Super Heavy Starship Moon Mission Needs a Ten-Ton Lunar Lander

Robert Zubrin indicates there is a need to stage the SpaceX Starship from low earth orbit or injection orbits for the moon and Mars. Missions to the moon would be far more efficient with a ten-ton lunar lander. This could be a mini-starship.

Mars mission should stage from the TMI (Trans Mars Injection) or low earth orbit.

The mini-Starship would look similar in size and shape to the Small Falcon Rocket. The Small Falcon Rocket was imagined a couple of years ago when Super Heavy Starship was called BFR. It would be similar size and possibly a bit smaller. It would fit inside the Starship and could be deployed from Starship.

Zubrin also mentions the problems of spreading large amounts of lunar material all over if there is attempt at landing the Starship on the moon. This would be a problem until there is landing pad made from lunar material turned into lunar concrete.

31 thoughts on “Super Heavy Starship Moon Mission Needs a Ten-Ton Lunar Lander”

  1. My understanding is that any (ballistic, in vacuum) orbit which begins on the ground ends on the ground. In other words, a pebble will make one orbit of the moon and then it’s going to hit dirt.

  2. Elon recently said: “If it’s long it’s wrong, if it’s tight it’s right”. The tightest plan for sending humans to the moon and Mars is for Starship to just land there – and be left in place if it’s too hard to return.

    Making special purpose little landers is what NASA is calling for dragging out the schedule and adding tens of billions in costs.

    Following this rule SpaceX started making Starship out of steel. They’re mass producing Raptor engines. They’ll make a lot of Starships and they’ll be quite cheap enough to destroy in testing or leave in place after missions even though they’re very reusable. They’ll be cheap enough to do the moon and Mars at the same time and every other appropriate use as well.

    Zubrin seems lost in trying to optimize for things that aren’t important now.

  3. If you’re just going to burn the containers along with their content, and then run everything through Sabatier to get methane, then you may as well go with polyethylene containers. It’s cheaper, strong enough, and the little bit of extra hydrogen isn’t an issue (you’ll just get a little more water out at the end). Besides, carbon fiber uses a polymer binder anyway (usually).

    There’s a bit of a handling challenge in how to load the carbon into the burner, but it’s not much more complex than other cargo handling. A Lunar base is likely to have the necessary equipment. But for an orbital fuel depot, I’ve previously suggested packing the carbon as a hollow graphite tube inside a metal tube, similar in construction to a solid rocket booster. You feed oxygen from one end, collect the burn products out the other end, and feed them to Sabatier as usual. That simplifies the zero-gee handling, but at the cost of the metal tube mass. But the metal can be reused in various ways.

    Come to think of it, that metal/graphite tube can substitute your drop tank.

  4. One thing that can be said about bringing the carbon by itself, instead of the methane, is that carbon is solid at any reasonable temperature. In fact, it has decent mechanical properties.

    Tankage: you don’t need any for the carbon. You can ship carbon fiber crates full of carbon fiber bags full of carbon granules. It’s carbon all the way down.

  5. Ideally you’d want to refuel in LEO (or an elliptical orbit) and EML2. If it’s with methane, you’d bring the carbon and water there, and make the fuel at fuel depot, to minimize the delta-v you spend on carrying them back and forth. The EML2 depot should give you enough delta-v to land on the Moon and return to EML2 without refueling on the surface, and it’s closer to Mars etc than EML1 is, in terms of delta-v.

    The next best option is bringing the carbon or methane with you to the Moon. There’s not much difference between the two, but bringing only carbon would give a bit more savings, allowing more payload. But most of the savings are in the oxygen, which is 80% of methalox mass.

    Long-term, you’d want dedicated hydrolox tugs for cislunar space, and high-Isp propulsion for cargo missions like asteroid mining.

  6. My miss was that you meant a drop tank in addition to the native tanks. I thought you meant instead of them.
    Thanks for the explanation.

  7. Superficially, it seems like there might be good reason to design hydrolox engine ships, to take advantage of lunar ISRU hydrogen and oxygen. Starship’s ability to lift cargo (like fuel) off the moon will be hindered if it has to land with enough methane to lift off.

