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Star drives we have argued about

At 3:35 PM 3/12/96, Kelly Starks x7066 MS 10-39 wrote:
>At 10:42 AM 3/12/96, David Levine wrote:
>>Kelly Starks x7066 MS 10-39 wrote:
>>> Dave, what have you forwarded to him of our recent correspondence?
>>> Kelly
>>Um, nothing.  Instead of forwarding huge loads of
>>mail, I thought a summary might have been better.  But
>>most of this is technically above me...  Anyone want
>>to give it a crack?

Propulsion systems

This section describes the various types of interstellar drive systems we
have considered.

Bussard Interstellar Ramscoop

This is a relativly old idea first proposed by Robert Bussard.  The idea
was that a ship could scoop up interstellar hydrogen and use it for fuel.
Since it wouldn't need to carry any of its fuel, it could accelerate to
high speeds without concern for high fuel to weight ratio's.

Unfortunately, we don't really know what's in interstellar space, but we do
know we are in a very thin part of it due to a recent (by galactic
standards) supernova in our area.  We might need a scoop thousands of
kilometers across to fuel the motors for a decent sized ship (See below).
Even if we could do that (which is highly doubtfull), straight hydrogen is
very hard to fuse, and doesn't fuse as quickly as we might need to run the
ships motors.

All in all we have no real idea on how to make such a ship work; and if we
could get it to work, we'ld find it doesn't work very well in this part of
the galaxy.

Ram Scoop collector

The problem with using a scoop, is their isn't much in interstellar space
to scoop up.  We found papers that proposed1000 km diameter scoops that
only weighed 200 tons.  Assuming your moving at 1/3rd the speed of light
(100,000 kilometers per second) with a scoop area of 1000 km across
(pi*R^2=pi(50,000,000cm)^2 = 7.854E15 cm^2).  You'd be scooping up the mass
in 7.854 E25 cubic centimeters of space.

A big question is the composition of interstellar space.  A classic
assumption is that there is nothing but about 1 atom of hydrogen in a cubic
centimeter of space.  More recently, people guess it might be less than
..054 atoms per cubic centimeter or as many as 10.  Even more recently than
that (say the last few months) it has been proposed that there may be a lot
of long-chain carbon molecules in space.  Perhaps 60-200 atoms / molecules.
These small, dark, heavy molecules might be the missing 90-99% of the mass
of the galaxy (euphemistically called "dark matter").

So far, no one really knows.  This is unfortunate, because the composition
of the interstellar medium makes a hell of a difference in the design of a
RAIR-based starship.  Since we don't know one way or the other, let's
assume one atom per cubic centimeter at a proton mass of 1.673 E-27 Kg.  At
0.333c, using the above design figures, our 1000 km in diameter scoop,
scoops up a ram flow of 131.4 grams per second.  Thats about 345 tons a
month.  Given that the scoop weighs 200 tons (and you really want the mass
at slower speeds) this really isn't very helpfull.  So this stardrive goes
into the impractical bin.

Multi-cycle Ram Augmented Interstellar Ramjet (RAIR)
Spring 1995

A drive idea I came up with, and origionally used as the assumed drive
system for Explorer class starship design, was a multi-cycle Ram Augmented
Interstellar Ramjet (RAIR).  It would scoop up reaction mass from
interstellar space like a pure ram scoop, but it would only use it as
reaction mass, not fuel.  It would accelerate this mass megnetically or
electrostatically using power from onboard fussion reactors.  The scoop
system could simultaniously scoop up fuel thrown ahead of the ship by a
fixed launcher back in our solar system.  So it wouldn't be limited in
speed by the fuel it carried on-board.

If you could load a 1/4th light year track in space with enough fuel to
keep the ship accelerating at 1g, the ship would (after 6 months) exit the
track at half of light speed (0.5c).  The ship could then switch to
accelerating external scooped mass using power suppied by fuel stored
on-board, or (more likely)coast to the target star.  Assuming Alpha
Centauri, in the later case it would coast for about eight years.

