Zubrin was struck by many good and powerful ideas in the Musk plan. However, Musk’s plan assembled some of those good ideas in an extremely suboptimal way, making the proposed system impractical. Still, with some corrections, a system using the core concepts Musk laid out could be made attractive — not just as an imaginative concept for the colonization of Mars, but as a means of meeting the nearer-at-hand challenge of enabling human expeditions to the planet.
Zubrin explains the conceptual flaws of the new SpaceX plan, showing how they can be corrected to benefit, first, the near-term goal of initiating human exploration of the Red Planet, and then, with a cost-effective base-building and settlement program, the more distant goal of future Mars colonization.
Robert Zubrin, a New Atlantis contributing editor, is president of Pioneer Energy of Lakewood, Colorado, and president of the Mars Society.
* Have the second stage go only out to the distance of the moon and return to enable 5 payloads to be sent instead of one
* Leave the 100 person capsule on Mars and only have a small cabin return to earth
* use the refueling in orbit and other optimizations to enable a Falcon Heavy to deliver 40 tons to Mars instead of 12 for exploration missions in 2018, 2020 etc...
* Reusable first stage makes rocketplanes going anywhere point to point on Earth feasible. Falcon Heavy would have the capacity of a Boeing 737 and could travel in about one hour of time anywhere
There are videos of the Elon Musk presentation and an interview with Zubrin about the Musk plan at the bottom of the article
Design of the SpaceX Interplanetary Transport System
As described by Musk, the SpaceX ITS would consist of a very large two-stage fully-reusable launch system, powered by methane/oxygen chemical bipropellant. The suborbital first stage would have four times the takeoff thrust of a Saturn V (the huge rocket that sent the Apollo missions to the Moon). The second stage, which reaches orbit, would have the thrust of a single Saturn V. Together, the two stages could deliver a maximum payload of 550 tons to low Earth orbit (LEO), about four times the capacity of the Saturn V. (Note: All of the “tons” referenced in this article are metric tons.)
At the top of the rocket, the spaceship itself — where some hundred passengers reside — is inseparable from the second stage. (Contrast this with, for example, NASA’s lunar missions, where each part of the system was discarded in turn until just the Command Module carried the Apollo astronauts back to Earth.) Since the second-stage-plus-spaceship will have used its fuel in getting to orbit, it would need to refuel in orbit, filling up with about 1,950 tons of propellant (which means that each launch carrying passengers would require four additional launches to deliver the necessary propellant). Once filled up, the spaceship can head to Mars.
The duration of the journey would of course depend on where Earth and Mars are in their orbits; the shortest one-way trip would be around 80 days, according to Musk’s presentation, and the longest would be around 150 days. (Musk stated that he thinks the architecture could be improved to reduce the trip to 60 or even 30 days.)
After landing on Mars and discharging its passengers, the ship would be refueled with methane/oxygen bipropellant made on the surface of Mars from Martian water and carbon dioxide, and then flown back to Earth orbit.
Zubrin's Problems with the Proposed Spacex System
The SpaceX plan as Musk described it contains nine notable features. If we examine each of these in turn, some of the strengths and weaknesses in the overall system will begin to present themselves.
1. Extremely large size. The proposed SpaceX launch system is four times bigger than a Saturn V rocket. This is a serious problem, because even with the company’s impressively low development costs, SpaceX has no prospect of being able to afford the very large investment — at least $10 billion — required to develop a launch vehicle of this scale.
2. Use of methane/oxygen bipropellant for takeoff from Earth, trans-Mars injection, and direct return to Earth from the Martian surface. These ideas go together, and are very strong. Methane/oxygen is, after hydrogen/oxygen, the highest-performing practical propellant combination, and it is much more compact and storable than hydrogen/oxygen. It is very cheap, and is the easiest propellant to make on Mars. For over a quarter century, I have been a strong advocate of this design approach, making it a central feature of the Mars Direct mission architecture I first laid out in 1990 and described in my book The Case for Mars. However, it should be noted that while the manufacture of methane/oxygen from Martian carbon dioxide and water is certainly feasible, it is not without cost in effort, power, and capital facilities, and so the transportation system should be designed to keep this burden on the Mars base within manageable bounds.
