In my talk on the commercial space panel at the Mars Society Convention on Saturday afternoon, one of the things I said that we needed was “gas stations on the road to Mars.”
I’d never really thought about it much, because I don’t think about Mars much, but I wondered just how advantageous it was, so I decided to take a break from the space safety stuff and play with the idea in a spreadsheet. I haven’t triple checked the numbers yet, but what I found blew me away. For a hundred metric tons of payload delivered to Mars orbit in the vicinity of Phobos, I looked at three mission scenarios: departure from LEO direct to Mars, departure from EML1 direct to Mars, and departure from EML1 with a fuel stop half way. For the latter scenario, you can deliver a ton of payload to Phobos’s orbit for less than a ton of propellant. For the EML1 scenario without refueling, it takes over ten times as much. For the LEO scenario, it takes over twenty times as much. That is, for the refueling scenario, you start with an IMEML1 (Initial Mass in EML1) of about a hundred and fifty tons, two thirds of which is payload bound to Mars (the total mission propellant needed is twice as much as you start with, because you pick up the other half, about 43 tons, on the way). For the non-refueling case in EML1, the IMEML1 is over a million tonskilograms, and for the LEO case, IMLEO is almost two-and-a-half million tonskilograms.
Such is the power of the rocket equation, and refueling.
What’s the catch? There are two. First, of course, it assumes the delivery of the propellants to EML1 and the gas-station orbit. If it were done chemically, the savings would go away, but it could be done electrically. The other catch is that it obviously both complicates and increases the time of the mission. I’d recommend it for cargo only. The gas station orbit I chose was at a distance of 1.25 AU, which turns out to exactly split the delta V between earth departure and Mars arrival, which means that you just refill the tanks at the gas station. That orbit turns out to have a synodic period with earth of about three and a half years, and with Mars of about five and a half years. But with the savings, you could afford to put a dozen stations in that orbit, which would give you an opportunity every month, and then you could take more of the savings and do faster trajectories, so you don’t have to wait for Hohmann line ups.
If anyone wants to check my numbers, I ignored the moon’s gravity, just doing a departure from EML1 starting at EML1 velocity with patched conics (same thing going into Mars orbit to co-orbit with Phobos). I used a stage fraction (ratio of propellant to total stage weight) of 0.85 and an ISP of 460 seconds (lower ISPs would just make the numbers even worse for the non-refueling scenario). I also didn’t do any aerobraking at Mars, so the numbers would work the same in either direction. So if PR wants to sell water, that’s the location for the gas station — 1.25 AU.
[Update a few minutes later]
Just looking at the difference between LEO and EML1, it seems insane to not use propellant depots with electric propulsion.
[Update a while later]
Let me put this another way. This implies that you could deliver a hundred metric tons (that is, 80% of a Saturn V payload) to Mars orbit in four Falcon Heavy launches. Two launches for electric depots (one of which has cryo propulsion, probably RS-68 class if you can start it in space, or a bunch of RL-10s if you don’t mind the gravity loss) and two for the payload. Less than half a billion in transportation costs to LEO. Leaves a lot of money left over for the other stuff.
[Late afternoon update]
OK, I found an error in the spreadsheet. As I note at the new post, the propellant advantage is not as dramatic as I indicate above, but it is still significant.
I hope Elon is among your readers.
I would bet money he is.
Actually, I kind of doubt it. He’s a pretty busy guy, and he’s never indicated to me that he does. He no doubt reads stuff that I write about SpaceX in other places, but I don’t think he frequents this blog.
Then I hope at least one of your regulars has his e-mail address and drops him a line.
By way of opening the vistas a bit, it seems to me that your multiple depots in 1.25AU orbit would be quite useful for sending “freight” to any destination beyond Mars too, such as the Asteroid Belt and the outer planets and moons. If Elon built out that infrastructure it would serve to cut the cost of his stated grand design of Mars colonization and could be sold as a service to NASA and outfits like Planetary Resources for doing likewise with unmanned exploration/exploitation missions to Mars, the Belt and all points farther out.
