Sunday, January 02, 2011

Mars Direct Without Super Heavy Lift

For a decade or two now the proponents of Bob Zubrin's Mars Direct have been bemoaning the lack of super heavy lift (any vehicle that can lift more than 50 ton to LEO). While few advocates claim a heavy lift launch vehicle is the only component of the Mars Direct plan which is missing, most consider it a necessary prerequisite.

Before I tell you why I think that's simply wrong, let's just make a quick list of all the other things we still need: decent life support, space suits for Mars, rovers that run on methane/LOX, the ISRU propellant production system, bigger entry/descent/landing systems than have ever been flown, portable field equipment for the science mission, artificial gravity generation, and the habitat itself. Oh yeah, and a space rated nuclear reactor.

The beautiful innovation of the Mars Direct plan was the use of the Mars atmosphere to produce the return propellant so it doesn't need to be carried all the way from Earth. Without this simple idea the size of the launch vehicle would have to be exponentially larger. The architecture is littered with examples like this. The use of direct aerocapture over aerobreaking or just using propellant to enter Mars orbit. The advance staging of the Earth Return Vehicle, etc. It's such a shame that this willingness to trade mission complexity for reduced launch capability has been be lost. So we sit on the ground, waiting for a big enough rocket.

Why can't we make do with the rockets we've got? Why can't we start now? While many of the technologies in the list above could be tested with the smaller launch vehicles we have now, it's hard to imagine how to develop the capability of putting 30 ton payloads onto the surface of Mars without first having the capability of throwing 30 ton payloads to Mars transfer orbit.

An LH2/LOX third stage, these days often called an Earth Departure Stage, with an initial mass of 100 tons in low Earth orbit, will do the job (dry mass 6328 kg). To get any smaller we need to start using multiple launches. The traditional approach is to send up the EDS and then send subsequent flights to fill it up. The primary difficulty with this approach is that the LH2 starts boiling off as soon as the sun hits it, so you have to get the propellant up there as quickly as possible.

I'd like to propose an alternative. Using a storable propellant would mean we could take as much time as we like to build the EDS but it would be more like 177 tons in low Earth orbit (dry mass 13359kg). While I'm going backwards, why stop? All that rendezvous and docking and propellant transfer is heavy and eats into the inert mass ratio, so amassing all this propellant is likely to be a slow process. How can we reduce it?

Most EDS designs call for more than just the one engine. This is because the entire burn has to occur in less than a few minutes to make the orbital calculations workable and avoid prolonged exposure to the radiation belts when sending the crew. If we're going to use multiple engines anyway, we could just use multiple stages firing in parallel. Of course, they'd have to be docked together pretty well.

Ok, enough stalling, how many flights do we need?

In just a few years, SpaceX will be fielding the Falcon 9 Heavy. It will be able to throw 32 tons to low Earth orbit. A storable propellant upper stage (dry mass 2226kg) can throw 5 tons to Mars transfer orbit and still give us 2.5 tons of propellant to perform rendezvous and docking with the other stages. A final flight is required to deliver the payload to the cluster of stages. As said earlier, these 7 flights can be stretched out over as long a period as is required. The total cost of each 30t Mars throw is $665M.

Compared to the reported $300M price for a launch of the proposed SpaceX super heavy lift vehicle, this seems like a pretty bad deal. The argument can no doubt be made that paying SpaceX $2.5B now and waiting for super heavy lift is a better idea. Remembering that SpaceX will be developing the Falcon 9 Heavy on their own dime, from a simple cost perspective, more than six flights of the super heavy lift vehicle have to be flown to justify the development costs - but more important than this, sitting on the ground and waiting is not the right choice.

Update April 12, 2011: Obviously SpaceX's announcement of the Falcon Heavy, which will take 53 tons to LEO and yes, they plan to develop on their own dime, completely changes the plausibility of this architecture. It is now conceivable to do just a few launches to build a big enough EDS. I still think storable propellant is worth the extra mass in LEO.. it's just more mature right now, and NOFBX in particular makes it even more viable.

