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Why have multiple rocket nozzles on scifi spaceships?


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Scifi spaceships have excellent delta v, so why do they have multiple rocket nozzles?

A few reasons below that I can think of:

Thrust vectoring. Also, to add to that, turning off some engines while leaving on others is a way of reducing thrust easily.

It looks awesome: Preferred style or imitation perhaps?

Having googled it, the advantage one big rocket nozzle confers are:

Fuel flow is better: Multiple fuel tanks linked to a single nozzle. Stop pumping from some of the tanks and you reduce thrust. With multiple nozzels attached to a single fuel tank, it takes longer for some of the fuel to reach some nozzles than if all fuel tanks were linked to the same nozxle. At least that is what I read.

In scifi though, all seems equal.

For example, if you have two spaceships with virtually endless high thrust delta v,  the advantage of multiple nozzles over one big one appear minimal.

The only advantage I can think of in that case that multiple nozzles would confer is thrust vectoring.

What do you think?

Edited by Spacescifi
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TWR: more engines equals higher thrust.

Nozzle Area. Generally, more nozzle area gives more thrust for a given chamber pressure. A single nozzle with the same area as multiple smaller ones may not fit the cross section of the ship. How would a Star Destroyer look with a single gigantic nozzle at the back?

Engineering problems: Larger engines tend towards uneven combustion / combustion instability problems, which require additional engineering work to solve. (Saturn V's F-1 engines)

Simplicity. It's easier to use multiple smaller off-the-shelf (already developed) engines than develop a whole new gigantic engine (Falcon 9)

Engine-out capability.

 

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1 hour ago, StrandedonEarth said:

TWR: more engines equals higher thrust.

Nozzle Area. Generally, more nozzle area gives more thrust for a given chamber pressure. A single nozzle with the same area as multiple smaller ones may not fit the cross section of the ship. How would a Star Destroyer look with a single gigantic nozzle at the back?

Engineering problems: Larger engines tend towards uneven combustion / combustion instability problems, which require additional engineering work to solve. (Saturn V's F-1 engines)

Simplicity. It's easier to use multiple smaller off-the-shelf (already developed) engines than develop a whole new gigantic engine (Falcon 9)

Engine-out capability.

 

So based on what you said, an ideal scifi ship rocket nozzle configuration would cover the entire rear end cross section of the ship with rocket nozzles only as big as an adult human hand.

That way you can do even more precise speed control.

Good idea? If not why not?

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Just now, Spacescifi said:

So based on what you said, an ideal scifi ship rocket nozzle configuration would cover the entire rear end cross section of the ship with rocket nozzles only as big as an adult human hand.

That way you can do even more precise speed control.

Good idea? If not why not?

Well, everything is a trade-off, and using a zillion engines that small would run into issues with cost and fuel-system complexity. Also bear in mind that nozzle area is proportional to the square of radius, so doubling the radius gives a four-fold increase in area. A large (car-tire-sized) nozzle would only need to be marginally bigger to add the area of a small (hand-sized) nozzle. While it is probably cheaper to build one big engine instead of ten small ones (depending on scale), developing and setting up a production line for another engine class (especially huge ones) is very costly and time-consuming,  And it's mainly the truly huge engines that run into combustion problems.

Regarding fuel delivery, engines are usually valved/throttled at the engines, not the tanks. so throttle response should not differ between single and multi-engine setups.

There are also placement constraints to consider (can't put an engine there for whatever reason).

Optimizing is a big part of engineering, with all the factors I listed to consider. Scifi ships are more interested in the rule-of-cool than sound engineering, so the ideal scifi ship rocket nozzle configuration is whatever looks the like the best combination of cool and function. Writers rarely need to justify those choices, unless it pertains to the story.

 

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50 minutes ago, StrandedonEarth said:

Well, everything is a trade-off, and using a zillion engines that small would run into issues with cost and fuel-system complexity. Also bear in mind that nozzle area is proportional to the square of radius, so doubling the radius gives a four-fold increase in area. A large (car-tire-sized) nozzle would only need to be marginally bigger to add the area of a small (hand-sized) nozzle. While it is probably cheaper to build one big engine instead of ten small ones (depending on scale), developing and setting up a production line for another engine class (especially huge ones) is very costly and time-consuming,  And it's mainly the truly huge engines that run into combustion problems.

Regarding fuel delivery, engines are usually valved/throttled at the engines, not the tanks. so throttle response should not differ between single and multi-engine setups.

There are also placement constraints to consider (can't put an engine there for whatever reason).

Optimizing is a big part of engineering, with all the factors I listed to consider. Scifi ships are more interested in the rule-of-cool than sound engineering, so the ideal scifi ship rocket nozzle configuration is whatever looks the like the best combination of cool and function. Writers rarely need to justify those choices, unless it pertains to the story.

 

 

Good informative post, yet combustion should not be an issue in my scifi because they just run the propellant over the hot reactor/power core. Could be fusion or something. That is enough to make a flame out the back end of the nozzle.So without combustion being a relavent factor, does that change any of your previous analysis?

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2 hours ago, Spacescifi said:

 

Good informative post, yet combustion should not be an issue in my scifi because they just run the propellant over the hot reactor/power core. Could be fusion or something. That is enough to make a flame out the back end of the nozzle.So without combustion being a relavent factor, does that change any of your previous analysis?

Not really. Scifi ships are generally more about aesthetics: Looking good, or looking like how people think it should look. Realistic engineering considerations take a back seat.

Although given the basis for your thermal-expansion drive, one nozzle per reactor core would make more sense, requiring much less heavy, high-pressure plumbing. Much like how the kerbal LV-N's are discreet, self-contained reactor+nozzle units, which aids redundancy.

