Post by Attero Dominatus on Jul 15, 2009 1:31:55 GMT -5
Originally I just posted a link to where you can design a realistic spaceship (see below), then recently I realized that it might be above most people's heads, so I wrote this for those who want a ship realistic enough to be plausible but not necessarily exact. If you still want to go into exacting detail (such as life support requirements, crew makeup and size, command structure, advanced ship and engine design, thermodynamic issues, realistic aliens, etc), then I would suggest Atomic Rocket, which offers a lot more information than I have written here.
MASS RATIO
This is the ratio of propellant mass to ship mass. In fact, think about this number before doing anything else, as it will determine how much you can take. Divide the ship's maximum propellant mass by its empty mass. Example: A ship carries 212 tons of propellant and masses 100 tons with the tanks dry, which makes for a mass ratio of 2.12. A high mass ratio will usually be necessary but if you have a jump drive or warp drive, you can get away with a lower one.
ENGINE
While chemical rockets CAN propel ships around the solar system and even beyond, as proven by the Voyager probes, you will be forced to wait several years just to get to reach Jupiter, much less Pluto. If you want to get around the solar systems in just months or weeks, you will have to use a nuclear rocket engine.
Your choice of engine will often decide your ship's purpose. A warship will likely use an engine that offers high Specific Impulse (see below) and high thrust, which pretty much limits you to a Nuclear Saltwater Rocket or a Gas Core Nuclear Rocket or a multi-terawatt fusion rocket. A scientific space probe will likely use an ion engine, which produces very little thrust but can operate continuously for years while solar sails and laser sails would probably be the most economical means of transporting light cargo loads and chemical rockets would be used for boosting payloads into orbit or to local bodies such as moons.
SPECIFIC IMPULSE
The specific impulse is a measure of a rocket's efficiency, often depicted in seconds (how long it will take to reach your maximum velocity or delta-v). Generally, higher numbers will be better but there are certain situations where you want a lower number, which corresponds to a higher mass flow rate and higher thrust at the cost of propellant efficiency. An engine that can change IsP on command will be essential for a rocket that can land and takeoff from planets and travel throughout the solar system.
Specific impulse in seconds is your exhaust velocity (see below) divided by 9.81^2 (squared). For a ship that can change IsP on command, assume this number is the maximum possible (this would be your ship's 'cruise mode' which would be used strictly for orbit-to-orbit burns).
EXHAUST VELOCITY
Like Specific Impulse, higher numbers are better most of the time, but in all but a few engine designs, higher EV will come at the cost of acceleration. You can get EV by multiplying IsP in seconds by 9.81^2. Again, if your ship can change its IsP on command, the number should be the highest.
THRUST
Multiply exhaust velocity by the rate of propellant consumption (mass flow rate) in kilograms per second. Example: 9,941 (exhaust velocity in meters per second) x 150 (kilograms of propellant shot out the back every second) = 1,491,150 newtons.
If your ship carries 212,000 kilograms of propellant and 1,491,150 newtons of thrust (150 kilograms a second) is being applied to it, it can maintain that thrust for 1,413 seconds.
To find the acceleration in meters per second, divide the thrust in newtons by the weight in kilograms. Example: 14,911,500 divided by 400,000 (the ship's total mass) = 37.2 meters a second. To find the acceleration in Gs, divide that result by 9.81. In this case, the result would be 3.79 Gs.
DELTA-V
The Delta-V is how fast your ship can go if its propellant tanks are allowed to run dry. It is a measure of the ship's 'range' in space. For example, if your Delta-V is 200,000 meters a second and you want to take off from Earth's surface and get into low earth orbit, you will need to accelerate to 8,600 meters a second. This leaves 191,400 meters a second to use.
To get your Delta-V, find the natural logarithm of your mass ratio (the 'ln' key on a scientific calculator) and then multiply it by the exhaust velocity.
PAYLOAD
Your total payload will typically be a small fraction of the ship's total mass, even with nuclear rockets. For a warship, this can get tricky because you will need that free mass for your weapons and ammunition as well as the supplies for the crew. Often, every single kilogram of mass will count, so crews will often be forced to take as little with them as possible. A good spaceship designer will leave an assigned number of kilograms as a safety overhead to avoid the sort of situation depicted in The Cold Equations. I would suggest at least 100 kilograms to accommodate a stowaway, but preferably more.
What is left between your desired payload and the ship's fuel mass will be the ship's dry mass. This would be the ship's structure, engines, life support equipment and maneuvering thrusters, as well as all of the systems needed to control the ship. You will want to use the lightest but strongest materials possible, which would mean either carbon fibers or carbon nanotubes, both of which are lighter than aluminum and stronger than steel. In order to keep things simple, you may want anywhere from a third to half of your dry mass be structural mass. If you want to be more exacting about the ship's structure, then I suggest visiting the link at the very top.
