Mission to Alpha Centauri

Iain Why do you need staging in space?

Jim The same reason you would need staging anywhere; it's a method of
Jim achieving the necessary mass ratio.

Maybe I was being unclear.

Yes, staging helps you get a larger mass ratio. But in
general, how do you get a large mass ratio?

- small engine mass relative to payload.
- small tank mass relative to payload.
- large propellant mass relative to payload.

If you are willing to have very low acceleration, then your
engine mass can be small (don't need so much thrust) and
tank mass can be small (don't have to support so much weight
for a given amount of mass). At 10 G acceleration, a single
stage mass ratio of 5 requires some pretty good engineering.
At 1 G acceleration, the same materials, etc should get you
a mass ratio of 50 in a single stage. At .01 G acceleration,
think about mass ratios of 5000.

Of course, at mass ratios of 5000, you have to start
thinking about other problems. Your engine burn will last
a very long time, and so your tanks and engines have to deal
with reliability problems ordinary rockets simply ignore.

My point was that if your trip is going to take months or
years anyway, you can afford to wait through engine burns of
days or months, and very low accelerations are perfectly
reasonable, and so very large single-stage mass ratios are
possible. So why bother with staging?

Is that more clear?

In article ,
Roger Stokes wrote:
Very low acceleration would prolong trip time so I assumed a certain minimum
acceleration of (say) 0.1g would be needed to actually be going at 0.2c for
most of the trip.

A useful rule of thumb is that 1G is roughly c/year. So at 0.1G, you need
about two years to accelerate to 0.2c. Somewhat lower acceleration would
be acceptable, but it can't be a lot lower or the acceleration time starts
to dominate the trip time.

At 0.01G, you are halfway to Alpha Centauri at engine cutoff. That is
grossly wasteful at interstellar speeds, because it means that your
average speed is rather less than your final speed. It takes about 30
years to reach Alpha Centauri that way, but it only takes about 45 if you
stop accelerating when you reach 0.1c. Doubling the delta-V is an awfully
expensive thing to do when it only reduces the trip time by 1/3.

Another useful rule of thumb: acceleration distance should be no more
than 10% of the total trip distance. For Alpha Centauri and a cruising
speed of 0.2c, that means you need about 0.05G minimum.

What values of space fuel tank mass/volume ratio are achievable today -
depends on acceleration, and type and volume of fuel I know but any figures?

Also depends *tremendously* on what's in them, what pressure they're at,
and on whether there are any aerodynamic constraints. With minimal
pressure, low acceleration, and no aerodynamics, it should be feasible to
make tank mass very low... *provided* the tank walls don't have to include
thermal insulation too. Which might be feasible out in interstellar
space, if you are very careful to shield the tanks from engine heat and
payload warmth.

The Daedalus first-stage tanks were 60m-dia titanium-alloy spheres
weighing about 25t each empty.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

"Elmar Moelzer" wrote:
Hey Henry!
Thanks for the explanation!
I do not really understand that smoke- cloud- thing in full detail. I
suppose they would be using some sorts of rockets to spread the gas- cloud
ahead of the mothership (when simply blowing it out in front of the
mothership the gascloud would continually drifft away further and further)?
Wouldnt hitting that smokecloud be dangerous too? I mean that this way one
would replace a potentially existing dangerous object with a certainly
existing dangerous object, right?

You're not thinking right. You've gotta think *relative*
velocities. So you spew a smoke or gas cloud out, what's
the relative velocity going to be? Low, a few km/s at
most if it's rocket exhaust going the wrong way. That's
nothing to sneeze at but it's easy to design a ship to
withstand the effects. But what's the relative velocity
going to be between the gas / dust and junk in space?
Yup, 0.2 c, 60 *thousand* km/s. 44 *million* times as
much relative kinetic energy per kg as with rocket
exhaust from even a high-performance nuclear thermal
rocket, 4 times that in relation to rocket exhaust from an
ordinary chemical rocket. That makes all the difference.
The relative kinetic energy per kg is sufficient to melt,
vaporize, and even ionize any material on impact. The
trick is throwing up enough mass in between your ship and
incoming debris to guarantee incoming objects interact
with enough mass to release enough energy to do that, but
that's an easy problem given the rarity of large objects
even in interplanetary space. Running into a several km
asteroid would be a problem, but for ordinary debris
(dust, pebbles maybe) that you are most likely to run into,
the gas / dust shield would vaporize it quite effectively.


