The incredible, expendable Mars mission
An analysis of NASA’s 2009 Mars Design Reference Architecture 5.0
by John Strickland
|The document covers what can only be described as an extreme example of complex, expendable design.|
Since this design evolved and was released several years before it became obvious that there was a lot of water and ice on Mars, and just before the first launch of a large, private rocket, it would be unfair to compare the 2009 scenario with what could be envisioned now. But with those provisos, it is reasonable to point out the limitations of the 2009 design in light of current knowledge and thinking.
A reference mission is intended to be the best and most realistic design at the moment of release, and a baseline for other future variants of it. However, the mission design’s default philosophy seems to have been to maximize the number of individual expendable vehicles and components, and minimize the amount of redundancy and reusability.
The document covers what can only be described as an extreme example of complex, expendable design. Even though a team of very capable people worked on the document, it is not easy to read, and there is no single good overview of the overall mission sequence. It took me many hours of careful reading to even figure out which vehicle the astronauts were supposed to use to land on Mars. Some of the tables as provided do not seem to add up as presented.
The primary mission design was intended to use nuclear thermal propulsion for it in-space components, with lesser attention given to a version that used only chemical propulsion. The nuclear choice was and still is very unrealistic, since (1) there will be no funding available for a revival of a nuclear thermal rocket program until after the SLS development program is finished, whenever that is; and (2) anti-nuclear sentiment is still very strong among the activists who will never go away. The program design did provide a chemical rocket alternative, but it did not provide good breakdowns of its propulsion needs. Nuclear thermal rockets for use in space are very desirable, and nuclear power reactors for use at surface bases are an absolute necessity, but the former should not have been included as a foundational technology for the baseline mission. My analysis and description here thus covers only the chemical mission profile alternative.
The mission was to be launched from low Earth orbit (LEO). As a result, the mission needs a large amount of fuel just to depart from LEO. The crew habitat vehicle with its Earth return propulsion module is said to be too large to use an aerocapture maneuver to reach Mars orbit, requiring an expensive propulsive orbit capture. The plan provides for three missions to Mars, lasting over a decade, and each landing at a different location. There is little or no redundancy among components, such as the nuclear power reactor for use on the surface. The mission uses essentially all expendable rocket and vehicle components, and does not leave any useful components in low Mars orbit or Earth orbit for use by the next mission. Thus, there is no advancement in safety or capability from one mission to the next.
The missions were to have been launched by the Ares V, the immediate predecessor to the current Space Launch System (SLS) heavy-lift rocket. These two boosters are roughly comparable, so that the mass that a single Ares could have launched into LEO on is similar to that for an SLS, with a maximum payload of about 120–130 tons. Since the missions are launched from LEO, each cargo vehicle uses two small expendable stages to boost it into its trans-Mars injection (TMI) trajectory, while the more complex crew vehicle uses three small expendable stages. A total of about 20 separate major vehicles or components are needed in LEO to assemble the three vehicles for each mission. This takes 12 Ares V flights; the number of SLS flights needed for the same mission design would be about the same.
|The mission uses essentially all expendable rocket and vehicle components, and does not leave any useful components in low Mars orbit or Earth orbit for use by the next mission. Thus, there is no advancement in safety or capability from one mission to the next.|
Each Mars mission uses two initial cargo vehicles sent on flights from LEO into a Mars transfer orbit. One cargo flight includes a Mars Ascent Vehicle (MAV, but referred to cryptically as a Descent-Ascent Vehicle, or DAV), to be used by the crew to return to orbit, along with fuel production equipment. The other cargo flight has a combination surface habitat and crew lander (SHAB). On reaching Mars, the two cargo flights each use a giant tubular aeroshell, 10 meters wide by 30 meters long, with each cargo item positioned inside one of the shells. One effective design feature is that the huge aeroshells also double as fairings for the cargo during launch from Earth. The shells, each with its own attitude control system, do an aerocapture into an elliptical Mars orbit using a single braking pass through the Mars atmosphere. The use of aerocapture is laudable, but the rest of the sequence is not.
