The Case for Mars, page 12
The next most important job on the mission is that of field scientist. Remember, the exploration of Mars is the raison d’être of a human mission to Mars. After those needed to get the crew to Mars and back, the next most important personnel are those essential to competently carry out the mission’s exploration goals. Since no science return would effectively be a form of mission failure, once again I recommend carrying two people for the job. One of the field scientists should be a geologist, oriented toward exploring the resources and understanding the geologic history of Mars. The other should be a biogeochemist, directed toward exploring those aspects of Mars upon which hinge the question of past or present life. The biogeochemist would also conduct experiments to determine the chemical and biological toxicity of Martian substances to terrestrial plants and animals, and the suitability of local soils to support greenhouse agriculture.
And that’s it. With two mechanics and two field scintists, we have the ability to split the crew into two groups in which no one is alone (one out in the field with a ground rover, say, while the other remains at the base camp) and someone expert at fixing malfunctioning equipment and someone capable of doing scientific work are present at all times. There is no need for people whose dedicated function is “mission commander,” “pilot,” or “doctor.” True, the mission will need someone who is in command, and a second in command for that matter, because in dangerous circumstances it is necessary to have someone who can make quick decisions for all without electioneering or debate. But there is no room for someone whose sole function is to manage others to get the job done. Similarly, there can be no one on board whose job description is “pilot.” The spacecraft will be capable of fully automated landing, and piloting skills would at most be useful as a contingency backup to the automated flight system during a few minutes of the two-and-one-half-year mission. If such a manual flight control backup is desired, then one or more members of the crew could be given cross training as a pilot (it’s much easier to train a geologist to be a pilot than a pilot to be a geologist). Finally, no ship’s doctor as such. The great Norwegian explorer Roald Amundsen always refused to carry doctors on his expeditions, noting that they were injurious to morale and that the large majority of medical emergencies that occur on expeditions can be handled just as well by experienced explorers. And, truth be known, behind their public relations facade, nearly all astronauts hate space doctors. You would too in their shoes—just think about trying to get a hard job done while somebody constantly jabs you with needles, wires, and thermometers. In place of a physician, all crew members will be trained in first aid, and expert systems on board and medical consultation from Earth will be available to diagnose readily treatable conditions (ear infections and the like). Such diagnoses could be assisted by having a crew member be someone who had either practiced general medicine at some point earlier in his or her career or who had been crossed-trained to a medical assistant’s level of knowledge, and equipping this individual with a country doctor’s black bag and a stock of broad-spectrum anti-biotics. The biogeochemist would be a natural candidate for such a cross-trained role. However, the idea of having a dedicated top-notch doctor on board who spends his or her time reading medical texts and honing skills by practicing surgery with virtual reality gear, or worse, being a pest by subjecting the rest of the crew to an in-depth medical study, is cumbersome and unnecessary.
To summarize in Star Trek terminology, what a piloted Mars mission needs are two “Scottys” and two “Spocks.” No “Kirks,” Sulus,” or “McCoys” are needed, and more importantly, neither are the berths and rations to accommodate them.
We can do the mission with a crew of four.
DIRECT LAUNCH
All interplanetary missions flown to date have been flown by “direct launch”—a launch vehicle lifts a spacecraft to LEO, and then uses its upper stage to throw the spacecraft on a trajectory to its planetary destination. It’s the way the Mariner and Viking missions reached Mars, and it’s also the way the Apollo lunar missions reached the Moon. No missions beyond LEO have ever been flown by lifting the payload to an orbiting spaceport and transferring it to a freshly fueled interplanetary cruiser just back from Saturn. No missions beyond LEO have ever been flown on an interplanetary spaceship constructed in space. The association in many people’s s of humans-to-Mars missions with such futuristic spaceship/spaceport scenarios has caused human Mars exploration to be relegated out of today’s world and into the world of “The Future.” But, if a manned Mars mission can be done by direct launch, then we can do it. Get rid of the spaceships and spaceports, and a human mission to Mars moves from the “parallel universe” of The Future into our universe. If we can do it by direct launch, then 90 percent of everything we need to send humans to Mars is available now.