    But landing carbon to make methane from lunar water wouldn’t be much better than landing the methane itself. Apparently there’s some carbon dioxide along with the water at the moon’s south pole, so maybe this isn’t going to be a problem, depending on how much there is.

    If more carbon is needed, the solution may be to bomb the moon with giant carbon fiber ‘sponges’ that can be shipped compressed but expand ~500x in volume as they are dropped, and might stay more intact than solid carbon, making collection easier. Long term the carbon could be sourced from Mars or asteroids.

    Longer term a hybrid solution is best – deliver enough carbon to the moon to make enough methane to launch a lot of water to fuel production/storage depots in Earth and lunar orbits (though using orbital depots creates tighter launch timing constraints). This also reduces the size of energy and fuel production systems needed on the moon’s surface.

  8. The point of using the drop tank is that it supplies additional propellant, so that the Starship itself can land with full tanks and a decent amount of payload. That, combined with the mass of the drop tank itself if you take it all the way to the surface, is what reduces the thrust to weight ratio enough that you can throttle to less than your weight in lunar gravity.

    The Starship’s launch mass fully loaded is 1,320,000 kg. In lunar gravity, that’s 2,100 kN of weight

    With six engines, you have a thrust of 12,000kN, just about equal to the Starship’s launch weight on Earth. Cut that down to just the 3 gymbled engines, and you’ve got 6,000kN. Throttle range on the Raptor is 50%, that gets you to 3,000kn.

    So if you add a drop tank with a dry mass about half the mass of a fully loaded Starship, and rely on it to supply your fuel to the lunar surface, your throttling issues are solved.

    That’s probably too heavy for an efficient drop tank, but if added payload was incorporated into the drop tank, you’re good.

    So you design the drop tank to include some of your payload, and show up on the Moon’s surface with well over the Starship’s rated 100,000 Kg payload capacity. The drop tank also doubles as your initial moon base!

    Yes, I didn’t mention it, but of course the fact that rocket exhaust is compressible is why its speed drops drastically when it makes that 90 degree turn on hitting the ground. The organized kinetic energy gets re-thermalized.

  9. Wouldn’t a drop tank make throttling worse? Less mass on descent means you need to throttle down more. Or are you talking about something else?

    > depends on the exhaust coupling to the rocks VERY efficiently. Not very likely, since the exhaust would be hitting the surface perpendicular, not tangentially.

    I don’t think the coupling depends on the angle per se. Consider if you open a faucet above a spoon. Even if the spoon is perfectly perpendicular to the water flow, the water isn’t reflected up. It sprays sideways, because it has nowhere else to go.

    I think the coupling efficiency has more to do with the compressibility of the exhaust. Some of it will get reflected back, carrying with it a portion of the kinetic energy (which will then thermalize via gas-gas collisions). If it was incompressible like the faucet water, it wouldn’t be able to be reflected into itself.

    Everything else is pretty much elastic collisions (molecules to hard rock and hard rock to other hard rock), so the kinetic energy should be conserved in those collisions, and only redistributed and redirected.

  10. Raptors’ thrust is 2000 kN (per Wikipedia), which gives a flow rate of ~540 kg/s, moving at ~3.7 km/s. Wiki says the engine diameter is 1.3 m, so assuming the exhaust plume has ~1 m^2 cross section, its density should be ~0.15 kg/m^3 (540 kg spread over 3700 m^3).
    Lunar regolith is 1.5 kg/m^3, so ~10 times denser.

    If the ejection mass ratio equals the density ratio, then the dirt won’t reach orbital velocity. But if it can be lower, then an ejection ratio between 5 and 2.5 seems plausible to me. In that case, some of it can remain in orbit.

    edit: On further thought, the exhaust is compressible, and the individual gas molecules are much lighter than the average dust particle (way more than a factor of 10). So a bunch of the kinetic energy will be directed back into the exhaust as reflected gas, where it will convert to heat through gas-gas collisions. So the amount of kinetic energy that will be transferred to the regolith is much lower, and the ejection ratio limits are correspondingly also lower.

