The Case for Mars (17 page)

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 t
he 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.

This ability to pile nearly unlimited but usefuredundancy into the Mars surface camp compared to what can be provided to the crew in transit is another reason why Mars mission planners should try to maximize the crew’s time on the Martian surface and minimize the time spent in transit.
Mission assets can be concentrated cumulatively on the Martian surface.
If this is done, then
the Martian surface becomes the second safest place in the solar system.

BACKUP OR ABORT?

In the past, many Mars mission plans were constructed around the following scenario: Days before arrival at Mars, perhaps on arrival, the crew of a Mars expedition realizes they have to abort the mission. Our concern now is not why they have to abort, but how. How do they reach a safe haven? Well, obviously they have to return to Earth, and, though they had planned for a lengthy conjunction-class surface stay, they have fortunately brought along enough fuel for an opposition-class quick return to Earth. They can power away from Mars and head for Earth via a Venus flyby. They don’t have to wait for a Hohmann transfer window to open, and who would in the case of an emergency? But let’s think about this. There are costs involved in planning a mission around this abort option, and they are not trivial. First off, such missions require the extra payload needed both for a long surface stay and for a long trans-Earth cruise, as well as the extra propellant to send all this stuff onto a very high-energy opposition-class trajectory. It is hard to imagine a more costly approach to mission design. Furthermore, if the abort is not exercised, all of the extra mass delivery entailed by such a strategy is for naught. Moreover, the opposition return trajectory subjects the crew to 1.5 years of continuous deep space radiation doses (probably in zero gravity as well), high solar radiation during a close-in pass through the inner solar system, and very high gloads at Earth return. All in all, such an abort return may be problematical to su
rvive, and obviously, even if the crew does survive, the mission is a complete loss from the standpoint of exploration.

In the end, mission plans of this sort would do little to increase effectiveness, yet greatly increase mission mass and cost. Fortunately, we can solve the problem of what to do in an emergency by questioning one very basic assumption: does Earth have to be the only safe haven? The answer is a resounding no. Rather than design the mission around Earth-oriented abort options, the right strategy is to base the plan upon creating a safe haven in advance on the Martian surface, and aborting to it as our primary option. Such a haven can be reached much more quickly by an outbound crew than Earth can, and is thus much more likely to represent a real source of help in the event of trouble. The primary abort option is thus the same as the primary mission mode, imposes no mass penalty, and its invocation still allows the mission to be carried out. There are secondary aborts that do not involve carrying out the mission, but
the mission is not designed around them.
Put another way, rather than design the mission around
abort options
, you need to design it around a hierarchy of
backup plans.
This is the way the Mars Direct mission handles things.

Let’s start the mission in LEO and see what kinds of aborts and backup plans are available to the crew as the mission proceeds. The first major event of the mission is the engine burn that will send the spacecraft onto trans-Mars injection (TMI). The total ΔV required to perform this maneuver is 4.3 km/s to put the spacecraft onto a fast conjunction, two-year free-return trajectory, which will get the crew to Mars in 180 days or so. However a ΔV of 3.7 km/s is good enough to send the crew on a 250-day minimum energy trajectory to Mars, so if the engine burn is successful to at least that point, the crew will be sent on its way to carry out the mission. If the propulsion system on the TMI stage should fail to provide a ΔV of 3.3 km/s—the ΔV required to escape from Earth—the spacecraft will be left in an elliptical orbit about the Earth. In this case, the crew will use the hab’s own propulsion system to dip the perigee (lowest point) of their orbit very slightly into the uppermost part of the Earth’s atmosphere. After a number of orbits the drag induced by this maneuver will lower the apogee (highest point) of the orbit to altitudes that can be reached by the Space S
huttle (such slow-aerobraking apogee-reduction maneuvers were successfully undertaken by the
Magellan
spacecraft at Venus in 1994), after which a small propulsion burn by the hab will raise its perigee out of the atmosphere, circularizing and stabilizing the orbit. Once this is done the crew can be retrieved by the Space Shuttle (although there’s not any rush; they have enough supplies for almost three years on board). If the TMI stage propulsion system should fail between 3.3 and 3.7 km/s ΔV, the crew can get back to Earth orbit by retrofiring the hab’s propulsion system; between its midcourse correction, Mars orbit propulsion maneuver system, and landing propellant, the hab has a total ΔV capability of 0.7 km/s—more than enough to negate the maximum excess ΔV of 0.4 km/s that could strand the crew between Mars and Earth. All of this, however, is hypothetical. A properly designed TMI stage would use multiple engines, each with a reliability for a burn of this length on the order of 0.99. The probability that two such engines would both fail is about 1 in 10,000, a negligible portion of total mission risk.

Once the TMI and midcourse burns have been successfully completed, the hab is targeted for an aerocapture pass at Mars. During the first 95 percent of the outbound flight, several options, including free-return aborts and powered flyby maneuvers, can be undertaken. However, once the lander has been targeted for an aerocapture trajectory (typically several days prior to aeroentry), the options of a free-return or powered flyby trajectory abort back to Earth become increasingly tenuous. At some point, on the order of several hours to one day prior to aerocapture, the ability to perform any trajectory abort is lost completely. But you have to make up your mind sometime, and the fact that the free return is available for the first 175 days of your 180-day trip is nothing to sneeze at.