RE: fuel chat

Since we're on the subject...

I generally play a very "near-future" version of FT: no FTL drives at
all
and the STL drives have realistic capabilities.  This isn't as as
different
from a "standard" version of FT as you might think; instead of requiring
weeks or months to travel between star systems, it takes weeks or months
to
travel between planetary systems.  At any rate, one of the things I've
had
to do was figure out what the capabilities of real engines are and how
they
could be put into the existing FT system.  This has equal relevance to
star-spanning games as well.  I assume that the engine percentages given
in
FTFB are fixed (we don't, after all, want to change the design system)
but
that the time and distance scales can be varied.  As it turns out,
varying
the time scale from the "standard" 15 minutes doesn't change things
much, so
given that this works well with other parts of the FT/DS/SG system, I
decided to hold this fixed as well.

While I realize that 1 MU=1000 km is the most prevalent choice for
people on
the list, a real ship simply can't maintain that acceleration for very
long.
I assume that a ship needs to carry at least 10 combat turns worth of
fuel/reaction mass and this must be included in the "engine mass" of the
design system (this of course eliminates some promising engines, see the
discussion below).  While I'm sure it's probably happened, I can't think
of
one of my FT games where the battle has gone on longer than 10 turns. 
I'll
put the spreadsheet up my website fairly soon, but until then here are
the
results for several different engines.	I am using the mass and Isp's
from
"Rocket Flight" just because the few I've been able to check
independently
from my sources have matched up and it gives us a common base for
comparison.  If anyone knows where the sources for "Rocket Flight" came
from, I'd love to find them...

   So without further ado...

Turn Length (sec):900	 Combat Turns in Drive: 10   Ship Mass: 600
Ship Thrust: 8

Column headings:
Drive Type-Quantity-Ve (km/s)-Thrust (kN)-Engine Mass (tons)-Burn Rate
(Tons/Turn)-Total Mass (tons)-Max Accel(m/s^2)-Accel/Thrust Pt-km/MU

COMPARATIVE EFFICIENCY
Liquid Chemical 	0.072	    0.33	120.10	0.144	24.02  
240.34	0.2002	0.0250
10.13
Solid Core Fission	3.999	  35.19 195.95	39.99	20.04	240.39 
0.3266	0.0408
16.53
Mass Driver			1.54	  46.20 30.80	231	0.92   
240.24	0.0513	0.0064	2.60
Ion Electric			0.6	  94.20 6	240	0.03   
240.34	0.0100	0.0013	0.51
JxB Electric			2.17	160.58	10.85	238.7	0.13   
240.02	0.0181	0.0023	0.92
Orion Fission Pulse	0.942	  40.51 247.75	188.4	5.19	240.25 
0.4129	0.0516
20.90
Gas Core Fission	6.51	195.30	149.73	195.3	4.49	240.22 
0.2496	0.0312	12.63
MPD				0.156	  48.99 3.12	240.24	0.01   
240.33	0.0052	0.0007	0.26
Orion Fusion Pulse	1.018	  74.31 297.26	203.6	3.66	240.25 
0.4954	0.0619
25.08
D-T Fusion			4.43	  97.46 478.44	44.3	19.57  
240.03	0.7974	0.0997	40.37
H-B Fusion			0.799	783.02	48.74	239.7	0.04   
240.15	0.0812	0.0102	4.11
Pion Annihilation	0.48	3763.2	23.52	240	0.00	240.03 
0.0392	0.0049	1.98
New Drive		2	5000.0	11852	200	4.27	242.67 
19.7533 2.4692	1000.01

I have assumed that each engine is scalable. For example, you wouldn't
normally talk about "0.6 of an ion engine".  This is not too terrible
since
in most cases you can get the same mass by not running the engine at
full
steam.	I have done these calculations using integer numbers of engines,
the
final scales changed somewhat, but not the choice of engine. The last
column
is the desired info.  This is the number of km per MU assuming the
engine
limitations specified by the FTFB design system.  I have used the
smallest,
fastest ship which should be the most restrictive case.   AS you can
see, to
get 1000 km/MU you need a truly phenomenal drive if you want to keep
your
engine and fuel within mass limits (there are other combinations
possible
other than the "New Drive" shown, of course, but they're all pretty
gross).

The best we can manage *today* would seem to be the Orion Fusion Pulse
(detonate hydrogen bombs behind the ship).  This allows 1 MU=25 km.  
The
best near-future drive, not surprisingly, seems to be the D-T Fusion
torch,
which allows 1 MU=40 km.  Incidentally, this sets 1 thrust point equal
to
almost exactly 0.01g which is a convenient round number.  Anti-matter
(pion
annihilation) drives, incidentally are -incredibly- fuel efficient, but
don't have the thrust to match what is seen in the game.

And that brings up an important point.	I'm not saying that the D-T
Fusion
drive, for example, is the -best- engine for space flight.  I'm just
saying
that it is the best one that fits the performance parameters and rules
restrictions we see in the game.  You could change the rules of FT (for
example, by not including fuel in the engine system mass), but then you
don't have the "basic" FT rules anymore.  Remember, my goal is to see
what
scales are necessary to make -FT- realistic, not to design a new game.

I think it's interesting that with 1 MU=25 or 40 km, sensors would have
no
trouble identifying everything on the game board.  This fits nicely with
the
basic rules, as no bogey markers are ever needed.   Sensors and stealth
would be critically important at the strategic level, however, and I am
working on a board game (for which FT can be used as the combat system
if
desired) which uses this fact.

One other note, not relevant to star-spanning games, I use the mass
assigned
to FTL drives in the design system to be extra fuel which is used for
strategic movement, i.e., travel between planets.  Generally this fuel
is
completely burned over the course of a few dozen combat turns and the
ship
then coasts on an intercept orbit to its destination - exactly as it
would
in the real world.  Ships that have more fuel to burn would be able to
make
intercept faster than would that had less.  There's even the possibility
of
using a different engine (such as an ion drive) for strategic movement,
but
FTL drives in FT are so tiny that it turns out you're better off just
using
the mass for extra fuel for your main drive.

Anyway, just something I thought I'd throw out for discussion.

Keith

Keith Watt
University of Maryland Astronomy
kwatt@astro.umd.edu