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Old 10-25-2009, 02:50 AM   #1
Ulzgoroth
 
Join Date: Jul 2008
Default [Spaceships] Fuel versus reaction mass

I'm wondering just how this stands with fusion and antimatter rockets. Particularly in relation to air rams.

A chemical or nuclear pulse rocket has both-in-one. Others usually don't. An electrical thruster uses whatever feeds it power as fuel. Nuclear rockets, except nuclear salt water, have a built in reactor (with how much endurance, anyway?) providing a lifetime supply of fuel.

Fusion rockets could fuse their reaction mass...considering that they're plasma thrusters, they must be directly exposing the reaction mass to fusing material at least. But then there's air ram mode. It still has to fuse something, but it doesn't burn off anything from its tanks. Is the fusion reaction fuel negligible compared to reaction mass? Is it assumed to be able to fuse the air?

Antimatter rockets, of course, use the antimatter which is included in their tankage as fuel. That may be a negligibly small mass (except for the pion drives, of course), but it's essentially the entire cost of fuel. So how do antimatter air rams work? They can replace the reaction mass, but obviously that does nothing to stretch the antimatter supply. Maybe they should carry extra antimatter for air ram mode?
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Old 10-25-2009, 04:08 PM   #2
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Default Re: [Spaceships] Fuel versus reaction mass

[QUOTE=Ulzgoroth;872486]
Nuclear rockets, except nuclear salt water, have a built in reactor (with how much endurance, anyway?) providing a lifetime supply of fuel.

About the same as any other nuclear reactor. Realistically somewhere around a year, give or take a factor of 10, for anything except a direct contact gas core system (which loses fissionables to the reaction mass stream)

Quote:
Fusion rockets could fuse their reaction mass...considering that they're plasma thrusters, they must be directly exposing the reaction mass to fusing material at least.
Nah, they could be some sort of electric rockets run on a fusion plant, or injecting the fusion exhaust into the reaction mass stream.

Quote:
But then there's air ram mode. It still has to fuse something, but it doesn't burn off anything from its tanks. Is the fusion reaction fuel negligible compared to reaction mass?
Yes, it is. Depending on the fuels used, fusion reactions liberate a few percent of the mass energy of their fuel. A fusion reactor uses something like 1/10th as much fuel as a fission reactor for the same power output. Conversely they use about 10 times as much fuel as an antimatter rocket with the same performance uses antimatter.
With the exception of low thrust high endurance designs with specific impulses up in the millions of seconds, fuel use is tiny compared to reaction mass.

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Antimatter rockets, of course, use the antimatter which is included in their tankage as fuel. That may be a negligibly small mass (except for the pion drives, of course), but it's essentially the entire cost of fuel. So how do antimatter air rams work? They can replace the reaction mass, but obviously that does nothing to stretch the antimatter supply. Maybe they should carry extra antimatter for air ram mode?
Probably. They'd continue to use antimatter at the same rate in air-ram mode as they would in rocket mode, so yeah, counting the antimatter as part of the fuel tank rather than separately doesn't work well here.
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Old 10-25-2009, 04:11 PM   #3
PK
 
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Default Re: [Spaceships] Fuel versus reaction mass

As I understand it (and I am not an expert here!), adding an air-ram mode to an engine means that it heats up and expels air instead of using any of its usual fuel. It takes the same built-in power source which would normally be used to drive its reaction engine and diverts that power over to an element which essentially overheats intaken air.

So, to reference your last example, no antimatter is involved. It doesn't slam antimatter into normal air -- it uses the energy which would normally contain and redirect the M/AM interaction to instead "ram" the air.

At least, that's how I understood it. I tend to just wave my hands at vehicles in a game. :)
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Old 10-25-2009, 05:45 PM   #4
Langy
 
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Default Re: [Spaceships] Fuel versus reaction mass

Your mixing up fuel and reaction mass, Rev. Fuel's the stuff that provides power - antimatter in an AM engine, fissionables in a fission engine, and whatever it is your actually fusing in a fusion engine. Reaction mass is the stuff that you expel out of the back of the ship. Air-ram mode replaces the normal reaction mass with air, but it doesn't effect the fuel at all.

The normal fuel is still used to heat the air - it's just heating air instead of hydrogen, water, or whatever.
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Old 10-25-2009, 05:51 PM   #5
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Default Re: [Spaceships] Fuel versus reaction mass

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Originally Posted by Langy View Post
Your mixing up fuel and reaction mass.
It doesn't help that Spaceships refers to "fuel tanks" rather than "propellant tanks". Or that for chemical rockets and torch ships the fuel is the propellant.
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Old 10-25-2009, 05:52 PM   #6
Ulzgoroth
 
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Default Re: [Spaceships] Fuel versus reaction mass

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Originally Posted by malloyd View Post
Probably. They'd continue to use antimatter at the same rate in air-ram mode as they would in rocket mode, so yeah, counting the antimatter as part of the fuel tank rather than separately doesn't work well here.
Well, that's easy enough to deal with. Just say that you can carry extra fuel without any tankage (since antimatter is only around 1 part per million of AM-boosted fuel), but such fuel only can be used in air-ram mode (and that fuel is mandatory for antimatter air rams).
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Originally Posted by Rev. Pee Kitty View Post
So, to reference your last example, no antimatter is involved. It doesn't slam antimatter into normal air -- it uses the energy which would normally contain and redirect the M/AM interaction to instead "ram" the air.
For the antimatter thermal and plasma engines, the M/AM reaction is just used to heat hydrogen or water reaction mass. AM pion is the only engine that uses matter/antimatter directly for propulsion.
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Old 10-25-2009, 06:06 PM   #7
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Default Re: [Spaceships] Fuel versus reaction mass

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Well, that's easy enough to deal with. Just say that you can carry extra fuel without any tankage (since antimatter is only around 1 part per million of AM-boosted fuel), but such fuel only can be used in air-ram mode (and that fuel is mandatory for antimatter air rams).
The problem with this (and all antimatter ship designs) is that antimatter storage is heavy. You need antimatter densities about a thousand times greater than those given in Ultra-Tech for the portable AM storage trap in order to fit the AM required for a single antimatter power plant in 1/10 of the power plant's mass. (Approximately as much as 3.5 milligrams of antimatter per pound of storage at TL10, assuming an AM power plant with 20% AM/M reaction->power efficiency). Antimatter plasma and pion rockets are much, much worse, requiring significantly more antimatter.
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Old 10-25-2009, 07:02 PM   #8
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Default Re: [Spaceships] Fuel versus reaction mass

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Originally Posted by Langy View Post
The problem with this (and all antimatter ship designs) is that antimatter storage is heavy.
This has always been the killer for antimatter power storage designs. We simply don't know a good way to store it where the energy tied up in the storage system isn't comparable to the energy content of the antimatter. And since we have to keep the storage system from tearing itself apart with stuff held together with chemical bonds, the storage system ends up weighing something comparable to an amount of chemical fuel containing hte same energy, which rather defeats the purpose.

I'd note that converting a space drive to an air-ram is fundamentally a poor idea for any drive where the energy supply has significant costs. Any drive has a thrust proportional to mass flow times square root (power consumption). What an air ram amounts to is taking an engine designed to use the least mass (and hence most power) possible and running it where mass is essentially free. In a sense, you are using the energy in the least efficient way your engineers are capable of arranging. If energy is not so cheap that the lifetime cost of the waste is less than the cost associated with keeping a second vehicle designed as an aircraft, or a second engine, it's a losing proposition.
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