Well, actually, I have his email address…
By way of opening the vistas a bit, it seems to me that your multiple depots in 1.25AU orbit would be quite useful for sending “freight” to any destination beyond Mars too, such as the Asteroid Belt and the outer planets and moons.
Yes, though for destinations other than Mars, it wouldn’t be an optimal location, because the plane split wouldn’t be fifty fifty. On the other had, other orbits for further fuel stops (perhaps in the belt itself) could prove useful for outer-planet missions.
You can use a high Mars orbit too, probably a Mars Lagrange point. It would be a very natural refueling point. Cycling between high orbits at the edges of the two gravity wells saves a lot of energy. It also makes propulsive capture a reasonable thing to do. Refueling is a very powerful technique. So is SEP.
Since Mars’ moons have essentially the mass of small rocks, I’m not sure there’s enough mass for such a point within the Mars-moons system — plus the interference of the smaller moon’s mass as it orbits at its much faster rate would (I think) tend to destabilize any such point if it did exist.
Therefore perhaps you’re referring to Lagrangian points involving only the masses of Mars and the sun. Those distances are much greater, and the only useful and reliable point, if stable, would be the one directly between Mars and the sun. Not being a rocket scientist I can’t say whether it could work.
But I think the best trajectory would be one that happens to more closely match the trajectory the chemical ship would be on when it approaches rendezvous with the way station. The less it has to decelerate to make the rendezvous, the better. A Lagrangian-style position wouldn’t offer that.
Wow.. http://en.wikipedia.org/wiki/List_of_Mars_trojan_asteroids
Therefore perhaps you’re referring to Lagrangian points involving only the masses of Mars and the sun.
Yes, a Sun-Mars Lagrange point, SML1 or SML2.
A Lagrangian-style position wouldn’t offer that.
I believe it is actually likely the cheapest option. Looking at the Wikipedia delta-v chart, the delta-v from EML1/2 to SML1/2 would be less than 2 km/s, which is less than a lunar landing. You could cycle back and forth without refueling at the far end if you had to, but there isn’t really a need. You can preposition fuel to both locations using higher Isp propulsion, maybe LOX/LH2 maybe the much more spectacular SEP. And none of it requires new technology. A 40-50kW SEP tug for prepositioning storable propellant can be built with mature technology only and mostly off the shelf components.
A delta-v of only 2km/s is very little, well within the capability of hypergolics. Mass fractions are just not going to be a problem, even with very conservative designs and high factors of safety. End to end efficiency still counts, but that is where the use of LOX/LH2 and SEP on those segments of the journey where they can be used with today’s technologies and systems would shine.
Not being a rocket scientist I can’t say whether it could work.
Something like this was proposed by the great Robert Farquhar in Huntress’ study for the International Academy of Astronautics:
The Next Steps In Exploring Deep Space
Okay. I didn’t catch the meaning of “SML” in your first comments.
It does seem odd to me though, to shoot for a part of Mars’ solar orbit that doesn’t actually contain Mars. 😉
Therefore perhaps you’re referring to Lagrangian points involving only the masses of Mars and the sun.
Yes, Sun-Mars L1 or Sun-Mars L2.
Not being a rocket scientist I can’t say whether it could work.
It was one of several options proposed by the great Robert Farquhar in Huntress’ study for the International Academy of Astronautics:
The Next Steps In Exploring Deep Space
A Lagrangian-style position wouldn’t offer that.
Actually, I think it might be close to optimal. According to the Wikipedia delta-v chart the delta-v from EML1/2 to SML1/2 is less than 2 km/s, which is less than is required for a lunar landing! With such small delta-v’s mass fractions and stage / spacecraft size limitations imposed by EELV-class fairings simply aren’t a problem. If you had to, you could even do a round-trip without having to refuel near Mars.
But there isn’t really any reason to avoid refueling near Mars, since it offers the potential of using higher Isp propulsion, even without new technology development, let alone with it. A 40-50kW SEP tug for prepositioning propellant could be built using only mature technologies and many off-the-shelf subsystems.
And end-to-end efficiency would still matter, even if the mass fractions don’t absolutely require it, so refueling near Mars should probably be a part of the plan from the start.
