26Aug2014

Mercury And Mars – An Insight

Planets

The inner planets all all rocky worlds. In order: Mercury, Venus Earth and Mars

MARS is, and will remain, the focal point of NASA’s manned space program. The budget realities of our time, and the times to come, will keep other manned projects on the back burner.Here’s an interesting story:

So far as NASA is concerned, manned Mercury exploration is not even on the stove. A greater appreciation for what Mercury can do to support Mars settlement would help the cause for settling both venues. So, what can Mercury do for Mars settlement?

Mercury and Mars have a synodic period of exactly 101 days. The synodic period, among other things, defines when a spacecraft can transfer between planets with the least amount of fuel. It also defines how often transfers can be made over a given period of time. In a decade, for example, there will be 36 launch window opportunities between Mercury and Mars, while there will be only four between Earth and Mars. Earth and Mercury have a synodic period of 116 days.

A Quick Transfer

Therefore, it is possible to transfer between Mercury and either Earth or Mars three times in any calendar year. The nine-fold synodic launch advantage enjoyed by Mercury is extremely important. It enables Mercury to send more payloads to Mars than any practical scheme launched from Earth while allowing individual payloads to be kept small enough to be launched by lower-cost launchers.

Mercury

Earth and Mars have a synodic period of about 780 days, or 2.13 years. In that same time, Mercury and Mars will have gone through seven synodic line-ups and are near to an eighth launch window. This means seven missions could potentially be flown between Mercury and Mars for each flown from Earth. A 7:1 launch window advantage is very useful. It allows strategies where much more modest launch vehicles can be used. Individual payloads can be smaller but, combined, can outstrip any conceivable launch program from Earth.

Speed Requirements

Velocity requirements for flights to Mars from Mercury can be up to four times those for Earth to Mars missions. This is the case no matter what propulsive technology is used. This is only part of the story however. The now defunct Ares V rocket originally contemplated for the Mars Direct plan was designed to put about 25 tons of actual payload on Mars’ surface. Two Ares V units could be launched simultaneously for a total of 50 tons on Mars at each launch window. Another way to express this would be to say mass for the Mars Base would increase at a rate of 25 tons per year. A chemical system using the same technology would only be able to deliver only a fourth of that, 6.25 tons, per flight from Mercury. But it could fly from Mercury seven times within the same 2.13-year period. There are two important implications to this point:

1) All we would have to do is improve the payload performance for the Mercury-based transport system by two tons and it would equal the SLS in payload performance, while the hardware involved would not be anywhere near as massive and expensive as SLS.

2) A Base on Mercury could, in principle, support Mars settlement with only chemical propulsion. Since chemical propulsion requires both infrastructure (‘support facilities’) and a logistical supply string, it is something we would do without if we could.

As a crude measure of what a mission costs, if only in relative terms, mission planners look at delta-v requirements for completing a mission. As an example, a mission going to the Moon’s surface is more expensive than a flight to the Earth-Moon L2 point because the lunar mission has the added delta-v of a landing maneuver – about 2.1 km/sec. For a flight to Mars from Earth, the delta-v to go from Low Earth Orbit (LEO) to Mars’ surface is slightly less than to get to the Moon’s surface, about 5.5 km/sec. Were they the same distance, Mars would be cheaper to access. Going from Mercury to Mars requires a delta-v, one way, of 17 km/sec. at best. As a result, we would expect a mission to Mars launched from Mercury to be three times more expensive than a flight to Mars. For this reason, sending cargoes from Mercury to Mars has been a non-starter using traditional rocket technology, even though it could be done. Fortunately, Mercury does not need to use conventional rocket technology in this role.

Seeking New Ideas

Solar sails are a game changer for Mercury operations. They are also a very misunderstood technology. Solar sails work by the effect of particles of light – photons – radiating out from the Sun. By definition, solar sails are a ‘continuous thrust’ propulsion system. They gain momentum with every second of exposure to the Sun. How much momentum is determined by the distance from the Sun; the area of the sail; the lightness of the sail’s material and any supporting structure, and the mass of payload the sail is to carry.