We’ve chosen the trajectory and the crew size. Now, can a realistic heavy-lift launch vehicle deliver, in no more than two tandem launches per mission, everything that’s needed to conduct a four-person Mars mission in accord with the flight plan we’ve chosen? Let’s see.
There is nothing magical about a heavy-lift vehicle—the United States built and operated one thirty years ago. The Saturn V booster that sent the Apollo astronauts to the Moon went into operation in 1967 after a five-year development program, and operated without a single failure for an eight-year period until 1973 when the last working unit launched the Skylab space station. The Saturn V could lift 140 tonnes to LEO. If we wanted equivalent capability today, one foolproof way of doing it would be to reengineer the dies and start producing Saturn V’s. There are other ways to get the job done, though. For example, using Space Shuttle hardware it’s possible to produce a heavy-lift booster in the same class by attaching a pod of four Space Shuttle main engines (SSME) to the bottom of a Space Shuttle external tank (ET), attaching two Space Shuttle solid rocket boosters (SRBs) to either side of the ET, and positioning a hydrogen/oxygen upper stage on top of the ET. This is the Ares booster design David Baker developed for Mars Direct. Depending on the thrust of the upper-stage motor used, an Ares could deliver between 121 tonnes (with a 250,000 lb upper-stage thrust) and 135 tonnes (with a 500,000 lb thrust SSME on the upper stage) to LEO. The Russians have a heavy-lift vehicle right now called the Energia. The existing model can only lift 100 tonnes to LEO, but an upgraded design, the Energia-B, boasts a 200 tonne capability. During the Space Exploration Initiative’s short life, NASA developed dozens of heavy-lift booster designs of all sorts with capacities between 80 and 250 tonnes. In short, if the United States wants a heavy-lift booster, we can certainly get one.
While on paper a booster can be designed to any size desired, reality is different. Some super boosters have been designed with 1,000-tonnes-to-LEO capability. Sounds great, but they would probably blow away Orlando when they took off (or at least the Kennedy Space Center). So let’s be exceptionally conservative and assume that the United States—today—can build a heavy-lift booster with a capability no greater than the one we fielded in the 1960s. Let’s baseline our booster at 140-tonnes-to-LEO capability, exactly the same as a Saturn V. Would such a launch system be good enough to launch the Mars Direct mission by direct throw?
Part of the answer to this question is given in Table 4.3, which shows the amount of payload that a single launch of our 140-tonne-to-LEO booster can deliver to the Martian surface after a preliminary aerocapture at Mars. The table shows variants for both the cargo and piloted outbound trajectory, and for the assumption that the third stage of this vehicle is either a state-of-the-art hydrogen/oxygen chemical stage with a specific impulse of 450 seconds, or a near-term nuclear thermal rocket (NTR) with a specific impulse of 900 seconds.
TABLE 4.3
Payload Delivery to Martian Surface from 140 Tonne to LEO HLV
The payload delivery capabilities shown in Table 4.3 assume that aerobraking is employed to capture the spacecraft into Mars orbit. This is clearly the optimal way to perform Mars orbital capture (MOC) in the Mars Direct missions because all the payload is destined for the Martian surface, and so it must carry an aeroshield in any case. Using aerocapture in the Mars Direct mission thus eliminates a significant propulsive ΔV essentially “for free.” If rocket propulsion had to be employed for this maneuver instead of aerobraking, the payloads delivered would be about 25 percent less. In mission plans such as that in the NASA 90-Day Report, aerocapture faced many technical difficulties. Aerobraking the enormous “Battlestar Galactica” spacecraft the plan called for would require huge aeroshields that could only be built in orbit, and that, as I’ve noted, is really not a credible proposition. Furthermore, the opposition-class trajectories employed by those missions hit Mars really hard, thereby increasing the heating and mechanical loads on the aerobrakes during atmospheric entry. Mars Direct uses lower energy conjunction-class trajectories that have lower entry velocities and thus lower heating rates and experience much lower aerodynamic deceleration forces. More decisively, the spacecraft that need to be decelerated in the Mars Direct plan are relatively small, so that aeroshields big enough to protect them can easily be made to fit inside the launch vehicle’s payload fairing. This can be done in one of two ways; either by using flexible fabric umbrella-shaped aerobrakes that fold around the bottom of the payloads, as in the original Mars Direct design, or by replacing the fairing of the launch vehicle with a rigid, bullet-shaped shell that fits over the payload from on top. Both are feasible, and when used with Mars Direct-sized payloads, either can be launched “all-up” without any need for on-orbit assembly. In addition, the guidance, navigation, and control requirements on Mars Direct aerocapture are less than in those plans where a subsequent Mars orbit rendezvous is anticipated, because it does not really matter exactly what orbit the vehicle captures into (since the orbit will be “erased” after the vehicle lands), so long as its orbital inclination is within the broad tolerances that will allow access to the designated landing site.
