Mars Semi-Direct with Falcon

RobS · 2012-03-20 22:44:34

I'm glad to be back, after 3 or so years. I am surprised no one has been discussing Zubrin's idea, discussed in last May's Wall Street Journal and elaborated on since, of using three Falcon Heavys to send a crew of two to Mars. The Dragon capsule would be outfitted with a 200 kg inflatable that would rely on the Dragon's life support equipment but would provide adequate housing space for the interplanetary cruise. Before aerobraking into Mars orbit, it would be stowed or discarded, and either reinflated or another hab would be inflated on the surface. The mision would rely on solar panels for power and would bring the methan and oxygen along for the launch from the Martian surface.

I gather that it had some pretty thin safety margins, but I don't recall what the criticisms of the idea were.

--RobS

JoshNH4H · 2012-03-20 23:35:03

Welcome back, RobS! And We have been discussing it a bit. While not on the scale of the Saturn V or the Ares V (188 Tonnes is still an incredible amount), it is quite a capable vehicle and it is certainly worth the added complexity of putting a few pieces together in orbit for the cost savings it brings. I would not be too surprised if something coming from Zubrin had slim margins, given that he tends to make pretty optimistic proposals.

I would assume that safety would be the big question. Three Falcon Heavies is 156 tonnes into LEO. If you don't have an earth departure stage, you have a pretty small payload going to Mars. Even if you do, it's a maximum of 70 tonnes, and really more like 50 or 60 to LMO or Mars (For the mass of your rockets and heat shields), even assuming that no delta-V is expended beyond what is required to get into the Mars Hohmann (e.g., you use aerocapture and then aeroentry without any rocket firings whatsoever).

It's certainly not impossible, but I would imagine that there wouldn't be too much margin to be found there.

What I really would like to see would be a launch vehicle made by strapping three Falcon Heavies to each other. That would be an extremely capable rocket, right there. I would predict a payload of 156 tonnes. But then, I predicted a payload of about 30 tonnes for the Falcon 9H, which became the falcon heavy.

RobS · 2012-03-21 07:04:17

Thanks, Josh, it's nice to be back. I hope I have the time to post a bit. If a solar thermal rocket is used to spiral the bulk of the mission to 90% or so of escape velocity, that improves the payload quite a bit. In Zubrin's book, he notes that a solar thermal rocket improves the payload to Mars by about 50%, if I recall. So one could assemble most of the mission in low Earth orbit and spiral it slowly to close to escape velocity, then send up the astronauts in a capsule quickly in a regular Falcon, dock, and light up a small trans-Mars injection stage to head for the Red Planet.

As for what to send to Mars, an inflatable, expendible hab changes everything. You can basically go to Mars in a version of the Mars Return Vehicle that normally would go to Mars unmanned. Rather than launching a heavy return hab, you launch a Dragon capsule with an expendible hab plus surface equipment. The astronauts go to Mars in the return vehicle for 2 years later. When they land, ideally they will land next to a fully fueled return vehicle launched two years earlier, and it will have additional cargo as well. You'd also want to launch two roboting cargo vehicles to increase the cargo available; one 2 years earlier and one with the astronauts, so you can manage the mission if one crashed. I think a system like this, which would use 4 launches (three heavies with the bulk of the mission and a solar rocket, one with the astronauts and their capsule) would be sufficient to send three astronauts to Mars, probably with reasonable safety margins.

RobS · 2012-03-21 09:07:43

Here are a few more pieces of information:

The "Ares" booster for Mars Direct was supposed to put 140 tonnes in low Earth orbit (The Case for Mars, 2d ed, p. 103). The same usable cargo could be launched to Mars with a nuclear upper stage by a booster with 85 tonnes to LEO, or with a solar thermal rocket, 100 tonnes to LEO (p.118). Two Falcon Heavies can put 108 tonnes to LEO, so two Falcon Heavies plus a solar thermal rocket can accomplish the same mission as one Ares. This is assuming that the solar thermal rocket uses a series of perigee "kicks" to slowly (over a month or so) spiral the cargo to about 90% of escape velocity, and the remaining delta-vee is accomplished by a chemical stage. This is necessary to avoid gravity losses. If you know anything about solar thermal rockets, they basically heat up a big block of graphite (or some other thermal storage medium) over several hours time, then can thrust for a few minutes before the block cools off. The specific impulse (or exhaust velocity) is in the same range as a solid-core nuclear, in other words, about 900 seconds (double the performance of LOX/LH2). I have seen thermal rocket designs on the web with Isps in the 850 to 1200 range (the latter being rather futuristic and untested). You can't have astronauts on board during the spiraling to a highly elliptical orbit because they'd pass through the Van Allen belts repeatedly and get heavy radiation dosages. So you launch the astronauts in their capsule, with the Trans Mars injection chemical stage, separately later.

With a system like this--a solar thermal rocket wouldn't be expensive to develop--the Falcon Heavy could be quite sufficient for a Mars mission, as near as I can tell.

GW Johnson · 2012-03-21 11:57:20

I think you have to consider what you can actually accomplish with 1-3 astronauts landed (hopefully not a one-way suicide mission) at one single site on Mars, and maybe just barely enough gear to go home. This matters little whether the return gear is prepositioned, locally produced, or carried with them.

Your not going to accomplish very much beyond flag-and-footprints and a couple of tow sacks of surface rocks. Even if you have a rover, the surface sampling will only be a few dozens of km apart. That's basically the same model as we used with Apollo going to the moon, and, in hindsight, that never really "explored" the moon.

It's been the probes since Apollo that did what "real exploration" we actually have accomplished (on the moon and on Mars). Exploration fundamentally answers two deceptively-simple but difficult questions: "what all is there?" and "where exactly is it?". A lot of the stuff we'd like to find is buried deep, sometimes very deep. And it is never, ever uniformly distributed.

I haven't seen in any of the Mars mission proposals, even Zubrin's, anything that addresses doing real exploration. Not in all these years since the "battlestar galactica" concepts first dreamed up in the 1950's.

But "real exploration" is exactly the prerequisite for siting bases, prospecting, and eventually establishing permanent settlements. You cannot utilize local resources effectively until you answer those two questions. And it is not easy to answer them. Some can be done by robots, some of it must be done with men. That's just life.

Mars is a lot farther away than the moon. For a robot, that's no problem, for humans it is. I have not seen since Skylab in the 70's a habitat spacious enough to support a mentally-healthy crew for the 2+ year round trip to Mars, with the kind of rockets we have.

