Fast interstellar propulsion in near term
IsaacKuo
capable of fast interstellar propulsion. Beam propulsion
requires upscaling lasers or particle beams far beyond the
state of the art. Fission rocketry lacks sufficient Isp for
fast interstellar velocities. Efficient fusion rocketry
requires new technology and/or science to work. Anti-matter
rocketry may require even greater technology/science advances.
I reject the conventional wisdom! Using modern fission
technology and near term magsail technology, I believe a
fast interstellar probe is acheivable. This relies on the
concept of "Particle Puff Propulsion", which may be
scaled up for a manned mission--even a return mission.
Here's a basic description:
-----------------------------------------------------------
A 25%c Probe in Our Lifetime using Particle Puff Propulsion
by Isaac Kuo
The principle of particle beam propulsion to accelerate a
magsail starship to high speeds is well known, but it
traditionally requires powerful long range particle beam
emitter technology.
Instead, I propose using powerful short range fission powered
beam emitters, acheivable with near term technology. These
disposable emitters are laid out in a long line along the
acceleration path of the starship.
Particle Puff Emitter:
_________________
/ |
/ __ /|SSSSSSSSS|
/ / \ / |TTTTTTTTT|
( ( )( | : <- hydrogen propellant
\ \__/ \ |TTTTTTTTT| <- fission fuel
\ \|SSSSSSSSS| <- fusion fuel
\_________________|
The particle puff emitter looks like a nuclear thermal rocket.
The "reactor" in the rear heats up hydrogen gas, which escapes
out the nozzle. Unlike a normal rocket, we only care about
the exhaust gas. The rest of the rocket vaporizes upon use.
It's a nuclear bomb.
The bomb is a basic Teller-Ulam fission-fusion-fission
design, with some hydrogen "propellant" inside the last stage.
Upon detonation, the final fission stage efficiently reacts,
creating a dense imploding plasma with 84MeV per fission
fragment. This heats the hydrogen propellant, which escapes
as a puff of gas out the rear nozzle at speeds up to 40%c.
Assuming a 2:1 ratio of fission fragments to hydrogen and
perfect thermal transfer, the hydrogen is given 56MeV of energy
for a puff velocity of around 33%c. This corresponds to a mass
ratio of 236:1. (This mass ratio only considers the mass of the
final stage plus hydrogen; there's also the overhead of the rest
of the bomb and stationkeeping rockets.)
Uranium:H Mass | Puff Velocity
---------------+--------------
236:1 | 33%c
120:1 | 29%c
30:1 | 20%c
14:1 | 13%c
By adjusting the amount of hydrogen in the "reactor", it's
possible to trade-off between mass ratio and puff velocity.
It's worth optimizing the puff velocity along the bombtrack,
starting with lower puff velocities where the starship is slow
and gradually increasing the puff velocity along the bombtrack.
With particle puff velocities of up to 33%c, accelerating a
magsail starship up to 25%c should be doable. This starship
could be a lightweight flyby probe, unburdened by massive
fuel tanks.
This mission can be done with near term technology. The
particle puff unit is similar to existing nuclear bomb designs,
which have been extensively studied and modeled. It would not
take a heavy extra investment to perform the basic initial
design. Underground or space testing of a modest number of
bomb units could provide information about the mass, velocity,
and directionality of the hydrogen puff.
The starship itself can be lightweight and compact, especially
if it uses an M2P2 sail. Either a traditional magsail or an
M2P2 sail needs further R&D and testing in space, but such
sails are potentially very useful for interplanetary missions
so it's worthwhile regardless.
Testing of sail performance can be conducted using plasma
thrusters and a modest number of puff unit tests conducted
at low speed. This is because the stresses on the sail are
greatest during the initial acceleration. As the sail moves
faster and faster, the "push" from each puff unit gets
smaller and smaller.
Assuming the magsail technology has already been developed
for other purposes, there is relatively little extra R&D
required to make this flyby probe a reality. With a mission
time of around 2 decades (including time for return data),
the project duration is reasonable and it's not plausible
that a later mission with improved technology could arrive
faster/sooner.
Isaac Kuo
BDH
magnetic nozzle would work better. Most of the energy being in neutrons
for most fission would work: I like He3 + Li6 propellant impacted by
more propellant from the spacecraft. A solar sail stage can give 40+
km/s; impact fission-fusion bomblets can get the spacecraft going fast
enough for impact fusion bomblets.