I never gave much thought to the nitty-gritty of nuclear-thermal engines. I suppose it would need a turbopump to pressurize the propellant to go through the reactor. What drives the turbopump? A portion of the exhaust gas? A separate steam loop, either directly or by electricity generated by the same reactor? I really don't know. But for the purposes of sci-fi those details can be ignored, unless it pertains to the story (pump breaks down, etc.)

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3 hours ago, StrandedonEarth said:

Not really. Scifi ships are generally more about aesthetics: Looking good, or looking like how people think it should look. Realistic engineering considerations take a back seat.

Although given the basis for your thermal-expansion drive, one nozzle per reactor core would make more sense, requiring much less heavy, high-pressure plumbing. Much like how the kerbal LV-N's are discreet, self-contained reactor+nozzle units, which aids redundancy.

I never gave much thought to the nitty-gritty of nuclear-thermal engines. I suppose it would need a turbopump to pressurize the propellant to go through the reactor. What drives the turbopump? A portion of the exhaust gas? A separate steam loop, either directly or by electricity generated by the same reactor? I really don't know. But for the purposes of sci-fi those details can be ignored, unless it pertains to the story (pump breaks down, etc.)

Neither had I, indeed, you understand it better than I!

Yet in conclusion, I have decided to only use rockets for RCS.

Scifi abilities on spaceships I prefer when they have obvious side effects. A cost as it were for defying our real understanding of physics.

So to ascend to space (not orbit) my ships will use an antigravity field.

The side effect cost is that the ship repels everything that can fall due to gravity away from it, everyting outside the ship, up to a certain foot radus. The rate of repulsion is the same rate of acceleration the ship is repelled away from the planet with. So no one should be near the ship during launch for dear they will fall backward several feet.

Inside the ship the effect is the opposite, as the crew us pulled at toward the center of the ship at the same rate of acceleration the are being repelled upward. Thus they feel horizontal g-force while ascending upward.

Once the ship clears the air up into space the antigravity drive will cease to accelerate, as it only accelerates away from a planet because detects the atmosphere and ground as one big object to repel away from with the shortest path (up).

In space the antigravity field will still pull the crew inward at whatever number of g's they are repelling outward, and it can be used to actually push stuff away from the ship, while pushing the ship away, (thus accelerating it so long as an opposing object is within the field's radus).

One can actually push stuff continuously this way since with the constant acceleration drive from via vacuum jets, one can accelerate enough to not be repelled away as they push asteroids and whatever else with their antigravity field.

Since this is supposed to be alien technology anyway, and they have FTL, and fuel tanks taje up cargo space, I decided to just use it for RCS.

Granted my ship's cannot fly around like airplanes in the air like star wars,  but they are perfect for space.

Besides, to fly any odd looking non-aerodynamic ship the way they fly the milkennium falcon would be both uncomfortable for the crew and inefficient.

Imagine what happenst to the cargo when they do a barrel roll? In space? No problem. And the design is so inefficient for air travel that it screams fiction to me.

Granted some may care less. But I am doing it my way because I care about that kind of stuff. Aliens I think are supposed to be wise, and making a ship that is shaped so that it will fly inefficiently through the air, even if the engibes have endless fuel, makes no sense to me.

So my spaceships will only perform best as spaceships.

 

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@Spacescifi, with fission reactors or magnetically confined fusion, the problems with clustering escalate dramatically: these designs invariably emit fields that directly interfere with each other’s core processes. Nuclear reactors are throttled through control of thermal neutron flux, but unshielded reactors also spew them in all directions, including towards other reactors, this keeping the entire cluster working effectively will be a bother - let alone selectively shutting down some of the engines. You thus end up with the added weight of partitions made from neutron poisons, but at least the solid-core nuclear rockets can be gimbaled so that as few as two engines would give you engine-out capability.

You can’t really gimbal gas/plasma-core engines, neither fission nor fusion. And the magnetic confinement fields of most fusion rocket variants would probably endanger nearby ships.

Thus yes, in a lot of cases a single engine is very sensible because it’s the only option.

6519132275ef6027ad8a1517a567c3a98b49caab

And the clusters of similar-looking engines of varied sizes essentially never make sense.

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7 hours ago, DDE said:

@Spacescifi, with fission reactors or magnetically confined fusion, the problems with clustering escalate dramatically: these designs invariably emit fields that directly interfere with each other’s core processes. Nuclear reactors are throttled through control of thermal neutron flux, but unshielded reactors also spew them in all directions, including towards other reactors, this keeping the entire cluster working effectively will be a bother - let alone selectively shutting down some of the engines. You thus end up with the added weight of partitions made from neutron poisons, but at least the solid-core nuclear rockets can be gimbaled so that as few as two engines would give you engine-out capability.

You can’t really gimbal gas/plasma-core engines, neither fission nor fusion. And the magnetic confinement fields of most fusion rocket variants would probably endanger nearby ships.

Thus yes, in a lot of cases a single engine is very sensible because it’s the only option.

6519132275ef6027ad8a1517a567c3a98b49caab

And the clusters of similar-looking engines of varied sizes essentially never make sense.

 

Interesting. So a proper fusion scifi spaceship that follows known principles can only look much like the ship on the expanse.

I have wanted to make many of the absurd spaceship shape designs seen in scifi practical (for alien style and variety), but where real life engine constraints matter, there really would be an optimum shape.

In other words,  scifi human and alien ships, if they use fusion, will both look much the same because of optimization of fusion engines.

Thanks for your informative post!

 

Edited by Spacescifi
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15 hours ago, Spacescifi said:

Interesting. So a proper fusion scifi spaceship that follows known principles can only look much like the ship on the expanse.

With the mere exception of “pure” laser fusion with no magnetic fields involved? Yes.

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