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Comment by lmilcin | Hacker News Reader

Depends on what kind of test that was supposed to be.

If they pumped it until it explodes it would be a kind of test (also called experiment) where it makes sense.

If you design something with a margin of safety, then pump it to working pressure and it blows, then it means you made a mistake. If you made a mistake it shows you don't know what is necessary to design a rocket (yet).

Seriously, they don't build skyscrapers or bridges and leave for 5 days to see if they topple to prove the design was correct.

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> If you made a mistake it shows you don't know what is necessary to design a rocket (yet).

Yeah, sure SpaceX doesn't know what's necessary to design a rocket.

Rockets are designed to have a vastly lower margin of safety. If they never fail in testing that’s a sign you’re over engineering it and sacrificing cargo capacity, which is a bad thing. That said, these failures should be uncommon.

Starship needs to be reusable dozens of times, with no refit between launches (only refueling). Fuel is cheap; the vehicle needs to survive. It also needs to be reliable enough to carry people. They're not using razor-thin safety margins on it.

Fuel is not cheap. Fuel is heavy and payload reduction is money lost.

Fuel is very cheap. SpaceX has built their whole business around the idea that it's worth trading payload for re-usability.

Fuel is cheap on a monetary basis.

_Lifting_ that fuel is NOT CHEAP. See also, the rocket equation.

But the primary cost of lifting fuel (if you have a reusable rocket) is fuel. Leaving us back at cheap.

But we're essentially arguing whether max payload per launch or payload efficiency to orbit is more important.

With exponential compounding, I wouldn’t be so sure that it’s still cheap…

Yeah, and with much higher required dv and more required stages this would be a huge problem. But we only need two stages to orbit when launching from Earth, so it's really not that big of a deal, and it turns out that optimizing for reusability ends up being a lot cheaper than optimizing for minimum fuel use. For example, it only costs $200k for a fuel load of Falcon 9 fuel. If you got rid of reusability you'd need less fuel and could save tens of thousands of dollars ... but at the cost of needing to build a new $54M spacecraft with each launch. Clearly that trade-off isn't worth it.

So yes, fuel is cheap.

Fuel becomes more expensive on reusable spacecraft. Granted, the first pound of fuel is cheap, but the costs keep rising.

With one and done you have a larger mass fraction to work with. Reusable spacecraft operate on much tighter constraints. Every single wasted pound in Space X’s design likely costs them 100’s of thousands of dollars in lost profit over time.

Not sure what’s to disagree with about that: “The reusable Falcon 9’s performance to GTO is listed at 5,500 kilograms. The same rocket in fully expendable version can lift 50 percent more payload — 8,300 kilograms. For the Falcon Heavy, the performance to GEO is about 2.8x that of the reusable version” https://spacenews.com/spacexs-new-price-chart-illustrates-pe...

Reusable means cost savings, but it has a dramatic reduction in cargo capacity. This means more trips to get the same mass into orbit, which means mire fuel directly used and every extra lb in the design is then carried up multiple times while further requiring more trips.

Which trips each cost much less because you don't have to build another rocket to do each with.

Or, equivalently, in one trip with a bigger rocket you have left over from another launch, because you didn't ditch it.

That’s great if you’re transporting large numbers of the same thing like star link satellites. However, many payloads are worth sacrificing the first stage. Just look through the “No attempt“ missions in 2018 for some of the reasons why: https://en.m.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_...

Which again demonstrates reusability is really expensive, though clearly not always as expensive as a new rocket.

Many of the "no attempt" 2018 missions were because they were reflights of older models of cores (pre-Block 5) that weren't worth keeping around. Expendable launches for Block 5 boosters are far less common.

Right, if you can throw away an obsolete rocket stage as part of profitably launching a much bigger load than you would have done with a re-usable one, it's hard to justify not doing it, and just selling it for scrap instead.

They are also removing new rockets. SpaceX is simply going to prioritize older ones for destruction assuming they can handle the payload.

Remember while the first stage is significantly more valuable, it's not just a question of fuel the upper stage is destroyed with both approaches. So, combined with significantly lower payloads the cost per kg does not drop as much as you might think.

SpaceX has built part of their business around the idea that it's worth trading a moderate amount of payload for re-usability as long as they can still reach the orbit their customers care about and it was considered an extremely risky bet before they made it routine.

It doesn't make fuel cheap, let alone very cheap.

Actually another significant part of SpaceX business is modern engineering and modern mean of production. They used to launch without recoverability for some high orbit payloads and they were competitively priced for that too.

Musk has said the cost of the methane for one Starship/Super Heavy launch will be about $1 million.

Fuel on the ground is cheap. Halfway to orbit, nothing is cheap.

It's still a lot cheaper than the rocket.

The reason the rocket is so expensive is that it has to lift a lot of weight, mostly fuel. I charge that to the fuel's account.