3. The large scale manufacture of methane/oxygen bipropellant on the Martian surface from indigenous materials. Here I offer the same praise and the same note of caution as above. The use of in situ (that is, on-site) Martian resources makes the entire SpaceX plan possible, just as it is a central feature of my Mars Direct plan. But the scale of the entire mission architecture must be balanced with the production capacity that can realistically be established.
4. All flight systems are completely reusable. This is an important goal for minimizing costs, and SpaceX is already making substantial advances toward it by demonstrating the return and reuse of the first stage of its Falcon 9 launch vehicle. However, for a mission component to be considered “reusable” it doesn’t necessarily need to be returned to Earth and launched again. In general, it can make more sense to find other ways to reuse components off Earth that are already in orbit or beyond. This idea is reflected in some parts of the new SpaceX plan — such as refilling the second stage in low Earth orbit — but, as we shall see, it is ignored elsewhere, at considerable cost to program effectiveness. Furthermore the rate at which systems can be reused must also be considered.
5. Refilling methane/oxygen propellant in the booster second stage in Earth orbit. Here Musk and his colleagues face a technical challenge, since transferring cryogenic fluids in zero gravity has never been done. The problem is that in zero gravity two-phase mixtures float around with gas and liquid mixed and scattered among each other, making it difficult to operate pumps, while the ultra-cold nature of cryogenic fluids precludes the use of flexible bladders to effect the fluid transfer. However, I believe this is a solvable problem — and one well worth solving, both for the benefits it offers this mission architecture and for different designs we may see in the future.
6. Use of the second stage to fly all the way to the Martian surface and back. This is a very bad idea. For one thing, it entails sending a 7-million-pound-force thrust engine, which would weigh about 60 tons, and its large and massive accompanying tankage all the way from low Earth orbit to the surface of Mars, and then sending them back, at great cost to mission payload and at great burden to Mars base-propellant production facilities. Furthermore, it means that this very large and expensive piece of capital equipment can be used only once every four years (since the feasible windows for trips to and from Mars occur about every two years).
7. The sending of a large habitat on a roundtrip from Earth to Mars and back. This, too, is a very bad idea, because the habitat will get to be used only one way, once every four years. If we are building a Mars base or colonizing Mars, any large habitat sent to the planet’s surface should stay there so the colonists can use it for living quarters. Going to great expense to send a habitat to Mars only to return it to Earth empty makes no sense. Mars needs houses.
8. Quick trips to Mars. If we accept the optimistic estimates that Musk offered during his presentation, the SpaceX system would be capable of 115-day (average) one-way trips from Earth to Mars, a somewhat faster journey than other proposed mission architectures. But the speedier trips impose a great cost on payload capability. And they raise the price tag, thereby undermining the architecture’s professed purpose — colonizing Mars — since the primary requirement for colonization is to reduce cost sufficiently to make emigration affordable. Let’s do some back-of-the-envelope calculations. Following the example of colonial America, let’s pick as the affordability criterion the property liquidation of a middle-class household, or seven years’ pay for a working man (say about $300,000 in today’s equivalent terms), a criterion with which Musk roughly concurs. Most middle-class householders would prefer to get to Mars in six months at the cost equivalent to one house instead of getting to Mars in four months at a cost equivalent to three houses. For immigrants, who will spend the rest of their lives on Mars, or even explorers who would spend 2.5 years on a round trip, the advantage of reaching Mars one-way in four months instead of six months is negligible — and if shaving off two months would require a reduction in payload, meaning fewer provisions could be brought along, then the faster trip would be downright undesirable. Furthermore, the six-month transit is actually safer, because it is also the trajectory that loops back to Earth exactly two years after departure, so the Earth will be there to meet it. And trajectories involving faster flights to Mars will necessarily loop further out into space if the landing on Mars is aborted, and thus take longer than two years to get back to Earth’s orbit, making the free-return backup abort trajectory impossible. The claim that the SpaceX plan would be capable of 60-day (let alone 30-day) one-way transits to Mars is not credible.