At low speed interplanetary trips, the drive would work like a conventional
ion rocket or mass driver.  Stored reaction mass would be fead into the
electromagnetic or electro static accelerator core.  (Unlike nomal thermal
rockets, an ion thruster works more efficiently with heavy atom ions.  So
I'll assume we are storing iron for reaction mass.) Power would come from
fussion reactors runing off stored fuel.  Specific impulse varies depending
on the exaust velocity of the expended mass.

Now for the bad news - slowing down.  We can't pre-load the deceleration
course track with fuel at the target star because it would be virtually
impossible across interstellar distances.  Carrying enough fuel /
reaction-mass to decelerate at the target star would be prohibitive unless
the coast speed was kept low or another breaking force is found.
Getting out of the starsystem and back up to speed would also be a problem.
If we decelerated using stored fuel.  We know we can get back up to speed
by refueling in the target star system.  Slowing down when we get home
would eaither require another magnetic brake, or use of a fuel launcher at

This seems a clumsy and unreliable method, and given that this system is
just a usless as a pure RamScoop (there still isn't enough mass out there
to make it worth bothering to scoop up), this drive wouldn't work eiather.

Internally fueled Fusion Rocket.

A fusion powered rocket could cross interstellar distances, and is a near
term enough technology to be considered likely for the mid 21st century.
Unfortunately the amount of fuel it takes to get such a ship up to a usable
speed (at least 1/5th of light speed is necessary, a 1/3rd or more is
highly desirable.) can be far to much for a ship to carry.  Possibly
weighing hundreds to thousands of times as much as the rest of the ship.

The following table gives a breakdown of the fuel to ship mass ratios given
fusion rockets of various specific impulses.  Specific impulse is a
standard way of measuring the performance of a rocket engine.  For example
a specific impulse of 1,000,000 (which gives an exhaust velocity of
10,000,000m/s); means that the engine gives 1,000,000 pounds of thrust, for
one second, for every pound of fuel consumed.  A specific impulse of
1,000,000 has long been a standard fusion engine performance number.  (For
comparison the best chemical engines have a specific impulse of 455.)
Current designs might exceed 1,500,000, possibly more than 2,000,000.
Which is fortunate since you'ld need a specific impulse of over 2,000,000,
with a 100 to 1 thrust to weight ratio, to be able to use this system to
boost the ship  So we would need to assume that fusion engines are
developed, and advanced quite a bit before we could use them.

For example for a fusion rocket with a specific impulse of 1,000,000.  If
you wanted to use such an engine to accelerate a ship up to 1/6th the speed
of light.  The ship would need to carry 147 times its dry weight in fuel
and reaction mass.  If you want to get to 1/3rd the speed of light, it
would need to carry 22,000 times its weight in fuel!  Obviously no
realistic ship could do this.  Yet if we could build an engine with a
specific impulse of 2,500,000 1/3rd of light speed becomes fairly

Specific impulse
(exaust velocity)
                Speed 50,000,000 m/s (1/6 light speed)
                                Speed 100,000,000 m/s (1/3 light speed)
2,500,000 sec
                7 to 1 mass ratio.
                                55 to 1 mass ratio.
2,000,000 sec
                12 to 1 mass ratio.
                                148 to 1 mass ratio.
1,500,000 sec
                27 to 1 mass ratio.
                                785 to 1 mass ratio.
1,000,000 sec
                147 to 1 mass ratio.
                                22,000 to 1 mass ratio.
500,000 sec
                22,000 to 1 mass ratio.
                                500,000,000 to 1 mass ratio.

