10-25-2009, 01:50 AM | #1 |
Join Date: Jul 2008
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[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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10-25-2009, 03:08 PM | #2 | |||
Join Date: Jun 2006
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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:
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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. Quote:
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10-25-2009, 03:11 PM | #3 |
Join Date: Aug 2004
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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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10-25-2009, 04:45 PM | #4 |
Join Date: May 2008
Location: CA
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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. |
10-25-2009, 04:51 PM | #5 |
Join Date: May 2005
Location: Oz
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Re: [Spaceships] Fuel versus reaction mass
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10-25-2009, 04:52 PM | #6 | |
Join Date: Jul 2008
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Re: [Spaceships] Fuel versus reaction mass
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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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10-25-2009, 05:06 PM | #7 | |
Join Date: May 2008
Location: CA
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Re: [Spaceships] Fuel versus reaction mass
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10-25-2009, 06:02 PM | #8 | |
Join Date: Jun 2006
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Re: [Spaceships] Fuel versus reaction mass
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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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