Meta rears its head again, thanks to recent discussion on the thread that refuses to die. Short form, one commenter asked about the implications of space warps, and another pointed out that as magitech they are outside the scope of this blog.
I've never formally defined the scope here, but in a practical sense it is surely true. In fact, regular readers here know that I am downright stodgy. (I told Orville, and I told Wilbur, and now I'm telling you - that thing will never work.) My justification is that at least the doors to space, orbit lift and basic space operations, are mature technologies.
The space age is about as old, mature, and well established as commercial jets. And we have in fact achieved all the basic operational Cool Stuff, including building building a space station, keeping it in service over years, and test flights to the Moon, even if we did it in a contrary order.
What keeps us from doing more of it is not technology but how expensive it has turned out to be. We've had that discussion before, and will no doubt have it again.
There are other reasons for my bias toward Realistic [TM] technology. One of them - the underlying one, in fact - is aesthetics, which I will get to in another post. But another reason is that I like to play with numbers.
Technologies 'indistinguishable from magic' are not very easy to quantify, or if you do, the numbers are equivalent to spell points. Magical and technomagic systems can and should mesh together internally, but their rules are as arbitrary as the rules of football. Which is why those arguments about Trek versus Star Wars are like arguments about why World Cup teams never go for a touchdown.
Torch drives are another matter. I have always been partial to torchships, from the evocative name to the plausible implied physics. A torch is conceptually a rocket engine, just using some high energy reaction such as fusion. Yes, controlled fusion has been 20 years away for 60 years, but it is hardly magical to suppose that by 2100 or so, after 150 years of trying, we might get the hang of it.
Let us say that our ship has a mass of 1000 tons, and a modest exhaust velocity of 300 km/s, a mere 0.001 c. In the name of further modesty we will set our acceleration at less than one third g, 3 meters per second squared. This ship is very much on the low end of torchships; Father Heinlein would hardly recognize her. If our torch has VASIMR style capability, we can trade specific impulse for thrust; dial exhaust velocity down to 50 km/s to develop 1.5 g, enough to lift from Earth. (But see below.)
Setting surface lift aside, let's look at travel performance from low Earth orbit. We will reach escape velocity about 15 minutes after lighting up, a good Oberth boot, but with this ship it hardly matters. Suppose half our departure mass is propellant, giving us about 180 km/s of delta v in the tanks. Since we must make departure and arrival burns, our transfer speed (relative to Earth) is about 90 km/s.
Holding acceleration to 0.3 g as we burn off mass, we'll reach 90 km/s in less than eight hours, at about three times lunar distance from Earth. (If we were going to the Moon, we'd need to swing around three hours out for our deceleration burn.)
For any deep space mission we'll end up coasting most of the way, and 90 km/s is some pretty fast coasting. Even this low end torchship lets you take retrograde hyperboloid orbits, which is Isaac Newton's way of saying you can pretty much point & scoot.
You will truck along at 1 AU every three weeks, reaching Mars in little more than a week at opposition, though up to two months if you travel off season. The near side of the asteroid belt is a month from Earth, Jupiter three months in season. For Saturn and beyond things do get prolonged.
Still, 90 days for the inner system out to Jupiter is nothing shabby, and there's no apparent High Magic involved.
How much power output does out modest torch drive have? The answer, skipping simple but tedious math, is not quite half a terawatt, 450 GW. This baby has 600 million horses under the hood. That's effective thrust power. Hotel power for the hab is extra, and so is getting rid of the waste heat.
Putting it another way, this drive puts out a tenth of a kiloton per second, plus losses.
And yes, Orion is the one currently semi plausible drive that can deliver this level of power and performance. I say semi plausible because, broadly political issues aside, I suspect that Orion enthusiasts gloss over the engineering details of deliberately nuking yourself thousands of times. Like banging your head on the table, it only feels good when you stop.
The good news, for practical, Reasonably Fast space travel, is that we don't need those near-terawatt burns. They don't really save much time on interplanetary missions - we could reduce drive power by 90 percent, and acceleration to 30 milligees - the acceleration of a freight train - and only add three days to travel time.
(Our reduced 'sub-torch' fusion drive is still putting out a non trivial 45 gigawatts of thrust power, which happens to be close to the effective thrust power of the Saturn V first stage.)
If you want Really Fast space travel, however, you will need more than this. Go back to our torchship and increase her drive exhaust velocity by tenfold, to 3000 km/s, and mission delta v to 1800 km/s, while keeping the same comfortable 0.3 g acceleration.
A classic brachistochrone orbit, under power using our full delta v, takes a week and carries us 270 million km, 1.8 AU. Add a week of coasting in the middle and you're at Jupiter. Saturn is about three weeks' travel, and you can reach distant Eris in 6 months.
Drive power output of our upgraded torchship is now 4.5 TW, about a third the current power output of the human race. Which in itself is no argument against it. Controlling the reaction and getting rid of the waste heat are more immediate concerns.
As for a true, Heinleinian torchship? Heinlein's torch is a mass conversion torch. He sensibly avoids any details of the physics, but apparently the backwash is a mix of radiation, AKA photon drive, and neutrons, probably relativistic.
Torchship Lewis and Clark, pictured above, is about 60 meters in diameter, masses in on the order of 50,000 tons, and in Time for the Stars she begins her relativistic interstellar mission by launching from the Pacific Ocean at 3 g. I don't know how to adjust the rocket equations for relativity, but the naive, relativity-ignoring calculation gives a power output of 225 petawatts, AKA 225,000 TW, AKA 53 megatons per second.
Do not try this trick at your homeworld. I don't know whether Heinlein never checked this calculation, did it and ignored the results, or did it and decided that a few dozen gigatons - 12 torchships are launched, one after another - was no big deal since it was off in the middle of the Pacific somewhere.
Maybe he only did the calculation later, because in other stories the torchships sensibly remain in orbit (served by NERVA style nuke thermal shuttles).
You don't need full Heinleinian torchships even for fast interplanetary travel, but there is another side to this whole discussion. Over a couple of hundred years, the era I've been calling the midfuture, would we really expect the propulsion technology to remain essentially unchanged?
In Starman Jones, Heinlein gives a wonderfully evocative picture of the passage of time and technology:
But directly in front of the gate hardly half a mile away was a great ship that he knew at once, the starship Asgard. He knew her history, Uncle Chet had served in her. A hundred years earlier she had been built out in space as a space-to-space rocket ship; she was then the Prince of Wales. Years passed, her tubes were ripped out and a mass-conversion torch was kindled in her; she became the Einstein. More years passed, for nearly twenty she swung empty around Luna, a lifeless, outmoded hulk. Now in place of the torch she had Horst-Conrad impellers that clutched at the fabric of space itself; thanks to them she was now able to touch Mother Terra. To commemorate her rebirth she had been dubbed Asgard, heavenly home of the gods.
Sometimes you just have to be propelled by the sheer power of cool.