drew
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Space ships, and how they work
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Jul 23 21:15 UTC 1995 |
"Beam me up, Scottie!"
"Lock Phasers on target."
"Ahead Warp factor two."
"Eye Eye Captain"
Nothing has so thoroughly captured my imagination as the concept of
the vehicle-that-can-leave-the-world. I have watched many TV series and
movies featuring these wonderous machines, from the relatively decent
to the downright cheesy, and eventually took up the task of learning
their workings. Viewed in the light of the new knowledge, these shows,
including that staple of space fiction _Star Trek_, were found wanting.
It seems that the writers of TV shows don't often bother to learn about the
subject on which they're writing. Or, as has once been commented to me,
"*somebody's* warped!"
This item will deal with a few of the major beefs I have with TV
space shows, and how they should be rewritten.
Scale and speed:
Space is *huge*. Nearby planets are hundreds of millions of miles
apart, and the Outer planets are in the billions of miles distant. This
is between 10,000 and 100,000 times the diameter of Earth. Covering these
distances, and for that matter even getting out of a gravity well, involves
speeds measured in miles per second. What this means is that spacecraft,
particularly in combat with one another, will spend most of their time too
far apart to track each other visually.
Consider two StarDestroyers measuring about a mile in length, in orbit
around Earth moving in opposite directions; and suppose that they are
oriented so as to expose the maximum of surface area to each other. Since
Earth low orbital speed is about 5 miles a second, the two ships will
pass each other at 10 miles a second. Human visual resolution is around
35 seconds of arc; so, providing that the ships are well illuminated, they
should be just barely visible to each other when about 6000 miles apart -
10 minutes travel time relative to each other. Seeing enough detail to
tell that it's a StarDestroyer will require them to be much closer together,
about 600 miles (1 minute of travel time) or possibly even less. A twenty
foot long fighter craft will be *much* less visible. You're not going to
be able to see it *at all* until it's 22 miles away, and about 2 seconds
from impact.
Thus in most interactions between ships, especially in deep space,
you shouldn't be able to see *squat* just by looking out a window.
Communication:
The scale of distances will even have an effect on communications. The
presence of instantanious communications in every sci-fi series has been a
continuing disappointment; a radio message will take a very noticeable
amount of time to get to its recipient. Communications can be near normal
as far as the Moon. However, communicating with Mars will involve several
*minutes* of waiting for each reply. And while M-net's MSEN connection has
been known to operate like this, it will make communication very different
from how it's usually shown.
In fact, *any* information that a ship receives is going to be out of
date in proportion to the distance it has traveled. This includes radar
and visual sightings, and it will have a definite effect in combat. At best,
you would be seeing where the target was when the light or radar reflection
left it. By the time the information arrives, the target will have had plenty
of time to move around; not to mention the time it will take for your shot
to get there.
Out further than 100,000 or 200,000 miles, a laser beam, or any kind of
attack which cannot change course on the way, is going to be ineffective. The
only weapon which will have a chance is a guided missile.
Motion:
In space, everything moves and keeps moving. If it didn't, everything
would immediately collapse into a gigantic black hole. Ships move, and
getting somewhere involves changing the motion. The known modes of ship
movement are basically two types: ballistic and accelerative. In the former,
the engines are fired for some nominal amount of delta-V, and the ship
coasts to its destination, where the ship is turned and the engines fired
in the opposite direction (or else reverse-mounted engines may be used) to
match speeds with the destination. The latter type involves running the
engines constantly, turning the ship around at the midpoint of the trip.
Whereas a car, plane, or watercraft has a maximum speed and range, a
spacecraft has a maximum *acceleration*, generally expressed in gravities;
and a maximum *delta-V*, which is the total amount of speed change that
full fuel tanks will allow.
_Babylon 5_, and the _2001_ movies, have this part right. All the
others seem to treat spacecraft as air or ground vehicles, slowing to a
stop whenever the power is shut off.
Fuel:
The _Enterprise_ never seems to need refueling; the "dilithium crystals"
somehow being able to recharge themselves. Several fist-sized glass cubes
provide a seemingly infinite amount of energy. The ships in the _Star Wars_
movies *never* refuelled, and nowhere were fuel requirements even hinted at.
In fact, even with total conversion of matter to energy, starships will
likely need frequent refueling, probably at every stop. Even changing vectors
within a solar system will take measurable amounts, let alone boosting to
relativistic speeds. Expect mass ratios much greater than unity.
Fortunately, exotic and expensive materials like "dilithium crystals"
probably won't be necessary. The most likely fuel will be an isotope of the
most common substance in the universe - hydrogen. Deuterium is available
anywhere there's water, and would also be present in the atmospheres of gas
giant planets like Jupiter and Saturn.
Ordinary hydrogen is even more plentiful, and can be used in a fusion
engine if a few catalysts are added to the reaction. Since hydrogen atoms
occur in small quantities throughout interstellar space, a catalysed fusion
engine can be fueled en route at low relativistic speeds (0.1C to 0.8C) if
a magnetic field can divert protons from far enough away from the ship. Such
a collector could also double as a screen to protect the ship from the
incoming proton radiation.