And nobody seems to want to face up to the need for artificial gravity, either. The only spinning designs have been "battelstar galactica" concepts from NASA mostly, or else complicated nonrigid cable things that cannot be course corrected without disassembling everything.

It doesn't need to be that way. But, you have to give up the Apollo flag-and-footprints model, and you have to face up to the question of safely sending healthy people all that way, and getting them back alive. That is not done with a minimalist approach. It is constraint driven.

Those constraints are a time limit of 1 year to endure zero gee, cosmic radiation near the tolerable limit but that we can't shield and that will be a career limit in one round trip, solar flare dangers than can be shielded, and the need for a Skylab volume for 3 (to no more than 6) men that is not jammed full of stuff either.

Back to "real exploration": it's a very long trip to Mars. If we're going to all the trouble of sending men there, then why not plan on more than one landing? Really do a proper sampling all around the planet. That's not a minimalist design, but it need not be "battlestar galactica" either.

And once you're past the 25 ton shuttle payload, anything you send can be assembled in LEO from docked payloads. It's the lowest cost per payload mass that counts. Falcon heavy is 53 tons at $800-1000 per pound. So who needs a gigantic launch rocket?

Just some things to think about.

GW

RobS · 2012-03-21 22:00:18

Thank you, GW Johnson, I very much like and agree with your post. I wonder what the implications of it are.

1. How many to send to Mars. I'd favor six, if it is practical, perhaps two ships of three each. If either one had difficulties, the other one could provide for everyone, especially if parts could be salvaged from the incapacitated vehicle. When the International Space Station had only 3 on board it could do very little because it took 2 crew full time to maintain the station! I suspect a Mars base would take one or two as well, so you need a minimum of three and preferably more.

2. Establishing a series of beachheads (one every 26 months) versus establishing a "Martian McMurdo." McMurdo, of course, is the hub of Antarctic operations, and having a well equipped hub has been invaluable for Antarctic exploration (which is about 5% the size of Mars!). I think safety favors a concentration of resources at one point initially, but that also reduces the range of exploration you do at first. A McMurdo can also accumulate habs so that there is a lot of pressurized volume per crewmember.

3. Practical range of a human crew. I refer to their surface vehicles here. Zubrin proposes a pressurized rover with a range of 1,000 kilometers, but I don't think anyone is going 400+ kilometers out and back until Mars has at least two such vehicles and probably has a surface crew of more than six, for safety reasons.

4. Human/robot interaction. If we had five or six telerobotic operated vehicles (TROVs) like a second-generation Spirit and Opportunity or like the Mars Science Lab, a surface crew could supplement a terrestrial crew quite nicely because they could control the vehicles live. If each TROV had a sample bin that could eventually be retrieved, that would be even better. In my Mars novel I envisioned robotic solar powered airplanes called "Sunwings" rather like the Helios that NASA experiemented with a few years back. The astronauts would assemble the pieces of the sunwing and test the vehicles. They could be flown down to a TROV robotically and snag a long mast with a hook. The mast was attached to the sample bin, thereby retrieving the samples and bringing them back to base, where some preliminary analysis could be done. Some samples could be selected for return to earth later. There may be other ways to retrieve samples, too; Zubrin proposed a vehicle with a built in thermal rocket. The solar power on the wings would heat up a beryllium engine and a pump would compress carbon dioxide from the atmosphere. Once the engine was hot and the CO2 tank full, the vehicle would run the CO2 through the hot beryllium engine and fly using the rocket exhaust perhaps 20 or 50 kilometers. It would then perform a rocket landing, examine rocks, recharge its propulsion system, and repeat. A balloon/TROV system might work as well. The idea would be to keep the crew busy inside the base with vital tasks spread out all over Mars.

5. Longer term, exploration expands via a transportation system of some kind. Maybe once there are a dozen people on Mars and considerable experience with vehicles has been accumulated, you send out expeditions to clear a track to an attractive geological site about 500 kilometers away, where you set up a small "oasis" (solar panels, Sabatier reactor, water supply, maybe a well, methane and oxygen tanks, maybe some emergency shelter and supplies). You return to base, process your samples, write up articles for Nature and JGR with a terrestrial support crew, then a few months later you go out with the equipment for a second oasis. You stop at the first oasis to refuel and do a bit a maintenance and set up a second oasis another 500 kilometers out. Perhaps sunwings drop ice blocks wrapped in plastic so you have a water supply at each one. A system like this could gradually develop a network of dirt tracks across the planet. Or perhaps larger sunwings can safely carry people and the expedition equipment can be kept out almost permanently, with crew rotation by air. But exploration strategies like this have to evolve over time as equipment improves. It may be a few people and a lot of robots will be plenty.

Last edited by RobS (2012-03-21 22:06:45)

JoshNH4H · 2012-03-21 22:14:20

Just a couple of comments, because there's been a lot of commenting going in a lot of threads and I'd like to get to as much of it as possible as promptly as possible.

Firstly, while the Solar Thermal Rocket is not inherently too complicated, things are always harder in practice than in theory. I don't doubt that it can be done, though.

My recollection is that Zubrin's Mars Direct requires two launches of his Ares rocket, and Mars Semi-direct three. Therefore three Falcon Heavies is not equivalent to that much payload. That is probably why Zubrin reduced his mission design from four crew to two.

There's no problem with orbital assembly, but it is very important to make sure that no matter what you do you're not compromising your mission.

RobS · 2012-03-21 22:42:26

Yes, you are quite right, Josh. With Falcon Heavy, we need at least two launches for every Ares launch if solar thermal is used and three if it is not. So a Mars Direct that uses two Ares--one to land the return vehicle, one to land the hab--would require 4 to 6 Falcon Heavies. But it may be possible to modify the Mars Direct plan with the use of inflatables and reduce that somewhat.

GW Johnson · 2012-03-22 10:01:12

Falcon-heavy will launch 50-53 ton payloads to LEO from Canaveral. We built the ISS with 15-25 ton payloads in the Shuttle. Assembly is not a problem. Cost will be, once the payload size threshold for practical assembly (25 ton) is crossed. Falcon-heavy is projected at $800-1000/pound of delivered payload.

Do you really think a government heavy-lifter will ever be that cheap? (I don't.)

If so, please tell me why, when shuttle was $1.5 billion for 25 tons. And Delta-4 is way above the competition curve between Atlas-5 and the Falcon family. Falcon is the better choice, by the way. I posted that data on "exrocketman" a little while back. It's a strong function of payload size, but flattens some as you get into the 20 ton + range.