This has all been discussed before, and will be discussed much more
with more technology and knowledge to consider well before it would be
implemented.
p.s.
I suggest you pick one group to post these to.
BDH
IsaacKuo
BDH wrote:
> Using tricks to increase particle speed is not necessary. A tapered
> magnetic nozzle would work better. Most of the energy being in neutrons
> for most fission would work:
Neutrons are not affected by a magnetic nozzle. Also, the neutrons
emitted by fission reactions are not very fast.
>I like He3 + Li6 propellant impacted by more propellant from the
>spacecraft.
Cute, but it's not even clear if you can break even that way, much
less propel a spacecraft with it. The propulsion method I propose
uses well proven high yield fission-fusion-fission.
>A solar sail stage can give 40+ km/s;
Too slow! I'm talking about 75,000km/s! That's a speed suitable
for fast interstellar travel. That's a speed which can give you an
interstellar mission spanning only 2 decades.
>impact fission-fusion bomblets can get the spacecraft going fast
>enough for impact fusion bomblets.
Which may work, or it might not even break even. The impact
fusion bomblets might act as a potent brake because drag
outweighs thrust.
Even if it does work, it's not a near term technology. Because
the drive can't work at low velocities, you need to test it with
something close to a full scale test. The Particle Puff Propulsion
I propose works at any speed and can be tested with small
scale tests.
>This has all been discussed before, and will be discussed much more
>with more technology and knowledge to consider well before it would be
>implemented.
With all due respect, this concept has NOT been discussed
before. It's a new concept. And it's possible in the near term.
>p.s.
>I suggest you pick one group to post these to.
Which nearly dead newsgroup do you suggest?
Isaac Kuo
BDH
capture/reflection thrust would be heavy and ineffective. I like
Li6+He3+D because it produces few neutrons. The fusion bomblets would
have no drag. The fission fusion impact bomblets can be tested with
things orbiting the sun in opposite directions. Your "Particle Puff"
concept has not been discussed before because it is ineffective and
unnecessary. Only a small amount of the fission energy would go to the
hydrogen, and the hydrogen would not squirt out in one direction. You
clearly did not understand my post.
IsaacKuo
BDH wrote:
> The initial speed for impact is what the sail is for. Neutron
> capture/reflection thrust would be heavy and ineffective. I like
> Li6+He3+D because it produces few neutrons. The fusion bomblets would
> have no drag.
That's an amazing claim, seeing as the kinetic energy for
an impact has to come from somewhere. If it's not from
the kinetic energy of the starship, then where?
Conceptually, you only have a propulsion drive if the
kinetic energy lost to the collision is overcome by the
kinetic energy gained by the detonation. That's a tall
order, considering the inefficiency of fusion reactions
we have made so far. We can't even acheive break-even
outside of a nuclear bomb. And within a nuclear bomb,
the efficiency of the fusion reactions is so low that they
really aren't much more energetic than fission reactions.
There's a reason why half the energy from an H-Bomb
comes from fission. It's even debatable whether a fusion
stage is even necessary on an atomic weapon.
>The fission fusion impact bomblets can be tested with
>things orbiting the sun in opposite directions.
This is only adequate for testing up to impact velocities
of twice the orbital velocity--maybe on the order of only
60km/s. If you're hoping to acheive interstellar travel
velocities of 30,000km/s or more, this is NOT a good
test. It's many orders of magnitudes too low to
reasonably predict performance of the full scale impact.
In contrast, the "impact" velocity between my proposed
particle puffs and the magsail is always relatively low,
and it get's lower with increasing magsail velocity--not
higher. The stresses on the magsail are at a maximum
at low velocities and get gentler with greater and greater
velocity. Therefore, low speed tests are adequate to
predict stardrive performance.
>Your "Particle Puff" concept has not been discussed
>before because it is ineffective and unnecessary.
You fail to understand the magnitude of the problems
with alternative stardrives, and how this stardrive
concept solves them elegantly.
>Only a small amount of the fission energy would go to the
>hydrogen, and the hydrogen would not squirt out in one
>direction.
Wrong on both counts.
