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
