Oddbean

In an interview, Elon Musk recently said that Earth has just the right gravity at which space travel [with chemical rockets] is theoretically possible. A bit less gravity and it would be easy. A bit more and it would be impossible. I'm not sure what the limit would be but the fact that we have enough in the tank to do a propelled landing tells me there is still some margin. But not much, as much of the breaking before landing is done by air friction. So while a bit bigger planet would not be able to land as smoothly, a much bigger planet basically needs nuclear propulsion to get off the ground. So what are the actual limits here? Earth invented life and went through snowball times and global extinction events. Bigger planets' intelligent life cannot get off their surface - or might not have a solid surface at all that's not covered by liquid hydrogen. Could intelligent life develop on a gas giant? If so, what would be the hurdles to even communicate out? How would one go about sending a radio signal from Jupiter? Would life on Jupiter even come up with radio technology given it's probably useless on their planet? Or can more complex molecules not develop at all on gas giants? In the other extreme, smaller planets, getting to space is "easy" but getting to life or intelligent life probably not. With less of an atmosphere, comets are more frequent to hit the surface. It needs less rock for "global" events. I don't understand enough about the origins of life to rule out non-carbon based chemistry but I would find it hard to believe life to spring into existence on a planet that has no liquid for chemistry to happen in.

I don't think this has anything to do with the size of the planet, but with its density. But yeah, the parameters for intelligent life to develop are probably extremely narrow, but hey, there are quadrillions of planets, so there will be more than enough with just the right conditions. And like you said, there are probably intelligent lifeforms out there that are so utterly alien to us that we can't even imagine how they would look like and they might find other ways to get into space.

For space travel, escape velocity and cost to leave the atmosphere are the main factors. I've seen estimates for earth-like planets but wonder if venus and mars are earth-like by their count. Mars might have had a more hospitable past but was it long enough to get anywhere near the time scales we had to develop intelligent life and space travel? Was it closer to asteroid belts? Does Venus provide conditions for complex molecules? A dense atmosphere sure helps to protect the surface but it also holds back rockets on their way out. But then again, dense atmospheres might not be a problem if lighter-than-air platform launches work: nostr:nevent1qvzqqqqqqypzq3huhccxt6h34eupz3jeynjgjgek8lel2f4adaea0svyk94a3njdqqstjpnthcg63v35v2gx5ls80m3nl4yc0yn5ufr63zkphta4d9yhhmguf0rcn

I think I can fly because of lightweight bones and windage. If we were on much bigger planet, we couldn't exist in forms that we are. I think Sun system is not for any complicated life system excluding Earth.

To understand how close we are to have too much gravity, just look at the payload-to-total-mass ratio. Except for SpaceX, the ratio is between 1% and 2%, meaning the useful stuff you can bring to orbit is only 1 to 2% of the total mass. 98-99% of the mass of a rocket is its structure and its fuel. This is due to what's known as the curse of Tsiolkovsky's rocket equation. A tiny bit more gravity and the 1-2% becomes 0%. SpaceX Falcons are around 4% payload mass. Saturn V was 4% too. Catching Super Heavy booster with the chopsticks means the booster doesn't need legs, which would be pretty heavy given that it's tall. By removing legs the payload-to-total mass ration improves quite a bit.

Great way to look at it! But somehow I'm not convinced the whole argument is as rigid as Elon puts it. 4% payload is just the state of the art and the rocket equation would allow to lift much more to space if we further improved the tech. Saturn V didn't land any boosters, so Starship is way ahead already. They could get well above 4% to orbit if they wouldn't carry fuel for the landing.

But for a bit more outlandish plan: Let's launch the vehicles from a platform in 30km altitude to almost completely eliminate the atmospheric drag at launch. If ChatGPT is right, 130t of helium would suffice to offset the 5000t of a fully loaded starship. Helium costs $100k/t, so for the mere starship weight we would need $13million. Make that $130million for a full 350 million cubic meters zeppelin/balloon/structure - costs of the order of magnitude of a single launch to LEO. At that altitude, winds are below 120km/h and drag less than 2% of an equally strong wind at sea level. How can we anchor a lighter-than-air platform of that size? It's size would be equivalent to a sphere with a radius of 440m - almost 900m across, so quite a mega structure. But a balloon or zeppelin shape would probably be the best, with the platform hanging below it. The rocket could dangle below it and enter into free fall before igniting the engines. It would lose as little altitude as necessary to safely fly around our platform. No trouble with blown up concrete at the launch site. The anchor site could be a single site so the zeppelin would align with the wind direction but the tether would need enormous tensile strength and provide for transport of parts, liquids, gas, data, electricity and ultimately humans. Catch at 30km or on the ground? I think, once it works well, returning boosters could be caught below the zeppelin again. Maybe not too close to not lose that mega structure but the maneuver would be quite similar apart from the zeppelin not having a static geo location and the booster needing to go a few meters sideways in the final approach.

