Subject:
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Rocket science, & some MOC ideas (Was: Combat strategies and tactics in space)
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Newsgroups:
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lugnet.space
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Date:
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Sun, 24 Oct 2004 13:45:32 GMT
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Viewed:
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1518 times
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(OK folks, last post, and I'll try to push it on-topic :-) And there are MOC
ideas down there near the bottom, honest)
In lugnet.space, Mike Petrucelli wrote:
> Ok, then you would be most qualified to clear this
> up (if you're willing anyway)
Ha! OK, I'll open my mouth really big to get both feet in at once, but...
here it goes.
> Assuming ships have the insanely high fuel efficiency
> seen in most sci-fi,...
Here's the heart of the problem: are you talking high *thrust* engines, that
can, say, accomplish a rapid delta-v (change in velocity), or high specific
impulse engines (that can accomplish a given delta-v using a minimum amount of
reaction mass). One reasonable way to talk about this in a LEGO world is what
fraction of the ship design is weapons/crew/structure, vs. what fraction is pure
fuel, vs. what fraction is engine
> wouldn't smaller ships (fighters) be more effective
> in actual combat senarios. Sure the capital ships would
> have better straight line acceleration but that would
> generally constitute running away. Wouldn't the lower
> mass of a fighter make it far more maneverable compared
> to a large ship.
Acceleration is acceleration. There is no difference between "straight line
acceleration" and "maneveablity". If I double the mass of the ship, and the
engines are a constant mass fraction of the ships mass, then I have twice the
engines and twice the thrust, resulting in the exact same acceleration for both
large and small ships. Any manuevering would be done by turning the ship using
offset smaller thrusters, (again, more massive ships could incorperate
larger/more thrusters, so the scaled linear acceleration should be the same) but
the angular acceleration depends on the moment of inertia, which scaled as the
mass as well as the linear size *squared*. It's that last that gives you a hint
of "smaller ships are more manueverable" - it implies scaling up ships size will
make for a slower rotating ship. The problem is that most of the time spent in a
direction change has nothing to do with the time it takes to rotate, but the
time it takes to use "main thrust" to alter your linear velocity. In short,
again, large & small ships should take very similar periods of time to "make the
same manuever" (like changing their velocity vector 90 degrees).
> Also wouldn't maneverability be more important
> in a combat situation that straigt line accelertion,
> unless you are trying to run away.
That depends on how combat is done. Consider that even with modern planes,
with closing velocities of Mach 2, more often it's target detection that counts
than manueverability.
> So what is your opinion on this?
As far as LEGO goes (pulling us back on-topic), I've never seen a huge point
in making such craft "realistic", because realistic is a whole lot of fuel and
very very little ship. The total delta-V availible is:
delta-v = Isp g ln( m_start / m_finish )
Isp = specific impulse of engine (depends on fuel and power
supplied, in a sense, *not* the # of engines!). Measured in
"seconds", best current is around 460 [s], while fission comes
in around 3000-7000 (gaseous core, using hydrogen as reaction
mass), and fusion tops out around 200,000 (but with a *very*
low thrust/mass ratio around 0.0001, implying top accelerations
of tiny fractions of a G)
g = acceleration of gravity on earth, 9.8 [m/s]
m_start & m_finish = inital and final masses for the ship; the
difference is fuel mass, and so provides a measure of how much
of your craft ends up being LEGO-uninteresting fuel tanks.
To put this another way, the reaction mass required for a certain total delta-v
is:
mass_fuel = ship_dry_mass e^(delta-v / (Isp g))
Take the highest Isp (specific impulse) engines we've got ("Isp" is
essentially a measure of exhaust velocity - the higher the better, as far as
fuel efficiency goes) around 530 seconds (H2/O2 SSME's on the space shuttle
orbiter are at 460, while the theoretical max for H2/O2 is 528), allowing *any*
ship to have a total delta-v (total velocity change it can manage) in the 10 kps
(kilometer per second) range implies that fuel outmasses ship by almost 7 to 1!
Since both H2 & O2 are significantly less dense than the ship structure, they
take up even more interesting LEGO space than that outrageous mass fraction
implies, probably at least 2-3 times that in voume.
