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Musk explains why SpaceX prefers clusters of small engines

arstechnica.com

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Re: Musk explains why SpaceX prefers clusters of small engines

#71

Post by Florin_Andrei »

cuspycode wrote: post #66
Florin_Andrei wrote: post #58

Earlier quoted context omitted.

The Soviets won the first round of the space race (until the mid-60s) because of multiple factors, but mainly because of the laser-focus at the highest levels to push the technology as far as it could go. It helped a lot that they had an engineering genius heading the program (Sergey Korolev), and the top politician during that time (Nikita Khrushchev) was a forward-thinking progressive (relatively speaking - please…

Korolev's contribution is even more impressive when you consider that he was imprisoned in the Soviet Gulag for many years (for political reasons), suffering under living conditions which probably shortened his life. https://en.wikipedia.org/wiki/Sergei_Korolev

Seriously, I can't comprehend the guy.

He starts studying liquid fuel rockets in the '30s, and does some amazing work probably trailing only the top German engineers in this field.

He's denounced by some envious low-lifer who wanted his job and is arrested (along with Valentin Glushko, another great rocket scientist) during the stalinist Great Purge at the end of the '30s, when a simple anonymous note was enough to get someone disappeared. They torture him, sentence him to death - but then he's commuted to hard labor in the gold mine, where the poisonous environment and poor conditions meant the average life expectancy was barely over one year. Loses all his teeth to scurvy.

Meanwhile his friends back in Moscow are lobbying with Lavrenti Beria (the KGB boss) to release him - they succeed and he's placed in the "easy prison" where a bunch of intellectuals were doing essentially white collar slave labor (with pencil on paper, sure, but no choice in the nature of the work) for the Soviet government. He's released towards the end of WW2.

Then Stalin figures he needs to catch up to the Germans in rocketry, so Korolev is rehabilitated, made colonel of the Red Army, and finally starts working again on his rocket engines. They copy a bunch of German designs first, use some German engineers (who were prisoners) to get them started. Then continue on their own.

He develops the first Soviet ICBMs, but that was just what paid the bills. He keeps pushing for a real space program. Launches Sputnik 1 into space. Leads the Soviet space program until the mid-60s.

When he died, he was working on plans for manned missions to Mars and beyond.

I mean, what motivates a person to keep forging ahead against such adversity? Death sentence, hard labor in the poison mine, years of imprisonment and disgrace - and then he builds and launches the world's first ever satellite. To say nothing of the fact that, like Elon Musk, he was a man of many talents: great engineer, very effective leader, and a good politician and lobbyist. It's amazing.

Re: Musk explains why SpaceX prefers clusters of small engines

#72

Post by robbiep »

allthenews wrote: post #69

There's something I don't understand about space travel. We made it to the moon in 1969, on the first computer to use ICs[1]. Since then we've seen monumental advances in computer science, material science, manufacturing, rocketry, and just about any other component of space flight. Why 50 years later is it still such a relatively difficult task to launch a rocket into space? Why is it still so expensive and failure…

Literally what you are witnessing now is the scaling of that. Rockets are expensive - there hasn’t been the opportunity for rapid iteration such as during the rise of automobiles or planes, so there have been less opportunities for the sorts of experiments such as what SpaceX, blue origin, armadillo before it, and the electron are doing.

Re: Musk explains why SpaceX prefers clusters of small engines

#74

Post by adventured »

allthenews wrote: post #69

There's something I don't understand about space travel. We made it to the moon in 1969, on the first computer to use ICs[1]. Since then we've seen monumental advances in computer science, material science, manufacturing, rocketry, and just about any other component of space flight. Why 50 years later is it still such a relatively difficult task to launch a rocket into space? Why is it still so expensive and failure…

Apollo cost ~$120-$150 billion in today's dollar.

Falcon Heavy cost $500m-$1b to get right.

Given some time, Musk could probably make it so a trip to the Moon costs $300 million, maybe less.

We've (they've) accomplished an extraordinary cost improvement.

It's still such a monumental task, because the laws of physics have not changed. Building a huge rocket, putting people in it, and firing it at the Moon, is not the hard part per se (not killing them in the process, and bringing them back safely, is). We could have done that all over again at any time if we desired to spend the money. The next level of difficulty, is turning it into a truly routine task, and building something on the Moon. That's a dramatic leap up from only exploring the surface of the Moon. We've avoided doing that, not because we can't, but because it's a cost benefit equation, and the benefit has not been considered worth doing at the cost, even as the cost gradually declined. Now that cost benefit equation has improved dramatically enough to favor it being worth doing.

Simply put, as a society we're not willing to spend $500 billion or $1 trillion to build a Moon base. But we might be willing to spend $50 or $100 billion over time to do it. We're not willing to spend $10 or $20 billion for a trip to the Moon, but we are probably willing to spend $300 million or even a billion.

Re: Musk explains why SpaceX prefers clusters of small engines

#75

Post by Obi_Juan_Kenobi »

aurizon wrote: post #38

A large portion of the efficiency of small engines comes from the reduction of "hoop stress". This is the linear tension in the wall of a pressure vessel(rocket engine) which varies as the square of the diameter. Twice the diameter = 4 times the pressure (hoop stress) the walls must take. A rocket is a special case of a balloon - with an expansion nozzle attached to couple the impedance of the combustion chamber to t…

Isn't this largely offset by the reduced efficiency of smaller engines?

