Re: fuel chat

[Nyrath the nearly wise <nyrath@xxxxxxxxx>]

Thomas Anderson wrote:
> force is rate of change of momentum, which is exhaust velocity times rate
> of flow of mass. acceleration of a ship is given by the exhaust velocity
> times the flow rate divided by the ship's mass. thus, if you multiply
> specific impulse by flow rate and divide by mass, you get the ship's
> acceleration in multiples of g. clever.

	Yep. You got it. 

	If:

	F    = Thrust (Newtons or kg mt/sec)
	Ve   = exhaust velocity ( mt/sec )
	Isp  = specific impulse (sec)
	Mdot = propellant mass flow ( kg/sec )
	A    = acceleration ( m/sec^2 )
	Mc   = ship's "current" mass ( kg )

	Then: 

	F = Mdot * Ve
	F = Mdot * g * Isp
	A = F / Mc


> > Metastable Atoms (e.g. Helium)                3,150
> 
> eh? (a) how is helium metastable? (b) how does a helium-fuelled rocket
> work?

	Atoms can be "excited" by thrusting their electrons into
	higher orbitals.  Such electrons quickly fall back into
	their base state, re-emitting the energy.
	Metastable atoms have their electrons excited in such
	a way that they stay in the higher orbitals.
	For a while, at least.
 
> > NERVA/DUMBO "Atomic Rocket"   Isp = 850 to 2950 seconds
> > GASEOUS CORE NUCLEAR REACTOR  Isp = 3570 to 7000 seconds
> > ORION "old Bang-bang"         Isp = 1000 to 5000 seconds
> > DAEDALUS (Fusion Microexplosions)Isp = 1e6
> 
> these are the options we have run over, i think. i have read of GCNRs. i
> thought they were of similar efficiency to scnr (nerva type), but
> apparently they are much, much better. better, in fact, than orion. 

	Yes, but Nerva rockets at least have the decency to
	keep their fuel elements of glowing radioactive death
	sealed in the reactor.  GCNR are much like slow
	reacting Orions, spraying vaporized plutonium
	like a radioactive crop duster.

	This wouldn't matter much in space, but 
	it would be unwise to use one to lift off into
	orbit.  At least not from a spaceport based
	on a continent you like.

> > ARCJET (Electrothermal)               Isp = 800 to 1200 seconds
> > MPD (Electromagnetic)         Isp = 2000 to 5000 seconds
> > ION (Electrostatic)           Isp = 5e3 to 4e5 seconds
> 
> so the ion drive has a higher Isp than Orion; i suppose it would have to.
> the thing is that it is very hard to have a high mass-flow rate (the other
> factor in determining thrust) with an ion drive, so an orion will give you
> a higher, less efficient thrust.

	Correctomundo. Once you figure in the ion drive's mass
	flow, the thrust can be measured in humming-bird power.
 
> > BORON FUSION                  Isp = 1e6
> > HYPOTHETICAL FUSION TORCH     Isp = 5e4 to 1e6 seconds
> > ANTIMATTER                    Isp about 3.06e7 seconds
> > PHOTON DRIVE                  Isp about 3.06e7 seconds
> 
> hmm, now this is getting a bit blue-sky. ultraviolet-sky? i've never heard
> of boron fusion before; sounds like a bit of a daft idea to me.

	No, actually boron fusion would be marvelous, if we could
	only figure out how to ignite the reaction.
	Here's the relevant section from rec.arts.sf.science:

nyrath@clark.net (Winchell Chung)
BORON FUSION
11             4 
  B5 + p -> 3(  He  ) + 16Mev
   5              2 
	that is, bombard Boron-11 with protons.  A complicated reaction ends 
with helium and no pesky nuclear particles. 16 million electron volts gives 
an exhaust velocity of better than 10,000 km/sec, which translates into a 
theoretical specific impulse of something over a million seconds.
	What's the catch?

schillin@spock.usc.edu (John Schilling)
	The catch is, you have to arrange for the protons to impact with 
300 keV of energy, and even then the reaction cross section is fairly 
small.  Shoot a 300 keV proton beam through a cloud of boron plasma, and 
most of the protons will just shoot right through.  300 keV proton beam 
against solid boron, and most will be stopped by successive collisions 
without reacting.  Either way, you won't likely get enough energy from the 
few which fuse to pay for accelerating all the ones which didn't.
	Now, a dense p-B plasma at a temperature of 300 keV is another  
matter.  With everything bouncing around at about the right energy, sooner 
or later everything will fuse.  But containing such a dense, hot plasma 
for any reasonable length of time, is well beyond the current state of 
the art.  We're still working on 25 keV plasmas for D-T fusion.
	If you could make it work with reasonable efficiency, you'd get 
on the order of ten gigawatt-hours of usable power per kilogram of fuel.

Paul Dietz <paul@interaccess.com>
	Unfortunately, this discussion ignores side reactions:
            11      12
	p +   B  ->   C + gamma
               5

        4     11      14
	 He +   B  ->   N + n
                 5    
	The first is quite a bit less likely than the ->3( 4He2 )  
reaction, but the photon is very energetic and penetrating.  The second 
reaction there occurs with secondary alpha particles before they are 
thermalized.

> excellent stuff! added to my collection of reference works cribbed from
> Nyrath ... shall i webbify this (and star-list?) or are you on the case?

	Go ahead and webbify it, it will be a while before I get 
	around to it.