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Comment by twic | Hacker News Reader

You want to get all the exploding done before you launch the rocket with crew aboard.

Saturn V could lift 140 tonnes to low earth orbit. Falcon Heavy can lift 54 tonnes to low earth orbit. I think that means it could get a mission to the moon as long as it weighed 38% of what Apollo did. That doesn't sound like a fun trip.

Unless you assemble the moon ship in earth orbit first. Falcon Heavy can launch Orion on its own, or the European Service Module that it requires. I imagine it could launch a lunar lander too. You would also need a booster to send the combination to the moon - the Saturn final stage that did that weighed 100 tonnes, twice what Falcon Heavy can launch. So, maybe you launch an empty booster, and send up a couple of tankers to fuel it on orbit? It's a lot of launches, and a lot of work to do in orbit. That's not impossible (and is cool as hell), but there would be a lot of work to do to make it possible.

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> Saturn V could lift 140 tonnes to low earth orbit.

Just to be pedantic, remember, that the only thing Saturn-V brought to LEO and left there was Skylab, with mass 77 tons. Other than that you counting parts of Saturn-V itself as part of payload, which is inconsistent - you don't count mass of Falcon-9 second stage as part of payload, for example, even though it gets to orbit.

Saturn-V is better characterized by the mass it leaves on translunar orbit, which is 48,6 tons. For LEO it's awkwardly lopsided with mass of 3rd stage and remaining fuel.

In other words, if you have a 140 tons satellite, you couldn't get it to LEO with Saturn-V. So the comparison is wrong.

Good points. I think the reason this is complicated is that the Apollo missions included one stage, the S-IVB, which was used twice, for getting into low earth orbit, and for departing for the moon. You have to count some fraction of that as mass lifted to orbit, but i don't know how much.

> Falcon Heavy can lift 54 tonnes to low earth orbit. I think that means it could get a mission to the moon as long as it weighed 38% of what Apollo did

You can launch the lander first, park it in lunar orbit, then launch the crew on a second Falcon Heavy, each having 38% of an Apollo worth of mass. With a little more creativity, you can launch the ascent vehicle on another Falcon Heavy and land it before the other two so that astronauts can rest assured that, unless they kill themselves during the landing, there is a ride back up waiting for them.

With that you can have 3 separate chronograms and each mission is simplified in relation to a single Apollo.

If you are unhappy with the mass budget, you can launch yet another series of missions with supplies and equipment to be landed before the ascent vehicles are landed. And, since in this case whatever lands doesn't even need to be pressurized, or delicate, landing it should be much easier too.

Now we are at 1.52 Apollo masses per mission. It took 4 launches to get there but, still, it's a lot of stuff for our astronauts to work with. Also, if any one launch fails, you can always try again.

Requiring the astronauts to execute a cross country EVA from the lander to the ascent vehicle seems like a huge risk. What if they land just slightly too far away?

They can land the Falcon 9 first stage in the middle of the drone ship. Any reason they can’t locate a perfect spot on the moon and land there exactly?

Earth orbiting GNSS provides excellent position information near the surface of the Earth but poorer position information near the surface of the Moon. That said, the Falcon 9 is equipped with a decent inertial navigation system.

Yep, not sure about the high altitude flight, but terminal guidance is certainly not GPS based. I think it's mostly radar & they even painted their landing zone with some special paint a while ago to get better radar returns.

Eh. Apollo 12 landed within 200 meters of Surveyor 3 with 1960s-era technology and after manually evading a bunch of boulders. These days, SpaceX can land a booster on a ship in the middle of the ocean after a hypersonic return. I think they could manage a precision landing on the moon.

Planning to land in a designated area is easy. Ruling out any deviations is a challenge. SpaceX knows booster landing on earth, but that may not be the the same on the moon. Moreover, it can afford to lose a booster every know and then here, but there it can't.

It can be done, but can it be guaranteed?

> but that may not be the the same on the moon

True. No wind or waves, everything standing perfectly still.

I'd say it's much easier.

Principal negative as I can imagine it is the constitution of the regolith. I remember reading that one of the concerns about the original lunar programs was how soft and deep the regolith might be at the landing site - if it's loose and silty and 6' deep, then that's going to get tricky.