So, the relativistic effects do not increase in a linear way? How strong
would the effect be at 0.4 light then (just so I can calculate this)?

Completely non-linear. It kinda has to be to go to infinity at
c. What you need is "gamma", which is the relativistic scaling
factor, it's sqrt(1-(v/c)^2). As you'll notice, it is 1 at v=0
and 0 at v=c. "Relativistic mass" increases as 1/gamma
(m0/gamma), as does time-dilation. Length contraction decreases
directly with gamma. You'll notice that 1-gamma is a mere 0.02
at 0.2 c, in other words, at 0.2 c "relativistic effects" only
manifest as a difference of around 2%. And 1-gamma doesn't
get up to 0.5 (i.e. relativistic effects of around 2x) until
0.87 c.

In article lekom.at,
Elmar Moelzer wrote:
I do not really understand that smoke- cloud- thing in full detail. I
suppose they would be using some sorts of rockets to spread the gas- cloud
ahead of the mothership (when simply blowing it out in front of the
mothership the gascloud would continually drifft away further and further)?

Yep, the mother ship would have little auxiliary vehicles to spread it.
The sub-probes probably would just have to blow it ahead, reducing its
effectiveness.

Wouldnt hitting that smokecloud be dangerous too? I mean that this way one
would replace a potentially existing dangerous object with a certainly
existing dangerous object, right?

Anything that's in the path of an object moving at 0.2c is toast. :-) The
only question is whether the 0.2c object is toast too. The assumption is
that the preservation of minor meteorites is not a priority for the probe.

So, the relativistic effects do not increase in a linear way? How strong
would the effect be at 0.4 light then (just so I can calculate this)?

Correct, they are strongly nonlinear. The formulas can get messy, but a
lot of the effects scale with 1/sqrt(1 - v^2/c^2), which is easy enough.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

"Elmar Moelzer" wrote in...
Hey Henry!
Thanks for the explanation!
[...]
So, the relativistic effects do not increase in a linear way? How strong
would the effect be at 0.4 light then (just so I can calculate this)?

Is this one of those 1/(1-beta) things? Dang, why does that seem so
long ago....someone musta shifted my quantum clock by 30 years.

/dps

(dave schneider) writes:

"Elmar Moelzer" wrote in...
Hey Henry!
Thanks for the explanation!
[...]
So, the relativistic effects do not increase in a linear way?

No, it's highly non-linear.


How strong would the effect be at 0.4 light then (just so I can
calculate this)?

Look up the formulae for relativistic time-dilation and the relativistic
Doppler shift --- you can find them in most freshman physics textbooks,
and in numerous places on the web.


Is this one of those 1/(1-beta) things?

No, it's one of those 1/sqrt(1 - beta^2) things.


Dang, why does that seem so long ago....someone musta shifted my quantum
clock by 30 years.

It's not the years --- it's the milage... :-T


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

(Gordon D. Pusch) wrote
[...]
It's not the years --- it's the milage... :-T


-- Gordon D. Pusch


;-)

/dps

Laser light sails are capable of sending payloads to stars.

Sunlight is intercepted by a large thin film concentrator. This
illuminates a solar pumped laser. The laser energy is directed
through a beam forming device. The beam is reflected by a thin film
mirror which causes momentum to be transferred to the mirror. A
payload is attached to the mirror which is carried to high speeds
along with the mirror.

No fuel is used aboard the craft during interstellar transit.

Bob Forward and others have proposed two-stage mirrors that separate
at the target star into a donut shaped annulus and a donut-hole shaped
central disk. The annulus is shaped to focus light on the smaller
disk as they separate. The smaller disk now carries the payload.
Momentum is transferred to both the larger annulus and the smaller
disk simultaneously. The large annulus continues to accelerate. The
small central disk is slowed. Ultimately, the payload enters the
target star system.

The payload can be equipped with microfission based nuclear pulse
rocket which gives it the legs to zip throughout the target star,
collecting data and beaming it back to Earth.

More advanced systems can be contemplated. Larger systems can carry
people and industrial cargo. Such systems, developed for
interplanetary settlement, can be adapted for use around target star
systems possessing suitable planets and planetoids.

Imagine that star colonies are established around nearby planetary
systems, complete with their own industrial infrastructure. Each is
equipped with their own laser light sail set up to send payloads to
neighbors around them. In this way, payloads can be sent back to
Earth from the colonies, incoming payloads can be slowed down using
local laser light energy thus reducing the cost and increasing the
payload, and payloads can be forwarded to destinations further
outward.

