| Topic: |
Science > Physics |
| User: |
"IsaacKuo" |
| Date: |
19 Oct 2007 10:09:44 AM |
| Object: |
Any limits on relativistic impact blast velocity? |
I'm trying to estimate the potential performance of a
kinetic impact powered rocket for interstellar propulsion.
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
The kinetic impactors are small guided projectiles
launched from the home system. They may be
accelerated by an X-ray free electron laser focused
by a 1km diameter zone plate. I'm comfortable with
analyzing these details.
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
As such, the explosion is roughly spherical in all
directions (slightly biased in the direction of the
impactor's original velocity).
What would the blast velocity be like, and how
efficiently could the kinetic energy of the impactor
be converted into kinetic energy of the blast
products? I suspect that a lot of energy will be
converted into thermal X-rays, which harms
efficiency. Does this put an upper practical limit
on the blast product velocity? Or does some
other physics limitation put an upper practical
limit on the blast velocity?
I'm hoping that this kinetic impact powered
rocket could acheive average exhaust velocities
of 20%c or more, in which case it's possible to
design a "fast" interstellar rocket without
stupendous mass ratios and without resorting
to stupendously expensive bulk antimatter.
But I don't know how to figure out how much
energy is lost to thermal radiation rather than
converted into explosion product velocity.
Thanks!
Isaac Kuo
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| User: "Uncle Al" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 05:24:08 PM |
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IsaacKuo wrote:
I'm trying to estimate the potential performance of a
kinetic impact powered rocket for interstellar propulsion.
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
You've got some real problems there. Think about it - conservation of
energy, conservation of momentum.
The kinetic impactors are small guided projectiles
launched from the home system. They may be
accelerated by an X-ray free electron laser focused
by a 1km diameter zone plate. I'm comfortable with
analyzing these details.
Good for you. It's *****. Gonna hit a gas puff with pellets?
Yeah, that's gonna do stuff.
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
As such, the explosion is roughly spherical in all
directions (slightly biased in the direction of the
impactor's original velocity).
Trying to hit two missiles together at 10 miles/second relative is
stooopid. Trying to do it relativistically at multiple AU separation
is not even insane. Here ya go, Bunkie:
1) Big proton accelerator
2) Big electron accelerator (believe charrge conservation)
3) Colinear beams for neutral atom recombination
4) Big catcher's mit on the far end - ya still gotta impossibly aim
it.
5) Diffusion of the collimated neutral beam. Pity.
Didn't yer momma ever tell you why countercirculating beam
accelerators are much better than stationary target accelerators?
It's not merely a linear improvement with energy, either
What would the blast velocity be like, and how
efficiently could the kinetic energy of the impactor
be converted into kinetic energy of the blast
products?
Bupkis on a good day. Have somebody else ride in the crew capsule.
[snip]
I'm hoping that this kinetic impact powered
rocket could acheive average exhaust velocities
of 20%c or more, in which case it's possible to
design a "fast" interstellar rocket without
stupendous mass ratios and without resorting
to stupendously expensive bulk antimatter.
Giggle.
1) Who pays the power bill at the proximal end?
2) When ya get to where ya goin', how do ya return? Radio and
laser communications are gonna suck - a huge line of hot plasma
between the sender and receiver.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2
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| User: "Sam Wormley" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 11:02:35 AM |
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IsaacKuo wrote:
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
Near-c velocity impacts result in plasma and showers
of subatomic particles.
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| User: "IsaacKuo" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 11:18:18 AM |
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On Oct 19, 11:02 am, Sam Wormley <sworml...@mchsi.com> wrote:
IsaacKuo wrote:
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
Near-c velocity impacts result in plasma and showers
of subatomic particles.
Sure. And at least at first, the kinetic energy in
the plasma and particles will be practically equal
to the kinetic energy of the impactor. There's
some small amount of energy absorbed in
ionizing the propellant gas, of course, but we
can more or less ignore this.
My question is what happens after the initial
impact. The plasma starts off hot, and thus will
be radiating a lot of thermal X-rays. How much
energy gets lost to thermal radiation while
the plasma expands and converts the brownian
thermal motion into bulk spherical expansion?
Isaac Kuo
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| User: "Sam Wormley" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 11:30:52 AM |
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IsaacKuo wrote:
On Oct 19, 11:02 am, Sam Wormley <sworml...@mchsi.com> wrote:
IsaacKuo wrote:
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
Near-c velocity impacts result in plasma and showers
of subatomic particles.