Sure, we have much better radar and composition maps of the lunar surface these days, but it strikes me that it's still not as 100% as landing on a barge's [presumably] metal surface or concrete and Cape Canaveral.

The tiny landing legs (more like landing pinkies) of Starship concern me a lot, but whatever would be going to land with this approach would have reasonably robust landing legs with feet that can handle soft surface.

I'm more worried about sandblasting the surroundings.

Aye, that's it! The huge and wonderful mass of a Starship plonking down three spindle-thin legs onto and into luna incognita.

I'm sure smarter people than I have thought about this, but it was a thought that struck me when I saw the mockups in the NASA lunar bid vid a wee while back.

Well... At least it's more than 3.

Still, by then the ship should be somewhat top-heavy and those six landing pinkies do not inspire much trust.

OTOH, the people designing this are smarter than we are and much more experienced than me in rocket design, so they may know one or two things I don't.

The thing is that if you are landing a two stage rocket (or a rocket with enough delta-v for ascent) you can abort the landing back to orbit (unless you are crazy enouh to do direct ascent from earth without entering lunar orbit).

Apollo basically had a switch + sensor based tigers to start an automatic segvence that shuts down and drops the first stage, starts the seconds stage and flies it back to lunar orbit.

If you don't have an ascent stage or enough delta-v, you have to land, potentially many kilometers of course or even crash if something goes very wrong.

That's a very good point. A lander without an ascent module can't abort to space. Once you do your deorbit burn, you will land no matter what.

That's a good reason to have multiple supply/equipment depots on the surface before you even land the ascent vehicles. Walk up to the nearest one, then drive the rest of the way.

Also that's one good reason to have spare ascent craft (or supply depots) parked on low lunar orbit that could be directed to pick up any stranded astronauts.

A last option is a LESS-like lifeboat assembled from lander components. Surface to orbit on a convertible has some appeal.

>A last option is a LESS-like lifeboat assembled from lander components. Surface to orbit on a convertible has some appeal.

Already considered in Apollo days:

https://en.wikipedia.org/wiki/Lunar_escape_systems

A very simple "rocket chair" using residual fuel from the LEM. There were also variants for regular use during exploration using only leftover propellant from the descent stage.

Flown manually all the way to lunar orbit due to guidance computer being to heavy back then:

"Guidance in typical LESS designs was simple: an 'eight-ball' to show spacecraft attitude, a clock to show time since liftoff, and a planned pitch program. The Apollo Guidance Computer used as an autopilot for the CSM and LM had a mass of around a hundred pounds and consumed a significant amount of power, so computer controlled flight was out of the question. This would be one of the few cases where an astronaut flew a rocket manually all the way to orbit, and with far less instrumentation than normal."

I remember reading somewhere the astronauts noted Surveyor 3 looked sandblasted, presumably because of the LM's ejecta. That's why the ascent vehicle should probably not be the first thing to land and, perhaps, should be located not too close to the crew landing spot.

This could be made into a movie. Astronauts stranded on the moon have to figure out how to survive until they can travel to the ascent vehicle.

There is a 1968 movie about that: https://en.wikipedia.org/wiki/Countdown_(1968_film)

And American astronaut is launched in a hurry due to the space race in a one-way Gemini based lander, lands off course and needs to find the shelter/ascent vehicle with limited supply of air. And at the same time the Soviet mission he was effectively racing with goes silent...

One think to note is that if you use cryogenic proplants for your lander or lunar tug, you might face issues with fuel and oxidizer boiling off over time (unless you go for very good isolation or use active cooling, both rather heavy), reducing your time window to have everything launched & used.

You can opt for hypergolic propellants that won't boil off, but you sacrifice performance and make ground handling more expensive and dangerous due to these being highly toxic.

You can use cryogenics for the landers and depots that don't stay parked in lunar orbit for long, but any assets with long shelf life should use hypergolics.

And now my mission profile has two different landing modules, one using cryogenics, one using hypergolics. Oh well... We can always go hypergolic if the mass budget is not a big issue.

