Re: Heavy Lift Design for Mining/Cargo Propulsion
- From: Willie.Mookie@xxxxxxxxx
- Date: Sat, 19 Apr 2008 16:30:46 -0700 (PDT)
Rocket engineering is a little more sophistticated, even in the
basics, than you put forth here;
Check out these articles
http://www.engineeringatboeing.com/articles/turbopump.htm
http://space.au.af.mil/primer/rocket_theory.pdf
These are chemical engines, there are similar articles for nuclear
thermal rockets. Then, there are nuclear pulse rockets, where the
mechanisms and numbers re way way different.
Basically if you run an engine continuously, you've got to inject
propellant against the chamber pressure, and that takes a certain
rather large percentage (around 3% iirc) of the power of the jet.
Which becomes MONUMENTALLY huge when you are talking about very hish
performance high thrust engines.
Furthermore, most regeneratively cooled engines, recycle about 1% or
more of the thermal energy to keep the throat from burning out. This
doesn't work so well when you move beyond nuclear thermal or chemical
engines. In fact the throat of a nuclear thermal rocket is cooler
than a chemical rocket. The major improvement in performance comes
from the use of liquid hydrogen - which is 1/9th the mass of hydrogen
enriched steam the next highest performing genreally available rocket
(when you burn hydrogen and oxygen in a 6:1 ratio)
When you talk nuclear gas core rockets with a continuous burning -
fuhgetaboutit. You're just too hot, and too energetic to use these
sorts of deals. That's what makes nuclear pulse so cool.
First when you have really really fast pulses of energy interacting
with things, you have very little spread of heat energy. The classic
example is an ember from a burning fire falling on your carpet. You
try to pick it up, and you get burned. You flick it back into the
fire and you are safe. Why is that?
Heat transfer is a diffusive process
Momentum transfer is not
So, that's the answer to dealing with really really high energies -
you hit something with energy fast enough, all the kinetic stuff takes
place before heat gets a chance to diffuse away. That's why they can
use ultra-fast laser pulses to machine C4 and not worry about an
explsion
http://www.galasers.com/pdf/double_pulse_machining.pdf
There is a cool drawing in my rocket text book back in the day, which
is in my attic in a box, and I don't want to go find it to even check
the title - anyway - it is a flow diagram showing the percentages of
energy that are generated in a thrust chamber and how they flow
around.
Now, when heat is not being flicked away through ablation, or carried
away through evaporation, into the vacuum, the only thing you've got
to cool things down, is radiation. Stephan Boltzman is your friend
in that case.
P = 5.67e-8 J/m2/K/s * Area * Temp ^ 4
Now, I did a calculation this morning showring that if you had a 150
meter diameter plate, you'd have something like 35,342 sq m of
radiative area (both sides of a circular disc) and you absorbed just
1/2 percent 0.5% of the jet energy INTO the plate (a really high
number btw for nuclear pulse, really low number for steady state
engines) - you'd have to be at the melting point of Boron to radiate
away the energy produced by a 70,000 metric ton force thrust jet to
accelerate a handy class space freighter (20,000 tons payload and
15,000 ton empty vehicle0 at two gees. So, your plate would be
tungsten or carbon composite. Thrust to weight would be something
like 14 to 1. (5,000 ton mass)
Having a large copper body doesn't change this. Look at the size of
the sun, its the temperature it is at because the surface area and
energy release are such that it rises to the temperature it needs to
be for those two things to balance - there's also an effect of
temperature on the SIZE of the sun - which is a whole otehr topic.
I don't want to say you're clueless in your commentary - but pretty
much - you are.
Most nuclear pulse spacecraft coat the active area with material that
gets ablated away by the rapid pulse. A flash bulb type blast is
better than a long duration blast for this reason. Not only is
ablated stuff potentially higher performing - more energy per unit
mass - but less material carries away more heat than if you relied on
evaporation rather than ablation processes. Check it out, flashing a
ton of water into a ton of steam each second absorbs something like
2.7 billion watts of energy, and flashing a ton of liquid ammonia into
a ton of gaseous ammonia, absorbs 1.1 billion watts of energy. The
Handy size ship at 70,000 tons of thrust, uses 14,000 kg/sec of
propellant (of which 70 grams per second is the atomic charge, and the
jet generates 17 TRILLION watts of power and 0.5% of that is 82.5
billion watts - which radiated from a 150 meter diameter pusher plate
heat it up to 2,900 K.
Like I said I worked it all out in detail, but mu computer froze up
and I had more important stuff to do until now, I'm just recounting
the numbers I got - I worked it out on a napkin at a coffee shop I had
breakfast at, and threw the source materials away! lol.
.
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