Monday, February 1, 2016

Interorbital Exchange - part 2, Mars cargo

 This entry covers near-term resources on and near Mars and how they might be transported.
I assume the propellant network described in part 1 has been built, or at least that lunar propellant is available at EML1.

Details after the break.

The short version of the below is that using hardware similar (or identical) to the part 1 Lunar infrastructure, cargo to and from Mars becomes relatively cheap.
A set of three NASA-reference mars missions could have their cargo requirements filled for a total of $11.4 billion (including fuel).
Nitrogen and argon from Mars could be as cheap as $400 per kg at EML1.

Again, let's start with the basics.

Mars has a surface gravity of 3.711m/s² or 37.6% of Earth gravity, and a surface rotation of about 0.24km/s.
Mars is blessed with a thin atmosphere, mostly carbon dioxide but with useful amounts of nitrogen and argon.
Martian soils are rich in perchlorates and relatively rich in water.
The north and south poles have permanent ice caps, mostly water ice under a layer of frozen CO2.
CO2 slabs violently sublimate in the spring, making the edges of the ice caps fairly dangerous in this season. Otherwise the dust storms are not a serious threat.
Takeoff and landing requires 4.1km/s; drag can save as much as 2.4km/s of this on landing.
Mars has two moons, Phobos and Deimos. Both are believed to be captured asteroids, probably C-type.
From EML1, Mars transfer costs 0.74km/s. Mars capture costs 0.9km/s and the move to low Mars orbit costs 1.4km/s. Trip total is just over 3km/s.
From low Mars orbit, reaching Phobos costs 1.4km/s and reaching Deimos costs 1.9km/s.
Launch opportunities to or from Earth (using a Hohmann transfer orbit) occur every 2.135 years, or about 26 months.

Taking all of this together we want a surface location near the equator and a depot in low orbit. If we find significant amounts of water at either Phobos or Deimos then we want a mining outpost there. It would be nice if we could set up a Phobos-anchored tether for orbit changes.

 Getting cargo to Mars is slow and expensive. The post in part 1 used NASA's DRM as a baseline to show how ISRU could save money on fuel. Let's look at other ways to save.
 For starters, let's use the same cargo tug as described in part 1. This is a 5.5 ton dry vehicle that would deliver 50 tons of payload to EML1 using 77 tons of Lunar fuel and a reusable 15-ton (fueled) drop tank. The tug and payload would refuel with 63 tons of lunar fuel and continue on to Mars with fully propulsive capture to low orbit. These tugs can be used as fuel tankers between Phobos and LMO and can also be used to return cargo to Earth (up to 52 tons to EML1 all propulsive).
 Once cargo is in low Mars orbit, a different vehicle is needed to land it. The cargo tugs are capable, but multiple atmospheric landings in Mars gravity are out of reach. Instead, let's use the Access to Mars SSTO ferry (Strickland, Gopalaswami). This is a 30-ton dry mass, reusable SSTO cargo ferry for moving payloads to and from the Martian surface. The vehicle is 14 meters in diameter, which poses a problem; we would need a larger-diameter booster, a way to assemble the vehicle in orbit or a way to launch such a wide payload on an existing rocket. The design could probably be scaled down, but extrapolating the performance of a smaller version is not trivial. I will assume the problem is solved without specifying how. If reliable fuel is available from Phobos then the lander might be redesigned to refuel both in LMO and on the surface, allowing it to carry more payload each way.
 The cargo lander is intended to fuel up, launch to orbit (with 5 tons of payload or excess fuel), collect a 25-ton payload and land. Repeat as needed. Each 50-ton tug payload would require two lander trips. NASA's reference mission involves 80 tons of cargo to the surface, so two such flights would be required. Note that the ~40-ton DAV would not be required at all; a crew version of the lander can ferry crew to and from the surface. That suggests that a single 50-ton payload could include all of the required surface habitat, science equipment, power, rovers, food, etc. The first mission would require a cargo lander and crew lander as well as an orbital depot, so the first mission would require three 50-ton cargo flights to establish infrastructure along with the surface cargo. Each additional mission would require only one cargo flight.
 Each lander flight requires 95 tons of fuel, and each mission requires two flights (one 25-ton cargo, one 20-ton cargo + crew). This 190 tons requires about 9 tons of ISRU plant and 2 tons of spares. The Phobos ISRU plant should provide 15 tons of fuel for each lander. We might also send a single tug flight back to Earth each period for a cost of 63 tons of propellant from Phobos. That totals about 7.4 tons of ISRU plant and 1.6 tons of spares. A cargo tug can haul fuel from Phobos to the LMO depot, delivering 37.2 tons per trip at a cost of 25.8 tons of fuel.

