Stephen A. Lawrence wrote:
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).
I don't understand this. It's not obvious to me how you give a
power rating to a rocket engine, save possibly by rating the energy
released by the fuel (most of which is typically lost as heat).
See Ref. 1, p. 73:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899_1992005899.pdf
This shows 4 nuclear rocket engines with a rocket engineer standing
between them. #4 is a Phoebus 2, 1967, 5000 megawatts output (!!!),
250,000 lb thrust. It is only about 3 times taller than the guy,
which is astounding to me. I did not know it was possible to generate
5 GW in an object this size without the whole thing melting.
Also, this engine uses a mere 121 kg of hydrogen propellent per second.
Anyway -- if I have this right -- 250,000 lb thrust = 1,112,055 N,
and 1 G of thrust on 1 kg of mass = 9.8 N. In other words, this
gadget could push 113,475 kg at a steady 1 G. 113 tons. And if that
mass was going from earth to Mars in 3 days (302,400 seconds) it
would use 37,000 tons of hydrogen . . . give or take. So that doesn't
work out very well.
Of course conventional rocket engines are not meant to run for 3 days
straight, but I would like one that does. So obviously we need a much
higher amount of energy imparted to each kg of propellant. The
Phoebus 2 imparts 41,322 kW per kg of propellent, which is way better
than a solid or bipropellant rocket, but still 30 times below the
1.25 GJ per kg imparted by an ion thruster. See:
http://en.wikipedia.org/wiki/Spacecraft_propulsion
As long as it's a rocket, the rocket equation dominates everything,
and you are stuck with delta-V being proportional to exhaust
velocity. So, you want as high an exhaust velocity as
possible. Ion engines can approach V_e=C, which is as good as it gets.
The exhaust velocity of an actual Ion engine is about 50,000 m/s, way
below the speed of light. Maybe there are newer ones that do better?
I would like to know what can actually be done with present day
materials and techniques. Anyway, this is only 30 times better than
the Pheobus, which is nice but . . .
What I want to know is, is it even theoretically possible to make an
Ion thruster of approximately the same mass as the Phoebus 2 that
produces 5 GW of power? It would be great if the thing could impart,
let us say, 1000 times more energy per kg to the propellant. That
would cut the 37,000 tons down to 37 tons, which is quite manageable.
It much less than the fuel burden of a 747 that flies its maximum
range (about 96 tons).
Heck, I'd settle for one that is merely 300 times better than the
best nuclear engine they could devises back in 1967. Is that too much
to ask for?
Still, we have a ways to go before we can drive that 50,000 ton
vehicle at 1 G continuously. I guess I will have to settle for flying
to Mars in a month (0.02 G continuous acceleration -- 25 days).
- Jed
