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Comment by choeger | Hacker News Reader

Yeah, about that "impractical" part. I think it's time to reconsider it. With the Falcon, SpaceX has made access to space if not cheap then at least very affordable. The sheer cost of the Mars missions, as an example, make the market launch price look like change. The design also has to be perfect due to this "once in a decade" mentality.

So I really think it's time to focus on "get there quickly" to make missions cheaper. Design a relatively simple vessel weighing around 40t or so with enormous ISP (electrical or chemical, whatever fits the bill) and then use it twice per year or even more often to send more or less standardized probes to whatever target you find interesting.

I am well aware that this concept sketch trivializes many aspects of space missions: Communication, power, navigation to name just a few aspects. But I still think that oversizing and then using the same design often would be much cheaper in the long run.

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I'm looking forward to lifting fuel into orbit as the next major progress item for this. A lot of the math changes if you lift fuel into orbit. Our mindset is currently very confined by the need to launch everything in one shot. I'm not sure exactly how fast you could get to the outer planets, but I expect it's at least enough that you would no longer need a "grand tour" to do it. (Also depends on whether you just get there fast as a fly-by or whether you want to go into orbit, I'm sure.)

Honestly kind of unsure why we're not hearing more about this already. The costs of it are an increasingly small fraction of what we're spending on these probes anyhow.

Aren't multi-stage rockets essentially "lifting fuel into orbit"?

I mean fuel as the payload, to be used as fuel for future missions via something rendezvousing with it in orbit and picking up the fuel. You could think of it as one more stage, mathematically that's how you might treat it, but from an engineering perspective it's a qualitative difference versus what we have now.

Yes and no. They're lifting fuel into orbit, but all at once. That has three implications of needing more thrust for the engine than if we did it in multiple trips

That's true, but there is extra energy cost associate with putting it in orbit and then breaking orbit. Orbital calculations are about velocity, and delta-velocity, more than they are about height-above-ground.

Getting above the atmosphere is a big win, and may well make it worth it to do the final fueling from orbit. But in the end, getting to orbit may be as little as 100 miles of a multi-billion-mile trip.

I find it weird how your first paragraph is correct, then your second paragraph kinda seems to ignore it?

The whole reason I'm interested is obviously not that lifting fuel a vanishing fraction percent of the distance is helpful. The whole reason is that with a full fuel tank that's already in orbit, a probe ought to be able to take off straight for a planet without having to wait for an energy-impoverished orbit to be available. Or take off on a much more energy-intensive orbit that involves more aggressive slingshot maneuvers, or whatever else; having more energy doesn't preclude using that energy cleverly as well.

I also find myself curious about what we could do with an ion drive... or set of ion drives... fed with lots of fuel. And maybe an RTG for power, or some other real power source other than solar (which isn't much of a source out there in the gas giants). Or fed by solar cells, but first it takes a swing around the Sun for the almost sole purpose of gathering solar energy and driving the ion drives with it. All sorts of interesting things become possible if we can just use multiple lauches.

The impractical part is really about carrying that much fuel. The problem is you to speed up and then slow down at the other end. Slowing down takes fuel (obviously) but you have tos peed up that fuel at the beginning as it increases the overall payload. That's what I mean by impractical.

New Horizons launched on an Atlas V 551. Falcon Heavy has a larger payload capacity so it might be more possible.

But if you're relying on an engine to decelerate more than gravity then that creates another problem: whgat engine do you use? Remember it has to probably survive in space for possibly 10+ years and fire successfully. The Apollo lander used a super-simple engine for this reason, so much so that it could only ever be fired once (to launch back into Moon orbit) so it couldn't even be test-fired [1].

So what you'd put on a deep space probe is a significant unsolved problem.

[1]: https://space.stackexchange.com/questions/36226/why-could-th...

If you look at the comment from NortySpock in this there was a study to get a probe direct to Neptune and with a Falcon Heavy and extra kick stage it was "only" 3800 kg.

This is still an order of magnitude more than previous deep space probes (Voyager, New Horizons), but still not huge. It is still significantly smaller than something like our current geostationary communications satellites.

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