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Old 07-07-2013, 08:07 PM   #11
Agemegos
 
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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Originally Posted by DaltonS View Post
Actually, the equation is international. Isp is measured in seconds
Isp (specific impulse) is not measured in seconds in the SI, only in US customary units.

I is impulse, measured in Newton-seconds, a.k.a. kg-m-s^-1. "Specific" means "per unit mass", so you divide by kg and get specific impulse (Newton-seconds per kilogram) in metres per second. Or in US Customary units, impulse is in pounds-force seconds (or poundal-seconds) and to get specific impulse you divide by pounds-mass (or slugs). You only get seconds if you divide pounds-force by pounds-mass and get a dimensionless number instead of an acceleration. You can't even do that in the Système International. Specific impulse works out identical to the effective exhaust speed.
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Old 07-07-2013, 09:38 PM   #12
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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Isp (specific impulse) is not measured in seconds in the SI, only in US customary units.
If it's labeled Isp, it's commonly measured in Seconds even under SI. If instead it's labeled as Ve, it's not measured in Seconds, despite the fact that these two names are measuring exactly the same thing.
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Old 07-07-2013, 10:10 PM   #13
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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If it's labeled Isp, it's commonly measured in Seconds even under SI. If instead it's labeled as Ve, it's not measured in Seconds, despite the fact that these two names are measuring exactly the same thing.
That is bizarre, freakish, and bound to lead to confusion. Also, not what I was taught in my physics courses.
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Old 07-07-2013, 10:21 PM   #14
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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That is bizarre, freakish, and bound to lead to confusion. Also, not what I was taught in my physics courses.
Yeah, well, common usage fairly often uses physically annoying notations. Seconds should really be G-seconds, but it's a useful enough notation and commonly understood.
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Old 07-07-2013, 11:21 PM   #15
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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That is bizarre, freakish, and bound to lead to confusion. Also, not what I was taught in my physics courses.
But of course specific impulse isn't much used in physics. In spaceflight literature it is by and large quoted in "seconds" even in modern papers where everything is in metric units. In aeronautical engineering, it's almost always quoted in Ns/kg - and never reduced to meters/second, because in fact nothing is moving at anything like that "velocity", which could conceivably exceed the speed of light for a big enough rotor and sufficiently efficient engine.
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Old 07-08-2013, 12:18 AM   #16
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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The classic rocket equation as derived by Tsiolkovsky and used by everyone who is not stuck in a unit system in which "pounds" might be mass or might be force, is:
You have no idea how fun explaining this to a classroom full of engineering students was.
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Old 07-08-2013, 02:01 AM   #17
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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You have no idea how fun explaining this to a classroom full of engineering students was.
I used to be an engineering student, once.
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Old 07-08-2013, 02:05 AM   #18
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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nothing is moving at anything like that "velocity"
Surely, by conservation of momentum, the exhaust must be doing so relative to the outlet.

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which could conceivably exceed the speed of light for a big enough rotor and sufficiently efficient engine.
I'm going to have to call for a citation on that one, because it will completely overturn my understanding of the limits of rocketry.
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Old 07-08-2013, 07:22 AM   #19
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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Generally, tanks, pumps and engines of the same size would be more likely to have the same volume flow rate rather than mass flow rate, which would mean thrust is proportional to density x Isp, - this matters quite a bit because pretty much everything is denser than hydrox, and therefore gets better thrust despite lower Isps - and endurance is unchanged. I crunched through a lot of numbers for GURPS Vehicles at one point - http://www.gurpsnet.org/Archive/Vehi...struction/V2ad for chapters 2 and 7.
Which explains why in Spaceships all the alternate reaction masses mentioned divide the delta-V by the thrust multiplier. The fuel tanks are constrained by mass, not volume, so fuel density is not the issue. (Not too realistic, I know.)
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Old 07-08-2013, 09:50 AM   #20
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Default Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel

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Surely, by conservation of momentum, the exhaust must be doing so relative to the outlet.
No. That's true if the thrust is provided by reaction to exhausting the fuel, but that's not actually the definition of specific impulse, which is the thrust of the engine divided by the weight of the fuel it burns. If you move a very large volume of air very slowly you can get lots of thrust for very little energy (and hence fuel) use. Turbofan specific impulses can break 100,000 Ns/kg, helicopter rotors routinely reach millions. It's more common to quote these as the reciprocal ("thrust specific fuel consumption") and multiply by 3600 to get them in per hour but its the same thing.
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