Odd O'blography

A Long Time Gone

Trigger Warning: This post contains math.

Reality is a harsh mistress and for interstellar travel, physics is a bitch. It takes light, which weighs almost nothing, more than four years to travel from our solar system to Alpha Centauri. How long would it take for even a very small spaceship?

Let's assume the ship has a mass of 15,000 kilograms (33,000 lbs.), about the same mass as a large motorhome. We'll limit acceleration to 1G (9.8m/sec²) for passenger comfort, and top speed to 0.8C because the energy required for acceleration goes up exponentially as we approach light speed. With this mass and acceleration, the trip would take 6 years each way. So, how much thrust would we need and how much fuel?

The thrust calculation is fairly simple, Force = mass x acceleration.

15,000 kg x 9.8m/sec² = 147,000 Newton (323,400 lb).

The problem is that we need to apply that thrust for approximately six months to accelerate and another six months to decelerate (in between, we'll coast for five years at 0.8C). And that's only one way. We also have to carry enough fuel to do it again on the return trip.

The fuel requirement is much more difficult to compute, but for simplicity, let's consider the Saturn V rocket that took Apollo 11 to the moon. It had three stages. Stage 1 held 2.1 million kg of fuel and provided 34,500 kilonewtons of thrust for 168 seconds. Stage 2 held 440,000 kg of fuel and provided 4900 kN of thrust for 6 minutes. Stage 3 held 110,000 kg of fuel and provided 890 kN of thrust for 8 minutes. The total fuel carried was 2.65 million kg and it provided a burn time of about 17 minutes.

If we assume that the Saturn V engines could be 'throttled back', then all three stages could provide 147,000 Newtons of thrust for about 880 minutes. So, one Saturn V booster would last only 14.7 hours. For two six-month burns, we'd need the equivalent of 924 of them to reach Alpha Centauri. And another 924 to get back to Earth. And even that very rough estimate is outrageously optimistic, because we have to accelerate all those billions of kilograms of fuel in addition to our 15,000 kg spaceship.

It's simply not possible to carry enough fuel to accelerate to near light speed with a conventional rocket booster. So, carrying fuel seems to be a non-starter for an interstellar mission. But what about an external push?

In my story Project Hermes  I propose using a solar sail to accelerate a tiny ship with a beam of sunlight. This light pressure is real, but it's not very strong (you can buy a radiometer on Amazon that demonstrates the effect). In the story I've imagined a solar boost satellite orbiting near the sun that amplifies the solar flux by a factor of a million. So, what would this concept look like?

We still need a thrust of 147,000 newtons to accelerate our 15,000 kg spaceship at 1G. And the solar flux near the sun can provide a push of 0.00008 newton per square meter. Multiplied by a million, the solar boost beam would provide a thrust of 80 N/m². So, our sail would need a surface area of 1840 square meters to produce 147,000 N of thrust. A round sail 48 meters in diameter would do the job nicely.

Is it possible to build such a boost satellite near the sun? Could our spaceship actually 'balance' on the end of the beam for six months? I don't know those answers, but the concept seems more feasible than carrying billions of kg of rocket fuel.

Of course, there's still the question of deceleration at Alpha Centauri and then the return trip to Earth. How does our spaceship accomplish that with very little fuel on board?

My solution in the story is to not slow down at all. I have the ship do a gravity slingshot maneuver and retrace its path toward Earth. As it approaches our solar system again, it's decelerated by the same solar boost beam.

Is such a maneuver possible? Is it worth it to make a 12-year trip for a 15-minute pass through the Alpha Centauri system? I don't know, but again, it seems more feasible than an impossibly huge rocket.