First, the majority of energy in the high yield fission
reaction goes to the fission fragments--out of 200MeV
for a U235 fission reaction, about 168MeV goes to the
two fission fragments. From here, thermal mixing with
the hydrogen transfers energy very efficiently. With
a 1:1 ratio between hydrogen atoms and fission
fragments, about 50% of the energy gets transfered to
the hydrogen. This is a large fraction; I accounted for
the fraction in the chart I gave.
Note that a loss factor of 50% is not a big deal for my
propulsion system; it just means that the particle puff
units need to be twice as powerful. In contrast, a loss
factor of 50% is a severe problem for an impact fusion
pellet ramjet--it halves the amount of thrust per pellet
while keeping the drag the same.
Second, the hydrogen will squirt out in one direction
because it's blocked on all other sides by dense
fission fragments. A tapered "nozzle" at the end can
help direct the hydrogen in one direction. This is
geometrically a throatless nozzle, which is surprisingly
efficient. Unlike a traditional cooled nozzle which
might cool the exhaust, this hot nozzle would consist
of hot fission fragments, and thus would not slow
down the exiting protons at all.
>You clearly did not understand my post.
I understand the fusion pellet ramjet concept better
than you do. I was once a strong proponent of this
type of interstellar propulsion. However, continued
failures to acheive break-even fusion reactions with
magnetic pinch or particle beam techniques have
sucked out my enthusiasm for the concept.
Magnetic pinch techniques correspond to pellet
compression in a magnetic ramscoop. Particle
beams correspond to pellet initiation due to high
speed direct impact. Neither technique is acheiving
break-even status.
That means that even if you assume PERFECT
efficiency of utilizing the energy from the fusion
reaction, you lost more kinetic energy initiating
the fusion reaction than you get out of it.
At the very least, it's going to take great advances
in fusion power technology before fusion pellet
propulsion can be seriously considered.
Isaac Kuo
BDH
This is only adequate for testing up to impact velocities
of twice the orbital velocity--maybe on the order of only
60km/s.
"
Wrong. If you'd looked up the orbital speed of mercury you'd see that
over 100 km/s relative velocity is reasonable.
Your knowledge of nuclear weapons also seems shallow:
"
There's a reason why half the energy from an H-Bomb
comes from fission. It's even debatable whether a fusion
stage is even necessary on an atomic weapon.
"
The fusion neutrons dramatically increase the efficiency of the fission
part even in small weapons, and true 2-stage fusion bombs do be most of
their energy from actual fusion. Learn the basics before designing new
stuff.
The spacecraft gives energy to start the reaction, but once the bomblet
is released it isn't part of the spacecraft any more, so the energy
comes from decreasing the mass of the spacecraft.
"
continued
failures to acheive break-even fusion reactions with
magnetic pinch or particle beam techniques have
sucked out my enthusiasm for the concept.
"
You can't get 50 km/s in space in opposite directions in a lab.
Completely different, in fact irrelevant.
"
With
a 1:1 ratio between hydrogen atoms and fission
fragments, about 50% of the energy gets transfered to
the hydrogen. This is a large fraction; I accounted for
the fraction in the chart I gave.
"
Show it.
"
Second, the hydrogen will squirt out in one direction
because it's blocked on all other sides by dense
fission fragments.
"
Except it isn't. The fragments are in a charge-neutral plasma, and the
hydrogen doesn't just get reflected. At the relevant energies
everything just mixes.
You know what, I don't have time for this. Have fun. I'll send you a
letter of apology when you're famous.
IsaacKuo
BDH wrote:
> "
> This is only adequate for testing up to impact velocities
> of twice the orbital velocity--maybe on the order of only
> 60km/s.
> "
> Wrong. If you'd looked up the orbital speed of mercury you'd see that
> over 100 km/s relative velocity is reasonable.
This depends on where you want to do it, of course, but
in any case the impact velocities are still several orders
of magnitude too small for a reasonable test.
> Your knowledge of nuclear weapons also seems shallow:
> "
> There's a reason why half the energy from an H-Bomb
> comes from fission. It's even debatable whether a fusion
> stage is even necessary on an atomic weapon.
> "
> The fusion neutrons dramatically increase the efficiency of the fission
> part even in small weapons, and true 2-stage fusion bombs do be most of
> their energy from actual fusion. Learn the basics before designing new
> stuff.
