As I mentioned last time, in The Future of Space Flight – Nuclear Propulsion, the nuclear thermal engine is a necessary step for moving cargo and fuel in near-Earth Space. This isn’t to say that nuclear engines aren’t capable of taking us to the outer planets, but the ISP gains still leave us shipping a huge amount of fuel. In the distant future, we may have a fuel depot in the outer planets. I can envision a robotic ice station in Jovian orbit, however, it isn’t near or necessary.
I’d rather not get into the depths of ISP, there is a Wikipedia if you need it, but lets just say that the push part of a rocket has 2 parts (theoretically) the mass of the engine and the mass of the fuel. The ISP really just talks about the fuel needed. ISP of 500 means that 1 tank of fuel gets you to … lets just say 10 km/s. If your ISP changes to 1000, you’d only need 1/2 a tank of fuel to get you to 10 km/s. If your ISP goes to 10,000 – you only need 5% of the fuel your first space ship needed. This leaves out that the engine might weigh ten times more. Meh, Rocket Science is hard.
What is best for near? Ion drives. The Nuclear Thermal engine has an ISP around 1000 s. Maybe, when they cross an MWatt, they may look at one of the enhanced propulsions, which may lead to 2000 s ISP. BUT, that starts trading on excess electric power, which is excess weight. (Not as excess as all that, since you could assume that the delivery of cargo included delivery of a working nuclear reactor, but, this works with Ion drives as well.)
Ion drive? From Hall Effect to VASIMIR, they involve the same things:
- Heat something up a lot. (Argon, Helium, Lead) till it becomes an ionized gas. (Shoot the lead with a laser, works fine.)
- Confine and heat the plasma
- Let the Plasma Escape, slowly, at great temperature.
The VASIMIR has a low-ish ISP, around a few thousand, but the thrust can be significantly higher than Hall Effect thrusters.
VASIMIRs are good for near planets, where the balance of thrust and low fuel use gives you short mission times. You may drop the Mars trip, a distance 1 – 3 AU depending on date, to six months or less. Very reasonable. Jupiter, at a distance of 4 -6 AU, would take about a year. However, at that time, we start getting in fuel to mass ratio issues again.
The Hall Effect Thruster may have an ISP on the order of 40,000.
What does that “MEAN?” It means that a ship with a Hall Effect thruster will have a very high final velocity. It may take years to reach that velocity. Hall Thrusters usually run about a ten micro G. A push felt that would leave an adult male weighing in at 220 lbs on the Earth, at about 1 gram on the spacecraft. Literally, you couldn’t feel the thrust.
Now, Alta’s Hall thruster has much lower ISP (factor of 10) and much higher thrust (factor of 10) than theoretical, but that’s one of the trade offs. If you design a mission, the farther away you are going, the lower, constant, thrust you can deal with if your ISP is high enough.
Your final velocity could easily be in the 10’s of km/s. Velocities like that let you get to Pluto with enough fuel left to park, not just fly by into infinity. Had the mission designers agreed to a better power supply for the New Horizon mission, then they could have selected an Ion drive, and be parked around Charon as we speak.
Not that I think Pluto is anything but a big comet, but think about a decade of data, instead of one picture. That’s the future of spaceflight.