Combat:
My major complaint here is with the depiction of "lasers" whose output
is visible from the side, and in many cases is noticeably slower than light -
in fact, slower even than a bullet. While a powerful enough laser can be
useful against ships or missiles, all that will be visible is a burning spot
of light on the target, especially in a vacuum.
A couple of other things are either ignored, or else when used, do far
less damage than they should. They are collisions and engine exhaust.
A quick calculation will show that a moderate sized ship which crashes
into a starport at orbital speed will have enough kinetic energy to obliterate
the starport. If the collision is at a significant fraction of lightspeed,
very little mass will do a great deal more damage.
Drive exhaust is similarly deadly. Any engine efficient enough to be useful
for interstellar travel is going to have a lot of power per unit thrust.
Gravity:
There are times when there *should* be gravity inside a spacecraft.
Whenever the engines are firing, for example, there will be what seems
to be a gravity field, with "down" toward where the engine exhaust is being
directed. For ships that spend most of their travel time accelerating, the
most sensible design would resemble a skyscraper rather than an ocean liner.
However, when the engines are off, everybody floats. This, combined
with some health problems caused by long term weightlessness, is why
ships and installations which spend long periods in free fall should be
built to spin, like the Babylon 5 space station.
_B5_ deserves credit for putting a new spin on space drama, but still
falls a little short, in that gravity seems to be Earth-normal regardless
of where in the station people are. In reality, gravity would vary from
zero at the axis, to maximum at the outer hull, with a little Coriolis
twist thrown in to make things interesting. Of course, if you're inside
the space station and you look out a window, the entire universe would
appear to be spinning.
I am informed that the maximum recommended rate of spin to avoid vertigo
problems is 1 RPM. This means that a space station or ship would need to
be 894 meters in radius to provide a full gravity. At 3 RPM, the previous
recommended rate, a full gravity could be produced with a 99 meter radius.
Thus, depending on the size of the ship, a full gravity may not be available.
When it comes to keeping bones in shape, however, a fraction of a gravity
is still better than none at all.
The usual practice of having artificial non-spinning gravity perpendicular
to thrust, and more or less constant regardless of what the ship is doing,
seems rather cheesy and suggests a lack of preparation on the writers'
part. It may be that, if artificial gravity generators are invented, ships
will be designed in such a matter. Even so, there *will* be circumstances
in which varying rates and directions of gravity are desired. Zero-G would
be ideal for moving heavy cargo, for example. Also, the gravity should be
just as likely to go out in a fight as any other system, since it's not
being produced by spin. (Speaking of which, if a ship is disabled while in
pitch or yaw, new gravity fields could show up for which the interior wasn't
designed. Smart naval architects will make sure that there are handholds
and ladder rungs everywhere.)
Differing gravity strengths will have an effect on physical activities
such as running and walking.
Time dilation:
Speed-induced time distortion has been known about since the 19th Century.
The math is, for the most part, rather simple, and anyone who is the least
bit serious about interstellar travel is going to hear about it. Yet it is
completely absent in every TV show. In fact, in at least one episode of _Star
Trek_, the presence of a person who is physically young but chronologically
old takes everyone by surprise. In an interstellar community, this sort of
thing should be commonplace.
Even with faster than light travel, time is very likely to be different
for everybody; and while a ship's computer will probably be able to calculate
the time and date based on navigational data, clocks are *always* going to need
adjusting.
Causality:
Related to the time dilation effect is the issue of "when" events occur.
Again, the notions of "now", "before", and "after" are going to have different
meanings for everybody, which is why the idea of "instantanious" communication
is so ridiculous. A faster-than-light trip is *always* going to appear as
a backward-in-time trip according to *someone's* sense of "now".
I had high hopes that _Babylon 5_ would do something about the causality
problem when I saw the jump gate in operation. Unfortunately, this show turned
out to be as oblivious as everyone else about it. Ironically, while everyone
else seems ignorant of causality, the makers of the original _Trek_ series
actually used it in three of their plots! Where _Trek_ is lacking is in
explaining why no private FTL ship owners have tried this stunt.
Causality is a tough nut to crack, and sci-fi writers can perhaps be
forgiven for ignoring it. Niven's _Known Space_ novels don't even address the
issue. However, causality can be dealt with by the introduction of a few
simple rules:
* Establishing a faster-than-light path between two points a distance x
apart must take time at least equal to x/c.
* Once established, travel through the new route can be arbitrarily fast,
even "instantanious".
* Changing the reference frame of the jump must needs involve re-establishing
the route, which again will take time of at least x/c.
* Establishing a jump point becomes more difficult the closer to a previously
established point and the faster relative to that point this is attempted.
What you have as a result is a region of explored/colonized space through
which traffic flows quickly, but which can expand only at lightspeed. This
might eventually meet up with another species' similarly expanding bubble,
which can result in anything from a decades-long border war to profitable
trade. Critical to holding causality in check is preventing ships from
instantly and arbitrarily choosing the reference frame of their jumps.