Conservatively (with current tech) for every 1 2x4 brick of LEGO
engines/weapons/crew, I'd need 17 2x4's of reaction mass / fuel. This does *NOT*
make for an interesting LEGO spaceship (not to mention, using up all my bricks
in "uninteresting" structure!).
Better tech helps, but not a lot. The best Isp I've got handy is for gaseous
core fission engines using H2 as reaction mass (Isp = 3000-7000) or fusion (with
criminally low thrust/mass ratios, limiting maneuvers to micro-G's; Isp =
200,000). Assuming an Isp around 10,000, for a 10 kps delta-v capability it
still implies that for every 10 kg of ship mass I'd need a little over 1 kg of
reaction mass, and (to get those ludicrously high Isp's) it would have to be
pure H2, with a correspondingly low densities; assuming the reaction mass is
1/3rd the density of the weapons/crew/engines, it implies my "semirealistic"
LEGO model have 3 2x4's of reaction mass tankage for every 10 2x4's of
"interesting" stuff. Still, I'm using a *LOT* of bricks just to make the tankage
realistic.
Final kicker - how much delta-v do you need? Well, to change your orbital
plain by 90 degrees just *once* in low Earth orbit (LEO) is about 11 kps. I
other words this massive, oversized (LEGO-uninteresting) ship could turn 90
degrees just *ONCE* in the way we ussually think of dogfights. And note again,
this makes no difference if you are small or large. Fighters have no advantage
here.
If you want to say, "but wait, I have gravity drives, or inertial dampeners,
or warp engines, or [insert technobabble here]", great, then have fun. Since you
are making it up, you can assume anything you want, and try to be
self-consistant... but you're likely to run afoul of contradictions very
quickly. Artificial gravity is a good example, but it's needed to make anything
simply realistic (i.e.- spaceship with a floor), so go with it. Just don't try
to defend it on technical grounds.
Now, for the actual on-topic part: why *NOT* make an interesting space MOC
that follows the laws of physics. Daedalus (an unmanned mission to Barnard's
Star, proposed in the 70's using microfission electron-beam imploaded fuel
pellets, two stages, with a very complex self-maintaining payload; top speed
around 10% c, but no decceleration capability at the target star), A minifig
scale Saturn V (LEGO has been nice enough to give us the LEM already; anybody
built a CSM to go with it?), a Valkyrie antimatter starship (Powel & Pellegrino;
crew model towed behind (yes, in the highly radioactive wake) of an antimatter
engine, and yes, that *is* a good way to do it), Cassini, Voyager (is the scale
set by the main dish, or can you work outside that to make it big enough to put
a lot of detail on it), and asteroid miner (done right; there's lots of
literature on it), Robert Forward's laser-propelled one-way manned mission to
Barnard's Star (the book was originally "Flight of the Dragonfly", later
re-released under the title "Rocheworld": it contains detailed appendixes with
measurements, power requirements, acceleration profiles, etc.). SPS's (solar
power satellites in high Earth orbit, using low mass truss systems that could be
well-modeled by the bridge truss components, for instance). A scale model of at
least the Mars landing and maybe transit stages of Zubrin's baseline Mars
mission (Mars Direct; plenty of options here, as Zubrin's plan has mulitple
slightly different modules, including surface stations landed before the
"minifiganauts" get there).
When I was a kid (about 5), I built an accurate (well, as accurate as I could
at 5) model of a two stage launch vehicle for a solar-powered satellite - the
bulk of the model (square, as I had no idea how to make a cylinder at the time)
was two huge empty stages, with a payload fairing on top that hinged out to
release the satellite. And while that may sound uninteresting to many of us, I
thought it was wonderful, because it was *real* - I could *really* play with a
rocket all my own, and make it do all the real rocket things, like inertial
motion. To some that will seem dull & boring - it certainly lacks weapons, space
combat, and the "space opera" aspect. But I loved it.
To finish up a too long, not enough on-topic post (sorry, I'm stopping now),
I *love* the stuff in .space, but arguing pro & con based on reality requires,
well, using reality.
--
Brian Davis
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