If not, then why do we need an expensive 100-ton lifter when 2 cheap 50-ton lifters will do the same job?

GW

RobS · 2012-03-22 10:25:23

I agree. It is sad NASA is spending billions on a booster than will probably be obsolete four years before it is finished. But NASA has a long history of bad decisions. It is driven more by politics and the need to create jobs in lots of Congressional districts than by science and technology, it would seem.

louis · 2012-03-22 16:34:29

RobS wrote:

I'm glad to be back, after 3 or so years. I am surprised no one has been discussing Zubrin's idea, discussed in last May's Wall Street Journal and elaborated on since, of using three Falcon Heavys to send a crew of two to Mars. The Dragon capsule would be outfitted with a 200 kg inflatable that would rely on the Dragon's life support equipment but would provide adequate housing space for the interplanetary cruise. Before aerobraking into Mars orbit, it would be stowed or discarded, and either reinflated or another hab would be inflated on the surface. The mision would rely on solar panels for power and would bring the methan and oxygen along for the launch from the Martian surface.
I gather that it had some pretty thin safety margins, but I don't recall what the criticisms of the idea were.
--RobS

Probably because such ideas have been aired by others - including me - well before Zubrin came out with that proposal. I'm glad to see he's dropped the completely unnecessary proposal for artificial gravity (or it sounds like he has from your description).

I would add in a pretty obvious element: over an 8 year period one could deliver a series of supplies to the target zone in robot craft. You don't need to load everything on to one mission.

I feel we are in a post-Zubrin age really. Elon Musk has a plan to get people to Mars. I don't think he's announced it in full detail yet, but it's pretty clear he knows how to do it and that he's determined to win the prize. So, Zubrin's pronouncements have far less impact. This is Musk's era.

louis · 2012-03-22 17:05:21

RobS wrote:

Here are a few more pieces of information:
The "Ares" booster for Mars Direct was supposed to put 140 tonnes in low Earth orbit (The Case for Mars, 2d ed, p. 103). The same usable cargo could be launched to Mars with a nuclear upper stage by a booster with 85 tonnes to LEO, or with a solar thermal rocket, 100 tonnes to LEO (p.118). Two Falcon Heavies can put 108 tonnes to LEO, so two Falcon Heavies plus a solar thermal rocket can accomplish the same mission as one Ares. This is assuming that the solar thermal rocket uses a series of perigee "kicks" to slowly (over a month or so) spiral the cargo to about 90% of escape velocity, and the remaining delta-vee is accomplished by a chemical stage. This is necessary to avoid gravity losses. If you know anything about solar thermal rockets, they basically heat up a big block of graphite (or some other thermal storage medium) over several hours time, then can thrust for a few minutes before the block cools off. The specific impulse (or exhaust velocity) is in the same range as a solid-core nuclear, in other words, about 900 seconds (double the performance of LOX/LH2). I have seen thermal rocket designs on the web with Isps in the 850 to 1200 range (the latter being rather futuristic and untested). You can't have astronauts on board during the spiraling to a highly elliptical orbit because they'd pass through the Van Allen belts repeatedly and get heavy radiation dosages. So you launch the astronauts in their capsule, with the Trans Mars injection chemical stage, separately later.
With a system like this--a solar thermal rocket wouldn't be expensive to develop--the Falcon Heavy could be quite sufficient for a Mars mission, as near as I can tell.

Solar thermal sounds interesting but I think starting from here with a Mars Mission plan we should stick with tried and tested technologies.

We should also remember that we can send much smaller loads to Mars - perhaps 3/4 tonne loads in a series of robot missions over several years to help prepare for the arrival of humans. I'd like to see maybe up to 10 missions like that delivering supplies to the surface in the target zone (with guidance from transponders on the ground and orbiting satellites).

louis · 2012-03-22 17:11:18

GW Johnson wrote:

I think you have to consider what you can actually accomplish with 1-3 astronauts landed (hopefully not a one-way suicide mission) at one single site on Mars, and maybe just barely enough gear to go home. This matters little whether the return gear is prepositioned, locally produced, or carried with them.
Your not going to accomplish very much beyond flag-and-footprints and a couple of tow sacks of surface rocks. Even if you have a rover, the surface sampling will only be a few dozens of km apart. That's basically the same model as we used with Apollo going to the moon, and, in hindsight, that never really "explored" the moon.
It's been the probes since Apollo that did what "real exploration" we actually have accomplished (on the moon and on Mars). Exploration fundamentally answers two deceptively-simple but difficult questions: "what all is there?" and "where exactly is it?". A lot of the stuff we'd like to find is buried deep, sometimes very deep. And it is never, ever uniformly distributed.
I haven't seen in any of the Mars mission proposals, even Zubrin's, anything that addresses doing real exploration. Not in all these years since the "battlestar galactica" concepts first dreamed up in the 1950's.
But "real exploration" is exactly the prerequisite for siting bases, prospecting, and eventually establishing permanent settlements. You cannot utilize local resources effectively until you answer those two questions. And it is not easy to answer them. Some can be done by robots, some of it must be done with men. That's just life.
Mars is a lot farther away than the moon. For a robot, that's no problem, for humans it is. I have not seen since Skylab in the 70's a habitat spacious enough to support a mentally-healthy crew for the 2+ year round trip to Mars, with the kind of rockets we have.
And nobody seems to want to face up to the need for artificial gravity, either. The only spinning designs have been "battelstar galactica" concepts from NASA mostly, or else complicated nonrigid cable things that cannot be course corrected without disassembling everything.
It doesn't need to be that way. But, you have to give up the Apollo flag-and-footprints model, and you have to face up to the question of safely sending healthy people all that way, and getting them back alive. That is not done with a minimalist approach. It is constraint driven.
Those constraints are a time limit of 1 year to endure zero gee, cosmic radiation near the tolerable limit but that we can't shield and that will be a career limit in one round trip, solar flare dangers than can be shielded, and the need for a Skylab volume for 3 (to no more than 6) men that is not jammed full of stuff either.
Back to "real exploration": it's a very long trip to Mars. If we're going to all the trouble of sending men there, then why not plan on more than one landing? Really do a proper sampling all around the planet. That's not a minimalist design, but it need not be "battlestar galactica" either.
And once you're past the 25 ton shuttle payload, anything you send can be assembled in LEO from docked payloads. It's the lowest cost per payload mass that counts. Falcon heavy is 53 tons at $800-1000 per pound. So who needs a gigantic launch rocket?
Just some things to think about.
GW

I think the mental toll of restricted living space is overstated. Lots of people can cope with it. However ,I think with an expandable (Bigelow style) habitat attached to a Dragon, we have the perfect solution I think. I have always envisage the Transit Vehicle as being Dragon + expandable habitat + service module + rocket + lander.