> Saturn V could lift 140 tonnes to low earth orbit. Falcon Heavy can lift 54 tonnes to low earth orbit. I think that means it could get a mission to the moon as long as it weighed 38% of what Apollo did. That doesn't sound like a fun trip.

While I haven't yet seen anything concrete, SpaceX's definitely wants to have orbital refueling. If I recall, that's actually one of SpaceX's requirements for Starship.

Orbital refueling would mean that lift capacity to LEO doesn't translate directly the range beyond LEO.

orbital refueling makes a lot of sense, do you know if there are plans to manufacture the fuel in orbit? That would be somewhat good practice for producing fuel on Mars. At lease they would know not to forget anything, no making a quick trip to Home Depot while in orbit.

I've heard about approaches for manufacturing fuel on the Moon, but I can't imagine how you might manufacture it in earth's orbit. What are the leading ideas around this approach?

edit: i know we can launch more fuel into orbit, and I really emphasize the "manufacturing" in orbit part.

Collect a water-ice asteroid or cometary nucleus or some such, park it in orbit, and electrolyse it.

I'm not aware of a less outlandish plan!

Now, if you want oxidiser, you could skim air off the top of the earth's atmosphere. I have no idea if that has been considered outside of science fiction [1], though.

[1] okay, here they're skimming nitrogen, but: https://en.wikipedia.org/wiki/Fallen_Angels_(science_fiction...

If you were electrolyzing water ice, wouldn't you get both hydrogen (fuel) and oxygen (oxidizer), by definition?

Yes. But if you only wanted oxidiser, you could skip the awkward "capture a cometary nucleus" step by scooping air. Since oxygen is much heavier than hydrogen, this could save you a lot in mass launched from the Earth.

Also if you had enough power (solar or possibly nuclear) you could use the captured gases directly for slightly inefficient propulsion by heating them NERVA style or accelerating them by a modifiedion engine.

I remember some concepts for very low altitude satellites that compensate for drag by running a solar powered ion ramjet. :)

The main problem would be getting the necessary atoms to your factory. It likely would be easier to construct fuel on earth or on the moon and send it up. (If your rocket exploded the ‘BOOM’ might be a bit louder if its cargo were rocket fuel instead of raw materials for rocket fuel, but the practical difference would be zero)

Your best bet would be to use mass that’s present in space, but unfortunately (1), there isn’t that much in orbit. In theory, one could scope up cosmic dust, tiny meteorites, and old satellites in orbit. It would be a challenge to scale that up, though. https://en.wikipedia.org/wiki/Stardust_(spacecraft) didn’t catch kilograms of stuff, even though it explicitly went for a comet.

Capturing a larger meteorite would work, but larger meteorites are harder to catch.

If you’re extremely ambitious, build a solar powered particle accelerator in space, use it to create electrons, neutrons and protons (https://en.m.wikipedia.org/wiki/Matter_creation), turn those into atoms, and build fuel from those (apart from the practical problem of scaling that up, there are ‘a few’ Ph.D’s to be earned there.)

(1) or fortunately, as having lots of mass there would make space flight dangerous (https://en.wikipedia.org/wiki/Space_debris)

About 80% of the methalox propellant used by the Raptor engine is oxygen. If you manage to scrub oxygen from the very low earth orbit (VLEO), and then bring it to LEO, you are in business. The Japanese Space Agency put a satellite in VLEO in 2019 [1].

[1]https://en.wikipedia.org/wiki/Super_Low_Altitude_Test_Satell...

Oxygen can be obtained from regolith on the moon.

I'm in-favor of robots on the moon bootstrapping a fuel and solar-furance refinery of materials construction plant. The speed of light delay is annoying, but not fatal for higher level remote supervision and planning of automated processes.

Manufacturing fuel in orbit requires sending the plant in orbit, extracting the raw materials, packaging them then bringing them up there and also involves sending a ship every time a maintenance is needed, unlike Mars where the raw material (water, CO2) are already nearby. ISRU (in situ resource utilization) is the best way to go, i.e. you bring the plant to Mars but at least you use the local raw materials.

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