The original impetus behind H-bombs was to get big explosions
while minimizing the amount of expensive fissionables. But
the need for super-huge explosions was reasonably questioned,
and fission fuel has become much less expensive.
With the blast radius of a nuke only scaling up with the cube-root
of its energy, you could get more destructive capability out of a
MIRV of smaller warheads than a single humongous warhead.
With the cost of fissionables no longer such a factor, the main
factor for ICBM applications is mass and compactness. For this,
skipping the fusion stage of a fission-fusion-fission design might
actually be beneficial. While fusion is theoretically much more
powerful than fission, the fusion that occurs in H-bombs is
horrificly inefficient--dragging its performance down to be
comparable with fission.
Nevertheless, I'm sticking with a conventional fission-fusion-fission
design for this propulsion concept. The fusion stage is basically
there to provide a shower of neutrons to cause efficient high yield
fission in the final fission stage. While a fission-fission design
might
actually be better for this application, I'm sticking with the heavily
studied fission-fusion-fission design to minimize technology risk
and R&D effort.
> The spacecraft gives energy to start the reaction, but once the bomblet
> is released it isn't part of the spacecraft any more, so the energy
> comes from decreasing the mass of the spacecraft.
Decreasing the mass of the spacecraft IS decreasing its
kinetic energy. Where did that energy come from? Hopefully,
the energy came from thrust gained from previous bomblets.
But you're still not gaining if you sacrifice more kinetic energy
than you get back.
> "
> continued
> failures to acheive break-even fusion reactions with
> magnetic pinch or particle beam techniques have
> sucked out my enthusiasm for the concept.
> "
> You can't get 50 km/s in space in opposite directions in a lab.
> Completely different, in fact irrelevant.
You can smash a fusion pellet with a particle beam, which
gives you up to 300,000km/s in a lab.
> "
> With
> a 1:1 ratio between hydrogen atoms and fission
> fragments, about 50% of the energy gets transfered to
> the hydrogen. This is a large fraction; I accounted for
> the fraction in the chart I gave.
> "
> Show it.
In a gas, the temperature of a material is defined by the
average kinetic energy per particle. If you double the
number of particles, and allow the temperature to
equalize, then the average kinetic energy per particle
will be cut in half. The fission fragments start off
with 84MeV per particle. The hydrogen particles
start off with 0MeV per particle. Thus, after thermal
mixing they will have 42MeV per particle.
> Second, the hydrogen will squirt out in one direction
> because it's blocked on all other sides by dense
> fission fragments.
> "
> Except it isn't. The fragments are in a charge-neutral plasma, and the
> hydrogen doesn't just get reflected. At the relevant energies
> everything just mixes.
At the relevant energies, even neutrons can be deflected
by neutron reflectors. A neutron even has to directly
impact a nucleus, whereas a proton will also be deflected
by a near miss due to electrostatic force.
There is mixing at the borders, but the protons are so
lightweight they just bounce around like crazy within the
forest of slow moving behemoths and quickly bounce
back out.
The motion of the protons is like a drunkard's walk.
If the drunkard starts his walk on the East side of town,
chances are high that he will hit the East side of town
again, after not too long. In this case, the border protons
start their drunkard's walk on the inner edge of the fission
"barrel". They move so fast that they're going to come
back out the inner edge of the barrel, and after not too
many bounces, typically.
> You know what, I don't have time for this. Have fun. I'll send you a
> letter of apology when you're famous.
Pity--you raise some interesting issues. I have answers
to those issues raised so far, but that's not necessarily
the case for all issues.
At the very least, I have an open mind about the possibility
of novel interstellar propulsion concepts out there. I've
been thinking off and on about plausible interstellar
propulsion concepts for years, and yet I never thought
of this Particle Puff Propulsion concept before.
Of course, you could say it's just my Bombtrack Propulsion
concept with a prettified name. However, I previously
only had vague ideas about how to implement the bombot
unit and only wild guesses about potential performance.
My wild guesses had been barking up the wrong tree in
any case--pinning my hopes on an efficient fusion reaction.
But even the best available fusion reaction only spit out
protons at 17%c, and it was really hard to buy it being
done efficiently even in a nuclear bomb.