9. The use of supersonic retropropulsion to achieve landing on Mars. This is a breakthrough concept for landing large payloads, one that SpaceX has demonstrated successfully in landing the first stages of its Falcon 9 on Earth. Its feasibility for Mars has thus been demonstrated in principle. It should be noted, however, that SpaceX is now proposing to scale up the landing propulsion system by about a factor of 50 — and employing such a landing techniques adds to the propulsive requirement of the mission, making the (unnecessary) goal of quick trips even harder to achieve.
Improving the SpaceX ITS Plan
Taking the above points into consideration, some corrections for the flaws in the current ITS plan immediately suggest themselves:
A. Instead of hauling the massive second stage of the launch vehicle all the way to Mars, the spacecraft should separate from it just before Earth escape. In this case, instead of flying all the way to Mars and back over 2.5 years, the second stage would fly out only about as far as the Moon, and return to aerobrake into Earth orbit a week after departure. If the refilling process could be done expeditiously, say in a week, it might thus be possible to use the second stage five times every mission opportunity (assuming a launch window of about two months), instead of once every other mission opportunity. This would increase the net use of the second stage propulsion system by a factor of 10, allowing five payloads to be delivered to Mars every opportunity using only one such system, instead of the ten required by the ITS baseline design. Without the giant second stage, the spaceship would then perform the remaining propulsive maneuver to fly to and land on Mars.
B. Instead of sending the very large hundred-person habitat back to Earth after landing it on Mars, it would stay on Mars, where it could be repurposed as a Mars surface habitat — something that the settlers would surely find extremely useful. Its modest propulsive stage could be repurposed as a surface-to-surface long-range flight system, or scrapped to provide material to meet other needs of the people living on Mars. If the propulsive system must be sent back to Earth, it should return with only a small cabin for the pilots and such colonists as want to call it quits. Such a procedure would greatly increase the payload capability of the ITS system while reducing its propellant-production burden on the Mars base.
C. As a result of not sending the very large second stage propulsion system to the Martian surface and not sending the large habitat back from the Martian surface, the total payload available to send one-way to Mars is greatly increased while the propellant production requirements on Mars would be greatly reduced.
D. The notion of sacrificing payload to achieve one-way average transit times substantially below six months should be abandoned. However, if the goal of quick trips is retained, then the corrections specified above would make it much more feasible, greatly increasing payload and decreasing trip time compared to what is possible with the original approach.
Changing the plan in the ways described above would greatly improve the performance of the ITS. This is because the ITS in its original form is not designed to achieve the mission of inexpensively sending colonists and payloads to Mars. Rather, it is designed to achieve the science-fiction vision of the giant interplanetary spaceship. This is a fundamental mistake, although the temptation is understandable. (A similar visionary impulse influenced the design of NASA’s space shuttle, with significant disadvantage to its performance as an Earth-to-orbit payload delivery system.) The central requirement of human Mars missions is not to create or operate giant spaceships. Rather, it is to send payloads from Earth to Mars capable of supporting groups of people, and then to send back such payloads as are necessary.
To put it another way: The visionary goal might be to create spaceships, but the rational goal is to send payloads.
Alternative Versions of the SpaceX ITS Plan
To get a sense of some of the benefits that would come from making the changes I [Zubrin] outlined above, let’s make some estimates. In the table below, I [Zubrin] compare six versions of the ITS plan, half based on the visionary form that Elon Musk sketched out (called the “Original” or “O” design in the table) and half incorporating the alterations I [Zubrin] have suggested (the “Revised” or “R” designs).