Sure. And at least at first, the kinetic energy in
the plasma and particles will be practically equal
to the kinetic energy of the impactor. There's
some small amount of energy absorbed in
ionizing the propellant gas, of course, but we
can more or less ignore this.
My question is what happens after the initial
impact. The plasma starts off hot, and thus will
be radiating a lot of thermal X-rays. How much
energy gets lost to thermal radiation while
the plasma expands and converts the brownian
thermal motion into bulk spherical expansion?
Isaac Kuo
You are missing the point--the sail is gone and most
likely the ship too. Try doing some calculations!
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| User: "IsaacKuo" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 12:41:23 PM |
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On Oct 19, 11:30 am, Sam Wormley <sworml...@mchsi.com> wrote:
IsaacKuo wrote:
Sure. And at least at first, the kinetic energy in
the plasma and particles will be practically equal
to the kinetic energy of the impactor. There's
some small amount of energy absorbed in
ionizing the propellant gas, of course, but we
can more or less ignore this.
My question is what happens after the initial
impact. The plasma starts off hot, and thus will
be radiating a lot of thermal X-rays. How much
energy gets lost to thermal radiation while
the plasma expands and converts the brownian
thermal motion into bulk spherical expansion?
You are missing the point--the sail is gone and most
likely the ship too. Try doing some calculations!
Okay, let's suppose the impactor has a kinetic
energy of 1000 joules. It impacts the puff of
propellant. Some energy gets converted into
X-rays. Let's assume a worst case scenario
that 100% of the energy gets converted into
X-rays. The percentage of that which hits
the ship may be 10%, depending on the
exact geometry. So, 100 joules of energy is
absorbed by the ship.
Let's say the ship is a torus 1km in diameter
and 10m thick. Thus, the 100 joule of energy
is spread out across about 30,000m^2 of
surface area. Each square cm of surface
absorbs 1/300 of a joule. That may raise
the temperature of the material by about
1/1200 of a degree.
That's nothing like "blowing" it away.
Note that there's no minimum energy requirement
on the impactor. It's not like a nuclear bomb
where there's a minimum critical mass.
Isaac Kuo
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| User: "Edward Green" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
21 Oct 2007 01:16:50 PM |
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On Oct 19, 1:41 pm, IsaacKuo <mech...@yahoo.com> wrote:
On Oct 19, 11:30 am, Sam Wormley <sworml...@mchsi.com> wrote:
IsaacKuo wrote:
Sure. And at least at first, the kinetic energy in
the plasma and particles will be practically equal
to the kinetic energy of the impactor. There's
some small amount of energy absorbed in
ionizing the propellant gas, of course, but we
can more or less ignore this.
My question is what happens after the initial
impact. The plasma starts off hot, and thus will
be radiating a lot of thermal X-rays. How much
energy gets lost to thermal radiation while
the plasma expands and converts the brownian
thermal motion into bulk spherical expansion?
You are missing the point--the sail is gone and most
likely the ship too. Try doing some calculations!
Okay, let's suppose the impactor has a kinetic
energy of 1000 joules. It impacts the puff of
propellant. Some energy gets converted into
X-rays. Let's assume a worst case scenario
that 100% of the energy gets converted into
X-rays. The percentage of that which hits
the ship may be 10%, depending on the
exact geometry. So, 100 joules of energy is
absorbed by the ship.
Let's say the ship is a torus 1km in diameter
and 10m thick. Thus, the 100 joule of energy
is spread out across about 30,000m^2 of
surface area. Each square cm of surface
absorbs 1/300 of a joule. That may raise
the temperature of the material by about
1/1200 of a degree.
That's nothing like "blowing" it away.
The torus no doubt motivated by the possibility that the impactor beam
will become momentarily misaligned with the puffs, and in fact blow a
hole in ship.
I also suppose the x-rays will pass through the thin sail without
absorption, and so be a complete loss as far as momentum transfer
(except for the fraction blocked by what we presume will be the
toroidal ship's shielding).
Well, I understand your question. :-) I don't understand how to
answer it. Can we extend the classical black body radiation laws into
this temperature range?
Note that there's no minimum energy requirement
on the impactor. It's not like a nuclear bomb
where there's a minimum critical mass.