HEAT
One of the biggest problems facing any spaceship is how to dispose of waste heat. Vacuum makes one of the best thermal insulators, which is why certain liquefied gases are stored in tanks with rarified atmosphere between the outside and the liquid inside. If you do not radiate this heat into space, then your crew will eventually die from heat exhaustion and your ship will melt. A warship will likely have radiator fins that can be retracted to protect them from enemy weapons fire and the waste heat will be stored inside of an internal heat sink until either the combat is over or the heat sink reaches is maximum capacity.
Waste heat also makes stealth in space all but impossible, though the main character's ship in Vigilance makes use of an internal heat sink and light refracting metamaterials to allow it to bend the background photons around it (though accelerating will still give the ship's position away).
COMMUNICATION
If your theater of war is a contested solar system, then communication will take several hours due to light speed lag. Unless you delve into certain theories like quantum tunneling or entanglement, there is just no way around it. Communication through wormholes is one possibility, but that would require knowing where they are and having the technology to open them (the casimir effect is said to be able to do this), and if you can communicate through them, then why not travel through them as well?
Also, communication will effect the entire civilization being depicted. If lightspeed lag is taken into account, then that would mean no instantaneous phone calls to your friends or even a rapid response to an invading enemy. Unless goods can be produced locally, on, say, Titan, then products will be very expensive because ordering said goods from the factories in the asteroid belt will take hours due to lightspeed lag, and then those goods will need to be shipped to Titan, which will take weeks, and the spaceship operators will need income to pay for the upkeep on their cargo ship, and space technology is not cheap. In fact, the communication itself may not be cheap because powerful radio or laser transmitters will be needed.
The vast distances involved would make paper money and gold of limited use because they must be transported across those vast distances to change hands, which would cost more than the currency's worth itself (especially true with precious metals) Money would mostly be electronic (unless the exchange of goods and money occur locally on a planet or moon or space station), but backed up by hard assets such as precious metals that change ownership but not necessarily physical location, which is the direction we are moving toward here on earth with things like electronic transactions. An electronic currency would make the economy easier for a government to control and abuse, if a government controls the network (and most likely it will).
On the flipside, distance and lightspeed lag would make revolt against a tyrannical government easier as long as the resistance was well coordinated and had access to laser or particle beam weapons to destroy a government base's communication transmitter before an attack.
WEAPONS
In space, combat will take place at millions of kilometers. Closer ranged fights will only be found around planets or moons: an example would be be an enemy's attempt to drive the allied ships away from low orbit to cut the allied ground forces from their space support, and even then, combat ranges will still often be in the distances of thousands of kilometers.
Beam weapons and missiles will be the primary weapons for ship to ship combat. Projectile weapons would rarely be seen except in cases where it is possible to counter the recoil (recoilless rifles which vent some of the propellant gas out the back are a good example) and even then they will likely be close range weapons, used primarily to defend against missiles and destroy boarding pods (assuming an enemy can make it past the ship's longer ranged weapons).
EDIT: Added paragraph about space economy.
MASS RATIO
This is the ratio of propellant mass to ship mass. In fact, think about this number before doing anything else, as it will determine how much you can take. Divide the ship's maximum propellant mass by its empty mass. Example: A ship carries 212 tons of propellant and masses 100 tons with the tanks dry, which makes for a mass ratio of 2.12. A high mass ratio will usually be necessary but if you have a jump drive or warp drive, you can get away with a lower one.
ENGINE
While chemical rockets CAN propel ships around the solar system and even beyond, as proven by the Voyager probes, you will be forced to wait several years just to get to reach Jupiter, much less Pluto. If you want to get around the solar systems in just months or weeks, you will have to use a nuclear rocket engine.
Your choice of engine will often decide your ship's purpose. A warship will likely use an engine that offers high Specific Impulse (see below) and high thrust, which pretty much limits you to a Nuclear Saltwater Rocket or a Gas Core Nuclear Rocket or a multi-terawatt fusion rocket. A scientific space probe will likely use an ion engine, which produces very little thrust but can operate continuously for years while solar sails and laser sails would probably be the most economical means of transporting light cargo loads and chemical rockets would be used for boosting payloads into orbit or to local bodies such as moons.
SPECIFIC IMPULSE
The specific impulse is a measure of a rocket's efficiency, often depicted in seconds (how long it will take to reach your maximum velocity or delta-v). Generally, higher numbers will be better but there are certain situations where you want a lower number, which corresponds to a higher mass flow rate and higher thrust at the cost of propellant efficiency. An engine that can change IsP on command will be essential for a rocket that can land and takeoff from planets and travel throughout the solar system.
Specific impulse in seconds is your exhaust velocity (see below) divided by 9.81^2 (squared). For a ship that can change IsP on command, assume this number is the maximum possible (this would be your ship's 'cruise mode' which would be used strictly for orbit-to-orbit burns).
EXHAUST VELOCITY
Like Specific Impulse, higher numbers are better most of the time, but in all but a few engine designs, higher EV will come at the cost of acceleration. You can get EV by multiplying IsP in seconds by 9.81^2. Again, if your ship can change its IsP on command, the number should be the highest.
THRUST
Multiply exhaust velocity by the rate of propellant consumption (mass flow rate) in kilograms per second. Example: 9,941 (exhaust velocity in meters per second) x 150 (kilograms of propellant shot out the back every second) = 1,491,150 newtons.
