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Old 04-13-2021, 07:07 AM   #1
the-red-scare
 
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Default [Spaceships] Engine size and performance

For the acceleration of a reaction engine to remain constant, the propellant mass flow rate has to scale with the mass of the ship. So a 3x increase in ship size means a 3x increase in mass flow, a 3x increase in thrust, and therefore equal acceleration.

Does that then mean the cross-sectional area of the nozzle throat (or equivalent region in an electric or pulse-type engine) also scales with mass? And therefore the surface area required for a given type of engine scales with mass?

What I’m getting at is, a SM +7 ship has twice the surface area of an SM +6 ship. Do its engines take up twice the surface area on the rear hull (scaling with area as most parts will) or three times the surface area (scaling with mass to accommodate three times the mass flow)?

If the latter (causing the engines to be ridiculously huge at large sizes) should performance decrease, perhaps in a Speed/Range kind of way, as ships get bigger? Could be a justification for “fast” fighters and “slow” capital ships…

Last edited by the-red-scare; 04-13-2021 at 04:14 PM.
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Old 04-14-2021, 12:01 AM   #2
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Default Re: [Spaceships] Engine size and performance

Spaceships doesn't concern itself with things like mass flow rate and surface area. And it should be noted that a SM+9 "engine" may be a single large engine or the same engine that a SM+8 ship carries mounted triple.
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Old 04-14-2021, 03:10 AM   #3
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Default Re: [Spaceships] Engine size and performance

The Spaceships design system doesn't concern itself with such realities. It simplifies everything as scaling linearly from SM+5 all the way up, when in real engineering there would be all sorts of non-linear breakpoints and limits. Of course, you can layer such limitations on top of the system if you like, according to your own tech paradigm, whether that's realistic rocket engine design or some FTL system that favours either small ships or large ones.
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Old 04-14-2021, 04:27 AM   #4
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Default Re: [Spaceships] Engine size and performance

As others have said spaceships does not go into such detail. It only cares about mass, not volume or surface area.

Quote:
Originally Posted by the-red-scare View Post
If the latter (causing the engines to be ridiculously huge at large sizes) should performance decrease, perhaps in a Speed/Range kind of way, as ships get bigger? Could be a justification for “fast” fighters and “slow” capital ships…
Basically: if you wish. But then the capital ships would have a smaller mass fraction of engines.

But a possible other reason for “fast” fighters and “slow” capital ships is that capital ships have less engines and fuel fraction because they require things like long term crew quarters and likely more armor, thus no need to go into surface area considerations. Whereas a typical fighter would have only a cockpit, weapons, engine and fuel with minimal or no armor.
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Old 04-14-2021, 06:05 AM   #5
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Default Re: [Spaceships] Engine size and performance

I know Spaceships doesn’t concern itself with these things. If it did, I wouldn’t need to ask about them. But I concern myself with these things, hence the question.

So does anyone know if rocket cross-sectional area should scale with mass flow rate?
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Old 04-14-2021, 08:36 AM   #6
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Default Re: [Spaceships] Engine size and performance

If nothing else about the engine changes, then yes. If you have to triple the mass moving through your engine, you need to triple the area through which it moves. (Area, so the radius of the nozzle goes up by a factor of sqrt(3), not 3.) Or, you have to change other things about your engine - pressure, temperature, other characteristics of the gas produced.

More than you wanted to know from NASA.

If the rest of the ship is the same (shape, but really just density), then if mass triples, the volume triples. If the shape is the same, then the dimensions grow by the cube root of 3. Since that nozzle area grows with a square root, the nozzle area does increase relative to the rest of the ship.

But then, even a Spaceships ship doesn't scale that way. The walls don't have to get proportionately thicker just because the room gets bigger, for instance. Structural members need to handle more force (same acceleration times higher mass); a quick glance at a beam support calculator suggests that does scale with mass (beam length and support positions as l^1/3, 3x force) -- which is going to mean in practice that the ship gets redesigned, not just scaled up.

But I'm not an engineer, much less a structural one.
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Old 04-14-2021, 08:56 AM   #7
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Default Re: [Spaceships] Engine size and performance

Assuming the same type of propulsion system (personally, I'm inclined to give "fighters" drives with high acceleration and low delta-v, while "capital ships" have low acceleration and high delta-v - replace delta-v with endurance for reactionless drives), I could see a paradigm where smaller ships are narrow and long while larger ones are wide and short; the smaller ones only have a few nozzles on the back, while the larger ones are absolutely covered in them. That's at the extremes - in general, the larger you get, the shorter and wider ships are, while the smaller you get, the longer and thinner ships are.
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Old 04-14-2021, 10:19 AM   #8
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Default Re: [Spaceships] Engine size and performance

Quote:
Originally Posted by the-red-scare View Post
I know Spaceships doesn’t concern itself with these things. If it did, I wouldn’t need to ask about them. But I concern myself with these things, hence the question.

So does anyone know if rocket cross-sectional area should scale with mass flow rate?
Nozzle area mostly scales with mass flow rate (with some variance based on design) but this isn't a significant limiting factor for most rockets we actually deal with in the real world.
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Old 04-14-2021, 10:26 AM   #9
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Default Re: [Spaceships] Engine size and performance

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Originally Posted by Anthony View Post
Nozzle area mostly scales with mass flow rate (with some variance based on design) but this isn't a significant limiting factor for most rockets we actually deal with in the real world.
In Spaceships where mass flow rate can vary by a factor of 300,000, seems like it might come up…
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Old 04-14-2021, 10:40 AM   #10
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Default Re: [Spaceships] Engine size and performance

Quote:
Originally Posted by the-red-scare View Post
In Spaceships where mass flow rate can vary by a factor of 300,000, seems like it might come up…
The idea that you can use the same designs with only minimal modifications over a size range of +5 to +15 (100,000 times) lacks real world justifications, a lot of systems have both minimum and maximum practical sizes or don't scale in a simple way, but Spaceships was always intended as quick and dirty, not an accurate simulation of anything.
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