Jed Rothwell wrote:
See:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899_1992005899.pdf
http://en.wikipedia.org/wiki/Nuclear_thermal_rocket
It is interesting to think about how one might apply high temperature
CF for a rocket engine. I am rewriting my book, based on the Japanese
edition which I just finished writing. I am thinking about beefing up
the rocket propulsion section. My problem is that I know next to
nothing about rockets, so I better run this subject by the audience
here, especially Ed Storms who is an expert in nuclear propulsion.
I would like to know how much mass of propellant a rocket would
require to launch from earth to orbit, and from earth to Mars. Based
on the Wiki paper it seems fission rockets from earth to orbit did not
have many advantages over conventional ones, but the transit to Mars
would be a lot faster.
A 50 MW engine described in Ref. 1 consumed 2.36 kg/s of hydrogen
propellant (0.05 kg/MW), and a 5 GW NERVA rocket that was the planned
would have consumed 121 kg/s (0.02 kg/MW). This 5 GW unit would have
been remarkably small. If I do my calculations right, it would produce
as much energy in one day (0.97 days) as a 100 kiloton nuclear bomb,
which is astounding.
For a deep space engine, people have been talking about using high ISP
Ion thrusters. According to Wiki and other sources, these have about
an order of magnitude better than liquid fuel rocket engines, but a
very poor power/weight ratio, and a very low propellant flow.
Apparently you cannot just increase the flow to any level you like.
Perhaps with a CF power supply you could generate 50 MW or even
gigawatts continuously for months.
http://en.wikipedia.org/wiki/Ion_thruster#Thrust
According to Wiki the best possible ion engine would be linear
particle accelerator for specific impulse of 30 million seconds (!)
but you cannot push much mass through one so the actual thrust is
negligible. It is not clear to me whether this is a design limitation
or whether it is because people do not have portable 5 GW electric
power supplies.
I am not sure what kind of generator would work for this.
Thermoelectric generation might work; electrohydrodynamics would be
great; but I was thinking perhaps one could use water to drive a steam
turbine, condense and recycle some of the steam (with large cooling
fins I suppose), and then reheat some of the other waste steam for
propellant.
Plan B might be a high temperature CF can be used to heat the
propellent (hydrogen or water) to high temperature gas, and perhaps
something like lasers with CF generated electricity then boosts the
gas temperature far above the melting point of the CF cell, kind of
like an inertial confinement hot fusion reactor. Of course converting
heat to electricity and using lasers would be energy inefficient but
as I said the idea would be to conserve propellant.
What we want are rockets that can achieve continuous 1 G thrust with a
payload of, say, 20,000 DWT (a small freight ship). Assuming the ratio
of ship to payload is the same as a Boeing 747, the empty ship would
weight about 30,000 tons. I have no idea how much the propellant would
weigh, or how much energy it would take. 1 G carries you to the moon
in ~3 hours, which is about as long as I care to be crammed into a
seat. I am not sure how long it would take to get to Mars at kind of
acceleration (of course it depends on how far away Mars is at the
moment) . . .
NASA says Mars is usually about 78,300,000 km away
(http://aerospacescholars.jsc.nasa.gov/HAS/cirr/em/9/2.cfm) and it
takes about 6 months to get there, but they figure it can be reduced
to 4 months (http://nssdc.gsfc.nasa.gov/planetary/mars/marsprof.html).
With constant 1 G acceleration I gather it would take around 3 days.
That's more like it! See:
http://www.cthreepo.com/cp_html/math1.htm
Enter 39 million for half the trip; ignore earth's gravity. This comes
out 2 days.
A more sophisticated calculator:
http://home.att.net/~srschmitt/script_starship.html
For Mars, enter 1.5 AU (from data shown below on this page), and 1 G.
It comes out 3.5 days. The longest trip in the solar system would be
17 days. Alpha Centuri is 3.5 years for the person on board, 5.9 years
earth time, taking into account special relativity.
20,000 DWT is fine for Mars, but for interstellar travel you want to
bring all your stuff. So let's Think Big. Even 30,000 tons is peanuts
by the standards of modern container ships. For interstellar travel
done right, I say take a fleet of 1,000 container ships, each with a
151,000 tons payload. Now that would take a lot of energy and a lot of
propellant!
- Jed
Jed I know something about rockets. The key irony of the cold fusion/
plasma fusion fight is that cold fusion powered plasma rockets is always
where interplanetary space propulsion was going. A plasma rocket is a
plasma reactor with a controlled leak. Unfortunately plasma reactors
don't actually work :-P Even with no leaks so one with a leak would be
even less likely to work! All is not lost however; with a bank of cold
fusion reactors providing the power we don't need stable break even in
the plasma reactor we can run the think hot and unstable. The result
would be fast 2000+ isp to Mars or elseware in the system.
In the atmosphere we could make fusion ram jets and fusion electric
fans that take a craft to the edge of space where much less fuel is
required to get into orbit. Above 30 miles you could actually dive into
orbit gaining most of your momentum from a crude high altitude gravity
slingshot effect.
It is possible to your a slow rocket design fueled with water using
fusion to both crack the water into Oxygen and hydrogen , super heating
both and feeding them to a rocket nozel. This would give a low Isp but
a good thrust with an easy to get and store fuel. Balanced with a ion or
plasma engine it would give a high thrust start and then a low thrust
high ISP sustained transit. In theory you can combined a Oxygen after
burner to the super heated hydrogen gas flow from a nuclear rocket or a
plasma rocket [assuming your also feeding in electrons to convert the H+
ions in combustible Hydrogen.
Take a look a Bob Zubrins work on mars direct.
http://en.wikipedia.org/wiki/Mars_Direct The plan has been accepted as
the only way to goes, even Nasa is using it in their design. A cold
fusion design is easy, CF units on a robot ship powers a powerplant to
make fuel on mars for the return of sample and people to earth a manned
ship is then sent when the return fuel is ready. CF makes it all very
much easier particularly if its a robust, maintenance free CF system.
I do not believe we will see a cold fusion thermal rocket using simple
boiled fluid; Such systems are compact and simple but the ISP stinks.
Also remember the Eugene Malove wrote on rocketry, The Spacedrive book
and I think he wrote on CF spacedrives early on. I'll hunt up a reference.
There was also some discussion of space propulsion way back in the
Fusion Facts period, Hal Fox is a rocket scientist and it would be
useful to chat with him on the subject. and check the Fusion facts
archive CD's for papers.
Please note: Many it seems have not heard of the high temperature
cartrage fed CF design. Gas discharge cells are charged and primed in
parallel on rack and then discharged in series in a heat engine or jet
turbine. The spent units are fed back to the rack for repriming or
recycling. in effect the fusion cells are used like ammunition in a
"gattling reactor". I think this design was cooked up by Hal Fox way
back in 1990-91. It seems to have been forgotten or never described to
new people in the field.