An important property of solar sails – one not possessed by any other technology – is that, once launched, they require absolutely no maintenance or servicing of any kind. Their useful lives are spent in flight and these lives can be very long. As long as the sail is able to control its orientation to the Sun, it will remain useful. Sails are often rejected for many mission scenarios because they are perceived as ‘slow’. They can take weeks to work up to escape velocity and mission velocities can take years to build up for even nearby planets. These issues, however, are more of a concern when departing Earth. At Mercury, a solar Sail is starting with, on average, eight times the photon flux as at Earth’s distance.

will try to realize a dream older than the Space Age itself: the deployment of a working solar sail

Like the sailing ships of yesterday – one day we will try to realize a dream older than the Space Age itself: the deployment of a working solar sail to Mercury, Mars and beyond.

If launches occur when Mercury is near perihelion, the flux value can be up to ten times the 1 au figure. This means a much faster rate of build-up in velocity. That ‘rate of build-up in velocity’ is referred to as the ‘characteristic acceleration’ for the sail. This is sometimes confused with ‘characteristic velocity’, which is about how fast a given object with known orbital parameters is going. Because solar sails require no propellant except solar photons, which are freely available in space, and because solar sails require no infrastructure or logistical support, they are a highly economical means of moving useful payload around in space. Launched from Mercury, they begin missions with characteristic accelerations eight times those for identical sails launched from Earth, suggesting much faster transfer times.

A Matter Of Flexibility

Another important attribute of solar sails is their flexibility in carrying different payload masses for a given size sail. As an example, a sail of 641,200 square meters in area could deliver a payload of 10,000 kilograms to Mercury in 1.6 years. That same sail could deliver a 20,000 kg load to Mercury in 2.5 years. . . or 30,000 kg in 3.3 years . . . or 40,000 kg in 4.1 years. A square sail with that area would be 820 meters to a side. A circular sail with that area would be 908 meters in diameter.

In either case, the total mass of the sail alone is just 3,880 kilograms. At least three, possibly four such sails could be launched into Earth orbit on a single Falcon 9 Heavy launcher. SpaceX currently quotes a price of $135 million for a Falcon 9 Heavy launch. If three sails are launched, the launch cost for each sail would be $45 million. That $45 million would establish the cost of transport using the sail in space. The more flights it can make, the cheaper each flight gets. As a working figure, a sail would be likely to have a useful service life of at least a decade. This suggests as many as ten round-trip flights for a sail. Initially, the sails are used to build up Mercury Base by delivering three cargo payloads per year.

Launch Cost Problems

Each cargo is 15 tons for an annual build-up rate on Mercury of 45 tons. These 15-ton cargoes are delivered to the sails by Falcon 9 Heavy’s at a cost of $9,000.00 per kilogram. Because sails get their ‘propellant’, solar photons, in flight for free, it does not matter very much where the sail is going. It will cost the same to deliver a ten-ton payload to Mars as to Mercury. Once the sail is launched, its operating cost is frozen as there are no more tasks relative to deploying or flying the sail requiring physical contact that have to be added to the cost. Its operating cost drops dramatically when the sail has met its planned lifetime of ten flights. There is good incentive to also drop the operating price accordingly.

Solar sails (also called light sails or photon sails) are a form of spacecraft propulsion using the radiation pressure

Solar sails (also called light sails or photon sails) are a form of spacecraft propulsion using only radiation pressure for delivery of personnel and payloads in space.

It should be noted that a solar sail payload delivery does have a ‘chemical cost’ associated with it: the cost of delivering the payload from Mercury’s surface to the sail. In the case of a payload from Earth, that cost would be 10 km/sec. to get the payload into LEO, plus another 3 km/sec. to get the payload to the sail’s orbit. Mercury is a much smaller planet, with much weaker gravity and a much smaller gravitational ‘sphere of influence’ within which sails would orbit. Mercury‘s escape velocity is 4.25 km/sec. and defines the highest possible orbit of a sail. Delivering a payload from Mercury’s surface to an orbiting sail can therefore be no more than 4.2 km/sec. This is less than one-third the delta-v required for Earth-launched payloads. Consequently, the propellant mass to be produced is one-third and the time and energy involved in that production are one-third. . .

Cost Comparisons

Mars benefits from solar sail delivery from Mercury because it can obtain large masses of common materials or items for much less than the ‘efficient’ SLS. Costs for payloads delivered to Mars’ surface using the SLS are around $71,000 per kilogram. This is almost eight times the cost of sail transport. Mars benefits also by being able to obtain payloads more frequently, by a 9:1 ratio, than Earth can supply, regardless of technology used. The cumulative effect of this factor is interesting. It allows a design to be completed nine times faster if construction materials are brought in from Mercury than if they are only brought in from Earth. The 9:1 ratio is the number of potential launch opportunities from Mercury (36) and the number from Earth (4) based on the assumption that the Mars Settlement is to be completed in ten years from first launch.