Fuel is cheap. Spaceships are not.

Speaking relatively here.. spaceships are still more expensive.

That's basically been the conventional thinking about rocketry historically - scientists and engineers looked at the tyranny of the rocket equation and choose to maximize payload per launch because they were largely funded by government or quasi government "cost plus" types of contracts. The design philosophy of working close to margins comes from this thinking.

SpaceX's greatest contribution has been to look at rocketry from the point of view of economics of running a rocket launching business as opposed to a "per launch cost plus" model realizing that fuel cost for launching a rocket is close to trivial compared to the facrication cost of making a rocket.

So their entire design philosophy is around reusability and reusability seems to push costs down low enough that you can get more payload up by simply doing more launches. While SpaceX is famous for not filing patents and instead protecting their IP using the trade secret approach and fabricating everything in-house, you can bet they'll be operating with more engineering headroom to get reliability and reusability.

Evidence for their bet having been right is the fact that they have grabbed virtually 100% of commercial and quasi-governmental launches and some part of US government launches. Other players in this space now almost entirely depend on defence contracts or national prestige contracts to survive.

Doesn't matter. If you take the conservative assumptions that go into manufacturing, say, a car and apply those to a rocket, you'll get something that won't even lift off the ground, much less make orbit.

Any rocket HAS to be very close to the limits of what materials used allow, and not the +50% margin of error you routinely seen in other fields. What you say about SpaceX is true, but only relative to the rocket industry as a whole. E.g. SpaceX uses cheap, available materials and simple designs over exotic composites and complex mechanisms. But they still operate close to the margins, as they have to in order to have any payload capacity at all. In fact, the move to steel probably reduced their mass margins even further.

EDIT: Actually maybe I shouldn't have used a car as an example: https://www.youtube.com/watch?v=pJdrlWR-yFM

The video is hilarious! Thanks for sharing!

My favorite part is the look on the engineer's faces when they're asked to turn a Robin Reliant into a Space Shuttle.

"Have you got a spare billion dollars?"

"No. That's why we came to you."

I think SpaceX has proven they know how to design rockets. But they are newcomers to building rockets from welded steel.

It is my understanding that the pressure vessels are carbon fiber composites. The skin is steel.

The skin is the pressure vessel, double walls are a waste of valuable mass.

They gave up on carbon fiber awhile ago.

*I think they still use small composite overwrapped pressure vessels for fluids they need less of than CH4 and O2, just referring to giving up on carbon for the main tanks.

Do you have a source?

My understanding is that the skin is the pressure vessel. The reason for going with stainless steel at all is because it maintains integrity at high temperature. Carbon fiber does not. So a stainless steel combined heatsink and structure can maintain integrity at 1000C or whatever, while carbon fiber+ceramic heatshield will need to hold temperatures below 400C or whatever.

And even though the fiber structure is much, much lighter, the amount of heat protection required is massive. And the stainless steel "cheats" by performing double duty. And you get a second free lunch because the heat protection requirements are slimmer.

It wouldn't be the first time I've been wrong today though. Nor the second.

Apparently I have not been following this as closely as I thought I was. This 12 meter diameter tank is from 2016. Not part of current design.

https://everydayastronaut.com/stainless-steel-starship/

Edit: "So in summary. SpaceX chose stainless steel over carbon composites because it’s about as light, it can handle higher temperatures which means less heat shield is need, which then makes it lighter, it reflects heat which means even less heat shield which again makes it even lighter, it’ll be cheaper and quicker to build AND it’ll look FREAKING AWESOME."

> But they are newcomers to building rockets from welded steel.

AFAIK everybody is.

Atlas was steel. The first US ICBM and an early space launch vehicle. To this day, the Centaur hydrogen upper stage with RL10 engine is steel. Balloon tanks. They inspired SpaceX heavily.

This and the Mk2 prototype in Florida are mostly experiments with manufacturing processes; I would not be surprised if there were a reliability problem with their methods that weakened the product.

Skyscrapers have been built again and again, and they have much different constraints. Starship Mk1 is the first of its kind.

I expect this was more likely a manufacturing mistake. It was assembled night and day under a very tight deadline.

> Seriously, they don't build skyscrapers or bridges and leave for 5 days to see if they topple to prove the design was correct.

Perhaps a better analogy is carving a violin top. Skyscrapers and bridges can be overbuilt without destroying their utility, but a violin has to be as thin as possible without breaking if you want it to function well as a violin.

Boeing intentionally snaps of the wings of prototype aircraft to find the breaking point/prove the margin of stress.

No, they don't. They load it to 150% of max design load. Sometimes they keep bending it until it snaps (777) and sometimes they don't (787 - no one wants to clean carbon fibre mess). Sometimes it snaps before it's reached and it's ok (airbus a380).

At least the pump is working and generating pressure.

Seems like the difference between unit tests and integration tests.

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