Relativity respecting space travel is so boring: who cares what
velocities we can achieve, if you are guaranteed everybody left behind
on Earth will be dead when you slow down at the other end. You are
also gambling that several hundred years in future Earth time, when
you are still making your time dilated journey, but nobody on Earth
had any part in sending you, the current population will simply turn
the foolish thing off, leaving you stranded in deep space without
anyway to change your trajectory, or reach any particular destination.
That is obviously what is going to happen!
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| User: "IsaacKuo" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
22 Oct 2007 10:47:13 AM |
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On Oct 21, 1:16 pm, Edward Green <spamspamsp...@netzero.com> wrote:
On Oct 19, 1:41 pm, IsaacKuo <mech...@yahoo.com> wrote:
On Oct 19, 11:30 am, Sam Wormley <sworml...@mchsi.com> wrote:
IsaacKuo wrote:
Sure. And at least at first, the kinetic energy in
the plasma and particles will be practically equal
to the kinetic energy of the impactor. There's
some small amount of energy absorbed in
ionizing the propellant gas, of course, but we
can more or less ignore this.
My question is what happens after the initial
impact. The plasma starts off hot, and thus will
be radiating a lot of thermal X-rays. How much
energy gets lost to thermal radiation while
the plasma expands and converts the brownian
thermal motion into bulk spherical expansion?
You are missing the point--the sail is gone and most
likely the ship too. Try doing some calculations!
Okay, let's suppose the impactor has a kinetic
energy of 1000 joules. It impacts the puff of
propellant. Some energy gets converted into
X-rays. Let's assume a worst case scenario
that 100% of the energy gets converted into
X-rays. The percentage of that which hits
the ship may be 10%, depending on the
exact geometry. So, 100 joules of energy is
absorbed by the ship.
Let's say the ship is a torus 1km in diameter
and 10m thick. Thus, the 100 joule of energy
is spread out across about 30,000m^2 of
surface area. Each square cm of surface
absorbs 1/300 of a joule. That may raise
the temperature of the material by about
1/1200 of a degree.
That's nothing like "blowing" it away.
The torus no doubt motivated by the possibility that the impactor beam
will become momentarily misaligned with the puffs, and in fact blow a
hole in ship.
This is a technical challenge, of course. For outward
acceleration, this isn't such a problem because the
size of the puffs can be larger than the ship. However,
this drive system might not even be used for outward
acceleration since it might make more sense to use
the laser directly with a ribbon sail.
For inward acceleration (acceleration toward the home
system), the problem is that each impactor must
actually pass through the starship loop before
impacting the propellant puff. Obviously, the propellant
puff provides no protection against a stray impactor.
My favored solution for that problem is to actually
have two propellant puffers. One emits a thicker
amount of gas on the far end of the ship. This is
an amount of gas large enough to fully absorb the
kinetic energy of the impactor. The other emits
a very thin amount of gas on the near end of the
ship. This is an extremely thin amount of gas which
doesn't do much more than vaporize and ionize
the impactor. This converts the impactor into ions,
which will then be funneled by the ship's magnetic
field into the loop hole.
This solution actually makes the guidance problem
much easier also, because the impactors don't
need to be as precisely lined up with the starship.
The magnetic field can somewhat funnel the
incoming particles.
I also suppose the x-rays will pass through the thin sail without
absorption, and so be a complete loss as far as momentum transfer
(except for the fraction blocked by what we presume will be the
toroidal ship's shielding).
The X-rays are too soft to penetrate either the
magsail material or the starship, but in any
case it's more or less a complete loss as far
as thrust or momentum is concerned. Even
if the all of the X-rays in the desired direction
were absorbed, the momentum/energy ratio
for the X-rays is 1/c. It doesn't get any worse
than that.
Well, I understand your question. :-) I don't
understand how to answer it. Can we extend
the classical black body radiation laws into
this temperature range?
I think so. It's not all that much hotter than
a fusion bomb, and classical black body
radiation is used to analyze fusion bombs,
right?
Note that there's no minimum energy requirement
on the impactor. It's not like a nuclear bomb
where there's a minimum critical mass.
Relativity respecting space travel is so boring: who cares what
velocities we can achieve, if you are guaranteed everybody left behind
on Earth will be dead when you slow down at the other end. You are
also gambling that several hundred years in future Earth time, when
you are still making your time dilated journey, but nobody on Earth
had any part in sending you, the current population will simply turn
the foolish thing off, leaving you stranded in deep space without
anyway to change your trajectory, or reach any particular destination.
