In the preceding articles, we considered the advantages and disadvantages of lunar mission architectures strictly from the point of view of the lunar base program itself and found that the currently fashionable quadruple launch, quadruple rendezvous plan was inferior to other options employing heavy-lift vehicles (HLVs) to accomplish missions in a single launch.
These conclusions, however, become even more forceful when one considers that the president’s directive calls for a Moon-Mars program, not just a Moon program. The quadruple plan accepts a plethora of additional development and recurring costs, and mission risks for the sole purpose of avoiding the development cost of an HLV. Yet, since the goal of the program is to get humans to Mars, an HLV and a potent trans-Mars injection stage will need to be developed anyway. So on a cost basis the quadruple plan will lose twice over, since there will be much more hardware to develop for Mars.
Furthermore, in addition to imposing the greatest mission risk for lunar explorers through its own excessive complexity, the quadruple plan also will increase the risk to Mars explorers, because the quadruple lunar plan will not test the Mars mission hardware. In contrast, using the HLV-large upper-stage-based options, hardware that is directly applicable to human Mars exploration will be developed and extensively exercised in advance in the course of the lunar program. This will reduce the risk of human Mars exploration and shorten the schedule required to transit from lunar activities to Mars exploration.
The quadruple plan also undermines the lunar program rationale. The purpose of the lunar base, as stated in the president’s directive, is to prepare the way for human Mars exploration. The lunar program as defined in the quadruple architecture does not do this, and thus would open the program up to the valid criticism that it is not, in fact, delivering the goods that it is promising.
Human Mars exploration offers the potential to resolve fundamental issues concerning the prevalence and diversity of life in the universe. These are questions of deep scientific, philosophical and public interest, and the commitment to the search for truth to answer them provides a strong and solidly rational intellectual foundation for the program.
A properly designed lunar transportation system could support this goal directly, and by doing so, make the lunar program much more defensible in the scientific, public and political arenas. This is critical for program success, because the extended schedule of the program requires that it survive through many changes of political fortune. By designing a lunar transportation system that is useless for Mars, the quadruple plan would threaten to fatally weaken the program.
To see why this is so, consider that, depending upon the mission architecture selected, a human Mars mission will require two to four times the launch mass of a lunar mission. Therefore, a hardware set that requires four launches and four rendezvous to accomplish a mission to the Moon will need eight to 16 launches and a similar number of rendezvous operations.
Thus the quadruple plan hardware set, which provides a foundation for a lunar program that is simply unsound, becomes utterly ludicrous when extended to Mars.
Now consider our recommended alternative: a Direct Return lunar mission architecture accomplished with a single launch of an HLV.
This booster employs a hydrogen/oxygen upper stage, which delivers 38 metric tons of payload from low Earth orbit to low lunar orbit. The delta-V (change in velocity) required to accomplish this maneuver is 4.2 kilometers per second, which is exactly the same as the delta-V needed to send a payload from low Earth orbit to Mars on a six-month trajectory. Moreover, if it should be decided to abort the mission, this particular trajectory will return the payload to Earth precisely two years after the date of departure.
This is the fastest free-return option that is physically possible for any Mars mission, and is therefore to be preferred to higher energy trajectories, which in addition to imposing much greater propulsion and mass penalties on the mission, are actually less safe, as they make free-return survival nearly impossible.
Thus an HLV with an upper stage optimized for delivery of substantial payloads from low Earth orbit to low lunar orbit is in fact the best system for sending humans to Mars.
Using such a system, it is possible to undertake human Mars missions without on-orbit assembly using either a two-launch Mars Direct or three-launch Mars Semi-Direct mission plan. (references: R. Zubrin and D. Weaver “Practical Methods for Near-Term Piloted Mars Missions,” AIAA 93-2089, and R. Zubrin “The Case for Mars,” The Free Press, Simon and Schuster, New York, 1996.)
For example in the Mars Direct plan, the first HLV delivers an unfueled Earth Return Vehicle to Mars, which then manufactures its return propellant by reacting a small amount of onboard hydrogen with carbon dioxide from the Martian atmosphere, which is 95 percent CO2.
After this is done, a second HLV launch sends the four-person crew to Mars in a large habitat module that is closely based upon the hab employed in the Direct Return lunar plan.
The crew lands their hab near the Earth Return Vehicle, and then for the next 18 months, conducts an intensive program of field exploration using the hab as their home and laboratory. At the end of 18 months, the launch window to Earth opens, and the crew transfers to the Earth Return Vehicle for a six-month flight home. The hab module is left behind on Mars, so that each time a mission is flown, another hab is added to the base, or alternatively, a string a mini-bases can be set up supporting field exploration on an extended geographic scale.
Alternatively, if a six-person crew is desired, the three-launch Semi-Direct plan can be employed. If necessary, mission mass margins could be greatly expanded by replacing the hydrogen- oxygen upper stage with a small (15,000 pounds thrust) expendable nuclear thermal rocket (NTR) stage.
NTR units of this size (and considerably larger) were ground tested in the United States in the 1960s, and their feasibility and performance is not in doubt.
Highly versatile, small expendable NTRs also could be used with medium-lift launch vehicles to enable potent outer solar system robotic missions with flight times to Jupiter of 2.7 years. Using such near-term technology nuclear stages, the trans-Mars throw capability of our heavy-lift booster would grow from 38 metric tons to 60 metric ton s.
We do not need giant futuristic space ships to go to Mars. We can do it with multiple launches of the same system we use to go to the Moon – provided we choose our lunar mission system design correctly.
Finally, there is the issue of program phasing. If we attempt our lunar program with a Moon-only hardware set such as that employed by the quadruple launch plan, then cost considerations will require abandoning the lunar base in order to shift the resources required to create and operate new flight systems for Mars.
However, if we design a hardware set that is appropriate for both the Moon and Mars, then lunar and Mars programs can proceed in parallel. Thus, for example, if the single-launch Direct Return lunar architecture with in-situ propellant production recommended in the previous articles is implemented, only two launches per year will be required to support continuous activities that visit 12 locations per year.
This would leave plenty of launch capability free to support human missions to Mars and the near-Earth asteroids as well. In contrast the quadruple plan would require four launches to support an anemic lunar program that visits only one site per year, making scrapping the Moon base a precondition for any further exploration.
If we wish to go to the Moon to stay, we must do it with flight systems that support Mars exploration as well. Such flight systems are also best for supporting the lunar base considered in isolation.
The right way to design a Moon-to-Mars program is not to create a Moon-only transportation system, and then drop it and build something else when it is time to go to Mars. Rather, the right way is to start by designing a minimum-cost Mars mission, and then define a lunar flight system using a modular subset of the Mars mission hardware.
Done this way, only one hardware set will need to be developed instead of two, Moon missions will validate Mars hardware directly, and lunar activities can continue after Mars exploration begins. Proceeding this way, the program will save tens of billions of dollars, decades of time and dozens of lives. Going this way, we can make our gains in space permanent.
It is the intelligent way to go.