 So, let's compare this to the baseline NASA plan over the course of three missions to Mars. The first trip requires three cargo flights to establish the orbital depot, Phobos ISRU base, tanker fleet and lander fleet. A habitat, science payload, ISRU facility, food and other supplies are delivered to the surface. Total cost is two landers (60 tons), three tugs (16.5 tons), 420 tons of Lunar fuel, a ~15-ton depot (200-ton capacity) and 16.4 tons of ISRU plant (with 3.6 tons of spares). That leaves 55 tons of payload available for the surface mission, with the understanding that the surface ISRU system will provide sufficient water and buffer gas.
 As a first-mission flight, the fuel alone will cost $2.24 billion. Using a middle of the road estimate of $30 million per ton for hardware (including surface payloads) that's almost exactly $5 billion. Launch costs would be three Falcon Heavy and a Falcon 9, about $500 million. Total mission cost, $7.75 billion. Note that this does not include the crew launch, transit habitat or crew recovery; these will be discussed in a later post.
 Future flights would require a single cargo flight, 55.5 tons of hardware ($1.67b) and 140 tons of lunar fuel ($135m). All other components are reusable and already present. Total pricetag for the cargo end is $1.8 billion for each additional mission. After about the fifth mission some of the infrastructure hardware will need scheduled replacement, so this cycle of one expensive mission and four cheap missions could continue indefinitely or the replacement costs could be spread across multiple missions. Still, $11.4 billion for three missions to Mars is cheap compared to $2.5 billion for MSL/Curiosity (which was money well spent) or the $156 billion for Apollo to put men on the moon.

 Looking past the baseline, the Mars surface base is able to supply nitrogen for breathing gas and argon for electric engine fuel. This can be carried back to EML1 in a reusable cargo tug that paid for itself by delivering its first payload to Mars orbit. The same tug can be reused multiple times. Ultimately the cost of shipping along this route is the cost of spares and replacement of the various ISRU bases, depots and tugs. With Earth launch costs in the $3 million per ton range, any advantage is worth pursuing. Cargo from Mars costs about 126kg of spares to Mars per ton delivered to EML1. Cargo to Mars costs about 76kg of spares to the Moon per ton delivered to LMO. Of course each trip requires the use of a cargo tug costing anywhere from $66-$165 million but in terms of ongoing costs we're looking at 6.78 tons from Earth to support a 50-ton cargo from Mars, a better than 7:1 ratio. Another way to say it is for $20 million in launch costs we can get 50 tons of argon and nitrogen (or Mars core samples or Phobos turnings, etc.) at EML1 for use elsewhere once the system is established, a savings of at least $130 million.


  1. It's counter intuitive but EML2 is closer than EML1. As you know, LEO to any high apogee is around 3.1 km/s. To park at an EML1 apogee takes about another ~.7 km/s.

    Instead of apogee at EML1, go closer to the moon. A apogee/perilune a .15 km/s burn suffices for capture with apolune at EML2. Another .15 km/s burn at apolune parks at EML2.

    EML2 is only 3.5 km/s from LEO. With a heatshield, EML2 is about .4 km/s from LEO


  2. Indeed it is. Since I'm looking at early days and all-chemical, the 0.3km/s difference is noticeable. EML2 would also give a slight advantage to periapsis speed for performing transfer burns to Mars, etc.
    I used EML1 throughout as sort of a worst-case; all of my numbers can be improved somewhat, and the cost figures in particular can be improved by at least an order of magnitude.