To deliver payloads, we can also employ an approach known as direct entry. As with aerocapture, aerodynamic drag against a planet’s atmosphere, not rocket propulsion, decelerates the payload. There is a difference, though. With aerocapture, the spacecraft dips into the planet’s atmosphere just enough to slow down and then reemerge from the atmosphere to place itself in orbit. In the case of direct entry, the entering spacecraft plunges deep into the atmosphere until all of its velocity is shed and then proceeds directly to landing. Most people consider aerocapture the better flight plan for a piloted Mars mission, because if the weather is bad, it allows the crew to assume a station on orbit until conditions improve for a landing. With direct entry, the vehicle is completely committed for a landing immediately after Mars arrival. Nevertheless, direct entry will be used on both the Mars Pathfinder and Mars Surveyor ’98 (unmanned) missions, currently scheduled for launch in 1996 and 1998, respectively. If these missions are successful, a data base will exist that may encourage mission designers to employ direct entry on the piloted Mars mission as well.
The bottom line in allof this, however, is the payload delivered to the surface. If chemical propulsion is used, then the unmanned cargo flight launched by a single 140-tonne-to-LEO booster can deliver 28.6 tonnes to the Martian surface, while the faster piloted flight can deliver 25.2 tonnes. Can a manned Mars mission be designed within these mass limits? If it can’t, we could always design a bigger booster or go ahead and develop the NTR stage. But let’s see if we can make it with nothing better than a Saturn V and chemical propulsion. If we can, then more advanced technologies or propulsion capabilities and their associated benefits are icing on the cake.
SUPPLIES FOR THE CREW
Is our mass delivery capability sufficient? Well, let’s take a look at the mission’s supply needs. In Table 4.4 we see the consumables required for each member of the crew per day for each leg of the mission and the totals required to support a crew of four in each of the two habitation systems, the hab (which houses the crew during the outward voyage and during the surface stay) and the Earth return vehicle (ERV) cabin. The numbers given under the need/man-day column are NASA standards (quite liberal in the washing water category, as you may notice), except that I have replaced 0.13 kg/day of dehydrated food with 1.0 kg/day of whole (wet) food. Such a mixed diet is much better for crew morale on a long mission than dehydrated rations only, and actually costs the mission very little in the way of added mass, since the water content of the whole food serves to make up for the losses in the potable water recycling system. The life-support system assumed for the crew is a fairly low-efficiency physical/chemical one that recycles 80 percent of the oxygen and drinking water and 90 percent of the washing water (which can be of lower quality). Such a system is much simpler and less power hungry than futuristic ones based upon a closed ecology wherein 100 percent of food, oxygen, and water will supposedly be recycled.