GW Johnson · 2012-03-22 17:43:21

Louis:

I sort of disagree about the living space requirement. Bigger really is better, ask any prisoner who ever served time in solitary. But, inflatables really make the best sense, provided access to the pressure shell is unimpeded by the equipment inside. You have to be able to patch leaks very quickly. There have been some hints about cramped living from the old Salyuts, Mir, and even the ISS. But not from Skylab. Too little space we already know is very bad from Gemini 7: 2 weeks of a 3 week plan nearly cracked that crew up (I know that was really, really, really cramped, though).

I agree with using a modular transit vehicle. I had slightly-modified Dragons as crew return vehicles in the mission design in my paper last August, at the Mars Society convention in Dallas. My transit vehicle was a stack of propellant modules, a nuke propulsion module, a 3-part habitat module, and two crew return Dragons, for a crew of 6. It was a long stack, so you could spin it end-over-end rather slowly for artificial gravity. (The landers went to Mars with all the landing propellant as separate vehicles. They were single-stage, reusable nuclear.)

GW

Glandu · 2012-03-23 02:40:44

+1 with GW. Modern submarines offer far more comfort than WWII tin cans; time of missions went from weeks to months, & comfort had to follow. You don't want a seaman with nukes getting crazy.

RobS · 2012-03-25 13:47:09

Last year, when I saw Zubrin’s plan to use a Falcon Heavy to send two men to Mars, I wondered how it could be modified to send a larger crew. His use of inflatable, expendable habs attached to a Dragon capsule prompted me to ask: would it be possible to combine the ERV (Earth Return Vehicle) and the Surface Hab into one vehicle? Mars Direct assumes two 140-tonne launches, one with four crew members and a 25.2 tonne Surface Hab, the other with a 28.6 tonne ERV (which masses more because it goes to Mars more slowly). If you launched the crew in the ERV, replaced the hab and ERV cabin with a capsule based on Dragon (let’s call it “Draco” instead) and an inflatable, and moved essential surface supplies from the Hab to the ERV, what would you get? My results are listed below. I have also made a few changes to the Mars Direct Plan.

A. “Draco” Crew Capsule: I am not sure we can use a regular Dragon to go to Mars because it has to accommodate the furniture of the inflatable and 30 months of supplies. Perhaps the unpressurized bottom section of the Dragon would have to be modified to be pressurized and accessible from the capsule. The canted escape rockets might have to be moved to the propulsion stage so they can be used in the Mars landing, or eliminated entirely (in which case, the crew would be launched in a separate vehicle).

“Draco” structure: 3 tonnes, the same as Zubrin’s ERV cabin.

Life support system: Zubrin has 1 tonne for the ERV and 3 tonnes for the surface hab. I used 1 tonne for the trip to Mars and back and 1 tonne additional for the surface stay (stowed in the propulsion module with the surface hab). I am assuming a smaller initial crew of 3, and that part of the life support system in the capsule can be moved to the surface hab.

Consumables: I have taken Zubrin’s numbers for 4 crew and modified them for 3. I have stowed the surface consumables in the propulsion module rather than in the capsule. If possible, I would include a growth unit of several hundred kilograms where the crew could grow herbs (aregano, mint, cilantro, etc.) and vegetables (carrots, lettuce, peppers) which would improve diet and morale. It would decrease the consumable mass somewhat, and possibly the life support system mass as well.

Inflatable habs: Zubrin used 200 kg of mass for a crew of 2; I have used 300 kg for a crew of 3. I assume the habs can’t be deflated and packed away—that would be quite difficult to do in zero gee—so I have included one for the trip out and one for the trip back. The surface hab is listed in the cargo manifest of the propulsion module.

Crew and EVA suits: Zubrin uses 100 kg each per astronaut.

Spares and margin: I have used 16% like Zubrin.

Aerobrake/Thermal Protection System: Same mass as Zubrin’s ERV aerobrake, needed to aerobrake into Earth orbit upon return. The return mass of 11.7 tonnes ends up being a tiny bit more than Zubrin’s return mass (11.5 tonnes).

B. Propulsion module: This is the vehicle that propels the crew capsule back to Earth. It also has a cargo storage area where surface equipment is stored (so you don’t have haul it down a ladder from the capsule on top of the vehicle). Since the Draco capsule has a mass virtually identical to Zubrin’s ERV, the propulsion module is the same. Presumably it is a two-stage vehicle.

Buggies: These are the “open rovers” in Zubrin’s surface hab. I suppose they are essentially ATVs. Note that Zubrin’s surface hab had a 1.4 tonne pressurized rover. This mass is widely understood to be too small to be of any use. I am assuming a pressurized rover (3-5 tonne?) will ride to Mars on a separate cargo vehicle.

Inflatable surface hab: I assume it is 500 kg; larger than the transit habs (they’ll be using it longer). It will use some of the life support equipment of the Draco capsule, which will require some dismantling and moving, and possibly reinstallation in the capsule before liftoff.

Electrical power: I am assuming solar rather than nuclear because of the political problems of the latter. ZTJ photovoltaic cells (according to http://emcore.com/assets/photovoltaics/ … sheet.pdf) are 29.5% efficient and mass 84 milligrams per square centimeter (840 grams, 0.84 kg, per square meter). The solar constant at Mars is 590 watts per square meter; 29.5% of that is 174 watts per square meter. You can get that only 8 hours per day on average, so you are down to 55 watts per square meter per hour for a 24 hour period. Let’s call it 50 watts, so we need 20 square meters per kilowatt (16.8 kg of panels). Multiply that by 50 kilowatts and you get a solar array mass of only 840 kg; less than a tonne. On the other hand, according to http://www.universetoday.com/21293/desp … -colonies/, a 100 by 100 meter solar array (10,000 square meters) will produce 100 kilowatts of continuous power on Mars; that would mass 8.4 tonnes (4.3 tonnes for 50 kilowatts). I will assume 4 tonnes for solar panels. Maybe this can be reduced to 2 or 3 tonnes; solar power technology is advancing rapidly. There is also a website (http://www.renewablepowernews.com/archives/1744) that speaks of an indium tin oxide coating that, when charged electrically, repels dust particles that have already settled on the panels. That will simplify cleaning.

Hydrogen feedstock and propulsion stages: Same mass as Zubrin’s ERV.

Propellant Production Plant: Same mass as Zubrin’s.

