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Comment by ncmncm | Hacker News Reader

Not just SLS. The whole notion of a "Lunar Gateway" makes even less sense. Like, absolutely none at all.

Its original purpose seems to have been a place SLS would be able to get to, and then SLS would have the purpose of getting to it, in the same weird way that the ISS was a place the Shuttle could get to, and the Shuttle was needed to resupply ISS. Until it wasn't; and now the ISS has no purpose or value. Don't need SLS to get to ISS, so make a "Gateway" that does.

Make no mistake, a "Lunar Gateway" is much worse than completely useless. If you want to go to the moon, go to the moon. If you want to go to Mars or an asteroid, go there. Stopping at lunar orbit, effectively halfway, just costs a huge amount of extra fuel to stop and then start again.

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"Stopping at lunar orbit, effectively halfway, just costs a huge amount of extra fuel to stop and then start again. "

Wait, what? The standard "go straight to the moon" mission plan (e.g. Apollo 11) has a required point where you are at a lunar orbit. There's no extra fuel cost to "stop" there; as you're spending fuel to slow down between lunar insertion and lunar landing, at some speed you inevitably are in a lunar orbit and can spend some time there at no fuel cost.

It does limit your flight profiles significantly though, as you have to use the same Lunar injection orbit

What's the actual benefit?

The "Lunar Gateway" is not planned to be in an orbit that would be optimal for any particular, expected lunar landing site, nor as rendesvous for a launch from the lunar surface. IOW, a different staging orbit would be better for any given descent and rendesvous. Stopping at a "gateway" first, too, costs more.

The only useful place for a "gateway" would be in low Earth orbit, with minimal cost to deliver fuel to, and useful for any destination except highly-inclined Earth orbit. Its value there is a result of the radically different character of Earth-launch vs. deep-space transport vehicles, being the natural point to transition between the two. Getting between Earth orbit and lunar orbit is, relatively, trivial, even for a crewed mission in an all-fired hurry.

Finally, the moon is a very specialized destination. There is no value in going or being there, beyond curiosity. Everything of practical interest or value is elsewhere, and a "Lunar Gateway" is useless for those.

I think this is in general how NASA often operates. Dr. Robert Zubrin has made the same criticism multiple times. Missions are invented in order to provide rationale for previous decisions, instead of it being the other way around.

If they want to target an emerging market they should figure out how to get a lot of fuel to orbit cheaply. Starship is going to need a lot of tanker launches and they’d probably pay for extra capacity if it was there.

It seems to me that like any system, the go to Mars plan will benefit from modularity. That’s what the Gateway might offer - multiple paths to Mars are possible.

A "lunar gateway" is much worse than useless for getting to Mars.

A side trip from low Earth orbit to the gateway takes nearly the same delta-V as to Mars. But unless you are willing to take weeks getting to the gateway, you use up most of a Mars transfer delta-V to stop there, and as much delta-V again to get to your transfer orbit to Mars. In other words, LEO-to-gateway-to-Mars takes almost 3x the delta-V of going directly from LEO to Mars.

I play a lot of Kerbal Space Program. It saves a LOT of fuel and mechanical complexity to break up space missions into specialized components. Engines that are able to perform well in atmosphere do not perform well in space, or vice-versa.

So Having a launch vehicle that does a rendevous with a LEO space station, then transfer the astronauts to a vehicle that is optimized just for the function of shuttling back and forth between an Earth space station and a Lunar space station, then have them board another craft that is optimized for landing on the Moon and taking off again, using minimal fuel because it doesn't need to carry the weight of the other components..

It makes designing a mission a lot more complex but at the savings of a lot of components and fuel, makes a lot more reusable components as well. Then when you add the ability to refuel in orbit by capturing asteroids, it really pays off.

Apollo used specialised vehicles too, you don’t need gateway stations for that. Conversely using a gateway station means you cannot choose optimal orbits and timings for your mission, you have to use the pre-established orbits and timings whether it suits you or not.

Suppose I want to send a mission to land on the moon at a site not on the orbital plane of the gateway station. For an all in one mission this is no problem, I can match my Earth launch plane and transfer plane to line up with the destination. With a gateway station the lander has to do two plane change burns, on on landing and again on ascent back to the gateway. If you play KSP you know just how expensive plane change manoeuvres can be.

ISU is a great opportunity for sure, but we don’t have it yet. That would be a great reason for building a way station and a great reason for going to it on other missions. But without that reason what’s the point?

From the article:

Once the Gateway is up and running, the PPE element will provide power (in the 60-kilowatt), high-speed communications, attitude control, and Solar-Electric Propulsion (SEP) capabilities. This will allow it to alter its lunar orbit as needed, giving crews greater access to the lunar surface than was ever possible with conventional missions.

So instead of launching the vehicle into whatever plane you need for a specific mission, they’re going to do a plane change manoeuvre for the entire station for each mission?

If it takes no fuel (from being solar-electric powered), and can be accomplished in parallel while the spacecraft on Earth is prepared, why not? Sounds reasonable to me.

Plane changes always require fuel. Solar-electric propulsion, in particular, requires fuel. Its high specific impulse means that it needs less fuel, but also would take many months for a small change of orbital plane.

Requires solar fuel, which is free...

Many months is ok, because it can happen in parallel with the spacecraft being prepared on Earth.

There is no such thing as "solar fuel". Nothing in space flight is free except failure.

It uses energy from the sun to turn xenon gas into plasma and shoot it into space. This makes it use 10 times less propellant than conventional systems.

The xenon gas is the propellant. This still comes from the earth so it's far from free. It's just more efficient.

Isn't this more or less the "container ship" metaphor? Forcing everything into containers means the ship carries overhead weight and some unused space, but that overhead is offset by the standardization of everything else in the logistics process.

A plane change can be expensive, but if there's a space station that can supply all the required fuel, who cares?

That fuel still has to get there somehow, so the cost is still being paid.

Any argument of standardization efficiency applies overwhelmingly moreso for a low Earth orbit transfer station. The only reason even to talk about a lunar station is politics: a lunar station seems more exotic, and lunar orbit is easier to get to than the surface.

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