Comment by Robotbeat | Hacker News Reader

Comment by Robotbeat | Hacker News Reader

Parent Full thread

This isn’t accurate. 20-30% larger diameter with a constant density would mean about 20-30% greater escape velocity (mass is radius cubed but gravity is inverse radius squared, etc). So instead of 11km/s escape, it’d be 13-14km/s. And instead of ~9km/s to reach LEO, it’d be about 11km/s. This is not at all a showstopper. Even in the 60s and 70s, we built rockets with enough stages to go land on the moon and return to Earth, which is a round-trip delta-v of like 14-15km/s, and the Soviets did this mostly with low Isp hypergolic propellants, not even hydrolox let alone an exotic propellant like nuclear thermal hydrogen or electric thrusters. We often just launch stuff directly to GTO or GSO instead of bothering with electric propulsion, so even today’s rockets (many of which also have high enough thrust to weight ratio, like an Atlas V 551) you could do it (and then probably rely more heavily on electric thrusters). Even with higher gravity losses.

It’d be more expensive, sure, but the idea that it’d be almost impossible is just not true at all. It’s a very common thing that even a lot of fairly smart people often say, though.

Not to mention other technologies like balloons that actually make climbing through the (denser) atmosphere easier on denser planets.

https://arxiv.org/html/1804.04727v2

A larger earth with the same average density of Earth that has 1.3 Gs of surface gravity would need around twice the fuel to escape into space and a system that could handle that extra fuel and longer burn without self-destructing.

The fuel itself would weigh more, and thus require more fuel to lift. The fuel to lift that fuel would also weigh more and would also require more fuel, etc, and the radius of gravity to escape would also be larger even though it would also be mediated somewhat by the inverse square rule.

I actually wouldn't be surprised if you needed more than twice the rocket fuel to leave a 1.3 G super earth that is 1.3 times it's diameter.

Not impossible, but definitely more difficult.

Twice the rocket fuel or half the payload. Again, regular rockets can do this today, like the Atlas V 551/2.

I think it was just a matter of speech. 20-30% doesn’t matter because such a planet doesn’t exist. It’s probably bigger, and so OP has a point

There are a bunch of exoplanets in that size range, and we could probably contrive chemical rockets with over 20km/s delta-v if necessary. Nuclear rockets 30km/s.

"Because such a planet doesn't exist" bruuuh do you know how many planets there probably are?

How many in Goldilocks zones with stable orbits for billions of years? With non-toxic soils and atmospheres? With water at the surface and in significant amounts?

Now, how many can humanity reach in the life time of frozen embryos?

And if it's more than 30% bigger then escape velocity is likely beyond the practical limits of science.

That is a totally different topic. We’re talking about a hypothetical, where Earth is 20-30% larger, not whether an identical copy of Earth but 20-30% larger actually exists nearby. We know such roughly similar exoplanets are common, though. In that size range and in habitable zone.

And again, 30% greater escape velocity just isn’t a challenge. Existing rockets could be used to reach LEO, like Atlas 551 (well, probably want 552 to reduce the gravity losses). And then you can use electric thrusters (which don’t have a hard Isp limit) to reach wherever.

Heck, we could probably reach LEO even if escape velocity was twice what it is. Even 3 times, if you include more exotic propulsion methods that we’ve done some testing for (nuclear thermal, etc). With staging, etc, it’s just a matter of a worse and worse mass fraction, not really whether it’s possible or not. And once you get to low orbit, electric propulsion allows much better scaling as you can adjust Isp higher (although reduces thrust and means it takes longer to do your mission).

Reply on news.ycombinator.com