If the construction material comes from Mercury, crews can come from Earth in greater numbers than any Earth-launched plan. If Ares V could deliver four people per launch opportunity supported with a second unmanned launch, then the Mercury option presented here should allow both launches* to carry crews, at least doubling the staffing at the Mars base. Why bother getting any construction material from Earth or Mercury? Why not just use Mars’ resources to build the settlement? That is profoundly logical and is in fact the current plan of most Mars settlement schemes. The important question is whether this is the most productive approach, particularly with Mercury in the issue?

Building A Future

Mars has all the resources it needs to build a large Settlement. Putting aside the undecided matter of the Settlement’s design and what materials, exactly, might be needed for it, we need to look at the issue in generalized terms. We already know three things: Mercury has construction material. 2) Mercury can transport those materials – regardless of their form – more economically than any system of transport from Earth. 3) Mercury can deliver those materials at 101-day intervals (give or take) once the deliveries begin.

A European Space Agency Solar Sail full-scale deployment test (20x20m). Credit: D. Kassing/ESA/DLR

To use Mars’ resources requires that we mine minerals from Mars’ surface, process then into raw stock material, then fabricate that raw stock material into useful items. If the ‘useful item’ is a pressurized building, there is the time involved in construction. All of this requires energy. A lot of energy. Whether it is melting basalt; smelting metals; baking bricks or forming metal stock into tubing, beams or whatever, it will place a heavy demand on energy. Mars has energy. Solar energy on Mars’ surface is virtually identical, most of the time, to the flux on Earth’s surface. Mars’ atmosphere is close enough to being a vacuum that smelters lose much less heat to air convection than they would on Earth. Mars can

readily set up solar furnaces to smelt locally-derived rock. The problem, for Mars, is that Mercury can smelt 20 times as much of anything than Mars can for a given solar furnace capacity. If smelting alone were the only issue, the path to take would be clear. Much lower energy costs for mining, processing and smelting done on Mercury would be decisive.

There is more.

Mars has water. Lots of it. Evidently, Mars has had periods (note the plural) of flowing water. In all probability, important metals are to be found in concentrations likely to be suitable as ores. This is bad news for the economics of Mars settlement. It means that mining operations (again, note the plural) are likely to be needed at a variety of locations for at least some metals. All of these mining operations need to be developed simultaneously before the materials can be applied to constructing large facilities. Utilizing Carbon-based composite materials would seem to be a solution, given the supply of Carbon available, but other minerals are still needed.

In any case, if the material overlaying Mercury’s water supply is similar to the tholins found in comets, as is theorized, Mercury is then in a position to make the same materials, at one-twentieth the energy cost as on Mars. ‘Composites’ in this context is an umbrella term that includes plastics, glass and fiber reinforcement elements, not just metal alloys. Mercury’s regolith is generally more alkaline than the Moon’s with higher percentages of Sodium and Sulfur, among other elements. Mechanically, however, Mercury’s regolith is essentially like that of the Moon’s.

This means the metallic content is more or less evenly distributed among the surface materials, but in low concentrations. However, at Mercury’s distance from the Sun, the abundance of solar-thermal energy is such that even these poor-grade materials are effectively as economical to process as true ‘ores’ would be on Mars – and all of the materials we need are available from a single mining operation.

Making It Reality

What evolves from these factors is a Mars Settlement that develops nine times faster than if it were developed from Earth resources alone at approximately a third the cost in propellant use. Once a determination of the ‘deadweight’ mass required for the Mars Settlement is available, it will allow us to create a more coherent set of estimates for Mercury’s production inputs. This is used in turn to determine how much material – and of what type – must be launched to Mars from Mercury to meet the construction schedule. The mass figure is itself derived from an anticipated population size for the settlement. This is difficult to pin down. That is where the study stands at the moment. To state the obvious, there is a lot of work to be done to fully flesh out the Mercury-Mars synergy suggested here. The Mars advocacy should have a say in this. Their input will be well taken. . .

*The careful reader my wonder why I am talking about Ares V and not the current SLS. That will be the subject of a future posting. Stay tuned. . . Written by: Bryce Johnson Word Online

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