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What's the scaling like for rocket engines? How much more efficiency (in various forms) could you get if you made 1 giant engine instead? (Disregarding reliabil

What's the scaling like for rocket engines? How much more efficiency (in various forms) could you get if you made 1 giant engine instead? (Disregarding reliability)

tksmith151 5y ago
I'm not a rocket scientist, but my understanding is that efficiency is derived from the exit velocity of matter from the engine. So the faster it can fire matter in the opposite direction the more efficient it is. There are some other factors as well like bell shape, which are generally designed to provide better efficiencies at different atmospheric pressures (ranging from sea level to vacuum of space). The main consideration after these factors is the mass of the engines. If multiple engines and one giant engine had all of these same factors equal, then I don't think there would be much difference in the efficiency of the rocket. Likely though, there are differences and other advantages to multiple small engines. The easiest one for me to think of is modularity. If you want a specific thrust for a payload it's much easier to add more small engines together than to try and throttle down the large engine to the correct amount, if that is possible at all.
KiwiJohnno 5y ago
Musk has actually talked about that before, during the early stages of Raptor development. He said they were downsizing Raptor from the originally planned size, because their analysis showed a smaller size was the sweet spot for maximum thrust/weight ratio.
mlindner 5y ago
There are a lot of issues you get from scaling an engine up, one of the major issues being combustion instability. The larger the "blob" of fuel and oxidizer mixed together that you're trying to burn, the more you will get things like eddy currents and shockwaves flowing through the material. These disrupt the combustion and make it uneven, possibly leading to feedback cycles where these elements self-reinforce eventually causing vibration or otherwise damage to the engine. The smaller the physical size of your combustion chamber the easier these things are. The largest single engine ever built, the F1 engine that pushed the Saturn V, had massive issues with this that were further made worse because the engines were largely designed without the aid of computers.
dkbrk 5y ago
The short answer to your question is that larger engines have very few advantages and many disadvantages. Specific impulse is the most obvious parameter of a rocket engine, but it's relatively less important for a first stage where thrust/weight is also a very important concern. Specific impulse depends on the chemistry, combustion cycle, combustion efficiency and nozzle design. There isn't any obvious reason why it would scale either way with engine size. Thrust/weight is very important for engines on a first stage because of gravity losses. Imagine if the rocket sitting on the pad had a thrust/weight of just 1.1; that would mean that at liftoff 91% of the thrust would be wasted just countering gravity. You both want a high thrust/weight at liftoff and a small dry mass to give the second stage the most momentum possible at stage separation. High thrust also helps for reuse in making the burn time of the first stage shorter (for fixed specific impulse and propellant load), which means that at stage separation the first stage isn't too far downrange and is easier to return to the launch site. Note that there's both the thrust/weight of the rocket as a whole, and of the engines themselves. The engines comprise a significant proportion of the first stage's weight, though, so looking at engine weight (and thrust to weight) is also useful. Consider the RS-25, Merlin 1D, and Raptor. Their specific impulses are 366s, 282s, and 330s (at sea level), so the RS-25 looks pretty good. But their thrust/weight ratios are roughly 60, 185, and 200 (target). The Merlin 1D was, I believe, the highest thrust-weight ratio liquid-fueled engine ever. This is one of the primary reasons, in addition to cost, it is considered so good, despite having such anemic specific impulse. You can optimize thrust/weight several ways. Firstly, you can try to cram more thrust out of an engine of a fixed size. This is done by driving up chamber pressures as high as you can, which might be easier with a smaller engine due to square-cube scaling. Secondly, you can make the engine smaller, without changing its thrust, and try to cram as many of them in as possible. Consider thrust/area as another important metric -- for fixed rocket dimensions, being able to cram more engines into the base is an easy way to get more thrust and improve the total thrust/weight of the stack. Another consideration, which has all too often been ignored in the rocketry business, is cost. More specifically $/thrust, if we're looking at a first stage. Here, smaller engines have a clear advantage in that your tooling doesn't need to be as large and expensive, and you're going to need more of them so you can start to leverage economies of scale rather than having each engine being an individual, artisan-produced artifact. That obviously has a limit -- there's a point at which more engines would make things more expensive, but judging from the thrust and cost of the Raptor, I would guess it's right around the optimal point. Finally, it's worth noting that, all other considerations aside, going bigger in rocket engines tends to make the engineering more difficult. The square-cube law makes heat flux in the combustion chamber scale roughly linearly with engine size, which makes cooling more difficult. And the larger the combustion chamber, the more at risk it is of combustion instabilities. Look, for example, at the Russian RD-170 engine. It looks like four engines but is actually one engine with four combustion chambers. They did that because while it looks more complex, it actually makes things easier.

