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Comment by bryanrasmussen | Hacker News Reader

>By launching your rocket from the ballon city and returning to orbit. (I'm struggling to see the novel problem.)

I believe they are having trouble envisioning how you would launch rockets from a balloon city without disturbing the equilibrium of the balloon city because it is assumed that rockets thrusting down with great force would damage the balloon city in a way it would not easily recuperate from.

I also find the idea difficult to understand, but assume that is because it is in an area I know nothing about and the problems that I think sound bad are actually totally solvable engineering problems otherwise it would not be have been suggested as a solution by engineers expert in that area.

on edit: changed rocks to rockets

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> how you would launch rockets from a balloon city without disturbing the equilibrium of the balloon city because it is assumed that rocks thrusting down with great force would damage the balloon city in a way it would not easily recuperate from

It's a floating platform. Same as the ones SpaceX lands its rockets on. Same as a gunboat firing projectiles.

Will a launching rocket impart force to the platform? Yes. But unless the platform is super weirdly balanced, or the rocket absurdly oversized for the platform, it will stabilise after rocking a bit. (You'd have to design the platform to be stable in winds, anyway.)

And if you do have an absurdly oversided rocket, you don't launch it from your platform. You float it off to the side on a dedicated launch "boat" and have it ditch its floaty as an ultra-early first stage.

>It's a floating platform. Same as the ones SpaceX lands its rockets on. Same as a gunboat firing projectiles.

It's a great comparison that helps me understand it a bit. In many respects, Venus's atmosphere is as heavy as an ocean. That said, I can still see how if you're talking about the upper atmosphere with pressure similar to what you would have on Earth, the force necessary to do a straight vertical takeoff imparted against a platform seems like it could cause problems.

But it might just be the Archimedes thing of "give me a prop large enough and deliver long enough and I can move the world" applied to atmospheric dynamics, e.g. enough buoyancy and you're good to go. I just don't know how much is "enough" when you're talking about Venus and if that runs into prohibitive engineering complexity that makes it different from our familiar Earth examples.

Not saying it can't be done but I think that one question at least, was reasonable.

Oops, that should be "and a lever long enough"

>And if you do have an absurdly oversided rocket, you don't launch it from your platform. You float it off to the side on a dedicated launch "boat" and have it ditch its floaty as an ultra-early first stage.

The ww2 german "v2 in a tube towed by a U-boat so it can get closer to its target" project being a decent conceptual example of this.

No need for a launch boat. Just light the fuse and eject it from your balloon. Think subsurface missle launch by an Ohio-class sub.

Launching the rocket from a balloon is not a problem. You can have a platform with a hole underneath the rocket's exhaust and the balloon itself can be a torus through which the rocket flies upwards. There would be minimal impact of a rocket launch on the platform if designed properly.

Heck, you could just drop the rocket and have it ignite (one hopes) with lateral thrust, clearing the launch aerostat.

Or consider the aerostat disposable (at a significant replacement cost, see: <https://news.ycombinator.com/item?id=45333674>).

But either way it's a bit less the firey-burney-explodey problem than the sudden loss of a few thousands tonnes of mass that would displace the equilibrium of the launch platform, should you care to re-use that, or the means by which such platforms (capable of supporting said thousands of tonnes of mass).

Just to put some hard numbers on it, a crewed Falcon9 (Crew Dragon) has a launch mass north of a half-million kilograms, or 500 tonnes.

The Russian Soyuz-FG, also human capable, has a launch mass of slightly over 300 tonnes.

If this spacecraft is only a shuttle to low-Venus orbit with another transfer craft for the flight back to Earth (or other points of interest) that should suffice. If the launch craft is intended for the full return trip of 100--250 days, things could get a bit cozy and interesting depending on the number, disposition, and fragrance of inhabitants.

Uncompleted thought: "or the means by which such platforms (capable of supporting said thousands of tonnes of mass)..." are replaced. Given lack of readily available solid substrates in situ a nontrivial challenge.

There is precedent: SpaceShipOne was successfully launched from an airplane [1].

The great force downward is (mostly) irrelevant if there is nothing below. Just hang the rocket between two towers over a void, with the atmosphere below.

[1]: https://en.wikipedia.org/wiki/SpaceShipOne#Launch_aircraft

>I also find the idea difficult to understand, but assume that is because it is in an area I know nothing about and the problems that I think sound bad are actually totally solvable engineering problems otherwise it would not be have been suggested as a solution by engineers expert in that area.

A refreshingly sober approach that I think is a lot more healthy than hip firing incredulous questions.

I agree though, I don't intuitively understand how it would work. I would think you would do horizontal takeoffs. Also Seveneves by Neil Stevenson gives some interesting examples of ways to escape gravity wells without rockets, but I won't spoil anything there.

I have a silly take on this: to avoid rocking the floating city you throw the rocket overboard, let it re-orient itself upright, then use the main thruster to slow down the fall and then move up.

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