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Comment by hughes | Hacker News Reader

It's interesting that there is no existing (or planned) launch vehicle that could perform a direct injection for this mission. Getting to Saturn is really, really hard, and without multiple gravity assists from the inner solar system it would be simply impossible.

Too bad it results in an 11 year transit time though! Flying direct would save almost 8 years of that...

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I don't think that's accurate, if you're willing to allow (perhaps multiple) kick stages. A naive Hohmann calculation will be misleading as you can do a massive Oberth burn deep in Earth's gravity well before leaving, and entry will be done aerodynamically (not propulsively).

Even spaceX's starship/BFR with LEO refuel wouldn't work?

Maybe answering my own question with some further thought. I guess even if it could get there, it likely couldnt return which would be a big waste of a reusable starship.

Why do you think it would be unable to return? I'm curious.

Naively, a round trip requires twice the delta-v, which means you need to launch with a lot more than twice the fuel.

Maybe they could produce more fuel and oxidizer in situ, and maybe Earth is a more attractive target for aerocapture, and maybe gravity assists line up better one way or the other, but it's not a bad starting point to assume a round trip is far more expensive than a one-way trip.

I'm not sure on what basis he thinks Starship might be able to get there in the first place.

Yeah that was my thinking. And I didn't have a basis for thinking it could get there in the first place which is why I asked the initial question. I didn't realize just how high the delta v requirements were for Saturn.

Yeah, I didn't mean to accuse you of making assumptions; I just wanted to call out one of the obvious well, actually replies that my own comment might have received.

It is an interesting question for sure, especially given that one of the stated goals for Starship (IIRC) is to enable exploration of the outer planets.

Sorry for the late reply. Titan has lakes of liquid methane rather than water. Raptor is a methalox engine. I think refueling on the planet would be quite possible.

There is no doubt in my mind that with the right equipment on the surface of Titan you can refine methane out of the lakes, electrolyze oxygen from the water ice of the crust, and fuel a Starship.

Can you fit that equipment in a single Starship, by mass and volume? Is that what we're talking about here? No idea.

The delta-v required for a trip to Titan, according to this[1], is 28.43 km/s. Optimistically, a fully re-fuelled startship in LEO has ~10km/s of delta-v to spend, afaik.

[1](https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of...)

Starship can do better than that if stripped down to expendable use. About 13km/s: https://twitter.com/elonmusk/status/1111798912141017089

Also, you're reading that delta-v map completely wrong. The red arrows mean you can aerobrake. So it only takes about 4.1km/s to reach Saturn if you start out at a highly elliptical (near escape) Earth orbit and do a large departure burn at perigee. (See here: https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary ...the 7.3km/s if the burn is done starting in LEO, minus the 3.2km/s benefit you'd get if starting near c3=0. Or you add up the numbers on your delta-v map that can't be done using aerobraking, and you get the same 4.1km/s delta-v if starting from Earth escape.)

...so a fueled up Starship could send a payload direct to Titan and still have enough propellant to propulsively brake back to Earth orbit immediately after sending the Titan payload on its way.

(But that's just one option... Lots of other vehicles could do it if you used refueling or some high performance kick stages or in orbit rendezvous.)

So it would work! Thanks for the analysis.

> Getting to Saturn is really, really hard

Why is that?

Delta-v.

Just the speeds required. This mission will effectively have a speed of about 17km/s faster than the speeds just to get to low Earth orbit after its final gravity assist.

https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of...

Gravity is a b..ch :-)

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