How much more efficient would Nuclear Fission rockets be?

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?

"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?

Rocket efficency is measured in terms of "specific impulse". Or to
use english units:

pounds thrust produced by the rocket divided by pounds mass of
propellent consumed per second = lb_thrust/lb_prop/sec= Isp

The very best chemical rockets use Liquid Hydrogen - Liquid Oxygen
propellent and get an Isp of around 450 in english units.

The nuclear powered NERVA rocket engines built and tested by NASA in
the 1960s got an Isp as high as about 900 if I recall right. One of
the basic reasons the Nerva project was shut down was that the Nerva
design had to be put in orbit by chemical rockets as it could not get
to orbit on its own as the thrust to weight ratio was too low.

Designs that were never tested full scale, specifically the "Dumbo"
project, should have been able to produce Isps of as high as about
1200. In addition it could reach earth orbit in one stage, again in
theory. Dumbo fuel elements were tested in nuclear reactors and found
to work well, however a full scale dumbo nuclear reactor design was
never built.

Another project was the Orian project that used a totally different
approach of detonating nuclear explosives (bombs) to propell very
large spacecraft that used a thick shield plate. In theory it should
work. Test models were built and flown in the 1950s using chemical
explosives to propell the model. In principle it should work fine,
and be very fuel efficent and able to lift huge payloads and go very
fast indeed. The enviromental ad legal concerns are the problem.
Lately a revised design that uses the principle of the Orian system
but micro nuclear explosions that are set off by pinching tiny pieces
of nuclear fuel rather than a traditional nuclear bomb is being
studied. The ISP on an Orian system in theory can be very high
indeed, as I recall and figure of 10,000 was considerd low.




I believe
conventional rockets are capable of speeds around 8km/s.

That is not correct. In thory any rocket can achieve any velocity if
you have enough propellent.

The formula is as follows:

DeltaV=Isp*g*LN(M0/M1)

Whe

DeltaV is the required change in velocity

Isp= specific impulse as discussed above.

g= gravitational constant 32.2 ft/sec/sec in english units

M0 is the initial mass of the rocket+ propellent

M1 is the final (after rocket burn) mass of rocket + remaining
propellent

and LN() is the natural log of whatever is inside the parenthesis ().



What sort of speeds
could a Fission rocket get up to?

A whole lot faster than a chemical fueled rocket with the same mass
ratio,

example assume that the mass ratio is 5 =M0/M1 then

the LH2-LOX rocket with and ISP of 450 is going at 7.11 km/sec

the Nerva rocket with LH2 Propellent heated in a Nerva nuclear reactor
to an ISP of 900 is going at 14.22 km/sec

the Dumbo rocket with LH2 Propellent heated in a Dumbo nuclear reactor
to an ISP of 1200 is going at 18.95 km/sec

and the minimal Orion rocket is booking along at 157.96 km/sec

"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 rough rule is twice the exhaust velocity. After that the required mass
ratios tend to get ridiculous.

LOX/LH2 will get you an exhaust velocity of maybe 4.4km/s in vacuum. The
most energetic chemical propellants I'm aware of is the tripropellant
LOx/LH2/beryllium, at about 5.3 km/s.

For a NERVA-type engine, figure about 8 - 9km/s exhaust using LH2.


Regards
Jonathan Wilson

(Mike Miller) writes:

"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?

Good chemical rockets have a specific impulse (Isp) of 450-460 sec^-1,
which is to say they can generate 450-460 pounds of thrust for one
second using one pound of fuel.

Good solid core fission rockets have a specific impulse of 1000, which
is approximately twice as efficient.

Nit: I_sp is not an "efficiency," since one is comparing apples to zucchini.
An "efficiency" is a _DIMENSIONLESS_ ratio that compares the output of
some "stufflike" quantity to the input of the _same_ "stufflike" quantity.

The terms "figure of merit" or "coefficient of performance" should be used
when one is reporting the dimensionful ratio of an output of one quantity
to the input of some different quantity.


-- Gordon D. Pusch

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

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.

(Gordon D. Pusch) wrote in message ...

The terms "figure of merit" or "coefficient of performance" should be used
when one is reporting the dimensionful ratio of an output of one quantity
to the input of some different quantity.

Granted and understood. I was just speaking colloquially.

Mike Miller, MatE

"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

Ian Stirling wrote in message .. .
Christopher M. Jones wrote:
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.

You snipped the full quote, here's what I wrote:
"Chemical rockets are capable of speeds up to the speed of
light, given sufficient mass ratios and staging (note:
quite a lot of dry humor there)."

The difficulty of obtaining more propellant mass than
the mass of the known Universe is merely an engineering
problem.

Christopher M. Jones wrote:
Ian Stirling wrote in message .. .
Christopher M. Jones wrote:
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.

You snipped the full quote, here's what I wrote:
"Chemical rockets are capable of speeds up to the speed of
light, given sufficient mass ratios and staging (note:
quite a lot of dry humor there)."

I know, that was why there was the Nitpick:.
It was also to point out to those that might just be lurking how
mind-bogglingly fast C is, compared to chemical rockets.

The difficulty of obtaining more propellant mass than
the mass of the known Universe is merely an engineering
problem.

"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