    For example, if only 50% of the kinetic energy is transferred to the regolith, the mass ratio limits are also 50% lower, since half as much energy can only accelerate half as much mass to the same velocities.

    So there’s a higher chance there won’t be enough energy to put any regolith in orbit. But it’s still not obvious that nothing will reach orbit. This needs more detailed computations.

  11. I’m not clear on this point: “By the time the exhaust is moving sideways, it’s moving a LOT slower.”
    Why would it be slowing down if it’s moving in vacuum? Or do you mean it’s slower after it hits the dirt?
    If the latter, then sure – because it already transferred part of its kinetic energy to that dirt.

    For a simple analysis of the question, I would start with the total kinetic energy of the exhaust, and try to figure out how much speed that can give to the dirt ejecta. The direction is much is more difficult to predict. Even if the exhaust hits the dirt straight down, the dirt pieces can still eject at various angles due to collisions with nearby dirt. This isn’t a two-body problem.
    (In fact, it would mostly eject sideways and at low angles, because its being forced down by the exhaust.)

    Vacuum Raptors’ Isp is 380 s, which gives an exhaust velocity of ~3.7 km/s, and specific kinetic energy of ~6.8 MJ/kg.
    Low Lunar Orbit velocity is ~1.6 km/s (estimated based on the altitude and orbital period), and Lunar escape velocity is ~2.4 km/s.
    Plugging that into the above kinetic energy gives the ejecta limits: if each kg of exhaust ejects between ~2.4 and ~5.3 kg of dirt, some of that dirt can remain in Lunar orbit, depending on its angle of ejection. The less dirt is ejected, the faster it will be ejected.

    The question is, what’s the actual ejection mass ratio.
    (continued in reply…)

  12. Zubrin is right on one thing, I would not want to land Starship on the moon until you had a hard landing pad for it. A tall thin rocket landing on soft ground may not be wise.

  13. I don’t entirely fault him, either. The guy is 67, and at 61 I know that even if you’re aging gracefully, by our age you’re getting kind of set in your ways, less flexible. I’ve certainly seen it in myself.

    But it isn’t good when you’re not responding rationally to people pointing out mistakes.

    Yes, the Starship takes more fuel to return to Earth. But it lands more payload on Mars, too, so it can land a larger fuel plant. If anything, the scaling laws favor the Starship.

    And he must be aware by now of the need to economize on engineering resources; Why wouldn’t you launch a larger mission requiring more fuel, to avoid having to design a whole new landing craft? It’s a very favorable trade.

  14. Mars synchronous orbit lies between Mars’ moons, which means if you set up a sky hook, Phobos is making a close pass several times a day. That doesn’t make it impossible, but it does make setting one up more complex.

    Anyway, you wouldn’t land the cable, you’d have to lower it from orbit.

    I think you’d actually be better off with a rotovator using Phobos as an anchor to put off having to make up momentum initially.

    Yes, there’s a lot to be said for having the first ship designed as a dedicated fuel plant. Though you *might* want the very first landing to be dedicated to scouting and site prep; Making sure the water you want is accessible, flatten the landing spot and unroll some glass fabric, install landing beacons. Maybe give Mars its own GPS constellation?

    So that subsequent missions have an easier time landing.

  15. I don’t fault Zubrin for not getting where things are going. I’m not entirely sure any of us really understand how cheap bulk access to LEO will change things.

    We are all byproducts of the 80s, 90s, and 00s where access to space was more expensive than anything you would do in space.

  16. SpaceX doesn’t care. The financial motivation for BFR/BFS is 100% reusability and a sharp reduction in the cost to reach LEO.

  17. The first Starship to land needs to be a bespoke ship and won’t return. It could do one of two things:

    1. The ship itself could be a Lunar Oxygen plant. Use the starship’s tanks to hold locally produced oxygen, enough O2 to fill a normal starship up for a return trip to Earth.
    2. If you want to get crazy ambitious then land a kevlar cable for a space elevator. That way future cargo takes the last leg of travel to the lunar surface without using any propellant.
  18. “but he really is stuck in his old space mindset”

    I love Zubrin but he is stuck in the trauma induced 1990s era of space flight when the only way to get anything done was to shave every gram of weight from every rocket and to make everything do at least two different things with triple redundancy.