While one cargo vehicle stays in Mars orbit, the other vehicle with the MAV then uses its giant aeroshell to enter the Mars atmosphere. The shell splits open and is thrown away during the final stage of entry, allowing the vehicle inside it to emerge and then descend to the surface on rocket power. Similar scenarios have been used in some other technical papers on Mars landing methods. No attempt was made to use expendable drag-enhancing devices. The fuel production equipment landed with the MAV then uses Martian carbon dioxide to produce liquid oxygen (but no methane fuel) and store it in the MAV’s tanks for the eventual ascent of the crew back into orbit. This idea was derived directly from Robert Zubrin’s fine Mars Direct scenario. No surface material such as Mars ice is used to make hydrogen (needed to produce methane), since this mission design was created before the realization that abundant frozen water lies just below the surface in many locations on Mars. There are also no mobile robots provided to do the work of digging up the ice for conversion to rocket fuel. The power is provided by a small nuclear reactor, which must be off-loaded and moved away from the MAV robotically before it is turned on. Fear that the reactor would be damaged if it was buried helped to kill the idea of an excavator for the mission.
Twenty-six months after the cargo launches, the assembly of a complex Mars Transfer Vehicle (MTV) in LEO is finished. It consists of a crew habitat module with solar panels for use during the long transit to Mars, an Orion capsule for Earth return, three small expendable Trans-Mars Injection (TMI) stages, an expendable Mars Orbit Insertion (MOI) stage, and an expendable Trans-Earth Injection (TEI) stage. A six-person crew is launched to the MTV on an expendable Orion capsule. This capsule that delivers the crew from Earth to the Mars-bound crew vehicle is then simply jettisoned. The MTV is then launched into a Mars transfer orbit.
When the crew arrives at Mars, it uses the MOI stage to enter Mars orbit via rocket propulsion, not via aerocapture. The composite crew vehicle as designed is simply too large to fit behind an aeroshell. This places the MTV in the same elliptical 250-by−33,793-kilometer orbit (with a one-Mars-day period) as the remaining cargo vehicle. The mission assumes the use of methane-oxygen RL10-type engines for operations at Mars. The crew then does a rendezvous with the habitat/lander module. They transfer into this large module, which is still inside one of the giant aeroshells, and from there descend to the surface the same way the first cargo payload does. The habitat/lander vehicle, with the crew’s food stores, also has a rover, which hopefully has enough range to carry the crew to the MAV if they do not land close to it. There is no provision for an abort back to orbit since the crew landing vehicle (SHAB) is inside the giant aeroshell until just above the surface, and that vehicle cannot be used for ascent. If the complex aeroshell fails during entry or fails to open, the crew is doomed. The crew habitat, which is part of that vehicle, leaves the crew exposed on landing legs high above the surface, with little radiation protection.
|The architecture would require the design, development, and construction of at least 10 different types of expendable vehicles during the decade before the first mission.|
The crew stays on Mars for about 500 days, making each mission about 900 days long. This is actually the preferred mission plan now, especially if a permanent base was being established. Each of the three missions creates a temporary base with essentially no backups or duplicates of critical equipment, such as power reactors and MAVs. The mission plan does provide for crew rovers with a range of 100 kilometers and the ability to return up to 250 kilograms of Mars samples to Earth. At the end of the mission, the crew walks or takes the rover to the MAV, which then uses its partly Mars-generated fuel to return to Mars orbit. There, they rendezvous with and transfer back to the MTV for the return to Earth. The MAV cannot be used for another trip and is jettisoned. The Trans-Earth Insertion (TEI) stage then pushes them out of Mars orbit into an Earth transit orbit. On the return to Earth, the transit habitat and all of its remaining food is jettisoned and whips past the Earth back into solar orbit. The Orion capsule that brings the crew back to a water landing is the only element to return to Earth, and its reuse is very doubtful.
For the entire three-mission Mars program as envisioned in 2009, it would thus take a total of 36 Ares V or SLS (130-ton version) flights to LEO to bring up all of the modules. At the rate of two SLS missions per year, it would thus take 18 years of production, and many years of very good storage, to accumulate this many boosters. The total number of major modules and components needed in orbit for the program is about 60, some of which share flights on the same booster. It would take between three months and a year to assemble each of the nine Mars-bound vehicles in LEO, with no provision for adding the cryogenic fuel after assembly. It would require the design, development, and construction of at least 10 different types of expendable vehicles during the decade before the first mission.