If it were done chemically, the savings would go away
That’s true for one-way payloads, but not for a reusable MTV, which could be very heavy.
so I decided to take a break from the space safety stuff and play with the idea in a spreadsheet
More people should do this. It’s amazing how much a Centaur-sized upper stage at L1/L2 can throw through TMI. We haven’t even come close to realising the full potential of technologies and systems we already have, and in some cases have had for decades.
Rand,
Would you mind sharing the spreadsheet you did for the calculations? Just trying to wrap my brain around them.
Yes, but I’m still cleaning it up so it’s almost comprehensible to someone else.
just doing a departure from EML1 starting at EML1 velocity with patched conics
Doing a powered flyby of Earth makes this more efficient.
Certainly does make the case for figuring out how to mine ices out of NEAs / NEOs and turning them into water. Cheers –
Rand, are you sizing this for a mission to the Martian surface? Manned or unmanned? If a surface mission, how much payload would make it to the surface? How much would return to earth? I suppose those questions might be beside the point for an exercise comparing refueling vs. non-refueling architectures, but I was just wondering where the 100 mT to Martian orbit is coming from.
Rand, are you sizing this for a mission to the Martian surface?
No, just matching orbits with Phobos.
How much would return to earth?
Nothing, unless it refueled at Phobos, in which case it would return as much as it brought, assuming another fuel stop on the way back. Though the return would be to EML1, not earth.
The 100 MT to Mars orbit was completely arbitrary, and not based on any thought given to what it would do. Large enough to be realistic in the context of a vehicle design, but not ridiculously so.
What losses do you incur matching velocities with the fueling station for rendevous, and would you have to maintain a fuel margin to allow for a return trajectory if there was a problem with the fuel depot (pump failure, etc)? Although any such margin would extend the fuel available for the normal return trip, how much would it eat into the overall weight savings?
Another question I have is how much a depot could save on the weight of consumables (food and water), lowering the initial payload weight, and whether it would make sense to make the first leg to the depot in a small cramped capsule, where the crew will mate with a larger, more capable, longer-endurance craft for the second leg.
What losses do you incur matching velocities with the fueling station for rendevous, and would you have to maintain a fuel margin to allow for a return trajectory if there was a problem with the fuel depot (pump failure, etc)?
I didn’t have any “losses” in velocity matching, other than accounting for the necessary delta V. I didn’t account for margins — that would be a refinement.
Another question I have is how much a depot could save on the weight of consumables (food and water), lowering the initial payload weight, and whether it would make sense to make the first leg to the depot in a small cramped capsule, where the crew will mate with a larger, more capable, longer-endurance craft for the second leg.
It has no benefits like that. Each leg of the trip would be several months, at least with Hohmann transfers, so it might not be desirable for crewed missions.
This is really interesting, the nerdmoot must provide more comments to read.
Ok Rand, I have a question, If you have a difficulty at a refueling station, is there a free or near-free return to Earth or a slower one to Mars so the crew can safely abort but short enough time-wise they won’t perish in the interm.
No, not in this scenario. Unless they carry enough propellant to get them back to earth, which defeats the whole purpose of the project, because that’s the same amount needed to get all the way to Mars.
When you drive out to somewhere, and count on being able to get gas along the way, you’re screwed if you run out, until someone comes long to help you out. This scenario assumes that there will be an infrastructure (a Triple A of the solar system) to support that.
insane to not use propellant depots with electric propulsion
If only sanity were the benchmark. Commercial free enterprise is the way to ensure sanity. Land ownership is the beginning of sanity.
All the elements exist to begin a mars colony now (or will be ready when actually needed.) The only thing missing is complete financing by realizing the value of ownership. Distributed ownership. Everybody risking their lives gets a claim. Everybody making it possible for anybody to risk their lives gets a claim based on the colonists claim.
Mitigate the risks as much as we can, but don’t delay because we don’t have all the answers. We can’t stay in rover mode forever, taking a month to do what a human could do in a day will hold us back for as long as we don’t take the plunge.
I’d like to see more work on long term closed cycle life support before an attempt to colonize Mars. I’d hate to see the colony fail because supplies from Earth got cut off. Propulsion wise the tech we have can do the job.