I'm not envisioning time dilated journeys. I'm just
thinking of high speed non-relativistic or low relativistic
missions to alpha Centauri and other nearby systems.
A probe to alpha Centauri with a cruise velocity of
..3c could arrive in 1.5 decades and start returning
data in 2 decades.
As far as long range manned interstellar missions
go--yes, I can see this sort of drive being used for those
sorts of missions eventually. But time dilation is
stupendously expensive. I find it more plausible that
it will eventually be done after we've mastered life
extension or cryogenic "cold sleep" or "we" have
been uploaded into or replaced by digital AIs.
Anyway, assuming we don't bother with ridiculously
expensive time dilation, this sort of drive would use
two starships instead of one. The first one contains
the payload. The second is a sacrificial drone which
contains the impactors. The second starship is
launched after the first one, but slightly slower so it
lags behind the first starship.
A few decades before arrival, the first starship uses
a magsail to brake against the interstellar medium.
This allows deceleration from high relativistic
velocities down to perhaps .5c or .6c. However,
ISM braking deceleration is proportional to the
square of the velocity, so it gets much less effective
the slower you're going.
For the remaining deceleration, the second starship
spits out the impactor missiles into a long stream.
While the starship has been lagging behind the first
starship, that was before the first starship used the
magsail brake. Now, the impactors are cruising at
high relativistic speed and will catch up with the
first starship.
From then on, the kinetic impact rocket acts as
I described before. The impactors smash into puffs
of propellant gas; the explosions produce thrust
against the magsail.
This mission requires no assistance from the home
system after launching the second starship.
BTW, thank you for a sane and sensible reply.
I was afraid that sci.physics had been completely
taken over by kooks.
Isaac Kuo
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| User: "Randy Poe" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 10:49:08 AM |
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On Oct 19, 11:09 am, IsaacKuo <mech...@yahoo.com> wrote:
I'm trying to estimate the potential performance of a
kinetic impact powered rocket for interstellar propulsion.
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
The kinetic impactors are small guided projectiles
launched from the home system. They may be
accelerated by an X-ray free electron laser focused
by a 1km diameter zone plate. I'm comfortable with
analyzing these details.
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
As such, the explosion is roughly spherical in all
directions (slightly biased in the direction of the
impactor's original velocity).
What would the blast velocity be like, and how
efficiently could the kinetic energy of the impactor
be converted into kinetic energy of the blast
products? I suspect that a lot of energy will be
converted into thermal X-rays, which harms
efficiency. Does this put an upper practical limit
on the blast product velocity? Or does some
other physics limitation put an upper practical
limit on the blast velocity?
I'm hoping that this kinetic impact powered
rocket could acheive average exhaust velocities
of 20%c or more, in which case it's possible to
design a "fast" interstellar rocket without
stupendous mass ratios and without resorting
to stupendously expensive bulk antimatter.
But I don't know how to figure out how much
energy is lost to thermal radiation rather than
converted into explosion product velocity.
Thanks!
Isaac Kuo
A few thoughts:
- If you smash hard, fast moving things into
your rocket, they're likely to penetrate and break
things rather than transfer momentum.
- You might want to read through the design
issues on "Project Orion", an old concept using
nuclear fission blasts to power a spacecraft.
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
Note that the explosion is well behind the rocket
and the propulsion comes from the shock wave of
hot gases.
- Randy
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| User: "IsaacKuo" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 11:09:16 AM |
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On Oct 19, 10:49 am, Randy Poe <poespam-t...@yahoo.com> wrote:
On Oct 19, 11:09 am, IsaacKuo <mech...@yahoo.com> wrote:
I'm trying to estimate the potential performance of a
kinetic impact powered rocket for interstellar propulsion.
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
[...]
- If you smash hard, fast moving things into
your rocket, they're likely to penetrate and break
things rather than transfer momentum.
The impactors don't hit the rocket, but rather a
puff of sacrificial propellant gas.
- You might want to read through the design
issues on "Project Orion", an old concept using
nuclear fission blasts to power a spacecraft.
Note that the explosion is well behind the rocket
and the propulsion comes from the shock wave of
hot gases.
This rocket works broadly like Orion, except the
explosions are caused by kinetic impact rather
than nuclear bombs, and a magsail is used
place of a pusher plate.
Compared to Orion, the potential exhaust velocity
is much higher, there is no minimum pulse size
imposed by nuclear warhead critical mass, and
there's no neutron radiation to worry about.
