atomic_fungus (atomic_fungus) wrote,

#3759: More starship stuff

Gonna quote the bit that kneecaps my SF universe the worst, then go from there:
One issue your ships face is that fusion drives are very low thrust because they are necessarily very low mass flow-rate. This is due to the fact that it becomes exponentially more difficult to fuse larger masses of fuel. Assuming the above spaceship is trying to accelerate at 500 m/s^2 (around 50 g) that works out to a mass flow rate of 1000 tons/second!

rho = m*a/v = (16,000,000 tons * 500 m/s^2) / 8,000,000 m/s = 1000 tons/s

And that's for a fairly extravagant fusion drive (exhaust velocity of 8,000 km/s). Even with a pure photon drive (v = c) the mass flow rate is ~30 tons/s. Even if the back of your ship is festooned with 1000 engines each with their own fusion chamber you'd still have to be fusing a ton per second in each. And that's not the half of it; the thrust power of the propulsion system is 64 Exawatts (6.4E20 W). If the ship is made of titanium and the propulsion system 99% efficient then it will be completely vaporized 1/45 of a second after it turns on its engines, purely due to their waste heat. Not to mention the Kzinti Lesson (this propulsion system is a planet-killer).
I'm going to go with the density of 0.2 tons per cubic meter, because I'm not using materials which are currently available to build starships and it lets me fudge the numbers in my favor, even if it's only by a very, very, very little bit.

Clearly, the propulsion model for starships in my universe simply does not work, and I'm glad to have had this discussion now rather than after [Release Candidate One] is published.

...but for one thing: I do have inertial damping technology.
In order of least change to most change, the possible solutions seem to be:

1) The inertial dampener lowers the effective mass of the ship so that the engines actually have very low thrust; they're actually pushing what is effectively a 100 ton mass at 50 g. This raises the question of where the exhaust stream goes back to massing what it should be and the edge effects relating to that. Best just not to mention them.

2) Enormously lower the acceleration rate of your ships. If you stick to fusion you'll be down to hundredths of a m/s^2 before the propulsion system stops melting nearby moons, sadly.

3) #2 and pick a higher-thrust propulsion system. I favor Gas Core Nuclear Thermal Rockets (GCNTR) and accelerations around 1/10 g for ships in the 2-10 km range. Also a mass ratio of about 3 works well (gives a delta-v around 70-80 km/s) for most month-scale interplanetary trajectories. Such a ship can maneuver swiftly enough for space combat to be interesting on a human-scale attention span, too. Very space-Jutland. The advantage of this is that you can drop the inertial dampening, which tends to be a bucket full of unintended consequences anyway.

4) Make up a completely science-fictional drive system that gives you the performance you want. This is where Larry Niven and Jerry Pournelle went with The Mote in God's Eye, positing a perfectly efficient force field that sucks up the energy of the fusion reaction and re-radiates it as photons in the desired drive direction. Note that in this case you'll still have the problem of the drive plume being able to burn off the atmospheres of Earthlike planets. Admittedly that might be a fun premise to play around with :-D
Inertial damping in my world ends up completely eliminating inertia of objects. That is to say, when you throw a ball while in such a field--even if you throw it as hard as you can--the instant your hand stops pushing it, the ball stops moving. Gravity takes over and the ball falls to the deck, and when it collides with the deck it stops moving instantly regardless of how bouncy it is.

Firearms don't work in inertial fields, either. The instant the bullet leaves the barrel and the expansion of gas stops moving it, the slug stops dead and falls to the deck. This is one reason the military arms troops with laser weapons; slugthrowers are useless aboard ships.

Further, I don't rely on pure fusion thrust to move the ship. The ship also carries reaction mass (as mentioned in the last post) which is heated by fusion and expelled from the main engines. This mitigates--but does not eliminate--the problems presented above. The damned ship can't carry enough reaction mass to toss out 1,000 tons per second; not for very long, anyway.

Later in the history of my universe, a totally inertial space drive is developed based on the principles of the inertial damper. That one converts power supplied by fusion into kinetic energy almost directly, with a very high efficiency (say, 80% or so) and therefore can provide stupid-high accelerations.

...some of the issues can be handled with minor changes. Going about 30 AU takes five days at eight gravities, and there's no real need for passenger vessels to be capable of 50 g. 10 g (100 m/s/s, since a Standard Gravity is 10 m/s/s in my world--and yes, Earth has a surface gravity of 0.98 Standard) would be plenty of boost. Still problematic, but less so than the 50 g maximum, and perhaps within the Margin of Handwave.

But not all of it. Because for the example given, that's still 200 tons per second; and even if most of it is reaction mass it still is too much. *sigh*

Even so, that's peak thrust. What if you keep it down to 1g? Suddenly it's 20 tons per second, and your trip time takes...two weeks. And 50,000 tons of reaction mass lasts 40 minutes at that rate.

...I'm beginning to wonder how Larry Niven got away with all his stories about Bussard ramjets.

Well: for one thing, 8,000 km/second is a glacial exhaust velocity for a fusion drive. 12% of C seems to be the commonly-accepted number (Google-fu to the rescue) and that's 36 million m/s, 36,000 km/s.

With that kind of exhaust velocity, you're talking about 5 tons of reaction mass per second, and that gives you 10,000 seconds of thrust from 50,000 tons of reaction mass. (3 hours and 45 minutes at one g. Better. Not good, but better.)

Of course that's for a proton-proton fusion reactor. I might as well wish for magic.

...all of which, of course, ignores the fact that I do have inertial damping technology in this universe, so I don't need to worry as much about this as I let on here. Maneuvering thrusters could consist of guys pissing off the corners of the ship with inertial damping! But with inertial damping, of course, suddenly your top speed is limited to your specific impulse, and 36,000 km/s is nothing.

So let's talk about the physics of the inertial damper for a moment, and see where that gets us.

Think, for a moment, of an inductor in an electrical circuit. When you apply a voltage across the inductor, it has a certain impedance; for a few moments no current flows. Forcing electrons through the coil forces it to build a magnetic field which gradually decreses the impedance, until the thing reaches a steady state where it appears to have no resistance to the voltage. When you remove the voltage, then, the current wants to keep flowing; and in the case of a really big coil that current will flow regardless.

This is analogous to moving a mass: you have to push on it to get it moving, and you have to push on it to make it stop.

So, in a way, pushing that mass is like applying a voltage to an inductor: the longer you apply the force, the faster the mass moves, but only up to the velocity at which the force is applied.

In a purely inertialess field, your exhaust velocity is your top speed.

But inertial damping doesn't have to work that way. In fact--as was suggested above--you could damp only parts of the ship, and use the undamped portion as a kind of flywheel.

...I think I've found the solution to my problem. This is how I can use reasonable fusion drives and still get 50 g of acceleration from them: instead of accelerating sixteen million tons, I'm accelerating--say--fifty tons, and the rest is inertially damped so it doesn't matter. It means I can't play tennis or golf or baseball aboard the ship while I am maneuvering in real space, but my stories already feature that, anyway.

*whew* I feel better now. I don't have to frantically rewrite anything.

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