Note: a specific impulse of 1,000,000 (A exhaust velocity of 10,000,000m/s)
means that the engine gives 1,000,000 pounds of thrust, for one second, for
every pound of fuel consumed.  This has long been a standard fusion engine
performance number.  (For comparison the best chemical engines have a
specific impulse of 455.) Current designs might exceed 1,500,000.  (See
write ups on Bussards Fusion reactor, and Bussards Plasma rocket) But none
have a specific impulse of 2,000,000 or more, or get a 100 to 1 thrust to
weight ratio.  So

Staged fusion ship

You start with a 1 billion ton fueled ship cluster, driven by10 million
tons of engines and support structure.  Those engines are assumed powerful
enough to push the whole mess with an acceleration rate of 10m/s.  (A star
ship that weighs as much as a thousand loaded super tankers, and is
powerfull enough to out accelerate a Corvette?  Yeah right.)

When you burn off 95% of your weight in fuel.  The ship cluster weighs 50
million tons, 20% of which is a first stage engine/structure that's WAY too
powerful.  You throw the first stage away and start a smaller second stage.
It weighs about 400,000 tons (about as much as 4 aircraft carriers) and
can push the 40,000,000 ton ship cluster at 10m/s.  When you burn that down
to 2,000,000 tons of cluster you throw that away that stage for a 70,000
ton ship with 5-10,000 tons of drive systems.  Which can use the remaining
390,000 tons of fuel to get itself into the system.

Total weight (In tons)
Thruster pack and stage structure.  (In tons)

So to get a 70,000 ton ship (with 5-10,000 tons of drive systems) into the
target system.  You need to launch out of this starsystem a one billion ton
fueled ship.  Even this assumes a 100 to 1 thrust to weight ration for a
fusion drive systems (which is questionable), and once you get where your
going, coming back is out (unless of course you scale the craft up
accordingly).  But it would give us huge fuel ratios for relativistic
flight.  So, in theory, a Multi stage fusion craft could get to the star.
Assuming of course you can find a billion tons of fusion fuel, and a ship
yard in space that can construct a ship the size of an asteroid! Which
means in practice the ship is unbuildable.

Assuming you could build it, how fast could it get?

Assuming the engines had a Specific impulse of a 1,000,000 (Which as I said
means it has an exaust velocity of 10,000,000 meters per secound), the
speed at the point where you burn out the fuel for each stages is:

Delta V per stage.  Stages have 100 to 1 fuel ratio.
Delta V per stage.  Stages have 20 to 1 fuel ratio.
46,000,000 m/s (.15 C)
30,000,000 m/s (.1 C)
92,000,000 m/s (.31 C)
60,000,000 m/s (.2 C)
138,000,000 m/s (.46 C)
90,000,000 m/s (.3 C)
184,000,000 m/s (.6 C)
120,000,000 m/s (.4 C)
230,000,000 m/s (.7 C)
150,000,000 m/s (.5 C)

Also note the diminising returns.  The secound stage doubles your speed.
The third stage increases it by 1/2.  The fourth stage by 1/3rd.  The fifth
stage by 1/4th.  This is due to the low exaust velocity relative to the
speeds your tring to get to.  Obviously an impractical system without much
better engines.

These numbers of course assume the ship has to carry the weight of its
fuel.  Obviously craft normally have to carry their fuel, but their are
some ways around it.

Externally Feed Fusion system.

We don't have to rely on the fuel nature left in interstellar space for us.
A fuel launcher somewhere in our solar system, could throw the fuel out in
front of where the ship is going to fly.  The ship scoops up the fuel as
its going along and uses it to fuel the engines.  This has several
advantages.  The ships engines only need to accelerate the ship itself.
(They don't even have to adjust for changing ship weights.) The fuel is
accelerated up by the launcher, and the ship would only need a fraction of
the fuel it would otherwise.  This means the launcher system (who's power
comes from unaccelerated fuel) takes up a large fraction of the load, and
the ship saves a lot of energy.