I'm not questioning the virtues of LH2, but we need both development to reduce cryogenic boiloff and an increased flight rate before it will be a plausible technology. We should be doing this development, and supporting the launch industry to increase flight rate, but there is no reason to wait for either to be successful before we start lofting heavy payloads to Mars to demonstrate the techniques required on the business end of Mars colonization.


  1. Anonymous5:58 AM

    So what's the rush to head to the gravity well so fast? Why not do some small precursors missions first.

    (a) Mars or Mars/Venus Flyby - still sexy and will prove out the transit habitat, who wouldn't want to be the first person to see Mars with their own eyes. It's Apollo 8!

    (b) PhD Mission - Retire aerobraking and field tools + habitat risk. Having a staging site at Ph or D reduces lander risk - only need to get as far as PhD. Pepper the surface with a couple small rovers to tele-operate - easier to control from PhD than from Earth. The rovers can be much less autonomous, i.s they cost less.
    Would be nice to build a PropDepot on PhD, probably Methane, LH2, LOX (unless there's enough power available then leave it as H2O)

    (c) ISRU - drop a PropDepot (Methane) on Mars.
    See how it does. Drop a habitat nearby.

    (d) Reusable Lander - Now's the time to go.

    Oh if we've got a PropDepot the EDS problem is much simpler . . . KeroLOX might be an even better initial step for the EDS. Change over to LH2/LOX later.

  2. (a) I would think a Mars flyby is going to be a very difficult mission to sell to any astronauts. you're looking at 2 years (or more) of being spam in a can for what probably amounts to no more than one week of being able to 'see Mars with your own eyes'. Apollo 8 at least was only a 6 day mission, and the astronauts were expecting to land on later missions (only Lovell got the chance.. and that was Apollo 13). 2 years on a Mars flyby means 'no more space for you', lifetime radiation exposure will be reached.

  3. if you're going to have to develop methane engines for the lander/ascent stage anyway, why not use methane/lox for the departure stage? boiloff is massively lower than hydrogen.. and you take a much lower performance hit.

    with a bit of planning you can probably share the engines (or at least the design) between the stages.

  4. Nathan Rogers8:07 AM

    Sounds fine to me. Any thoughts on engine choice?

  5. Anonymous8:10 AM

    Isn't' the 'Mars Direct' mission actually a Mars Surface rendezvous architecture ?

    Wasn't the ISRU concept kicking around the back rooms of NASA in the early 70's ?

    Wasn't the Martin Marietta plan sized by their team of engineers to specifically use the launchers they were trying to sell ?

    Something about repetition redefines truth.

  6. There are a lot of issues and details but the only real missing component is the lander. Storable fuel means you don't need heavy lift.

    Put a spaceship in earth orbit and the rest is obvious. Def: spaceship; a vehicle that takes humans from the orbit of one body to another. Requires: enough internal volume and delta-V. Capable of ongoing missions for scores of decades with resupply and refueling. Components have already been tested in orbit.

  7. Nathan Rogers9:02 AM

    That 32 tonnes to orbit payload capacity of Falcon 9 is to a very low orbit. Also 10% of that figure is reserved for spacex (can't remember why but basically there is 3 tonnes that cannot be used for payloaad).Realistically we'd want to assemble the craft higher up so may drop to the 25tonne range. Still - 25tonne transit craft and lander is good enough for 2 people. Say 15tonnes on the surface.

    What fuel would be used? Hydrogen peroxide?

    Can I request that this idea be worked out in more detail as it is interesting.

  8. Anonymous10:00 AM

    The RL-10 has been successfully run on methane.
    Injector modifications and wire inserts in the cooling jackets were required.