Our starting assumptions: The ship begins the mission in a circular low Earth orbit with an altitude of 350 kilometers and an associated orbital velocity of 7.7 kilometers per second (km/s). Escape velocity for such a ship would be 10.9 km/s, so applying a velocity change (DV) of 3 km/s would still keep it in a highly elliptical orbit bound to the Earth. Adding another 1.2 km/s would give its payload a perigee velocity of 12.1 km/s, sufficient to send it on a six-month trajectory to Mars, with a two-year free-return option to Earth. (In calculating trip times to Mars, we assume average mission opportunities. In practice some would reach Mars sooner, some later, depending on the launch year, but all would maintain the two-year free return.) We assume a further 1.3 km/s to be required for midcourse corrections and landing using supersonic retropropulsion. For direct return to Earth from the Martian surface, we assume a total velocity change of 6.6 km/s to be required. In all cases, an exhaust velocity of 3.74 km/s (that is, a specific impulse of 382 s) for the methane/oxygen propulsion, and a mass of 2 tons of habitat mass per passenger are assumed. A maximum booster second-stage tank capacity of 1,950 tons is assumed, in accordance with the design data in Musk’s presentation.
Using the improved plan to send 40 tons (3.3 times more) to Mars with Falcon Heavy
Consider what this revised version of the ITS plan would look like in practice, if it were used not for settling Mars but for the nearer-at-hand task of exploring Mars. If a SpaceX Falcon Heavy launch vehicle were used to send payloads directly from Earth, it could land only about 12 tons on Mars. (This is roughly what SpaceX is planning on doing in an unmanned “Red Dragon” mission “as soon as 2018.”) While it is possible to design a minimal manned Mars expedition around such a limited payload capability, such mission plans are suboptimal. But if instead, following the ITS concept, the upper stage of the Falcon Heavy booster were refueled in low Earth orbit, it could be used to land as much as 40 tons on Mars, which would suffice for an excellent human exploration mission. Thus, if booster second stages can be refilled in orbit, the size of the launch vehicle required for a small Mars exploration mission could be reduced by about a factor of three.
In all of the ITS variants discussed here, the entire flight hardware set would be fully reusable, enabling low-cost support of a permanent and growing Mars base. However, complete reusability is not a requirement for the initial exploration missions to Mars; it could be phased in as technological abilities improved. Furthermore, while the Falcon Heavy as currently designed uses kerosene/oxygen propulsion in all stages, not methane/oxygen, in the revised ITS plan laid out above only the propulsion system in the trans-Mars ship needs to be methane/oxygen, while both stages of the booster can use any sort of propellant. This makes the problem of refilling the second stage on orbit much simpler, because kerosene is not cryogenic, and thus can be transferred in zero gravity using flexible bladders, while liquid oxygen is paramagnetic, and so can be settled on the pump’s side of the tank using magnets.
Dawn of the Spaceplanes
Toward the end of his presentation, Musk briefly suggested that one way to fund the development of the ITS might be to use it as a system for rapid, long-distance, point-to-point travel on Earth. This is actually a very exciting possibility, although I would add the qualifier that such a system would not be the ITS as described, but a scaled-down related system, one adapted to the terrestrial travel application.
For a rocketplane to travel halfway around the world would require a DV of about 7 km/s (6 km/s in physical velocity, and 1 km/s in liftoff gravity and drag losses). Assuming methane/oxygen propellant with an exhaust velocity of 3.4 km/s (it would be lower for a rocketplane than for a space vehicle, because exhaust velocity is reduced by surrounding air), such a vehicle, if designed as a single stage, would need to have a mass ratio of about 8, which means that only 12 percent of its takeoff mass could be solid material, accounting for all structures, while the rest would be propellant. On the other hand, if the rocketplane were boosted toward space by a reusable first stage that accomplished the first 3 km/s of the required DV, the flight vehicle would only need a mass ratio of about 3, allowing 34 percent of it to be structure. This reduction of the propellant-to-structure ratio from 7:1 down to 2:1 is the difference between a feasible system and an infeasible one.
In short, what Musk has done by making reusable first stages a reality is to make rocketplanes possible. But there is no need to wait for 500-ton-to-orbit transports. In fact, his Falcon 9 reusable first stage, which is already in operation, could enable globe-spanning rocketplanes with capacities comparable to the DC-3, while the planned Falcon Heavy (or New Glenn) launch vehicles could make possible rocketplanes with the capacity of a Boeing 737.
Nextbigfuture notes that reusable first stages are now technically functioning but safety and reliability would need to be improved by about 1000 to 10,000 times for point to point manned travel.
SOURCES- Spacex, Zubrin, the New Atlantis