Fuel to land on Mars and aerobrake: Zubrin doesn’t give numbers for these items. I am assuming a delta-v of 700 meters per second, methane/oxygen, and an aerobrake mass equal to 16% of the mass being protected. Here’s the resulting table:

A. Crew Capsule
Capsule structure 3.0* (same as ERV)
Life support system 1.0* (same as ERV)
Consumables, out 2.5 (six months outbound)
Consumables, return 2.5* (six months inbound)
Medical equipment 0.2*
Inflatable hab, outbound 0.3
Inflatable hab, inbound 0.3*
Electrical Power (10 kw) 0.3*
Reaction control system 0.5*
Furniture and Interior 1.0 (0.5*)
EVA suits 0.3*
Crew 0.3*
Spares and Margin (16%) 2.0 (1.0*)
Aerobrake 1.8*
TOTAL, CREW CAPSULE 16.0 (11.7*) (basically, the same as the ERV)

B. Propulsion Module
Two buggies (rovers) 0.8
Inflatable surface hab 0.5
Life support system 1.0
Consumables, surface 2.7 (eighteen months)
Surface equipment    0.8
Electrical Power    4.0 (50 kw)
Hydrogen feedstock 6.3
Propulsion stages    4.5
Propellant Production Plant    0.5
Fuel to land on Mars 8 tonnes
Aerobrake 8 tonnes
TOTAL 37.1 tonnes
GRAND TOTAL 53.1 tonnes

Note that once the vehicle lands on Mars, the fuel and aeroshell are gone, resulting in a vehicle mass of 37.1 tonnes. This compares with the 25.2-tonne hab and 28.6 tonne ERV of Mars Direct.

Year 1: One such vehicle would be sent to Mars without crew and landed. We could replace the 0.3 kg of crew with surface robots to unroll and install the solar array, and replace the 0.3 kg of EVA suits with a science station. A 50-kg array will take about 2 years to convert the hydrogen feedstock into methane and oxygen. You’d want to land in the northern hemisphere and not in the dust storm season to be sure of enough power.

Year 1: I would also send at least one unmanned cargo vehicle. A Falcon Heavy can put 10.5 tonnes of cargo on Mars. I would include: a 4 tonne pressurized vehicle, 4 tonnes of additional solar panels (50 kw), a 1 tonne greenhouse, and a 1.5 tonne deep drill (for water).

Year 3: One vehicle would be sent to Mars with a crew of 3 and the cargo manifest above. Their main objective would be to set up a base, including the greenhouse, drill for water, and conduct preliminary exploration. They’d have 150 kw of power, two inflatable habs, a greenhouse, and a pressurized rover. They’d return to Earth in the first vehicle and leave the second one for the next crew’s return. If they secured a good water supply, the next vehicle would not need to fly out 6.3 tonnes of hydrogen and could fly cargo instead. It could also fly a crew of four or even 5 (assuming the capsule and inflatables can accommodate than many). Year 3 would also include a cargo flight with an additional pressurized rover, experimental industrial equipment (metal carbonyl, plastics, a lathe), and a second greenhouse.

How do we get the 53-tonne Draco and Propulsion Module from low Earth orbit to Mars? A Falcon Heavy can push 21 tonnes to a Hohmann trajectory to Mars and 17.5 tonnes to a 6-month trans-Mars trajectory (with LH2/LOX propellant). 53.1 tonnes/3 = 17.7 tonnes. So the Draco plus Propulsion module are massed almost exactly right to be pushed to Mars by a chemical propulsion system launched by 3 Falcon Heavies. One would launch two boosters with LOX, the third with the hydrogen and the rest of the LOX (to reduce hydrogen boil off) and launch the crew last.

Alternately, a solar-thermal engine expelling hydrogen at 9 km/sec can accelerate the complex to 3.1 km/sec and a hydrogen/LOX kick stage can accelerate it another 1.2 km/sec to a total of 4.3 km/sec (which is the average delta-v from low Earth orbit to a 6-month trajectory to Mars). Such a combination requires a 53 tonne propulsion system to push 53 tonnes to Mars; in other words, we now need 2 launches to send 3 astronauts to Mars rather than 4. The solar-thermal rocket would have a 100 pound thrust, which requires a solar mirror 50 meters in diameter! Disadvantages: Someone has to devote a hundred million or two developing and testing the solar-thermal engine; it takes several weeks to spiral the complex through the Van Allen belts, so the astronauts have to be shuttled up separately later. Advantages: It would roughly double the cargo one could land on Mars per Falcon launch, and would reduce launch expenses.

Summary: This plan lands one 37.1-tonne (minus landing fuel and aeroshell) vehicle on Mars rather than two vehicles in the 26-tonne range. That saves mass and reduces the danger that one vehicle will crash and leave the other one unable to complete its mission. The first mission would involve only 3 crew members, unless we launched two vehicles with three each. Once the base has a water supply, each vehicle could transport 4 or 5 instead.

louis · 2012-03-25 18:10:17

Lots of food for thought there, Rob S.

Some points:

1. Greenhouse agriculture is too problematic to begin with I think. I think we should run with artificial lighting.

2. I would give priority to brick making, so that the first colonists can experiment with creating (pressurised) habitats and food growing areas. Construction would be through trenches being dug and "Roman brick" arches being built over the trenches.

3. It's easy to forget the mass allowance for air locks, probably one of the bigger items (though opinion seems divided on this with some saying kevlar type material can substitute for heavier steel).

4. What is wrong with an Apollo style plan where your lander craft is relatively small, if you can land all the required supplies separately in a series of robot landings. You do make allowance for some pre-landing.

RobS · 2012-03-25 19:00:56

I've done a little more checking of my figures, because the numbers for trans-Mars injection didn't look right. Mars Direct lands 28.6 tonnes of usable payload on Mars via a Hohmann trajectory and 25.1 tonnes via a 6-month trajectory, in both cases using a 140-tonne Ares. Those numbers are 20% and 18% of the 140 tonnes to low Earth orbit respectively. To land 37.1 tonnes on Mars, we need 185.5 tonnes in orbit for a Hohmann trajectory and 206 tonnes in orbit for a 6-month trajectory, assuming a LOX-hydrogen stage. These require 3.5 Falcon launches and 3.9 Falcon launches, respectively. So we are talking, in either case, about 5 Falcon launches; 4 for the trans-Mars injection stage and one for the vehicle itself.

