How much more efficient would Nuclear Fission rockets be?

"Rats" wrote in message ...
Using existing technology if we were to build a Fission rocket then how much
more efficient would than a Chemical rocket would it be? I believe
conventional rockets are capable of speeds around 8km/s. What sort of speeds
could a Fission rocket get up to?

The speed of a rocket propelled projectile is given by;

Vf = Ve * LN(1/(1-u))

Where

Vf = final velocity (in km/sec)
Ve = exhaust velocity (in km/sec)
LN(...) = natural logarithm function
u = propellant fraction

Since 15% of a vehicle is typically structure, its pretty difficult to
get a vehicle that has more than 85% propellant fraction. The whole
SSTO affair attempted to reduce that 15% to something like 5% - before
everyone gave up.

So, let's use .85 to .90 as the propellant fractions of interest and
can compute stage velocities knowing exhaust speeds.

For a nuclear rocket exhaust speeds range from 9.0 km/sec to 20.0
km/sec depending on type (more on that below)

For chemical rocket exhaust speeds range from 2.5 km/sec to 4.5 km/sec

So, stage velocities range from 4.7 km/sec to 46.0 km/sec! The best
chemical rockets as you point out have stage velocities of 8.0 km/sec
- ideally. Taking air drag and gravity losses into account, this
drops down to around 6.5 km/sec - which means you need a stage and a
half or two stages to get to LEO.


Type Exhaust u Vf
Solid Chemical 2.5 0.85 4.743
2.5 0.90 5.756
Liquid Chemical 4.5 0.85 8.537
4.5 0.90 10.362
Nuclear Thermal 9.0 0.85 17.074
9.0 0.90 20.723
Nuclear Pulse 20.0 0.85 37.942
20.0 0.90 46.052

But, if we could keep structural fractions in line for nuclear thermal
or nuclear pulse rockets, we could go to the moon and planets in a
single stage - with these rockets, which would essentially solve the
problem of practical interplanetary travel.

There are other approaches. There's the hypothetical 'scramjet'
technology. Here you attempt to use oxygen from the air to reduce
your effective propellant fraction, but then you face the fact that
when you blow on a flame, it goes out! And that's what happens to
ramjets that travel too fast. The flame blows out. Then you've got
the secondary issue of inlets and such - which tend to raise your
structur fraction to well above 15% There have been interesting
suggestions to resolve these problems. You could eject the propellant
at some speed from the vehicle so that it moved more slowly relative
to the air - and then detonated near the skin of the aircraft so that
propulsive effects were produced by the interaction of the shock wave
and skin. But, this presupposes the shock waves travel faster than
the vehicle. Which isn't always the case at high speeds!


Nuclear rockets (or equivalently laser heated rockets) delivery more
energy in a given mass of propellant, so they have higher exhaust
speeds. These type of rockets have been built

http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000143.html

NERVA - Nuclear Energy Rocket VEhicle Application - irc - produced as
much thrust as the SSME of today, but had an exhaust speed of DOUBLE
the SSME. This has a huge impact on the propellant fraction
requirements. A Space Shuttle External tank with 7 NERVA rockets
attached to its base, ensconed in heat sheild tiles, with four SRBs to
help at lift off - could form a SINGLE STAGE that could place 560,000
lbs into LEO - more than 10x the payload of the space shuttle.

A single NERVA rocket on a slimmed down ET - forming a 560,000 lb
payload - could send 100,000 lbs to Mars or the moon and bring it back
- and be reused!

Not too shabby.

Of course each NERVA engine at peak output puts out 5 GW of thermal
energy. That's 35 GW of nuclear power! About 10x the output of
Three mile island, in a space the size of a small office!

Of course this is nothing when compared to NUCLEAR PULSE ROCKETS -
like ORION.

http://www.astronautix.com/lvfam/orion.htm

These have exhaust speeds of 20 km/sec and more.


Laser Propulsion - is a way to replace the nuclear reactor with a
remote laser beam.

LSD - Laser Sustained Detonation - is a way to replace the nuclear
bomb in a nuclear pulse rocket with a laser beam

http://optics.nasa.gov/ast.html

Which has the potential to produce exhaust speeds of 20 km/sec or more
- giving us access to the solar system, without nuclear bombs and
nuclear reactors!

Of course, we still need to power the big lasers needed. This could
be done by sunlight!

http://www.vs.afrl.af.mil/News/99-22A.html

The URL above had a cool photo of an inflatable mirror that suggested
concentrating mirrors kilometers across could be built and launched
with very little mass. This has since been removed and is not in the
archives. So, if anyone can point to a new one, let me know.