The heart and soul of the Particle Puff Propulsion concept
is the design of the Particle Puff unit. The big "aha" was to
look at fission instead of fusion, since the heavy atomic
weights help instead of hurt. Once I had that big "aha",
it wasn't long before I figured out how to make a particle
puff emitter with only a minor change to existing bomb
designs.
Isaac Kuo
BernardZ
mec...@yahoo.com says...
> With particle puff velocities of up to 33%c, accelerating a
> magsail starship up to 25%c should be doable. This starship
> could be a lightweight flyby probe, unburdened by massive
> fuel tanks.
>
I remember reading that because of meteors and dust in space, the limits
of a starship would be about 10% with existing technology.
--
Only in movies do guys pick the engagement ring.
Observations of Bernard - No 89
IsaacKuo
--------------------------------------------
Particle Puff Propulsion - Theory of PPD mechanism
Particle Puff Propulsion is similar to Particle Beam Propulsion
except the magsail is accelerated by short range "puffs" of
particles from one-shot "particle puff devices" instead
of a long range particle beam emitter. These PPDs are laid out
along the acceleration path, so they activate one-by-one as
the magsail passes by. Each PPD needs to have stationkeeping
thrusters and control electronics.
The key to this concept is the design of the particle puff
devices. The PPD is a slightly modified Teller-Ulam
fission-fusion-fission device, with a tube of hydrogen
propellant gas within the main fission stage.
The operation of the fission-fusion-fission reaction is
conventional, so I won't explain it here. The important
thing is that it causes the tube of fissionables to
implode and react efficiently. These heavy fission fragments
are at solid densities, but with a temperature of ~84MeV
per particle, it's a hot dense plasma.
The question is how energy from the hot fission fragment
plasma is transfered to the hydrogen propellant and directed
out the rear nozzle.
PPD rocket chamber:
u | fusion stage |
r |___________________________________________| /
a | | /
n | fission stage __|/
i | .-"
u | ________________________________/
m | / :
| / :
| ( hydrogen propellant :<--- plastic "plug"
| \ :
s | \_______________________________:
h | \ <--- throatless
i | "-.__ nozzle
e | fission stage |\
l |___________________________________________| \
d | | \
| fusion stage |
This is a cross section of the PPD's rocket chamber.
The geometricy is tapered instead of straightwalled
so that as the fission stage implodes it has a
"zipper" effect from the front to back, squeezing the
hydrogen propellant out the rear like squeezing
toothpaste out of a tube.
Of course, everything is a hot plasma rather than a
solid material so it's not appropriate to directly
use our intuitive notions of squeezing material.
Nevertheless, the practical effect is to expel the
hydrogen out the rear at up to 40%c. What's really
going on?
First, let's consider what happens at the boundary
between the hydrogen and the fission fragments. The
fission fragments have average atomic weights of
116, so they really don't get moved around much by
the much lighter protons bouncing off them. A proton
entering the plasma of fission fragments gets bounced
around to high speeds in random directions, quickly
absorbing kinetic energy and leaving back to the
interior quickly. (The thickness of the fission stage
is far greater than the mean path of the proton so it
has little chance of penetrating all the way through.)
This process exchanges heat at the boundary practically
instantly.
The fast moving protons quickly equalize the temperature
within the chamber, but the slow moving fission fragments
at the boundary can't receive heat from the rest of the
fission plasma quickly. This seems to limit the amount
of energy dumped into the hydrogen working gas to the
heat energy of only the innermost layers of the fission
fragments.
However, there is an additional heating mechanism which
essentially transfers kinetic energy from the hot outer
layers to the inner layers and ultimately to the
hydrogen--compression heating. The hot outer layers
will expand, imploding the inner layers and the hydrogen
gas. This does work to the inner layers and hydrogen
gas, heating it up in the process.
Ultimately, both direct thermal mixing and compression
heating tend to equalize the temperature of the fission
fragment plasma and the protons. Thus, to a rough
approximation the kinetic energy of the protons can be
calculated as the weighted average by number of particles
of 84MeV per fission fragment and 0MeV per proton
particle. For example, assuming 9 protons per fission
fragment would mean 84MeV/(1+9) = 8.4MeV.