Read between the lines of Table 4.4 and you’ll immediately note the huge advantages Martian resources give us. In addition to manufacturing fuel, the ERVs produce copious amounts of water and oxygen. Without the ERV chemical processing plant, we would need to ship out an additional 7 tonnes of consumables with the hab. This would increase the required consumables from 7 tonnes to 14 tonnes, which since we only have the capability of delivering a 25-tonne hab, would be very difficult to accommodate. The 9 tonnes of water each ERV produces provide an excess over NASA nominal water requirements and that should be a real plus for the morale of a hardworking crew on a desert planet. For these reasons, in Table 4.4 there is no requirement to transport oxygen or water to support the hab surface stay. We also see that each hab flies out to Mars with enough food for an 800-day mission, which gives it more than enough provisions to handle a two-year free-return abort. In the latter case, the crew in the hab will have to exploit the 5 tonnes of methane/oxygen propellant in the lander stage to provide extra water and oxygen (unneeded as propellant in the event of a free return, which is concluded by aerocapture into Earth orbit), and reduce their use of wash water to 40 percent NASA nominal levels. This will be uncomfortable and bad for morale, but it could be endured and survived, which is the only issue in the event of such an abort. Also, in Table 4.4, there is no wastage of potable water shown because potable water lost due to inefficient recycling is made up by water added to the system from the use of whole food.
TABLE 4.4
Consumable Requirements for Mars Direct Mission with Crew of Four< />TABLE 4.4p>
Given these consumable requirements, the mass allocations for the ERV cabin and the hab can be assigned, and are presented in Table 4.5.
TABLE 4.5
Mass Allocations for Mars Direct Mission Plan
The ERV payload shown above will, after landing, convert its 6.3 tonnes of hydrogen feedstock into 94 tonnes of methane/oxygen propellant and 9 tonnes of water. Of the 94 tonnes of propellant produced, 82 tonnes will be used by the ERV for rocket propulsion to return the crew to Earth, while 12 tonnes will be available to support the use of ground vehicles using internal combustion engines. If we count only the water and the 12 tonnes of rover propellant, and add them to those other parts of the ERV payload that are useful while on the Martian surface (such as the ERV cabin with its power and life-support system, the power reactor, the EVA suits, light truck, etc.), we find that each ERV payload delivers 36.5 tonnes of useful surface payload. The first Mars mission crew will have with them on the Martian surface two ERVs (the precursor, which made its propellant in advance of the crew launch, plus the backup, which flew out in tandem with the crew) plus one hab (which has 24.7 tonnes of useful surface payload). This adds up to 97.7 tonnes of useful surface payload available to the crew, roughly four times that of the traditional opposition-class mission featured in the NASA 90-Day Report (which had more than double this mission’s initial launch mass). The surface payload available to the crew includes four pressurized volumes capable of supporting life: the hab, the two ERV cabins, and the pressurized rover. The crew thus has many safe havens available in case the primary life-support system in the hab should malfunction. In addition, they have 12 EVA (extra-vehicular activity) suits, five motorized vehicles (the pressurized rover, the two open rovers, and two light trucks), five primary power supplies (two 80 kWe nuclear reactors plus three 5 kWe solar power systems in the hab and the two ERVs), five backup power supplies (the engines on each of the motorized vehicles can be used to turn a generator), a thousand kilograms of combined field and lab scientific equipment, 14 tonnes of consumables from Earth plus 18 tonnes of Mars-produced water and 24 tonnes of rover propellant, plus two chemical plant systems either of which is capable of producing oxygen from the Martian atmosphere at a rate roughly fifty times that required by the crew for life support. The plan must therefore be considered extremely robust. And in case that’s not good enough for you, the redundancy can be multiplied further by taking advantage of the first launch window in which no crew is sent to Mars to send a complete hab, loaded with supplies but containing no crew, to accompany the precursor ERV to the first landing site (thus making the program’s launch schedule two heavy-lift booster flights every other year, including the first). In that case, the crew would have available to them six habitable volumes including two complete habs, plus two complete ERV cabins, plus . . . but I think you get the point. No program of exploration on Earth has ever been conducted with anything approaching this level of backup redundancy. And we’ve done it all with 1960s technology Saturn V’s, chemical propulsion, and no on-orbit infrastructure, assembly, or docking operations, or orbital rendezvous of any type at any point in the mission.