Problems are that unless the ship is flying to a starsystem with a
operating fuel launcher.  It can't fly any faster then a speed it can
decelerate from using its onboard fuel reserves.  Also, this only works
when your close enough to the launcher that it can accurately launch the
fuel to you.  Once your out of range, your stuck with the fuel in your

A fuel launcher based system (and the beamed power system listed below) has
the advantage of eliminating the need for the ship to carry the heavy fuel
(and power systems).  Which improves the ships power to weight ratio
significantly.  But the systems are difficult to do, limit range, and don't
seem to help us to slow down.  (See details in Externally Fueled fusion

Beamed power

Beamed power (or fuel launchers) have the advantage of eliminating the need
for the ship to carry the heavy fuel (and power systems).  That improves
the ships power to weight ratio significantly.  But the systems are
difficult to do, limit range, and don't seem to help us to slow down.

Beamed power systems are most effective as microwave sail craft.  But
powered electromagnetic drives, or laser pumped drives are also possible.

	Microwave sail craft.

The idea of a microwave sail is that you hang out a parachute like wire
mesh.  To the microwaves the mesh looks like a smooth reflective mirror.
(Just like in the mesh radar dishes.) The microwaves bounce off the sail,
pushing it, and the attached ship, forward.  Just like a solar (or photon)

This idea has several advantages.  Its efficent.  The ship doesn't need to
carry a complex drive system.  You can even leave the main power system
back home where all of the solar systems infastructure can get at it for
repairs or improvements.  Unfortunatly, it also has several disadvantages.
You have to be able to hit the ship with the beam across interstellar
distences.  Since the ship will be light years away.  You can't aim at it
because your aim will be years out of date.  The system also can't slow the
ship down, since the presure is straight away from our star system.

Robert Forward in his Roche World serries of science fiction novels had a
laser sail craft that got around this problem by having a two part sail.
You drop the outer ring of the sail which, under robot control, tuned
itself into a concave reflective mirror lens.  The mirror would focus the
light back toward the smaller, now reversed, inner sail.  The outer sail,
now free of the weight of the ship would be blown forward at fantastic
accelerations, but the reflected light could decelerate the ship.

This system is simple in a theoretical sence, but impossible in a real
world sence.  The droped reflection mirror would be under tremendous
presure from the power beam from earth.  This would rapidly accelerate it
away from the ship.  Soon it would be so far away that time dialation would
make it impossible for the mirror to aim at the ships retro mirror.

Fluctuations in the beam density would cause the mirror to ripple and dance
like a kleennex in a jetwash.  The mirror systems would need to be
continuously surfing this light preasure to keep the mirror on the beam,
and forcing the mirror back into precise curvature (with a mirror that
could be hundreds to thousands of miles across), and aiming the reflected
beam at the receading ship.  Since light speed limitations meen it can't
see where the ship is, it will have to calculate a probable possition,
based on its best guess of its beam precision and the ships deceleration
mirror efficency.  Errors are inevitable, even ignoring random effects of
errosion and system breakdowns; and any error will mean the mirror is
missing the ship.  Which would doom the crew.

	Microwave powered electromagnetic drives.

Like the microwave sail above, a beam of microwaves is beamed to the ship.
Unlike above the power is collecte and turned into electricity to power the
ships motors.  Also unlike above, this can give reverse thrust.
Unfortunatly it might not be able to generate enough thrust to slow down.
Much to our annoyance we found the sail effect on the collector would
provide as much forward thrust as the engine could give going backwards,
even if we assumed 100% efficency.


As anyone whos ever seen an episode of Star Trek knows.  Anti-matter can be
destroyed to create tremendous amounts of energy.  Pound for pound a
Anti-matter / matter reaction releases over a hundred times as much power
as a fusion reaction.  Unfortunately, though it releases more power; this
power is harder to directly use to power the ship, and it is far more
dangerous to handle.  If we could synthesize the tens of thousands of tons
of antimatter this would take.  It would have the potential of exploding
with a force of hundreds of millions of H-bombs.

We do not have the technology needed to synthesize, store, or ship anti
matter on this scale, and are not likely to get it by 2050.


Kelly Starks                       Internet: kgstar@most.fw.hac.com
Sr. Systems Engineer
Magnavox Electronic Systems Company
(Magnavox URL: http://www.fw.hac.com/external.html)