The Case for Mars (18 page)
Since an orbital rendezvous is not required in the Mars Direct mission plan, the accuracy of the capture orbit is unimportant, so long as the inclination of the orbit is such that it permits access to the surface site (that is, the inclination of the capture orbit must be greater than or equal to the latitude of the desired landing site). With this in mind. so long as the crew simply captures into an orbit around Mars, they’ll be able to descend to the surface outpost that was delivered in the previous opportunity. This relaxation of aerocapture accuracy requirements translat
es into a relaxation of aerocapture guidance, navigation, and control requirements, significantly enhancing the attractiveness of aerobrake technology for the orbit capture maneuver in Mars Direct. If the maneuver is unsuccessful to the point that the hab will not capture into orbit, the crew could use the propulsive capability of the lander (up to 700 m/s) to augment the performance of the aerobrake. The crew might now be unable to descend to the surface in the hab, but they would be captured into orbit around Mars. They would then have two potential options. First, the crew could remain in orbit and rendezvous with either of the ERVs (the precursor or the one that followed them to Mars, either of which could be flown up to them by remotequiremen) after 600 days in orbit. They could then transfer to the ERV and return to Earth. Alternatively, they could wait in Mars orbit for only 90 days or so until the ERV that is following them to Mars arrives, and then rendezvous with it prior to its landing. They could then have the option of transferring some propellant from the ERV to their hab, thereby enabling a hab landing (but sacrificing the ERV). Or, they could transfer to the ERV and land in it, leaving the hab behind in orbit. This could be done immediately after rendezvous, if there already was a hab on Mars to support surface operations (left behind by the crew of another mission). If not, the landing could be delayed, allowing the crew to spend most of the Mars stay time on orbit (where they could use the large supplies of consumables and ample quarters aboard the hab), and then perform a short-duration surface mission using the quarters of the two ERVs as their surface base.
However, since a safe haven and the possibility of completing the mission successfully exist on the surface of Mars, clearly the best option is to go there. For this reason, the aerocapture maneuver will choose to err on the side of entering the atmosphere too deeply, rather than risk skipping out into interplanetary space. Since the Mars Direct plan does not require capturing the ship into a loosely bound, highly elliptical orbit (as traditional missions do—such orbits require less propellant to get out of), the craft can be targeted toward a more tightly bound, slightly elliptical or circular orbit about Mars, which would make a skip-out almost impossible. If the ship enters the atmosphere too deeply to capture into a stable orbit, the crew can simply land the
hab. After all, the plan is to ride it to the surface anyway.
Eliminating the need for a Mars orbit rendezvous prior to descent represents a major addition to mission safety, because it jettisons the need to cut it fine with an aerocapture maneuver that might risk skip-out. But, of course, Mars Direct exchanges an orbital rendezvous for a surface rendezvous. What about that? Well, let’s consider. The surface rendezvous plan used in the Mars Direct mission also has several levels of backup to assure mission success. First of all, the ERV has been on-site for the two years prior to the crew’s arrival, affording the opportunity to deploy robotic roving vehicles to fully characterize the rendezvous site well in advance of the crew’s arrival, as well as to place a transponder on the best possible landing site identified in the vicinity. The ERV also mounts a radio beacon much like an instrument landing system transmitter at an airport that gives the crew exact position and velocity data during approach and terminal landing. We should remember that both
Viking
landers touched down within 30 kilometers of their targeted sites without active guidance, while the Apollo manned lunar landers were able to land within 200 meters of a target
Surveyor
spacecraft With the aid of a feedback targeting control system and a guiding radio beacon, the landing should be within a few meters of the targeted point. However, if the landing should prove inaccurate by tens or even hundreds of kilometers, surface rendezvous can still be achieved through the use of the ground rover carried in the hab, which has a one-way surface range of up to 1,000 kilometers. Because the crew has landed in a full habitat, and not a short-duration small landing vehicle, they have the ability to support themselves for a long period of time if they should land in an isolated location, and therefore third and fourth levels of backup are also available. As the third level, if the landing rendezvous fails by distances of planetary dimensions, the second ERV following (by several months) the piloted hab to Mars can be redirected to the hab’s landing site. As a fourth level of backup, the entire crew has landed on Mars in a hab carrying enough supplies for a two-year surface stay. If all else fails, they can simply tough it out and wait for the next launch window when more supplies and another ERV can be sent out to them.
