The Case for Mars (19 page)

For this reason, I and several others argued long and loud for this option. However, in the early 1990s, when this debate was underway, NASA had not yet
accepted Mars Direct, and the 10,000-lb thrust NTR would have been much too small to send Battlestar Galactica to Mars. So, because of NASA planners’ unwieldy mission design, engines between 75,000 lb and 250,000 lb thrust were baselined. Furthermore, many of the people who had rallied around Borowski were representatives of institutions that hoped to receive large infusions of cash for the job of building the new giant test facility, and they pressured him accordingly. In addition, Borowski’s bosses in the NTR program were NASA managers who generally favored the idea of making the NTR a big, long-duration program, and therefore were opposed to any shortcut, small-scale, quicker, cheaper approach. So, in the end, the big engine school won out. NASA dawdled away the SEI opportunity, drawing up plans for a $6 billion NTR program dedicated solely to SEI, with big facilities and a twelve-year development timeline. When SEI was canceled, the NTR went with it. Once the program was terminated, the rats jumped ship, leaving Borowski to argue for starting a small NTR program. For now everything is on hold.

If the United States wanted to, I believe that we could launch a small NTR program that could produce a flight-ready, 10,000-lb thrust, 850 seconds Isp engine within four years for between $500 million and $1 billion. These estimates are based upon detailed discussions and studies done in conjunction with NERVA veterans and other experts working in industry and at several national labs. The cost is not trivial, but it is on the order of the cost of a single Space Shuttle launch and would create a whole new spectrum of space capabilities for the nation. Because such an engine has all sorts of potential applications, its development would be a wise thing to undertake whether we are planning on sending humans to Mars or not.

There’s no denying however that pulling off a space nuclear program is a tall order nowadays. So on the principle that half a loaf is better than none, a group of engineers associated with the U.S. Air Force’s Phillips Lab in Albuquerque, New Mexico, have been pushing for the development of a solar thermal rocket system, and as of this writing, appear to have gotten a small development-and-flight-test program going. The STR is an old concept that was first proposed by German V-2 veteran Krafft Ehricke in the 1950s, but it has never been flown. Concentrated sunli
ght supplies the power for an STR, thereby eliminating the nuclear baggage, but because of the diffuse nature of solar power, it’s hard to make an STR with a thrust of more than 100 lbs or so. Furthermore, for obvious reasons, the system is completely ineffective in the outer solar system. Because the thrust is so limited, the STR could not be used to push a Mars Direct spacecraft all the way from LEO to trans-Mars injection. But it could be used in a long series (taking several weeks) of maneuvers known as “perigee kicks,” in which the engine is fired for about thirty minutes each time the spacecraft passes through the lowest portion of its orbit. This would raise a Mars Direct spacecraft from LEO to a highly elliptical orbit just short of Earth escape. From this orbit the spacecraft would fire off to Mars with a short chemical burn, while the STR stage would either be expended or cycled back to LEO to lift another Mars spacecraft. Since the STR ΔV required to lift the spacecraft to near escape is about 3.1 km/s, while the total trans-Mars burn has a ΔV of 3.7 km/s (for cargo) to 4.3 km/s (for crew), the STR is accomplishing 72 to 83 percent of the trans-Mars propulsion job. It thus offers benefits comparable to, but somewhat less than NTR.

What would be the benefits of these systems for Mars Direct? As we have seen, they would not be used to achieve fast flights to Mars. Short of the introduction of very futuristic propulsion systems (fusion engines, anti-matter, etc.) that do not use ballistic trajectories, the right trajectory to take humans to Mars is the two-year free-return option, which takes about 180 days to reach Mars regardless of what propulsion system is used. But what the STR or NTR would allow us to do is take a lot more payload for the same launch mass. As we have seen, the use of NTR allows the delivery of 60 to 70 percent more payload to Mars than hydrogen/oxygen chemical propulsion when used for the trans-Mars injection burn of the mission on our chosen trajectories. The STR would allow about 40 to 50 percent more payload to be sent than chemical propulsion allows. Therefore, if we use the same 140-tonne-to-LEO booster that we baselined for our chemically propelled missions, the use of these propulsion systems would allow expansion of the crew size to about six (three mechanics, three field scientists—no doctors!) with wider mass margins available for all mission components.

