split” derivative of the “all-up” approach. However, the initial mass in Low Earth Orbit (IMLEO) required for such a mission was, like IMIS, still extremely high. http://www.astronautix.com/graphics/z/zmem69.jpgAbove: Two of Von Braun's spacecraft, similar but smaller than the IMIS craft; here shown in convoy. An example of a "horizontally-split" mission profile.
The next noteworthy progression of the HTM concept may be the Planetary Society’s Mars Expedition plan of 1983. Similar to the “FLEM” concept of 1966, this mission plan took advantage of a flyby trajectory to return the crew to Earth – thus saving mass by not having to brake the Earth return habitat (or ERV) into Mars’ orbit, or propulsively depart such an orbit. The ERV would be sent without crew and slightly ahead of the crewed interplanetary craft. The crew would descend through the bottleneck of EDL in a 54 tonne MEM, this time 13m in diameter to provide the necessary low ballistic coefficient (although apparently still too high by the benefit of later analysis). After ascending into Mars’ orbit, the crew would rendezvous with the Mars departure stage from which they had recently departed, and use it to “catch up” to the ERV on its flyby trajectory. Such a minimalist mission plan resulted in a very low IMLEO requirement. However, like FLEM, the simple flyby trajectory entailed subjecting the crew to an unreasonably long return duration; 500 days in deep-space, before the crew re-entered Earths atmosphere in a small re-entry capsule. There was also only one chance for the crew to rendezvous with the ERV on its flyby trajectory. If this critical maneuver failed, the crew would have been lost.
Next was the Case for Mars II mission study. This may have been the first true “vertically-split” mission architecture; whereby surface assets were proposed to be sent ahead and landed on the surface *before* the crewed vehicle departed the Earth. Also for the first time, ISRU was proposed. Both of these would have resulted in the requirement for much lower surface payload masses, and consequently also lower IMLEO requirements, had a whoppingly-large crew of 15 not been specified for each mission! Emphasis was also placed on reusability of most of the mission elements.
During the remainder of the 1980’s many more HTM mission concepts were proposed. Some explored the “vertically-split” approach of sending elements ahead of the crews, and some also explored ISRU. Most of the conservative mission plans, however, stuck to the traditional “all-up” architecture; specifying small MEM-type vehicles which would deliver a small crew to the surface for only short durations (usually 9 to 60 days) before providing the means to ascend to the awaiting orbital vehicle – much like the tried-and-true LOR method used for the (then still recent) Apollo landings. IMLEO requirements were high, orbital assembly often specified, and little science return was afforded from the short-duration surface stays.
In 1991, Dr. Robert Zubrin came up with an interesting and ambitious proposal he called “Mars Direct”. Like the Case for Mars mission studies, Zubrin proposed sending an ISRU unit ahead of the crews to generate consumables and fuel needed for their forthcoming surface stay. Also, quite ambitiously, he proposed that this ISRU capability could be utilized to not only provide consumables and fuel for the crews surface activities, but also to generate all of the propulsive fuel necessary to send the crews not only through the ascent to Mars’ orbit, but also to escape velocity and onto the trans-Earth trajectory. This feat would have been accomplished via a single ERV sent to the surface ahead of the crews. A small nuclear reactor onboard it would provide the power necessary to generate the large quantity of fuel required to send the crew and their return habitat through ascent and departure from Mars. The crew of four would be sent to Mars within their surface habitat using a conjunction-class trajectory, with enough supplies onboard to last 550 days on the surface, before using their rovers to perform a surface-rendezvous with the awaiting ERV. Both the Hab and ERV would be launched into a direct trans-Mars trajectory using a single Saturn V class HLV each. Thus IMLEO requirements are kept low, and the need for orbital assembly is eliminated. The long surface stay allowed more science to be conducted for each mission. However, many viewed such a mission plan as too technologically ambitious to be credible – citing a myriad of issues, such as hopelessly optimistic technology assumptions, and lack of adequate mass margin. Above: Zubrin's minimalist Hab and ERV landers, shown in close proximity on the surface of Mars. An example of the "vertically-split" surface-rendezvous mission profile. Note the very small presurized rover that is squeezed in with the Hab lander. Later NASA DRMs delivered a much larger rover alongside the smaller MAV lander.
The Mars Semi-Direct architecture proposal that followed (also 1991) might be interpreted as a partial rebuttal to some of these concerns. Instead of specifying such a wildly ambitious “do it all” ERV vehicle, the function of ferrying the crew from Mars surface back to Earth was split up into two parts, to be performed by two separate vehicles: A Mars ascent vehicle (MAV), which needed only to generate enough fuel for the crew to ascend to Mars’ orbit, and the Earth return vehicle (ERV) which was pre-placed in Mars’ orbit – and which would perform the propulsive maneuver needed to send the crew back to Earth. This was exactly the Mars orbital rendezvous (MOR) mode of operation that earlier mission plans had specified. Combined with the other elements of the Mars Direct architecture (pre-sending unmanned assets, maximal utilization of ISRU, no orbital assembly) the result was the more robust “Mars Semi-Direct” plan – perhaps a natural culmination of the “vertically-split” approach.
For the next two decades the core of the Mars Semi-Direct philosophy appears to have gone un-trumped. Theoretical work appears to be focused on providing definition of more realistic mass and technology requirements, rather than on exploring wildly different mission options. NASA’s own DRM series of architectural studies is perhaps one example of this. Other variants of the Semi-Direct approach have also been explored – such as Wilson & Clarke’s MarsOz (which attempted to better define EDL requirements as well as explore photovoltaics as a potential replacement for surface nuclear power), and Bonin’s Mars for Less (which brought the idea of orbital assembly back in to allow use of EELVs rather than HLVs).