SpaceNut · 2012-03-25 20:10:44

Probably more launches as what we want to build for a particular section will most likely have station keeping fuel/thrusters or engines, air lock/docking ports and duplicate powering sources to allow for the waiting time between each sections subsquent link up to happen, before the train can speed out of the orbit that we have chosen to assemble the beast in....
To make matters more unsure of the structure that Mars Drirect used was on a 10 meter diameter not the smaller Falcon unit is at....

RobS · 2012-03-25 20:37:26

Interesting questions, Louis.

Greenhouses: Have you seen any studies that suggest greenhouses are problematic? I haven't, and suspect they would work just fine. Imaging a cylinder with its axis oriented north-south. Inside the airtight plastic there is a silvered thermal blanket, closed tightly against the cylinder at night. At dawn, the thermal blanket covering the eastern side is lowered, letting in the sun. The silvered blanket on the western side is still up and reflects sunlight that would pass over the crops down onto them. The western blanket is lowered a bit before noon and the eastern blanket is raised, reflecting the light of the westering sun down onto the crops. At sunset, the thermal blanket is up on both sides to keep in the heat. With this system, rather than getting 40% as much sunlight as it would at Earth distance, the plants would get about 60%. There are many areas on earth where cloud cover reduces sunlight almost that much. By raising the carbon dioxide level in the greenhouse by several times (not too much for people to breathe, but enough to aid the plants), by optimizing the water and nutrients, and avoiding all insect damage, Martian plants should grow just about as well as terrestrial plants do. There’s no reason to add a kilowatt per square meter of artificial light. If the average person needs 100 square meters of cropland, that’s 100 kilowatts of power per person; a huge amount to provide.

Brick making: Zubrin loves the idea, but I simply can’t picture spending a billion dollars and a million a year in salary to send someone to Mars to lay bricks in a space suit. A one-tonne inflatable bubble would be much cheaper and faster. We don’t build all brick buildings on earth any more for a reason: they’re too labor intensive. We erect a steel structure and pour concrete. If I were building on Mars, it’d be easier to assemble rebar and concrete molds in a spacesuit than lay brick, and then fill the molds with Martian concrete by machine.

Air locks: Yes, I forgot about them. Airlocks and pressure tunnels to connect structures together are going to add maybe 2 tonnes to the cargo manifest I laid out.

Lots of small landings: The problem is that right now more than half of all landings on Mars have been failures. Do you want to assemble a mission out of, say, six small-cargo landings, or 10, and take the risk one will crash or go astray? How will you complete your primary mission with 90% or 84% of your equipment? If you send two of everything, you’ve doubled your launch costs. No, one is better off with as few landings as possible, all else being equal. All else is not equal if: the Martian atmosphere makes large landings impossible or the boosters you are using are small and you have lots of Earth-orbit assembly you need to avoid.

louis · 2012-03-26 14:32:05

RobS wrote:

Interesting questions, Louis.
Greenhouses: Have you seen any studies that suggest greenhouses are problematic? I haven't, and suspect they would work just fine. Imaging a cylinder with its axis oriented north-south. Inside the airtight plastic there is a silvered thermal blanket, closed tightly against the cylinder at night. At dawn, the thermal blanket covering the eastern side is lowered, letting in the sun. The silvered blanket on the western side is still up and reflects sunlight that would pass over the crops down onto them. The western blanket is lowered a bit before noon and the eastern blanket is raised, reflecting the light of the westering sun down onto the crops. At sunset, the thermal blanket is up on both sides to keep in the heat. With this system, rather than getting 40% as much sunlight as it would at Earth distance, the plants would get about 60%. There are many areas on earth where cloud cover reduces sunlight almost that much. By raising the carbon dioxide level in the greenhouse by several times (not too much for people to breathe, but enough to aid the plants), by optimizing the water and nutrients, and avoiding all insect damage, Martian plants should grow just about as well as terrestrial plants do. There’s no reason to add a kilowatt per square meter of artificial light. If the average person needs 100 square meters of cropland, that’s 100 kilowatts of power per person; a huge amount to provide.
Brick making: Zubrin loves the idea, but I simply can’t picture spending a billion dollars and a million a year in salary to send someone to Mars to lay bricks in a space suit. A one-tonne inflatable bubble would be much cheaper and faster. We don’t build all brick buildings on earth any more for a reason: they’re too labor intensive. We erect a steel structure and pour concrete. If I were building on Mars, it’d be easier to assemble rebar and concrete molds in a spacesuit than lay brick, and then fill the molds with Martian concrete by machine.
Air locks: Yes, I forgot about them. Airlocks and pressure tunnels to connect structures together are going to add maybe 2 tonnes to the cargo manifest I laid out.
Lots of small landings: The problem is that right now more than half of all landings on Mars have been failures. Do you want to assemble a mission out of, say, six small-cargo landings, or 10, and take the risk one will crash or go astray? How will you complete your primary mission with 90% or 84% of your equipment? If you send two of everything, you’ve doubled your launch costs. No, one is better off with as few landings as possible, all else being equal. All else is not equal if: the Martian atmosphere makes large landings impossible or the boosters you are using are small and you have lots of Earth-orbit assembly you need to avoid.

1. Whenever I've debated this with greenhouse farming enthusiasts they've ended up admitting there are problems. These basically come down to two issues I think. The first and most important is the dust storm problem. In an extended dust storm your plants will die or at best become stunted, and you will have to engage in a lot of external maintenance to prevent the exterior of the greenhouse from gathering dust.

The second relates to construction. I am not sure it is that easy to produce a transparent expandable farm hab. Alternatively, the challenges of maintaining pressure in a more traditional greenhouse are great, whether you use real glass or some form of plastic. Assembly would also be very challenging.

But with an enclosed habitat, you have complete control over the growing environment. Dust storms may affect a PV energy system, but you can compensate for that by (a) ensuring you have sufficient number that can give you the desired minimum power even in a max. dust storm (the lowest PV energy was abiout 20%) and (b) storing your solar energy as methane which in turn can generate electricity.

I think that for the very early years of the colony enclosed artificial farming is the best way forward.

2. Mars bricks. Bricks are v. versatile and well suited to Mars conditions (where laying of concrete is quite a challenge in extreme cold). I think if you are embarrassed at the idea of brick making on Mars then you aren't really taking colonisation seriously. If you have to import tonnes of habitat structure on every mission to Mars, you are depriving yourself of tonnes of other equipment e.g. PV panels, steel making equipment, fertiliser, more rovers etc

3. Air locks - This is certainly an area where we need more research. I am sure there must be better ways of locking in pressurised air than steel bulk head doors. I have suggested we should investigate ice (and others like GW have suggested icecrete). BUt we will need quite a few of those, as we will need discrete pressurised areas for farming, industrial activity, rover maintenance etc.