Because it uses in-situ propellant for ascent, the Mars Direct plan does not have t
he capability for abort-to-orbit during descent. Once you start for the surface of Mars, there’s no turning around. However, it is extremely doubtful that any lander, fully fueled for ascent though it might be, could really ascend successfully to orbit by taking off from the back of an aeroshield buffeting its way at hypersonic speeds through the Martian atmosphere. (Such a maneuver would require flying the ascent vehicle
through a hypersonic shock wave
to the rear of the aeroshell, and then turning it around in midair to shift its engines from a deceleration to acceleration orientation!) In exchange for giving up the illusory option of abort-to-orbit during landing (which the crew of a traditional mission landing in a fully fueled ascent vehicle would like to believe they have, but don’t) the Mars Direct crew gains an element of real safety. That is, they know, not only before atmospheric entry but even before they have left Earth, that they have a fully fueled Earth return vehicle that has already survived the trauma of landing waiting for them on the Martian surface. Furthermore, in carrying out their own landing, they will be traveling in a large and sturdy hab containing multiple pressurized compartments and an up-and-running long-duration life-support system, and one that is almost empty of rocket propellant at the time of touchdown. In contrast, the crew who descends in a fueled Mars ascent vehicle will have to make their landing in a small vehicle with minimal long-duration life support—one that is filled to the gills with high explosive rocket propellant.
As discussed in the previous section, because the Mars Direct mission concentrates its assets on the surface, and not in orbit, all required systems to sustain the crew during the long 600-day surface stay are multiply backed up, with the degree of redundancy increasing as the sequence of missions proceeds and hab after hab is added to the set of available surface facilities. When it comes to return to Earth, the crew has two complete Earth return vehicles with them on the surface, either of which can get them home without any other assistance and both of which can be checked out manually prior to departure. This is a radical improvement over the situation in a traditional mission plan, where a crew must ride the only available Mars ascent vehicle to a mission-critical Mars orbit rendezvous with a mothership that may have been in orbit for up to a year and a half with nobody minding the sto
re and little in the way of onboard resources to effect repairs. The Mars Direct crew can personally check out their ERV before they commit themselves to ride on it, and have all the resources of the Mars base camp behind them if they need to effect repairs or adjustments. And if both ERVs are unsatisfactory, they can just sit tight at the Mars base because another hab loaded with supplies and another ERV will arrive within a few months of their scheduled departure time. In that case, they would have to extend their Mars stay by two years over the original plan, but it sure beats dying.
ADVANCED TECHNOLOGY OPTIONS
The transportation system used by the Mars Direct plan as described so far in this book can be executed with all existing technology: Saturn V’s or equivalent heavy-lift boosters, chemical propulsion, and so forth. But certainly if some more advanced technologies should materialize, the plan can and should be prepared to take advantage of them. While many forms of advanced space transportation systems have been proposedface12;nuclear and solar electric (ion drive) propulsion, solar and magnetic sails, fusion and even anti-matter rockets, to name some of the most prominent examples—only a few of these systems have the potential of materializing within the time frame of interest to initial manned Mars missions. These are nuclear thermal rockets (NTRs) and the closely related solar thermal rockets (STRs), which could replace chemical rockets for space transportation, and single-stage-to-orbit vehicles (SSTOs), which could replace expendable multi-staged heavy-lift boosters for launch from Earth. That is not to say that nuclear electric ion drives and magnetic sails and other advanced systems are infeasible. Quite the contrary, they are thoroughly feasible and will probably dominate interplanetary commerce a century from now. For that reason we shall discuss them further in some of the later chapters of this book that deal with the more futuristic aspects of Mars colonization. However, just as Columbus would not have traveled very far if he had held his expedition on the dock until an iron steamship or a Boeing 747 was available for trans-Atlantic transport, so the first generation of Mars explorers will have to settle their hopes upon a more primitive
set of technologies than will be available to travelers of a later era. Columbus crossed the Atlantic with vessels designed for Mediterranean and Atlantic coastal traffic. It was only after European outposts were created in the Americas that the technology driver came into being to propel naval architecture from Columbus’s primitive craft to three-masted caravels, to clipper ships, to ocean liners, and to airliners. Similarly, establishing human settlements on Mars will drive the creation of more advanced forms of space propulsion. For that reason, up till now we have based our discussion of Mars missions entirely upon the current primitive state of space technology. That’s the conservative approach. But there are technologies that could potentially be put into play in the relatively near future that could significantly improve mission performance or cut costs. Let’s take a look at them.