With an atomic weight difference between the fission
fragments and the protons of around 100, the difference
in their speeds will be a factor of around 10. Thus,
the taper rise/run in the chamber geometry should be
greater than 1/10 to give the hydrogen enough speed
to get out of the way. With insufficient taper, the
hydrogen will instead get "trapped" by the imploding
chamber walls and largely mix with the fission fragment
plasma instead of escaping rearward.
Isaac Kuo
IsaacKuo
BernardZ wrote:
> In article <1132176252.3...@z14g2000cwz.googlegroups.com>,
> mec...@yahoo.com says...
> > With particle puff velocities of up to 33%c, accelerating a
> > magsail starship up to 25%c should be doable. This starship
> > could be a lightweight flyby probe, unburdened by massive
> > fuel tanks.
> I remember reading that because of meteors and dust in space, the limits
> of a starship would be about 10% with existing technology.
I didn't think we knew enough about the interstellar medium to
really know what such limits would be. I find it hard to believe that
with interstellar space being so empty that there wouldn't be some
reasonable way to shield the starship. It's not some ethereal Starwisp
or light sail.
Isaac Kuo
Wayne Throop
: The key to this concept is the design of the particle puff
: devices. The PPD is a slightly modified Teller-Ulam
: fission-fusion-fission device, with a tube of hydrogen
: propellant gas within the main fission stage.
If nobody's already proposed it, I think they should be called
Powerpuff Gadgets, or PPGs. If it's already been proposed, then... nevermind.
They're coming through and fighting
And everyone they're shocking
You know no one can stop them
All because of the chemical X
--- Cherish, "Chemical X"
Wayne Throop thr...@sheol.org http://sheol.org/throopw
IsaacKuo
Wayne Throop wrote:
> : "IsaacKuo" <mec...@yahoo.com>
> : The key to this concept is the design of the particle puff
> : devices. The PPD is a slightly modified Teller-Ulam
> : fission-fusion-fission device, with a tube of hydrogen
> : propellant gas within the main fission stage.
> If nobody's already proposed it, I think they should be called
> Powerpuff Gadgets, or PPGs.
That's funny. ;)
But I desire to relate the name to Particle Puff Propulsion,
which in turn is related to Particle Beam Propulsion.
Hopefully, the Beam->Puff change will convey both the
similarities and differences between the concepts.
My choice of "PPD" as the bombbot acronym is a nod
to the board game Starfleet Battles. In SFB, a PPD is
a "pulsed plasmatic device", or something like that.
It doesn't bear any relation to the concept I'm proposing,
but it is one heck of a cool long range weapon in SFB.
> If it's already been proposed, then... nevermind.
That's a good question. I'm really very curious whether
anyone else has considered or studied the possibilities
of bombtrack propulsion. ("Bombtrack Propulsion" is
another name I made up; if others have proposed it
they may have come up with a different name.)
Isaac Kuo
Erik Max Francis
> I remember reading that because of meteors and dust in space, the limits
> of a starship would be about 10% with existing technology.
Perhaps there might be a noticeable limit in interplanetary space (but I
seriously doubt it would be anything as low as 10% c). But
_interstellar_ space is really fabulously empty.
--
Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
Well, it is quite simple. I fly close to my man, aim well and then he
falls down. -- Oswald Boelke
IsaacKuo
> Particle Puff Propulsion - Theory of PPD mechanism
I've been thinking more and more about how the PPD design works,
and one things been really bugging me about it--neutron contamination.
The basic idea is to accelerate a small number of protons per fission
fragment, but if we have efficient fission then that means there's
already 1+ neutrons around also. I've been racking my brains trying
to think of a way to get the neutrons out of the way before they
disastrously cool my concept out of doable-ness.
At the same time, I've been worried about neutron radiation in general
because of the close proximity between the starship and the detonation.
Once the starship gets up to speed it's not such a problem because
the neutrons can't catch up to the ship, but it's a problem near the
start of the track. This is exacerbated by my current desire to use a
near term conventional aluminum magnetic coil for the magsail.
I've thought of a solution, and it actually greatly improves the
potential
performance of the propulsion system. Basically, the final fission
stage acts as a very fast fission fragment rocket, also pushed inward
by fast 17%c protons and neutrons from the fusion stage. The inner
layers of this stage essentially act as neutron absorbers/reflectors
preventing contamination of the hydrogen propellant chamber with
neutrons (any neutrons which get carried along for the ride out the
exhaust puff are an inefficiency which reduces thrust to the magsail).
