How much more efficient would Nuclear Fission rockets be?
Rats
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?
Mike Miller
> 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. For the same mass fraction of
fuel in the ship (say, if both rockets had 9 parts out of 10 of their
mass as fuel/reaction mass), that equates to about twice the delta-V.
> I believe conventional rockets are capable of speeds around 8km/s.
>From a hypothetical "dead stop," the Saturn V Apollo stack had
11.3km/s (plus more, if you count gravity losses of about 1500m/s)
just to fling the large command/service module & LEM stack to the
moon. The CSM+LEM then had the juice to brake into lunar orbit
(~900m/s) and leave lunar orbit (~900m/s), while the LEM was able to
land on the moon (requiring about 2000-2400m/s, IIRC) and return
(another 2km/s). So the overall Saturn V stack and CSM+LEM totalled up
to a LOT more than 8km/s.
> What sort of speeds could a Fission rocket get up to?
The first question is, "relative to what?" A probe leaving Earth with
a 5km/s speed relative to Earth may approach a comet head on at
20km/s.
Kinda depends on the rocket. There's a lot of little engineering and
launch site details that'll affect the final delta-V. For example, the
favored hydrogen reaction mass of nuclear fission rockets requires
larger, heavier tanks relative to the mass of fuel carried than
chemical fuels. Nuclear rockets are heavier per pound of thrust than
chemical rockets. A chemical rocket will usually have lower gravity
losses launching from Earth than a fission rocket, an advantage that
doesn't matter as much for launches from the moon or deep space.
Mike Miller, MatE
Christopher M. Jones
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). Modern chemical rockets
with existing technologies are capable of a lot of delta
V, perhaps up to 20 km/s or more if you want to stretch it.
8km/s is about what is necessary just to get to low Earth
orbit. It takes more to get to geostationary orbit, or to
escape velocity.
Anyway, delta V mixes together a lot of different aspects
of rocket performance. What you want instead is exhaust
velocity or the closely related metric "specific impulse"
or Isp (basically exhaust velocity divided by Earth's
gravitational acceleration, given a figure in units of
time, seconds are standard). For a given rocket stage,
the mass ratio (wet mass to dry mass, including payload
mass in both) scales exponentially with the ratio of
delta V to exhaust velocity. Thus, if you make lighter
fuel tanks, or lighter structures, or lighter engines you
can increase the maximum mass ratio for a rocket stage
and increase its delta V. Or, you can stack rocket
stages and throw away the dead weight when they're used
up and achieve a higher total delta V that way. Or, you
can increase the Isp of the engine and boost delta V or
decrease mass ratio for a specific delta V that way.
Because of the exponential, boosting Isp pays off pretty
big. Of course, all these things are interrelated in
convoluted ways that make everything quite a bit more
complex than a simplistic characterization would assume.
For example, switching to a higher Isp engine usually
also affects other things like the mass fraction of
propellant tanks and such-like.
"Fission rockets" come in a huge variety of designs.
The simplest and most direct is a conventional solid
core fission reactor which heats up a propellant
passing through the core, this is called a Nuclear
Thermal Rocket or NTR. The advantage of this design
as a rocket is almost solely due to the ability to
heat up propellant to high temperatures without
chemically reacting the propellant itself. The
achievable propellant temperatures are actually lower
than for decent chemical rockets, but an NTR can heat
unmodified Hydrogen to higher temperatures than than
any chemical rocket (which would necessarily have to
react and use up most of the Hydrogen). Since
Hydrogen has an extremely low atomic mass (2) it has a
higher average molecular velocity at a given
temperature than other, heavier molecules. And thus,
it has a higher effective exhaust velocity than
rockets which generate exhausts predominated by
heavier molecules. Isps for solid-core NTRs easily
reach to twice what top of the line chemical rockets
are capable of. Put that in your exponential and
smoke it! That means that, all things being equal
*ahem*, you either double your delta V or *square
root* your needed mass ratio (e.g. 4:1 instead of
16:1). However, all things are not equal, as NTRs run
with a list of caveats about an arm's length long on
how the effect everything else. Especially since the
reliance on the extremely low-density Hydrogen as the
sole propellant dramatically affects the achievable
mass ratios for a given stage. In general, these
designs are best for secondary stages on top of a
chemical first stage that gets up out of the
atmosphere and going at a decent speed (a few km/s).