If your ship carries 212,000 kilograms of propellant and 1,491,150 newtons of thrust (150 kilograms a second) is being applied to it, it can maintain that thrust for 1,413 seconds.
To find the acceleration in meters per second, divide the thrust in newtons by the weight in kilograms. Example: 14,911,500 divided by 400,000 (the ship's total mass) = 37.2 meters a second. To find the acceleration in Gs, divide that result by 9.81. In this case, the result would be 3.79 Gs.
DELTA-V
The Delta-V is how fast your ship can go if its propellant tanks are allowed to run dry. It is a measure of the ship's 'range' in space. For example, if your Delta-V is 200,000 meters a second and you want to take off from Earth's surface and get into low earth orbit, you will need to accelerate to 8,600 meters a second. This leaves 191,400 meters a second to use.
To get your Delta-V, find the natural logarithm of your mass ratio (the 'ln' key on a scientific calculator) and then multiply it by the exhaust velocity.
PAYLOAD
Your total payload will typically be a small fraction of the ship's total mass, even with nuclear rockets. For a warship, this can get tricky because you will need that free mass for your weapons and ammunition as well as the supplies for the crew. Often, every single kilogram of mass will count, so crews will often be forced to take as little with them as possible. A good spaceship designer will leave an assigned number of kilograms as a safety overhead to avoid the sort of situation depicted in The Cold Equations. I would suggest at least 100 kilograms to accommodate a stowaway, but preferably more.
What is left between your desired payload and the ship's fuel mass will be the ship's dry mass. This would be the ship's structure, engines, life support equipment and maneuvering thrusters, as well as all of the systems needed to control the ship. You will want to use the lightest but strongest materials possible, which would mean either carbon fibers or carbon nanotubes, both of which are lighter than aluminum and stronger than steel. In order to keep things simple, you may want anywhere from a third to half of your dry mass be structural mass. If you want to be more exacting about the ship's structure, then I suggest visiting the link at the very top.
HEAT
One of the biggest problems facing any spaceship is how to dispose of waste heat. Vacuum makes one of the best thermal insulators, which is why certain liquefied gases are stored in tanks with rarified atmosphere between the outside and the liquid inside. If you do not radiate this heat into space, then your crew will eventually die from heat exhaustion and your ship will melt. A warship will likely have radiator fins that can be retracted to protect them from enemy weapons fire and the waste heat will be stored inside of an internal heat sink until either the combat is over or the heat sink reaches is maximum capacity.
Waste heat also makes stealth in space all but impossible, though the main character's ship in Vigilance makes use of an internal heat sink and light refracting metamaterials to allow it to bend the background photons around it (though accelerating will still give the ship's position away).
COMMUNICATION
If your theater of war is a contested solar system, then communication will take several hours due to light speed lag. Unless you delve into certain theories like quantum tunneling or entanglement, there is just no way around it. Communication through wormholes is one possibility, but that would require knowing where they are and having the technology to open them (the casimir effect is said to be able to do this), and if you can communicate through them, then why not travel through them as well?
Also, communication will effect the entire civilization being depicted. If lightspeed lag is taken into account, then that would mean no instantaneous phone calls to your friends or even a rapid response to an invading enemy. Unless goods can be produced locally, on, say, Titan, then products will be very expensive because ordering said goods from the factories in the asteroid belt will take hours due to lightspeed lag, and then those goods will need to be shipped to Titan, which will take weeks, and the spaceship operators will need income to pay for the upkeep on their cargo ship, and space technology is not cheap. In fact, the communication itself may not be cheap because powerful radio or laser transmitters will be needed.
The vast distances involved would make paper money and gold of limited use because they must be transported across those vast distances to change hands, which would cost more than the currency's worth itself (especially true with precious metals) Money would mostly be electronic (unless the exchange of goods and money occur locally on a planet or moon or space station), but backed up by hard assets such as precious metals that change ownership but not necessarily physical location, which is the direction we are moving toward here on earth with things like electronic transactions. An electronic currency would make the economy easier for a government to control and abuse, if a government controls the network (and most likely it will).
On the flipside, distance and lightspeed lag would make revolt against a tyrannical government easier as long as the resistance was well coordinated and had access to laser or particle beam weapons to destroy a government base's communication transmitter before an attack.
WEAPONS
In space, combat will take place at millions of kilometers. Closer ranged fights will only be found around planets or moons: an example would be be an enemy's attempt to drive the allied ships away from low orbit to cut the allied ground forces from their space support, and even then, combat ranges will still often be in the distances of thousands of kilometers.
Beam weapons and missiles will be the primary weapons for ship to ship combat. Projectile weapons would rarely be seen except in cases where it is possible to counter the recoil (recoilless rifles which vent some of the propellant gas out the back are a good example) and even then they will likely be close range weapons, used primarily to defend against missiles and destroy boarding pods (assuming an enemy can make it past the ship's longer ranged weapons).
EDIT: Added paragraph about space economy.