This is incredible--it means an imploding uranium cylinder moving at
up to 17%c. This heavy dense uranium plasma will be an excellent
proton reflector, squeezing protons out of the tapered imploding
walls at up to 15 times its own velocity! Of course, relativistic
effects kick in long before 255% of the speed of light.
But essentially, I now see particle puff propulsion as potentially
useful for even VERY high relativistic speeds. The limiting factor
is the sheer amount of energy required to send vehicles up to
near-c speeds. But if you've got the economy to handle the
energy cost, this is a way to do it.
Isaac Kuo
IsaacKuo
which can reach high relativistic speeds for potential
interstellar missions within our lifetimes. It's similar to
particle beam propulsion, except it uses many short range
particle puff devices rather than one long range particle
beam emitter. The starship is propelled using interaction
between charged particles and the magnetic field of a large
aluminum magnetic coil.
Previously, I had described the mechanism of the particle
puff device in terms of a thermal rocket. Fission heats
up fission fragments to 84MeV; these transfer heat to
hydrogen propellant protons, and the resulting protons
fly out the exhaust at upwards of .3c. Assuming this
mechanism, puff speeds of .4c are not acheivable.
However, I have realized that the thermal rocket model is
NOT the best model to use. Instead, far greater performance
is possible using a light gas gun model. In a light gas
gun, a heavy weight is accelerated by conventional gunpowder.
This is limited in speed by the energy density of the
gunpowder. The weight compresses hydrogen gas, which is
then capable of propelling the hydrogen to much higher speeds.
u| fusion |
r| ______|
a|_______........--------"""""""" |
n| fission _______|
i| _______.......-------"""""""
u| / |
m|( H |
| \___|___
s| """""""-------......._______
h|_______ fission |
i| """"""""--------........______|
e| |
l| fusion |
d
Upon device detonation, D-T fusion bathes the fission stage
with neutrons moving at around .17c. This causes the outer
layers of the fission stage to ablate away; acting as a
fission fragment rocket. The inner layers thus implode at
up to .17c. This is the "heavy weight" in a light gas
gun. It's also the "gun barrel".
The hydrogen starts off at the rear of the "gun barrel".
As the walls of the "gun barrel" implode, the hydrogen gets
accelerated down the barrel. Most of the hydrogen gas
is compressed into a small bundle caught in the conical
apex of the implosion "zipper".
Assuming the barrel is composed of atoms with an atomic
weight of around 100, imploding at .1c, it has a kinetic
energy of about 500MeV per particle. This can compression
heat the protons up to 500MeV, or about .75c. If the
"zipper" closes at less than .75c, then the hydrogen
puff should be able to squeeze "out of the way" rather
than getting stuck between the walls slamming together.
Thus, this PPD design is capable of puff velocities as high
as perhaps .75c or even higher. However, I don't think
higher velocities are desirable.
For near term missions, I think the "sweet spot" for good
velocities would be between .25c and .45c. Go slower,
and the mission time is too long. Go faster, and the
energy cost is obscene.
Isaac Kuo
Damien Sullivan
>I didn't think we knew enough about the interstellar medium to
>really know what such limits would be. I find it hard to believe that
>with interstellar space being so empty that there wouldn't be some
>reasonable way to shield the starship. It's not some ethereal Starwisp
>or light sail.
Standard estimate is one hydrogen atom per cm3, is it not? Fabulously empty as
Erik says. But in a light-year each cm2 of your surface would run into 1e18
hydrogen atoms, impacting with efficient-fusion level energies. About 1e-9
kg, or 1e6 J/cm2/10-years.
Depends on how the energy is absorbed, I guess. If running into a proton
causes some heat which can be readiated away, then no big problem. If it
causes direct damage then you're absorbing 1e6 J/cm2 which seems like it could
erode 10 cm of diamond shielding, or vaporize a meter of ice.
If you run into a milligram dust particle (much bigger and rarer), of course)
that'll be an instant point source of 1e9 J, 0.25 tons of TNT.
How likely is that? I don't know. But at 1 atom/cm3 a square meter of hull
will run into 1e22 atoms, 10 milligrams of matter. Most of the particles you
run into is individual atoms; what form most of the *mass* takes is another
thing. It's not hard to imagine running into milligram dust particles being a
near-certainty. A large stick of dynamite on every square meter of hull, per
light year. A habitat would have a couple meters of dense shielding anyway,
for general radiation, but a flyby probe may need more than originally
anticipated.