In this way, an expanding network of laser beam spaceways can be
constructed giving humanity access to the billion or so stars
contained in the Perseus Nebula surrounding the Sun in our arm of the
Milky Way galaxy.

Imagine now, that a collection of nearby star colonies coordinate
their affairs through radio telescope and arrange to collide large
shaped iron masses at considerable speeds at a mutually agreed upon
location. Black holes created in this way can themselves interact to
create all sorts of weird effects. Among them, is the creating of new
black holes from the decay of the vacuum due to the powerful gravity
effects of the black holes you built up from iron. In this way, huge
collections of black holes - black hole dusts - can be created. Such
dusts can interact via all the forces of nature, as well as temporal
loops. This opens up a whole new era of physics and engineering,
leading to among other things, a sort of superluminal travel involving
time machines.

Once we have time machines and gravity drives based on the decay of
synthetic black holes we will have the ability to traverse the visible
cosmos in very short times scales (hours) and return home about the
time we left. If in our journeys we discover naturally occuring time
violating regions, we will then have the potential of travelling back
in time before the epoch described above - essentially giving
intelligence access to all of space and time - from the very beginning
of the cosmos to the very end.

At this point we can say travel through space and time has achieved
all its major goals. Unconstrained by technical limits, we are then
constrained by our imagination only.


* * * LASER LIGHT SAIL REFERENCES * * *

Introduction to Laser Light Sail concept;
http://ffden-2.phys.uaf.edu/213.web....ightsails.html

Actual experiment with laser light sail;
http://www.space.com/news/lasersail_000301.html

More laser light sail stuff;
http://www.itsf.org/brochure/solarsail.html

http://www.sciam.com/article.cfm?col...A8809EC588EEDF

http://solarsail.jpl.nasa.gov/introd...ails-work.html

http://www.portal.telegraph.co.uk/connected/main.jhtml;$sessionid$D5TRTHIAABN4TQFIQMGSFF4AVCBQ WIV0?view=DETAILS&grid=P8&targetRule=10&xml=%2Fcon nected%2F2002%2F02%2F19%2Fecnaa519.xml

http://www.sff.net/people/Geoffrey.L...ghtsail89.html

http://www.niac.usra.edu/files/studi...df/4Landis.pdf

* * * SUN PUMPED LASER REFERENCES * * *

http://hep.uchicago.edu/solar/laser.html
http://www.nap.edu/books/0309045770/html/97.html
http://www-mpl.sri.com/projects/ted5985-1.html

* * * MICRO-FISSION REFERENCES * * *

http://www.engr.psu.edu/antimatter/Papers/ICAN.pdf
http://www.spacetransportation.com/a.../1A_Smith.html
http://highwaytospace.com/ast/presen...s/1a_smith.pdf

* * * SYNTHETIC and COLLIDING BLACK HOLE REFERENCES * * *

http://abcnews.go.com/sections/tech/...y.current.html
http://www.sciencenews.org/20020803/fob4.asp

Matthew Kwan wrote:

(Henry Spencer) writes:

Wouldnt hitting that smokecloud be dangerous too? I mean that this way one
would replace a potentially existing dangerous object with a certainly
existing dangerous object, right?

Anything that's in the path of an object moving at 0.2c is toast. :-) The
only question is whether the 0.2c object is toast too. The assumption is
that the preservation of minor meteorites is not a priority for the probe.

However, the use of smoke-clouds could make two-way interstellar travel
difficult, depending on the diffusion rate of the clouds. One thing that a
smoke-cloud won't protect against is another smoke-cloud moving in the
opposite direction.

Presumably traffic in each direction could be assigned "lanes", but these
would have to be pre-negotiated.

mkwan


Stellar proper motion. A straight line to Alpha Centauri 'now' would
pass through a slightly different region of space if made later. The
next nearest target, Barnard's Star, is one of the best examples of
this.

Remember, we're talking interstellar space, not the relatively
junk-laden confines of Earth orbit....

In interstellar space, minimal. It does need a shield on the front
against gas and fine dust, and some attention to cooling of the shield.
Hitting even a pebble would be deadly, but such objects are extremely
scarce out there.

Unfortunatly that's a long "slow wake zone" till you reach debris-free
interstellar space.