Larger combustion chambers are highly desirable because you can increase the maximum heat in the center of the combustion chamber. By varying the fuel mix, the outer portions can burn cooler to avoid damaging the chamber, while the center burns extremely hot for better efficiency. Basically, the better the ratio of cross-sectional area to circumference, the better the engine.

There's also the issue of having to replicate the turbopumps for each engine, all of the plumbing and gimbaling mechanics (though only the center engine on F9 has full gimbal control), etc.

Of course, there are downsides to large chambers such as combustion instabilities, some scaling like you've mentioned, production and economics, overall vehicle design, etc.

Basically, there are many design tradeoffs in rocket engine design. After all, it's rocket science!

Re: Musk explains why SpaceX prefers clusters of small engines

#76

Post by Florin_Andrei »

shmerl wrote: post #68
Florin_Andrei wrote: post #61

Earlier quoted context omitted.

The main reason why they failed is - they pushed testing into production, just like MySpace, because their funding and leadership disintegrated at the beginning of the program. See my other reply in this thread.

Sure, their management was erratic. But the bottom line, their combination wasn't technically reliable (why, is already what you are saying).

This meme of the "inferior Soviet technology" needs to die in a fire. It's soothing for the fragile Western conservative ego, but it's just not true - not, at least, with regards to space.

The failure of the N1 program was a leadership issue, period. Just like America trailing the Russians at first was a leadership and lack of focus issue. One team kept winning, then they lost their captains and their funding. Meanwhile, the other team got great leaders and awesome sponsors and started busting their collective ass really hard around the same time - and the wheel of fortune suddenly turned. Funny how that works, isn't it?

What's strange is that the true events sound a heck of a lot more "american" (work hard and reap the laurels, or slack off and be a loser) and yet you keep hearing the same bullshit all the time as to how "the Russian stuff was just inferior". It's not true. And it's not a good story to tell your kids.

Re: Musk explains why SpaceX prefers clusters of small engines

#78

Post by rocqua »

allthenews wrote: post #69

There's something I don't understand about space travel. We made it to the moon in 1969, on the first computer to use ICs[1]. Since then we've seen monumental advances in computer science, material science, manufacturing, rocketry, and just about any other component of space flight. Why 50 years later is it still such a relatively difficult task to launch a rocket into space? Why is it still so expensive and failure…

It has to do with the rocket equation [1].

Getting into orbit requires a lot of 'delta-V'. Part of this is gravity (needing to move up the gravity well) but more important are speed (you need to go really fast to stay in orbit) and aerodynamic drag when moving through the atmosphere.

There have been no real improvements to the exhaust velocity of rocket engines (at least none that work to get into orbit). As such, to get more delta-V we need to do that with the dry-full mass ratio.

This means we really care about the dry mass of our rockets. This in turn pushes engineering to the limits. The sweet-spot between weight-savings and reliability is skewed more towards the weight-savings side.

Beyond the rocket equation, there is the issue of little iteration being possible. Launching rockets is expensive, so it is not done often. Moreover, failing to launch a rocket is even more expensive. This makes it take a long time to figure out what works by real life testing. Combine the weight skimming with the massive forces and complexity of a rocket, and you get some problems that can essentially be found only by real life tests.

As real life tests are few and far between, it just takes longer to learn the lessons needed. This is something space-X has gotten quite good at, by always iterating on their design and doing tests. This is helped by the relative cheapness of launches for them. Cheap launches means you can try more per launch.

[1] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Re: Musk explains why SpaceX prefers clusters of small engines

#79

Post by rocqua »

simonh wrote: post #11

One thing not mentioned is throttle range. Larger engines can’t throttle down as low as smaller ones. That’s irrelevant for launches, but matters crucially for landing. I know the F9 engines can’t throttle down to a hover, even on a single engine with the fuel tanks mostly empty, but I’m sure they did all their landings up until the last few on a single engine that throttled down to its lowest setting for a reason. C…

The recent falcon heavy launch (and some falcon 9 launches before it) actually had the boosters land using 3 engines. So going by ratio, it should still be able to land if it had 3 larger engines.

This 3 engine landing failed for the centre core, which is why it was lost. Specifically, there wasn't enough 'lighter fluid' to relight all 3 engines required, only one engine was lit. Thus the booster tried a 3-engine landing on a single engine, and hit the water at 300mph IIRC.

Re: Musk explains why SpaceX prefers clusters of small engines

#80

Post by shmerl »

Florin_Andrei wrote: post #76
shmerl wrote: post #68

Earlier quoted context omitted.

Sure, their management was erratic. But the bottom line, their combination wasn't technically reliable (why, is already what you are saying).

This meme of the "inferior Soviet technology" needs to die in a fire. It's soothing for the fragile Western conservative ego, but it's just not true - not, at least, with regards to space. The failure of the N1 program was a leadership issue, period. Just like America trailing the Russians at first was a leadership and lack of focus issue. One team kept winning, then they lost their captains and their funding. Meanwh…

> This meme of the "inferior Soviet technology" needs to die in a fire.

Technology turns out to be inferior, when quality control is superseded by political agenda. It should be quite obvious.

See also: https://en.wikipedia.org/wiki/Nedelin_catastrophe

> The failure of the N1 program was a leadership issue, period.

Surely leadership failure is a key factor, but there was also an economical one. Space race was very expensive.

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