Alternatively, the superior throw capabilities of these systems could be used to red
uce the size of the required launch booster, while keeping all payload allocations the same. Instead of a 140-tonne-to-LEO booster, the mission could be launched with a booster with a capability in the 85-tonne (for NTR) to 100-tonne (for STR) to LEO range. The former figure is about the capability for a “Shuttle C” (basically, a Shuttle ayloa stack but with an empty payload fairing replacing the Orbiter space plane, a launch vehicle that NASA estimates it could develop rapidly for between $ 1 and $2 billion, a lot less than a Saturn V-class vehicle). The latter figure (100 tonnes) is the current launch capability of a Russian Energia booster, although the comparatively narrow Energia payload fairing would have to be widened to accommodate the bulky hydrogen propellant that an STR or NTR driven mission would require.

But may be the mission can be done without a heavy-lift vehicle at all. The United States has initiated a very ambitious program to develop a fully reusable single-stage-to-orbit (SSTO) vehicle. This program was inspired by space visionaries Gary Hudson and Max Hunter, and then given a big boost by the successful demonstration of a subscale, suborbital, reusable rocket (the McDonnell Douglas DC-X) in a “quick and dirty” program sponsored by Col. Pete Worden’s team in the Ballistic Missile Defense Organization. (Bill Gaubatz, the DC-X program manager, pulled it off for $60 million, which is a useful fact to throw in the face of the next person who tells you that something you want done will have to cost $10 billion and take forever.) The project, since taken over by NASA and currently known as “X-33,” faces many technical hurdles, because assuming hydrogen/oxygen rocket propulsion (as all the X-33 designs in circulation do), the SSTO needs to have a dry mass equal to only 10 percent of its mass when filled with propellant. This will be very difficult to do from a structural standpoint, as the hydrogen fuel is very bulky and the vehicle must be armored with a thermal protection system that can withstand reentry (expendable vehicles can be built a lot more flimsy). In order to make the SSTO work, many advances beyond the current state of the art will have to be implemented in lightweight structural materials, engines, and thermal protection systems. There is no guarantee that the required technical achievements will be accomplished. Still, a major national effort in this direction seems about to be launched, and American ingenuity has rarely failed when adequately funded and t
hen pitted against a problem of this kind. However, the funding picture for SSTO, while promising today, is problematical in the long term. NASA has placed the program on a seventeen-year timeline, and it is impossible for me to believe that any political consensus can be maintained over such a long period. Unless the program schedule is accelerated, it will almost certainly fail. But let’s say that the program succeeds. What then for Mars Direct?

Well, in order for the SSTO to be really useful for a Mars Direct mission, we would want to have a version whose engines can be made to burn both hydrogen/oxygen and methane/oxygen. (A straight methane/oxygen SSTO might also do. According to SSTO leader Max Hunter, a methane/oxygen system is just as promising for SSTO applications as hydrogen/oxygen. The greater density of the methane fuel allows for more compact and therefore lighter tanks, thereby compensating for its lower specific impulse compared to hydrogen.) This is not impossible; Pratt and Whitney RL-10 engines, which are designed for use with hydrogen/oxygen, have been run successfully with methane/oxygen on the test stand. Furthermore, some Russian rocket engine technology reportedly allows hydrogen/oxygen engines to be run alternatively with kerosene/oxygen, which is more of a stretch than a hydrogen/methane/oxygen tripropellant system (because methane is a lot more like hydrogen than kerosene is).