  19. I have heard the ejecta argument from several sources.
    Two things I am wondering about in this regard.

    1. The Raptors are way overpowered for landing on the moon. Maybe the RCS engines that Musk has previously mentioned would be strong enough to land Starship? At least make the touch down soft enough after the ship has slowed down high enough above the surface.
    2. Those cargo containers next to the engines could house something that could be dropped prior to landing. Some sort of inflatable or unfolding landing platform or a kind of ejecta shield, maybe? They could drop that from a high altitude and let it unfold prior to landing on it.
  20. Seems to be the drop of payload from 150t to 10t more than cancels out any gain in efficiency.  6 people instead of 100 people?  Plus mini starship will not afford the same level of radiation protection and probably will not be able to go as fast either so 9 month instead of 4 month..  Does not make sense.

  21. Not going to happen: The exhaust from the Starship engines is directed straight down. It hits the surface, and it’s not hitting some optimized airfoil, it’s hitting a dirt surface. By the time the exhaust is moving sideways, it’s moving a LOT slower.

    Then the low density exhaust, moving much slower, has to transfer momentum to the rocks. Even as it is slowing down and dropping in density due to being spread out.

    I’d be shocked if the the ejecta managed to reach a quarter of the exhaust velocity, and that for the dust, when landing on an unprepared surface.

    There’s no question that, if you’re landing a Starship near a base, you want to be doing it on the other side of a hill, or inside of a crater. And you’ll want a landing pad that’s swept clean as soon as you can get it.

    But you’re not going to be threatening assets in orbit. That’s just fear mongering.

  22. He’s trying to fit the Starship into his pre-designed Mars Direct mission. That’s what is going on here. He doesn’t want to accept that the Starship allows you to go big.

  23. Yup, pretty much. His program to minimize fuel requirements also minimizes payload. Sure, the full Starship requires more fuel to return, and so a larger fuel plant. It can also LAND a larger fuel plant.

    Yes, that requires more refueling flights in Earth orbit, too. But it’s a much more productive mission in terms of payload to Mars surface. And the Starship, if it works, makes those refueling trips relatively cheap.

    Basically, he’s just proposing to use the Starship as a stage in launching his original Mars Direct mission, instead of exploiting the full capabilities of the Starship.

    I continue to suggest that a Starship to the Moon flight would best use a drop tank to increase delta v to the Lunar surface, which you’d then leave behind on the return to be available as material and hardware for the lunar base. This solves the throttling issue, and gives the Starship the capacity to land on a much less level surface. And the empty drop tank becomes payload; Large SS tanks delivered to the lunar surface will be valuable!

    Finally, his suggestion that the landing Starship could launch rocks into lunar orbit and beyond depends on the exhaust coupling to the rocks VERY efficiently. Not very likely, since the exhaust would be hitting the surface perpendicular, not tangentially.

  24. I don’t get it … more mass is better…. period… why Would anybody want to go backwards in progress.

  25. Sounds like NASA propaganda…. try to trick spacex into not doing as good of a job so they don’t get their ass kicked on SLS Tantanic money waster….

  26. I wonder if Zubrin doesn´t really understood what´s the aim behind Starship proposal: The real function of starship is to colonize Mars. To go to the moon or other missions are collateral benefits of the system.

  27. Wasting their time and money in making another lander is the best way to ensure the schedule isn’t met.

    I like Zubrin most of the time, but he really is stuck in his old space mindset of going there, planting a flag and taking some nice pictures, plus do some ISRU for research and to enable a couple more similar missions.

    While SpaceX really is about converting themselves into the conveyor belt to the Solar System.

    Just put stuff you want in the Moon or elsewhere onboard one of their frequent, reusable launches and get done with it.

  28. At least lunar ISRU is a stated option in this stuff. Been the main goal for 40 years for those who want to develop Space!

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