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Comment by lawlessone | Hacker News Reader

They seem very fragile?

The one at 0:49 exploded just from tipping over.

Perhaps because they're huge i guess.

They are literally thin walled fuel tanks with some rocket engines strapped to the bottom. A soda can dozens of meters tall would be an appropriate analogy.

They're probably only built to withstand forces associated with a nominal launch to save on weight; being able to withstand tipping over is unnecessary if you can avoid tipping over in the first place.

Every pound of weight you add to the rocket is a pound of payload that you can't put into space.

It's not technically 1:1 on the first stage. It varies a bit from launcher to launcher, but it's in the ballpark of 4:1 (you lose a pound of useful payload for every four pounds you add to the first stage).

Your point is spot on though. Don't waste mass to account for failure modes that you can just avoid in the first place by landing upright.

> They seem very fragile?

They're extremely fragile. In fact, they're stored pressurized because laying on their side would damage them without the pressure keeping the walls rigid.

My understanding is that they don't need pressurization while being stored, but do need it to withstand flight loads. This is in contrast to, for example, the older Atlas rockets, which had to be pressurized constantly and would collapse otherwise.

Of course I can't find any authoritative source on this, although there are a lot of random internet folks like myself repeating the claim. And I guess I'm throwing one more on the pile here.

I unfortunately can't find an authoritative source, either, but I recall a /r/SpaceX discussion where it was stated that hurricane prep includes maintaining power because they're (lightly) pressurized at all times.

I wonder if they might maintain pressurization to increase strength in the event of some mishap, but it's not an instantaneous disaster if pressurization is lost.

I recall reading that they have equipment to maintain pressurization while the boosters are trucked to the launch site as well, so it's not just flight loads, in any case.

Yeah, they aren't balloon tanks (which absolutely require pressurization), but they do keep them pressurized as it improves their structural stability.

They're pressurized with nitrogen generally, just to keep them clean and dry inside. They're quite capable of standing or lying down while unpressurized though.

You're correct - the Atlas (ICBM as well as the current Centaur second stage) design that requires constant pressurization is known as a balloon tank.[0] SpaceX doesn't use balloon tanks, but it does pressurize tanks with nitrogen during transport to keep the tank walls rigid and prevent damage.

[0] https://en.wikipedia.org/wiki/Balloon_tank

Why nitrogen? Is air too corrosive?

Huge and thin. Proportionally, the walls are thinner than a soda can's.

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