Searching for this, all I can find is https://en.wikipedia.org/wiki/Rockoon where the balloon is lost with each launch. What's the catch? Is such a cable not feasible? A 35km Zylon cable would weigh 50t to only support its own weight. Increase that cable weight budget by x20 to 1kt and it should still be little compared to the above example of 50kt of lift capacity. Zylon is commercially available at about $200/kg. For this project that would cost $200M. For a reusable rocket launch platform, that's still peanuts, right?

There have been many tether-based designs to bring stuff to LEO. It looks feasible on paper but it's an open question whether it would work in practice. It's certainly a lot of fun to imagine a world with these gigantic tethers. There are many unsolved challenges: a) to build it b) to make it work, c) to make it cost-effective And you can't only consider the costs of mass-to-orbit by starting from 30km altitude. You first need to bring your payload from the ground to that platform. All in all, it looks cool but the starship design is really at the frontier of what is currently possible. Reaching 4% for SpaceX is not just having 1 killer feature that changes everything, it's rather the accumulation of a very significant number of small improvements regarding materials, strucutre, fuel, engines.

Ok, I spent way too much time these past days on this but I learned a lot. I learned that my original assumption that atmospheric drag was significant to Starship was wrong. Some Wikipedia article quoted the drag loss with "1500 to 2000 m/s" while a Saturn V which is most comparable to Starship reduces this to a mere 40m/s on ascent. I learned that there is such a thing as gravity loss. The idea here is that due to the atmosphere, the rocket cannot accelerate horizontally as would be ideal to quickly accumulate orbital speed. It has to start vertically and fly at a steep angle for a while, which contributes little to nothing to the orbital speed. All components that only fight gravity without adding speed are "gravity loss" and that is also quoted with 1534m/s for the Saturn V. With a high launch this could be lowered but by how much? A space nerd guessed 500 to 1000 m/s given the atmosphere up there is less than 1% of ground level. 1000m/s would translate to an extra 18 tons of payload to LEO per launch. According to https://aerospace.csis.org/data/space-launch-to-low-earth-orbit-how-much-does-it-cost/ that would be worth $27M per launch.

Getting to 30km height ideally costs just the potential energy and electric lifts are pretty close to achieving that, so it would be desirable to get out of thick atmosphere that way. Is it possible with today's materials to build a blimp that can just float for years without needing a full replacement? I think it could be compartmentalized. If the blimp has individual chambers such that a sever damage in one point can be compensated by pumping gas to a different chamber while fixing that spot, while smaller damage gets repaired by autonomous robots, while using at least one redundant tether so you can maintenance or replace these, too, ... you could have the whole structure maintainable. Then it comes down to leakage cost and longevity of the materials. If you have 50 launches per year, you get a life time budget for construction and maintenance of $1.35B/year.

Rockets expend about 20% of their fuel just to get out of the thick atmosphere. TIL there was/is a company that tried to glide all the way to space ... Two weeks ago, I had explored how to get to the edge of space with lighter-than-air vehicles: nostr:nevent1qvzqqqqqqypzq3huhccxt6h34eupz3jeynjgjgek8lel2f4adaea0svyk94a3njdqy88wumn8ghj7mn0wvhxcmmv9uq3uamnwvaz7tmwdaehgu3dwp6kytnhv4kxcmmjv3jhytnwv46z7qpqzdv2qavyr8ayje5k3gjge77l96nupl73pe2swjfuw02k6vwfaujq5x4nua I learned so much about rocket science exploring the idea of a 30km high "zeppelin" or blimp ... but got attacked viciously on reddit's r/askEngineers and r/rocketry: https://www.reddit.com/r/AskEngineers/comments/1g700qg/why_are_heavy_lifter_rockets_not_launched_from/ (OP got deleted) https://www.reddit.com/r/rocketry/comments/1g7qwug/how_much_would_access_to_space_improve_if_we_had/ But I learned that the biggest of rockets have virtually no drag loss but they still have a loss from atmospheric launches called gravity "loss" which comes from the fact that in the initial phase, the rocket has to go up and not sideways while sideways is what you need to get orbital speed. The flatter it accelerates, the more it accelerates in the right direction. That loss is easily 20% of the total budget to low earth orbit. Given that payload makes up only 4% of the total "wet" mass of a rocket, that's a big deal and I still believe, building a 30km high platform held in place by tethers to launch heavy lifter rockets is an idea worth exploring. Anyway, today I found out there was a company that went even one step further. At 30km, the rocket would still have to start at an angle and could not use ion thrusters from the start Ion thrusters would allow higher top speeds with the same amount of fuel. That company - http://www.jpaerospace.com suggested a platform 42km high with the space ship being a giant, lighter than air mono wing that would gently accelerate and climb from there all the way to orbit. And they did not stop at reddit posts. They did build some small scale models but it looks more like rudimentary hobby scientist builds than the future of space travel.