A variant of this method would be to have the jump routes be naturally
occurring, possibly by means of black holes, as in Haldeman's _Forever War_.
Causality should not be a problem here, since changing the reference frame of
a jump would likely require accelerating one of the black holes, which of
course would have the mass of a large star.
Worlds:
Most of the action in space shows seems to take place in breathable air,
mostly around room temperature. In reality, hostile atmospheres will certainly
far outnumber breathable ones, and even many oxygen-rich planets will have
atmospheric taint. More of the action should take place on airless worlds if
for no other reason than because they are there.
Breathable air is not the only valuable resource, though its value alone
would be sufficient reason to put factories elsewhere. Metals and minerals
would be as likely to be on unliveable worlds as on liveable ones. Corrosive
attmospheres might also be useful for certain industrial processes.
Gravity will vary from near zero on asteroids to above normal on large,
dense planets. People will endure a bit more weight if there's something in
it for them or if the place is otherwise pleasant.
APPENDIX:
Typical distances
Distance Travel time Radio response
Place from Earth at 1 gravity time
----------------------------------------------------------------------------
Moon 240,000 miles 3.5 hours 2.6 seconds
Sun 93 million miles 2.85 days 16 min 40 sec
Pluto 3 billion miles 16.2 days 9 hours
Alpha Centauri 4.3 light years 3.56 years ship 8.6 years
5.93 years global
Sirius 8 LY 4.5 years ship 16 years
9.75 years global
Vega 28 LY 6.65 years ship 56 years
29.9 years global
Rigel 800 LY 13 years ship 1600 years
802 years global
Galactic core 30000 LY 20 years ship 60000 years
30002 years global
Engine performance:
Engine Oxidizer/ Spec Imp 1G Boost time for mass ratio of
type Fuel propellant (sec) 1.5 2 5 10
------------------------------------------------------------------------------
Solid rocket (typical) 200 1.4 min 2.3 min 5.4 min 7.7 min
Liquid rockets
Hydrogen Oxygen 460 3.1 min 5.3 min 12.3 min 17.7 min
Hydrogen Ozone 607 4.1 min 7.0 min 16.3 min 23.3 min
Hydrides O2 or F2 700 4.7 min 8.1 min 18.8 min 26.9 min
H+ free radicals 2130 14.4 min 24.6 min 57.1 min 81.7 min
Metastable atoms 3150 0.4 hr 0.6 hr 1.4 hr 2.0 hr
Fission U-235 Hydrogen (avg)
Solid core 800 5.4 min 9.2 min 21.5 min 30.7 min
Liquid core 1450 9.8 min 16.8 min 38.9 min 55.6 min
Gas core 5000 0.6 hr 1.0 hr 2.2 hr 3.2 hr
Ion (*) Various Ionizates 200000 0.9 days 1.6 days 3.7 days 5.3 days
Fusion Hydrogen isotopes 3750000 2.5 wks 4.3 wks 10.0 wks 14.3 wks
Total con Anti- Matter
-version matter (**) 30E+6 4.6 mon 7.9 mon 18.4 mon 26.3 mon
Engine 1G Boost time for mass ratio of
type 20 50 100 1000
------------------------------------------------------------------------
Solid rocket 10.0 min 13.0 min 15.4 min 23.0 min
Liquid rockets
Hydrogen Oxygen 23.0 min 30.0 min 35.3 min 53.0 min
Hydrogen Ozone 30.3 min 39.6 min 46.6 min 69.9 min
Hydrides O2 or F2 35.0 min 45.6 min 53.7 min 80.6 min
H+ free radicals 1.8 hr 2.3 hr 2.7 hr 4.1 hr
Metastable atoms 2.6 hr 3.4 hr 4.0 hr 6.0 hr
Fission
Solid core 39.9 min 52.2 min 61.4 min 92.1 min
Liquid core 1.2 hr 1.6 hr 1.9 hr 2.8 hr
Gas core 4.2 hr 5.4 hr 6.4 hr 9.6 hr
Ion (*) 1.0 wks 1.3 wks 1.5 wks 2.3 wks
Fusion 4.3 mon 5.6 mon 6.6 mon 9.8 mon
Total con
-version 2.8 yrs 3.7 yrs 4.4 yrs 6.6 yrs
* Power is most likely from a nuclear generator. Typical thrust/mass
ratios are much less than 1 lbf/lb.
** or artificial quantum singularity
References:
Heppenheimer, T.A. _The Man Made Sun: the quest for fusion power_
Boston: Little, Brown c1984.
Mallove, Eugene F.; Matloff, Gregory L. _The Starflight Handbook: a pioneer's
guide to interstellar travel_
New York: Wiley, c1989.
Niven, Larry, ed. _The Man-Kzin Wars_ Volumes 1-5
Baen Publishing Enterprises, New York, NY, 1988-92.
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