In that role they have the potential to really shine.
Beyond that there are roughly a gazillion fission
rocket designs ranging from gas-core NTRs (which can
achieve higher temps and thus higher Isps), Orion,
Medusa, NSWR and some even more exotic beasties.
There's no hard upper limit on Isps for these, but
easily tens, hundreds, and thousands of times higher
than modern chemical rockets are pretty safe bets.
alfred montestruc
> 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)
Where:
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
Jonathan Wilson
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
Gordon D. Pusch
> "Rats" <ru...@chump.com> wrote in message news:<c37vjt$d25$1...@news.f.de.plusline.net>...
> > Using existing technology if we were to build a Fission rocket then how much
> > more efficient would than a Chemical rocket would it be?
>
> Good chemical rockets have a specific impulse (Isp) of 450-460 sec^-1,
> which is to say they can generate 450-460 pounds of thrust for one
> second using one pound of fuel.
>
> Good solid core fission rockets have a specific impulse of 1000, which
> is approximately twice as efficient.
Nit: I_sp is not an "efficiency," since one is comparing apples to zucchini.
An "efficiency" is a _DIMENSIONLESS_ ratio that compares the output of
some "stufflike" quantity to the input of the _same_ "stufflike" quantity.
The terms "figure of merit" or "coefficient of performance" should be used
when one is reporting the dimensionful ratio of an output of one quantity
to the input of some different quantity.
-- Gordon D. Pusch
perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;'
Ian Stirling
> "Rats" <ru...@chump.com> wrote in message news:<c37vjt$d25$1...@news.f.de.plusline.net>...
>> Using existing technology if we were to build a Fission rocket then how much
>> more efficient would than a Chemical rocket would it be? I believe
>> conventional rockets are capable of speeds around 8km/s. What sort of speeds
>> could a Fission rocket get up to?
>
> Chemical rockets are capable of speeds up to the speed of
> light, given sufficient mass ratios and staging (note:
Nitpick:
Say one stage has a 5Km/s ISP, and each stage is double the weight
of the last.
300000/5 (neglecting relativity) is 60000 stages.
I make that a mass ratio of
63057948700178233572600261579236409495216587841434361062005234596045
40006238697171501101348715304065265065961166212456929797807660184547
23814941962225280444496680617986892514285389144879868315356003294016
<snip some 263 lines>
If the final payload is one atom, the first stage will weigh considerably
more than the whole universe.
Mike Miller
> The terms "figure of merit" or "coefficient of performance" should be used
> when one is reporting the dimensionful ratio of an output of one quantity
> to the input of some different quantity.
Granted and understood. I was just speaking colloquially.
Mike Miller, MatE
Carey Sublette
"Ian Stirling" <ro...@mauve.demon.co.uk> wrote in message
news:4OG8c.23859$h44.3...@stones.force9.net...
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/question.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
Christopher M. Jones
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.
Ian Stirling
> Ian Stirling <ro...@mauve.demon.co.uk> wrote in message news:<4OG8c.23859$h44.3...@stones.force9.net>...
>> Christopher M. Jones <vege...@dualboot.net> 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
>>
>> 63057948700178233572600261579236409495216587841434361062005234596045
>> 40006238697171501101348715304065265065961166212456929797807660184547
>> 23814941962225280444496680617986892514285389144879868315356003294016
>> <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.
Sander Vesik
Sounds like a design that involves two white holes one of which
outputs hydrogen and the other oxygen might do it ;-)
--
Sander
+++ Out of cheese error +++
william mook
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
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