  9. Anonymous11:15 AM

    There are at least a couple of plans around that do manned Mars landings without the heavy lift launch vehicle. I think the most prominent one is the Mars Drive Consortium's "Mars for Less" plan, which uses some 13 Falcon 9 Heavy's to essentially carry out NASA's/ Zubrin's "Mars Semi-Direct". Other ideas include just sending smaller spacecraft like in "Mars to Go" or sending large missions by refueling with ULA depots that have low boiloff technologies built under the "Flexible Path" plan.

  10. Anonymous11:55 AM

    "I would think a Mars flyby is going to be a very difficult mission to sell to any astronauts."

    It's a matter of retiring technical risk. There's a reason why NASA itself wanted to do the Mars/Venus Flyby first. Go read up on the old studies. Prove the equipment out first before you drop someone onto Mars and then can't get them home.

    Plus they get to see Mars AND Venus. It's not like there are solid plans to land on Mars. Something is better than nothing.

  11. Yeah, this nonsense about having trouble getting astronauts to do a particular mission is freakin' absurd. If the shuttle era has taught us anything it's that the astronaut core don't care what the mission is, they just wanna fly.

  12. G'day,

    Two comments:

    1) The planetary mission to do first is a Venus mission not a Mars mission. Not a landing of course but a mission to a high elliptical orbit around the planet. Its a lot easier then any Mars mission and would allow the tele-operated exploration of the planet.

    2) Why not use the Lagrange points to avoid the boil off problem? Don't have the components hanging about in LEO, but blast them to say; the Moon Earth L4/L5. Once the assembly is complete use a gravity assist from the moon Earth system to send it off to Mars.



  13. If you combine this with refueling at a Langrange point and returning to it propulsively so you can reuse the MTV (as proposed by Huntress et al the IAA study), then you can still use LOX/LH2 for LEO -> L1/L2. This still doesn't require cryogenic depots, because you could launch your LEO to L1/L2 stage fully fueled on an EELV or Falcon Heavy, which is good enough to transport dry MTV components. Use of three body trajectories to L1/L2 (3.2km/s instead of 3.8km/s) further recaptures some efficiency as well as allowing larger pieces. Another advantage is that you now need much less thrust and therefore much less clustering of engines.

    The same scheme works just fine for the moon, NEOs and even unmanned probes. We could establish a large market for propellant long before we had cryogenic depots and with TRL-9 technology.

  14. Another trick would be to use a Mars Lagrange point too. Propulsive braking of the MTV into a Lagrange points is much cheaper than braking into low Mars orbit. Only a much smaller lander would have to brake into LMO. On top of that, you could use SEP to preposition storable lander propellant to a Mars Lagrange point and/or LMO. The SEP tug could start from LEO if it is disposable (or capable of repeated van Allen crossings without deterioration, which would require some technology development) or from an Earth or moon Lagrange point (both reachable with 3.2kms trajectories) if it isn't. This allows you to recapture even more efficiency. This could even turn out to be more efficient than using LOX/LH2 from LEO.

  15. Anonymous11:16 PM

    "The planetary mission to do first is a Venus mission not a Mars mission. Not a landing of course but a mission to a high elliptical orbit around the planet."

    At the same time NASA did the study on the Venus or Mars/Venus mission, they did a Manned Venus Orbiter study. BeyondApollo did a good writeup. Again, this is with Apollo-level of technology - matter of fact they were going to use a pair of LEM decent engines in the Apollo SM instead of the SPS.

  16. Of course, a much cheaper way to get there, is to send one-way missions only. IOW, set up an underground base in a tunnel, with nuke power, and then send a small crew (4-6) there on what is told to be a one-way mission.
    In doing so, from that point on, the crew is devoted to survival, expanding the base, and some explorations. Upon getting fully established and not finding any other issues, then we send a new crew.

  17. G'day,

    Some more thoughts. Mars Direct is a product of the early 1990s. There has been some important developments since then. Including , not in any particular order:

    1)Flight tested electric ion engines.
    2) Proven inflatable modules
    3) Better understanding of Lagrange points ie Interplanetary Super Highway
    4) Commercially available large rockets
    5) An operational Space Station.
    6) Confirmation of water at Mars.