4. Multiple landings: I don't think that failure rate reflects the work of NASA or ESA since 2000. However we could certainly cope with a 10% failure rate. If this is going to be a colonisation effort, then we will need two of everything, twenty of everything for all the subsquent missions until we have created a fully functioning mini-industrial framework on Mars.

RobS · 2012-03-26 15:08:37

Interesting points, Louis. I view dust storms on Mars the way I view winter in Indiana: there are times you do agriculture and times you don't. Even in dust storm season, there are probably some crops you can grow. You just have to plan. Perhaps you supplement daylight artificially then. Yes, plastic enclosures won't be 100% transparent; in fact, they may be translucent. Translucent might actually be better because a point source produces shadows and a diffuse light source will hit all the leaves all the time. But the earth's atmosphere isn't transparent, either; it absorbs 10% or so of the usable daylight. I throw three sheets of translucent plastic over my goldfish pond in late October and remove it in late April (this year, mid March!) and there is always an algae bloom and vigorous growth and grass and weeds underneath when I remove it, because there's early warmth, even if there is a reduction in light.

I also wonder whether, by the time we get to colonization, we won't have developed some genetically modified crops that are designed for Mars's lower light levels. Some plants deal with partial shade much better than others.

Bricks: My problem wasn't making them, but laying them! It's labor intensive and we'll have a shortage of labor. Maybe robots can lay them, though. If it's too cold on Mars for concrete to set, it's also too cold for mortar to set. My guess is that construction will be carried out inside bubbles, so people can work in shirt sleeves and the4 temperature is comfortable for people and for terrestrial construction processes. The bubble might even be temporary if the resulting structures are air tight. I wrote a Mars novel once where a 40-meter "biome" bubble was placed on top of a prepared duricrete pad, building bubbles were inflated inside the "biome" bubble, buildings were then built inside their bubbles and siding was put up outside the building bubbles so that they just looked like buildings (even though they were airtight) and a steel frame was erected around the buildings and an agriculture terrace was completed above them. Thus each building had several meters of soil and water and vigorous agriculture above them to reduce cosmic rays and the courtyard between the two buildings was a pretty space with fruit trees, flowers, low vegetables, an eating area, and a basket ball court. All inside the "biome" bubble. Something like that might be possible eventually. It would involve importing plastic bubbles from Earth, at least at first, but construction conditions would be much easier.

Lots of small payloads: I'd avoid that when starting the effort to explore Mars, if possible. Later, demand for cargo (imports and exports) will be so high the Martian colonists will be using reusable shuttles.

RobS · 2012-03-26 16:38:23

Actually, Louis, I've been thinking about your question about small payloads, and maybe I have misunderstood you. What do you mean by "small"? And when do you favor small payloads?

For the first mission or two, I wouldn't favor dividing the mission into a bunch of small pieces, because the landing technology won't have been perfected yet and because you need to maximize your chance of success; otherwise, you could lose public opinion and the funding for future missions. You need to get the crew and the essentials of their mission to the surface in the fewest landings possible, so as to avoid the possible problem of losing essentials. Zubrin's Mars Direct does that pretty elegantly: three vehicles, two being identical Earth Return Vehicles to guarantee the crew can get home. If you lose one of the two ERVs, you still have the other. If you lose the hab, you lose the crew. If we can reduce that even further, to two ERVs (one with the arriving crew) maybe that's better. Or maybe not.

But if you plan to expand beyond arrival of 3 or 4 astronauts every 26 months (and leaving before the next crew arrives) then yes, cargo landers are essential. The first two or three manned vehicles won't be bringing a body imager and there's a good chance no one will get sick enough to need one, but eventually 10 tonnes of medical equipment, then 20, then 30 tonnes of medical equipment will be needed on Mars. Those don't need to come with a crew; better that they come slowly on a Hohmann trajectory. And if they are lost, they can be sent out again 26 months later.

The size of the cargo lander will have a lot to do with the size of the terrestrial boosters. The Falcon heavy, which can launch 53 tonnes to low Earth orbit, can land 11 tonnes on the Martian surface, according to Zubrin. That's possible if the 53 tonnes includes a properly sized LOX/liquid hydrogen TMI stage. Of the 11 tonnes, probably 1 tonne will have to be the fuel tanks for the landing fuel, a cargo platform, and landing legs. So a Falcon Heavy could land 10 tonnes of useful cargo on Mars. I could see Elon Musk adding TMI stages and landers to his rocket factory in southern California and mass producing them. If he can put 53 tonnes into low Earth orbit for $100 million, he can probably get 10 tonnes of medical equipment to Mars for $200 million, or $20,000 per kilogram (about the cost of the space shuttle to get cargo to the International Space Station!). Once that's done a dozen times, the technology will become more reliable and costs will go down further.

The next stage beyond that may be a reusable Mars shuttle. Cargo would be aerobraked into Martian orbit; the Falcon Heavy can put 14 tonnes into Martian orbit. I suspect a fairly small unmanned vehicle (total mass, 30 tonnes or so, fully fueled, if my fast back of the envelope calculation is right) would be able to take off from the Martian surface, go into orbit, rendezvous with the cargo pallet, and deorbit it. Such a vehicle could also launch Martian products into Martian orbit or boost them to trans-Earth injection (eventually colonies will have exports, after all; gold, rare metals like platinum, and heavy water are all possibilities).

RobS · 2012-03-26 20:38:24

Here's a little reusable Martian shuttle I've designed. Since it complements the Falcon Heavy(which can put 14 metric tonnes into Martian orbit) I have designed it to pick up 14 tonnes of cargo in Martian orbit and bring it to the surface or deliver the same mass from the Martian surface to low Martian orbit. I have named it Falconet, for the world's smallest raptor and in homage to the Falcon Heavy.

Falconet structure: 3 mt plus 2 tonne aerobrake, total = 5 mt (metric tonnes)
Propellant capacity: 52 mt (CH4/LOX, Isp = 380, Exhaust velocity = 3.72 km/sec)

Lift capacity Martian surface to low Mars orbit (LMO) [Δv 4.1km/sec, mass ratio 3.01]: 25 mt (nominal: Falconet, 14 mt cargo, 6 mt propellant for return to Martian surface [Δv 1.0km/sec propulsive, mass ratio 1.3]) With no payload and a full 52 tonnes of propellant: 18.9 mt propellant reaches LMO, enough to push Falconet and 14 tonnes to Earth [Δv 2.3km/sec, mass ratio 1.86] and return empty Falconet to surface.