Thermal rockets, either nuclear or solar, are the most likely candidates for a space propulsion system capable of replacing chemical rockets. The idea behind such systems is very simple. A heat source, either a nuclear reactor or a parabolic mirror focusing sunlight, heats a fluid to very high temperatures, turning it into ultra-hot gas that is then expelled out a rocket nozzle to produce thrust. In other words, a thermal rocket is just a flying steam kettle. The performance of such a system is limited primarily by the maximum temperature the engine materials can tolerate, and this is generally believed to be in the neighborhood of 2,500° Centigrade. The highest exhaust velocity, and thus specific impulse, obtainable by such a rocket will be supplied by the propellant gas having the lowest possible molecular weight. Therefore, hydrogen is the propellant of choice for thermal rockets. An NTR or STR using hydrogen propellant can achieve a specific impulse of 900 seconds (9 km/s exhaust velocity), twice that of the best hydrogen/oxygen chemical rocket engines.
And thermal rockets are not just a theory. In the 1960s, the United States had a program called NERVA (Nuclear Engine for Rocket Vehicle Applications) that built and ground tested about a dozen NTR engines in sizes ranging from 10,000 lb thrust all the way up to 250,000 lb thrust. These engines really worked, and really delivered specific impulses of over 800 seconds, far beyond the wildest dreams of any chemical rocket engineer. Wernher von Braun planned to use NTRs as the propulsion system for the manned Mars mission NASA hoped to follow Apollo with by the early 1980s. But when the Nixon administration pulled the
plug on NASA’s post-Apollo Mars plans, the NERVA program went down the drain as well The engines were never flight tested and the ground test facilities were left to rust. Many of the veterans of the NERVA program are still around, although most are approaching retirement age. Even as I write, their priceless expertise with such systems is evaporating. Still, feasibility has been demonstrated.
During the period when the Space Exploration Initiative (SEI) was still alive, a faction in NASA led in spirit (but not in command authority) by Dr. Stan Borowski of NASA’s Lewis Research Center in Cleveland made an effort to revive America’s NTR research and development program. This effort, which I vigorously supported, faced many hurdles in the political environment, not the least of which was the fact that the SEI’s enormous projected price tag had disposed Congress not to spend a penny on anything associated with it. There were other problems too. In the 1960s, the anti-nuclear movement had not yet materialized as a serious political force and it was routine practice to test NTR engines in the open, with their potentially radioactive exhaust spewing straight up into the air of the Nevada test site. This procedure would not pass muster today. Instead, modern NTRs would have to be tested inside closed facilities containing scrubbers that would remove all radioactive products from the exhaust gases before releasing them into the environment. Depending on the size of the NTR engine, such a facility could be very large and costly, possibly on the order of a billion dollars, and the required environmental permits needed to build one could hold the program up for years. There was an existing facility, called the LOFT, already certified at the Idaho National Engineering Lab that, with minor modifications, could have been used to support testing a
small
NTR engine of perhaps 10,000 lb thrust. This would have saved a great deal of time and money. Such a small NTR would have been
large enough
to push the relatively small Mars Direct spacecraft out of LEO onto a trans-Mars trajectory, and would also have been
small enough
to be useful for a multitude of non-SEI applications, including sending unmanned probes to the outer solar system and military satellites to geosynchronous orbit. These other missions had real budgets, whereas SEI did not.
