Comment by rbanffy | Hacker News Reader

Comment by rbanffy | Hacker News Reader

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> 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.

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.

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.

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."

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.

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...

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