Required surface payloads, however, remain high for all of the mission concepts explored above – 25 to 60 tonnes – two orders of magnitude above what has been able to be accomplished by robotic craft to date. Furthermore, the payload mass fractions specified (often 60% or higher) would seem hopelessly optimistic in comparison to the payload mass fractions of the robotic landers (closer to 30%). In 2006, Grant Bonin, the lead designer of the Mars for Less mission concept, acknowledged this discrepancy, stating that both Mars for Less and Mars Direct were unrealistically optimistic in their EDL assumptions. Even NASAs own DRMs may not be immune, with recent EDL studies indicating that their assumed EDL methods are not viable in their current form, and will have to be revised. So while there have been many purported “showstoppers” in the quest for Humans to Mars, the task of getting massive payloads through the bottleneck of EDL is perhaps still the most relevant – and, arguably, still remains unresolved. In short, the pieces are still too big.
In the Semi-Direct architecture, the “biggest smallest piece” that must be sent to the surface is the surface habitat. The surface habitat must be large, and heavy, because it is the means by which the full compliment of crew are accommodated in a modicum of comfort for the entire 500 to 600 day surface stay duration. In the original Semi-Direct variant, the crew lands inside their surface Hab. This is to provide some degree of safety – so that the crew is not at risk of being isolated from the means of their long-term accommodation if their means of surface transport were to fail. Likewise, their means of surface transport (in the form of a pressurized rover) is commonly delivered with the Hab as well. Making surface transport available allows the crew to get to the MAV and ascend to the awaiting ERV in orbit in the event that the systems onboard the Hab fail, or in any event where the alien surface turns out to be somehow inhospitable. Another variant of the Semi-Direct architecture recognizes that a pressurized rover is an awkwardly bulky item, and is more easily accommodated with the MAV lander (rather than with the more volumous Hab). It should be noted that this arrangement usually requires that the rover has the capability to autonomously remove itself from its lander and drive across uncertain terrain to rendezvous within the approximate vicinity of the crewed Hab lander upon its arrival.
Upon the successful arrival of all the mission elements, the crew are tasked with using the rover as the primary means by which they will access sites of scientific interest in order to conduct geological field work. The degree to which such field work is necessary dictates the capability of the pressurized rover. Most mission concepts specify a few weeks of on-site capability – necessitating quite a large and capable rover. Sorties away from the base site are performed frequently – as the crews must periodically return to the main Hab or other surface assets for recuperation, resupply, and refueling. This mode of surface operations can be called the “commuter mode”. Above: An example of the "commuter mode" of surface operations. The crew use the large pressurized rover to commute between their fixed hab and sites of interest.
After 500 to 600 days on the surface conducting field-work and other activities, the crew is expected to be able to use the same rover to drive to the MAV, from which they will ascend to the ERV waiting in orbit which will return them to Earth. For various reasons, a crew of at least 4 is usually specified per mission, from which the minimum mass of the Hab lander is dictated.
In a theoretical attempt to reduce the mass of this “biggest smallest piece” required to be delivered through the bottleneck of EDL, two conceptual operations are performed upon the Semi-Direct architecture:
The first step takes a page from Von Braun’s “horizontally-split” paired mission arrangement and conceptually splits every vehicle of the Semi-Direct architecture into two smaller vehicles. Thus the 4-crew Hab is divided into two half-sized 2-crew “mini Habs”, and likewise for the MAV and ERV. With this simple step, the mass of the “biggest smallest piece” may be seen to be effectively halved. And having two of every vehicle may provide additional safety benefits just as Von Braun envisioned for his 1969 Humans to Mars mission concept, in the same way that the Lunar module provided crucial life support functions to the crew of Apollo 13.
The second step involves conceptually combining the functions of the Hab and the pressurized rover into a single small “Mobile Hab” vehicle. The logic of this step becomes apparent when it is realized, as a result of taking the first step, how small this new 2-crew “mini Hab” unit (both in terms of volume and mass) may be. Landing legs and struts are conceptually replaced by wheels and suspension, in the same manner as the forthcoming robotic MSL rover. The resulting vehicle is not much larger than some pressurized rover concepts. With only one vehicle per lander, packaging is also thus no longer a problem. With Mobile Habs, the limitations of having only 2 crew in each vehicle is circumvented by the ability to land the two separate crews nearby each other and have them rendezvous on the surface, so that they can cooperate during surface activities – just as Von Braun envisioned for his mission concept of 1969. Above: An example of the "mobile home" mode of surface operations. The Mobile Hab vehicles envisioned for MP4, with only two crewmembers each, would be much smaller than the vehicles shown here.
Thus we have the essence of the mission profile that we shall call “MP4”: A vertically and
horizontally-split mission profile.
Although in the concept presented henceforth, one more conceptual step is made: Instead of having two fully independent MAV landers, one of these is swapped for an additional un-crewed Mobile Hab-derived vehicle – a “Mobile Lab” of sorts. This vehicle we shall call the “ASH”, for “Auxiliary Surface Habitat” (the function of which will be explained later). The reason for this step is that we intend to send the MAV ahead, instead of concurrently, of the crews arrival. This enables us to place more confidence in the operation of a single MAV. There are also other reasons, both technical and mundane.
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