I'm pretty sure there will be a significant amount
of (non-penetrative) thermal X-rays to worry about.
But I don't know how to calculate how much energy
is lost into thermal radiation vs how much gets
converted into blast product kinetic energy.
Isaac Kuo
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| User: "Igor" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
20 Oct 2007 12:48:28 PM |
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On Oct 19, 11:09 am, IsaacKuo <mech...@yahoo.com> wrote:
I'm trying to estimate the potential performance of a
kinetic impact powered rocket for interstellar propulsion.
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
The kinetic impactors are small guided projectiles
launched from the home system. They may be
accelerated by an X-ray free electron laser focused
by a 1km diameter zone plate. I'm comfortable with
analyzing these details.
What I'm wondering about is the sort of performance
can be expected from the kinetic impact. Let's
assume that the impactor has a near-c velocity
and the mass of the propellant puff is an order of
magnitude greater than the impactor's rest mass.
As such, the explosion is roughly spherical in all
directions (slightly biased in the direction of the
impactor's original velocity).
What would the blast velocity be like, and how
efficiently could the kinetic energy of the impactor
be converted into kinetic energy of the blast
products? I suspect that a lot of energy will be
converted into thermal X-rays, which harms
efficiency. Does this put an upper practical limit
on the blast product velocity? Or does some
other physics limitation put an upper practical
limit on the blast velocity?
I'm hoping that this kinetic impact powered
rocket could acheive average exhaust velocities
of 20%c or more, in which case it's possible to
design a "fast" interstellar rocket without
stupendous mass ratios and without resorting
to stupendously expensive bulk antimatter.
But I don't know how to figure out how much
energy is lost to thermal radiation rather than
converted into explosion product velocity.
Thanks!
Isaac Kuo
The physics of collisions is based on momentum and energy, not
velocity.
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| User: "Puppet_Sock" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 12:02:19 PM |
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On Oct 19, 11:09 am, IsaacKuo <mech...@yahoo.com> wrote:
I'm trying to estimate the potential performance of a
kinetic impact powered rocket for interstellar propulsion.
Ok.
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
Supposing that you can actually aim these things
well enough to get them close to your departing ship.
Which is nearly as big a technical leap as all the
other stuff you are hand-waving away here.
Do not forget that you will need to be doing this for
a long time for any useful speed to be built up.
Consider how long it takes at 1 g to get to 1/10th
the speed of light. So you will be firing these little
packages for many months at least, while the
ship is moving out of the solar system.
Now, you have to deal with what happens to a
mass that gets hit with enough xrays to accelerate
it to relativisitc speeds. It turns to plasma is what.
And damned hot plasma. And it disperses. By
the time it gets to your ship it's 1000s of km across
and most of it misses the ship entirely.
So, suppose you find some marvy way to keep the
mass together. Relativistic mass hits gas behind the
sail. Microscopic fraction of total kinetic energy is
transferred to the gas. The mass continues on
with nearly all of its kinetic energy, and punches
a hole through your sail. Wave bye to the
impactor mass.
And if it *did* work, you'd be leaving your gas
cloud behind. That means you have to be letting
out this cloud of gas the entire time you are under
thrust. Seems like you will need a lot of gas.
The kinetic impactors are small guided projectiles
launched from the home system. They may be
accelerated by an X-ray free electron laser focused
by a 1km diameter zone plate. I'm comfortable with
analyzing these details.
Um. Sure you are. You are comfortable with it
because you have no clue. You are focussing
an xray laser with what now?
If you have an x-ray laser, why don't you use a solar
sail and catch the x-rays directly? If you can aim
masses at relativisitic velocities, you can certainly
aim a laser over a distance of a few AU.
Socks
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| User: "IsaacKuo" |
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| Title: Re: Any limits on relativistic impact blast velocity? |
19 Oct 2007 01:11:47 PM |
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On Oct 19, 12:02 pm, Puppet_Sock <puppet_s...@hotmail.com> wrote:
On Oct 19, 11:09 am, IsaacKuo <mech...@yahoo.com> wrote:
The basic idea is that the rocket ship has a magsail
and a tank of inert propellant gas. It puffs propellant
gas behind or in front of the magsail (depending on
whether acceleration or deceleration is desired).
Relativistic kinetic impact causes the propellant puff
to explode, producing thrust as the resulting plasma
is deflected by the magsail.
Supposing that you can actually aim these things
well enough to get them close to your departing ship.