    I would think these developments should make interplanetary exploration a lot easier.

  18. Anonymous11:32 PM

    SpaceX isn't going to develop F9H "on their own dime" any time soon. They don't have the money. Somebody's going to have to want it badly enough to pony up the development cost for it. Which also won't happen any time soon. You should either 1) include the F9H development cost in your overall program cost, or 2) assume you'll use Delta IV Heavy. But I expect that either one of those choices would still be much less money that building a new super heavy lift rocket, especially if NASA does it to Congressional specifications.

  19. Anonymous, whoever you are, SpaceX have said they are developing F9H for the commsat market.. on their own dime. I don't make shit up and I stand by my statements. Please login if you want to question me.

  20. Sorry, Trent. That was me. I was just too lazy to log in. I did not know they had said that. Did they give a time frame? I really doubt they have anywhere near enough money right now, but I suppose they could raise it with an IPO.

  21. Bill, thanks for reminding me that dismissing Anonymous so readily is hazardous. Apparently you can buy a Falcon 9 Heavy launch right now for the listed price - that's how confident they are that they can field it. When you think about it, it's not so wild an expectation.. it's basically just extending the hold down system.

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  23. Anonymous6:28 PM

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  24. Anonymous12:14 AM

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  25. Pete Hague2:29 AM

    Quick question:

    The spacecraft in the image appears to be from a proposal by Reaction Engines, who specifically designed that Mars mission around the Skylon spaceplane (this is why all the modules, apart from the attached Orion capsules, are roughly the same size - its the size of the payload bay).

    I appreciate that the TR level of Falcon 9 Heavy is quite a bit higher than Skylon, but it is very unlikely that what you are proposing would happen before 2020 anyway, when Skylon is due to start operating (assuming the pre-cooler system can be demonstrated to be viable)

  26. David Icard3:28 AM

    One common launcher for Mars exploration would be Russia's Soyuz booster. If the various craft in Earth orbit and on the surface of Mars could be split up into their smallest components then the Soyuz might be enough. I would envision the expedition leaving Earth in a convoy. Each astronaut would have his own ship. Each rover, fuel plant, power plant, life support, or rocket engine would would fly a seperate mission in the convoy. The astronauts would have to get all of these parts together on the surface of Mars in order to get back to Earth. Putting the convoy together would take years in orbital construction. Would solid payload assist modules wait out the years till the convoy was ready to head out to Mars? Welcome comments.

  27. I think we need to invest in heavy launch vehicles, if we want to go out and explore the solar system. The best way to do this use to modular systems built around the the RD-170 engine and its less powerful siblings (rd-180,rd-190). Rocket stages built around these engines already exist, eg, the Zenit first stage.

    We then have a single core stage consisting of 1-4 RD-180 engines and surrounded by several strap-on single engined RD-180 (or family) stages. Plus Upper Stages tailored for each mission.

    Essentially what I am stating is nothing but the current Energia launcher, but it can be a cooperative international effort. Such a LV is cheap because it has already been developed and no new R&D is necessary. No development is necessary. Just configure the core and strap-ons in different ways to launch anywhere between 20 tonnes to 200 tonnes to LEO.

    This is the best way to go.

    May be the Americans can develop the upper stages and the space craft, Russians can provide the boosters and we are ready to go to not just Mars, but also Uranus and Pluto.

  28. I think some American LV (is it the Delta V ?) already uses the Russian Engines, so no ego problems here. Lets make it a cooperative effort and succeed. The duties for each module can be split between different countries such as the greenhouse can be handled by the Russians, the small nuclear reactor can be designed by the Americans, the Japanese and the ESA can contributed their might and I think even India and South Korea can offer their solutions.

  29. What happened to Russia's Energia? Isn't that a heavy-lift rocket that can bring 100 tons to LEO?