Hacker News Reader

Comment by tablespoon | Hacker News Reader

> The thing that's impressive to me, in addition to the sheer size of these machines, is the engines. The Raptor 2 is groundbreaking. And there's 30+ of them between both stages. It's just incredible, and there's nothing like it that's been done.

I may be wrong, but isn't the reason there are so many engines on their rockets is because SpaceX isn't good at building big rocket engines?

View on HN

>I may be wrong, but isn't the reason there are so many engines on their rockets is because SpaceX isn't good at building big rocket engines?

No, that's completely wrong. There are challenges in running a bunch of engines, but it's critical to pull off for reusability. All rocket engines have a strict floor on how low it's possible to throttle them. Even 40-50% is quite hard, and I believe the Raptor is 40% though with the goal of getting somewhat lower. And for old space expendable rockets that was plenty.

But to land a rocket, you necessarily need to be dealing with bringing it down when it's nearly empty. Rockets are mostly fuel by mass, and when that's expended are very light. The thrust necessary to lift a fully fueled rocket up is massively ginormously more then would be needed for its empty weight. So if you light up at full power even for a moment it's going to fly right back up again. You want a low enough minimum thrust/weight ratio (TWR), ideally below 1 (so you can gradually descend/hover), though a rocket can get by with a bit above with a "hover slam approach" approach like the F9 uses. But it can't be that far above. But you can't just throttle a rocket engine down to 1% either. What to do?

The answer is multiple engines. Throttle is limited, but you can always turn an engine off. So if you have 10 engines which each can throttle to 50%, your effective ability to throttle from max thrust is .5/10 or 5%. With 30 engines at 40% would be .4/30 = 1.3%. Very helpful. Also means more potential redundancy for landing.

There are challenges with plumbing, harmonics, control etc which are real, so as always with rockets complex tradeoffs, and big engines have their own issues. Small engines are also easier gimble, another valuable thing for landing, and to mass manufacturer which is a whole extra aspect for SpaceX, Starship is about economics remember. But there are many solid technical reasons for the choice.

Not only this, you have to have the ability to throttle while also being able to land if an engine fails, because Starship is going to carry people.

Many smaller engines is cheaper than one big one as you can streamline manufacturing. Also large engines develop combustion instability much easier. They're also harder to cool because of the square-cube law (more heat, less surface area).

It also gives you engine out capability and also the ability to effectively throttle the rocket in a much more fine grained way by shutting off engines.

I don't think they have even attempted to build a large engine. But SpaceX is setup like an assembly line rather than some bespoke engine maker. A larger engine would require larger tooling, 3d printers, etc. Also with 30+ engines its less of an investment if one goes bad, just swap it out.

They always have the option of designing another larger engine later.

> Also with 30+ engines its less of an investment if one goes bad, just swap it out.

also less bad to lose ~3.3% of your total thrust than 100% of it.

Depends on failure modes: many small engines rolling dice about losing thrust is nice, many small engines rolling dice about blowing up is not.

With more smaller engines, you have a lot more options how to control your flight. You can throttle them down individually, gimbal them etc.

The only downside is that if any of the engines explodes catastrophically, it will take the entire rocket down. This used to be a big problem in the Soviet N-1 lunar rocket, but contemporary technology can mitigate it quite a bit: with enough telemetry and computer power, you can determine that the engine is acting funny before it explodes, and you can switch it off. Losing one engine out of 30 isn't fatal.

Also, with computer modeling, a lot of the engine design and testing process takes place on screen, so many failure modes are debugged early.

Yes, but it's not just SpaceX; nobody is good at building big rocket engines.

I don't know much about it, but I'm interested to know why the space shuttle solid rocket boosters were so much more powerful than the F1 engine. 2 of them produced about 87% of the thrust of the 5 F1 engines on the Saturn V.

I thought combustion chamber instability was the biggest issue with the big engines. Are solid rockets much more stable?

Solid rockets don't really have that problem in the same way. The instability happens in the thrust charmer of a liquid engine resulting from non-uniform burning. The chamber is not very big if you compare it to the size of the tank. Once you press that non-uniform hot gas threw the neck out into the nozzle the expansion happening in the nozzle also expands those imbalances. So you can end up with different pressure pressure in different part of the nozzle leading to vibration and potentially the nozzle ripping itself into pieces.

Solids rocket are like a candle with a hole threw it. They burn inside out, not from the button to the top. So there is not really a thrust chamber in the same way. A channel goes from buttom to the top inside the booster. So the 'thrust chamber' is essentially the hole length of the booster. And all that material gets pushed down in the direction of the nozzle.

I have never heard any rocket engineer explicitly explain this, but my assumption is that on the way down when everything is pushed threw the neck the gas has time to mix enough so that the instabilities are not that large anymore. I also think the pressure is lower in a solid.

Solids have higher thrust but lower efficiency. The thrust comes from the fact that in the solid fuel mix there is lots of other stuff that need to form the solid. Heavier elements that don't participate in burning (thus less efficient) but add thrust.

Solids also have the draw back of being incredibly loud and having massive vibrations.

Solids also can have 'stability' problems as in that if they are not correctly manufactured, they might not burn uniformly and that can lead to a whole lot of problems and additional vibrations too.

Almost no new rocket company uses solids. Its far to expensive and operationally dangerous. At best you can reuse some outer shell, but not the expensive part.

No. There are other good reasons to use lots of small engines. Redundancy. Ability to throttle down (very important for Starship). You can use the same engines on both stages. Easier to gimbal.

My understanding was that if one goes out then they still have 32 left

There is an upper limit to scaling. Bigger engines do not mean more power / weight. Bigger is also harder to build and maintain. SpaceX is finding the sweet spot.

Reply on news.ycombinator.com