Which is nearly as big a technical leap as all the
other stuff you are hand-waving away here.
The things are self guided. Each impactor is a
microchip with DLP style micro-panels for
maneuvering. In a DLP chip, the micro-panels
are mirrors which rotate to change the direction
light is reflected in. In these chips, the micro-panel
selectively blocks or lets through waste heat
light to maneuver.
Each chip is essentially an x-ray laser sail,
which directly absorbs the momentum of x-ray
photons and emits waste heat photons.
Do not forget that you will need to be doing this for
a long time for any useful speed to be built up.
Consider how long it takes at 1 g to get to 1/10th
the speed of light. So you will be firing these little
packages for many months at least, while the
ship is moving out of the solar system.
The x-ray laser in question gradually accelerates
a stream of chips over a very long distance.
Assuming the base of the chips are made of
graphite, an acceleration of 10 gees should be
feasible without melting the chip. Let's assume
a much lower acceleration of 3 gees. It takes
about a hundred days to reach high relativistic
speeds, during which time they have traveled
over 8000AU.
Focusing a visible wavelength laser over this
distance would require a stupendously huge
aperture. However, with an X-ray laser it's
possible to do this without outrageously sized
optics.
A free electron laser tuned to the lead k-edge
transition can be efficiently focused by a
lightweight zone plate coated with lead.
By varying the thickness of the lead between
0 and 0.1mm, a gabor zone plate pattern is
possible. This absorbs half of the beam
energy while focusing the remaining half
onto a diffraction limited spot. The diffraction
limits allow useful beam spreads over
interstellar distances. For example, a
1km diameter zone plate can focus the
beam onto a 700m diameter spot that's
4.3 light years away. At a distance of 8000AU,
the spot size is only 22m. The stream of chips
can easily cover that area.
So, suppose you find some marvy way to keep the
mass together. Relativistic mass hits gas behind the
sail. Microscopic fraction of total kinetic energy is
transferred to the gas. The mass continues on
with nearly all of its kinetic energy, and punches
a hole through your sail. Wave bye to the
impactor mass.
Umm...no. The kinetic energy is almost all
instantly transfered to the gas. And you
obviously don't know what a magsail is,
because it already has a huge hole in it.
It's a large loop.
And if it *did* work, you'd be leaving your gas
cloud behind. That means you have to be letting
out this cloud of gas the entire time you are under
thrust. Seems like you will need a lot of gas.
Umm...that's exactly how the rocket is supposed
to work. The puff of gas is propellant. When
it explodes due to impact, it's gone. The
propellant explodes into high velocity plasma
which becomes the rocket's exhaust. The
magsail deflects about half of the particles,
to produce thrust.
The kinetic impactors are small guided projectiles
launched from the home system. They may be
accelerated by an X-ray free electron laser focused
by a 1km diameter zone plate. I'm comfortable with
analyzing these details.
Um. Sure you are. You are comfortable with it
because you have no clue. You are focussing
an xray laser with what now?
A gabor zone plate. The free electron laser is
in solar orbit, solar powered. It relies upon the
SASE principle to emit a laser in a very narrow
conical beam. With an electron accelerator
length of 100m, it's possible to emit a beam
which has a spread of 1km per light second.
The 1km zone plate is stationed 1 light second
away from the free electron laser. The
reason for this extreme distance is to make
the zone plate easier to manufacture. The
spacing of the zone ridges is proportional
to the distance from the laser, and inversely
proportional to the lens diameter. By placing
the zone plate 1 light second away, the
spacing of the zones is 0.1mm.
If you have an x-ray laser, why don't you use a solar
sail and catch the x-rays directly? If you can aim
masses at relativisitic velocities, you can certainly
aim a laser over a distance of a few AU.
Because the laser sail only works in one direction.
It can be used to accelerate the starship outward,
but not for decelerating the starship at the destination,
nor for accelerating it back for a return journey.
The kinetic impact powered rocket can be used
to decelerate at the destination and to accelerate
for a return journey. The starship is shaped like
a loop, so the impactors can pass through the
central hole before impacting with the puff of
propellant gas.
The interstellar capability of the X-ray laser isn't
wasted, though. The impactors need the
external power of the laser in order to perform
the guidance maneuvers that keep them on
track.
The physics and math for accelerating and guiding
the mini-sails is something I've already worked out.
It's the effects of the kinetic impact which I'm not
certain about.
Isaac Kuo
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