Lift capacity Martian surface to Phobos [Δv 5.5km/sec, mass ratio 4.4]: Falconet plus 6.8 mt (or, Falconet plus 1.75 tonnes payload plus return fuel)

Lift capacity Martian surface to Deimos [Δv 6.0km/sec, mass ratio 5.02]: Falconet plus 5.3 mt

Lift capacity Martian surface to Earth [Δv 6.4km/sec, mass ratio 5.6]: Falconet plus 4.25 mt

Some assumptions: That there is a small unstaffed station in low Mars orbit with two "Canada arms." The 14 tonnes of cargo aerobrakes into low Mars orbit and rendezvous with the station, which grabs it with one arm. The Falconet comes along and the arm docks the cargo to the top of the Falconet, which them deorbits it. What would be even better, if possible: the Falconet's empty ascent tanks are grabbed by the other arm and separated from the Falconet; the vehicle is then reassembled in this order: aerobrake/engines/descent tanks, cargo, ascent tanks; then the Falconet returns to the surface. The cargo is now a few meters off the ground, rather than being perched 10 or more meters up.

I have assumed a propulsive delta-v of 1 km per second for landing on Mars and no use of parachutes.

The Falconet can carry 14 tonnes up from the surface, which is grabbed by a Canada arm, and still has enough fuel to deorbit 14 tonnes of cargo. Thus it serves as an essential link in a two-way transportation system. To send the 14 tonnes to Earth, however, a second Falconet launch needs to arrive at the low Martian orbit station with 18 metric tonnes of fuel, sufficient to push the Falconet and 14 tonnes of cargo to trans-Earth injection and return the Falconet to the Martian surface.

The Falconet can also be used to send astronauts to either Phobos or Deimos from the Martian surface, but it will have to refuel (either on the moon or at the low Martian orbit station on the way up) to return to the Martian surface.

By the way, the Falcon Heavy second stage is 3.66 meters in diameter and 10 meters high, masses 3.1 mt, and holds 48.9 mt of RP1/LOX, for a total mass of 52 mt. The Falconet would have to be slightly longer because methane is less dense than RP1. If it is otherwise identical to the Falcon Heavy second stage, it could benefit from the reusable technology Space X is developing for that stage.

A Falconet could be orbited by a Falcon Heavy with 9 metric tonnes of cargo and 40 tonnes of LOX/methane fuel and it could propel itself to Mars and land on the surface. Thus the first flight would carry less cargo, but subsequent Falcon Heavy launches could carry more.

Last edited by RobS (2012-03-26 21:19:39)

louis · 2012-03-27 15:43:28

RobS wrote:

Here's a little reusable Martian shuttle I've designed. Since it complements the Falcon Heavy(which can put 14 metric tonnes into Martian orbit) I have designed it to pick up 14 tonnes of cargo in Martian orbit and bring it to the surface or deliver the same mass from the Martian surface to low Martian orbit. I have named it Falconet, for the world's smallest raptor and in homage to the Falcon Heavy.
Falconet structure: 3 mt plus 2 tonne aerobrake, total = 5 mt (metric tonnes)
Propellant capacity: 52 mt (CH4/LOX, Isp = 380, Exhaust velocity = 3.72 km/sec)
Lift capacity Martian surface to low Mars orbit (LMO) [Δv 4.1km/sec, mass ratio 3.01]: 25 mt (nominal: Falconet, 14 mt cargo, 6 mt propellant for return to Martian surface [Δv 1.0km/sec propulsive, mass ratio 1.3]) With no payload and a full 52 tonnes of propellant: 18.9 mt propellant reaches LMO, enough to push Falconet and 14 tonnes to Earth [Δv 2.3km/sec, mass ratio 1.86] and return empty Falconet to surface.
Lift capacity Martian surface to Phobos [Δv 5.5km/sec, mass ratio 4.4]: Falconet plus 6.8 mt (or, Falconet plus 1.75 tonnes payload plus return fuel)
Lift capacity Martian surface to Deimos [Δv 6.0km/sec, mass ratio 5.02]: Falconet plus 5.3 mt
Lift capacity Martian surface to Earth [Δv 6.4km/sec, mass ratio 5.6]: Falconet plus 4.25 mt
Some assumptions: That there is a small unstaffed station in low Mars orbit with two "Canada arms." The 14 tonnes of cargo aerobrakes into low Mars orbit and rendezvous with the station, which grabs it with one arm. The Falconet comes along and the arm docks the cargo to the top of the Falconet, which them deorbits it. What would be even better, if possible: the Falconet's empty ascent tanks are grabbed by the other arm and separated from the Falconet; the vehicle is then reassembled in this order: aerobrake/engines/descent tanks, cargo, ascent tanks; then the Falconet returns to the surface. The cargo is now a few meters off the ground, rather than being perched 10 or more meters up.
I have assumed a propulsive delta-v of 1 km per second for landing on Mars and no use of parachutes.
The Falconet can carry 14 tonnes up from the surface, which is grabbed by a Canada arm, and still has enough fuel to deorbit 14 tonnes of cargo. Thus it serves as an essential link in a two-way transportation system. To send the 14 tonnes to Earth, however, a second Falconet launch needs to arrive at the low Martian orbit station with 18 metric tonnes of fuel, sufficient to push the Falconet and 14 tonnes of cargo to trans-Earth injection and return the Falconet to the Martian surface.
The Falconet can also be used to send astronauts to either Phobos or Deimos from the Martian surface, but it will have to refuel (either on the moon or at the low Martian orbit station on the way up) to return to the Martian surface.
By the way, the Falcon Heavy second stage is 3.66 meters in diameter and 10 meters high, masses 3.1 mt, and holds 48.9 mt of RP1/LOX, for a total mass of 52 mt. The Falconet would have to be slightly longer because methane is less dense than RP1. If it is otherwise identical to the Falcon Heavy second stage, it could benefit from the reusable technology Space X is developing for that stage.
A Falconet could be orbited by a Falcon Heavy with 9 metric tonnes of cargo and 40 tonnes of LOX/methane fuel and it could propel itself to Mars and land on the surface. Thus the first flight would carry less cargo, but subsequent Falcon Heavy launches could carry more.

Sounds like an excellent proposal.

I think there are a number of ways of cracking the EDL problem but this one sounds good in that we can get a lot of tonnage back to Earth - which would be very, very valuable in terms of revenue it could generate, to fund future missions.

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