It was entitled 'inflate.jpg'

Solar pumped lasers that reside at the focus of a mirror that
intercepts billions of watts of solar energy could beam controlled
amounts of energy safely to rockets rising from Earth as well as
rockets flying across the solar system.

MEMS - Micro mechanical - rockets could form a propulsive skin that
used intercepted laser energy to provide an extremely safe, reliable,
and quiet ride!

http://www.me.berkeley.edu/mrcl/rockets.html

Think of inkjet print head technology that delivers ink precisely to a
large area of paper - adapted to delivering rocket propellant instead
of ink precisely to a large array of rocket nozzles - each very tiny -
and you have the idea.

A computer controls a propulsive skin that provides forces across the
vehicle very efficiently.

http://www.memsjournal.com/Newsletter.htm

You may write to subscribe!

"Carey Sublette" wrote in message hlink.net...
"Ian Stirling" wrote in message
...
Christopher M. Jones wrote:
"Rats" wrote in message
...
Using existing technology if we were to build a Fission rocket then how
much
more efficient would than a Chemical rocket would it be? I believe
conventional rockets are capable of speeds around 8km/s. What sort of
speeds
could a Fission rocket get up to?

Chemical rockets are capable of speeds up to the speed of
light, given sufficient mass ratios and staging (note:

Nitpick:
Say one stage has a 5Km/s ISP, and each stage is double the weight
of the last.
300000/5 (neglecting relativity) is 60000 stages.
I make that a mass ratio of

63057948700178233572600261579236409495216587841434 361062005234596045
40006238697171501101348715304065265065961166212456 929797807660184547
23814941962225280444496680617986892514285389144879 868315356003294016
snip some 263 lines

If the final payload is one atom, the first stage will weigh considerably
more than the whole universe.

Or, to work the problem the other way - starting with the mass of the
Universe (3 x 10^55 g,
http://curious.astro.cornell.edu/que...php?number=342) we have a total
mass ratio of 1.8 x10^79 (assuming the payload is one hydrogen atom) we can
make the mass ratio larger by a factor of 1836.1527 by assuming the payload
is an electron, but this is supposed to be a *chemical rocket* using
reactions between atoms and we wouldn't want to get *ridiculous* about this!

This gives a maximum of 263 stages (with the same assumptions as Ian's) and
a burnout velocity of 1300 km/sec or 0.44% c.

(Note that a large majority of the mass of the Universe is invisible,
non-baryonic dark matter. This calculation assumes that this mass can be
converted to usable fuel.)
Carey Sublette


A much more interesting calculation follows...


First some data;

http://woodmansee.com/science/rocket...tellar-19.html
http://www.astro.umn.edu/~larry/CLAS...t%20Sails2.ppt
http://www.astro.ex.ac.uk/people/aa/...ght_sails.html
http://www.islandone.org/APC/Sails/01.html

We can accelerate 500 tons of payload at 1 gee with about 1,500 TW

The sun produces 386 billion TW.

Intercepting 1% of this light and putting it to use to accelerate
light sails means that 1.28 billion tons of payload could be
accelerated (and deaccelerated).

If acceleration is 1 gee and the speeds are 1/3 c, then we can say
that 3.86 billion tons of material can be sent to interstellar points
of call each year.

If 10% of the sun's output were captured and used in this way 38.6
billion tons of material can be sent to interstellar point of call
each year.

If other stars are setup to support two-way travel, they could
dispatch material onward from their locations including sending
material back to Earth.

So, let's say 20 billion tons of material can be sent to the nearby
stars each year, and 20 billion tons comes back. Using around 10% of
the sun's total output we can achieve this.

With a population of 10 billion, that's 2 tons per person.

This says that if economics can be worked out everyone everywhere
could travel among the stars using only a small fraction of the energy
the sun radiates uselessly away into space every year.

Interstellar travel (at 1/3 light speed) can be commonplace.

People living in space homes, communicating by radio telescope, could
form a sort of advanced interstellar culture, no problem.

Oops! Off by a factor of 1,000 - 20,000 billion tons of materials per year, sorry.

Carry on...

(william mook) wrote in
om:



This says that if economics can be worked out everyone everywhere
could travel among the stars using only a small fraction of the energy
the sun radiates uselessly away into space every year.

Interstellar travel (at 1/3 light speed) can be commonplace.

People living in space homes, communicating by radio telescope, could
form a sort of advanced interstellar culture, no problem.

Interesting. People going everywhere at .333C, and getting nowhere
particularly interesting within a human lifespan.

Doesn't sound like much fun. But does suggest that interstellar trade
and communication is still possible, and perhaps even practical.

--Damon