What's the scaling like for rocket engines? How much more efficiency (in various forms) could you get if you made 1 giant engine instead? (Disregarding reliability)

tksmith151 5y ago

I'm not a rocket scientist, but my understanding is that efficiency is derived from the exit velocity of matter from the engine. So the faster it can fire matter in the opposite direction the more efficient it is. There are some other factors as well like bell shape, which are generally designed to provide better efficiencies at different atmospheric pressures (ranging from sea level to vacuum of space). The main consideration after these factors is the mass of the engines. If multiple engines and one giant engine had all of these same factors equal, then I don't think there would be much difference in the efficiency of the rocket. Likely though, there are differences and other advantages to multiple small engines. The easiest one for me to think of is modularity. If you want a specific thrust for a payload it's much easier to add more small engines together than to try and throttle down the large engine to the correct amount, if that is possible at all.

KiwiJohnno 5y ago

Musk has actually talked about that before, during the early stages of Raptor development. He said they were downsizing Raptor from the originally planned size, because their analysis showed a smaller size was the sweet spot for maximum thrust/weight ratio.

mlindner 5y ago

There are a lot of issues you get from scaling an engine up, one of the major issues being combustion instability. The larger the "blob" of fuel and oxidizer mixed together that you're trying to burn, the more you will get things like eddy currents and shockwaves flowing through the material. These disrupt the combustion and make it uneven, possibly leading to feedback cycles where these elements self-reinforce eventually causing vibration or otherwise damage to the engine. The smaller the physical size of your combustion chamber the easier these things are. The largest single engine ever built, the F1 engine that pushed the Saturn V, had massive issues with this that were further made worse because the engines were largely designed without the aid of computers.

dkbrk 5y ago

The short answer to your question is that larger engines have very few advantages and many disadvantages. Specific impulse is the most obvious parameter of a rocket engine, but it's relatively less important for a first stage where thrust/weight is also a very important concern. Specific impulse depends on the chemistry, combustion cycle, combustion efficiency and nozzle design. There isn't any obvious reason why it would scale either way with engine size. Thrust/weight is very important for engines on a first stage because of gravity losses. Imagine if the rocket sitting on the pad had a thrust/weight of just 1.1; that would mean that at liftoff 91% of the thrust would be wasted just countering gravity. You both want a high thrust/weight at liftoff and a small dry mass to give the second stage the most momentum possible at stage separation. High thrust also helps for reuse in making the burn time of the first stage shorter (for fixed specific impulse and propellant load), which means that at stage separation the first stage isn't too far downrange and is easier to return to the launch site. Note that there's both the thrust/weight of the rocket as a whole, and of the engines themselves. The engines comprise a significant proportion of the first stage's weight, though, so looking at engine weight (and thrust to weight) is also useful. Consider the RS-25, Merlin 1D, and Raptor. Their specific impulses are 366s, 282s, and 330s (at sea level), so the RS-25 looks pretty good. But their thrust/weight ratios are roughly 60, 185, and 200 (target). The Merlin 1D was, I believe, the highest thrust-weight ratio liquid-fueled engine ever. This is one of the primary reasons, in addition to cost, it is considered so good, despite having such anemic specific impulse. You can optimize thrust/weight several ways. Firstly, you can try to cram more thrust out of an engine of a fixed size. This is done by driving up chamber pressures as high as you can, which might be easier with a smaller engine due to square-cube scaling. Secondly, you can make the engine smaller, without changing its thrust, and try to cram as many of them in as possible. Consider thrust/area as another important metric -- for fixed rocket dimensions, being able to cram more engines into the base is an easy way to get more thrust and improve the total thrust/weight of the stack. Another consideration, which has all too often been ignored in the rocketry business, is cost. More specifically $/thrust, if we're looking at a first stage. Here, smaller engines have a clear advantage in that your tooling doesn't need to be as large and expensive, and you're going to need more of them so you can start to leverage economies of scale rather than having each engine being an individual, artisan-produced artifact. That obviously has a limit -- there's a point at which more engines would make things more expensive, but judging from the thrust and cost of the Raptor, I would guess it's right around the optimal point. Finally, it's worth noting that, all other considerations aside, going bigger in rocket engines tends to make the engineering more difficult. The square-cube law makes heat flux in the combustion chamber scale roughly linearly with engine size, which makes cooling more difficult. And the larger the combustion chamber, the more at risk it is of combustion instabilities. Look, for example, at the Russian RD-170 engine. It looks like four engines but is actually one engine with four combustion chambers. They did that because while it looks more complex, it actually makes things easier.