If you go at 0.25 c instead of 0.1 c then scale everything up by 6.25...
-xx- Damien X-)
IsaacKuo
>>I didn't think we knew enough about the interstellar
>>medium to really know what such limits would be.
>>I find it hard to believe that with interstellar
>>space being so empty that there wouldn't be some
>>reasonable way to shield the starship.
>Standard estimate is one hydrogen atom per cm3,
>is it not? Fabulously empty as Erik says. But
>in a light-year each cm2 of your surface would
>run into 1e18 hydrogen atoms, impacting with
>efficient-fusion level energies.
I'm not worried about hydrogen atoms. The ship's
magsail can be run at low power to shield against
them. I'm imagining a probe with a long endurance
fission reactor, with enough power to run a decent
radar sensor and maybe an active laser sensor
throughout the journey. These active sensors can
detect interstellar debris and planetoids, if any,
which are simply too small/cold/distant for us to
study from the solar system. It wouldn't consume
much more power to run even a plain aluminum coil
magnetic sail throughout the journey.
>If you run into a milligram dust particle (much
>bigger and rarer), of course) that'll be an instant
>point source of 1e9 J, 0.25 tons of TNT.
Against macroscopic projectiles, I favor the use of
a thin foil shield in front of the ship. If it
runs into a macroscopic particle, the collision
instantly converts it into a shower of plasma which the
magnetic field can deflect.
An interesting interstellar precursor mission would
be to launch a swarm of high performance solar sails
into interstellar space and see if they run into anything.
These are essentially small "bubbles" of reflective
material which are deployed from a spacecraft during a
flyby near the Sun. Light pressure pushes the tiny
spheres outward at high speed, although not good enough
for a fast interstellar journey.
If there's a collision, sensitive telescopes could
estimate the size of the debris based on the intensity
of the explosion.
Isaac Kuo
IsaacKuo
IsaacKuo wrote:
> Against macroscopic projectiles, I favor the use of
> a thin foil shield in front of the ship. If it
> runs into a macroscopic particle, the collision
> instantly converts it into a shower of plasma which the
> magnetic field can deflect.
Hmm...on second thought, a better shield might be some sort of
sticky "silly string" or "spider web" wire, or some low density
foam. This forms a low density matrix in front of the starship.
If a macroscopic particle hits it, the ethereal shield material
converts it into a trumpet shaped blast of plasma, which can
then be deflected with the magnetic shield.
With a thin disc shield, the disc needs to be thick enough to
vaporize any incoming projectile with the material from just
a speck. That puts a particular minimum frontal area density
on the shield. With an ethereal foam or wire matrix shield,
a much lower frontal area density is acceptable. This is like
the theory behind modern chobham tank armor--traditional
metal armor needs to be very thick to prevent penetration by
a HEAT warhead because a HEAT warhead concentrates its force
through a narrow channel. Chobham armor defeats a HEAT warhead
by spreading out its force into a conical zone of foam. The
frontal area density of this armor is lower than traditional
armor because of this.
Obviously, there's a concern about the ability of this
ethereal shield to handle repeat collisions. For this,
the shield may be replenished by spraying more "silly string"
or foam as necessary. Another possibility is to use a series
of rotating shells. Each collision would punch a conical
trumpet shaped hole through them, but as the shells rotate
the holes no longer line up. Another possibility is to use a
material which is "springy" so that it naturally expands to
fill out any gaps.
Isaac Kuo
Damien Sullivan
>I'm not worried about hydrogen atoms. The ship's
>magsail can be run at low power to shield against
>them. I'm imagining a probe with a long endurance
How does a magsail protect you from neutral hydrogen atoms?
-xx- Damien X-)
IsaacKuo
A hydrogen atom is made up of two charged particles,
a proton and an electron. When they hit a magnetic field,
the proton is pulled in one direction while the electron is
pulled in the opposite direction. This force pulling them
apart increases with velocity--and these atoms are hitting
the starship's magnetic field at .25c. In short, they may
start off as one neutral particle, but when they hit the
magnetic field they get ripped apart into two charged
particles.
Isaac Kuo