Okay, let’s say that’s what we’ve got. The SSTO has a dry mass of 60 tonnes, carries 600 tonnes of propellant (86 tonnes hydrogen and 514 tonnes oxygen), and can deliver a payload of 10 tonnes to low Earth orbit. So we fly one of these things to orbitith 10 tonnes of payload needed for the Mars mission and leave it there. Then, through a series of more than twenty additional SSTO flights, we deliver another 200 tonnes of propellant to the orbiting SSTO along with another 30 tonnes of cargo. (This “cargo” includes 20 tonnes of liquid hydrogen that will not be burned as propellant on the outbound trip, but used instead as hydrogen feedstock for the in situ propellant production process on Mars. It can still be stored along with the propellant hydrogen in the vehicle’s fuel tanks, though.) So now we have an orbiting SSTO loaded with 40 tonnes of cargo and enough propellant to send it off on a Minimum Energy trajectory to Mars. We’ll call this
spacecraft “ERV/SSTO 1.” Off it goes, to aerocapture and land on Mars with the full cargo carried by the regular Mars Direct ERV (any SSTO designed for Earth reentry has more than enough thermal protection to deal with a Mars reentry). As in the standard Mars Direct plan, it would now deploy its reactor and start its propellant plant to turn its 20 tonnes of hydrogen cargo into 332 tonnes of methane/oxygen bipropellant (320 tonnes for the trip back, 12 tonnes for the surface rovers) plus 9 tonnes of water. (It must produce far more methane/oxygen than in the standard Mars Direct plan because it is a single-stage vehicle, whereas the Mars Direct ERV is a two-stage vehicle, and it carries a relatively massive structure required for reusable operations. Both of these exert a price when it comes to propellant requirements.) While this is going on, another SSTO has lifted to LEO carrying 10 tonnes of cargo. A series of 24 further flights by another SSTO loads the first with another 20 tonnes of cargo, an additional 220 tonnes of propellant, and on the final flight, the crew. This second SSTO, the “Hab/SSTO 1,” now has a crew, 30 tonnes of cargo, and enough propellant to send it to Mars on a 180-day fast-conjunction trajectory. Presumably, the loading of the second SSTO is timed so that it ends shortly before the opening of the Earth-Mars launch window. That being the case, and the refueling of the first SSTO having been completed on the Martian surface, the crew can then set off for Mars. Arriving at the Red Planet 180 days later, they rendezvous on the surface with ERV/SSTO 1. Shortly after the crew arrives, a second unmanned cargo SSTO arrives at the site, ERV/SSTO 2, and starts making propellant for the next manned mission (and otherwise provides the crew of Hab/SSTO 1 with a backup) in analogous manner to the standard Mars Direct mission sequence. The crew remains on the surface for 600 days, and then abandons their Hab/SSTO 1 on the surface, flying ERV/SSTO 1 back to Earth. Shortly after they depart Mars, another SSTO (Hab/SSTO 2), containing a crew of four astronauts will arrive at the base to continue exploration, trailed in convoy by another unmanned Earth-return type SSTO, ERV/SSTO 3. The crew of Hab/SSTO 2 will return to Earth in ERV/SSTO 2, and so forth, the sequence of missions continuing in this way indefinitely, with each mission adding another Hab/SSTO to the base. All SSTOs
that do not remain on Mars return to Earth for reuse, so nothing is expended, making such a plan potentially highly economical.

Note that each piloted Mars mission conducted in this way requires a total of forty-nine SSTO flights. This would be totally ludicrous if the SSTOs were to operate in any way similar to existing launch vehicles, with flight rates on the order of one per month. However, if as SSTO proponents advertise, the SSTOs could be made to operate more in the manner of airplanes, with quick turnarounds and flights on the order of several per week, or faster, it’s conceivable that this could be a viable plan. It’s also a very high-tech approach, however. In addition to the demand that the SSTO achieve as yet unap-proached performance and operability, the scenario also requires that both liquid oxygen and liqid hydrogen be transferred from one orbiting SSTO to another in zero gravity. Now both liquid oxygen and liquid hydrogen are cryogenic (ultra-cold) fluids, and no zero-gravity transfer of cryogens from one tank to another has ever been done. It is an operation fraught with problems. The cryogens would freeze a flexible bladder if you tried to use such a device to move the fluids from one tank to another, and pumps won’t work because in zero gravity there is no way to get a fluid to go to the point of suction (the pump gets to bite out one mouthful of fluid and after that just sits there with an empty space in front of it). It might be possible to settle a tank by slowly accelerating the vehicles with rocket thrusters, or rotating them on a spinning platform, and capillary and other devices that rely upon surface tension to control fluid movements have also been proposed. In addition, at least for oxygen, there is the possibility of controlling fluid motions with magnets. (Liquid oxygen is paramagnetic—you can pick it up with a magnet.) In short, while the situation is not hopeless, a lot of work needs to be done before this plan can be regarded as credible.

So for now, my bets are on old fashioned Mars Direct, complete with expendable heavy-lift boosters, chemical propulsion, horse-drawn rovers (well, not quite), and the rest of the primitive paraphernalia of our current Dark Age of space exploration. There may be better ways of getting to Mars, and when they come to hand we will use them. But chances
are they won’t materialize until and unless we use what we’ve got now to get to Mars and get the ball rolling. What was it the old salts use to say about who and what conquered the seven seas? Iron men and wooden ships, not wooden men and iron ships. So it will be with Mars. We can do it with what we have now.